This process avoids pinching by designing the tube periphery to be smaller than the desired finished product. The process starts with a round tube (a) & fully closing the die (b) before filling it with water (c). Often one of the results is undesired ripples in the tube wall roughly perpendicular to the tube centerline as shown in (b) & (c).

A 2nd more significant characteristic is the difference between the tube and die cavity peripheries. As a result, the cross sectional corners are unfilled when the die is closed & material thickness is the same as the bent tube blank.

In HPH2 (c) water fills the tube. Pressure is applied such that material stretches into previously unfilled areas of the die cavity. Increasing pressure completes forming as in HPH2 (d).

A key characteristic is uneven wall thickness, often in the pattern shown in Figures HPH2 (d) and HPH1. Both show internal pressure stretches the material into the corner, creating tensile stresses in the tube wall. The wall thins because as forming progresses from HPH2 (b) to (c) and (d) more of the tube wall contacts the die cavity surface. Pressure needed to stretch the material is high enough to make it to stick to the die surface and stretching occurs in a decreasing portion of material, which concentrates wall thinning.

Strategies to reduce wall thinning are employed and work well. Examples are end feeding, lubrication, annealing & high n-value material. End feed is only effective for 500 mm from the tuber end. The other 3 options increase piece cost.