My HDPE products have generally retained the off-white colour of the source filament but recently the results are a much darker grey. Is this a sign that the temperature is too high( I have not changed the setting from 240 Celcius that I have been using since I started)? What else might it be?

Jon

What kind of extruder are you using. The bushings on the old Mk II tended to generate a really fine black iron grit some of which got into the extruder barrel with the filament. That could darken plastic. If you are using one of the old threaded bolt or rod filament pump designs take the extruder apart and see if you have a lot of black iron dust inside. If you do, that may be what is causing the grief.

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Yes I have never seen HDPE change colour due to temperature but it does go grey from pollution from the bearing of my extruder.

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[www.hydraraptor.blogspot.com]

Thanks for the feedback. I am pleased not to have problems with the temperature control. Looking at the extruded filament with a lens I can see a fine black line which is presumably the dust from the bolt and washer which take the load of the pressure roller in the BitsFromBytes laser-cut extruder.