Underwood platen knobs are usually pretty durable. Occasionally I’ll get in a typewriter that has one or both in crumbles. This week I decided to make silicone molds for the Underwood Portable universal typewriter platen knobs.
I start with one of the reference machines and clean up each knob to make sure it is as “perfect” as possible. When I say perfect, I really mean something that will duplicate well after being on a machine for 80 plus years!
I use Alumilite molding products as they have numerous options for mold making and varying hardness of plastics. There are many other manufacturers out there. Pick your favorite.
I make a mold box mostly from Legos. This works well as I’m dealing with small parts and it does not take many to make one of the right size. The Goldilocks approach; not too small and not so large you waste material. After cleaning up the master pattern (those knobs from the reference machine) I fill the bottom of the box with silicone putty to set each knob so they do not move around when I pour the first layer.
Then the second layer is poured over the knobs. Wait a day then gently pry the material apart and you have your mold to create copies.
I remove the original knobs and carve out sprue holes to pour the acrylic two part material.
I add black dye to part A before mixing in part B. Pour the mixture into the mold, force out as much air as possible then into a painters pressure pot to force out any remaining tiny air bubbles. When things turn out OK, a nice duplicate is born.
The blue silicone mold will last for years giving consistent platen knobs that look pretty damn close to the original.