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Underwood Portable Platen Knobs

April 8, 2020 By Paul

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.

original knobs

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.

left knob is the original, right knob is the copy
Top knob is the original, bottom knob is the copy

The blue silicone mold will last for years giving consistent platen knobs that look pretty damn close to the original.

 

Filed Under: Article

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