The process sequence
The process sequence for a typical
hydroforming operation follows the progression illustrated in Fig. 1. Fluid pressure
within the tube is increased after the die closes to force the material into the
deformation zone. The fluid is typically water with a rust inhibitor. During this process,
axial feeding and internal pressure are controlled simultaneously to improve the
process’s material-shaping capabilities.
The cross section of the tube is often
stretched beyond its yield point (2 to 3 percent elongation) to prevent spring-back and
achieve tight tolerances.
A complete cycle consisting of pre-form
loading, forming, depressurising and unloading ranges from 15 seconds to more than a
minute.
A companion process, called pressure
sequencing, is used at the same pressure range. It consists of applying a pre-pressure,
and varying the applied pressure as the dies are closed. The pre-form consists of a
straight, bent, twisted or locally deformed tube. Die splits must be designed to allow the
pre-form to drop into the cavity and the finished part to be extracted from the cavity.
The pre-form must have sufficient dimensional repeatability to drop readily into the
hydroforming cavity; conversely, the dies must be able to tolerate the expected variations
in the pre-forms. The design limits of the cavity split lines for insertion of the
pre-form into the hydroform cavity may constrain the final complexity of the finished
parts.
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