Home > Knowledge > What causes mouldings (and moulders) to be off-colour?

What causes mouldings (and moulders) to be off-colour?

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.

What causes mouldings (and moulders) to be off-colour?

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

For more of Charlie’s blogs why not read more of our knowledge base articles?

For information on all the specified grades that we can offer please contact us today or try our polymer search.

Posted in Engineering, Polymers,

Find the exact grade you require

Improve on your current price

Get the material you need faster

Our no risk Polymer Search

  • We’ll conduct a no obligation search so you don’t have to
  • We can supply the exact grade you need so you can meet your spec
  • Minimum quantity is just 25kg so you can get just what you need
  • If we can’t find your grade of plastic there is no cost to you
  • Products can be dispatched by express shipping if required

Chat to us first as we’re happy to answer any of your questions.


Request a Quote

  • Our team are available from 8am-5pm and we always aim to get back to you the same day. If you would rather contact us immediately please join us on live chat
    or give us a call on 0141 952 1900.
  • This field is for validation purposes and should be left unchanged.