44 “Breaking Out” the mold

PPE

• Eye and ear protection.
• Close toe shoes should be worn in all lab areas.

General Technique Tips

Preparation

Before making any cuts, visually inspect the plastic device to identify stress risers: Look for cracks, cuts, notches, sharp corners, folds in the plastic (creased during thermoforming) or sudden changes in geometry. If you find any stress risers, drill a hole (~1/8”) at the end of the stress riser, which essentially changes the geometry from a sharp point (which creates extreme stress concentration) to a smooth, rounded feature. The hole creates a broader area to distribute the stress rather than concentrated at a single point.

After evaluating the thermoformed plastic, it is time to cut trimlines with the cast saw. It is often helpful to remove the proximal excess plastic first by a circumferential cut. If the plastic is too broad to fit into the bench pipe vice, place the mold on a secure surface and cut close to the top of the mold to remove the excess plastic (Image 1).  Removal of the proximal excess plastic makes it easier to stabilize the mold for the trimline cuts.

• For below knee sockets, it is often helpful to drill ¼” holes just proximal to the indentations at the hamstring tendons and at proximal to the lowest part of the PTB trimline.  This can help reduce notches in these areas from cast saw cuts (Image 2).   PETG is highly notch sensitive.
• The first cut on an AFO or KAFO should be made perpendicular to the long axis of the mold (Image 3). Specifically, this means making a full circumferential cut around the proximal part of the mold before cutting along the longitudinal axis. Thermoforming plastic over a mold creates residual tensile stresses – stresses that remain in a solid material after the original cause of the stresses has been removed. Cutting circumferentially around the top of the mold relieves the stresses in the longitudinal direction, thus reducing much of the material stress. Residual stresses also play a role in thermoplastic warpage, e.g., plastic opening after making the longitudinal cut.

Cut the plastic to the trimlines

Hold the cast saw near the blade , close enough to maintain control while still being able to operate the trigger lever (if present). This grip improves precision and stability and allows you to use your thumb on the mold for added support (Image 4). If sharp corners or precise cuts are necessary, considering changing to a smaller or notched blade (Image 5).  Make short cuts – once through the material, lift the blade out before moving to the next cut. Avoid dragging the blade continuously through the material because this can overheat the plastic, which can cause it to melt and adhere to the blade.

Break out the mold

Once trimlines are cut, it is time for “breaking out” which tends to be where material fracture occurs most.  It is important to have patience and exercise caution  to avoid material failure.  There are various methods to break the plaster out of the mold.

• Hitting along the length of the pipe sends impact force through the pipe and to the brittle plaster and chisels send direct impacts to the plaster. That impact force is also translated to the plastic.Any stress risers in the plastic will experience the impact at higher magnitudes and potentially expand.
• Pneumatic chisels can speed up the process and also require extreme caution during use.
• Care must be taken to not place the chisel too close to the wall of the socket.
• A rubber mallet can be used to strike the outside of the socket. Never hit the edges of the socket or areas that would create isolated areas of impact.

If the mold is cylindrical and has no undercuts, the plastics Cylindrical shapes can be hit off with a hammer and wood, or be blown off with compressed air.  The benefit of this method is that the mold can be saved. However,  this also must be performed with extreme caution as plastic can fracture and built-up air pressure can be extremely dangerous.  Avoid standing in front of a socket if using compressed air to force the socket off.