Aspects concerning sheet bending with rubber pad.

Hancu, Liana ; Paunescu, Daniela ; Borzan, Marian 等

Abstract: This paper presents some experimental results concerning sheet bending with an elastic die. The testing machine is INSTRON 1196 and the die was realized by the authors with the possibility to change the pad. Three different pads were used: a metallic one and two made of rubber with different hardness. Tests were performed with four different radii for the punch. Punch load is studied both in metallic die and by using a rubber pad. The influence of punch radius and punch stroke upon punch load is also presented. The accuracy of the specimen is also discussed. Proposals for geometrical dimensions of the pad are made.

Key words: bending, rubber, stroke, radius, accuracy

1. INTRODUCTION

Sheet metal forming with elastic tools includes applications such as bending of sheet, bulging of tubes, cutting and deep drawing.

Bending with elastic tools means that one half of the forming tool, either the punch or the die, is replaced by a pad of rubber like materials, called elastomers.

The main advantage of rubber forming in comparison with conventional process using two steel tools is the fact that only the steel punch is needed, while an inexpensive and universally elastomer pad replaces the die.

The pressure is one of the most important characteristics of the process of forming with elastic tools (von Ende, 1991). If the dimensions of the work piece are too large, or in case the construction of the tool is not properly done, the pressure necessary for bending might be in exceed and the forming press might not be able to do it (Hancu & Achimas, 2005).

In this context, it seems important to determine the most accurate technology of elastic bending in order to have the smallest energy consumption possible.

2. GENERAL CONSIDERATIONS

Bending tool used for tests is shown in figure 1. It is a die for bending V parts that can also be adapted for a metallic pad. Sheet material is A3 SR EN 10130:2007 with thickness of 1mm. Bend specimens has the following size: 50x50x1mm.

The main parts of the tool are the punch and the rubber pad, which is fastened in a steel die. The punch radii are: R=2mm, 5mm, 10mm and 15mm. Th e material used for pad is rubber (polibutadien and silicone type) with hardness of 66 Shore A, respectively 46 Shore A. For comparison, a conventional steel die was also used.

Tests have been performed on a universal testing machine INSTRON 1196 that is equipped with an automatic system for recording the load variation diagram, as an electrical sensing device measures the load. The class of precision of 0.5 ensures an error of exactness of [+ or -]0.5% from the applied load; the relative error of coming back to zero is 0.25% from the maximum limit of measuring, while the threshold of sensitiveness is of 0.05% from the maximum limit of measuring. Punch speed is 50mm/min.

[FIGURE 1 OMITTED]

Tests were performed using calculated punch stroke for each punch radius, using the conventional metallic die, in order to have the specimen completely fixed between the die and the punch, when bending is finished. In this way, four different strokes ([s.sub.u]) were obtained.

3. EXPERIMENTAL RESULTS

At the beginning of the bending process, the specimen lies on the pad and the punch is then advanced against the specimen. The test is performed not only with the calculated stroke [s.sub.u] but also by using a punch stroke with 10 mm higher and also 5 and 10 mm lower than the calculated one. The results, for a certain punch radius of 15mm, are presented in figure 2.

The same tests are performed using different radii for the punch, both in conventional die and in rubber ones. The resulting relationship between punch load and punch radius appears in figure 3.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

It can be seen that punch load is decreasing with punch radii if H stroke is used. This is due to the deformation of the sheet, which is higher if the punch radius is smaller and if the punch stroke is properly chosen.

The deformation of the pad is almost constant and remains the same, no matter the value of the punch radius is (Hancu et al., 2004). Punch load is considerable higher for bending with elastic pad and that has to be known for choosing the proper forming press.

It is known that a higher punch stroke will generate a greater accuracy of the specimen (Wilhelm, 1972). The problem is that the accuracy obtained with higher punch strokes is not so great, so the energy consumption that is needed is not justified (Hancu & Achimas, 2001). Work piece angle after load removal are measured with a CMM ZWEISS machine called ECLIPSE 550. The results obtained in this way for elastic spring back are presented in figure 4.

Spring-back angle depends strongly on punch radius: it grows with increasing punch radius. Using a pad of rubber like material, spring back angle is considerable smaller than with a metallic one. This happens because the elastic material is deformed during bending with a force that presses the sheet layers on the punch. This causes a calibration of the work piece that is higher if the radius is smaller.

Comparing the two materials used for elastic pad, one can see that if the hardness of the rubber like material is higher, the accuracy of the specimen is also higher because there is a greater pressure upon the specimen during bending.

[FIGURE 4 OMITTED]

The geometry of the pad is very important in order to obtain high quality for the product. It has to be high enough for performing high strokes, but as small as possible for low costs.

In figure 5 are presented two kinds of pads: first is in one piece (left) and the other one (right) is placed on round shapes in order to facilitate high strokes.

[FIGURE 5 OMITTED]

The following dimensions are recommended:

* 2[s.sub.u] [less than or equal to] H [less than or equal to] 3[s.sub.u], (knowing that [s.sub.u] is the maximum stroke

of the punch and H is the height of the pad);

* h = H - [s.sub.u];

* b [less than or equal to] 0,4B, (when b is the part's width and B is the pad's width)

Another proposal for the shape of the die which does not have the disadvantage of the round shapes moving, is presented in figure 6.

[FIGURE 6 OMITTED]

4. CONCLUSIONS

Bending with elastic pad is a technology that brings the advantage of a greater accuracy of the work pieces. This is explained by the fact that if an elastic pad is deformed, a great pressure is obtained. This pressure will create a calibration of the work piece that is higher if the radius is smaller and if the punch stroke is greater. There is a certain value for punch stroke depending of the punch radius, that corresponds to the point were the layers of the specimen are completely fixed between the pad and the punch. For strokes higher than this, the accuracy of the specimen grows with such small values and the load becomes so high that it is not recommended to be used in manufacturing practice.

5. REFERENCES

von Ende, A. (1991). Untersuchnungen zum Biegemformen mit elastischer Matrize, (Researches concerning bending with elastic dies), PhD thesis, University of Erlangen--Nurnberg, Munchen, Wien, Hanser

Hancu, L. & Achimas, Gh. (2001). Researches Concerning the Accuracy of the Specimen in Bending with Rubber Dies, Acta Technica Napocensis, review from Technical University of Cluj-Napoca, no.44, pp.75-78

Hancu, L.; Achimas, Gh.; Iancau, H. & Paunescu, D. (2004). Numerical Simulation for Sheet Bending with Silicone Rubber Pad, Proceedings of microCAD International Scientific Conference, Miscolk, March, 2004

Hancu, L.; Achimas, Gh. (2005). Spring back Reduction for V Bended Parts through Elastic Pads, Proceedings of The 8th ESAFORM Conference on Material Forming, Academy Press, ISBN 973-27-1174-4, pp.497-503, Cluj-Napoca

Wilhelm, H. A. (1972). Bending with Elastic Tools, Forming Institution, Stuttgart.

Table 1. Calculated punch strokes

Punch radius [mm] 2 5 10 15
Punch storke [mm] 18,75 17,51 15,50 13,35