It’s been said that America’s favorite color is shiny, and I agree.
Pre-preg is great. Really, it’s a wonderful material—high strength-to-weight ratio, easy to engineer, highly repeatable—but there’s just one problem…it’s ugly! The usual method of using a cloth wick and a breather layer to remove excess resin during the curing process leaves a dull, abrasive finish that just isn’t what we are used to in this industry. Usually, your only options are either to be okay with ugly or do a lot of post-production work to make it pretty. Some people sand the material smooth and spray it with a coating, and some actually laminate over it to achieve a nicer finish. Both of these methods work but require investing a lot of extra time. I was never really satisfied with either of these options, so my team kept working to figure out some way to get a really nice finish on our pre-preg components without having to do any of the crazy, time-consuming things I’ve seen people try to make this stuff acceptable to the consumer. I mean, let’s face it—it can be an amazing material, but if it’s ugly, we can’t sell it. After a lot of trying, we finally got the solution down.
The fix actually came from some early problem solving. When we first started to work with this material, everyone said that the water in the plaster molds was a major problem for the pre-preg, so a lot of people suggested using dental plaster, extra hardeners, or all kinds of voodoo fixes that would help maintain the strength of the mold while it was being subjected to the extensive drying cycles. Most of this came from the idea that the experts were using PVA bags as their inner surface! PVA is smooth and easy to apply, but it’s water permeable! If water was the problem, why on earth would we use a permeable barrier? We fixed the problem by using a very thin polyethylene inner layer. It’s cheap, easy to apply, allows us to use the same plaster mold material we use for all our other molds, and no matter how wet the mold is, it never interferes with the pre-preg! One byproduct of this technique was that the inner surfaces of the devices we fabricated were beautiful! Their outsides, however, were not that nice looking. We tried PVA, we tried silicone, and we did have some success, but we just couldn’t get the outside to look like the inside.
The obvious answer was to use a polyethylene layer on the outside as well, but it wasn’t that simple. Pulling an inner layer of polyethylene is easy—you just use a separator (a fabricating hose) to wick the air out—but if we used a fabricating hose between the inner and outer layers, it left a texture on the inside of the outer piece of plastic, which then transferred to the outside of our pre-preg. If we didn’t use a separator, the material cooled in random patterns and wasn’t smooth.
So the trick became pulling a very thin layer of polyethylene over the polyethylene inner layer without using any kind of wicking layer that would leave a pattern on the inside of the outer layer. We tried a few different parting agents until we finally settled on Liquid Wrench® Dry Lubricant with Cerflon. It is designed to go on wet and then dry out, leaving a layer of Cerflon (a combination of PTFE and boron nitride) behind. This effectively stops the layers of plastic from sticking together and still gives a very smooth surface.
At this point, you might be wondering how we made up for the volume of the pre-preg material between the two layers of plastic since we didn’t allow for that in forming the outer layer of plastic. The answer is simple: we didn’t. We just cut the outer layer a few centimeters beyond our intended trim lines, layed up our pre-preg, and then applied the outer layer of plastic over the pre-preg material. By adding a few layers of fabricating hose over the outer layer of plastic and covering that with a PVA bag to act as our vacuum vessel, we could pull an effective amount of vacuum, and as the material ramped up to temperature, it “reformed” to fit the mold.
The process is simple: Modify the mold as standard. Add one layer of fabricating hose and pull a uniform layer of polyethylene over the entire mold. If no liner is to be used, then 1/16 in. is fine; if you plan to use a liner, you can pull a layer consistent with the thickness of liner you will use (1/8 in. liner = 1/8 in. polyethylene, for example). Sand down any seams that may get in the way, spray with the Liquid Wrench dry lubricant, and let it dry. A quick buff of the lubricant, and you’re ready for your next layer of polyethylene. For this layer, 1/16 in. is fine, and feel free to stretch it as thin as you can. Remove the entire outer layer. Trim this outer layer to extend just a few centimeters beyond your desired trim lines and discard the remainder of the second layer.
Now, just lay up your pre-preg in the predetermined pattern directly on top of the first layer of plastic and then apply the second layer over that. You still need to be concerned with removing as much excess resin as possible, so apply a few layers of smooth wicking agent to the outside of the entire mold and then apply a PVA bag to the outside of that. Apply vacuum as usual, and the layup will be de-bulked. Use a slightly longer ramping time to ensure that you evacuate as much resin as possible, then give it a short heat cycle to ensure the best curing of the final piece. The result is a component that is strong and attractive, with no postproduction effort required.
Also published in the May 2010 edition of the O&P Edge. © 2010 O&P Edge
Update – August 17, 2010
I created a pre-preg PTB brace, utilizing the techniques outlined above. The orthosis was super lightweight with really good axial loading capabilities.