Best practices for getting rid of degraded specks in molded parts

[BizzyCon]

Hi everyone!

I am hoping that you will be able to suggest best practices for avoiding degraded specks in molded parts. We make a lot of clear parts so they are very apparrent. We have a lot of ideas floating around but are hoping to identify industry best practices that we can follow.

More specifically we are using PET, and have been getting both black (fully degraded) and brown (partially degraded) specks in the parts we are making. They are worse (statistically) during startup and then reduce to a constant rate during continued operation. Most of the specks are black (~80%). The average size we are dealing with is <0.050" diameter if the spec is approximated by a circle with some outliers as large as 0.3-0.5" (larger specks are brown or partially degraded). The overall frequecy is <1% at peak. There is no apparant correlation to cavity (leading us away from the hot-runner/hot tips).

We are looking at drying, barrel/screw, manifold, and startup/shutdown procedures. We have verified correct operation of heater bands/thermocouples etc.

Obviously from a quality standpoint we want 0%. In reality we need to reduce this defect as much as possible through maintenance and processing best practices.

What do you think? I understand that degraded material is inherent in plastics processing. What I want to know is how much better we can expect to get if we implement more best practices and what those practices would be.

I'm sure I'm not the only one dealing with something like this. Hopefully this can end up being a great resource for others who want to get into reducing degraded material in their molded parts.

Thanks!!!!



- Bizzy

 

[patprimmer]

Presuming they are not in the raw material and are generated in the process.

It is very unlikely the degradation is occurring before the compression zone on the screw.

Most common sources are:-

1) To much back pressure.
2) To high a compression ratio.
3) To short a compression zone.
4) Material to cold and viscous when it hits the compression zone.
5) Dead spots in the flow path or mechanical damage to screw, barrel, check valve, nozzle or hot runner systems is holding material until it degrades then releases. This might include less thermally stable material from a previous job.
6) Check valve is to restrictive or has sharp edges.
7) Check valve not sealing.
8) If it has a vented barrel, burned material in the vent.


Regards
Pat
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