Rubber-Pad Forming,Rubber-Pad Forming,Rubber-Pad Forming
RUBBER-PAD FORMING, also known as flexible-die forming, employs a rubber pad or a flexible diaphragm as one
tool half, requiring only one solid tool half to form a part to final shape. The solid tool half is usually similar to the punch in a conventional die, but it can be the die cavity. The rubber acts somewhat like hydraulic fluid in exerting nearly equal
pressure on all workpiece surfaces as it is pressed around the form block.
Rubber-pad forming is designed to be used on moderately shallow, recessed parts having simple flanges and relatively
simple configurations. Form block height is usually less than 100 mm (3.9 in.). The production rates are relatively high,
with cycle times averaging 1 min or less.
The advantages of the rubber-pad forming processes compared to conventional forming processes are:
· Only a single rigid tool half is required to form a part
· One rubber pad or diaphragm takes the place of many different die shapes, returning to its original shape
when the pressure is released
· Tools can be made of low cost, easy-to-machine materials due to the hydrostatic pressure exerted on the
tools
· The forming radius decreases progressively during the forming stroke, unlike the fixed radius on
conventional dies
· Thinning of the work metal, as occurs in conventional deep drawing, is reduced considerably
· Different metals and thicknesses can be formed in the same tool
· Parts with excellent surface finish can be formed as no tool marks are created
· Set-up time is considerably shorter as no lining-up of tools is necessary
The disadvantages are:
· The pad or diaphragm has a limited lifetime that depends on the severity of the forming in combination
with the pressure level
· Lack of sufficient forming pressure results in parts with less sharpness or with wrinkles, which may
require subsequent hand work
· The production rate is relatively slow, making the process suitable primarily for prototype and lowvolume
production work
Equipment. The hydraulic presses used in most flexible-die forming are similar to those described in the article "Presses
and Auxiliary Equipment for Forming of Sheet Metal" in this Volume. Some processes use special machines, which are
described in this article in the discussions of the specific processes. In most applications, only one solid tool half is
specially made. The tool half can be made of epoxy resin, zinc alloys, hardwood, or other inexpensive material, as well as
aluminum, cast iron, or steel.
Equipment is available with cycling rates as high as 1500 per hour. Some flexible-die forming methods have been applied
to high-volume production, such as the forming of deeply recessed taillight reflectors for automobiles, and the deep
drawing of toaster shells (Example 3).
The application of rubber pads in press-brake dies is discussed in the article "Press-Brake Forming" in this Volume. In the
past, flexible-die forming methods were designated by specific processes: Guerin process, Verson-Wheelon process,
trapped-rubber process, Marform process, Hydroform process, SAAB process, and Demarest process. Modern technology
has reduced this list, categorizing the methods into three basic groups: rubber pad, fluid cell, and fluid forming. Detailed
applications of these rubber-die forming processes to specific metals are available in the articles "Forming of Stainless
Steel,"

Rubber-Pad Forming
The Guerin process is synonymous with the term rubber-pad forming. An improvement over the Guerin process is the
Marform process, 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. Another variation of the Guerin process is
the trapped-rubber process, in which the forming force is provided by a hammer instead of a hydraulic press. Like the
Marform process, the trapped-rubber process can be used for deeper draws and results in less scrap due to wrinkling than
the basic Guerin process. The design and construction of the ASEA Quintus rubber-pad presses are further refinements of
the Guerin and Marform processes.
Guerin Process
The Guerin process is the oldest and most basic of the production rubber-pad forming processes. Its advantages are
simplicity of equipment, adaptation to small-lot production, and ease of changeover.
Some metals that are commonly formed by the Guerin process are listed in Table 1. Titanium can be formed only if the
workpiece and the form block are both heated. The resulting deterioration of the rubber pad often makes the process too
costly, as compared to forming by conventional dies.

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