Injection molding defects are frustrating. All processors can agree on that. The worst defects are the ones that are sporadic and not easily diagnosed. With defects ranging from flash, short shots, splay, to a number of other possibilities one particular defect can be specifically frustrating. Poor color dispersion on a finished part can be a nightmare for processors. Most of us have been there. You’ve got your process dialed in, cycle times are in check and the molding machine is running well. Then you’ll notice something a little off about the color of the part.
Whether it is a slight shade change that occurs throughout a multi-cavity tool or a streak or patch of color discrepancy on a larger single part molding, the issue is poor color dispersion and there are some things to consider that could relate to the root cause of the issue.
If the root cause of the poor color dispersion is caused by the machine, then there are a couple of things to consider. Molders have the option now days to use either concentrate or liquid color for proper mixing of the desired color. Masterbatch is also used in many cases and is compounded as part of the production resin and greatly decreases the chances of poor color dispersion. When using concentrate or liquid color a certain let down ratio is used to generate the proper color of the finished product. Lowering the barrel temperatures can sometimes induce better mixing of the colorant. Increasing the back pressure can also help get the desired results of proper mixture in the screw and barrel. Lastly, as far as the machine is concerned, making sure that the auxiliary equipment being used to measure the let down ration and mixing of the colorant is calibrated correctly so the processor is actually getting the desired amount is extremely important.
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