Shape-dependent variations

During power pressing, the hydrostatic material pressure varies in different parts of the tool.

fig1

Variations in pressure on the tool inlet side.
The tool manufacturer has designed the tool with bearing lengths (friction surface in the extrusion shaping pin) approximately 1.5 times material thickness.
The pressure in the outer parts of the tool is lower than in the centre of the tool surface. Because of this, the flow through the tool must be controlled either by ensuring that bearing lengths in the centre of the tool 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.
P P/2 P

fig2

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.