Assignments
Assignment
Group assignment
- Review the safety data sheets for each of your molding and casting materials, then make and compare test casts with each of them.
- Compare mold making processes.
Individual assignment
- Design a mold around the process you'll be using, produce it with a smooth surface finish, and use it to cast parts.
Summary - Group Assignment
Safety Data Sheet Highlights
Before working with any molding or casting material, it is essential to review the Safety Data Sheet (SDS) for each product. Use the tabs below to explore the material properties, the required equipment, and a visual comparison of each material.
In the group assignment we designed a small brain figure to test and compare two different molding materials available in the lab. The objective was to make the same mold with both materials, observe how each one behaves during mixing, pouring, curing, and demolding, and compare the results side by side.
We used Silicone Rubber (with catalyst) and F20 Plus (two-part, no catalyst) to produce two molds of the brain figure. Each material has different handling requirements, mixing ratios, and mechanical properties.
Two completed brain molds. Left: Silicone Rubber. Right: F20 Plus. Both cast from the same polycarbonate master mold.
Material Comparison
| Property | Silicone Rubber | F20 Plus |
|---|---|---|
| Mix type | Base + catalyst | Two-part (A+B) |
| Ratio | 78g silicone + 2g catalyst | 1:1 by weight |
| Food safe | No | Yes |
| Flexibility | Very flexible | Slightly firmer |
| Surface detail | Good | Very good |
| Cure time | 6 to 8 hours | 4 to 6 hours |
| Ease of demolding | Good | Very good |
Observations
The Silicone Rubber requires careful attention to the catalyst ratio. Too much accelerates curing and can cause the mix to become lumpy before it is fully poured. The F20 Plus was easier to handle since both parts mix at equal weight and the pot life is more predictable. Both molds captured the brain surface texture well, with F20 Plus showing slightly sharper detail on the smaller folds.
Silicone Rubber is more economical and flexible, while F20 Plus is better when surface detail or food safety matter.
Brain Mold Results — Photo Documentation
assets/Week13/brain_silicone.jpg
Silicone Rubber brain mold
Brain figure cast using Silicone Rubber with catalyst. Mix ratio: 78g silicone + 2g catalyst. Cure time: 6 to 8 hours. Good flexibility and surface detail capture on the cortex folds.
assets/Week13/brain_f20plus.jpg
F20 Plus brain mold
Brain figure cast using F20 Plus two-part material (1:1 by weight, no catalyst). Cure time: 4 to 6 hours. Slightly sharper detail on the smaller cortex folds compared to silicone rubber.
Both brain molds placed side by side after demolding. The rectangular cavities show the brain figure in both materials for direct comparison.
For more detail on the Group Assignment, visit the official Fab Academy page:
Visit Fab Academy ULima →Individual Assignment
For this assignment I designed a small dog figure and machined the mold out of a polycarbonate block using the Roland MDX-540. Then I made silicone molds from that master and cast the final piece in chocolate, resin, and other materials. Below I'm documenting each step of the process.
Final molds and casted pieces — silicone molds with the dog figure cast in different materials.
3D Design in Autodesk Inventor
I designed the figure in Autodesk Inventor. I started from a reference image, created a sketch following the outline, extruded it with a slight taper angle so the silicone would release cleanly later, and added some details with fillet and chamfer tools.
The taper angle is important — if the walls are perfectly vertical, the silicone grips the inside and it's really hard to pull out without tearing it. Around 8 to 10 degrees works well.
assets/Week13/inventor_sketch.jpg
assets/Week13/inventor_final.jpg
Machining the Mold — Roland MDX-540
With the STL file ready I used SRP Player to set up the toolpaths for the Roland MDX-540. The material is a polycarbonate block. The process has two passes: roughing to remove most of the material fast, and finishing to get a smooth surface.
Model size and orientation
I imported the STL into SRP Player and checked the dimensions. The dog figure came out at 71.35 x 70.00 x 23.39 mm on the small version and 122.01 x 119.70 x 40.00 mm on the larger one. I kept the XYZ ratio locked and set the top surface as the face to cut.
Type of milling
I chose "Better surface finish" over faster cutting time because for a mold, surface quality matters a lot — any mark left by the tool shows up in every single cast. I also selected "Model with many curved surfaces" and "Cut top only" since this is a single-sided open mold.
Workpiece setup
I entered the actual dimensions of the polycarbonate block: 140 x 88 x 37 mm. The material was set to Polycarbonate in the dropdown. Model placement was set to Center. Both Roughing1 and Finishing1 toolpaths show in the Tool List as ZCB-150.
Roughing pass
The roughing pass removes material fast to get close to the final shape. I left a 0.20 mm finish margin on purpose that thin layer protects the shape and lets the finishing tool do a clean last cut. The surface looks rough and steppy at this stage, which is normal.
Finishing area setup
I added a 5 mm margin on all sides so the tool wouldn't miss the edges. The cutting strategy was "Contour + Scanline" — it first traces the outline of the shape and then fills the surface with very tight parallel lines.
Draft angle
I enabled "Make sloped" and set it to 10 degrees. This means every wall tapers slightly outward, which makes it much easier to pull the silicone out later. Without it, vertical walls grip the cast material tightly and it can tear on the way out.
Tool: R2 Ball end mill
I switched to a ball end mill for the finishing pass. Unlike the flat square end mill used for roughing, the rounded tip can follow curved surfaces without leaving flat steps. For a mold that's going to get filled with silicone, this makes all the difference.
Finishing parameters
Cutting in progress
I let the machine run. It's a long process — the roughing alone took around 6 hours. I cleaned up the chips with a vacuum cleaner a couple of times during the process to keep the area clear. These long cuts are just part of the job when you want a quality surface.
Mold complete
Once finished I removed the polycarbonate block and inspected the surface. No visible toolpath lines, no ridges. The cavity was clean and smooth — exactly what you need before pouring silicone. Very satisfying to see the shape fully machined into the block.
Materials
I used three different silicones. Click each one to compare their properties, mix ratios, and how I used them.
Making the Silicone Molds
Once the polycarbonate master mold was ready, I poured silicone into it to create the flexible molds I'd actually use for casting. The key things here are getting the mixing ratio right, pouring slowly to avoid bubbles, and waiting long enough for the silicone to cure.
assets/Week13/mixing.jpg
assets/Week13/pouring.jpg
assets/Week13/cured_mold.jpg
I poured the silicone slowly and in a thin stream from a height to help break up any air bubbles before they get trapped. Once poured, I gave the mold a few light taps on the table to bring any remaining bubbles to the surface.
Casting
With the silicone molds ready I tried casting different materials in them. I used the A-10 mold for chocolate and an epoxy resin for the other pieces.
I melted chocolate and poured it into the A-10 silicone mold (food safe). Once it cooled and hardened I peeled the mold back and the piece came out cleanly with all the detail preserved.
assets/Week13/chocolate_pour.jpg
assets/Week13/chocolate_result.jpg
For the resin I used a 3:1 mix (resin to hardener). I poured it into the mold and left it to cure for 24 to 36 hours. Important: always apply a release agent before pouring resin — without it the piece bonds to the silicone and you can't demold it cleanly.
assets/Week13/resin_mix.jpg
assets/Week13/resin_result.jpg
Results
Here are the final molds and casted pieces. The white and blue molds are made of A-20 and A-10 silicone, both food safe. The brown piece at the bottom is the chocolate cast.
Final result: 4 molds produced with different silicone materials. From left to right, Silicone Rubber, A-10, A-20, and the cast chocolate piece at the bottom.
Final molds (black polycarbonate master, blue and white silicone molds) and the casted pieces.
Understanding Positive and Negative Molds
In molding and casting, the terms positive and negative describe the relationship between the original form and the mold. Click each tab to understand the difference and see how they relate in this project.
Problems and Solutions
It was my first time using these materials so there were a few things I had to figure out as I went. These are the main ones:
Problem 01
Air bubbles got trapped in the silicone mixture while I was pouring it.
Solution 01
Pour slowly in a thin continuous stream from a height, and tap the mold lightly on the table after pouring to bring bubbles up.
Problem 02
The first silicone mix came out too stiff because the proportions weren't right.
Solution 02
Follow the exact mixing ratio from the technical data sheet. For silicone rubber it's 78g silicone + 2g catalyst.
Problem 03
When demolding the resin piece it stuck to the silicone and came out damaged.
Solution 03
Always apply a release agent to the mold before pouring resin. Without it the resin bonds to the silicone surface.
Problem 04
The silicone was hard to remove from the master mold because the walls were too vertical.
Solution 04
In future designs, always add a taper angle (8 to 10 degrees) to the walls in Inventor so the mold releases cleanly.