A 15-minute intro to resin casting
A quick overview of prototyping and part duplication with RTV-2 silicone molds and castable polyurethanes.
At one point in my life, I penned an epic, 110-page opus on hobby manufacturing. In retrospect, this was a goofy idea; the treatise was too long for those looking for quick advice, and it covered too much ground to survive the competition from monetized, SEO-tuned “how to” guides that have come to dominate the web.
I had more luck turning small portions of the guide into bite-sized articles, such as the intro to involute gears. Today, I’d like to cover another bit: the use of engineering polymers to make custom parts at home.
The process boils down to making a flexible mold, pouring in a liquid plastic, and then letting the material polymerize. The original geometry can be CNC machined, 3D printed, or sculpted by hand.
Compared to 3D printing, the process has two important advantages. First, casting resins have far superior mechanical properties, ranging from nearly-indestructible engineering plastics to bouncy rubbers. Second, because the low-viscosity liquid conforms perfectly to the original pattern, the copies are flawless. The process replicates every detail, down to picking up greasy fingerprints.
To illustrate the workflow, let’s take the manual approach to cloning a LEGO brick. The first step is to take a bit of sulfur-free modeling clay and sculpt a tapered pedestal for the original part. A couple of dimples are randomly added to aid with mold alignment down the line:
Next, some outer containment is added to keep the moldmaking rubber in. The containment can be almost anything; in this instance, a truncated disposable cup held in place with hot-melt glue should be fine.
With that out of the way, a two-component, platinum-cure silicone is poured in, forming a perfect negative impression of the contents of the cup:
My go-to rubber is CHT QM-270; for see-through molds, Silicones Inc P-592 is hard to beat. Hardware store silicone caulk won’t do: it adheres strongly to most materials, doesn’t set in thick layers, and tends to shrink as it cures. Pound-for-pound, it also costs a lot more.
Once the rubber is set, the putty is plucked out, but the original part is left in place. A mold release agent, such as Stoner A324, is applied to prevent a second pour of silicone from sticking to itself. Then, the other half is cast, and the original brick is removed.
The resulting geometry is shown below. Note that the halves can be aligned perfectly thanks to the registration dimples; and that there is a cavity for casting resin where the original part used to be:
The final step is to pour a casting resin into this cavity and let the material cure. For replicating a LEGO brick, a tough and rigid polyurethane — such as RAMPF IE-3075 or BJB TC-854 — would be ideal. Both products can be freely dyed every time you’re mixing a batch.
To facilitate the resin flowing into every crevice, vents or sprues are sometimes added to the mold. This can be done by temporarily attaching wires to the original part, or by cutting channels in the cured rubber with a craft knife. For this particular mold, it would be prudent to have a vent connecting to the tubular boss on the underside of the brick.
To avoid air entrapment, many hobbyists also invest several hundred bucks in a small pressure pot, such as California Air 255C. When the liquid is placed at 40-60 psi, small bubbles are crushed or dissolve into the resin, making the problem go away.
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For years now me and my daughter had lots of fun casting resin using molds we ordered online, adding dyes, metal dust, glitter etc - cool thing to do with your kid. Have to level up and try making our own molds.
For those of you in the EU, where the silicons and resins mentioned might not be available, I'll share what I found after quite a bit of research and trial-and-error.
The best components I found are: Bluesil RTV-3450 as the modeling silicone, and Sika Biresin F50 as the polyurethane resin. Their parameters are spectacular, and they have worked very well for me in my projects. Their only disadvantage is that they are difficult to pigment, being rather dark to start with. Unfortunately, for some bizarre reason hobby stores tend to sell outdated inferior stuff, I guess hobbyists tend to stick to proven solutions rather than look for new things. So these components are hard to buy in small quantities.
Runners up, which are more easily available: Xiameter RTV 4234-T4 as the silicone and RenCast 5146 for the resin. These are easy to buy, but are not as good, specifically the viscosity of both is much larger, which makes them harder to use.
I wish more people pursued that hobby in my area (Warsaw, Poland), so that we could do a group buy of the good stuff. I'm having trouble using up everything I buy within the best-before period.
Also, thank you for publishing that guide years ago. It was helpful and inspiring!