Replacement Rubber Parts Part of the Flexowriter pages by Douglas W. Jones THE UNIVERSITY OF IOWA Department of Computer Science

Contents

• Introduction
• Making Plaster Moulds
• Casting Replacement Parts
• Additional Thoughts

Introduction

The most beat-up parts of our Singer/Fridden 2201 Flexowriter were its rubber feet. One was entirely missing, and one was only half there. Fortunately, the other two feet were intact, so I used one of them as a master to cast a plaster mould and then I used that mould as a master to cast four replacement rubber feet. Note that, judging by the photos I have seen of older Flexowriters, they used rubber feet identical to the ones discussed here. This page is a tutorial on how I created these replacement feet. Click on any photos for an enlargement.

Making Plaster Moulds

The plaster mould

I used plaster of Paris to make a mould of one of the better feet. This is not the ideal material for this process, and as a result, the mould only survived through the casting of four feet, three of which were good enough to use. Since I only needed two feet, however, that was sufficient.

I used a shallow plastic sour cream container to form the plaster mould. The inside of the lid of this container was flat, so I planned on using that as the reference surface for moulding the foot. Therefore, I cut out the former bottom of the contaner so I could pour the plaster there, being careful to leave a flat rim parallel to the plane of the lid.

The next problem was dealing with the screw hole in the foot. I found a piece of brass tubing that fit snugly in the hole and cut it to length so that the tube would completely line the screw hole and extend into the plaster far enough to lock it in place. To assure that it would not pull out of the plaster, I drilled a hole across the end of the tube that would be embedded in the plaster and put in a length of paperclip wire. I bent the ends of the wire into curls so they would solidly grip the plaster.

Before inserting the brass tube into the original rubber foot, I greased the foot with a thin layer of Vasilene, and then inserted the brass tube in the screw hole and used the tube to place the foot on the flat surface of the plastic container. Finally, I sealed the end of the brass tube with a blob of beeswax to keep the tube from filling with plaster.

I then snapped the remainder of the container in place -- just a ring of plastic to act as a dam for the plaster. I added a spiral of soft iron wire around the rubber foot (about halfway to the plastic dam) to reinforce the mold. You can see part of this spiral on the surface of the plaster in the photo. After mixing the plaster with cold water (for slow setting) and stirring well, the plaster had the consistency of a thin pourable paste. I poured this slowly over the top of the foot, until the level of the paste was above the rim of the container, held in by the surface tension of the paste.

While the plaster was still relatively liquid, I jiggled the pool of plaster gently and then fished around in it with an unbent paper clip in order to release any bubbles. Once the plaster had hardened to a firm paste, I used the edge of a straight piece of wood to shave the upper surface of the plaster down flush with the plastic rim.

I waited 24 hours before coming back to invert the now hard plaster mould and remove the lid to expose the original rubber foot embedded in plaster.

With the lid off, the next job was to make part two of the mould to fill the hollow in the base of the foot where the mounting screw fits. I cut a piece of brass tube that telescopes into the tube that goes up the center of the foot and is just long enough that the end of the tube is flush with the end when it is firmly seated. Again, I drilled a cross hole in the tube and inserted a paperclip, bent into a spiral to fit entirely inside the hollow in the foot.

With the brass tube out, I lightly greased the hollow in the foot with Vasilene, using a cotton swab to apply it in a uniform thin layer. I then inserted the brass tube, plugged the end of the tube with beeswax, and mixed a small amount of plaster and poured it into the hollow. The procedure for releasing air bubbles and scraping the surface level with the flat surface of the foot was identical to the first pour, but with a much smaller volume of plaster involved.

I waited another day to let the plaster harden, and then used the unbent paperclip, with the end bent into a hook, to poke the wax plug (and a bit of plaster) out of the brass tube. The paperclip hook is included in the photos here. Using the paperclip to pull up on the cross-wire through the tube, the top half of the mould would not release.

To make the mould release, I used the blunt end of a pen to poke at the rubber foot hard enough to slightly deform the rubber. I did this over essentially the entire exposed flat surface of the foot. When poking near the edge of the foot, I always angled my poking so that it tended to pull the rubber away from the adjacent plaster.

Having completed this exercise, pulling up on the small upper half of the mould released it. Removing the rubber foot from the larger bottom half of the mould was harder. Eventually, I used brute force, a pair of needle-nose pliers with serrated jaws. With these, I could (just) get a grip on the exposed rubber far from the edge of the mould. If I grabbed hard enough to distort the rubber, I could pull upward (briefly, before the pliers snapped loose). Repeating this process on opposite sides of the foot, it eventually pulled free from the mould.

I then waited a week for the mould to finish hardening and thoroughly dry.

Casting Replacement Parts

Moulding a foot

I used a rubber product called Versimold to make replacement feet. This is a silicone rubber compound that can be worked somewhat like a cross between Silly Putty and modelling clay. This compound can be vulcanized by heating it to 250°F (120°C). The vendor recommends using a hair dryer, but for the relatively large volume of our feet, an oven makes more sense. I used a toaster oven.

Before filling the plaster mould, I greased it using a cotton swab to spread a thin film of Vasilene over the plaster. This may not be the right release agent, but it seemed pretty clear that some kind of release agent was needed.

My first attempt at creating a rubber foot was only a partial success. I rolled some Versimold compound into a long snake and coiled it around the interior of the mould, pressing it into place with my fingers and then adding more until the mould was just filled. I baked this at 250°F (120°C) for about an hour in order to make sure that the rather heavy mould was thoroughly and uniformly heated, and then I left it in the toaster oven for another hour to allow it to cool without any thermal shocks that could crack the plaster mould.

The problem with filling the mould incrementally was that the joints between the different bits of rubber pushed into the mould were imperfectly filled, leaving visible folds on the surface of the finished foot. You can see the defects in the side of the finished foot in the left foreground of the photo. The foot behind it is a much better one made as described below.

For my next three feet, I avoided this problem by rolling a ball of Versimold that seemed a bit bigger than the finished foot, and then forcing a pen through the ball to form the screw hole. I then shaped the ball into a very rough foot shape using my fingers before sliding it off of the pen and nestling it into the freshly greased mould.

Finally, I set the smaller top half of the mould on the ball and pressed it down, telescoping the two pieces of brass tube and compressing the ball between the two mould halves so that the excess silicone compound bulged up around the top of the mould.

I used a putty knife to trim off the excess, using a sawing motion while holding the blade almost parallel to the plaster surface. I always held the knife edge radial to the center hole in the foot, and always sawed toward the acute edge of the foot in order to avoid pulling the rubber away from that edge of the mould.

For the first three feet I made, I always removed the center plug from the foot before baking. This turned out to be a smart move, because when I left the center plug in place to bake the final foot I made, I could not remove the center plug without breaking it. The foot shown in the mould in the photo has just been trimmed but the center plug has not yet been removed.

After baking each foot, it was as hard to remove it from the mould as it had been to remove the original foot. Using the blunt end of a ballpoint pen to make the rubber squirm helped release it, and then, because the rubber was softer than the original foot, I could use my fingers to roll one of the acute edges inward and pull it free from the mould.

Additional Thoughts

In retrospect, Versimold was not the ideal material, although it is certainly better than broken and missing feet. It vulcanizes well, but the resulting rubber is significantly softer than the rubber of the original feet. It would be nice to find a harder rubber that could be worked as easily. I contacted Rowe Industries, the manufacturer, and John Donato, the firm's president, had this to say:

The problem with using plaster of Paris is that the curing temperature for Versimold (250°F, 120°C) is very close to the temperature needed to completely calcine the plaster (266°F, 130°C), converting it from the mineral gypsum to the mineral anhydrite. This change weakens the plaster and this explains, in part, why our mould degraded significantly during our short production run. Had we baked the moulds at a higher temperature, calcination could release water quickly enough for steam pressure to shatter the mould. Fortunately, we saw no evidence of this problem, but clearly, had we used a harder rubber, John Donato's comments about needing higher temperatures make this a more pressing problem.

It may be that some of the harder plasters in the Hydrocal family would work better, particularly plasters designed for metal casting. It may also be that Versimold itself could be used to make the mould, so long as there is a suitable release agent to prevent the part being moulded from sticking to the mould during curing.

We used Vasilene as a release agent. This may not be the best release agent to use with Versimold. John Donato had this to say: