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The critical aspect of the extrusion process is the relationship between extrusion ratio and the applied extrusion pressure.

The extrusion ratio E is a function of the required change in CSA and is determined by E = π(R²-r²)/A

(R= Billet adius   r=Mandrel radius   A = CSA of extruded section)

The Specific Extrusion pressure P is given by  645.16F/(π(R²-r²)) where F is the extrusion force in tonnes

These combine into the overall formula

                                               Log P = log(ρ ln e) + 2μL log e/(R-r)

( ρ = Resistance to deformation at specified temperature, μ = Coefficient of friction of glass lubricated extrusion, L = Billet length)

Beyond the basic formulae are considerations of complexity and as this example shows a section with a simple circle of the same cross sectional area but with very different surface areas and levels of complexity.