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Injection Molding and Temperature

Based on the grade you are using, check for the coefficient of linear thermal expansion (CLTE). It will depend on at which stage your weld lines are formed. If you have a high molecular weight material and the weld area is during the initial stages of fill, the weld strength may be good in both climate. If your weld area is towards the end of fill, a grade with higher elongation property might help. If you face the problem in an assembly, you have to check the CLTE of both the matching surfaces. Without having the complete details, it would be difficult to provide appropriate solution.

When you are molding a part, the mold temperature and melt temperature may be same irrespective of climate. So the environment at molding is uniform (assuming that the material is pre-dried if it has moisture). If moisture is present, the weld strength would be low and at the same time you will notice surface defects. The molecular chain would break and the material strength would be low; as good as low molecular weight material. The stresses developed due to climate change can cause problems. Thus, you have to analyze the problem whether it is in individual part or in an assembly.

Ambient temperatures and R.H. WILL influence the precise conditions that the material experiences as it makes its way through the tool. I'm confident that if the tool was fitted with thermocouples just near to the mold surface near to weld-line location and also with melt temperature sensor (Kistler and others offer these, even combined with pressure transducer function: another useful parameter to monitor for this purpose) also near the weld-lines, you would see differences, even with exactly the same barrel temperature and cooling medium settings. For clarity, it's arguably better to refer to ambient air conditions (temperature and R.H.) than "climate change" as this latter may trigger very different thoughts amongst some readers!

Good points about location of weld-line (early or late in flow path), but as this is fixed and material is fixed, the variables under scrutiny here are surely related to ambient environment only.

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