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13. Molding and casting

mold_cut

Overview of week 13 assignment

  1. Group assignment
    1. 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
  2. Individual assignment
    1. design a mold around the process you’ll be using, produce it with a smooth surface finish that does not show the production process toolpath, and use it to cast parts
    2. extra credit: use more then two mold parts

1. Group assignment

For more information, see the Week 13: Group assignment page.

2. Individual assignment

Because I'm planning to make a mobile 3D printing robot using omni-wheels or mecanum wheels for my final project, I tried molding custom wheels.

  • Omni wheel:
    An omni wheel has rollers mounted around its circumference, typically at 90°, allowing it to move forward/backward and slide sideways with minimal friction.
  • Mecanum wheel:
    A mecanum wheel has angled rollers (usually 45°) mounted around its perimeter, enabling movement in any direction (omnidirectional) through coordinated wheel rotation.
Feature Omni Wheel Mecanum Wheel
Roller Angle 90° 45°
Movement Control Requires holonomic configuration Built-in omnidirectional capability
Smoothness Smoother sideways motion May cause vibration during motion
Drive Complexity Simpler kinematics More complex control algorithm

Although I haven't decided whether to use omni-wheels or mecanum wheels, I started modeling a mecanum wheel.

A. CAD modeling

I referenced some mecanum wheels on Printables (Mecanum Wheel | Evolv3D (Printables)) as the base structure and ended up with six rollers angled at 45 degrees.

mecanum-wheel

The cross-section of the 45-degree angled rollers needs to follow a specific curved shape to maintain constant contact with the ground while rotating. I created the curve by projecting a circle on a 45-degree angled plane, then used that intersection curve to revolve and form the roller shape.

section

There are still improvements needed for it to function well as a mecanum wheel, such as adding bearings, but I went ahead and started making the rollers.

B. Mold design

I wanted the rollers to be soft enough for better grip, and instructor Tamiya-san suggested using MOLD STAR 31T, which has a Shore Hardness of 30A, similar to a gel shoe insole.

The mold design was a little tricky because the rollers are small and long, and also need a central hole for the shaft. I split the mold into three parts (or four, including the shaft), first fixed the upper and lower molds with the shaft in place, then poured the resin, and finally installed the top cap to push out any excess resin.

mold_design

The upper and lower molds were made from machinable wax, the cap was laser-cut from acrylic board, and the shaft was a bamboo skewer with a diameter of about 2.75 mm as a substitute for a 3 mm shaft.

C. Mold making

I used the MDX-15 to machine the upper and lower molds from machinable wax. They were milled as a single block, which I later cut into two pieces using a wire saw.

mold_wax

I didn’t check the estimated milling time beforehand, and after starting the finish pass, I realized it was taking too long. So I stopped it halfway and created a new finish process with a 0.5 mm path offset, instead of the previous 0.1 mm.
Due to the mid-process change, there was a difference in surface resolution, and the top surface was uneven because of an error in my work area settings. So, I sanded the top to make it even.

Please see the Week 13: Group assignment page for the detailed process.

D. Molding

I assembled the mold, secured it with a clamp, measured 10g each of resin A and B, stirred them within the 5-minute pot time, and poured the mixture into the mold. Then I installed the cap to push out the excess resin.

molding

molding

After 23 minutes of curing, I released the molds and got a roller:

  • It turned out a bit softer than I expected.
  • There were small bubbles in the resin, likely due to stirring too quickly.
  • Because of the mold tolerance, there was significant flash. I later trimmed the flash using a utility knife.

molding

E. Alternative solutions

Although I found molding to be efficient if done accurately, it required a lot of manual work with this non-automated setup. So, I started considering alternative solutions to make rubber rollers:

3. Files

Afterthoughts

  • Mold making is not so agile.
  • Although I often design plastic parts and negotiate manufacturability with moldings, I learned a lot by doing it myself.