Molding and Casting#

This week’s group assignment is designed to familiarize ourselves with the the molding and casting materials we will be using.

  • Review the safety data sheets for each of your molding and casting materials,
  • Make and compare test casts with each of them.

The Material Line-up#

As part of the FabAcademy inventory, we have access to two different types of molding materials and two different types of casting materials, all by Smooth On.



The Smooth-On Document Library holds all the Technical Bulletins and Safety Data Sheets of their products.

  • Mold Star 16 FAST (Platinum silicone)↗︎
  • SORTA-Clear 37 (Water white translucent silicone)↗︎
  • Smooth-Cast 300 (Ultra-low viscosity casting resin)↗︎
  • Smooth-Cast 325 (ColorMatch Series plastics are urethane resin)↗︎

A quick comparison of the main features:#

type Mold Star 16 SORTA-Clear 37 Smooth-Cast 300 Smooth-Cast 325
Color Blue Water Clear Translucent White Clear-ish Amber
Pot Life 6min 25min 3min 2.5min
Cure Time 30min 4hours 10min 10min
Mix Ratio(Volume) 1A:1B 1A:1B 1A:1B 1A:1B
Mix Ratio(Weight) 1A:1B 1A:1B 10A:9B 115A:100B

Stirring and Bubbles#

The 2-part materials need to be mixed properly, if that is not the case, they will not work as expected. The problem is, that stirring introduces air-bubbles. This creates a challenges, especially if the pot time is in the range of 3 min. Mix, stir, pour, de-gas (if available). Too fast, air bubbles. Too slow, materials starts to hardens.

The Safety Data Overview#

The safety data for the molding and casting material is basically the same-

  • Wear protective clothing, gloves and glasses, so that the material does not touch your skin.
  • Don’t eat or drink it. Don’t eat or drink anything in the working area.
  • If swallowed, do not trigger nausea, call a doctor.
  • Ventilate. Don’t inhale.
  • Don’t do anything stupid with the material.

Test Casts#

For test casts we could either use normal plastic cups, empty PET bottles. Or we could go to the ¥100 shop and get some character silicon molds. We did the later.


We measured an poured all 4 materials, we introduced air bubbles on purpose and did not use the vacuum chamber to reduce the bubbles. (Thank you @Rico for borrowing the vacuum chamber).

Results are mixed.


But pretty and nice and かわいい。


What we learned: Do not cast silicon in silicon. It will be difficult to remove and some details will get lost when peeling.