Materials, The Basics’…. part 3

In the past two tips we have discussed some basic differences between the types of plastic families. It was also mentioned, but not stressed about packing pressure. For that lets’ continue with the basics and packing pressure.  The definition that we shall use for packing pressure is the pressure used to pack out the part to a desired shape/size per our dimensional specifications.

With an amorphous material the material softens and has low shrinkage, the packing pressure is usually on the low side. By the low side what is meant is that if the fill pressure is 10,000 psi than the packing pressure could be anywhere form 2500 to 5000 psi.  [Note this is 1,000 psi hydraulic, to 250 to 500 psi hydraulic using the intensification ratio of 10:1] . Another way to think of it is that packing pressure can start at 25% of fill and be adjusted from there. The finished packing pressure is typically set at a point that there are no sinks and the part dimensions are within tolerance.  The biggest issue in packing of the parts is that we over pack the parts. This than results in a part that could be brittle, warped and or fails at the function it was designed for.

With crystalline materials they melt, have the high shrinkage as a result of this, thus the packing pressure tends to be on the high side. By high side it is meant that if the 10,000-psi used in the above example to fill the part, the packing pressure may start at 6,000 psi and even exceed the fill pressure to achieve the finished characteristics desired. Or we start at 60% of the fill pressure and adjust form there. Underpacking is a major problem with these materials, which lead to voids, stress, and warpage.

When packing out a part as stated above most if not all will measure the parts and adjust their pressure accordingly to achieve the correct (supposed) dimensions.  Shrinkage which is a difficult topic onto itself, is best understood in the manner of the part is too big, reduce packing pressure, to, too small increase the packing pressure. A major key here is to measure the cavity and or core and compare those to the finished part to understand what is happening in the process. What I am trying to say is calculate your actual shrinkage results and adjust from that point. In optimization of packing pressure the use of bracketed pressures usually gives a range that the pressure can be set at and still achieved a good part. What this means is if we find 5000 psi works we would than decrease by 2500 psi and if the parts are full run samples to be checked later as to size and function. Also at the same time we would increase the pressure 2500 psi (watching for flash) and run samples than checking as when decreasing the pressure.

The above is looking at things from the basics, other factors such as gate freeze time, clamp pressure, and tool constructions all must be considered too.

Thanks for the time.

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