HOT-KNIFING & FLAME POLISHING
Hot-knifing is a technique used to cut plastic off the cast when a metal part is vacuum formed underneath. It is useful in many applications such as when metal uprights are vacuum formed under plastic, when metal bands are formed under plastic, or to cut out a minimal amount of plastic at a 90 degree posterior stop. It is also critical in the fabrication of the Integrated Ankle.
While the technique is not new, it is not widely used or understood. The idea, in cases of metal parts under plastic, is to cut the plastic away without damaging the metal underneath. In the case of a 90 degree posterior stop, you would use this technique when the cast is at 90 degrees already and you can't afford to lose any plastic material at the mechanical stop area, compromising its position.
The preferred method is to clamp a utility knife blade in a vise grip and heat it up with a propane torch. The utility knife type razor blade is preferable since it is thicker and longer than other types. This method puts a lot of heat, instantly, to where you need it. The blade is reusable and very cheaply replaced, and old blades can be put back in service. This method also makes use of readily available tools, so you won't have to buy anything you don't already have. It is best to have a propane torch with a trigger so that you don't have to leave the torch burning (risking injury to your plastic and your self, not to mention wasting fuel).
With the blade clamped tightly in a vise grip, heat it with your torch to the point where it is glowing red, then carefully cut the area desired . It will require several cuts to separate the plastic, but with a couple of tries and some reheating, it will come apart. Be sure to push down on the blade and do not draw the blade across, as it will scratch the metal surface. Cut a little past the area where the metal is, as this will allow you to cut the rest of the plastic with a cast saw without getting too close to the metal. Once the plastic has been hot-knifed, cut the rest of the plastic as usual and remove it from the cast. If the plastic is vacuum formed around the metal part, its edge can be trimmed with a sharp razor blade and made smooth on the finishing wheel or by flame polishing the edge.
Flame polishing is a trick used to smooth hard to reach areas, by carefully heating the edge of the plastic with a torch. The plastic should turn glossy looking without being overheated. Be careful not to burn or warp the plastic, just heat it to the point where it looks wet. Flame polishing has the effect of not only smoothing the plastic, but it also removes any small nicks that may create potential fracture sites.