Difference between Moulding shrinkage and thermal expansion

[Bozo_Sam]

Hi there,

I am trying to make the simulation of injection molding. I use ABS plastics. After cooling down the "warpage" which to my knowledge should be the final deformation of the part after it has cooled to room temperature, is about 1%. This to me is rather strange since the mold shrinkage of the material is "Moulding shrinkage, free, longitudinal - % 0.4 – 0.7".
On the other hand, the thermal expansion of the material is 0.0001. And since the part cools form about 130C to 30C after being taken from the mold, this corresponds well to the 1% shrinkage that I am getting from the simulation.

My question is: Which one of the two parameters is more important for the final shrinkage of the part. It seems like thermal expansion plays a bigger role, although I have always been taught that the plastic molded part is going to shrink for roughly the Moulding shrinkage and not the thermal expansion.

Thanks in advance,

Uroš

 

[Compositepro]

I find your question very confusing, even after reading it a dozen times. Mold shrinkage is primarily due to thermal expansion, but is also influenced by a number of molding process parameters, such as mold temperature, injection temperature, injection pressure, injection rate, and injection flow patterns.

For example, plastic can be injected into a cold mold so that it freezes solid on contact. Then as freezing progresses through the thickness, the shrinkage is toward the mold walls rather than toward the center of the part, which is still molten. You get different dimensions if shrinkage is toward the wall or toward the center.

 

[Bozo_Sam]

Hi there and sorry for the late reply. I am sorry for my poor explanation. I have mixed the linear and volumetric shrinkage and after some research, this topic is clear now.
I do have another puzzle though. When I perform the analysis and measure the shrinkage after final cooling, the "in-plane" (I have a plate-like part where one dimension is a lot smaller than the other two) shrinkage in the "x" and "y" direction is measured as 0.8%. But when the software advises me how much the part mold should be scaled, the proposed factor is 0.6%. I am not sure why the 0.2% the difference? Shouldn't the scaling of the mold be equal to the final overall shrinkage?

 

[Harry.]

Could you use 0.6% as a test and measure it to see if there is a 0.2% difference or not?

 

[Harry.]

Also, I've just had another thought, how are you measuring the shrinkage? Maybe use precision calipers such as insize venier calipers

 

[Bozo_Sam]

Hi Harry,
I use the measuring tool implemented in Autodesk. So what id does is, it messures the deformation aftwr the shrinkage and decides it with the original length.

I was wondering that since I am getting different scaling in x y and z, and also the fact is that the my part also bends quite a lot. So maybe that effects the optimal scaling value.

 

[Harry.]

The software won't be able to consider all factors, such as if the ABS is reinforced with fillers and fibres or pigments which will increase shrinkage.
There is a thread here about shrinkage issues and calculations that you might find useful:

https://community.ultimaker.com/topic/6369-pla-shrink-factor/