Polycarbonate at times can be tricky to thermoform. Below are some common problems and helpful hints to eliminate these problems:
Webbing
Webbing is a common problem caused by an excess of material. The excess is often a result of over stretching during the heating process. Often something as simple as reducing the heating cycle can eliminate this problem. Take up blocks or pushers can be used to manipulate excess material, but this can be time consuming and difficult to accomplish if the webbing occurs in an area where aesthetics are required. In a worst case scenarios, redesigning the mold may be necessary.
Mark-off
Mark- off may occur when the mold surface is too cold or the mold surface has imperfections that transfer onto the surface of the part. Mark-off rarely occurs when cold forming where temperatures are around 350 F. For high-temperature thermoforming, heating the mold prior to forming may eliminate the problem. Fine sanding of the mold or coating the mold with a compound like bondo(tm) may also help. Aluminum molds, where the mold temperature can be adjusted, work well when making high optical quality parts.
Bubbling
There are 2 reasons bubbling occurs during the forming process. The first is the polycarbonate is not sufficiently dried. All polycarbonate sheet above 0.060 inches should be dried at 250 F for a specified amount of time based upon the thickness. If excessive moisture is the cause, the bubbles will appear within the sheet and not on the surface. If this occurs additional drying time is necessary. The second reason is over heating the sheet. This rarely occurs with polycarbonate since the degradation temperature is so high. However, it can occur. If the bubbles are clustered on the surface of the sheet, then this may be a sign of degradation. Try lowering the forming temperature or distancing the sheet from the heating elements.
Mold hang-up
This is when it is difficult to remove the part from the mold. Imperfections in the mold (such as undercuts) can lock the part onto the mold. Molds should be design with generous radii and adequate draft. A 5 degree draft angle is recommended on all vertical surfaces. The mold should be designed to accommodate for shrinkage. Female molds should be design to compensate for the 1/8 inch per foot shrinkage and male molds for the 3/32 inch per foot shrinkage. It is also important to remove the part from the mold before it is completely cooled.
Tearing
Tearing at the mold edge is sometimes caused by inadequate clearance between the frame and the mold. Hot sheet may stretch to much without a 1/2 to 1 inch clearance.
Sheet Pullout
Sheet pulling out of the mold is typically caused by sheet that has cooled below its forming temperature. After locking the sheet into the frame, heat the sheet to a higher temperature.
Warpage
Do not allow the sheet to cool on the mold below 250 F. When the sheet hits 250 F, remove it so that air can cool it evenly on both surfaces. This even cooling will help prevent the part from warping. Heating the frame to a minimum of 250 F prior to forming will help prevent warping of the flange.
Uneven detail
Non-uniform heating causes uneven forming detail. Check the temperatures and the spacing of the heating elements vs. the spacing from the sheet. Do not bring the sheet closer than 2 -3 times the distance between the heating elements. Air drafts coming from open doors can chill the sheet in spots. Screen areas that may be effected by cool air drafts.
Stress Crazing
Thermoforming at low temperature or uneven heating of the sheet may result in stress in the part. This stress is relieved by spider cracking in the part as known as “stress crazing”. Stress crazing can develop over time, on its own if the part is highly stressed. More often crazing develops when a highly stressed part is exposed to an incompatible cleaning agent or solvent.
To check for stress in the part, view the part through polarized film. Stressed areas will appear to have a concentrated rainbow effect.
Stress can be alleviated by annealing the part. However, it is more economical to heat the sheet evenly and throughly during the forming process so that minimal stress is introduced into the part.
Service temperature
The maximum continuous service temperature (MCST) is the temperature that a formed part can withstand over time without an objectionable change in shape occurring. The higher the forming temperature, the higher the MCST, because of the low forming stress. The MCST of a formed part varies from 230 F at low forming temperatures to 250 F at high forming temperatures.
For additional information on polycarbonate sheet, contact Liz Grimes – Technical Director 215-872-3532/lgrimes@highlinepc.com