Be sure read Part 1 and Part 2 of Model Makers — The Professionals

Stop Motion by Susannah Shaw. Reprinted with permission.

Casting Foam Latex
Remember to first brush your mold with a release agent (Figure 2).

The basic process for mixing foam latex is:

• foaming up to desired volume at high speed

• de-ammoniation at mid-speed * refining cell size at low speed

• gelling agent addition

You will need good ventilation when mixing latex as it gives off ammonia fumes. Depending on temperature, humidity, mixer type and size of the run, this process can take anywhere from 15 to 30 minutes. (Runs smaller than 150g of latex are not recommended.)

The following two sets of instructions are meant only as a guide and are for 150g of latex using either the Kenwood Chef mixer widely used in the U.K. (which has a choice of speeds), or the Sunbeam Mixmaster used in the U.S., (using the small bowl). The Sunbeam produces a foam of superfine mixture. It has a highly efficient beating action, driving off ammonia very quickly. However, the mixer is less efficient when used in a large bowl.

1. Shake all components well and into the mixing bowl, accurately weigh out:
   • 150g latex

   • 20-30g foaming agent

   • 20-30g curing agent

2. Foaming — foam at high speed for 3-5 minutes or until the desired volume is reached.

3. De-ammoniation — reduce to medium mix speed for 3-5 mins.

4. Refining — reduce to lowest speed for 5-6 minutes.

5. Gelling agent — at the end of the mixing time add 5-10 ml of gelling agent to the foam. Mix thoroughly for 60-90 seconds. (The addition of a latex color at this stage will give an indication as to when the gelling agent is fully mixed in.) (The longer times given are recommended as a starting point for 300g of latex.)

6. Put your armature in place in your mold. The PTFE tape on your armature stops any of the brass from the armature discoloring the latex. Then fill the mold by hand, brushing the latex in to ensure coverage (see Figures 01-09).

7. Press together two halves and weigh them down or clamp them. Then inject the foam down one of the channels. The injection hole should direct the foam to the core. Let the foam work itself around the whole mold. When you know the latex is coming out of every vent, plug the escape vents with wet clay or English Plasticine (American clay will melt in the oven).