Colour change on PSVO ?

Due to small order quantity, I used colour pigment ( powder form ) on PS frame retardent grade base for the injection moulding process. A strange occurrence is that colour tone changes from light colour to dark colour and than back to light colour again. It happened each time with same batch of material in the hopper. The colour is never even although with constant barrel temperature,constant temperature controller etc, What causes this phenomenon and can any members advise from this ?
Thanks & Regards

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How do you feed the powder pigment? In case you tumble blend the PS pellets and the powder pigment, there might be segregation issue in the hopper, due to the huge difference in particle size. To avoid this, if that's your problem, you might want to feed the powder pigment downstream, after the hopper. Or you prepare a colour pigment/PS-masterbatch and feed that with the PS-pellets. It could also be a compatibility issue between the colour pigment and the PS flame retardant. It's hard to tell from here ...

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Hi RoWi,
Thank you for your response. Dry powder blending is widely practise with small order qty. I checked the hopper and did not find powder remnant in the feed hole. Initially I thought the cause was due to material being heat sensitive. It is quite problematic that the colour tone varies in each production run.
Regards

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In this case I suspect that your colour pigment is incompatible with the flame retardand PS. Have you considered this?

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Hi RoWi,
This pigment had been in use for 10 years without any problem, I do not know if the supplier change recent formula due to RoHs requirement standard ?
Regards