This method of preventing pinching is less intuitive for most people than HPH. It is avoided by forming the tube filled with low pressure water as the die closes on it. At any cross section its periphery is essentially equal to the die cavity.

As seen in Figure LPH2, the die halves contact the round tube in view (a). The normally round starting shape is forced toward the intended shape as the die closes in LPH2(b). The tube is filled with water (signified by shading) in 6(c) and low pressure is applied while the die continues to close.

What distinguishes this process is that the periphery or circumference of the starting tube is essentially equal to the desired periphery of the finished part. Coupling this with low pressure water to keep the tube wall in contact, with the die cavity wall (but still able to slide) effectively harnesses the large mechanical force caused by the closing press and die. This force resolves into compressive components that act parallel to the tube wall as shown in Figure LPH1. The water resists unwanted deformation inward (ie crushing or ripples in the metal), the die outward, which facilitates a controlled reshaping of the cross section throughout the length of the part.

This combination of hydraulic and mechanical forces used during the cycle facilitates complex part forming. Using low pressure water when the die is closing allows material to slide on the cavity surface and get pushed into place using the power of the press.

As the die comes closed, forming is almost complete. Pressure is increased to a higher level (normally <60 MPa) to produce the final form of the part as shown in LPH2(d). Holes are punched at this stage since the backing force provided by the water is greatest. The other reason for this higher pressure is to flatten the planar areas, not to finish forming the corners. It takes much less pressure for the former than the latter.