Guerrilla guide to CNC machining, mold making, and resin casting
Copyright (C) 2013 by Michal Zalewski (lcamtuf@coredump.cx)
You are viewing chapter 4 of 8. Click this box for table of contents.

4. Resin casting and you

As discussed in section 1.3 of this guide, resin casting is a pretty amazing, simple, and user-friendly process that comes equally handy in CNC prototyping, 3D printing, and in manual DIY work; all its sophisticated uses aside, you will end up using it to replace broken parts in appliances or toys, make unique gifts, or even encapsulate backyard flowers or bugs (don't deny it).

Alas, the online market for moldmaking and casting supplies is dominated by several companies that cater chiefly to artistic users, and sell expensive products with poor mechanical properties and little utility in high-precision engineering work. In that spirit, even if you are familiar with artistic resin casting using epoxies, polyester resins, or polyurethanes from Alumilite, Smooth-On, and similar sources, you will be probably surprised by how much better your results can get.

4.1. Raw materials

4.1.1. Machinable stock

Well, before we dive into the world of casting resins, we should briefly revisit the choice of materials you can use to make master patterns. This part of this chapter is specific to projects that rely on CNC milling, so if you are interested in replicating manually crafted or 3D printed parts, you may want to skip ahead a page or two (this site provides a good overview of how to build patterns by hand - and if you have any other questions, feel free to drop me a mail or stop by /r/resincasting).

Still here? All right! Of course, milling machines are not particularly fussy, and will cut almost anything that is softer than tungsten carbide, but rigid enough to stay in one place; still, some materials are more predictable than others, and produce better results. Prime choices include rigid engineering plastics such as polyurethane, epoxy, polyester, ABS, polyamide (Nylon), or acetal (Delrin); many varieties of hard woords; aluminum, brass, and other soft metals and alloys; and more exotic choices such as printed circuit boards, hard waxes, plaster, etc.

Conversely, common materials that machine with greater difficulty or offer sub-par surface finish include stringy, low-melt thermoplastics (PET, polycarbonate, some grades of polyethylene); rubbers and other stretchy or squishy polymers (including some grades of PVC and most polystyrene foams); plywood and particle boards (including MDF); and exceptionally hard or highly abrasive stuff, such as steel, stone, or glass. Of course, many of these materials can be still cast or formed using CNC-machined molds and dies.

Poor choice of working materials is one of the most common mistakes made by hobbyist machinists; quite a few people stick to workpieces that offer poor accuracy, get damaged easily, gum up the tool, or simply cost way too much. For moldmaking purposes, your best bet is one of the little-known materials: an extremely accurate, low cost piece of plastic known as a medium-density modelling board, originally devised for the automotive industry. It's essentially a mix of medium strength polyurethane, and a combination of soft fillers such as calcium carbonate and aluminum hydroxide. There are many types of machinable boards, but the one we are interested in has a density of about 0.70-0.78 g/cm³, and vaguely resembles wood:

Prototyping boards of this particular variety include Huntsman RenShape 460 (or sligthly less dense BM 5460), Axson ProLab 65, Sika SikaBlock M700, BCC MB2001, Necuron Necumer 651, and several more. The material is typically sold in bulk, in sheets of about 50 x 150 cm, 25 mm thick. This may sound like a lot, but I recommend buying a full board, rather than grossly overpriced cut-to-size bits. The material lasts me for about a year, and costs about $12 per liter (roughly $250 for the whole thing). It's much less than what you'd pay for a similar slab of HDPE or acrylic - and it machines easier, too. (Planks of dense hard wood, when glued together and planed, may be a cheaper alternative for uncomplicated parts.)

Buying prototyping boards is actually pretty easy. If you are in the US, you can simply go to Freeman Supply, and order RenShape 460 online (search for item #075229). In other places, simply look at the manufacturers' websites and find local distributors, then send out several e-mails or make some calls (online ordering isn't common in the industrial world). Be aware that prices may vary significantly, so shop around.

Alongside with the board, you may want to order a matching board repair putty; it's a fast-curing, polyester compound that can be used to fix minor damage to your molds, or even completely fill a previously created cavity to reuse a particular workpiece for a new project. If you are ordering online with Freeman, go with their Quik-Fil; otherwise, ask the distributor for a matching product - they will be able to advise.

What else? Oh, about the only minor drawback of the medium-density boards is that they have a very fine but perceptible grain, as shown in this magnified image:

This grain normally has no appreciable effect on dimensional accuracy, but imparts a satin finish that will transfer to any transparent, water-clear parts. Of course, you can create high-gloss molds by coating the pattern with paste wax or a similar sealer (carnauba wax is particularly good); or you can always simply polish the final part - but both these options affect dimensional accuracy, and can be annoying when working on complex molds. The alternative is to use a more expensive material known as a tooling board - made out of solid, dense polyurethane, with no perceptible grain. Boards such as RenShape 5169 or BM 5272 cost up to 50% more, need to be machined 20-40% slower, and cause some wear to the tool - but they scratch this particular itch.

Note: RenShape 460 is relatively easy to cut with a hand saw; in fact, it's comparable to soft woods. That said, the extra labor may be annoying in the long haul, so it makes sense to have a decent jigsaw nearby. You can get one for around $35; blades designed for hard woods will cut the material very quickly and last for years.

4.1.2. Silicone rubbers

In order to replicate the parts laid out inside your pattern cavity, you need a flexible and durable substance to take an impression of the desired shape, and use it as a mold for the final product of your work.

There are several types of castable rubbers that could be useful for this purpose, but silicones are hard to beat. There are quite a few formulations that combine ease of use, excellent mechanical properties, perfect dimensional accuracy, no odor, no toxicity, and temperature resistance up to 300° C. On top of that, silicones come with an inherently non-stick surface, which helps greatly in casting work.

Almost all the silicone formulations you can find on the market come as a viscous goo consisting of long, linear, partly polymerized chains of siloxanes; that nominally non-reactive soup is then combined with a suitable cross-linker and a catalyst. The reaction between these components quickly turns the goo into a very bouncy solid; this can be initiated in several ways:

In other words, you almost certainly want to stick to RTV-2 platinum-catalyzed silicones, unless you are working on life-sized castings (at that point, the cost of silicone can become prohibitive).

Before discussing specific products, let's have a quick look at the notable characteristics that will come up in product datasheets for these rubbers, and review their significance to our work:

All right, that's it! Other parameters are either uninteresting, or are not advertised consistently. To help you with the selection process, my top recommendations for mechanical projects would be:

You should pick just one of these; if you're undecided, go with QM 262 or XP-592. If you need other options... well, Silicones Inc and Quantum are the most interesting US-based companies I know of. Other choices include Polytek, GT Products, BJB, and Smooth-On - but in my opinion, their selection is much less impressive, and the options I have tried pale in comparison with the ones mentioned on the recommended list.

US market aside, globally, Bluestar Silicones (Rhodia) is fairly ubiquitous; you may also want to check out ShinEtsu, Wacker, Zhermack, Huntsman, Axson, or Dow Corning - depending on where you are located, they all offer some interesting choices. But then, one reader living in Norway reported that placing an international order with Hobby Silicone for QM 262 actually turned out to be much cheaper than locally available alternatives (whoa).

As far as pricing goes, platinum cure silicones cost around $35-$45/liter when bought in one liter cans, or about $30-$35/liter in 4-5 liter pails; for example, XP-592 costs around $120 for 4 l, while QM 270 and QM 262 in 5 l quantities fetch $140 and $170, respectively. Unreacted liquids should survive at least 2-3 years without significant deterioration, as long as you keep them in tightly closed containers, away from sunlight, moisture, and excess heat - so getting a full pail is not a bad idea.

Note: resin manufacturers in the States use a somewhat confusing scheme for describing the size of their two-component kits: "1 gallon kit" usually means that you are getting about one gallon of whichever component is needed in greater quantity; and a matching amount of the other one. If the mix ratio is 10:1 (as is the case with most platinum cure silicones), the gain is minimal - but for resins mixed 1:1, you are actually buying two gallons or so.

To further confuse you, the same does not apply to sizes specified in pounds - "15 lbs" means that you are getting just enough to cast a 15 lbs blob of plastic or rubber. Be sure to account for these differences when shopping around: a lower price is not always a better deal.

4.1.3. Rigid polyurethanes

All right, let's talk about the materials you can employ to actually make final parts!

Polyurethanes are an incredibly interesting and versatile class of two-component, addition cure polymers. They use two principal reagents, mixed in comparable quantities: a non-volatile isocyanate and a complex alcohol (polyol). Some formulations trade some or all of the polyol for a polyetheramine, resulting in a material that is more properly called a polyurea. In any case, the two components are usually combined with variable amounts of chain modifiers, usually chemically similar to the primary polyol; and possibly surfactants, plasticizers, fillers, and so on. The whole thing is then catalyzed with a wide variety of organometallic compounds (bismuth, zinc, tin, zirconium, aluminum, or similar); with tertiary amines such as 1,4-diazabicyclo[2.2.2]octane (DABCO / TEDA); with diazoles such as 1,2-dimethylimidazole (DMI); or with something pretty close to that.

Modern polyurethane chemistry lets you manufacture everything from soft foams, to high-performance rubbers, to faithful, often superior imitations of many other rigid engineering plastics - all that without having to go bankrupt on injection molding equipment, and while using only fairly safe and predictable chemicals. They greatly outperform more familiar resins, such as epoxies or polyesters, and in hobbyist workshops, are much less dangerous to work with.

The only downside of high-performance polyurethane systems is that they generally require a basic vacuum rig - a small pump and a suitable container to remove any dissolved gases from the mix. Products that do not require degassing are readily available, but usually don't perform as well as their peers. (We'll talk about the required harware later on, but it's pretty compact and doesn't cost a lot.)

Anyway - if you are aiming to make functional prototypes, it is probably prudent to start by stocking up on a polyurethane resin that lets you produce hard, rigid, and shock-resistant parts. Once more, let's have a look at some of the key things worth highlighting in a datasheet:

All right, ready for some recommendations? Here we go:

At this point, unless you have a specific itch to scratch, it's perfectly OK to order just IE-3075 directly from IPI. Their "1 gallon kit" (actually around 6.5 l) sells for about $110. "Quart kits" are also available if you need to try it out first.

If you want to shop around, I don't think it makes sense to look beyond Innovative Polymers - not if you're in the States; they have a remarkable selection of unique, user-friendly products designed specifically for manual casting, and great customer service. They take direct orders, and also have several local distributors. If you're skeptical, you can have a look at products from Smooth-On, Alumilite, BJB Enterprises, or Freeman - but you will not find anything that even comes close to that selection. If you are in Europe, checking out Huntsman and Axson is not a bad plan.

As with silicones, polyurethane resins are pretty stable and have a fairly long shelf life - but they are fairly sensitive to sunlight, humidity, heat, oxygen, and moisture. It is a good practice to buy several 100-250 ml polypropylene or HDPE bottles (example) for intermediate storage of the amounts of resin you plan to use within a month or so - and keep the original containers sealed and blanketed with inert gas. The inert gas can be just a burst of "canned air" (difluoroethane or tetrafluoroethane), but if you want to save money in the long haul and have some room in your workshop, it makes more sense to invest around $150 in a small nitrogen tank, a regulator, and a hose. This setup will last for months, and it's only about $10 to refill at a nearby Praxair location or so.

In any case, if you store the resins properly, you can expect most of them them to maintain their properties for at least 2-3 years; manufacturers usually give much more conservative guarantees, but take them with a grain of salt. Remember to agitate the containers if the components separate. Oh, some formulations may crystallize if kept below 10° C or so, but this process can be reversed easily. Prolonged storage in crystallized form is not advisable, as it may lead to the formation of insoluble dimers.

Caution: although most of the polyurethane formulations you will encounter are reasonably safe, there are some unfortunate exceptions. We'll talk about this a bit more in chapter 7 - but for now, definitely watch out for:

You won't find any of the problematic components in resins from Innovative Polymers - but other manufacturers sometimes show less restraint. Request and study material safety datasheets (MSDS) when in doubt.

4.1.4. Flexible polyurethanes

Flexible polyurethane elastomers are an interesting alternative to silicones. You don't necessarily need to buy any, but they may come handy if you wish to make functional rubber parts, and your silicone is not pigmentable, or has insufficient strength.

Key advantages of these rubbers include 30-50% lower price, and much better performance toward the upper end of Shore A (above 40 or so): several times higher tear strength, much less pronounced tear propagation, excellent abrasion resistance, lower coefficient of thermal expansion, lower viscosity, and the ability to pigment the system as seen fit. On the flip side, polyurethanes exhibit some exotherm-caused shrinkage in larger castings, and adhere to many other plastics, making them less desirable for creating negative molds; they also tend to have slightly worse rebound characteristics, and limited temperature resistance (they get soft around 70-90° C, and deteriorate somewhere between 150 and 200° C - so casting low-melt metals, for example, is out of question).

The parameters to look for in these compositions are similar to these for silicones, with the exception of pot life and viscosity - here, the advice provided for polyurethanes is more pertinent. When reviewing the datasheets, pay close attention to the ratio of tensile strength to elongation at break, because there can be striking differences in the rigidity of various products, especially those rated 70 Shore A or above; some of them are only somewhat flexible, and will not be suitable for making parts such as tires, rollers, or transmission belts.

Note: This goes both ways: not all Shore A polyurethanes are particularly rubbery, and not all Shore D polyurethanes are necessarily very rigid. For example, some 60 Shore D polymers are highly elastic and can be stretched up to three times their original length, even though their surface feels hard as nails. They resemble the rubbers used in certain garden hoses, shopping cart wheels, etc.

As for recommendations: you should go with Innovative Polymers HP-21xx series; I've tried quite a few other products, and nothing else comes close. They are relatively inexpensive ($25 per liter) and feature superb "true rubber" mechanical characteristics, long pot life, and good cure profiles. For example, HP-2170A is a super-stretchy 70 Shore A rubber with tear strength of 42 kN/m, far surpassing most silicones. There are also softer variants, down to 50 Shore A (HP-2150A); and more rigid but still surprisingly flexible ones, up to around 60 Shore D (HP-2160D). In fact, it's possible to blend them to achieve intermediate properties as needed for a particular project.

All the products in the HP-21xx line take several days to polymerize at room temperature; if that's too slow and heating them up is not an option, you can add some separately purchased catalyst to get overnight cure with no real trade-offs.

4.1.5. Epoxies, polyesters, and so on

If you have dabbled in resin casting before, chances are, you used epoxy or styrene-based polyester resins, rather than polyurethanes. These options are popular with hobbyists because of their broad availability, low price, and less onerous processing requirements (i.e., less sensitivity to mixing ratios, moisture contamination, etc). That said, I think it's a bad idea to use these materials in precision casting work, for a couple of reasons:

Of course, I don't want to demonize these polymers. Polyesters are sometimes useful for bonding and repair applications; and epoxies are extremely useful as high-performance glues and laminating or potting resins. Both epoxies and polyesters can be superior in lay-up composite applications, too, in part owing to their improved bonding capabilities. You can have a look at the products sold by Freeman Supply, or check out the low-cost, water-clear epoxies available from Polymer Composites Inc; just don't expect them to be a sensible match for PU for the processes discussed in this guide.

4.1.6. Pigments and dyes

Adding colors to your castings is fairly easy. One option is to find an artist store, and shop for dry, non-toxic organic pigments. Such pigments will work equally well with polyurethanes, silicones, and just about anything else - but may be relatively painful to disperse or to blend with any accuracy. The other choice is to purchase coloring pastes where pigments are already dispersed in a non-reactive (plasticizer) or reactive (polyol) medium - but these won't work in silicones.

For dry pigments, Kremer is probably the best source online. For reactive dispersions, you can ping Innovative Polymers (send them your picks from this RAL chart); Eager Plastics has a pretty good selection of non-reactive pigments and dyes, too.

Beyond that... selecting your palette is a matter of personal preferences, but here are some quick tips:

If you need inspiration, here are some nice picks:

A small bag or bottle of a pigment will cost somewhere between $5 and $20, and should last for years.

Tip: You can, of course, opt to paint your parts instead of adding pigments to the resin; acrylic and polyurethane lacquers can be used alike. For adding text or other ornamental elements to machined parts, you may also want to consider a low-cost vinyl cutter, such as Silhouette SD or Roland Stika SV-8. These devices are fairly affordable, and the results look amazingly good - especially if a layer of clear coat is applied on top.

If another cutting machine is too much, you can also simply equip your CNC mill with a specialized drag knife to get comparable results.

4.1.7. Other additives

Adding colors aside, many other properties of cured plastics can be altered in profound ways by introducing certain easily available, low-cost additives. It's almost impossible to provide an exhaustive overview of all the available choices, but several use cases are definitely worth calling out:

Of course, multiple types of fillers can be combined; in particular, it may be useful to add some glass fibers to lightweight materials filled with hollow glass spheres, to maintain acceptable flexural strength. If you are wondering what to buy up front, it's not a completely bad plan to get some Scotchlite iM30K, plus 0.8 mm glass fibers, and a bit of DPGDB (also useful for preparing pigment dispersions and such). Other fillers are not nearly as essential.

4.2. Casting workshop

Now that we have the selection of resins, pigments, and fillers sorted out, it's time to briefly chat about the workshop equipment you will need to get the ball rolling. The list isn't particularly long, but even when it comes to something as inconsequential as mixing cups, making the wrong choice will unnecessarily complicate your life.

4.2.1. Vacuum pump and chamber

Insufficient mixing may cause a range of problems with finished parts - but vigorous stirring will almost always introduce some air into the resin. This problem aside, bubbles of air may get trapped inside mold crevices as you pour the mixture in - even if your mixing skills are beyond reproach. Last but not least, some resins may simply liberate some amount of dissolved gases once the polymerization reaction begins; IE-3075 is an example of that. Vacuum degassing solves all these problems, and is not as scary as it may sound.

Even if you are on a tight budget, you should get a low-cost vacuum pump capable of getting pulling around -1000 mbar of vacuum (that's about 10 mbar absolute, or -29.5 inches of mercury). I ordered mine from VIOT: click here for an entry-level model that sells for around $100, and should work fine; I have a $140 model with a higher flow rate, and it served me well. You will also need some sort of a vacuum chamber to hold the mixing container and the mold itself; it's possible to rig something together on your own, for example using a sturdy cooking pot and a cover made out of thick polycarbonate - but low-cost vacuum dessicator chambers work fine in that capacity, and start around $50 or so. If you are planning to work on relatively small projects, or opted for a pump with a relatiely low flow rate, Bel-Art #420100000 is a good choice; otherwise, model #420430000 will accommodate larger pieces, too.

(For a bit more money, you can also find some good-looking purpose-built degassing chambers from several sellers on eBay.)

About the only other piece of this puzzle is a hose to connect the pump with the chamber. I use 1/4" Kuriyama K7160 Polyspring, which I ordered from these guys; it's about $4 per meter. Other hoses can be used, too, but they need to be vacuum-rated, which in practice means that they need to be either fairly rigid, or reinforced with a metal spring to avoid collapsing as soon as you turn the pump on. Beyond that... well, getting some vacuum grease for o-rings and other parts of the vacuum chamber is not a bad plan (link); it also comes handy for several other purposes every now and then - for example, for preventing caps on resin bottles from seizing in storage.

4.2.2. Auxiliary tools

It goes without saying that for any sort of precision work, you need a reasonably well-equipped workshop to begin with. I am assuming that you already have that - and that basic tools such as clamps, needle files, or several grades of fine sanding paper, are always within reach.

With that in mind, there are several less obvious, minor items that will be useful specifically when casting mechanical parts, and that we haven't mentioned before:

That's about it! For working with small quantities of resin and pouring it into complex multi-part molds, you may also find it useful to get a box of single-use, two-element 10 ml syringes with no rubber seals (e.g., from eNasco), and some blunt-tip dispensing needles.

4.2.3. Mold releases and other useful chemicals

Mold release is a material that forms a protective barrier between the mold and the resin you will be pouring in, and makes it easier to demold final parts. The use of mold releases is optional when casting silicones in polyurethane patterns, or polyurethanes in silicone molds, because these materials don't adhere to each other in a particularly strong way - but still, a properly selected release agent makes demolding easier, and prolongs the life of any mold by reducing its exposure to reactive chemicals. On top of that, if you ever want to cast polyurethanes in polyurethane molds, or silicones in silicone molds, a robust adhesion barrier is simply a must.

I have tried many different mold releases over the years, and my top pick is, unquestionably, Stoner A324; this spray-on agent is probably lecithin-based, and beats most of the silicone, PTFE, or zinc stearate releases that I have tried before. They sell it for about $5 per can, and this should last you for about 1-3 months of hobby work. Stoner ships for free if you order a full box of 12 cans ($60).

If you can't obtain this product in your market, silicone-based mold releases are your second best choice, although some varieties may gradually permeate and swell silicone molds. Releases based on mineral oils, PTFE, zinc stearate, polyvinyl alcohol, and so on, usually don't perform well in high-precision, vacuum-assisted casting work. The manufacturer of your silicone rubbers may be able to recommend some specific, locally available picks.

That aside, you may want to also grab a wax-based mold release: they dry to form a hard, polishable, permanent layer that not only serves as a barrier, but should improve the surface aspect of polyurethane patterns. Low-viscosity brush-on formulations, such as Synlube 531, should work well; more viscous liquids, such as AdTech MR-1, may need to be diluted with naphtha when working with intricate shapes, but it's not a hard thing to do. Hardware store paste waxes that contain carnauba wax (link) work fine for simple shapes where dimensional accuracy is not critical, but high-gloss finish is desired - and if you dilute them to a water-like consistency, you can use them for complex models, too. In all cases, once the wax is dry, you can simply buff it with a soft cloth or a brush.

Note: waxes should not be applied to silicone molds, because they will inevitably crack and peel off - and the solvents used are usually damaging to silicone rubbers, too. In general, with any new mold release, always test it by soaking a piece of silicone in it for about 5-10 minutes; any substantial or permanent change in dimensions would be a reason for concern.

Mold releases aside, you may find it useful to get some of the following, largely optional chemicals:

4.2.4. Optional: temperature-controlled oven

Temperature-controlled ovens are not essential in casting work, but they let you perform several time-saving tasks:

Low-cost toaster ovens are the obvious solution, but somewhat unfortunately, they have extremely poor temperature accuracy and stability, especially on the lower end of the scale. If you are an accomplished DIYer, you may be able to grab one and simply equip it with a more accurate, digital temperature control circuit. That said, if you are willing to part with few hundred bucks and have some floor space, you can also try getting a real laboratory oven, such as this one ($400); hot air sterilizers and dryers may offer a smaller and slightly more affordable alternatives in some markets, too (example, around $300).

If you don't have that much money, or simply don't have enough room, don't worry; you can do just fine without this piece of equipment, provided you are willing to wait a bit longer for your castings every now and then.

4.2.5. Optional: pressure pot

There are some complex, multi-part molds where it may be hard to consistently avoid air entrapment, even with the aid of vacuum; on top of that, there are some resins that tend to be difficult to fully degas, or that will develop bubbles of carbon dioxide when not mixed perfectly well, or when exposed to residual humidity. That latter set of problems is particularly evident in mercury-free water clear poulyrethanes, such as the vanilla version of Innovative Polymers OC-7086.

To improve your odds when dealing with such tricky cases, it helps to have a pressure pot; the idea is to increase ambient pressure surrounding the mold to about 3-4 bar, thus crushing and dissolving back any existing bubbles, and discouraging the formation of new ones. Sure, it's a brute-force solution - but can you argue with its results?

Pressure casting equipment is more bulky, more expensive, and somewhat more dangerous than vacuum pumps (due to much higher pressure differentials) - and for most part, isn't necessary; get it only if you have plenty of room, and you are either forced to work with water clear resins that don't tolerate non-pressurized casting, or you are willing to spend at least $200 to improve your yields a tiny bit (say, from 85% to 97%).

If you want to take this route, pressure pots start around $80 (link); nicer ones fetch as much as $300 (link). To operate it, you will also need a compressor - and these start from $100 from fairly noisy units from the hardware store, to $180 for relatively quiet-running ones; standard pressure hoses and fittings add another $20 or so.

4.3. Your first casting projects

All right, all right - enough with all the theory and shopping tips. It's time to dig out that wax pattern that you have made before, and turn it into a finished work of art! This section is all-text, but if you need visual aids, this photolog is probably good to look at.

Caution: similarly to many workshop and household chemicals, casting resins can be harmful if misused. In particular, they may react violently when mixed with incompatible substances; cause severe irritation or lasting damage if splashed into your eyes; and will emit dangerous vapors if overheated, burned, or intentionally aerosolized.

Please refer to product safety datasheets (MSDSes), and to section 7 of this guide, for an overview of known risks, material handling recommendations, and disposal procedures. Do not proceed with any hands-on experiments until you have done so.

4.3.1. Making a silicone mold

Resin casting is fun, but you need to remember that once the components are mixed, the reaction will proceed no matter what; it's important to plan accordingly: read this section fully and memorize all the steps beforehand, and have all the necessary supplies and information within reach.

First, you need to estimate the amount of resin you will need: for CNC-machined or 3D-printed shapes, simply ask your CAD application to calculate the volume of the master, and subtract this value from the volume of a dummy box of the same height, width, and depth. To have a comfortable margin, add about 15% to the result, or 10 ml, whichever is greater. Then, multiply that volume by the density of the resin (check the datasheet; it's usually between 1.1 and 1.4 g/cm³). The resulting figure is the weight of the material you need to prepare.

With these calculations out of the way, prepare the following stuff:

Although accidents are unlikely, you should still try to minimize potential damage: if there are any LCDs or other expensive gizmos in your workshop, consider moving them a bit farther away, or covering them with plastic sheeting. Don't wear your best clothes, and if you have a carpeted floor or expensive hardwood, cover the area that is most likely to suffer in case of a spill.

When you are ready to go, place the mixing cup on a scale, then tare it. Agitate both components of the resin in their original containers, and then use a clean tongue depressor or a spoon to pour about 20 g of silicone into the mixing cup. Add a suitable amount of catalyst (the ratio is usually 10:1, but check the datasheet), start the timer, and begin mixing thoroughly for honest 3 minutes; be sure to repeatedly scrape the sides and the bottom to avoid leaving any unmixed resin in these spots.

Next, place the cup under vacuum; the mix will initially rise as the bubbles expand, and then collapse back; you should keep it under vacuum for another 1-2 minutes past that point. If the resin gets dangerously close to overflowing during the initial rise, simply release the vacuum (i.e., yank out the hose), and try again; several cycles of that should do the trick - and next time, use a larger cup.

The next step is to pour some of the resin into the mold created in chapter 3; use about half of the required volume or so. Place the mold on a strip of aluminum foil, and put that under vacuum; this will help the resin conform to even the most intricate shapes with no effort on your end. After about 2-3 minutes, you can slowly release the vacuum, pour the remaining amount of resin (use a bit more than necessary), and set the whole thing aside for several minutes to allow any bubbles to rise to the surface (or collapse back into the resin). If there are any stubborn bubbles on the surface at that point, you can apply a gentle burst of compressed air to get rid of them.

Finally, cover the entire contraption with a flat, clean sheet of polypropylene; lay it down gradually, starting at one side, to avoid air entrapment. When done, weigh it down with something reasonably heavy - around 500 g should do - and brace the whole thing, so that the cover doesn't slide off. Be sure to check the timer at this point - has the entire process taken more than about 12-15 minutes? If yes, why?

In any case, give it about 12 hours or so, until the resin remaining in the mixing cup is firm and tack-free. If you are impatient, placing the mold in a temperature-controlled oven at around 50° C will cut the curing time down to 1-2 hours or so - but don't go too high, given that this particular master is made out of wax. When the rubber is ready to demold, pull off the cover, and then use a dental hook or a similar tool to pry the rubber off near the corner of the mold. Remove it fully and inspect the result. if it looks flawless - as it should - you may want to briefly post-cure it at around 100° C for 30-60 minutes, and in the meantime, pat yourself on the back!

Here are several questions that may be on your mind:

Okay, okay - time for some polyurethane fun!

4.3.2. Casting plastic parts

Polyurethane casting is not dramatically different from working with silicones, but you have to be swift, and pay more attention to detail. Quite simply, with a resin such as IE-3075 or TP-4052, you will probably only have about 6 minutes to go through all the steps - so there is no time to look around for paper towels or a mixing stick.

In general, before starting, you should go through all the preparation steps outlined in the previous section; and when done, you should also do the following:

With all the preparations taken care of, you are ready to roll. Place the mixing cup on the scale, tare it, and pour the required amount of isocyanate; in our case, 10 g will do. Next, gently pour the appropriate amount of polyol (for IE-3075, this will be 8.9 g), start the timer, and begin mixing very thoroughly, frequently scraping the walls. Most polyurethane resins, IE-3075 included, require at least 90 seconds of mixing to progress from a dispersion to a proper solution when mixed in a small quantity - and if you stop sooner, the cured material may not cure properly. Do it right.

When done, place the container under vacuum, and turn on the pump. The resin should take no more than about a minute to rise and collapse back; if it's taking much longer, your vacuum chamber may be too big, or the pump may be inadequate or malfunctioning (e.g., due to an open gas ballast valve, or due to contamination). If, on the other hand, everything went as expected - and the bubbles have collapsed in a timely manner - you should now pour some of the resin into the mold cavity (to fill it roughly halfway, taking care to cover any detail where air entrapment is likely), and place the mold under vacuum for another minute or so. Don't worry if it never stops bubbling at this stage; that's OK.

After a brief round of degassing, release the vacuum gently, place the mold on a previously prepared flat surface, and add the remaining resin, until it overflows. Grab the polypropylene cover and carefully lay it on top of the mold, using the technique outlined for silicones. You should brace the cover against something, so that it doesn't slide off, and weigh it down with around 200 g (larger molds can be clamped with several kg of force, but this particular one is relatively easy to squish).

Well, that's it! Before you go, check the timer; if the process has taken more than six minutes, you should figure out how to improve it. In any case, leave the mold alone for at least 3 hours (or more, depending on the resin used), and when you come back, confirm that the material left over in the mixing cup is tack-free and hard as nails. Next, gently flex the polypropylene cover to detach it from the part, and extract your casting from the mold.

Hopefully, the result is perfectly fine; that said, the likelihood of mishaps is higher for polyurethanes than it is for silicones - so if something isn't right, don't despair. Here's a quick summary of the most common issues I have seen:

If you are seeing any issues, it's important to narrow the problem down right away, while the number of variables is still fairly low. If you are out of ideas, don't hesitate to ping me at lcamtuf@coredump.cx; I may be able to help. A good place to discuss your experiences or showcase your work may be /r/resincasting, too!

Anyway - if everything turned out to be just fine, you may want to briefly post-cure the part and any leftover material. Place it at around 100° C for one hour, and then play with it to get a sense of its physical properties: try drilling a hole in or scratching the surface of one of the leftover bits, and see how hard it is to break it.

4.4. Miscellaneous polyurethane casting tips

This section is just a quick a collection of random notes that should come handy in real-life projects, but that I wanted to keep out of your first casting job. Enjoy!

4.4.1. Meta: not all resins are alike

Before we dive into various advanced topics, you should know that there are significant differences in the handling characteristics of various polyurethane resins, even if the advertised cure times and ultimate physical properties of the material are roughly the same. In particular, be aware of the following:

Because of all these striking differences, don't take everything you see in this guide as universally applicable to every formulation on the market; and in the same vein, don't expect your own experiences with product A to be fully applicable to product B. If in doubt, request a sample of any new product you are considering, or simply ask.

4.4.2. Adding pigments and fillers to resins

Let's start with something simple. Non-reactive (i.e., plasticizer-based) liquid dyes and coloring pastes can be mixed into the working amount of isocyanate, before adding any polyol; when taking this route, just try to stay under 2% by volume (around 8-10 drops per 10 ml); if you find yourself routinely having to add more more, consider switching to dry pigments or a higher-yield dye or a reactive carrier - because past this point, solvents used in the dye will be affecting the properties of the part.

Reactive coloring pastes designed specifically for polyurethanes use a polyol as a base; that's the case for pigments from Innovative Polymers. Their main benefit that they can be added at much higher levels without completely messing up the properties of your parts. In principle, you should subtract the weight of the added dye from the required weight of polyol - but in a quick experiment with IE-3075, I found that using the nominal amount of polyol results in improved strength. Results in other resins may vary.

When working with dry pigments, there is a bit more legwork involved. If you simply dump the pigment unceremoniously into the liquid, it will probably clump together - and stay that way. To avoid this, you need to place the desired amount of material in an empty mixing cup, tare it, and start adding isocyanate drop by drop, mixing constantly, until you end up with a homogenous, runny paste (siloxane surfactants can make the process easier, too). Once the paste looks good, you can gradually add the remaining isocyanate while constantly mixing - and you should be all set.

Tip: if you have a high-yield, hard-to-disperse powdered pigment that you keep coming back to, it may make sense to make a custom coloring paste for future use. Simply disperse it thoroughly in an inert plasticizer (e.g., dipropylene glycol dibenzoate, discussed earlier; silicone oil works for platinum cure rubbers) or in a suitable polyol, and pour that into a dropper bottle.

Of course, as noted earlier, you should ensure that the material is moisture-free; in tricky cases, premixing the resin or adding zeolite should help. Glass-based fillers, such as Scotchlite or milled fibers, may benefit from being pre-treated with a silane coupler, too.

4.4.3. Premixing the system

Premixing is one of the simplest and lesser known tricks that can help solve many of the problems that crop up in some polyurethane casting jobs. For example, it can dramatically reduce shrinkage without affecting cure time; lower the risk of cure inhibition; halve the time needed to fully mix the resin in small batches; and greatly reduce the sensitivity to moisture, to the point of making many systems suitable for surface coating applications. These benefits stem from the reduced reactivity and improved compatibility of a partly polymerized liquid. It's not a silver bullet, but for many formulations, it's pretty close to being one.

The only price to pay for premixing is an increase in viscosity, which typically isn't a big deal if you have started with a low-viscosity system such as IE-3075; and the added expense of about 5 minutes of work and about one hour of waiting per every batch that you intend to cast.

If you want to try it out, the recipe is very simple: measure the desired amount of isocyanate, add all the fillers and dyes you want to have, and then introduce between 10% and 20% of the necessary amount of polyol; this mix needs to be stirred thoroughly, degassed - and then stored in a covered cup, blanketed with an inert gas, for about 1-3 hours. At that point, the remaining polyol can be mixed in, and the resin can be cast.

Keep in mind that premixed resins will have short shelf life: the viscosity will keep increasing, and in presence of a catalyst, the isocyanate will more aggressively react with ambient humidity and deteriorate. You should premix only the amount you intend to use right away.

Oh - in case you are curious, this graph shows the impact of premixing on the exotherm for 10 g of IE-3075, and how it compares to the use of fillers. The measurements were taken using a thermocouple submerged in a small, insulated polypropylene cup, approx. 30 mm in diameter:

The X axis is time in seconds. The Y axis is temperature in °C. The resin and the room are initially at around 20 °C.

4.4.4. Adjusting room-temperature cure speed of existing resins

Here's another tidbit you won't find in any other hobbyist reference on resin casting: it is possible, and in fact fairly easy, to chemically slow down many polyurethane systems to significantly reduce shrinkage; and to accelerate slow-curing ones to get your parts sooner or have fewer artifacts in thin sections of your molds. Why bother, you may ask? Well, it not only saves you money, compared to buying several resins for different applications - but perhaps more interestingly, it enables you to come up with custom-tailored cure profiles that are of no commercial interest to the manufacturer.

In essence, there is a wide variety of catalysts used in castable polyurethane resins. Every catalyst behaves differently: some are highly active at room temperature, some kick in only later on, when the resin has warmed up due to exotherm. Some are better at driving the early stages of polymerization, but stop shortly thereafter; some have a sustained effect until the very end. Some are highly selective toward the desirable isocyanate-polyol reaction, and some don't mind catalyzing the isocyanate-water reaction - which leads to the formation of bubbles of CO2. Some are very stable, and some deteriorate when exposed to open air and other substances, which may cause inhibition or poor surface cure. But there is no single product that gives you the very best on all fronts.

For this reason, manufacturers combine various catalysts to reach a compromise that makes sense for their intended customers - but these parameters aren't necessarily ideal for your needs. For example, OC-7086 is a resin designed for larger castings; when dealing with tiny parts, it will cure too slowly, and with far too much sensitivity to ambient moisture.

Thankfully, you can fix this on your own.

4.4.4.1. Speeding things up

Ideally, if you wish to use OC-7086 or HP-21xx - or accelerate any other finicky resin - you should get bismuth neodecanoate from Santa Cruz Biotechnology or Krackeler. The cost is around $35 for 250 g, and that amount will last you forever. The catalyst isn't dangerous, but both of these places have a blanket policy of shipping only to commercial addresses. If you can't have it shipped to work, ping the folks who run Chemsavers.com - they should be able to get it for you and ship it to your home for a very modest premium.

Bismuth neodecanoate is a syrupy liquid which needs to be diluted with a plasticizer (e.g., DPGDB), a suitable polyol, acetone, or something else of that sort. Depending on the resins you are working with, you may have to experiment with dosage, but typically, levels between 50 and 500 ppm will be enough. For example, to "fix" OC-7086, you can prepare a 4% solution in plasticizer, and add it at about 1-2 drops per 10 ml of isocyanate as you are getting ready to mix it with a polyol. To speed up HP-21xx, you will need a solution closer to 50%, added in similar quantities.

Now, if this particular bismuth compound is hard to find where you live, don't despair! A decent alternative is tin(II) 2-ethylhexanoate, also known as stannous octanoate. This substance is commonly sold as an accelerator for condensation-cure silicones; for example, a nearly pure form is available under brand names such as Smooth-On Accel-T, Quantum Silicones QSil STO, or Bluestar VICURE #2. Just be careful not to buy anything based on dibutyltin dilaurate, dimethyltin dineodecanoate, or a similar tin(IV) compound: they will work great, but also happen to be a lot more toxic.

As with bismuth, the appropriate dosage varies depending on the resin; for OC-7086, a 1% solution, added at 1-2 drops per 10 ml of isocyanate, is a good starting point. Note that the compound is a bit more harmful than bismuth - handle it with care.

Bismuth and tin aside, there are several other, more exotic options to choose from. They may offer very specific benefits, such as improved curing characteristics in particular systems, or no subsequent inhibition of platinum silicones (which are somewhat sensitive both to bismuth and tin). If you need additional guidance, click here to expand a section with some rough notes.

Random rant: as noted above, most suppliers of lab chemicals are no longer willing to ship to residential addresses. Such restrictions make some sense for haz-mat materials - but the companies simply won't do any business with you, even if all you're trying to buy is salt or glucose.

There are two reasons for this. First, there is a growing number of government agencies - ranging from DHS, to DEA, to CPSC (yes, that's right!) - that don't want people to make anything ranging from illicit drugs to bootleg fireworks. Companies that sell to individuals face a hodgepodge of regulations and vague reporting requirements, and risk police raids and other serious consequences if they mess up. Second, there are liability concerns: if a kid loses an eye and his parents sue - well, even if the manufacturer prevails in court, there are still legal expenses and bad PR to deal with.

Because of this, it simply makes no sense for most of them to cater to the hobbyist market at all - shipping to a commercial address creates a pretense of due diligence, no matter how weak it may be.

To keep chemistry alive as a hobby, I urge you to support the remaining few places that did not succumb to this trend; in particular, consider going with eBay sellers or friendly outlets such as Chemsavers even if you have an opportunity to order certain chemcials directly from the manufacturer for less. Just stay away from ScienceLab.com.

4.4.4.2. Slowing down the reaction

In many types of polyurethane formulations, it is possible to slow down the reaction by converting the catalyst to a less ractive complex. In particular, systems that rely on amine catalysts (and do not contain reactive amines as crosslinkers or any other vital components of the formulation) can be slowed down with strong, non-oxidizing acids that react with the catalyst to form a largely inactive ammonium salt. In the same spirit, some of the less obnoxious thiols and certain other substances can chelate a variety of organometallic catalysts.

In products such as IE-3075 or OC-7086, you can get good results with p-toluenesulfonic or methanesulfonic acid, both of which are available from Chemsavers for around $20. Methanesulfonic acid is slightly more convenient, because it is liquid at room temperature; but p-toluenesulfonic acid is pretty easy to directly dissolve in polyol. Levels around 0.1-0.5% by weight should have a very pronounced effect; just be careful not to go overboard: excess acid may react with isocyanates and mess things up.

This graph shows the impact of p-toluenesulfonic acid on the curing exotherm of IE-3075, using the setup outlined earlier on:

If PTSA or MSA are not easily available in your market, a much less potent but possibly still acceptable alternative is sulfamic acid, a common cleaning compound available on eBay and Amazon for just a couple of bucks. The main problem with this compound is its relatively poor solubility in polyols and in most organic solvents. A saturated solution in n-methyl-2-pyrrolidone (NMP) may be your best bet; it will need to be added at a level closer to 1-2% by weight, which isn't exactly ideal. The solution is also not stable in the long haul, so prepare only as much as you intend to use in a couple of days.

What else? If you really can't get any of the above, you can try tartaric acid. Along with several other weak, aliphatic hydroxy acids, this compound will inhibit the reaction to some extent, although it shows some interest in side ractions that may liberate bubbles of carbon dioxide or impart a yellowish hue to your parts. On the upside, it's a common food additive, available pretty much everywhere; and it can be easily dissolved in acetone.

Note that PTSA, MSA, and sulfamic acid are all highly corrosive; use gloves and eye protection whenever working with concentrated solutions.

Oh, one more thing: keep in mind that while adding catalysts to a resin is guaranteed to make a positive difference, adding a particular inhibitor is not. Of the fast-curing resins discussed in this guide, I never found a way to significantly slow down TP-405x, but almost everything else seems to be a fair game.

4.4.5. Blending several resins together

Every now and then, you may be hoping to modify the properties of a resin in a way that goes beyond what's possible with non-reactive fillers, plasticizers, and so on. Other times, you may be interested in changing its cure speed in a situation where the methods outlined in the previous section are impractical or simply don't work.

Well, the good news is that you can do quite a bit without resorting to making your own formulations from scratch. First of all, if you have two resins with comparably reactive isocyanates or polyols, and similar catalysts, you may be able to simply mix them together as-is. For example, let's say that you own HP-2150A and HP-2160D, and want to create a range of tough elastomers. The mix ratio is 100:43 for the first resin, 100:20 for the second one, and you want to blend them at a ratio of roughly 2:1 to get a rubber around 70 Shore A. In this case, suitable mixing amounts may be:

Of course, if the systems are based on dissimilar chemistry, the resin may cure prematurely, not cure at all, or have disappointing mechanical properties. Even in this case, not all is lost: you may be able to get somewhere by starting with a single resin, and then partly or completely substituting one of its components with that belonging to another system. There are situations where it won't work, and situations where it will give you useful materials with faster or slower cure profiles, and mechanical properties somewhere between these of its constituents.

The challenge with this second approach is figuring out the correct mixing ratio for isocyanate coming from product A, and polyol coming from product B; the manufacturer won't tell you how many reactive NCO and OH groups are there in each of the components, and without this information, you have to resort to trial and error. The correct ratio is usually between 100:30 and 80:100, and you can pinpoint it by doing several tests and selecting the range that resulted in the highest indentation hardness (Shore D durometer costs about $25-$50 on eBay); guessing the ratio within 5% should be fine in most uses.

4.4.6. Heat-accelerated cure

As noted earlier, polyurethanes and silicones begin polymerizing the moment you mix the components; by the time you reach the demold time, the reaction is mostly over - but some cross-linking may continue for many days or weeks at an exponentially decaying rate. As this process goes on, the properties of the part will keep approaching these advertised in the datasheet.

If you are impatient and want to demold your castings sooner than normally possible, but don't want to sacrifice pot life or deal with chemical catalysts, placing the mold in a temperature-controlled oven will typically cut the time in half per every 10° C over ambient. Alas, the combination of significant thermal expansion of silicones (0.025%/C), and the somewhat lower but still noticeable expansion of rigid polyurethanes (0.005%/C), will probably affect dimensional accuracy of the part - so if you are aiming for snap fits, it makes sense to keep the mold at room temperature for as long as you can, and then bake it at no more than perhaps 40° C.

For an already demolded part, post-curing is a valuable process that involves fewer trade-offs, and lets you reach the final properties of the material in hours, rather than weeks; since the resin is already largely polymerized, and is not confined in an expanding mold, its own thermal expansion is less likely to have lasting effects. It's important to ramp up the temperature gradually, though, so that the part doesn't get too soft. I suggest one hour at 40° C, followed by 30 minutes at 60° C and 80° C; the cycle can be wrapped up with 1-3 hours at 100-110° C. Note that many polyurethanes begin to deteriorate around 150° C, and that for transparent formulations and flexible rubbers, this limit may be even lower.

4.4.7. Multi-part molds

All right, all right - enough with chemistry. But there's one more topic that may help you in casting work. Sooner or later, you will need to make parts with complex features on multiple sides. When replicating hand-made shapes, the process is usually very intuitive; for example, the geometry can be submerged in a blob of silicone that is carefully dissected with a scalpel or a box cutter, and put together later on.

The process for designing accurately meshing multi-part molds in CAD software isn't much more complicated, but may take some effort to wrap your head around it. Let's say you want to make a part with a cross-section as shown on the left (and some additional features that prevent us from simply laying the shape on its side):

The first step is to make a regular mold similar to what we would do for one-sided parts, but also add a small pedestal around the geometry - this will serve as a registration mark. The second mold is simply the same part and its pedestal, flipped around; this top mold will neatly slide into the bottom one. Voila!

In more complex molds, the parting line may be located less conveniently, and may not allow all the air to escape on its own. In these cases, the mold will typically have a sprue through which the resin is poured in, and strategically placed vents to allow the air to escape from tight spots; a reservoir of resin on top of the spruce will offset for shrinkage in large parts, too. All in all, it's not hard, but when it comes to that, you will need to practice a bit.

Click to proceed to chapter 5...