Introduction
I have a need to develop rigid and flexible mechanical components for the ABSS project using 3D-printed masters. After some research, I found that a common method used by artists for sculpting involves creating moulds using silicone for subsequent casting. Silicone is suitable for this purpose because it has different flexibility grades and is inert. It adheres only to itself and a few other materials, making it very useful.
I’ve never tried to cast anything before, so this is a big chunk of learning. Let’s get started.
A Quick Test
When it comes to making moulds, the easiest type to create is the single-sided mould. This kind of mould is usually used when the copied object has a flat surface. The process is quite simple. First, place the object or pattern onto a flat surface and temporarily glue it in place. Next, spray on some release agent and let it dry. Then, mix the silicone according to the instructions provided by the manufacturer. Pour the silicone from a height next to the object and let it level itself out. For my test, I used Smooth-On Mold Star 20T silicone rubber and their Ease Release 205 release agent.
After the mould is cured, apply a spray of release agent and allow it to dry. Then, mix a small amount of the casting material (in this case, Smooth-On VytaFlex 30 polyurethane rubber), pour it into the mould, and let it cure. Keep in mind that VytaFlex takes longer to cure than the silicone. After a few hours, it was solid but still slightly tacky or gummy. Removing it from the mould was easy, and it was set aside overnight to fully cure. Now, the stickiness is gone.
The results of this quick test are quite good, with all the fine details reproduced. The soft 30A shore polyurethane rubber separated perfectly from the mould. This gives me confidence that these materials are compatible and that progressing to larger, more intricate shapes will be straightforward.
Making a Two-Part mould
To make a two-part mould, you must first prepare the pattern by filling the bottom cavities with modelling clay and making it fairly flat. Then, push the object into a clay bed at the bottom of the mould box. Flatten and seal the edges around the object to minimize leakage, and then spray a release agent and allow it to dry. Finally, cover the clay, mould box, and object with the release agent.
Firstly, tools. Working with clay is easy, but having some proper silicone rubber clay modelling tools makes it even easier. I’ve been using polymer clay from FIMO, hopefully, free from sulphur. Any sulphur in contact with the silicone will prevent the curing process, which is bad. In fact, sulphur contamination in any material, even the containers used for mixing, will cause problems with the cure. It’s also useful to have a set of small brushes for brushing on sealants and release agents. A decent scalpel is also a must for cutting clay, splitting open moulds, and general tidying up.
To begin with, we needed a test model. This simple shape has a non-flat bottom and some sharp points. I easily created this shape in Fusion 360 by first making a quick sketch and then using the extrude operation. I realize that it wasn’t the most efficient way to constrain a sketch in Fusion, but I did it quickly to get the printer started as soon as possible!
The model was exported as an OBJ mesh file and then printed using my Elegoo Saturn resin printer with ChituBox. I opted for Modforms Crys100 clear resin, mostly because it was already loaded in the machine.
Printing and Supports
As seen below, I didn’t put much effort into placing the support structure, which created quite a mess. I failed to angle the flat top surface of the model properly with respect to the build plate and made the support pillars too short. As a result, resin pooled around the top surface and thickened the support structures excessively. After removing the support, the result was less than satisfactory. However, since this was just a simple test, I simply sanded it down, cured it, and continued with the process.
Cleaning and Curing
I needed an inexpensive curing option for ABSS and couldn’t rely on leaving it out in the sun in the UK. So, I created a simple curing system. I was told that silicone rubbers, especially platinum-cure silicone rubbers, can react to the resins used for 3D printing, so I had to be extra careful with cleaning. For simplicity, I opted for a simple two-stage manual alcohol cleaning process.
I have two containers of isopropyl alcohol (IPA) that I use to clean my printed parts. One of the containers is labelled “dirty,” and the other is labelled “less dirty.” After printing a part, I clean it in the “dirty” IPA container to remove the bulk of the support structure. Then, I transfer the part to the “less dirty” IPA container for a final cleaning. Both containers are later exposed to UV light using my curing machine, which causes any remaining resin to solidify and settle at the bottom. I can then filter off the solidified resin using a paper towel stuffed into a funnel and reuse the IPA for another day.
For this print, I also gave the part a final bath in an ultrasonic cleaner for about 6 minutes at 60 degrees C with plain water. As an aside, I could have used water throughout this process as the Magoforms Crys100 resin is marketed as water-washable, but I got into the habit of using IPA, so I just had it all to hand.
After printing the pattern, it went through the final post-processing steps. This involved quickly sanding down the support side to remove any bumps and subjecting it to two 180-second curing runs in the curing machine. Over-curing transparent resins like this is usually not recommended, as they can turn yellow and look bad. However, in this case, the goal is to ensure that no uncured resin remains and that the appearance of the printed object isn’t a concern.
Sealing and Preparing the Mould Box
When using 3D-printed objects for casting, it’s crucial to seal them properly. This will ensure that any porous surfaces are closed and that chemicals do not bond where they shouldn’t, which could damage the cast. Smooth-On’s ‘Super Seal’ can be used to seal the object. It should be sprayed and brushed around the object. The instructions recommend applying at least two coats. The first coat should dry for five minutes, and then the second coat should dry for about half an hour.
After using the Super Seal (or a similarly packaged mould release), be sure to follow the instructions on the bottle. If you’re using the spray bottle version, remove the spray head, bag it, and tightly replace the lid. This product will evaporate from the spray head overnight, so be mindful of that. I once lost about 20% of a bottle over a weekend because I forgot to do this. Also, it has an unpleasant smell.
I’ve created a simple mould box using a plastic drinking cup. I cut off the bottom of the cup, which I specifically bought to mix the materials I’m using. I slice the polymer clay and mould it into the bottom of the container. It’s important to have clean hands or wear gloves to prevent contamination. I’m using polymer clay because many natural water-based clays may contain sulphur, which can interfere with the platinum curing process.
I haven’t researched if tin-cure silicone rubbers are sensitive to sulphur; that’s a task for another day.
Just a heads up: I miscalculated the amount of clay I needed to buy. I didn’t properly read the Amazon listing and missed the weight. As a result, I had to compromise on the depth of the clay bed, which is generally not a good idea. It’s crucial to have enough clay to create the bulk of the second mould half. If you don’t have enough clay, the second mould half will end up too thin and weak, which is not helpful.
Preparing the Object for Casting
In this part of the process, the goal is to create a primarily flat parting line by sealing the bottom of the object with clay. To achieve this, I filled the bottom of my master shape with clay and cut it off to be flush. Then, I created a seal around the object and the mould box by pushing the object into the clay bed and using modelling tools to push on the surrounding clay. This step helps to prevent silicone leakage during pouring. I used a flexible mould box, which was a bit too flimsy and distorted during handling. This resulted in the clay being compressed and the object being disturbed, which was a bit of a nuisance. If I had used a rigid mould box, this minor problem would have been solved.
Registration
Before pouring silicone over the model, adding registration marks into the clay bed is crucial. These marks are typically a series of indentations made approximately 5mm deep into the clay bed, away from the model, all around it. The indentations create small bumps on the surface of the mould, which help the second half of the mould to key to the first half. To ensure that the mould halves are correctly assembled, it’s best to make the pattern non-symmetric. However, achieving perfect asymmetry by hand may be challenging, so this is not an issue in practice. The goal is to prevent any part of the mould from slipping with respect to its mating half and to keep everything aligned as accurately as possible.
Release Agent
After completing the clay moulding process and embedding the object with registration marks, the clay surface and the object’s top surface must be coated with a release agent. It’s also essential to apply the release agent on the sides of the mould box, as any area where the silicone may come in contact needs a coating. I recommend using Smooth-on EaseRelease 205 spray and brushing it around a bit. Allow it to dry according to the instructions. Applying heat from a hot air gun set to 100 degrees will help speed up the drying process.
First Half Clean Up
After the silicone had fully cured, I cut off the bottom of the mould and carefully removed the clay bed. I tried not to disturb the object in the mould half, but its shape made it quite loose anyway. I used a wooden pick to remove the clay pieces, then cleaned the remains with white spirits (mineral spirits in the US), using a little heat to evaporate them.
Building up the Second Mould Half
To create a two-part mould, we must make a pathway for the casting material to get into the centre of the mould, as well as a path for air to escape. I used a pair of wooden picks to create two passages in the silicone top mould that led to the pattern. These passages must touch the shape to create a continuous passage from outside the mould to the central cavity. Technically, the passage leading into the mould is a sprue, which, together with gates, allows the casting material to flow into all parts of the pattern. The second passage is known as a vent and allows air to escape. It also indicates that the material has flowed far enough into the mould to reach the vent.
I had to use aluminium tape to build up the edges of the mould since I didn’t use enough clay at the bottom of the moulding box. The box was small enough, and the silicone weight was minimal, so the thin tape was sufficient to support the liquid. Once everything was secure and the wooden picks couldn’t move, I mixed up enough silicone for a second pour and let it cure.
The top half of the mould came out pretty nice for the first go. At the top left, the end of the sprue and vent touch the edge of the pattern impression. If this didn’t quite touch, you should adjust with a knife or perhaps a rotary tool to ensure casting material gets to the central cavity. You could use multiple passages.
The registration structure is evident at the top right. This is essential to keeping the mould halves in alignment and controlling leakage.
Casting into the Mould
I used VytaFlex 30A polyurethane rubber, the same material I used before. To assemble the mould, I added some polyurethane to the bottom half. Then, I attached the top half and wrapped it up with a rubber band. To avoid confusion, I marked the orientation of the mould with a Sharpie.
I found that my sprue was not flowing easily, and I forgot to form a larger opening at the top to allow material to be easily poured in. Obviously, I would tip it all over the top, and none would enter the mould. Plan b was to suck up the polyurethane liquid with a small gauge syringe and inject the mould directly. This worked well. I injected enough material for it to leak out of the vent (together with visible air bubbles), then rotated the mould to various angles and repeated. I could not get any more air out, so I assumed the mould was full, retracted the syringe and set it aside for curing.
Large air bubbles trapped in the cavity have resulted in significant voids on the upper surfaces of the part. There is also some minimal flashing and leakage. The main objectives of this test were to ensure that the materials are compatible, the process is running smoothly, and the dimensional accuracy.
Results
The images presented above demonstrate that it’s feasible to create intricate shapes with sharp angular features using soft polyurethane rubber, which has a 30A hardness that resembles very soft rubber bands. In addition, relatively good dimensional accuracy can be maintained while casting such shapes.
The materials used in the process are compatible with the silicone moulds, and polyurethane curing is perfect. However, air egress is expected to be an ongoing issue and will require careful planning for future castings. The shapes can be easily copied with high dimensional accuracy, and the resulting objects are structurally stable, at least for small pieces.
Next Steps
There’s still a little development to be done in this area, but the basics are there. I should complete my adjustable casting box to prepare for casting larger objects. Also, I need to try other grades of polyurethane rubber and try mixing different rubbers to achieve different hardness factors. This may become important for creating the rubber sole with embedded sensing.
The piezoresistive sensor work may require some flexible rubber conductive pads to be cast, so I need to research that and prepare some test pieces to evaluate.