Here at Highline Polycarbonate we are committed to providing useful information in support of the products that we sell. This blog post is the first in a series of posts on how to form polycarbonate sheet into practical shapes for commercial use. This short series will cover high-temperature thermoforming, low-temperature thermoforming, cold forming, brake forming and strip heating and troubleshooting. Let’s start with high-temperature thermoforming.
High-temperature thermoforming.
High-temperature thermoforming is used when a deep draw or sharp detail is required. This type of thermoforming typically requires pre-conditioning of the polycarbonate sheet to prevent moisture bubbles from developing in the sheet during processing.
Pre-conditioning or pre-drying is required for sheet thicknesses above 0.060 inches (1.5mm). Sheet at or below 0.060 inches does not require drying because entrapped water vapor normally escapes the surface before it develops into a bubble. In thicker sheets entrapped moisture as low as 0.04% can cause permanent bubbles within the sheet. It is recommended that the sheet is dried in a hot air circulating oven, preferably vertically in a rack that allows the hot air to circulate around the sheet. It is not recommended that the sheet be dried with the masking on or stacked horizontally on a pallet. Doing this may (1) cause the masking to fuse to the sheet, (2) not allow moisture to escape properly and (3) cause the sheets to fuse together.
The recommended drying temperature is 250 F (121 C). Using 250 F allows the sheet to expel the water vapor efficiently with little risk of deformation. The chart below provides recommended drying times at 250 F for specific sheet thicknesses to obtain a moisture level below 0.04%.
These times are approximations. Oven temperatures may vary. An increase in drying temperature will speed up the drying process somewhat, but it is important to note that polycarbonate will rapidly lose its rigidity at 370 F (188 C) and above.
The rate of reabsorption of moisture is a function of relative humidity and ambient temperature. Dried Polycarbonate sheet can be maintained indefinitely by storing it in a room with a dew point of 20 F or less.
Some thermoplastics like acrylics have what is known as a thermoforming memory. This thermoforming memory means that the sheets can be formed and reheated over and over again. Polycarbonate does NOT have thermoforming memory and rapidly loses heat during the thermoforming process. This makes thermoforming polycarbonate rather tricky. The quick loss of heat and rapid stiffening rate make hot air circulating oven impractical for high temperature thermoforming of Polycarbonate. Polycarbonate has roughly a six second window between heating and forming at 0.118 inches (3mm) and only a ten second window for 0.236 inches (6mm). Only infra-red (electric or gas fired) ovens are recommended.
Electric heating is more controllable and efficient than gas fired heating. Single sided heating is acceptable for sheet thickness of up to 0.080 inches (2mm). Double sided heating is recommended for sheets above 0.080 inches because it speeds up heating time and ensures that the sheet is heated thoroughly. Zoned heating is also a plus when forming polycarbonate sheet. It is recommended that percentage-on timers be set for 20% on the top and 37% on the bottom for 3.5 minutes when forming a 0.118 inch (3mm) sheet. Increasing the percentage-on timers to 25% for the top and 50% for the bottom may reduce the heating time to 90 seconds. Heating elements placed too close or too far away from the sheet can also cause problems. The distance of the sheet from the heating elements should be 2-3 times the distance between each heating element. For example, if your heating elements are spaced two inches apart, then your sheet should be 4 to 6 inches above and below the elements. Improperly heated sheet, one that is not heated to its core can build up stress. The build up of unintended stress is the reason for failure in most thermoformed parts.
The high-temperature thermoforming window for polycarbonate sheet is between 370 F (188 C) and 415 F (213 C). For high definition forming, 415 F is recommended. Sheet temperature can e monitored by the use of a thermometer, IR gun or the sag method.
The sag method of determining when a sheet is ready to form is done by using forming equipment with an electronic eye set at the proper distance. The electronic eye senses when the sheet has sagged the appropriate distance and activates the platens to begin the forming process. The sag method is independent of thickness. Some sag depths based upon sheet dimensions are listed in the table below.
Mold temperature is also important for producing successful parts. Because of the high temperatures used to form polycarbonate, wood molds are not recommended unless the production quality requirement is minimal. Aluminum molds are the best choice for long lasting molds. Aluminum molds should be maintained between 200 to 210 F to prevent chill lines and distortion appearing on the finished part. Epoxy and polyester molds will last longer than wood, but have a poorer heat transference than aluminum, resulting in an average of a one minute longer cooling cycle.
A minimum 5 degree draft angle is recommended on all vertical surfaces to allow for easy removal. Tooling should be oversized to account for the 1/8 inch (3mm) shrinkage per foot for female molds and 3/32 inches (2.4mm) shrinkage per foot for male molds.
A couple of extra tips before we wrap up. If you are using the sag method, allow ample space (daylight) between the frame and mold to allow for the appropriate sag distance. If possible use a moving oven rather than a moving frame to prevent sway. If you are using automatic molding equipment, use fast operating platens to assist with the short forming time. Don’t forget infra-red heaters above and below for sheet with a thickness above 0.080 inches (2mm). Use zone controls and shading to produce high quality, low stress parts. Also use a heated frame to prevent warping.
For additional information on high-temperature thermoforming of polycarbonate sheet, contact Liz Grimes – Technical Director 215-872-3552 /lgrimes @highlinepc.com.