deep drawing

Abstract

Deep drawing is an important process used for producing cups from sheet metal in large quantities. The deep drawing is affected by

many process variables, such as blank shapes, profile radius of punch and die, formability of materials and so on. Especially, in order to obtain the optimal products in deep drawing process, blank and die shapes are very important formability factorIn this study, we investigated the effects of blank holder and die shapes, using five kinds of blank holder and die shapes.We measured the distribution at blank holder force (BHF) according to the ratio of sheet metal drawing, under the constant conditions of profile radius of punch and die.Deriving from the experimental studies, it is clarified that angle of blank and die at deep drawing process influence blank holder force distribution and ratio of drawing.

Introduction

Deep drawing process, one of sheet metal forming methods,

is very useful in industrial field because of its efficiency.

In deep drawing a sheet metal blank is drawn

over a die by a radiused punch. As the blank is drawn radially

inwards the flange undergoes radial tension and circumferential compression [1]. The latter may cause

wrinkling of the flange if the draw ratio is large, or if the

cup diameter-to-thickness ratio is high. A blank-holder

usually applies sufficient pressure on the blank to prevent

wrinkling [2]. Radial tensile stress on the flange being

drawn is produced by the tension on the cup wall induced

by the punch force. Hence, when drawing cups at larger

draw ratios, larger radial tension are created on the flange

and higher tensile stress is needed on the cup wall. Bending

and unbending over the die radius is also provided by this

tensile stress on the cup wall. In addition, the tension on

the cup wall has to help to overcome frictional resistance,

at the flange and at the die radius. As the tensile stress that

the wall of the cup can withstand is limited to the ultimate

tensile strength of the material, the draw ratio possible in

deep drawing is usually limited to about 2.1 or 2.2, to draw

deeper cups recourse being made to special drawing processes

such as hydro-forming [3], hydro-mechanical forming

[4], counter-pressure deep drawing [5], hydraulicpressure-

augmented deep drawing [6], etc. These processes

are relatively slow (compared with the deep drawing or

redrawing process) and the draw ratios are limited to 3.5

or 4 at most. However, a conventionally-drawn cup can

be redrawn twice or more to obtain draw ratios of the

order of 5, 6 or even larger values.

In this study we have aimed to increase deep drawing

ratio and to decrease blank holder forces by giving an angle

to die and blank holder

Deep drawing experiments

2.1. Material

In this study, the test material used is DIN EN 10130-91, with low carbon and high quality formability and a thickness of 1 mm. Tensile tests were carried out in the directions of 0 , 45 , and 90 to the rolling direction.

The mechanical property in the tensile direction is indicated in Table

1. The tensile strength of specimens was measured through the tensile test

MOHR+FEDERHAFF+LOSENHAUSEN with setting load speed as

2 mm/min

http://www.4shared.com/photo/Mo2SikOD/1ok.html

Experimental method and procedure

After observing the punch and die geometry of Fereshteh and Montazeran

[7], it was decided to manufacture similar geometries for comparison

results. The punch has a diameter of 46.2 mm and die has a diameter

48.5 mm. This gives 1.15 mm of clearance. Fig. 1 shows the applied equipment,

hydraulic press (60 t) with a die cushion to control blank holder

force (BHF) and limit switch to determine a stroke of upper ram according

to the processes. Fig. 2 shows a new type deep drawing die.

Table 2 shows the angles of the frame for each type blank and die

shapes up to the five processes. For profile radius of the punch and die

the experimental conditions are constant. The punch profile radius (Rp)

was fixed on 10 mm. and die profile radius (Rm) was fixed on 8 mm.

The punch velocity was fixed on 4 mm/s. Lubricant for operation of deep

drawing was used a soluble oil lubrication for plastic working

http://www.4shared.com/photo/dQteBbEs/2ok.html FIG 2

http://www.4shared.com/photo/0i9ZX25N/3ok.html TABLE 2

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3. Results and discussion

During a deep drawing operation, workpiece is subjected

to type of radial stress and longitudinal stress. There

is also a compressive stress normal to the element which is

due to blank holder pressure [8]. Fig. 3 shows the influence

on deep drawing ratio of a angle. When a angle has

increased, deep drawing ratio has increased. During the

experiments, when a is 0 , limit deep drawing ratio is

b = 1.75; However, when a is increased, b has been

increased up to 2.175

Fig. 4 shows that blank holder force is decreased by

increasing a angle for constant value of b. As a is set over

the die and blank holder, radial stress decreases and at the

longitudinal and circumferential direction stress increase.

This is due to easy flow of the sheet metal into the die cavity.

Fig. 5 shows that the wrinkle at the bigger values than

2.175 b values is occur. If a is further increased, the radial

force will decrease. In this case, a angle at the die and blank

holder will not be important and wrinkle will occur.

Fig. 6 shows the influence on blank holder forces of a

angle. When a angle increases, blank holder force decreases.

http://www.4shared.com/photo/ITusACbc/5ok.html FIG 3

http://www.4shared.com/photo/ikPMGHMt/6ok.html FIG 4

http://www.4shared.com/photo/vmYTXx43/7ok.html FIG 5

http://www.4shared.com/photo/rOXxO3I1/8ok.html FIG 6

http://www.4shared.com/photo/mfuTmmO-/9ok.html FIG 7

However, blank holder force gives an optimum value at

a = 15 . Blank holder force has been impeding to the wrinkle.

So at the production, these forces are taken at the large

values. However, a angle set over at the die and blank

holder decreases the blank holder force. Here, a increases

the axial forces. Finally, blank holder force decreases to

3002 from 10,362 N.

4. Conclusion

In this study we carried out experiments on deep drawing

die by setting five different angles over the die and

blank holder. The following main conclusions can be

drawn from this study.

(1) Limit drawing ratio limit b, related to a, has increased

without failure to 2.175 from 1.75 (Fig. 7)

(2) Blank holder force for amin = 2.5 has decreased to

3002 from 10,362 N for constant b. At the same time,

for different values of a, it shows similar decreases.

This is an advantage for the die design.

(3) Energy and cost of die are decreased with this die

type. Especially in the die manufacturing it can be

used at the first stage drawing instead of twice and/

or three times deep drawing.

References

[1] Alexander JM. An appraisal of the theory of deep drawing. Met Rev

1960;5(19):349–409.

[2] Eary DF, Reed EA. Techniques of press-working sheet metal. New

Jersey: Prentice-Hall; 1974. p. 100–172.

[3] Panknin W, Mulhauser W. Principles of the hydroform process.

Mittleilungen der forschungrges Blechvererbeitung 1957;24:269–77.