Stress In Plastic Sheet, Rod, Tube
You know how bad it is when you get stressed out, with all the day to day challenges and issues? Well, plastics get stressed too.
The process of producing plastics into sheet, rod and tube is a stressful manufacturing process.
Stress free plastic manufacturing
- The process to make a plastic sheet, plastic rod or plastic tube requires the plastic resin to be heated to temperatures of 350° – 500º to get the resin to flow through the machines.
- The material is then pushed through a die at high pressure.
- This heat and pressure cause many stresses to affect the plastic material.
- The cooling of the plastic shape is critical
- If the plastic cools to quickly, the surface cools and the center is still very hot, this is a major cause of internal stress.
- Processed to quickly and voids or cold spots cause poor quality sections.
- Processed to slowly and materials burn in the die and visible burn spots appear on the surfaces – bad material
- Proper speed provides homogeneous material that has cooled at a uniform rate through the sheet yields a very low stress sheet
The stresses from this production can be very significant and can ruin your parts. One common prevention method is to anneal or stress relieve the raw material prior to machining, and may require stress relieving during machining if large amounts of material are being removed. The stress relieving process is a slow heat and cool process, contact us for details on your material or project.
Here is an example of a highly stressed plastic sheet, it is 1.250″ thick glass filled nylon sheet. The manufacturer sent us this sample to show the ‘quality’ of their material. We attempted to cut off a strip to do some testing, and you can see the results. This material warped so much it stopped our saw! We never did buy anything from this supplier.
Compression Molded Plastic Sheet and Brownies…
Yes, you read that right, brownies. I still remember the wonderful smell of my Grandma’s house when she was baking her special 2″ thick brownies. The smell was great, but I also remember the critical timing in the oven. She would open the oven and peek in, and close it quickly so as not to let the heat out. Then, when they looked about right she would open the oven and slip a toothpick into the center of the pan and pull it out again slowly. I can remember asking, “Grandma, why do you put in the toothpick into the brownie?”. The answer was, that the outside cooks quicker than the inside, and it takes a while to get the brownies to cook all the way to the center.Then, when they were all cooked, I would have to wait while they cooled. And, sometimes, if I didn’t wait quite long enough, when I would take a bit the center would burn my tongue. Ouch!
The interesting thing is, that plastics are about the same. Most plastics have a very low co-efficient of thermal conductivity (that is sure a mouth-full), which means that heat and cold don’t travel through the plastics well. This means that it takes a while to ‘cook’ the plastic all the way through, and not just the surfaces. And, just like my brownie story, the plastics take a long time to cool. If the outside of the plastic cools much faster than the inside, then large amounts of stress develop.
The process of compression molding of the plastic sheets is ideal for eliminating stress. The sheets cook slowly, so the entire sheet, including the center, is heated to a uniform temperature. The temperature is held at temperature to insure a fully cooked sheet (you can’t use a toothpick here). Then, over several hours, the temperature of the sheet and the entire compression molding press is lowered very s-l-o-w-l-y. This allows the center of the sheet to cool at the same rate as the external surfaces, and this yields a very stress free sheet. The premier manufacturer of this high quality compression molded sheet is Westlake Plastics. We have been a master supplier of the Westlake Plastic materials for over 35 years.
What should you do to eliminate stress?
If you are experiencing problems with your plastic materials, such as twisting, warping, ‘potato chipping’ or any other sign of stress, contact us. It is probably not your fault. There are dozens of manufacturers that produce engineering and mechanical plastic shapes, they are not all the same. During the production of materials, the process should include a secondary annealing step to remove stress from the newly product. By removing all stress from the sheet or rod you are insured of a ‘clean slat’ before you start machining or finishing. We only provide materials from top rated manufacturers, we truly care about the materials you receive from us.