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Colorant influences nucleation of plastic resin

I will not go down the road around the tooling or processing assumptions in this matter but concentrate on the fact that it's a black colored part, and the black color has been introduced by using a masterbatch. There are a number of theoretical possibilities linked to the coloration as such and around the formulation of the masterbatch. The black colorant is likely a carbon black but could potentially also be a back iron oxide.

All colorants influence the nucleation of plastic resins, especially semi-crystalline resins. The actual nucleation rate / speed / effect may depend on chemistry and surface chemistry of the colorant, the particle size of the colorant and the particle size distribution (dispersion) of the colorant. There are some different morphologies (structures) of CB's and there are different surface chemistries / treatments. The nucleation effect on the resin as such may be one problem. The chemistry may be one cause of the problem. The particle size of the CB may be too coarse (too big). This may be one reason for the problem. The dispersion of the carbon black may be insufficient, which is the responsibility of the masterbatch producer since further dispersion of the carbon black cannot be done in an injection molding machine. This may be one reason for the problem. The dispersion aid in the masterbatch may be another problem if not chosen properly. The carrier of the masterbatch may be a problem if not chosen properly. The actual amount of pigment in the part may be a problem. Pigments will very seldom enhance physical properties of plastics but may frequently decrease them - especially if not chosen carefully. The pigment content may be too high for the design / assumptions but another reason could also be variation of the pigment content, which may come from both the MB supplier but also created by you in house by variation on your dosing equipment on the injection molding machine.

The above may all be single or combination causes for the problems.

Suggestions:
Establish qualitative knowledge of the details of the masterbatch used linked to above mentioned factors. By a good co-operation with your masterbatch supplier, this should not be a problem to discuss.
Investigate by DSC if the colorant introduced influence the Tc of the resin vs the neat resin. May be complemented by actual measurements on shrinkage between colored and non-colored parts as especially LDPE is difficult to measure statistical Tc differences on.

0,4 - 0,5 % of carbon black is a relatively high amount in final parts! To have 4 - 5 % would be extremely rare and in more than 20 years working with plastics I have not really stumbled into any such cases unless it was a wrong doing. In black pipes, which are designed to be outdoor for very long period of times, the praxis is to have 2,5 % of carbon black inside.

Please remember that the difference between 0,4 % and 0,5 % in final is that 0,5 % is 25 % more.

0,5 % of carbon black in a olefine resin will normally mean that some aspects of the strength of the plastic is affected. If it on top of that has other "issues" as I mentioned earlier, then the effect can be relatively large. The design may swallow some variables but if you instead have a borderline design (e.g. part thickness borderline), a higher pigment content and / or other causes may create a problem.

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