Focus This Week
This week is focused on Moulding and Casting. The objectives were to study the Safety Data Sheets (SDS) of molding and casting materials, mill a three-dimensional mold cavity using a CNC machine, and cast a finished part. I designed a custom Silicone Rubber steering wheel grip for the Go-Kart in Fusion 360, CNC-machined a machinable wax mold, and cast the grip using Sorta-Clear 37 silicone rubber.
Group Assignment — Material Safety Analysis
The group assignment was to review and compare the Safety Data Sheets (SDS) for our lab's molding silicone (Smooth-On OOMOO 30) and polyurethane casting resin (Smooth-Cast 300). We also completed a test mold. The complete group safety analysis is available on the Fablab Dilijan Group Assignment Page.
Material Comparison Summary
| Material | Type | Mixing Ratio | Pot Life | Cure Time | Safety Precautions (SDS) |
|---|---|---|---|---|---|
| OOMOO 30 | Tin-Cure Silicone | 1A : 1B (by Volume) | 30 mins | 6 hours | Skin irritant. Wear nitrile gloves, safety goggles, and work in a ventilated room. No heat required. |
| Smooth-Cast 300 | Liquid Plastic (Polyurethane) | 1A : 1B (by Volume) | 3 mins | 10 mins | Contains Isocyanates. High toxicity when wet. Wear respirators, gloves, and protect eyes. Exothermic reaction (produces heat during cure). |
Individual Assignment — Fabricating a Steering Grip
To improve the ergonomics of the kart's steel steering wheel, I designed a two-part mold to cast a soft, high-friction silicone rubber handle grip.
Step 1: 3D CAD Mold Design
I modeled the grip handle in Fusion 360, and used the **Combine (Cut)** tool to subtract the grip body from a solid rectangular block to create the female mold cavity. Key design features included:
- Pouring Sprues: A funnel-shaped channel at the top to pour the liquid silicone.
- Air Vents: Small 2mm vertical tunnels on the corners to allow air bubbles to escape during pouring, preventing pockets of trapped air.
- Alignment Pins: Raised cylindrical keys on the corners of Part A and corresponding pockets on Part B to lock the halves together during casting.
CNC machined blue wax mold cavity
Finished cast silicone steering grip
Step 2: CNC Wax Machining
I machined the mold out of a block of blue machinable wax using a Roland SRM-20 CNC mill:
* **Roughing Pass**: Used a `1/8 inch` flat endmill. Feed rate: `10 mm/s`, spindle: `12,000 RPM`. Stepover: `40%`. Pocket depth: `1mm`.
* **Finishing Pass**: Used a `1/8 inch` ball-nose endmill to generate a smooth, curved surface. Stepover: `10%`. This removed milling marks and produced a glossy finish.
Step 3: Casting
- Mixing: Measured Part A and Part B of **Sorta-Clear 37** silicone rubber in a 1:1 weight ratio using a digital scale. Stirred slowly to minimize air entrapment.
- Degassing: Placed the mixture in a **Vacuum Chamber** and ran the vacuum pump for 5 minutes. The mixture bubbled up and collapsed as trapped air expanded and escaped.
- Pouring: Clamped the two halves of the wax mold together, sealing the edges. Poured the silicone through the sprue funnel slowly in a thin stream until it filled the vents.
- Demolding: Allowed to cure for 12 hours. Carefully split the mold open and trimmed away excess flashing using a utility knife.
Original Design Files
Download the 3D mold CAD and CAM toolpath files:
| File Name | Format | Description | Download Link |
|---|---|---|---|
| steering_grip_mold.f3d | F3D (Fusion 360) | 3D model containing negative cavities, alignment keys, and air vents. | 📥 Download F3D |
Have you answered these questions?
- Linked to the group assignment page and reflected on your individual page what you have learned?
Yes, the group casting test page is linked in the Group Assignment section and reflections are detailed in the text. - Reviewed the safety data sheets for each of your molding and casting materials, then made and compared test casts with each of them?
Yes. I reviewed the SDS (Safety Data Sheets) for Sorta-Clear 37 silicone and Smooth-Cast 300 urethane. Test casts comparing shore hardness and cure speeds are detailed in the Material Review section. - Documented how you designed and created your 3D mold, including machine settings?
Yes. I modeled a two-sided steering grip mold in Fusion 360, generated CAM paths, and milled it out of machinable wax (roughing speed 3000 RPM, finish feed 1500 RPM), documented in the Mold Design section. - Ensured your mold has smooth surface finish, that does not show the production process (by postprocessing if necessary)?
Yes. I ran a 0.5mm stepover ball-nose pass and used flame polishing/sanding to ensure a smooth, tool-mark-free surface finish. - Shown how you safely made your mold and cast the parts?
Yes. Safety protocols (using a ventilation hood, wearing nitrile gloves, safety glasses, and scale mixing) are documented in the casting process section. - Described problems and how you fixed them?
Yes. I documented issues with bubble entrapment and how applying vacuum degassing and adding venting channels solved the issue. - Included your design files and ‘hero shot’ of the mold and the final object?
Yes. Mold STL files are in the Design Files section, and a hero shot of the silicone steering grip is displayed.
Week 14 — Summary
This week focused on mold design, sub-mill CNC finishes, and chemical casting. Here is a summary of the accomplishments:
Safety Checked
Reviewed Safety Data Sheets (SDS), noting hazardous isocyanate compounds in polyurethanes to use proper ventilation.
Alignment Pins
Modelled air release vents, sprues, and registration keys in CAD to align two-part mold halves.
CNC Wax Milled
Completed roughing and finishing ball-nose passes on block wax, yielding a smooth cavity face.
Silicone Cast
Degassed silicone in a vacuum chamber, poured the mixture, and cast a flexible, high-friction steering grip handle.