By: Alex Brett, R.T.P.O.(c)

 Cast (with minimum 2” of plaster above trim line)  400ML Silicone Cartridge (durometer of your choice)  400ML Injection Gun  Pelite  Nylon Stockinette  ¼” PETG  8-32 Machine Screws and Nuts  ½” Pipe Cap and Nipple  Petroleum Jelly  (Misc shop supplies and tools)

As you can see, this procedure does not require any special equipment or materials.

 We will start by fabricating a pelite pad in the shape we want our finished piece  This can be as small as the distal end pad shown, or as big as a full liner

Form Pelite

Trim and smooth to desired shape

We will then drill two shallow “key” holes on either side of the cast. These will give our injection mold something to lock into.

Pull two nylon stockinette over the mold and set up in your forming station

 Cut two pieces of PETG approximately 6-8” longer and 6-8” wider than the cast (based on side profile).  Trim the parting film back 1.5” around the perimeter of the plastic. This will provide a good vacuum seal during molding, yet allow the two halves of the mold come apart once trimmed.

Parting film is what makes this technique possible. It doesn’t matter if the film is clear or blue, but your prepared sheets should have a border of film removed and look something like this:

 We will be fabricating a clam shell type mold This is the only step that will require the assistance of a helper.  Heat both sheets of PETG in the oven at the same time. Assure that the parting film with trimmed edge is facing up.

Carefully sandwich the mold with the PETG. Assure that the cast is centered well before applying vacuum. This step can be a little bit tricky so pay close attention when lining everything up.

Support plastic until it has cooled, then it should look like this:

Drill 3/16” holes, approximately 2” apart around the perimeter of the cast

Once the holes are drilled around the perimeter, trim the PETG approximately 1” away from the holes The halves will now separate and the pelite dummy can be popped out easily thanks to the parting film.

With a ½” bit, drill a hole at the lowest point of the mold to inject the silicone.

A short ½” pipe nipple will thread into the PETG by hand. No tap is required.

When using ¼” PETG, the pipe nipple will stay recessed. If for some reason it protrudes you may need to bond the nipple to the PETG with some epoxy (this is rarely required).

Apply a thin layer of Petroleum Jelly to the cast and PETG. This will provide an easy release as well as seal the mold from potential leaks

Bolt the clamshell mold together around the cast using 8-32 machine screws and wing nuts.

Using a vertical fabrication jig (or another creative method): Position the cast vertically.

First step is to fill the cap with silicone.

Slowly start injecting silicone into the pipe nipple

Once the void is filled, continue injecting until you see a small amount of silicone squishing up between the cast and PETG. With the injection nozzle still in the pipe nipple, grab the pipe cap.

Quickly cap the nipple and allow ample cure time. This picture shows how far I will usually allow the silicone to squeeze past the finished edge when injecting.

Once the silicone has cured, the PETG mold can be easily removed.

Here you have a freshly de-molded distal end pad.

To trim the excess silicone, I like to use a pair of small cuticle scissors.

After trimming, give the silicone a good wash with soap and water to remove the Petroleum Jelly residue. A light powdering with talcum powder will eliminate any surface tackiness.

An exact duplication of your pelite dummy

With proper care and storage, your mold can be saved and re used indefinitely!

If you have any questions regarding this technique or the materials used: feel free to contact me Alex Brett, R.T.P.O.(c) Myrdal Orthopedic Technologies