Rubber Forming of metal sheet

Rubber Forming

Rubber Forming

Rubber forming processes are well known in the aircraft industry; about half of all sheet metal parts are made using these processes. The large impact of rubber forming processes is related to two important features of the sheet metal parts production in the aircraft industry: a large variety of parts (differences in sizes, shapes, thickness, etc.), and the relatively small product series of each part (typically in the order of 500-1000). These two features make it necessary to use production processes, which are very flexible in operation and offer low tooling costs. Rubber forming processes offer these characteristics.

Rubber forming is not only of interest for the aircraft industry. Also Small and Medium sized Enterprises (SME's) may have the same production features like small products series and a large variety in different parts. During research in national (IOP) and in European (CRAFT) sponsored projects the possibilities for these companies have been investigated, resulting in the application of the process in a few companies. Usually the applied materials and shapes differ from the ones used in the aircraft industry: steel alloys vs. aluminum alloys and deep drawn parts and flanged parts for the SMEs and the aircraft industry respectively.


Figure 1 The lower bed of the press

Principles

The forming principle is simple: a product related die is placed on the lower bed of the press (figure 1). On top of this die the blank is positioned and a soft die of rubber is forced over the rigid die and blank, deforming the blank into its required geometry. The tooling or die costs are low, only one die is required for the manufacture of a sheet metal part, and accurate and time consuming positioning of the die is not necessary. During the process cycle the soft tool deforms elastically over the blank and the die, applying a large pressure (typically in the order of 100 MPa or 1000 bar) over the blank. When the process cycle is complete the soft tool springs back to its original geometry. In addition, the soft tool is a universal tool, which can be used for a wide range of different parts.
Rubber forming processes can be divided in processes using a solid rubber pad (figure 2a) and processes using a fluid cell covered by a rubber membrane (figure 2b). The first process is more easily transferred to SME's, since this requires only the application of a rigid container filled with a solid rubber pad. Another difference during rubber forming is related to the dies that are used: a male or a female tool. Male dies are used for the manufacture of flanged parts (figure 3). The blanks for these parts are precut to their final dimension before forming. The parts made on female dies are like deep drawn parts (figure 4), although their complexity is less. After deformation the parts have to be trimmed to their final dimensions.
In addition to the advantages already mentioned (low cost tooling, versatility), the process offers several other interesting advantages. First: due to the rather soft rubber the top surface of the sheet will not be damaged or scratched under normal circumstances. This offers the opportunity to deform sheet with pre-coated surfaces and sheet with special surface layers like Clad-layers as used in the aircraft industry. A second advantage is that the overall shape and all kinds of detail can be formed in one process cycle. This advantage stems from the (nearly) uniform pressure applied over the entire surface of the blank. Other advantages are the cheap tooling materials that can be used for the dies, since the product series is limited and the short change-over time from one part to another.



The process also has some restrictions and disadvantages. The first one is the relatively long cycle time. This will limit the number of parts that can be produced cost effectively. Another disadvantage is the large forces that are required. Not only the blank, but also the soft tool has to be deformed. Third, the complexity of parts is limited, when compared to processes like deep drawing and matched die forming processes. Finally, the wear of the soft tool is also a disadvantage. After about 10.000 to 20.000 process cycles the soft tool has to be replaced. In most cases however, special wear surfaces or wear pads are used, so that not the entire soft tool need to be replaced.

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Solid rubber pad process used for flanged parts



Solid rubber pad process used for flanged parts



Deep drawn part produced by a fluid cell covered by a rubber membrane

 

Naam auteur: Calvin Rans
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