How To Deal With Part Shrinkage
Scientific injection molders and quality control personnel know that part shrinkage is not uniform throughout the injection molded part. The plastic shrinkage near the gate area of the injection mold cavities is less than the shrinkage at the far end of the mold cavities.
So, the question to ask is “Why does the shrinkage of the part vary? Shouldn’t there be uniform plastic shrinkage throughout the part?”
There are three main reasons for variations in part shrinkage. They are:
- Plastics are compressible approximately ½ to 1% per thousand PSI
- There is less pressure at the far end of the mold cavity because when plastic flows, it loses pressure. This results in fewer plastic molecules at the far end of the part for a given area as compared to an area near the gate
- With fewer molecules comes more molded part shrinkage
So what can a molder do to reduce and minimize these shrinkage differences down the length of the part?
Since the primary problem is related to pressure variations within the mold cavities, the molder needs to do something to reduce the pressure differences down the length of the cavity. If you are a follower of Paulson’s teaching method that there are only “4 primary processing variables” in the injection molding process, you can determine which of these four variables must be changed – plastic pressure, plastic temperature, plastic flow rate or plastic cooling rate.
Of course, we’ve already given you the main answer above. It is plastic pressure that is the culprit and therefore plastic pressure that must be changed. More specifically, you must reduce the pressure loss that occurs as the plastic flows down the length of the mold cavity. The question that remains is what will reduce this pressure loss?
If you’ve taken our injection molding courses or been to one or more Paulson Plastics Academy seminars, you know that in order to reduce pressure loss, you must reduce the plastic viscosity (resistance to flow) during mold filling. This can be done in two ways, increase the melt temperature or fill the mold faster.
Which machine controls would you use to accomplish this? Write your suggestions in the comments section below.
One other thing to note regarding plastic part shrinkage… If you are allowed to change the inherent viscosity of the plastic (a higher melt index number), that would also reduce the pressure loss through the cavity.
You won’t be able to completely eliminate the shrinkage differences but they can be minimized with the correct machine control adjustments.