Found: 4A accessory for typing with mouth stick

+FarmerPotato · September 21, 2021

I know little about this. I found it in a box of my dad’s TI stuff from before 1986, mostly related to assistive technology (Vocaid, prototype Magic Wand Speaking Reader, and correspondence with schools and conferences.)

I guess that this is a keyboard guide for use with a mouth stick. The holes guide the stick to prevent hitting two keys at once.

It almost fits the 4A keyboard. There are rails on the sides to keep it in place. I estimate that it shrunk 5% from its intended dimensions. It feels like UV or epoxy resin, but I can’t decide if it was machined from a block, or if there was a mold. Some evidence either way.

I had fun trying to deduce this...

I guess it is 2-part epoxy, which is cast in a silicone mold, for making a prototype. The big giveaway is that the whole piece is warped. 2-part epoxy is prone to shrinkage and warping as it takes 24 hours to set. The left and right sides are cut off crudely.

A silicone mold would have copied an original. Original probably milled out of a rectangular metal bar. There was one goof, which could only happen if a round cutter was creating an original (not cutting channels into a mold directly.)

Signs that it was designed for later injection molding: There is a big sprue on one end, which is where the plastic enters in injection molding (TI had a large plastics factory in Lubbock.) There is mold flashing below every key hole. The holes have a slight taper for mold release (perfectly vertical surfaces would create friction and the finished part wouldn't come out of the mold).

An injection mold is made from two perfectly planed steel surfaces. The bottom half would have channels cut into it for the plastic to flow into. The top half of the mold would press down flat to meet the bottom face. Only the keyboard guides and sprue are cut into the top.

The parting line is where they meet. You want maximum contact here, as about 70 tons of force are applied. The force prevents "flashing" where the plastic pushes the mold apart and seeps out a little.

Anyhow, I had fun trying to deduce this.