43 Fab Tips: How to Avoid Plastic Spreading?
- Melt temperature: Heat plastic above the melt temp so stress is relieved.
- If you aren’t sure of the proper oven temperature to heat your plastic, consult with your supplier. It does vary with different manufacturers.
- Get good vacuum
- Some people think that if they can put the plastic on vacuum for a short while & then leave a socket on the mold overnight, and that it will be fine the next day. But that is not true if it has not been kept under strong enough vacuum long enough. Your vacuum should have a minimum of 27 inches of mercury or 13 PSI.
- Ambient temperature
- Avoid residual stress in the fabrication process: Pulling any plastic over a warm mold will always increase the chances of a successful pull. When pulled over a cool/cold mold, it shocks the inside of the plastic, therefore the cool down process is not as effective. (Just be sure a mold that has been warmed isn’t dried out or you’ll cause another whole set of problems.)
- Thermoplastic Set Temperature
- The hot plastic should be left on vacuum until it is completely cooled. A good rule of thumb is if the plastic is pulled over a cool/cold mold leave on vacuum at least twice as long as it took to heat it. If pulled over a warm mold leave on vacuum at least three times as long as it took to heat it.
- Machine direction: Align the plastic sheet so that MD is parallel to the leg (i.e., parallel the calf or thigh).
- Be sure you use a quality product. If it is not orthopedic (medical grade) quality material, you could have problems. Also, quality polypro will have an approximate shrinkage of 1% to 2.5%.
- Be sure it is a stress relieved product. (?? – maybe not relevant if we heat plastic above melt temp)
definition
In materials science and solid mechanics, residual stresses are stresses that remain in a solid material after the original cause of the stresses has been removed. Thermoforming plastic over a mold creates tensile stresses. If the plastic cools too quickly and polymers do not have a chance to reform, stress will be present in the plastic. This leads to plastic warpage.
temperature at which a thermoplastic will be safe to remove from a mold without distortion