Pressure Drop / Shrinkage

The understanding that as a cavity is filled within a tool there is pressure lost across the cavity is most important.
Simply stated, the pressure at the gate end of the part and that at the end of fill are not exactly the same. The results being now there are not the same number of molecules packed uniformly throughout the part.  This non-uniformity yields to a differential in shrinkage within the part. This may even lead to warp in the part. Other effects could be scuffing and sticking upon ejection from the cavity.  By measuring the product and steel, you may notice this non-uniform effect. In most cases the results are within spec, production goes on, but in some cases it is not, and we need to figure out what to do.

Measuring the pressure drop through the cavity will give the data as to how much of a pressure differential exists.
This is performed at the standard fill rate, plastic temperature and mold temperature for the tool, (i.e. where you were molding).  If the differential in pressure is great or end of fill pressure is extremely low the mold temperature or the plastic temperature could be raised. Next increase one or both of the temperature areas, re-do the test, and note what happens to the pressure drop.  How does the part measure out?  What are the results?    This allows for data generation and gives great insight to cause and effect.

Example:
 A 6-inch diameter part, highly polished and showing signs of scuffs on the part, 1-degree of draft.  The mold temperature was in the normal range for the material. A pressure transducer located at end of fill in the mold indicated approximately 2000 psi.  The temperature was increased 50 degrees F. resulting in a reduction of the scuffing by approximately 80%. There was also an increase in end of fill pressure by 1500 psi. Further, there was a decrease in pressure lost through the cavity. Note: pressure transducers make it easier but are not necessary to generate the data.

The use of a pressure drop showed that we had improved our pressure within the cavity and the part quality was improved too.
For reviews of how a pressure drop is performed refer to Silveys' Tech Tip May '96.

Thanks for the time.

Steven L Silvey
Sr. Technical Service
General Polymers Division
Ashland Distribution