Monday 22 May 2023

Injection Moulding of POM - Checking Thermal Degradation (Rule of Thumb)


Injection Moulding of POM - Checking Thermal Degradation

Hello and welcome to this new Rule of Thumb post, discussing how to check the thermal degradation of Polyoxymethylene (POM) during injection moulding.

Setting the optimal melt temperature during processing is important for POM since it cannot be thermally stretched too much. Polymers such as Polyamide and Polyesters can handle higher set temperatures better during processing compared to POM which develops gases as result of degradation.

What are signs for thermal degradation? 
Typical signs of POM degradation are: 
-melt shows a foamy structure when leaving the injection nozzle
-it has a strong odor
-the nozzle tends to spill material out
-black and brown specks
-black flow lines on moulded part

How to test for thermal degradation?
Once moulding with POM has started and has run a couple of cycles, the machine will be stopped at the end of the dosing time. Now the machine stands still for 10 minutes (natural POM) or 2 minutes (coloured POM). After the waiting time is over, injection of the melt into the open is done. During ejection, check if there is formation of foam. After cooling down the melt cake, the swimming test is done. If the melt cake swims, then there is too much formation of foam and the material is thermally damaged. 

For POM, the recommended melt temperature is 215 +/-5ºC at which the melt for standard grades will remain stable without degradation (residence time is around 30 minutes).

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Thank you for reading and #findoutaboutplastics

Herwig Juster

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[1] DuPont - Serie Teil 3: Spritzgießen von Polyoxymethylen (POM)

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