Machinery's Handbook, 31st Edition
RUBBER PAD AND HYDROFORMING PROCESSES 1435 height of a rigid form block. During the process cycle, the rubber pad deforms elastically over the form block and the blank, applying a strong pressure. The pressure that the soft die exerts on the blank is uniform, so that the forming process creates no thinning of the material, but the radii are more shallow than those produced in conventional dies. The following formula can be used to determine total pad forming pressure: p A F 2000 # = where p = total rubber pad pressure (psi); F = capacity of press (tons); and A = area of pad (in 2 ).
F, v
Rubber pad
Container
Final part
Blank Tool
Workpiece
Press plate
Fig. 28. Guerin Forming Process An improvement over the Guerin process is the Marform process (see Fig. 27), which features the addition of a blankholder and die cushion to make this process suitable for deeper draws and to alleviate the wrinkling problems common to the Guerin process. The advantages of the rubber pad forming processes compared to conventional forming processes are the following: a) For forming a workpiece, only one part of the tool (punch or die) is necessary. b) One rubber pad or diaphragm takes the place of many different shapes, thicknesses, and kinds of tools, returning to its original shape when the pressure is released. c) Tool material is low cost and easy to machine. d) No tool marks are created during forming, so parts with very fine surfaces can be formed. e) Set-up time is usually shorter than in conventional forming operations. However, these processes also have some disadvantages: a) The rubber pad and diaphragm have limited lifetimes. b) The production rate is relatively slow. c) Rubber pads or diaphragms exert less pressure than conventional die, resulting in less sharply formed workpieces that usually need some hand finishing. Superplastic Forming and Diffusion Bonding Conventional metals and alloys will extend in tension no more than 120 percent, re- gardless of the temperature or speed with which the metal is pulled. However, it has been known since the 1920s that some materials could endure enormous tensile strains without necking. This phenomenon, called “superplasticity,” has been scientifically investigated. In the beginning, activities were primarily concentrated in research laboratories and were entirely directed towards the exploration of basic material science. The materials investi gated appeared not to be sufficiently attractive for real production. But this changed with the development of supersonic aircraft with high requirements for power density and skin temperature. A general definition of the term “superplasticity” was formulated for the first time in 1991 during the International World Conference on Superplasticity of Advanced Materi als: “Superplasticity is the ability of a polycrystalline material to exhibit, in a generally isotropic manner, very high tensile elongations prior to failure.” Some materials developed for superplastic forming include
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