Processing With Flame Retardant Polymers

This e-mail can to me recently regarding the complications associated with the processing of flame retardant materials…

I am processing with PC/ABS C6600 FR grade material and it is difficult to maintain a consistent part quality. Recently I am getting gas marks and gas deposition on the mold surface after processing for a few hours. Proper venting is provided at the cold slug wells, ejector pins, parting line, and vent pins.

We are following the process information from the material specs. Is there any further requirements needed to process this material?
My Response
A material such as this is a flame retardant grade of PC/ABS. By itself, PC/ABS can be a difficult material to process as it tends to have a very narrow processing window. What many people do not realize is the thermal instability of flame retardants.
It may seem counter-intuitive… but flame retardants tend to have extremely high degrees of thermal instability! These flame retardants which were added to the material often degrade easily due to shear, temperature, and moisture.
Initially, the material should be dried, even if it comes in sealed bags… and you should be molding the part in a machine which is using 40-80% of the shot size to ensure a very low residence time. 
Next, measure the actual melt temperature and the dewpoint of the material at the feedthroat and then compare these to the recommendations. You should also reduce the screw speed & back pressure, and increase the screw delay to reduce shear and barrel residence time.
You can always increase the number of gates on the parting line… since more gates don’t cause flash. If you instill a schedule for the routine cleaning of the mold, this will also help prevent the gas buildup.
An in-mold rheology test will best determine when shear thinning occurs during fill, as well as visually demonstrate the fill time where the rate of filling degrades the polymer.
When your process is stabilized and functioning properly… you really need to document the process based on the process outputs such as melt temperature, fill time, part weight at end of fill, plastic pressures, etc.
Additional Thoughts
The science behind today’s polymers can create some materials which really perform great in their applications, but require very delicate processing. When the correct process is reached it is critical that you document the process and not just the machine settings so it can be repeated.

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