Guerrilla guide to CNC machining, moldmaking, and resin casting

• Hanita - full end mill range (note: most are "e8")
• Nachreiner - carbide finishers
• Harvey Tool - all tools (note: some have non-symmetrical tolerances)
• OSG - end mills (HTML only)

• Hanita 7N2201021 - square, 2 flutes, TiAlN, diameter 1 mm (0.98), reach 10 mm, length 50 mm, or
  Harvey 35440-C3 - square, 2 flutes, TiAlN, diameter 1.016 mm, reach 12 mm, length 63 mm
  Price: $30-35
  Two comparable, good, and somewhat pricey cutters for low-speed, precision machining of small features, such as larger gearwheels, very small screw holes (diameters needed for 1.2, 1.5, and 2 mm screws), slots, motor shaft mounting holes (1 - 2 mm), and other fine detail. Reach length of about 10 mm is very generous, but small enough to make the cutter reasonably easy to handle. These tools would not tolerate experimentation well, though - play by the book or lose $30. Short-reach (2-5 mm) alternatives that cost under $15 are easy to find and require less care - but will fall short in some applications.
• Hanita 401403000 or 402403000 - square, 4 flutes, TiAlN, diameter 3 mm (2.98), full-length reach, length 63 or 75 mm
  Harvey 33708-C3 - square, 3 flutes, TiAlN, diameter 3.175 mm (3.15), full-length reach, length 63 mm
  Price: $15-$25
  Lower-cost baseline cutters for almost all operations in my workshop. A great and robust choice for roughing and finishing in prototyping materials. These tools can take a lot of abuse, can reach real deep, and remove material efficiently. You can get two, as you will likely be tempted to experiment a bit - and breaking this one won't be too painful.
  You may also want to have a look at Harvey 35508-C3 - a more expensive ($35) tool with stub flutes and reduced neck; it may offer better finish in deep pocketing operations, and is less prone to chatter; so are shorter-reach cutters (although the utility of these is, again, somewhat limited).
• Hanita 402103000 - ball, 2 flutes, TiAlN, diameter 3 mm (2.98), full-length reach, length 75 mm
  Hanita 422845-000030 - ball, 4 flutes, TiAlN, diameter 3 mm (2.98), full-length reach, length 75 mm
  Harvey 32308-C3 - ball, 4 flutes, TiAlN, diameter 3.175 mm (3.15), full-length reach, length 63 mm
  Price: $20-$25
  Good all-around cutters for working on organic shapes, or as a roughing end mill for molds where using a square cutter all the way through the cutting process results in undesirable tool marks.
  A more expensive but possibly better stub flute equivalent is Harvey 35708-C3 ($35).

• Hanita 463200400RT - square, 2 flutes, TiAlN, diameter 0.4 mm (0.38), reach 1.5 mm, length 38 mm
  Harvey 73016-C3 - square, 4 flutes, TiAlN, diameter 0.406 mm, reach 1.5 mm, length 38 mm
  Price: $15-$20
  Low-cost cutters for (shallow) machining of very fine features and routing in relatively thin material. Might be of limited use in larger scale projects, but will give you a good opportunity to explore the fine machining capabilities of your unit, and to experiment with tiny gearwheels, very small designs powered by pager motors, etc. Note that unless you play by the book, you are almost guaranteed to break it right away. They will also break if dropped from as little as few cm on a soft surface.

  Also consider Hanita 7N2200410 or Harvey 35415-C3 as more expensive but longer-reach alternatives for more complex gearwheels. Long-reach cutters down to 0.3 or 0.2 mm may make sense in some applications, too; but don't buy them up front - you definitely need to master 0.4 mm cutters first.

• Hanita 7N2202041 - square, 2 flutes, TiAlN, diameter 2 mm (1.98), reach 30 mm, length 75 mm
  Harvey 48978-C3 - square, 2 flutes, TiAlN, diameter 1.981 mm, reach 30 mm, length 63 mm
  Price: $35
  Great cutters for medium-precision work, including pocketing 3 - 4 mm screw holes, many slotting operations, etc. They are too large for making small gearwheels and so forth, but can be operated much faster, and cut much deeper, than 1 mm cutters. You might also want to consider 1.8 mm diamter, 20 mm reach cutter instead - which is suitable for cutting 2 mm screw holes.
• Hanita 401406002 - square, 4 flutes, TiAlN, diameter 6 mm (5.97), full-length reach, length 75 mm
  Harvey 15216 - square, 4 flutes, diameter 6.35 mm, full-length reach, length 63 mm
  Price: $10-$30
  Good roughing cutters for large size, large volume objects, assuming you want to do this type of work. This cutter is nearly impossible to destroy when working in prototyping materials on medium power mills, so in all cases, buying one is enough.
• Hanita 401006002 - ball, 4 flutes, TiAlN, diameter 6 mm (5.97), full-length reach, length 75 mm
  Harvey 74499-C3 - ball, 4 flutes, TiAlN, diameter 6.35 mm (6.325), full-length reach, length 63 mm
  Price: $20-$30
  Good roughing and finishing cutters for medium to large volume objects with curved, organic shapes. Like the previous one, nearly indestructible.

7. Getting stock material (and a bit on the purpose of moldmaking)

The aforementioned boards in this range resemble wood, are non-abrasive, fairly lightweight, robust, and can be easily cut with a jigsaw (but do not expect to cut a 50 mm board with a hand saw, unless you are the master of Zen). They can be also machined at amazing speeds and produce relatively little volatile dust. Multiple slabs can be seamlessly glued together to make larger workpieces, too. Although some folks try to keep costs down by starting with other media - most commonly, pine, fir, plywood, PVC, or acrylic - in my experience, it's just not worth the effort: you will wear out your tools, waste a lot of time, and still not get anywhere near the results your machine is made for.

          .- silicone poured in                                                    .- polyurethane poured in
          |                                                                        |
  __      v     __                         __________                              v
  ::|    __    |::                        |@@@@@@@@@@|         rotate          ___    ___                          ___
  ::|___/##|___|::           ->           |@@@/  |@@@|           ->           |@@@|__/@@@|           ->           |##/  
  ::::::::::::::::                                                            |@@@@@@@@@@|
      
  Positive form (PU)               Negative cast (silicone)                                                       Final part (PU)

7.1. Silicone rubbers for molds

• Condensation cure compositions that use organic tin compounds as catalysts (commonly dibutyltin dilaurate) to trigger polymerization. These are the most common, affordable mold making silicones, although their performance might be lower than that of the addition cure resins, viscosity higher, and they might have limited shelf life once polymerized.
  Organotin compounds are also somewhat irritant or harmful if misused, and have a slight but not particularly pleasant smell (sometimes likened to stale beer). In the quantities used, nothing to lose sleep over, as long as you follow certain basic rules laid out later.
• Addition cure RTV-2 rubbers that use platinum catalysts to trigger polymerization. Cure rate may be accelerated or decelerated quite spectacularly by varying temperature within a convenient range.
  These compositions generally enjoy superior physical characteristics, including much lower shrinkage while curing and idefinite shelf life of the polymerized product - and their components are non-toxic and odor-free. They are more sensitive to contaminants and incompatible chemicals until cured, however - so for example, if you want to dye them, you will likely have to use non-reactive, powdered organic pigments.

• Stiffness (derived from tensile strength and elongation at break):
  When it comes to mold making, you want the rubber to be flexible enough to permit parts to be removed effortlessly - but it must be stiff enough to maintain dimensional accuracy when laid on a reasonably flat surface, filled with a liquid polyurethane resin, and then clamped or covered and pressed down for the resin to cure.
  There are several indicators of stiffness, most of which are not consistently advertised by all manufacturers; a ballpark estimate, however, may be trivially derived from the two values you can usually look up: tensile strength and elongation at break.
  Tensile strength by itself documents how much force per surface area needs to be applied to a standardized test specimen to break it by uniformly pulling it apart. The parameter itself is of secondary significance for determining how durable the mold is - tear strength, explained later on, is more important. That said, this parameter comes handy for the calculation we are about to make.
  The other relevant parameter, elongation at break, denotes deformation at maximum tensile strain, usually expressed as a percent (0% is no elongation). If you divide tensile strength in MPa (1 MPa = 145 psi) by elongation at break expressed as a ratio (1 + elongation_at_break / 100%), you will arrive at a reasonably decent ad hoc indicator of how the rubber deforms in proportion to the force applied.
  I advise shopping for silicones where this factor is in the 1 - 4 MPa range. Rubbers below 1 will be progressively more squishy, and in some cases, will require careful mold engineering to maintain dimensional accuracy; while rubbers over 3 or so will be stiff enough to make removing them from the master mold a bit of a challenge (although this can be managed by the introduction of draft angles or pull tabs). For relatively shallow molds (up to about 15 mm deep), silicones closer to 5 MPa should be fine, too.
• Hardness:
  Indentation hardness describes how much the material deforms under a localized, surface pressure; this is not a very interesting parameter by itself, but it is used as the primary key by which silicone rubbers are sorted in product catalogs. For a given class of polymers, it is vaguely correlated with stiffness, but not very reliably so. Still - it's a good way to quickly narrow down your searches.
  Reasonably user-friendly silicone resins typically come with hardness between 3 and 60 Shore A for the cured rubber (Shore A is a 0-100 indentation measurement scale devised for elastomers); some harder compositions exist, but with very few exceptions, they become progressively less useful for general applications due to worse physical characteristics and very high viscosity. Silicones under 10 Shore A are jello-like, and used primarily for special effects (faux skin), "gel" bicycle seats, and so forth; 20 Shore A is a squishy, stretchy rubber (think: rubber band); 40 Shore A is firmer, like rubber seals, pads, and so forth; and 60-80 Shore A is still very flexible, but much like a tire or a pencil eraser or a tire - easy to bend but largely impossible stretch by hand. The scale ends with materials that have a hard surface that can't be indented with a finger or a fingernail.

  Silicones over about 20 Shore A are marketed as suitable for mold-making; but I strongly recommend to sticking to resins at or above 40 Shore A. In fact, 55-80 Shore A works even better for many types of precision work, if you design your molds well.

  Keep in mind that hardness and stiffness of polyaddition resins can be trivially lowered by mixing in 5-50% of silicone oil (though at the expense of tensile strength), so if you select a very hard composition, you will be still able to produce soft parts where needed; going the other way round is harder: certain grades of fumed silica may help, and so will curing the resin at an elevated temperature - but there is not that much latitude.
• Tear strength:
  This parameter is a close relative of tensile strength, but describes the force needed to break a standardized specimen by applying a non-uniform stress to the material, simulating the process of tearing it at the edge. It is a better indicator of mold and part longevity than tensile strength itself, as the force needed to break the mold this way is much lower, and more likely to be accidentally applied when demolding parts. It also varies more significantly from one composition to the other.
  Roughly speaking, silicone resins can be divided into "low tear" and "high tear" varieties, denoting how well they respond to this type of abuse. "Low tear" compositions are somewhat cheaper (10-20%), but molds made from them often barely last enough to make several complicated parts in a row, and they are not a very good material for tires and other final components that are expected to withstand abuse.
  For moldmaking rubbers, tear strength of at least 15-20 kN/m is preferable (1 kN/m = 5.7 ppi, pounds per inch). Strength under 15 kN/m means the rubber will be fairly fragile. Around or below 10 kN/m, the material will frequently not survive demolding intricate parts. Some compsitions that go up to 30 kN/m are available, and can survive almost any abuse.
  As usual, this parameter is just a rough estimate of how the material will really perform: tear propagation is a complex topic.
• Catalyzed viscosity:
  Flow characteristics of liquids are difficult to parametrize, yet very important for casting: resins that flow poorly and exhibit high surface tension, will be significantly more difficult to work with, as they would trap air in tight spots, and prevent bubbles from rising to the surface and bursting fast enough. Dynamic viscosity is usually the only parameter given by manufacturers, and within a specific class of formulations, it usually gives you a good picture of how the resin will perform next to its close relatives - but remember that it does not enable you to compare the ease of use across different chemistries!
  With silicones, you can generally expect viscosities up to 10,000 mPa*s to be very easy to work with, largely self-degassing, and easily conforming to complex molds. Viscosities closer to 20,000 or 30,000 will require careful application, possibly with a brush or a syringe, to reproduce intricate details - or a quick round of in-mold degassing with a vacuum pump. Compositions getting closer to 50,000 - 100,000 end of the spectrum will almost always require vacuum, or a lot of really mundane and error-prone work, when dealing with complex, miniature shapes (but are perfectly OK with large and simple geometries).
  Of course, even within somewhat comparable formulations, things can get weird. Polytek PlatSil 71-40, a resin with a viscosity of 25,000 mPa*s, is very hard to degas (it expands very significantly before any of the bubbles burst, and needs a fair amount of time under vacuum). ShinEtsu KE-1310, a 70,000 mPa*s composition, is actually slightly easier to handle; and QM 280, a 90,000 mPa*s rubber, is the easiest of them all, probably because of its dilatant flow characteristics that make bubbles pop very early on.
• Pot life (also known as gel time):
  Mixed resin begins gradually polymerizing right away, which results in increasing viscosity. The effect becomes noticeable around 25% through the "pot life" period; and by the time 100% is reached, the resin is no longer free-flowing and self-leveling, which makes it nearly impossible to work with.
  For silicones, something around 30 minutes might be OK for low viscosity compositions; closer to 40-90 minutes is much better when dealing with high viscosity variants. In most cases, cure can be easily accelerated if needed by raising the temperature to about 30° (with some minor trade-offs) - but it's more difficult to slow down, so stay on the safe side.
• Shrinkage:
  As the molecular structure of the resin changes when curing, some changes in volume may occur, most commonly resulting in a slight reduction in overall volume of the substance. Typically, prototyping resins are carefully engineered to minimize this effect - but you should still check this parameter.
  Different manufacturers seem to have different testing protocols, but the most common methodology test a specimen of 500 x 50 x 5 mm or so, and examine reduction in length expressing it either as a percentage, or as mm/m ratio (multiply this by ten to get percentage - obviously); some companies use a 4 mm specimen instead - and this yields a slightly lower figure. In the States, shrinkage may be also measured using a D-shaped mold, about 250 mm long, as per the ASTM D-2566 standard; Huntsman commonly uses a 19 mm diameter ("mold #0"), Freeman uses 22 mm, while Innovative Polymers uses 25 mm ("mold #1"). The good thing about standards is that there are so many of them to choose from!
  In general, condensation cure silicones should exhibit shrinkage under 0.6%, which is, unfortunately, not always negligible: a 20 mm gearwheel may turn out about 0.1 mm smaller than designed. Some condensation silicones with shrinkage closer to 0.2% can be spotted, and are obviously better. Addition cure compositions, on the other hand, are usually well under 0.1% - in practical uses, often better than 0.01% or so - and for all intents and purposes, you simply don't have to care.
  IMPORTANT: In addition to the inherent curing shrinkage, silicones are also appreciably affected by thermal expansion; the coefficient of thermal expansion is fairly high in silicone rubbers - about 0.02% to 0.03% per 1° C (up to one order of magnitude higher than the coefficient for prototyping boards and parts made from rigid polyurethane resins). Because of this, if the rubber is not yet sufficiently polymerized, raising the temperature will increase apparent shrinkage: the material will polymerize to conform to the mold at an artificially increased volume, only to shrink to a smaller volume when cooled down. Worse yet, even if the rubber is cured at room temperature, but then heated up appreciably with the polyurethane resin inside - the resulting parts may be warped or otherwise deformed due to changes in the dimensions of the mold.
  In other words, addition cure silicones are preferred, and going more than several degrees over (or below!) the temperature at which you intend to use the mold is not recommended during the process - that is, unless dimensional accuracy is not critical.
• Color:
  Transparent (well, translucent) rubbers are essential for making multipart molds for non-CNC items (so that a submerged template part can be carefully cut out from a cured mold), and in general often make your life much easier, allowing you to spot air bubbles and other problems in the castings before they set. These resins are, however, a bit more expensive, usually a lot harder to degass, and simply available in a fairly limited selection - so don't get your hopes up. They are also not crystal clear in layers thicker than 1 mm or so, so their utility in decorative applications is limited.
  For non-translucent rubbers, the color matters if you want to use the resin to cast any final parts (e.g., rubber tires) out of it. Some casting resins are neutral white, light gray, or beige; these compositions take pigmentation reasonably well. Unfortunately, many compositions are "helpfully" dyed blue, green, or red, to provide an indicator of proper mixing - and in these cases, you lose this freedom.
  I would not consider the color to be a matter of top importance, as polyurethane elastomers are a cheaper way to make rubber-like parts anyway - but when picking between two otherwise similar silicones, it's definitely something to keep in mind. Some manufacturers, when asked, may be able to provide dye-free variants of their products, too.

7.2. Alternative to silicones: polyurethane rubbers

The price for polyurethane rubbers should be around $16-$30 per kg.

7.3. Rigid polyurethanes

• Stated purpose:
  Amusingly, as with polyurethane elastomers, this is often more telling than any measurements. If the manufacturer hints that the product is meant to simulate engineering plastics (e.g., ABS, polyamide / Nylon, polyoxymethylene / acetal, etc) for prototype parts - great. If they instead talk about tooling fixtures, conceptual prototypes, scale models, and so forth - be wary, as the resin might have some shortcomings that are not evident when looking at cold, hard numbers alone. For example, flexural strength in thin sections may be disproportionately low. If in doubt, ask.
  For the same reason, you probably want to stay clear of resins designated for machine vacuum casting, unless they have a long pot life (over 10 minutes or so) and do not require heated molds. The designation is usually there for a reason, and may mean that the resin is particularly painful in terms of mixing, degassing, etc. Some vacuum resins, such as Innovative Polymers VA-274, are perfectly fine; but most others are not fun to work with.
• Presence of fillers:
  The addition of plastic, mineral, or metal fillers may improve hardness, thermal conductivity, or abrasion resistance of the resin, impart an interesting finish, or reduce shrinkage. On the other hand, heavily filled systems will be viscuous, may be difficult to degas and pigment, and may be more brittle in some uses. Shorter shelf life due to the filler settling down is also common.
  Your primary rigid casting resin probably should not be a filled one. In cases you need to change the properties of an unfilled resin, nothing stops you from adding fillers on your own - and we will discuss this process later on.
• Flexural properties:
  To understand the mechanical properties of a particular resin, you need to look at a couple of parameters created to describe the material's behavior in a variety of scenarios. Perhaps the most important pair are flexural strength and flexural modulus. The first number describes the sustained bending pressure (force per surface area) at which a standardized test specimen (around 10 x 4 x 80 mm) breaks - the lower it is, the more fragile the material. Flexural modulus, on the other hand, describes the ratio of the bending force to the degree of elastic deformation observed in a test specimen. The higher the modulus, the stiffer the part.
  In principle, a component with a high flexural modulus but low flexural strength is rigid, but brittle (a good example is a biscuit); a component with low flexural modulus and high flexural strength will not break easily, but will deform significantly under load (think plastic PETG bottles).
  Polyurethane resins vary greatly when it comes to flexural parameters, depending on the intended purpose and the additives used; flexural strength usually lies somewhere between 30 and 120 MPa. Aim high: standard resins under 80 MPa may be fragile enough to be a major limiting factor in some designs. For rigid general-purpose resins, there is little or no reason to settle for less than 100 (which is roughly equivalent to acrylic glass or polycarbonate - try breaking a CD to get a picture).
  When it comes to flexural modulus, most of the time you want your rigid components to, well, stay rigid. Acrylic glass and ABS both are in the 2.5 - 3 GPa range here; unfilled polyurethanes that go slightly higher - into the 2.7 - 3.1 GPa zone - are commonly available and of interest.
  Adding low-cost fillers to an unfilled system can easily double or triple both these parameters, resulting in a plastic of superhuman strength; we'll talk about this later on.
• Thin section behavior:
  While flexural parameters are among the more important indicators of the robustsness of a plastic, the picture isn't quite complete yet. Consider a soda bottle made out of PETG: flexural strength of the polymer it is made of is somewhere around 70 MPa, while the flexural strength of acrylic glass is closer to 110 MPa. But go on and step on an empty soda bottle, and then on an an acrylic picture frame (cost of stitches not refunded), and compare the results: chances are, PETG will emerge vicious. The important difference is that in thin sections, PETG has a much higher ability to elastically deflect and store energy: at 1 mm or below, it may quite simply be impossible to break it by (non-repetitive) bending alone, even though it snaps easily in a thick layer used in the standard test.
  This behavior actually matters in some cases: a real-world scenario where this property is of critical significance are sub-millimeter gear teeth exposed to high torques and shocks when starting or braking: if the thin material can deflect momentarily under critical stress, it may make all the difference between normal operation and a catastrophic failure.
  The complex, non-linear flexural and shear behaviors in thin layers are usually not tested or documented in resin datasheets - but can be to some extent inferred from two other parameters: elongation at break (a parameter familiar from rubbers) and notched Izod impact strength - a measure of the vulnerability of the material to violent impact. Elongation of around 5-15%, and impact strength of 0.3 - 0.5 kJ/m² (convesion hint: 1 kJ/m² = 2.1 ft-lbs/in²) are commonly seen - and mean that much like with acrylic glass, acetal, or polystyrene, even very thin films of the plastic should be expected to be fairly brittle. But then, there are quite a few rigid resins with elongation in the 30 - 100% range, and impact strengths between 3 and 15 kJ/m² - and these will exhibit superior flexural characteristics in thin sections, more closely resembling ABS (Lego bricks), polypropylene (Tic-Tac lids), polyamide (servo gears), or polycarbonate (shatter-proof windows).
  You don't have to buy these impact-resistant, flexible plastics - but at the very least, be aware of their existence; they may come handy.
  (Just FYI, you can also see plenty of data on known polymers and composites on this excellent page.)
• Hardness:
  Just like with silicones, this should not be the primary selection criteria, but is a good way to quickly find the resins worth looking at. For rigid plastics, a separate scale, Shore D, is commonly used. Typical polyurethane resins span between 50 and 90 Shore D; I recommend sticking to the 80-90 range, as this usually maps to excellent rigidity, dimensional stability, scratch resistance, and polishability; but high elongation and high impact strengths justify excursions into the 65-70 Shore D world.
  (For the curious: Shore A measurements use a force of about 800 g and a flat-tipped indenter with a 0.8 mm tip. The penetration is then measured from 0 to 2.5 mm. Shore D, on the other hand, uses 4500 g and an indenter with a sharper 0.1 mm ball point. For polyurethane and silicone resins, 50-90 Shore A usually maps sort-of linearly to around 10-40 Shore D, but outside this range, even ballpark conversions are not very useful. )
• Pot life:
  Many polyurethane resins react very quickly - pot life between 30 seconds and 4 minutes is pretty common. This may sound like enough if you just want to pour the resin in a simple mold - but if you need to degas it after mixing, then apply it to a narrow, multi-part mold - you will run out of time. Quickly curing resins are also more exothermic, limiting the maximum size of a casting you can make without overheating the system or seeing excess shrinkage due to thermal expansion of the liquid material or the mold.
  To complicate your life a bit, while silicones have a fairly linear cure profile, polyurethanes are less predictable: some may exhibit a long period of low viscosity followed by a snap cure, while others may get difficult to handle 2/3 through the nominal pot life. Plus, many resins undergo a gradual phase change, requiring very thorough mixing (90 seconds or more) before you can move any further; if you cut corners, the result will not cure properly. All in all, I strongly recommend opting for systems that give you at least around 8 minutes to wrap things up; 10-20 minutes is even better. Don't overdo it, though: while there are some resins with pot lives of 40, 60, or even 100 minutes, they take forever to fully cure, especially on cold days or in thin layers - and tend to be annoyingly sensitive to moisture.
  Much like for silicones, the total curing time before the part is ready to be used might sound discouraging on the slower compositions (2 to 24 hours to demold, plus up to a week to reach final properties) - but once the resin is reasonably solid, further polymerization can be safely accelerated by carefully increasing the temperature. That said, this process needs to be done with care: overheating a liquid resin may ruin it altogether, and you also need to be mindful of the thermal expansion of the mold and the material, as discussed earlier (thermal expansion of cured polyurethane is, again, about an order of magnitude less pronounced).
  In the end, you will need to work out the right cure schedule depending on the resin you're using; for one of my favorite resins with a ~10 minute pot life, I allow at least 90 minutes at room temperature, until the resin is fairly hard, followed by 60-90 minutes at 30° C. At that point, the mold should be allowed to cool down (putting it on a working fan speeds this up significantly; cold water is not recommended, as it causes a very rapid and uneven cooling, which may contribute to warping). Once this is done, the part can be removed and put in an oven at 60° C for 30 minutes, 80° C for another 30, and 100° C for the last 2-4 hours. As long as the heating and cooling is gradual and even, and the part is properly supported, this preserves dimensional accuracy perfectly well.
• Mixed viscosity:
  You can find systems with viscosities starting in the 50-100 mPa*s range, but there might be some strings attached (such as higher shrinkage or reactivity). Resins around 300-1,500 mPa*s are common, and very convenient to work with. I do not recommend going much past 3,000 mPa*s or so, especially if pot life is under 10 minutes; polyurethanes behave somewhat differently than silicones and seemingly have a higher surface tension, so, say, 6,000 mPa*s is pretty challenging to degas and cast right in the comparatively shorter timeframe allotted.
• Shrinkage:
  Optimally under 0.1-0.2% ("D-2566 mold #0" or the EU method), but don't read too much into this parameter: it's somewhat removed from reality. In principle, if the mixture remains at room temperature during the cure, the change in dimensions will be usually completely negligible, similar to addition cure silicones; but because the polymerization reaction itself is exothermic, the temperature may rise significantly when casting large-volume parts - and the thermal expansion of the still-liquid resin will affect dimensional accuracy.
  The problem with the standard shrinkage test is that it involves a bulky slab of resin, and so, the exotherm plays a central role (and is to a large extent inversely proportional to the resin curing speed). But even if you were to cast something of that shape, the result you'd see would be different: the measurement is done for a steel mold, while a silicone mold will usually have sections that deflect more easily than the rest - and the volumetric shrinkage will focus there, rather than being distributed uniformly across the entire part.
  All in all, look at shrinkage as a comparative metric, and weed out the extremes, but don't assume that it translates into the results you will be seeing. In practical terms: when working with an unfilled resin with a pot life of about 10 minutes, you will probably see virtually no shrinkage when casting a part that is 30x10x1 mm thick, because the curing exotherm is very low. But make the part 10 mm thick, and the peak exotherm may hit 50° C or so; in this case, in the top center section of the mold, the size may be off by 0.2 mm or so.
  Thankfully, whenever you suspect this will become an issue, you can use one of several simple workarounds:
  1. Mix and match: mechanical properties of the cured resin are dictated chiefly by the polyol; therefore, substituting some of the isocyanate with a less reactive variant will slow down the reaction significantly, but not have a substantial impact on the parameters of the final part. In particular, it's possible to blend IE-3075 and TD-283-18 isocyanates, and use the standard IE-3075 polyol, at a ratio of 50:50:78 - and get a decent, slow-curing resin with very little exotherm and practically zero shrinkage.
  2. Partly pre-cure the resin: pre-mix the desired amount of isocyanate with around 20% of the required amount of polyol; then set this mixture aside for 30-60 minutes to polymerize and cool down, before actually adding the remaining amount of polyol and casting. This increases viscosity (which to some extent can be undone by adding several drops of a plasticizer), but gives you up to a ten-fold reduction in shrinkage.
  3. Add an inert moderator: any substance that keeps reactive molecules apart can reduce shrinkage by up to 40% without appreciably affecting mechanical properties. Plasticizers are probably the best choice: around 10% of dipropylene glycol dibenzoate (available for cheap from Eager Plastics as EP9009) will achieve that goal at the expense of only slightly reducing flexural modulus and tensile strength. Powdered aluminum is a good alternative if flexural modulus is more important than color.
  4. Cast in several layers: this eliminates shrinkage almost completely, even in very large parts. The technique is also useful for making polyurethane rubber molds.
  5. Make the mold thermally conductive: for relatively small parts, adding 1:1 by volume of copper powder (50 - 200 µm or so) to the silicone mold itself can help reduce shrinkage.
  6. Allow shrinkage to be offset: add a reservoir of resin and a thin channel through which it can be sucked into the mold. The ducts can be cut off easily later on.
  7. Let shrinkage happen in non-essential locations: intentionally make a section of the mold thin-walled, and reinforce the rest.
  8. Compensate in software: shrinkage is hard to reliably predict in flexible molds (at least without specialized FEM software), but you can do this by trial-and-error if really necessary.
  Note that the first two techniques can be combined. If you are curious about the outcome, I graphed the pretty much worst-case in-mold exotherm (for a very heavy and compact blob of plastic) for various casting strategies and additives; you can check it out here.
• Color:
  There is absolutely no reason to buy a polyurethane resin that cannot be pigmented. All white or slightly off-white (light beige, gray, lightly straw-colored) resins that aren't heavily filled should respond very well to appropriate pigments. You most certainly want to have this liberty - so avoid resins that are already "helpfully" dyed blue, black, or somesuch; as well as ones with strong natural hues.
  If the manufacturer says the resin has a color such as "amber", "caramel", "cognac", or "tan", ask for a photo or some advice, as it's impossible to tell how pigmentable the resin may be from such a description alone: Huntsman RenCast 6491 and Innovative Polymers TP-4020 are both described as "tan", but the color of the former is essentially tea-colored, while the latter has a subtle cream color that poses relatively little problem.

(Comparable Nylon-like products include Huntsman RenCast 6492, 6470, 6419, etc; but they are either difficult to get, have uncomfortably short pot life, an undesirable hue, or are up to three times as expensive.)

In Europe, on the other hand, you should probably go with Huntsman's RenPIM 5219, also sold as RenCast 5146. It is pretty close to the IPI product on all counts - and likewise, very cheap (especially by EU standards). It has a hardness around 85 Shore D, ~100 MPa flexural strength, modulus of ~3 GPa, 20 minutes pot life, and a neutral white color that is easy to dye as desired - just a tad more opaque than IE-3075.

Spare for slightly elevated sensitivity to moisture, the cure profile of these resins is otherwise excellent: great surface cure, long low-viscosity stage, virtually no brittle stage, good tolerance for heat-acceleration.

...

With the basics covered, let's talk about more exotic choices that you probably don't need to buy up front, but should be aware of. For example, you might want to look up Inovative Polymers TP-4004: it has rigidity and hardness comparable with polypropylene (70 Shore D, flex modulus of 800 MPa), and a modest flexural strength - but a very high elongation at break (50-70%) and high impact resistance (3-4 kJ/m²); this makes it nearly impossible to break the material by bending sections up to 2-3 mm thick - an ideal choice for gears and other small features subjected to shocks. The polyurea chemistry imparts high temperature resistance, and - unfortunately - a relatively high price ($30 per liter or so); RenCast 6486 seems to be its HDI-based relative from Huntsman, but with a rather steep price tag of over $50 per liter. Interestingly, despite their substantial flexibility, the surface hardness of these resins makes them easy to machine and polish.

Other resins worth exploring are transparent compositions; they cost up to 2-3 times as much as their translucent or opaque friends, but they have an array of obvious, decorative or quasi-decorative applications; nothing beats a transparent gearbox. Innovative Polymers has an unlisted resin named TD-283-18, which is ridiculously user-friendly and tolerant of non-pressurized casting; its properties are pretty close to IE-3075 otherwise. They also carry several other formulations, such as VA-274 or OC-7086, but these are a lot more sensitive to casting conditions, and may be hard to befriend if you don't have a a pressure pot. The clarity of all these resins is unmatched by epoxies and polyesters. Axson PC 521, Freeman 1090, Crystal Clear 200, etc, are comparable in many aspects - but are mercury-catalyzed, so I don't really recommend them.

Transparent resins fetch up to $40 per liter. Unreacted components really must be kept in a dark place and shielded from heat and air to prevent slight yellowing due to oxidation. If you don't store them in a cabinet, their shelf life will be very limited (perhaps 6 months or so).

The one thing you should probably not get excited about are high flexural modulus resins such as TP-4020 or VA-288. They are created simply by taking some other resin (e.g. TP-4006) and pre-filling it with glass fibers - but using IE-3075 as a base and filling it manually results in better flexural properties, so don't buy them unless genuinely necessary (temperature resistance is their other selling point, and that's of note in some uses).

...

Random note #1: Some types of polyurethane resins can be blended with similar compositions, and will form viable copolymers: for example, IE-3075 can be modified by mixing in IE-40A or IE-70A to create a range of tough, semi-rigid elastomers, without the need to buy a separate resin. The trick does not work with all types of polyurethanes, though: for example, if a system consisting of a fairly non-reactive isocyanate and a reactive polyol is mixed with a system that relies on the opposite configuration, the reactive isocyanate from one system will immediately cross-react with the reactive polyol from the other (turning the resin into a sticky mess), while the outcompeted non-reactive components may fail to further polymerize at all. This is the fate of most attempts to mix a chemistry based on MDI with one based on more volatile isocyanates; or a traditional polyether polyol system with an amine-cured one. Last but not least, even if the reaction works out, you are not guaranteed to end up with the desired mechanical properties. Bottom line is, if you want to experiment, be prepared for some successes and some failures. You can always ask the manufacturer for advice first. You may also want to get Shore A and Shore D durometers off of eBay to check the properties of the resulting rubber ($35 each); or to devise your own simple testing protocols to comparatively examine the properties you care about. For example, testing flexural strength and modulus can be accomplished with a kitchen scale and a ruler.

Random note #2: Many rigid resins may also be rendered soft and somewhat flexible by adding between 5% and 40% of a plasticizer, such as the aforementioned dipropylene glycol dibenzoate. Similarly to silicone oil added to rubbers, such plasticizers do not chemically participate in polymerization - and so at higher concentrations, they will significantly decrease the tensile strength of the plastic. In non-critical applications, this option is more cost-efficient than resin blending, though.

Random note #3: As with polyurethane rubbers, crystallization and the settling of fillers are the two conditions to be aware of during long-term storage.TP-40xx products are particularly prone to crystallization, and the filler will settle in TP-4020. The procedure for correcting this problem is the same as discussed earlier.

7.4. Alternative plastics: epoxies and polyesters

7.5. Adding colors to resins

• It is easy to get dull earthy colors from more vivid pigments by mixing them together or adding neutral opacifiers, but it is not possible to make dull colors more punchy. So begin by completing a competent palette of brilliant, vivid colors before moving on to more artsy, exotic choices. The aforementioned liquid dyes and studio colors are a good starting point.
• It is harder than it may sound to accurately mix high-yield pigments in small quantities to get exactly the color you want, and especially for transparent colors, it does not work the way you might think it does: mixing pure red and pure blue dyes, for example, does not yield transparent violet; you will get black or off-gray instead, because the red dye absorbs blues, and the blue dye stops all transmitted red. In addition, some pigments can't be mixed in even more prosaic cases: fluorescent dyes exhibit an interesting quenching effect, where they lose their special properties if used in excess or mixed with each other. So, don't try to save a buck by buying just sorta-primary red, blue, and yellow dyes, and assuming you would be able to get every other color easily.
• Most synthetic, organic pigments are safe to use in plastic resins (e.g., phthalocyanate, thraquinone, pyrrole, perylene, alizarin, dioxazine based ones). This also includes metals and other chemicals encapsulated in acrylic or phenolic resins. Some organic dyes will dissolve or form translucent dispersions in transparent resins and give beautiful, rich colors. Some will make the resin opaque.
• Many natural organic pigments made from plants or animals are a gamble - and not to mention, they are pretty expensive. I would advise against going there.
• Some inorganic pigments are safe, particularly the ones based on silica and zirconium silicate doped with various metals, but a good number of them is not. Titanium white and most iron oxide pigments are OK; Prussian blue is not. Powdered metals are a mixed bag, as some of them may prematurely catalyze the resin.
• Be wary of pigments that are either known to be outright harmful (check labels), or are based on particularly toxic / bioaccumulating compounds; the latter, even if not soluble or harmful in their original state, may eventually break down and migrate if exposed to UV radiation or certain chemicals, so it's best not to take risks. More about this in the "health" section later on.
• Stay away from any ready-made dilutions, pastes, liquid suspensions, etc, unless specifically recommended for polyurethanes - or be prepared for some nasty surprises.
• Read descriptions for any unusual properties, such as limited compatibility with carrier media, poor lightfastedness, the need for wetting agents, etc. Be alert.

• Fluorescent palette, for the road-sign-grade punchy colors:
  Kremer 56150 (lemon yellow), 56100 (light green), 56400 (pink), 56300 (orange-red) is a good starterset; additionally, you might want to consider 56450 (violet), 56050 (light blue), 56200 (golden yellow). Pigment 56000 (white) is an interesting animal, too: it's actually translucent and not masking, but it acts as a powerful optical whitener (and adds the ability to glow in UV light) when added to some other colors.
  All these cost about $20 (EUR 10) for 100 g, an amount that should last for years, and are very easy to uniformly disperse in a resin. Their downside? Fluorescent dyes are pretty much impossible to photograph: not only the color will be very off, but the detail will likely be washed out. They are also not very lightfast, so long-term outdoors applications willbe an issue.
  Note that there is no "true" daylight fluorescent blue pigment. Most manufacturers produce it by adding optical whiteners and UV-fluorescent components, resulting in worse performance in daylight compared to yellow or red. Blue, violet, and green pigments may be hence less spectacular under a tungsten lamp on your desk - they will, however, light up nicely in UV or blue light. A must-have for party robots!
• Studio colors, for the low-cost general, crisp "plastic grade" hues:
  Say,Kremer 55100 (lemon yellow), 55300 (dark red), 55500 (light blue), 55600 (dark blue), 55700 (light green), 55900 (violet), plus maybe 55470 (pink) or 55450 (bordeaux) depending on your personal taste. These generally cost $10 for 100 g (EUR 2-3), with comparable efficiency. They are less striking than fluorescent pigments (exactly the shades you see in most plastic products around you), and do not glow in UV - but a wider palette and darker colors are available. Very easy to disperse.
• Opacifiers:
  Kremer 46200 (titanium white), 47400 (spinel black). Both have an insanely high yield, so you will never ever run out of them if you get 100 g or so ($10 / EUR 4). You can use to control the brightness and opacity of a resin ("natural" opaque polyurethane is translucent in thin sections), to mask out some pigmenting mistakes, and to easily achieve shades of gray. Note that they will be fairly taxing to disperse uniformly in a resin (start with a small amount of plasticizer or isocyanate).

• Assorted individual pigments:
  There are several interesting organic pigments that nicely complement these basic palettes. One example is DPP red (Kremer 23180), a deep and pretty hue of red historically used for Ferrari cars, and really striking when polished to luster; other examples include high-yield bright yellow (23650), nice orange (23178), grass green (23010), or primary blue (23050). Or, dioxazine violet (23451) - a very dark, steel shade. As a general rule, they will be tricky to disperse, and sometimes, making a paste with a minimal quantity of a resin or a surfactant / plasticizer might be the only way to properly wet them.
  If you shop for these, it's best to read up on standard pigment naming schemes ("Color Index") to avoid duplication, and to look up additional samples on the web. Some useful notes on various pigments may be found at this site. It deals primarily with watercolor painting, but has an extensive database of pigment descriptions, CI numbers, and so forth.
• Transparent dyes:
  Only if you intend to make clear polyurethane castings or want to mix colors. If you want to experiment, grab Kremer 94400 (ORASOL lemon yellow), 94402 (ORASOL gold yellow), 94406 (ORASOL orange), 94412 (ORASOL red), and 94414 (ORASOL blue). You will pay about $10 (EUR 4) for 5 g, but this amount should suffice for several bucketfuls of a resin. Note that these dyes are pretty challenging to disperse uniformly, and 5-10 minutes of constant mixing might be required; preparing a solution in plasticizer is my preference.
  Alternatively, you can buy, say, Eager Plastics EP7701: scarlet red, violet, royal blue, lime green, bright orange, lemon yellow (should be $5 a pop, with a slightly lower yield) - but as noted, these premade solutions will almost certainly not play well with polyaddition silicones.
• Fancy pigments and glitter:
  If you have some money to spare, and are curious: Kremer 56550 is a phosphorescent blue dye, 56650 is red; 50440 is a red-metallic pearlescent pigment that looks pretty stunning, and 50180 is gold. Aluminium-based glitters are 50800 for gold, 50701 for silver; 54850 is fine copper powder (looks rusty at first, but turns shiny metallic once polished); etc. Transparent powdered glass pigments give pretty interesting effects in transparent resins, too. Be sure to check out Paint with Pearl store, too - they have some very interesting pearlescent and metallic pigments for less.
  Note that some of the large-grain glass and metal pigments would inevitably settle in a mixture, given enough time. You can either apply them directly to the sides of a mold, where they would stay; or you thicken the resin by adding fumed silica (Cab-o-sil) or pre-reacting some of the isocyanate and polyol first.

7.6. Creating reinforced plastics

8. A word on solvents

• Isopropyl alcohol: for various cleanups, removal of excess grease, soldering paste, water-soluble contaminants, etc. Safe on almost all plastics, although might reversibly soften some types of polyurethane rubber (use this to restore it to original shape if deformed under strain). Low toxicity in normal use, minimal environmental burden.
  Ethanol is a superior alternative, but a pure variety is considerably more expensive due to miscellanous taxes. In some markets, denaturated alcohol is a good choice; elsewhere, it may contain dyes or aromatizers that make it less useful or nauseating to work with.
• Light naphtha: non-polar solvent for waxes, paints, uncured silicone, etc. Will reversibly (and often quite grotesquely!) swell silicone rubber. Toxicity will differ depending on contaminants and exact composition (e.g., presence of benzene), but OTC solvent varieties (say, VM&P naphtha) should be only modestly harmful; they do have a more pronounced environmental impact, however, and for some annoying reason, are mostly banned in California.
  Tip: if applied to silicone rubber topically, this solvent makes it possible to glue it using cyanoacrylate glues, otherwise an impossible task. Some companies would actually sell you grossly overpriced naphtha as a "surface activator" for joining silicone rubbers (I mean, $10 for 20 ml).
  Heavy napthas are an acceptable alternative, but take considerably more time to dry, and many varieties have a more nauseating smell.
• Ethyl acetate: softens and possibly dissolves many plastics, some very quickly (polystyrene, PMMA). Useful for preparing plastic-plastic bonds, etc. Reasonably low toxicity and not a major environmental burden. It is also a good tool for speeding up the degassing process for high-viscosity or heavily filled polyurethane resins.
  Acetone and methyl ethyl ketone (MEK) are good replacements, but attack plastics more aggressively. They also happen to be (sloppily) regulated in some markets as precursors to some drugs, sometimes requiring IDs or silly in-store paperwork to purchase them.

9. Release agents

10. Casting tools and other workshop stuff

• Small bow saw with metal and wood blades,
• Scalpels or box cutters,
• Heavy duty scissors,
• Small wire cutters,
• Precision pliers, tweezers, etc,
• A set of precision screwdrivers,
• A set of needle files,
• A decent vise and clamps,
• A reasonably small hobby drill,
• Good, precision soldering iron (Weller WMRP rocks, if you are too rich).

• Precision weighing scale:
  Mixing ratios for all casting resins are typically given in parts by weight, rather than parts by volume, simply because weight can be measured more reliably and is not significantly affected by air bubbles, temperature differences, etc. You probably have a kitchen scale already, but for consistent results, it is strongly advisable to get a more precise device. Digital scales with 0.1 or 0.05 g resolution, and capacity up to 1 kg start at about $10-$20 (say, from there), and work surprisingly well for pretty much all the uses imaginable; when you need to measure 1 kg or more, which is pretty unlikely, you can fall back to a kitchen scale.
  I generally do not recommend settling for resolution of less than 0.05 g or so, as less precise devices would limit your ability to work with small quantities of resins. For example, on a scale with 1 g resolution, where you need to mix 10 g of an addition cure silicone resin with 1 g of catalyst, but may easily end up preparing 0.6 g or 1.4 g (both of which will read as "1"), grossly exceeding the specs. This would force you to prepare, say, 50-100 g minimum even for the smallest job, and throw away the excess - so much for the savings.
  Polyaddition silicones and several types of polyurethanes are very sensitive to mix ratios, and will not cure well with as little as 1% off; other resins are more forgiving, but you should aim to always keep the error under 2%.
• Micron-scale dial indicator with a base (optional):
  A cheap but useful measurement tool that allows you to check for runout of an end mill (i.e., the eccentricity of its rotation). It is not a necessity, but a good habit, to check runout after every tool change, as some factors, such as dirt on the collet or on the tool, or insufficient clamping force, may easily cause the tool to not only rotate around its axis, but also swing up to 0.02 mm to the sides, reducing finish quality and dimensional accuracy.
  Gauges are fairly inexpensive, in the $25-$50 range (example), so there is little or no reason not to get one. Be sure to grab a model with 0.01 mm gradation (0.0005") or better. Magnetic base with an articulated arm is very useful, too.
• Vacuum pump and hoses (optional but strongly recommended):
  Casting resins contain small amounts of dissolved atmospheric gases. The solubility of these gases may be significantly reduced as the resin polymerizes, and so the material may develop tiny air bubbles that may reduce its strength and affect surface finish. Furthermore, as a result of pouring and stirring, more viscous formulations may develop suspended air bubbles that take longer to emerge and burst than the resin actually remains liquid; and last but not least, reactions with moisture in pigments or in the air may contribute some more.
  A way to address all these problems is a process called degassing, where ambient pressure in a container with the resin is quickly and significantly decreased, as to facilitate a rapid evacuation of all gases from the solution itself; existing bubbles will expand to large sizes and raise rapidly, and dissolved gases will leave the resin likewise. Although many resins give reasonably good results without degassing, I strongly recommend degassing all the components prior to mixing, and doing another quick round with catalyzed resins too, to improve physical properties and detail reproduction. Not doing so gives much less predictable results.
  Atmospheric pressure is about 1.01 bar (101 kPa), and perfect vacuum is 0 bar. Vacuum pumps might have their performance expressed in terms of the maximum pressure differential they are capable of producing (for example -800 mbar, meaning that the usual final vacuum will be about 210 mbar on a typical day), or the ultimate vacuum attainable regardless of fluctuations in air pressure (for example, 50 mbar absolute).
  Be careful to get this parameter right - the difference between -900 mbar relative and 900 mbar absolute is obviously pretty dramatic; for proper degassing, you will benefit from a pump capable of getting to at least 20 mbar absolute / -990 mbar relative or so, preferably in less than a minute. Ideal pumps are closer to 2-5 mbar absolute or so (for US buyers: one "micron" is about 0.013 mbar).
  Depending on your budget and space available, you can either get a dual-stage rotary vane vacuum pump, or a two- to four-stage diaphragm one. Rotary vane units are more common, fairly cheap, moderately noisy, and somewhat bulky (slightly smaller and quieter than an airbrush compressor), but they also enjoy higher flow rates in a particular price range. They usually run wet - with about one to three glasses of mineral oil serving as a gas displacement medium and a lubricant - and need to have the oil replaced every year or so in lightweight use. The cost is very low - maybe $5 or so - but there is some hassle involved (in principle, you should have this oil disposed of the same way you are expected to dispose of motor oil). The advantage of having the mechanism protected with an oil is that any resin aerosols or dusts accidentally sucked into the pump are less likely to stick to pump components, and can be drained with the next oil change, however.
  Diaphragm pumps, on the other hand, are small, quiet, largely maintenance free, and can sometimes deliver a higher vacuum - but are more pricey, slower, and much more sensitive to contamination. They are easier to disassemble and clean if necessary, but it is definitely a more time-consuming and tricky process than just a simple oil change; you might want to consider an appropriate intake filter with the pump to prevent this from happening.
  Rotary vane units are available from multiple sources, as they are commonly used for HVAC servicing. A suitable device should cost around $100 if you look around - for example, VIOT offers a nice VPB1.5 unit for $117; I use their VPD5 unit, which goes for about $170, and has a much higher flow rate. VIOT folks seem to be nice and ship promptly.
  When it comes to suitable diaphragm pumps, you can expect to pay $200-$300; Gardner Denver Thomas has a nice selection, such as their 70110037 (7011ZVR/2,2/VVN/DC) four-stage unit, which goes to 10 mbar absolute, but isn't exactly fast.
  Once you select the pump you want, you also need to get a degassing chamber - a container where a cup with your resin can be placed, and vacuum applied, in a safe manner. You can get one for about $70 from a place such as Ted Pella, along with a suitable wire-reinforced PVC or polyamide hose (my favorite is Kuriyama K7160 Polyspring).
  You can also try building a makeshift chamber, for example from a medium size, robust mason jar - but be wary of the risk of an implosion; if using glass or rigid plastic, wrap the container in thick cloth, tape it over thoroughly, or put it in a wooden box. By the way - don't expect "vacuum" food storage containers to be actually capable of withstanding proper vacuum - been there, done that, had to collect the pieces.
  Along with the pump, consider getting a silicone-based vacuum grease, such as Dow Corning 891-7. You can use many other greases to seal joints and lubricate o-rings in vacuum chambers, but these formulations last longer and work better - and because of their high viscosity, non-reactivity, and superb purity, also work very well for non-vacuum applications (e.g., temporarily sealing multi-part molds, resealing resin containers and preventing their caps from seizing, etc).
• Pressure pot (very optional):
  Even with vacuum degassing, casting certain moisture-sensitive or fast-curing resins can sometimes be tricky: small mistakes may result in trapped air or carbon dioxide bubble formation; depending on your luck, this may strike at the least opportune times. The problem is particularly pronounced for certain (but not all) mercury-free water clear Shore D resins.
  In industrial settings, this is often countered with brute force: pressurizing the poured resin at 3-4 bar minimizes the risk of any trouble; existing bubbles are dissolved back, and the formation of new ones is much less likely, even if you mess things up.
  Working with high pressures is more dangerous than with just vacuum, and requires a separate set of specialized tools: a small pressure pot, such as this one from Amazon ($70+) or a more suitable and robust flat-bottom tank from Paint Sprayers ($320), and a generic compressor capable of providing the required pressure (at least $100; closer to $200 for quiet models). This setup is relatively bulky, so I consider this to be an optional luxury - but if you live in a single-family house, this option is worth considering - particularly for intricate multi-part molds, where in-mold degassing is simply very difficult.
• Temperature controlled oven (optional):
  At room temperature, many resins take between 4 and 24 hours to become safe to (gently!) handle; this is sometimes described as "demolding time" in manufacturers docs - although small and complex parts might be actually still be too fragile to remove from molds at that time. Another 2-14 days must pass for the resin to achieve final physical properties. This process can be greatly accelerated by placing the part in a higher temperature: a ballpark estimate for polyurethanes is that every 10° C cuts the curing time in half; polyaddition silicones are accelerated even more.
  So, to get your parts in just a couple of hours, you almost certainly want to be able to experiment with increased temperatures. Unfortunately, the safe range for many resins is very narrow: 70° C might be OK; at 100°, shrinkage of the resin, or warping due to thermal expansion of the mold, might be excessive; and clear epoxy or polyester resins may begin to develop a yellowish hue. At as little as 140°, some polyurethane rubbers may begin to deteriorate or swell (particularly true of Innovative Polymers IE-x0A, IE-50AC, and RenCast 6401-1) - and thermal decomposition takes center stage for almost all products around 180-200°.
  You might have some luck using your regular convection oven, assuming it has basic temperature control (well, baking chemicals where you later prepare food is discouraged), or with one of these cheap tabletop toaster ovens with adjustable temperature (maybe $30 at Wal-Mart) - but it is a bit of a gamble, as many of these devices offer very sloppy temperature control in the range we need the most - 40 to 90°. For example, my oven heats up to 90 degrees when set to 60. More importantly, these devices are really not designed to run for, say, five hours, or to keep heat very well - so you're wasting energy and risking equipment failure or worse.
  Still, if you want to take this route, you can - just be sure to buy a meat thermometer (an old-school, analog, all-metal one) and put it inside the oven, then try to be around through the process.
  A better option is to get a proper heater. Laboratory and industrial ovens are bulky, but in many markets, may be a viable option (example). Otherwise, one of the more affordable and portable options are hot air sterilizers / dryers for medical puporses. One such device is Tau Portable; the unit should be selling for about $300. All these devices offer very precise temperature control, and design that permits them to run for long hours in a very power-efficient manner. Be sure to examine chamber dimensions - at least 30 x 15 x 6 cm is a good idea for typical mold sizes.
• Digital calipers and other measurement tools:
  You will absolutely, positively need to accurately measure many third-party components such as motors, shafts, ball bearings, or even wires, to ensure proper fits and troubleshoot problems; manufacturer specs will be often unavailable or not worth that much, particularly when dealing with surplus components. If you do not have one already, it's a good idea to get small digital calipers (15 cm or so is fine), necessarily with a resolution of 0.01 mm or better. Ball bearings and motor shafts must have perfect press-fits, so do not try to be a cheapskate. There is a broad selection even on Amazon, and some $15-20 should get you a quasi-decent one.
  Furthermore, from time to time, you have an use for more precise measurements - for example, to troubleshoot machining problems - so consider getting a digital micrometer, too. These are more expensive and less convenient to use, but offer resolutions of 0.001 mm. Typical price tag is $70-$100 for digital units (and again, Amazon - surprisingly - has some decent deals).
  Lastly, a measuring magnifier (example) or an inspection / dissection / stereo microscope may be extremely useful for troubleshooting certain geometry issues; for example, figuring out why your 0.5 mm tooth gears do not mesh as expected. When coupled with a decent camera, you can actually use these tools to capture close-ups of the parts and overlay CAD data to check for any potential issues (not to mention all the other fun things you can do with this setup). This will also come handy for examining end mills that were dropped on the floor or had a cutting mishap: you can't tell with your bare eyes, but subtle flute damage is pretty evident at a magnigication of 30x or so (example).
• Jigsaw:
  Cutting prototyping boards by hand is very time-consuming and annoying. A decent 600-800W corded jigsaw from any hardware store, with blades designed for fast cutting of hard woods, work wonders. Try to get an unit that could be attached to a vacuum cleaner, to minimize the amount of airborne dust produced - and make sure that the blades purchased are long enough for the thickness of your prototyping boards.
  "Budget brand" jigsaws are best to be avoided; not only they tend to break, but are also generally pretty underpowered. The difference between a 200W no-name and 700W Hitachi jigsaw is pretty significant. Wireless saws are also usually a poor idea, as you need to pack some punch to cut through a 50 mm solid plastic board with a reasonable speed. You should be able to get a decent one in the $80-$150 range (do not go crazy for features such as tilt bases, guides, etc - your CNC machine will do the dirty work, you just need to feed it at least sorta-rectangular stock).
  (Jigsaws are one of the safer power saws, but it is still possible to cut off a finger or two if you misuse them. Read and follow manufacturer's safety guidelines, or learn about survival of the fittest the hard way.)
• Deburring tools:
  These simple, swivel-action manual devices are used to quickly, conveniently, and safely remove extra burr that may be left in some materials around edges, or appear on mold joints. These tools have pretty unassuming appearance - but you will soon find they work great for all types of minor manual finishing jobs for plastics and light metals, and are much more useful than files and knives, particularly on curved shapes. You can do witout them, but they are inexpensive ($5 to $15) and very handy - so why not?
  There are several companies that make such devices, and you just need to get an entry-level, general use one with a blade intended for plastics and soft metals; Noga has RB1000; C. P. Enterprises has DT-2; and just about anything along these lines from any other manufacturer should do.
  Another useful tool is a scraper, which is a very fine fixed cutter that allows small amounts of material to be removed safely, without the risk of cutting too deeply or damaging nearby surfaces; one such example is Noga SC8000.
  Finally, for deburring the teeth of tiny gearwheels, it's useful to have a small, hard nylon brush (such as this). For resins such as IE-3075, the brush is a lot faster than any other tool. For semi-flexible plastics such as TP-4004, combine it with freeze spray: the plastic is a lot harder and more brittle at -50° C.
• Fine sanding paper:
  Stock up on several grades of waterproof carbide sanding paper. Hardware stores usually have pretty ridiculous prices, but you can get 50-sheet packs on Amazon for cheap from sellers such as Industrial Supply. Try to get about eight sheets of each gradation (or something close): 100, 320, 600, 1200, 2500. These, when used progressively, can be used to bring any opaque plastic surface to satin shine, and will be indispensable for correcting occasional tool marks, reducing friction in sleeve bearings, etc.
  You might be also tempted to look around in hobby supply stores and find sheets of 4000, 6000 and 12000 sanding paper (or, alternatively, just a soft cloth and an appropriate polishing paste). With the help of these gradations, every opaque or transparent plastic, as well as metal, can be brought to perfect luster. Kingsley North has some 4000, 6000, and 8000 sheets, and polishing pastes, in stock; and several slightly more expensive types of 12000 cloth can be found on Amazon.
  One thing you should not waste your money on are sanding or polishing attachments for a Dremel-like power tool. Although they work very well on wood and metals, they suck for plastic molds - and most of them are also too big to safely use for this purpose. You can still use the tool with thin rolls of hand-rolled sanding paper to gently reach into tight spots, or use some non-abrasive felt or cloth attachments for buffing waxes. This Proxxon tool may be a bit more useful for polishing, too.
• Quality glues:
  Glues would be pretty essential for some of your work, and cyanoacrylates ("instant glues") are one of the best all-around choices for plastic parts; epoxies also find some uses when stonger bonds for dissimilar materials are required, but are not needed nearly as often. Ditch grocery store instant glues, however, and shop for "industrial" products: you will find compositions specialized for a variety of uses (binding plastics, metals, flexible compositions for rubber, etc), in various viscosities (including thixotropic variants), with specified adhesion and tensile strengths, and with a wide variety of curing times. They also come in very convenient, larger containers with decent applicators and caps.
  Thixotropic ASI SI-GEL cyanoacrylate is my favorite; you can find similar products from Loctite, Freeman, Loxeal, etc. The price per ml is actually not higher than the grocery store variety.
  Remember that the key to getting a good bond, especially with parts that are still covered with residues of release agents, is to clean the surface with xylene, ethyl acetate, or acetone. Skip this step, and you could be just as well using Elmer's glue.
  You might also want to consider getting a small hot glue gun from your local hardware store, plus a pack of adhesive bars. Just look for the cheapest, reasonably sized type - you do not need fancy temperature controls or so, $10 will do. Hot melt adhesives are very useful for temporarily attaching and encapsulating elements, etc - and in most cases, can be removed later on (a small amount of alcohol makes them peel of easily).
  For structural applications, you may also want to look at two-component epoxies, such as Devcon 20645 ($5 for 70 g tubes).
• Grease:
  For periodic maintenance of ball screws and other moving parts of your machine, you will probably need lithium grease (follow manufacturer's advice). For lubricating small gears and other fine mechanical parts for your robot, I recommend a dry PTFE grease, such as this or this one. PTFE-based dry grease is pretty useful in miniature mechanisms; generously applied wet greases tend to introduce extra drag on fast-moving surfaces that come very close to stationary ones.

• Canned air (or a nitrogen cylinder if you have room):
  Polyurethanes are humidity-sensitive; some compositions to a greater extent, some to a lesser. In any case, it is advisable to keep opened containers under a blanket of a heavier-than-air, chemically inert gas. Such a gas may also be used to shield any exposed resin during the curing process, for example in laminating, surface coating, potting, and in similar applications.
  Nitrogen is the primary choice in industrial applications, but many other dry gases will do. In particular, the gases used in "canned air" dusters (difluoroethane and tetrafluoroethane) should work just as well, and are easier to obtain and packaged more conveniently.
  Go with the cheapest duster you can find - just be sure it's not laced with Bitrex or other additives meant to discourage inhalant abuse; it's not that Bitrex would react with the resin in any way, but it tastes pretty bad and gets on your hands, clothes, etc, and it gets there easily. Stoner offers a Gust brand duster with no additives, in 12-can packs at $4 per can, complete with free shipping.
  Naturally, getting a medium-size nitrogen tank, a regulator, and a hose is also an option. The upfront investment is about $150, but it lasts for months, and you can then refill it for $10 or so (for example, at a nearby Praxair location) - so it's more cost-efficient in the long run.
• Molecular sieve, grade 4A (optional):
  Sounds serious, but it's a term for porous, powdered zeolite. The material is excellent for selectively trapping certain molecules on its surface because of its highly uniform pore size; grade 4A is particularly suitable for removing water without interfering with the chemistry of the resin. This fine powder has a neutral color, costs relatively little ($30 for 2 kg) - and when a small amount is baked at about 200-250° C, then added to a resin or a pigment, it can keep the water away, especially on cool and humid days. Not really a necessity unless you have a particular problem to solve, so just take note of its existence for now.
• Thick, transparent, heavy duty plastic sheeting:
  When loosely draped over the mill, well clear of rotating parts and radiators / cooling inlets, this serves as an excellent and low-cost airborne dust and minor projectile protection mechanism that provides easy access to the unit and visibility from all angles. Thick varieties of sheeting stay in place, do not wrinkle, and are unlikely to interfere with the cutting process (but you don't get to sue me if this turns out to be a filthy lie).
  You may also be tempted to build a permanent case from wood, acrylic glass or polycarbonate, of course; that is somewhat safer.
• Thin double-sided adhesive tape:
  A thin, non-foam-based double-sided adhesive tape will come handy for affixing prototyping boards to machine table. Get some from a hardware store (but really, a film-thin one used for wallpapers, not a foam variety!). Tesa 4970 is an industrial tape with excellent strength, commonly used for amateur CNC work. In the States, Tesa 4965 is perhaps easier to find, and should be just as good.
• Syringes and veterinary or laboratory needles:
  Locate an on-line medical / veterinary supply store and order a box of 100 10 or 20 ml, two-element, single use syringes (eNasco worked for me). Pick possibly the cheapest set ($20-$40 in the States, much less elsewhere), preferably with all-plastic construction (no rubber seals, etc- seals do not survive the chemistry of some resins, and may affect the cure of silicones). Syringes are remarkably useful for getting components out of containers, carefully measuring them, and applying catalyzed resin to hard-to-reach spots on the mold. The 10-20 ml range is optimal; much larger syringes are nearly impossible to operate with viscous liquids.
  Also consider getting a box of 100 1.2-1.8 mm x 20-50 mm needles. These might come handy for filling very narrow holes in the mold with resins, or sucking out any trapped air spotted after pour. Straight-tipped, dull dispensing needles for laboratory purposes are also available from several places, including Amazon, and are a good alternative.
  PS. If you live in one of the eleven US states that require a prescription to buy syringes, congratulations - your elected officials are morons. Your other option are laboratory pipettes with replaceable polypropylene tips - but they cost more, at least up-front.
• Stainless steel spoons or measuring cups with handles:
  Silicone resins typically come in relatively large cans; if that is what you are stick with, you will find it somewhat cumbersome to transfer 200 g of silicone using a 20 ml syringe; pouring straight from the can is fast, but messy.
  Metal spoons and kitchen measuring cups are a perfect solution. Just be sure to get a smooth, high-quality, plated one (i.e., shiny). This reduces the chance of any unexpected interactions with addition cure resins. Plastics, wood, and porous materials are to be approached with caution.
• Single-use beakers or specimen containers:
  Made of thick, chemically resistant polypropylene, these are ideal for mixing resins of all sorts. Ted Pella has a good selection of Tri-Stir polypropylene cups for about 10-20 cents a piece. I would recommend against using some of the flimsier beverage cups instead - made of polystyrene and other lower grade plastics, these are not guaranteed to take aggressive resins well (plus, are easy to accidentally puncture or crush, and tend to shrink even in moderate heat). The added advantage of polypropylene is that polyurethanes do not stick to it particularly well, so the containers can be cleaned and reused much of the time.
  It is also useful to buy a bunch of polypropylene or FLPE/HDPE laboratory bottles (they are available in bulk on Amazon and in many other places). In particular, I found it useful to store a smaller, "working" amount of resins, fillers, plasticizers, and dispersed pigments in bottles ranging from 20 to 250 ml or so. Avoid LDPE, though.
• Tongue depressors or chopsticks:
  All resins must be mixed thoroughly to initiate proper polymerization. Single-use wooden chopsticks and tongue depressors work equally well, although when using the latter, it's useful to clip it to get a flat edge on the business end. Ted Pella is a good place to go for tongue depressors (two cents a piece); Amazon and restaurant supply stores might have chopsticks in stock.
• A set of small, cheap brushes:
  For applying viscous resins to molds, and applying demolding agents where needed, brushing off dust. Better to have them handy.
• Airbrush (very optional):
  Some demolding agents (e.g., polyvinyl alcohol) and some lacquers achieve perfect, glossy surface aspect only when evenly sprayed, and brushes will unavoidably leave some streaks. If you have some money to spend, consider purchasing a decent gravity-fed airbrush, plus a reasonably small compressor capable of at least 4 bar intermittent pressure and about 25 l/min of air flow.

  A decent airbrush should cost about $70 to $130 (Paasche Talon and Badger 100 LG are both a decent choice; Sagyma SW770 is also fine). The compressor will add another $100 if you just go to the hardware store, or $150 if you buy a dedicated one (sample selection); the compressor can be shared with a pressure pot, but in this case, look for a reasonably powerful model to make sure that pressurizing the pot won't take forever (at least 1/2 hp is advisable).

  Using airbrushes properly takes some learning, but there are quite a few tutorials for model makers and painters on the web, so look around. They also double as a superior and in the long run, more economical alternative to compressed air for cleaning or drying molds.

• About 50-100 pieces of polypropylene, cut to size (2-4 mm thick):
  Cut to a dimension roughly comparable to the scale of parts you would be making. When casting polyurethane or epoxies in one-sided molds, putting them on top of an open mold and weighing them down ensures a smooth, dimensionally accurate surface and eliminates denting on foaming that may occur in humid environments due to isocyanate - water reactions. Polypropylene is preferred by the virtue of not sticking to most resins, and being more flexible and resistant to chemicals. Acrylic is an acceptable substitute, but demolding agents will be required, and even with them, trouble may still occur.
  Polypropylene, acrylic glass, and many other common plastics in arbitrary sizes can be ordered in bulk for cheap from multiple places that supply them for advertising industry (for signage, displays, etc); just get the cheapest variety from your local outlet. Online, a good source is Professional Plastics, although it might be not the cheapest one.

• Containers and tools used for silicone oil, wax demolding agents, surfactants, and so on: wipe clean, or rinse with water and a detergent and dry out.
• Containers and tools used for non-catalyzed epoxy, polyurethane, polyester, and silicone components: minor leftovers can be simply wiped clean; if there are more substantial amounts left, it's probably wiser to just discard the container.
  Any syringes and bottles that can be stored without making a mess can be, obviously, reused for the same chemical over and over again.
• Containers and tools used for catalyzed resins: pour out excess resin prior to setting, wait for the remainder to cure, peel away the film. If there are any minor uncured patches, wipe clean if possible, or discard the container if not.

11. Safety and health

11.1. Power tool safety

11.2. Vacuum and high pressure

11.3. Noise considerations

11.4. Dust considerations

11.5. Dealing with chemicals

11.5.1. Silicone rubbers

11.5.2. Polyurethanes

• Phthalate plasticizers: there is some controversy around certain phthalate plasticizers, most notably dibutyl phthalate (DBP). This plasticizer is fairly uncommon in polyurethanes, but appears in a handful of compositions - e.g., Innovative Polymers IE-90xx rubber. Acute toxicity of DBP is modest, but some scientists are concerned about long-term, subclinical exposures to extremely low concentrations of DBP leaching out of cured plastics, primarily PVC, and disrupting certain hormonal processes in human body. While phthalates are still very commonly used (and therefore, your exposure is largely independent of your hobby work), their use is restricted in products such as children toys. In case these fears are warranted, it may be safer to stay away from DBP, DEHP, and so forth, when producing parts that are meant to be extensively handled, worn, or come in contact with foodstuffs. Other plasticizers, such as benzoates (say, dipropylene glycol dibenzoate - DPGDP), seem to be safer, and come handy for example in preparing pigment dispersions; as mentioned earlier, Eager Plastics offers DPGDP under the name of EP9009.

• Mercury catalysts: another controversial (but increasingly rare) additive is phenylmercuric neodecanoate (or some other mercury carboxylate). Used in minutiae quantities (0.05 - 0.4%), this substance served as a highly selective polyurethane catalyst that greatly favors the isocyanate-polyol reaction to the undesirable isocyanate-water one - hence imparting superior humidity resistance. It is now mostly phased out, but still making some appearances in transparent or elastic resins, especially on the US market, where it is relatively unencumbered by blanket environmental regulations. Examples of mercury-catalyzed resins include Smooth-On Crystal Clear 2xx, Freeman 1090, or RenCast 6401; all the Innovative Polymers products are mercury-free.
  The catalyst is one of the less harmful organomercury compounds, and is present in quantities small enough to be of very little concern during normal casting operations, so this part actually isn't all that scary; more interestingly, it is not clear what the long-term fate of mercury in the cured resin might be. In the 60s, researchers assumed that the large molecule would be immobilized in the polymer matrix, but recent studies of polyurethane rubber floors catalyzed with phenylmercuric acetate revealed that after several decades of use, some of them give off elemental mercury vapors at an unexpectedly high rate. This could be due to manufacturing or application errors, UV degradation, or something else - but it's a good reason to refrain from using mercury-catalyzed resins to make toys, jewelry, and other everyday items.

11.5.3. Epoxies

11.5.4. Polyester resins

11.5.5. Pigments and dyes

That said, not all is roses. Key risk factors to look out for:

• Most pigments are finely powdered and may easily become airborne. As with any generic dust, do not inhale, and wear at least a basic single-use dust mask when doing something particularly messy, or working in confined spaces with poor ventilation,
• Some pigments may contain silica as a byproduct of the manufacturing process, as a way to encapsulate otherwise chemically active dyes, or as a method to achieve desired visual properties. If so, additional respiratory risk is present, as outlined in earlier sections. In most cases, powders consist of amorphous silica particles 40 - 200 micrometers in diameter, which is believed to be reasonably safe - but double-check this, and exercise caution anyway,
• As noted earlier, some pigments, particularly inorganic ones, are either known to be outright harmful (check MSDSes), or are based on particularly toxic / bioaccumulating compounds that may be released when the pigment degrades or is attacked by resin components.
  General guidance is hard to provide. In principle, lead, mercury, and arsenic are top offenders, and should be avoided at all costs. Pigments based on cadmium, chrome, nickel are typically problematic, too. Cobalt, manganese, barium, antimony, and copper may be somewhat harmful in some compositions, depending on solubility, reactivity, manufacturing contaminants, etc - but probably don't need to be treated with extreme suspicion. Zinc, tin, iron, aluminium, magnesium, titanium, and common alkali metals are of least concern. Google around for MSDSes and health warnings, particularly if you intend to make toys, jewelry, or other items that would be extensively handled or worn by humans. There are also some pages listing potentially dangerous pigments, for example this one.
  As usual, there is no need to overreact, but it is also unwise to buy and use potentially harmful pigments if equally good non-toxic equivalents exist.

It is particularly important to avoid harmful pigments when spraying coats with an airbrush or a paint gun. For such applications, stick religiously to reasonably safe compositions.

In addition, not every pigment is guaranteed to be compatible with every possible resin; be sure to perform initial tests on a small quantity of material, and immediately contain and discard the sample, then cease to use the dye, if an unexpected reaction is evident (through color change, premature polymerization, inhibited polymerization, foaming, smoke, etc).

11.5.7. Solvents, glues, demolding agents

11.6. Staying legal

12. What next?

Feel free to bug the author with any any specific questions, suggestions, concerns, flames, etc - the address is lcamtuf@coredump.cx, and I will be delighted to hear from any of the three people who actually care about this topic, and made it this far.

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