Two common defects caused by issues with the resin or resin additives used during injection molding are delamination and discoloration.
Delamination, when a finished part has a layer of flaky material at the surface, hurts both aesthetic of your part and its strength. Caused by moisture contamination of the resin pellets or by other contamination of the melted resin with a dissimilar resin, or by release agents in the mold, delamination is the result of the resin being prevented from bonding.
A number of methods, both simple and more complex, can be used to prevent delamination. If moisture is the issue, pre-drying the resin pellets or increasing mold temperature will help. If mold release agents are the cause, a mold redesign that places more focus on the ejection mechanism will help to eliminate mold release. If it is caused by cross-contaminated resins, that will need to be replaced with virgin material
Discoloration is simply when a finished part is a color different than intended. Caused most commonly by leftover pellets in the hopper, too hot of a barrel temperature or leftover resin in the feed zone, the problem can be addressed by thoroughly flushing the hopper and feed zone of a machine in between processes, thus preventing discoloration as a matter of course. Purging compound can also be effective to remove unwanted color or resin.