Thermoforming Tips #3: Removable Foam Liner
by Steve Hill
In orthotics, we frequently line orthoses with foam. Patient comfort is the most common reason we do this but there are others. Reduction of friction points and insensate diabetic feet are two more.
Another, less often encountered reason, is to compensate for the edema brought about by any number of conditions. It is in this situation that foam liners can be utilized as a removable boot, taken out when swelling is greatest and replaced when swelling has subsided.
Of course, the most common use for removable foam is in the fabrication of a soft foot bed, made replaceable with fresh, un-compacted inserts. CROW boots usually use this application of the removable foam UCB. The orthotist can simply discard the old, filthy and compacted UCB and replace it with a fresh one, fabricated with the initial orthosis.
Under normal fabrication conditions, foam wants to stick to the plastic vacuum formed around it. Both materials are made with essentially the same type of plastic, in the PVC (polyvinyl carbonate) family and both become very sticky and bonds to themselves readily. That trait is a big advantage for thermoforming in general, but makes fabrication of removable foam very difficult indeed.
There are several ways to make foam part from the plastic which has been formed over it. One tried and true way is to coat the area of foam with liquid soap. The soap acts as a parting agent and it works very well. Nothing wants to stick to foam so coated.
The disadvantage of this method is that it’s very messy. As you can imagine, clean up is a real bear. The soap runs all over the cast under the heat and pressure of thermoforming, and then everything must be washed in warm water. We did it this way for a while but we were loosing valuable fabrication time. There had to be a better way.
Another method makes this process much easier but it requires the use of an industrial release agent made by Miller-Stephenson*. This spray is so cool that it doesn’t even have a name, just the alpha/numerical designation of MS-136W. You know a product is serious when the name is just a bunch of letters and numbers. Like WD-40 or AK-47. The description on the label is “PTFE Release Agent, Water Base, For Hot Molds” and I think that says it all. Since it’s water based, it’s relatively safe and environmentally friendly.
For arguments sake, we’ll do a CROW boot. First, pull and shape your foam insert. Be sure to perforate it well so that you can achieve vacuum through it. Pull one layer of stocking over the cast and apply the removable insert to it. If it doesn’t want to stay on the mold, glue it in place with just a trace of contact cement around the edges.
Next, spray the insert with MS-136W and pull two to three stockings over the foam and cast. Make sure the knot in the end of the stocking is out of the way of the plastic pull. When pulling CROW boots, they will typically get another layer of foam between the insert and the plastic, but pull the rest of the brace with whatever else your patient may require.
After vacuum is achieved and the plastic is removed from the mold, the insert should just fall right out. If you’re still having issues with the insert sticking to the inside surface, add another stocking. This will increase the barrier between each sticky side and insure release.
When using this new release agent, we haven’t had to clean up any residue, even if we intend to glue the liner back in place. Fabrication time and accuracy have been improved and my cat hasn’t had any hairballs since.
*Miller-Stephenson, 203-743-4447, (www.miller-stephenson.com)