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How does temperature effect injection moulding physical properties?

For example the climate at the time of injection moulding effects the weld line strenght. The difference is between summer and winter months. There is a difference due to the climate. Process conditions so far have not shown the improvement, it needs to match the physical property of parts moulded in winter months where air is cooler and lower humidity.

My first reaction to this — and admittedly slightly "outside of the box" - is that the ambient air that is between the advancing melt fronts will be rapidly compressed (to an extent dependent on how easily it vents versus how quickly the air volume is reduced) and heat up. Clearly, it is not going to reach as high a temperature in winter as in summer, nor will the lower amount of moisture it contains be as likely to get entrapped between the melt fronts and potentially weaken them as they join. I must confess that I've never before thought about this phenomenon.

In higher humidity conditions, apart from the increase in water content in the air in the tool void prior to fill, maybe you're getting condensation at times (if the press is held-up between shots), in which case you will be pushing lots of water into the weld-line. Worth noting, also, is that even if you have the same barrel settings and water temperature and flow settings throughout the year, actual conditions that the material encounters (precise melt temperature at point of entry into the injection mould and actual mould surface temperatures) will vary, because of the R.H. and temperature differences. Hire a thermal imaging camera at relevant times and look at the readings on the parts at point of ejection. You'll see differences, I'm sure.

There are possibilities to employ vacuum-assisted venting (by default, this would pull moisture out with the air) in the mould void prior to fill. Also, as I've seen with multi-impression PET preform moulding, air-conditioning enclosures around machines.

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 moulding a part, the mould temperature and melt temperature may be same irrespective of climate. So the environment at injection moulding is uniform (assuming that the material is predried 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.

I used to assist periodically in a moulding shop in China, with >90% RH at >40°C (104°F) and had not been made aware of this problem. That is not to say, of course, that it wasn't there.

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