We are often asked to provide recommendations on plug materials based on a specific project. Usually the processor will ask us, “Which material should we run with PP/PS/ PET, etc.” And while we are happy to engage with toolmakers and thermoformers, there are many elements beyond our scope that need to be taken into consideration. However, we strongly believe that using the right material under the right conditions can make a dramatic difference in the end product.
The following is our response to a typical customer inquiry. We are writing about in our blog today because it is a universally relevant response. As always, we would love to hear from other processors who have different experiences. (All names are changed or removed to protect the innocent!)
“In this [PP cup] case, I think the answer is simply to have your customer reduce the temperature and fine tune the process. PP has a very narrow window for forming temperatures and the range your customer is at is very high. This is common when the customer has past experience with DELRIN due to the high thermal conductivity of DELRIN, which takes heat out of the sheet very quickly so the PP must be run at higher temperatures. (The thermal conductivity of DELRIN is over 2000% higher than XTL!)
Several factors that may help in the comparison of materials:
- XTL, FLX and DELRIN all have the same service temperature of 350F/176C.
- Thermal conductivity for each material (which controls how fast the material absorbs heat from the sheet ):
- XTL: 0.17 W/m0K (0.10 BTU/hr-ft-0F)
- FLX: 0.11 W/m0K (0.065 BTU/hr-ft-°F)
- DELRIN: 4.32 W/m0K (2.497 BTU/hr-ft-°F)
- Thermal expansion for each material (which controls how much the plug expands as it is heated):
- XTL: 58 x 10-6 m/m/0C (0.000058 m/m/°C) (32 x 10-6 in/in/°F)
- FLX: 42 x 10-6 m/m/0C (0.000042 m/m/°C) (23 x 10-6 in/in/°F)
- DELRIN: 85 x 10-6 m/m/0C (0.000085 m/m/°C) (47 x 10-6 in/in/°F)
The true meaning to the application of the numbers above comes from realizing that thermal conductivity has a double impact. First, the higher the number, the faster heat is absorbed from the sheet. (This results in the need to run the material hotter than is recommended and results in “chill marks” that show where the plug contacts the sheet. It also means the process typically must run slower to allow time for the sheet to form.) Second, the higher number means the plug itself will absorb heat faster, which means it will expand more quickly. (This results in the need to run more pieces at startup to allow the plug to stabilize in temperature and size.)
For thermal expansion, the higher the number the more the plug will expand as it is heated. This means the user must start with a plug that is smaller at room temperature than is needed for best material distribution. Many pieces must be run to stabilize the plug as it expands over time.
If you focus strictly on the numbers though, it would indicate FLX is the best choice. FLX does work very, very well with PP and we often recommend it. In the case of your customer though, you can see from looking at the plugs that there is much material buildup. This generally results from running the sheet at too high a temperature creating a buildup of melted plastic on the plug. The scratch marks and chipped- out sections indicate the material has been scraped or chiseled away from the plug and damaged the plug surface. In this case, we would suggest XTL is the better alternative because it is so much tougher and more resistant to chips and dings.
If the customer turns down the temperature (even a drop of 5 degrees will help) the parts will run better, they will likely be stronger in crush strength and the material will not stick to the plugs.”
Needless to say, our email exchanges can be very in-depth, but we wouldn’t have it any other way. Got any questions on your own application? Let us know by filling out our basic form here.