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Synthetic polymers play a role in almost every single commercially manufactured item on the planet. Plastics are not just ubiquitous, but extremely versatile: some of them are incredibly stretchy, while some are hard as nails; some are crystal clear, and others come in all colors of the rainbow; some can survive extreme temperatures, and yet others can stop a bullet mid-flight.

When you think about all this, it’s hard to believe that even for hobbyists well-accustomed to 3D manufacturing, engineering-grade plastics are still taboo. Sure, we may own 3D printers, but the output from affordable PLA and ABS extruders doesn’t even come close to the strength or variety of cheap injection-molded parts. The owners of CNC mills also have no reason to be smug: most of them shy away from plastics altogether, or resort to inexpensive but poorly-performing materials such as acrylic or HDPE.

Oddly enough, there is no reason why things need to be that way. There is a pretty safe, low-cost, and hassle-free technique that lets you make incredibly tough and precise parts in your workshop, in a matter of hours. The only problem is that almost all the available tutorials about this process – known as resin casting – are written by artists, with other artists in mind. Applying their approaches in engineering projects is usually not a good plan.

Fabricating patterns for single-part molds

If you follow the usual advice for replicating hand-made parts, you will be instructed to coat or submerge the original item in a flexible material, such as alginate or silicone rubber. Once this is done – and the shell is fully cured – you will be asked to cut this rubbery blob open and retrieve the original model. At that point, you will also fill the hollow void with another thermosetting compound that takes the shape of the initial part – and clamp the entire thing shut.

This process works, but has a number of drawbacks, especially if you want to get predictable results – and want to get them fast. For example, it can be taxing to build an appropriate box for a mold and then properly attach and orient the original model inside of it. It can be just as tricky to remove the original part without ruining the model, the shell, or both. Heck, even putting the mold back together without compromising the dimensional accuracy of the resulting part is quite a challenge on its own.

Luckily, all these problems can be avoided when dealing with computer-designed parts: all you need to do is to attach the desired geometry to a CAD-sketched mold cavity, forming a positive mold known as a pattern – and then manufacture them together in a single step. We will talk about more complex examples later on, but for now, let’s take a part with a flat bottom and limited undercuts. In such a scenario, the pattern can be as trivial as this:

Casting a Gear
Designing a simple pattern for a single-part rubber mold.

Adding a box around the part has a fairly modest impact on 3D printing, but makes a huge, positive difference for CNC milling work. That’s because you can quickly machine a pattern on top of an oversized piece of stock material, leaving most of it intact for future use. This saves you money, time, and eliminates the worry about fixturing the part so that it doesn’t fly away from underneath the tool.

Machining a Mold
CNC mill finishing a complex mold for a planetary gearbox on top of a random block of stock material. Machining these gears directly would be a lot more challenging and wasteful.

Patterns can be made out of any material that processes easily and produces predictable results. With 3D extruders, this usually means PLA. For CNC mills, machinable wax and HDPE should work acceptably well, although I always recommend a more predictable and cheaper alternative: medium-density prototyping boards. My favorite variety is RenShape 460 — a lightweight and fairly durable substance that machines like butter, but delivers amazingly fine detail:

Male Mold
Close-up of a CNC-machined pattern in RenShape 460. This material costs a few cents per cubic inch and is actually less expensive than machinable wax.

Producing negative molds

With a pattern cavity ready to go, we need to find a suitable material that will form a flexible, negative mold. In artistic applications, such molds are commonly made out of latex, alginate, polysulfide rubbers, or tin-cure silicones. All these options have significant downsides — ranging from poor strength to marked toxicity — and contribute to the perception that moldmaking is a messy affair.

In precision work, the only worthwhile choice is two-component, platinum-cure silicones. These materials are non-toxic and odor-free; on top of this, their mode of polymerization ensures incredibly high dimensional accuracy. The resulting rubber is remarkably tough, exhibits excellent rebound characteristics, is resistant to chemicals, and comes with an inherently non-stick surface that easily detaches from the pattern and from the final parts. In short, platinum silicones are one of the coolest polymers you get to play with at home.

Pouring in the Resin
Pouring silicone into a machined pattern.

Over the years, I have tried about a dozen different moldmaking rubbers, many of them from popular, hobbyist-oriented manufacturers – and to put it kindly, it is very easy to end up with a variety that is far too viscous, far too fragile, or far too soft for small parts. I have put together a detailed guide for selecting just the right formulation; if you don’t have time for that, I think that two products really leave the competition in the dust: the easily pourable, low-viscosity Quantum Silicones QM 262 and the nearly indestructible, translucent Silicones Inc XP-592. Both of these products cost around $15 per pound, and usually come in 10-12lbs kits.

With the moldmaking composition selected, the whole casting process is almost embarrassingly simple: the resin is mixed for several minutes, poured into the mold, and covered with a flat sheet of plastic for a couple of hours. That’s really about it.

Sample Mold
Cured silicone mold. Note the flawless pick-up of sub-millimeter detail, including tiny protrusions and thin walls.

Well, to be fair, there is one more step worth taking in between: to ensure excellent reproduction of intricate detail without biting your nails, it’s wise to get a vacuum degassing rig. The term may be scary, but the device isn’t: a small and relatively quiet vacuum pump costs around $100, and a shatterproof polycarbonate vacuum jar sells for less than $60. Two or three minutes under vacuum ensure that there is no air trapped in between the mold and the still-liquid resin – and you are done.

bubbling resin
Vacuum degassing helps avoid air entrapment in a fast, easy, and reliable way.

You can also use several other hacks, such as applying the rubber with a brush or a syringe – but frankly, except for very simple geometries, it’s just not worth your time.

Casting final parts

As soon as the negative mold is ready to go, you can start making final parts. The workflow is very similar to the steps discussed above: a two-component resin (optionally pigmented with commonly available, super-low-cost dyes) is thoroughly mixed, poured into the flexible mold, briefly degassed under vacuum, and finally covered with a flat sheet of non-stick plastic – polypropylene or HDPE will do. That’s it: the polymerization reaction kicks in, and in a couple of hours, the finished parts can be removed from the mold.

weighing down the casting
Final parts on their way; today’s color is blue. Molds are covered with a sheet of polypropylene and weighed down to ensure flatness and avoid flash. Since the mold is made from a fairly rigid rubber, it maintains excellent dimensional accuracy even under heavy load.

The casting process itself is simple, but selecting the right plastic can be a challenge: many of the popular resins sold under hobby brands are smelly, harmful, exhibit significant shrinkage, cure too quickly or too slowly, or simply produce flimsy parts. It may not matter if you are making a decorative paperweight – but for a functional mechanical assembly, it probably will. To avoid unpleasant surprises, you should stay clear of epoxies and polyesters; polyurethanes are a much better choice. This family of remarkably versatile polymers can faithfully approximate almost every other type of plastic or rubber – and offers excellent resistance and dimensional accuracy, too.

Alas, most of the polyurethane compositions marketed to artists and other DIYers are almost guaranteed to disappoint in one way or another. To avoid common pitfalls, you can have another look at this in-depth guide; but in short, I strongly recommend sticking to Innovative Polymers, a little-known manufacturer from Michigan. They do not seem to target hobbyist audiences, but they carry a remarkable range of top-notch, workshop-safe plastics for demanding applications (and for taxidermy – your guess is as good as mine!). In particular, their IE-3075 resin has no match: it is exceptionally strong and rigid, outperforming injection-molded Nylon or ABS in almost every way. Just as importantly, it’s very easy to work with and dirt cheap, selling for less than $8 per pound.

sample parts
Micro-scale parts made out of Innovative Polymers IE-3075 (dyed orange) and OC-7086 (water clear). The accuracy – CAD model to finished part – is better than 5 µm.

Innovative Polymers also carries a selection of comparably tough, UV-resistant crystal-clear resins (e.g., TD 283-18), a range of indestructible, stretchy rubbers with superb abrasion and cut resistance (HP-21xx series), and a lot more. Really, if you are in North America and want to get going with resin casting, it’s a crime not to give them a try.

A word of caution is in order: although the resins mentioned in this article are safer than most of the formulations employed in artistic work, they are still based on fairly reactive chemicals, and need to be treated with respect. Before starting any casting projects, familiarize yourself with product safety datasheets and review common-sense workshop safety tips.

But what about more complex parts?

Good question! We glanced over this topic earlier on, but it’s time to catch up. For geometries that can’t be easily cast in single-part molds – for example, because they do not have a flat bottom, or because they have pronounced undercuts – you need to build a mold that consists of two or more interlocking bits. The idea may sound intimidating, and involves a bit 3D problem-solving – but in most cases, the task isn’t as hard as it seems. One of the many possible approaches is shown below:

complex part
Creating patterns for a two-part negative mold. A very thin sprue is also added near the parting line to allow the resin to be poured in with a syringe.

In fact, multi-part molds may simplify your life. Directly manufacturing the part shown above would be difficult, because low-cost 3D printers tend to struggle with overhangs, while entry-level three-axis CNC mills can’t cope with undercuts (unless you manually rotate the workpiece in the middle of a cutting job). The split-pattern approach makes this problem go away: all the individual molds have simple shapes, and are combined to cast a monolithic part with a more tricky geometry later on.

Final words

Resin casting is not a silver bullet – but it offers compelling benefits even for one-off parts, and is not getting the recognition it deserves in the DIY 3D community. Once you get a hang of the process, the overhead involved can be surprisingly low – and in many cases, the approach may actually simplify your projects, working around the inherent limitations of 3D Printers and CNC Mills.

In exchange for taking the slightly longer route, resin casting gives you the ability to quickly crank out parts with almost any mechanical properties, from true rubbers to exotic composites. In fact, even if you limit yourself to a single type of a polyurethane resin, you still get a surprising degree of flexibility: fillers such as milled glass fibers or glass microspheres can be used to make rock-hard composites or ultralight syntactic foams by just throwing a spoonful of commodity powder into the mix.

Best of all, once you have a negative mold or two, you can replicate your parts in large quantities, at almost no cost, and much faster than you could ever print or machine them. To change their appearance, you just need a drop of dye and a good stir. No other approach even comes close to that.

A microminiature planetary gearbox made using the processes and products discussed in this article – and not much else.

This post was originally published on May 2, 2013 and WordPress comments are now closed.

Michal Zalewski

Security engineer by day, a maker of tiny plastic gears by night.


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