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There is no generic formula for this and wall thickness is only one of many variables. All of the following and more are other variables that can greatly change the cycle time:
  • number of cavities in the mould
  • type of material
  • any slides, core pulls, or unscrewing mechanisms will all add time
  • robot for part or runner removal will add time
  • geometry of the part
  • design of the mould
  • type of injection moulding machine that is running it
  • runner type
  • gate type
  • the shop that runs it
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 Nigeria, 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.
Injection Mould ClampFor big PP parts with undercuts, how do you evaluate the additional clamping force needed to keep the wedges (slides) closed and avoid flash? I want to develop a method to predict Injection Mould Clamping force for big PP parts with a lot of undercuts (8 big wedges in the mould). At the moment, I am using moldflow to predict tonnage but I am always missing 25% in comparison to the real life! I know that this is due to the side pressure on the wedges which makes them moving backward and the part is flashing. Do you have any idea of how to predict the right clamping force? Or what mould design change (bolster plates thickness, wear plate angle,...) could reduce the clamping force?
In comparison to the shear stresses in the flowing polymer melt, the gravity force is very small. (You can do a simple order of magnitude comparison to confirm this). However, I have seen cases in Gas Injection Moulding where the melt sagged around the gas core during the gas pressure holding phase. This is reasonable. If all melt in the cavity is pressurized to approximately the same pressure, then the shear stress are zero (or near zero) and so the gravity force is the only significant directional force acting on the melt. (To put it another way, the buoyancy of the gas core causes it to rise up through the polymer melt. Of course it won't displace the frozen skin layer).

I once had a Chinese injection moulder argue with me about this on a project where the weld lines where located in a bad location for the part's mechanical performance. He added gates (From one to four) and rotated the tool in the press. When the weld lines moved on the part he naturally said "See! Gravity does make a difference!"
In the injection moulding process, the mould cavity is filled by pressure-driven flow, in fact the material is injected very fast into a mould through small gates. The melt is at high temperature in the filling phase and some viscous heating also occurs due to friction. This increases the melt temperature even more and thus change (lowers) the viscosity. The gate dimension so effect a lot the material behavior.

As viscosity is a function of pressure, the pressure dependence of viscosity may become important in this kind of process. In addition to shear also extensional deformation has a significant effect, due to viscoelasticity.
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