One of the downside of 3D printing is that every incremental piece is going to take as long to produce as the very first 3D print. While this is ok for single runs like prototyping of desings, sometimes you just need to ramp up the production where you need many more of the same model to be produced. Thinking out of the box a little brings us to using Silicone Molding and Resin Casting method to replicate many more pieces of our 3D prints in a much faster time. The additional benefit of this is that if your 3D print needed some post-processing after being printed (such as support removal or sanding), this would allow your to save on that post-processing time and recast just the final piece instead.
Having said that, it’s still very much a matter of experience / art of geting recasting just right so even the process we have documented here is far from perfect – but still here it is in the spirit of sharing!
First, you would need to construct a watertight mold box. Having some spare arcylic pieces make it easy to quickly cut up some pieces, sized against the piece to be mold and casted in quick time.
First, superglue is used to hold the arcylic pieces together in the shape of a box.
Next, there will be the need to plug up the holes and keep everything tight, so a little shot of hot glue around the seams help.
Just to be extra safe, a final layer of scotch tape holds everything together and ensure that the enclosure if watertight so the the silicone rubber mold material does not leak out.
Now that the mold box is ready, the first thing to do is to use it to build the rubber mould in which the replicated pieces will be cast in. For us, we make use of Smooth-On’s Mold Star 16. This is a A-B mix which needs to be mixed in a 50-50 ratio by volume.
To achieve the 50-50 mix, we use 2 identical plastic cups and mark off a mark at the same height. These 2 cups will then be filled with the Part-A and Part-B of the Silicone Ruibber Mixes.
Once the 2 volumes are measured out, pour one of them into the other slowly (so as to avoid unnecessary air bubbles from being formed in it) and start to stir slowly, again to avoid bubbles. Scrapping the sides of the cup is also important to ensure that the mixture is thoroughly mixed, with a consistent color. As the Part-A and Part-B of the mix have different colours, any noticeable streaks of colour in the mixture would mean that it has not be completely and equally mixed. Before pouring the mould, it’s also advisable to transfer all of the mix into a new plastic cup for a final sitr to avoid residual unmixed parts of the mix which remain stuck on the sides and bottom of the cups.
While it is important to ensure that the Silicone Rubber Mold is fully mixed, it’s also good to remember that there is a limited working time with this Silicone Rubber Mold material so as soon as it is totally mixed, you can pour it slowly from height into a corner of the mold box (of course with the model already set inside it). Pouring as a thin stream into a corner and allowing the mixture to find its own level will ensure that more of the air bubbles dissipate and do not affect the mold quality. Once the 3D model has been totally covered, it would probably take around 15-30 mins for it to set completely.
Once the ruber mold has set, this is time for the replicating part of the process where we work with Smooth-On Smooth-Cast 320 Resin, which will be the final mixture that cures into a off-white solid recast of our model. Once again, its a very easy to use 50-50 by volume mix of two parts of the chemical.
Being that these componenet mixtures of the Smooth-Cast 320 are much more liquid and that we don’t need a lot of it for the models, we preferred to scoop them out of the bottles with a spoon (different ones of course for the 2 bottles to avoid cross-contaminatoin). Smooth Cast 320 has a much shorter working time than the Silicone Rubber so working fast is key here. Being more liquid means that its easier to mix thoroughly also. Do note that this mixture is chemically exothermic so will produce a little bit of heat while curing, though it’s nothing much to be worried about.
Once ready, pour the A-B mix of Smooth-Cast slowly into the rubber molds prepared. The liquid will start off with a somewhat yellowish colour and slowly “bloom” into off-white as it cures and hardens. Depending on the complexity and size of the model, this is another 15-30 minutes process. The reaons you’re seein a weird loopy model on the right is because we took too long to get things right and the resin has already began to set during the pour.
This is what it looks like when all the resin recasts have properly set.
Demoulding the casts are as simple as bending the Silicone Rubber molds and yanking the pieces out. The Silicon Rubber is pretty flexible and durable so they can take quite a lot of beating without deforming. These are some of the more successful recasts but even then, the gear model suffered badly due to pockets of air bubbles being trapped in the Silicone Mold making process.
A zoom in of a coin we recasted shows the amazing level of details which an be captured!
So well, that’s a quick look into silicone ruber mold making and resin casting as an additional process to 3D printing! =)