Did you know that in many cases, comparisons with nature and humans can be used to explain facts in plastics technology?
Today's term: Dynamic pressure, holding pressure, holding pressure - nothing happens without pressure!
We all know it. Homework, tax returns, ironing laundry... As long as there's no pressure, things happen for a long time. Nothing at all!
The situation is very similar in injection moulding - without pressure, hot plastic does not become a finished moulded part.
Dynamic pressure - the internal pressure builds up
You know, there's this one task.... But it doesn't do itself. It's like an inner traffic jam of thoughts and a guilty conscience. You take heart, prepare yourself and get started just before the deadline gets closer and closer.
Applied to the injection moulding process, this is the phase before the hot plastic compound is injected into the mould. In the injection moulding cylinder of the SG machine, the screw conveyor should transport the still cold granulate on its way to the tip under the influence of external heat and internal friction (Frictional heat) liquefy and homogenise. Without a counter-pressure, the so-called back pressure, which is directed against the direction of movement of the screw, the screw would simply push the granulate forwards. The mass cushion „accumulated“ in front of the screw would be completely unprepared for the subsequent injection process.
Emphasis - Now pull it off!
You've started - the biggest challenge of all. Now you have to see it through. You can now complete the last 10% of the task.
Holding pressure is the extra push on the last metre so that the project succeeds completely. Without holding pressure, the injection moulded part would not be completely filled. The reason for this is that the hot plastic mass shrinks quickly as it cools in the injection mould. Holding pressure ensures that the screw, which acts like a piston, continues to try to push plastic into the cavity during injection. This continues until the moulded part can no longer absorb any more mass. Burr formation, sink marks and warping are avoided.
Holding pressure - stay tuned!
You're done - theoretically yes, but is the result really sustainable?
The cavity, which is completely filled under holding pressure, is literally under pressure and would like to give in to this pressure - but we hold on until the injection point has cooled down. Only now can we release the pressure.
Without pressure, almost nothing works - just like in real life.