When mouldings are not the intended colour, the first thing to check is the raw material, particularly the dosing rate, if you are using masterbatch, and the quality of regrind.
The change of colour may make the mouldings more yellow or brown. The problem probably is thermal decomposition of the base polymer. This is due to excessive times at elevated temperatures. Possible courses of action are as follows. Reduce the melt temperature, if possible, but certainly reduce the time the melt sits in the plasticising cylinder or in the hot runner system, by reducing the overall cycle time, reducing the cushion in the cylinder and delaying the plasticisation stage.
Moving the job to a machine with a smaller plasticising capacity cylinder will also limit the time at high temperature. However the decomposition may have been triggered at the drying stage. This could be as a result of selecting temperatures and dwell times beyond the recommended drying conditions. As a rule of thumb, the rate of decomposition doubles with every 10oC rise in temperature. Some thermoplastics are sensitive to high shear rates and the temperature rise this induces in narrow feed and cavity channels. In this case, reducing injection speed can help.
If the discolouration is more localised, in spots or streaks, the problem may still lie in contamination of the raw material and regrind. However, localised decomposition can result from some material being held up in dead spots in the cylinder and mould feed system. If spots or streaks are still present after purging the system, checks should be made on the non-return valve on the screw tip and hot runner feed system. This may establish where material is being held up and exposed to multiple cycles.
Black spots at end of flow are more likely to be caused by ‘diesel burning’….see a previous blog.
This mouldings article was written by Dr.Charlie Geddes for Hardie Polymers
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