An employee confronted his management with this proclamation he found on a website… I was asked to explain why this was an incorrect philosophy for a 21st century processor.
Plastic is compressible. We must take the injected shot past its ability to compress so that it acts like a hydraulic (non-compressible) fluid to completely fill the mould. As the cavity fills, there is no measurable pressure because it is pushing air out. Once filled, the plastic compresses. At some point in time it is so compressed it is no longer a compressible fluid but responds as if it were a hydraulic fluid. It is at this point that the cavity is fully pressurized and the process switches from fill to hold, just before the spike on the curve. The screw is slowing down, but the pressure is now packing. The only way to hold pressure on the part is to maintain a cushion. The smaller the cushion is, the higher the amount of pressure we can apply on the melt. However, if it bottoms out, there will be no pressure on the melt. If the cushion cannot be maintained, the amount of pressure on the melt will be inconsistent.
This is the philosophy behind the older, pressure controlled, machines. This process theory focuses on, and requires, a fully pressure limited process.
Aside from the reciprocating screw, velocity-control is the most important advancement in the injection molding of thermoplastic polymers… The theory above requires that you neglect this feature completely.
In 99% of the cases, such a process will be significantly less reliable and more machine dependent than a robust, velocity-controlled process with a short shot during 1st stage fill.
There are always opposing positions… but you would be hard-pressed to find any successful consultant or educator who would subscribe to the theory espoused above.
0 Comments Leave a comment