We're done with the technical stuff, but stay with me for a bit longer: even for DIYers, workplace safety is no laughing matter. You will be dealing with power tools and reactive chemicals, and so if you want to pursue this hobby, there are inherent risks you simply have to understand and accept. It's entirely possible to lose an eye or set your house on fire; there are certain common-sense steps you can take to minimize the probability of mishaps, but the odds will never go down to zero.
It is also your sole responsibility to investigate and follow all the applicable HOA rules, city ordinances, and other regulations - especially as they relate to purchasing laboratory supplies and chemicals, the use of power tools, and the disposal of unreacted chemicals. In many places, the laws are sensible or non-existent; but exceptions happen. If you accidentally ignore a piece of obscure, boneheaded legislation - well, nine out of ten times, nobody will know or care, but do you really want to be that one unlucky guy?
OK. With all that in mind, this chapter provides a quick but non-authoritative overview of some of the most obvious safety considerations and health risks for CNC machining and resin casting. There are no guarantees that this information is accurate, complete, or up-to-date, so do your own research. Do not blindly trust one random dude on the Internet!
Small CNC mills are fairly safe, compared to most other power tools - but you still need to read and follow the instruction manual, and in general, exercise common sense in everything you do. The primary risks are getting your hand pinched between the rotating tool and the workpiece, or between any other moving parts; having loose clothing or hair caught by the spindle; or being hit by any debris ejected during the cutting process.
Becaue of this, you should really stay clear of the machine while it's running a job - after all, you have no business fiddling with it. If you really need to make some sort of an adjustment, or perhaps simply vacuum off accumulated shavings, it's imperative to pause the cutting process first. It is also a very good idea to wear basic eye protection at all times; although the odds of a broken tool or other sharp material hitting your eye are very low, the damage can be significant.
All that aside, high-speed CNC lathes and high-power CNC mills (over 1 kW or so) have their own safety rules that I will not cover here. These devices are often powerful enough to break bones, sever limbs, or eject a loose workpiece with a speed sufficient to kill or maim. If you have one of these, the use of robust safety covers made out of sheet metal, polycarbonate, or wood, is definitely a must.
Any type of dust can aggravate existing respiratory conditions; prolonged and heavy exposure may also lead to the development of brand new ones. That said, most of the noteworthy materials give off relatively little airborne particles when machined in a sensible way - and so, your overall exposure to nuisance dust will probably not change in an appreciable way.
When working with prototyping boards such as RenShape 460, airborne dust is produced in modest amounts, and has no known, specific health risks; the same goes for many other common plastics. Covering the mill with plastic sheeting for the duration of the job (well clear of any rotating parts and inlet vents) is a good practice, even if just to keep your workplace tidy; building or buying a proper enclosure is an option, too. In extreme cases, a simple N95 particulate respirator (link) is good to have at hand.
That said, there certainly are materials that pose an elevated risk; to avoid really unpleasant surprises, always investigate the substances you are milling, sanding, or otherwise dispersing in the air. You should be particularly wary of anything that contains crystalline silica (quartz), asbestos, elemental metals, toxic pigments, and any other harmful substances that may be absorbed through the lungs.
In essence, be sure to handle any powdered fillers and pigments with care, and do not mill, sand, or cut rocks, and mystery plastics of unknown composition - and you should be fine.
Note: there is no compelling evidence that amorphous glass products, such as milled glass fibers or 3M glass microspheres, share the dangers of crystalline silica. You still don't want to breathe any of that stuff in, but these fillers should be safe to use in their intended way - and a tiny amount of dust should be OK.
Vacuum isn't particularly dangerous to work with, but you should be aware of the risk of implosion. Plastic vacuum chambers should be inspected for cracking or crazing, and should never be exposed to high temperatures, substantial quantities of solvents, strong acids and bases, and so on. Sketchy-looking or homebrew chambers can be secured with packing tape, shrink-wrap foil, fabric, and similar materials that will retain debris if things go wrong. Other than that, you have very little to worry about; oh, if you are working in a confined space, it may make sense to vent the vacuum pump to the outside, so that the relatively small quantities of gases liberated from the resin - as well as the mist of mineral oil - don't end up right under your nose.
Pressure casting is more dangerous; that's in part because the pressure differentials are much higher than for vacuum casting (especially if something goes wrong), and because all the energy is released outward if the container fails. Because of this, you should never try to rig your own pressure chambers or any other parts of the system, and not even think about removing any safety valves, regulators, and so on. Follow the instructions religiously, especially when it comes to draining the compressor; do not exceed manufacturer-provided pressure ratings; always double-check that the system is depressurized before trying to open the chamber; and disconnect everything completely when not in use. With common-sense precautions, catastrophic failure is unlikely - but rest assured, bad things have happened to quite a few people who should have known better.
Oh, one more thing: if you are inclined to get a a nitrogen tank for blanketing resin containers in storage, go for a small one - capacity of 1 cubic meter (40 cubic ft) is more than enough. Store it horizontally or attach it to the wall to make sure it can't fall over, and always use a regulator.
Platinum cure silicones are believed to be essentially harmless; both the siloxane resin and the catalyst are non-reactive and show virtually no toxicity in animal studies. You should always read material safety datasheets for the specific product you are using, but chances are, the only risk you have to worry about is that the material is sticky, and spills can be annoying to clean up (naphtha and other nonpolar solvents can help). Beyond basic workplace hygiene, no special precautions should be necessary.
Now, I don't recommend using condensation cure silicones, but if you decided to choose that option, you should be aware that they often use a small amount of dibutyltin dilaurate, dimethyltin dineodecanoate, or a similar tin(IV) compound, to catalyze the reaction. The substance is corrosive, and more troublingly, exhibits some chronic toxicity. It is present at around 5% in the catalyst component, and usually at less than 1% in the finished product - but since it will leach out in favorable conditions, you should probably not be using condensation cure silicones to make anything that is routinely worn or handled by humans, or that comes into contact with food. When mixing these resins, it's best to use latex or vinyl gloves; basic eye protection is not a bad idea, too. Storing the catalyst in a suitable cabinet and out of reach of children is important, too.
Polyurethane resins consist of several very distinctive components, and it's probably useful to discuss them separately. Here's what you can typically find in the MSDS:
Isocyanates: polyurethane formulations used for prototyping normally rely on fairly non-volatile isocyanates, such as liquefied methylene diphenyl diisocyanate (MDI), hydrogenated MDI (HMDI / DMDI), or isophorone diisocyanate (IPDI); products where these isocyanates are already partly polymerized are common, and in this case, the prepolymer may be not listed in the MSDS at all.
In general, all these substances are fairly reactive, can be damaging to mucous membranes, and upon longer exposures, will irritate skin. Fortunately, they polymerize to form an inert substance soon after being exposed to water, alcohols, and many other common substances - so they pose little danger to the environment, and do not bioaccumulate.
Isocyanates tend to be pretty dangerous if inhaled - enough to earn some of the water-clear HMDI and IPDI formulations a nice "toxic" label in Canada and in the EU. Nevertheless, the extremely low vapor pressure of these chemicals - around one million times lower than water - means that if you are using them for casting reasonably sized parts at room temperature, you have very little to worry about. Within the scope of this guide, the main things to avoid is getting the substance into your eyes and spilling it in large quantities. Use eye protection, wear latex or vinyl gloves, and keep your workplace safe and clutter-free.
Now, as hinted earlier, several manufacturers resort to more reactive and more volatile isocyanates, such as toluene diisocyanate (TDI) or hexamethylene diisocyanate (HDI); I recommend avoiding these, but if you have no choice, really good ventilation, gloves, and safety glasses are absolutely a must.
Note: a small proportion of industry workers frequently exposed to significant concentrations of isocyanates eventually develop hypersensitivity to the compound. Although this is very unlikely to happen when working with room temperature casting formulations, you should obviously seek medical attention if you have difficulty breathing within several hours of working with this or any other exotic chemical.
Polyols / polyetheramines: most polyester or polyether polyol compositions are viscous, non-reactive, non-volatile fluids with relatively high molecular weight, chemically related to substances such as polypropylene glycol, glycerine, xylitol, and so on. They tend to have rather low toxicity in their intended use, and aren't particularly reactive. Not all products are the same - but unless something stands out in the MSDS, you probably don't have to worry (of course, common sense still applies).
Some systems may also use lower molecular weight amines as crosslinkers, or higher molecular weight polyetheramines to replace traditional polyols. Low molecular weight amines tend to be a bit more biologically active and irritating; a chlorine-containing amine originally used in many polyurethane formulations - 4,4'-methylenebis(2-chloroaniline) - was found to be a probable human carcinogen. A number of less reactive replacements emerged since then, and are expected to be much safer based on their chemical structure and animal data. In other words, there is no reason to panic, but it definitely makes sense to handle the formulations with some respect. A good sign of high amine content is a yellow-to-caramel color of the polyol component, alongside with a slight but disagreeable smell.
Catalysts: these substances are of special interest, because as opposed to the core reagents outlined above, they usually remain as-is in the cured product - and if the circumstances are right, may leach out of it in small quantities, particularly out of Shore A elastomers.
Modern polyurethane chemistry is usually catalyzed with much less than 1% of a carboxylate of bismuth, zinc, tin(II), or a related metal; or with a handful of reasonably selective amines, imidazoles, and so on. Most of the contemporary metal-based catalysts are extremely unlikely to be of any concern - especially at the concentrations used in the resin. The non-metal catalysts have also been studied extensively and carry no known risks at these levels - but as noted earlier, they belong to large and interesting families of bioactive compounds.
As a relic of bygone days, several manufacturers still resort to toxic or bioaccumulative organometal catalysts, such as mercury neodecanoate or dibutyltin dilaurate; for example, that's the case for many products from Smooth-On and Freeman. In theory, the cured plastic should be safe to handle under normal circumstances. In practice, the mobility of the organomercury compound may be increased by factors such as incorrect mixing proprtions, UV degradation, exposure to elevated temperatures, or the action of solvents and household chemicals.
In the end, I certainly wouldn't use the latter class of resins for jewelry, toys, or food applications - and I wouldn't want to worry about the exposure due to spills and other workshop accidents. For this reason, I strongly recommend staying away from such products, especially since the alternatives are usually just as good; simply check the MSDS to confirm.
Note: if you are purchasing any concentrated catalysts or inhibitors to modify the performance of your resins (as discussed in section 4.4.4), be sure to do your homework. For example, common bismuth, zinc, and tin(II) carboxylates have very modest acute toxicity and no major chronic risks. Many other organometal catalysts - for example, based on tin(IV), cobalt, or mercury - are something you really don't want to keep around in concentrated form. Triethylenediamine and its friends sit somewhere in between: they are flammable, irritating, and fairly reactive, but probably not particularly toxic per se.
Plasticizers, fillers, surfactants, etc: while most resins don't contain any other questionable additives, it's useful to briefly scan the MSDS for anything else out of the ordinary. For example, several manufacturers add phthalate plasticizers - such as butyl benzyl phthalate (BBP) or dibutyl phthalate (DBP) - to some of the more flexible polyurethane rubber formulations. While these substances have very low acute toxicity, animal studies raise some concerns about long-term subchronic exposure through children toys, food-contact items, and so on. No need to panic, but in such applications, it's best to stick to safer alternatives.
As with any chemicals, you should avoid mixing polyurethanes - and isocyanates in particular - with potentially incompatible substances; as hinted earlier, this includes alcohols, water, but also strong bases, oxidizers, amines, and metal salts. Rapid polymerization, decomposition, or other funny developments may ensue. Isocyanates also decompose in a matter of minutes in dimethyl sulfoxide (DMSO), so do not use this solvent as a carrier for dyes, catalysts, and so forth.
In the same vein, don't mix unreasonable quantities of polyurethane resins in a single batch. Runaway exothermic reactions can become dangerous if the resin gets so hot that the mixing container melts away, or something catches fire. If you need to dispose of a large quantity of unwanted resin, polymerize it in batches; or at the very least, use a large, shallow pan and work outdoors.
Last but not least, because of their irritant properties, uncured resins should be stored safely, and kept out of reach of children and other creatures other than the intended user.
These considerations aside, cured polyurethanes are one of the safer, more stable, and least controversial plastics out there. Even if you mess up the mixing ratio, the unreacted isocyanates will simply eventually react with moisture, unreacted polyols will not pose a threat, and nothing bad should be leaching out of the material otherwise. Like most plastics, and organic materials in general, polyurethanes release a fair amount of toxic substances during thermal decomposition - carbon monoxide being by far the most significant problem - so try to resist the urge to burn them if at all possible.
Throughout this guide, I have mentioned quite a few chemicals that won't be covered in this chapter in special detail. If you plan on getting any of them, you should take care to obtain and read the appropriate material safety datasheets, and understand all the associated risks. In no particular order, here are some of the things you should know:
Epoxy resins: many of these products use corrosive and somewhat volatile compounds. Chronic toxicity may be a concern with daily exposure. When mixing larger quantities, beware of significant exotherm - it's usually more pronounced than in polyurethanes.
Polyester resins: styrene is highly volatile and flammable, and its vapors are harmful. On top of that, the standard catalyst is a strong oxidizer, and the resin typically cures with an extremely pronounced exotherm.
Pigments and dyes: toxic pigments based on lead, mercury, arsenic, cadmium, chromium, and nickel are available on the market, but should be avoided. Shop specifically for modern, less harmful alternatives. Avoid pigments that contain crystalline silica of respirable size - and in any case, handle the material with care.
Glass fibers: thin strands of glass are easily embedded in the skin, and can give you an annoying but generally harmless rash that takes 2-3 days to go away; getting the material into eyes can be a more serious issue, so use gloves, clean up promptly, and don't touch your face until then. If you get a rash, scratching it will usually only push the fibers deeper; soaking the affected area in water and applying an over-the-counter anti-itch cream (antihistamine) will offer relief.
Common solvents, glues, demolding agents: unless indicated otherwise, you should assume that all these products are extremely flammable and that their vapors are harmful. Prolonged exposure to very high levels of certain solvents may lead to CNS effects, reproductive toxicity, and so on; over time, lasting damage is a possibility, too. Naphtha, xylene, toluene, benzene, methylene chloride, and NMP are of special concern.
All the other stuff: several substances mentioned in this guide may be described as "reasonably safe", but that doesn't necessarily mean they are non-toxic if aerosolized or ingested in significant quantities; that they don't react in funny ways with incompatible materials; or that they won't cause damage if rubbed into your eyes. Before buying any chemicals on eBay, Amazon, or anywhere else, look up their safety datasheets first.
Tip: MSDSes are targeted at industrial uses, and err heavily on the side of caution; the datasheet for table salt says that the substance is "harmful in case of ingestion"; that skin contaminated with it must be flushed with water for "at least 15 minutes"; that protective clothing and gloves should be worn at all times; and that proper ventilation is necessary. They also warn that you shouldn't let this chemical enter the environment.
And that's not even the worst example: Sigma-Aldrich has a serious datasheet for pure water that says you should use proper glove removal techniques and... wash hands after handling this dastardly chemical.
Do not ignore the advice provided in these documents, but use common sense when evaluating your personal risk, and the safety measures that are appropriate given the amounts you are working with, the way you are handling the substance, the frequency of exposure, and so on. It's always good to pay close attention to LD50 values; chronic health risks; and any mentions of carcinogenicity, developmental toxicity, etc.
It hopefully goes without saying that you should store all the materials discussed in this guide away from sources of ignition, excessive heat, and so on; that you need to keep them away from children; and that if you are pregnant or breastfeeding, you should avoid any non-essential exposure to exotic chemicals, no matter what the MSDS says.
Click to proceed to chapter 8...