VARATIONS IN PRESSURE ON THE DIE INLET INSIDE
The die manufacturer has designed the die with bearing lengths (friction surface in the extrusion shaping pin) approximately 1.5 times material thickness.
The pressure in the outer parts of the die is lower than in the centre of the die surface. Because of this, the flow through the die must be controlled either by ensuring that bearing lengths in the centre of the die are longer than those at the outer edges, or by increasing the material thickness of the outer sections of the profile, which also increases the flow.
PROCESS-DEPENDENT PRESSURE VARIATIONS
The aforementioned pressure ratio is not constant during the press cycle.
PRESSURE VARIATIONS DURING PRESS CYCLE
Put simply, it can be said that the pressure ratio is not constant during the press cycles. It varies according to the position of the stem. It remains constant at the beginning, but suffers from a rapidly increasing equalisation of pressure towards the end.¨
Since the material flow is of critical importance to the profile shape, these pressure variations have an enormous impact on the extrusion obtained.