In my company I use pom mix with black masterbatch to product gear for five diffrent type of gear. But I notice some kind of white powder coveing on the suface of the gear if I keep for more than three months, under the scope it look like dandraf & this only happen in one type of gear. Does any person know why ?
POM, especially co-polymer, tends to degrade relatively easily, compared to other polymers, and the deposit you see is low MW POM.
You could try to reduce your temperature settings and/or shear conditions in your barrel and/or mold (gate, runners etc.). You ought to check also your barrel for hold-up spots. In case you use co-polymer you may want to test h*mo-polymer instead, or a lower molecular grade of POM, if your application allows that. Also, are you sure your black masterbatch is compatible with your grade of POM (->
ask your supplier)?
A sign for overheating due to temprature, shear and/or hold-ups can be a strong smell (formaldehyde) near the molding machine ...
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POM, especially co-polymer, tends to degrade relatively easily, compared to
This could indicate that the design of this "one type of gear" causes more shear than other parts do. You ought to compare the runner system, gating, cooling system, design of the gear etc., with other types of gears, which do not show this deposit! What are the differences between the design and tool system of the "bad" and the "good" gears?
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The white powder maybe low molecular weight POM that is formed from formaldehyde, but could be something like thermal or UV stabilizer, or could even be the dispersant used to make the color masterbatch. A simple FTIR on the white powder should identify the powder on the surface.
A few questions:
Is the POM you are using a copolymer or homopolymer? Are you using the same black masterbatch, the same percent masterbatch loading, and the same percentage of regrind for all the five type of gears? Do you mold all five gears on the same machine, and under identical conditions (shot size, runners, melt temperature, etc.)? Is the gear with the white powder smaller or larger than the other gears? Drying the resin before molding may eliminate or reduce the problem, but it drying may not help at all; you just have to try it.
Regards,
Ernie
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