Various applications of hydraulic counter-pressure deep drawing

Journal of Materials

ELSEVIER

Processing Technology 71 ( 1997) 160- 167

eep drawin

Various applications of hydraulic counter-pressure Takeo Nakagawa a+*, Kazubiko Nakamura i( lnsrirurr oj’hhsrrial

Science, Unicersity qf Tokyo.

b Cltibct Itatiture

of’ Techolog~~,

’ Amino Engineering,

555,

2-I 7-l

b, Hiroyuki

7-22- 1 Roppctttgi. MSmto-ku.

Amino c

Tok.vct 106. Jctpcttt

T.scccictmtt~~u, Narctsltitto. Citibu 275, Jqtatt

Misottodariu,

Fccjiriomiya,

Slthtohcr

4 IS, Jctlwtt

Abstract

Hydraulic counter-pressure deep drawing present numerous merits. many of which are not understood properly. In addition to clearing most forming restrictions, it is also capable of forming tapering shapes and complicated shapes. Economical advantages such as reduction of sheet metal material costs and die costs should also not be forgotten. This report summarizes the features of hydraulic counter-pressure deep drawing by introducing several actual examples of applications. 0 1997 Published by Elsevier Science S. A. Keywords:

Hydraulic counter-pressure;

Deep drawing; Applications

1. Introduction

2. Principle of hydraulic counter-pressure deep drawing

It has been 40 years since hydraulic counter-pressure deep drawing was first attempted and more than 30 years since it was actually applied to production. In the beginning, it was pursued as a method to overcome the restrictions of deep drawing. The use of its merit of enabling tapered panel forming (this requires many forming steps) to be incorporated in the manufacturing process was the first actual application of the method. Looking at the developments after that, by making use of the various features of this method, its applications broadened. Lately, the method is also being applied to the forming of automobile body parts, apparently due to its die manufacturing cost-reduction merit. Most of the scientific papers on hydraulic counterpressure deep drawing describe analytical results focusing on its advantages in clearing forming limits. But it must not be forgotten that this method has many other features, which are being used extensively. This report classifies various examples of hydraulic counter-pressure deep drawing.

In hydraulic counter-pressure deep drawing, forming is performed by filling the die cavity with oil or water in the conventional sheet metal deep drawing, as shown in Fig. 1. As forming is carried out, the hydraulic pressure rises, the relief valve works so that the liquid flows out or the liquid flows out from the gap between the die and the flange of the sheet through the shoulder of the die. This method was studied by the Japanese Professor Kasuga and others at Nagoya University in the beginning [l] and application to production was first carried out by SMG of Germany. This method has various names-pressure lubricating deep drawing, hydromech, acquadraw and fluid former. In Japan, it is generally known as hydraulic counter-pressure deep drawing. Looking at the forming process, it is the same as the normal deep drawing process, except for the fact that the die cavity is filled with liquid so that hydraulic pressure is applied during the forming process, but this makes a huge difference. In the normal deep drawing process, blank holding pressure is applied to control the blank but essentially no other force except to the punch and die is applied. Thus the mere application of hydraulic pressure from the bottom creates a huge impact on the forming process. Fig. 2 shows the details of the change in hydraulic counter-pressure during deep drawing 121. First when the punch moves and enters the die and forming starts,

-* Corresponding author.

0924-0136/97/917.008 1997 Published by Elsevier Science S.A. All rights reserved. PII SO924-0136(97)00 163-5

the hydraulic

pressure increases rapidly. At the same time, the sheet metal is pressed firmly agai~xt the base of the punch by this hydraulic pressure. hydraulic pressure reaches the relief valve p the liquid starts flowing out of the valve. In addition. by setting the valve pressure 1 strong Icvcl. the ljyiaid will flow outside from the angc part via the die shoulder. The conditions for flow out from the Rlangc depend on the blank holding force mainly. In whichever case, the liquid flows out as the pzlnch moves, so that the hydraulic pressure during maintained at a constant level. Sometimes, from the flange is intermittent and the hydraulic prssure shows a slightly As forming out

more

pulsating

reaches

easily

motion.

its final stages, the

fro

tiange

the liquid

nnd

flows

the

hydraulic pressure drops. The punch force becomes great it.; hydraulic pressure is added in addition to the normal deep drawing force. A larger blank holding force is iihso Spring ,3ble pluge - _---

.

Mild

steel

Valve 1

er

Pure Aluminum Cushion 10

0

20

30

40

50

60

70

80

Punch stroke/mm

necessary for the hydraulic pressure imposed on the flange to prevent the flow of liquid from the Range, or to attain a high hydraulic pressure at a low blank holding pressure.

Fig. I. Hydraulic counter pressure deep dra\vinp.

\ Bulging

Blank

k

Punch

Die

-Sheet metal

I

pressure

a) Direct

chamber method

pressure

chamber

I

I b)

Indirect

method

c

Fig. 5. Radial pressure assisted hydraulic counter pressure deep dra\vl Fig. 2. Typicai counter pressure-stroke curve [?I.

[4] (Chiba institute Technology. Japan).

162

Punch

ia

(bl

Fig 6. Operational sequence for hydraulic counter ‘-pressure reverse re-drawing [5]. (a-~b) First drawing (conventional drawing); (c) re-drawing (hidrat& counter-pressure drawing with radial pushing).

Sometimes an O-ring is attached to the die surface as a seal.

3. Features of hydraulic counter-pressure deep drawing Hydraulic counter-pressure deep drawing was initially developed to control the frictional force metal during deep drawing and clear the forming restrictions. But it has various other features compared to simple cup shape deep drawing. These features are described briefly in the following.

Forming restriction depends on the comparison between the tensile resistance of the sheet metal at the shoulder of the punch supporting the punch force and the deforming resistance of the sheet metal by drawing of the flange. These resistances are the original deforming resistance of the material added with the frictional force. By adding hydraulic counter pressure, the increase in the frictional force when the sheet is pressed against the punch shoulder causes the tensile resistance of the sheet metal to rise. Consequently. the partial thickness reduction at the punch shoulder induces material breakage in normal deep drawing, but due to the counter-pressure, breakage occurs at the small walls of the deformed shell, or in some cases, at the overhang part near the die shoulder. This means that the frictionincrease effects on the punch surface prevents breakage at the parts contacting the punch, resulting in a sharp increase in the breakage resistance. On the other hand, in terms of the deformation resistance on the flange, haff of the friction on the die

and blank holder decreases sharply when the liquid flows out and becomes liquid lubricant. This means that the flow in resistance of the material at the flange drops considerably. This drop in the flow-in resistance of the material and the increase in the tensile resistance of the material are both produced by the addition of the hydraulic counter-pressure and works to increase the deep drawing limit ratio. As a result. it allows forming to be carried out in one process while the normal deep drawing method requires two processes including the re-drawing process.

In deep drawing, a tapered punch base (not flat) will result in shrinkage deformation of the overhang section in the circumference direction and wrinkling will be formed at the tapered area in most cases. To prevent this, in the normal deep drawing method, there is no other way but to increase the blank holding force to elongate in the radial direction and absorb extra materials in the overhang section. Increasing the blank holding force invites breakage from the punch shoulder and so the blank holding force cannot be increased arbitrarily. Therefore even if it involves extra work, forming is performed by controlling the generation of body wrinkles through many steps. But the design of a process consisting of several steps for controlling body wrinkles requires expertise, and at the same time, such process causes considerable surface scratches like shock lines on the product. In hydraulic counter-pressure deep drawing, the overhand section is stretched by the hydraulic pressure and the sheet metal is pushed against the punch side in the initial stages of the forming process. This stretching causes elongation in the radial direction and absorbs

the extra materials in the circumference direction. At the same time, the parts at risk of breakage move to the external circumference and the breakage resistance rises. As a result, tvhen the blank holding pressure ing1y. elongation in the radiaB direction of the overhang section also increases so that the estra materials in the circumference direction can be absorbed.

In deep drawing, the forme part suffers considcrabfe mismatching t.0 the shape of be punch and die due to spring back dfter forming. This causes dimensional accuracy of the formed part to drop. To reduce such spring back, the best solution is to increase the tension in the radial direction. ydraulic counter-pressure deep drawing allows this tension in the radial direction to be increased in the forming process, and as a result. reduces spring bllck itself. Furthermore. accuracy problems also resuh if the sheet metal itself does not contact the punch and die closely in the fomnng process. Hydraulic counter-pressure deep drawing also solves this problem to a considerable extent by applying consistent static hydraulic pressure in the sheet thickness direction and ensuring the contact of the sheet metal with the punch surface.

qf’ lodixd distribution

3.4. Cotmd titicktzrss

tliitulittg

ntd

Bf hollous and protrusions exist at the bottom of the formed product. normal forming methods use [emale dies lo shape 11x2 sheet meld in the final stages by sandwiching it between the paunch and remale die. In hydri:uIic counter-pressure deep drawing. the female die can bc substituted by the hydraulic pressure itself in most cases. thus eliminating the use of the female die. Normal female dies cost nmre than male dies to produce and require time consuming labor for adjusting to the male die and adjusting the clearance between the male and female dies. Of course sharp and complicated shapes at the bottom of the formed parts cannot be obtained just by hydraulic pressure.

The hydraulic count-pressure deep drawing demonstrates several merits. To put these to use, it is necessary to satisfy the requirements for main equipments such as the press machine. The following summarizes the requirements of these equipments.

cotui.~trttt

In conventional deep drawing, thinning of the materials in contact with the punch shoulder is the greatest and breakage occurs from those areas. Some products require this thinning of parts to be reduced as much as possibfe in order to attain the strength functions of the products. In some cases, the number of forming steps has to be increased to clear this restriction. In hydrauiic counter-pressure deep drawing, the sheet metal is pushed against the punch by the hydraulic pressure, to maintain the punch force using frictional force. Naturally, this controls localized reduction of the thickness at the punch shoulder. Furthermore, although the thickness of the side walls increase towards the rims of the formed parts. This results in greater tension in the radial direction applied throughout the forming process as well as on the external circumference and therefore the increase in the thickness of the external circumference is reduced. Consequently, in hydraulic counterpressure deep drawing, the thickness distribution of the product is on the whole consistent compared to normal deep drawing.

Fig. 7. Very deep cup which cannot processes in a single step.

be forlned by conventional

Fig. 8. Tapered shell often formed with body wrinkles in conventional processes.

contrary, to raise the shape accuracy blank holding pressure and hydraulic final stages of the forming process and decrease pressure halfway through the total press capacity.

Fig. 9. Atitomotive oilpan which cannot be formed in a single step by conventiona! processes.

Fig. IO. Two complicated bottom shape panels formed with prcbulging prior to deep drawing.

4.1. Hydraulic counter-presswe

by raising the pressure in the occasionally lo process to save

In general press forming methods., blank holding is constant. kately, press machines which vary the blank holding pressure halfway through the forming process are being used in some cases. In hydraulic counter-pressure deep drawing, the arbitrary variation of the blank holding pressure during forming is indispensable. This is because when the hydraulic counter-pressure is high, it is c!osely related to the blank holding pressure. As the liquid leaks from the blank holding flange side, increasing the blank holding pressure increases the hydraulic pressure and decreasing it decreases the hydraulic pressure, To increase the hydraulic pressure particularly during the initial and final stages of the forming process requires the blank holding pressure to be raised. In addition to control body wrinkles, the blank holding pressure must be changed during the forming process.

Needless to say, the die cushion is used as an ejector of the product. But the die cushion can be used efficiently for other purposes. Because hydraulic counterpressure deep drawing does not use the female die, should hollows and protrusions exist at the bottom of the formed panel, the hydraulic pressure will not be sufficient to ensure close adherence of the sheet metal attd accurate transcription. In this case, a female die is attached to the die cushion partially and the sheet metal is pressed with the die, or a female die is attached to one part of the base of the die cavity to carry out forming in the final stage of the stroke. Furthermore, in hydraulic counter-pressure deep drawing, the punch force is a high value because of the hydraulic counter-pressure applied. One solution for this is the use of a die c&lion as shown in Fig. 3. to reduce the punch force.

control

In normal hydraulic counter-pressure drawing, the maximum pressure is set and this maximum value needs to be variable. However, in complex shape forming, it is desirable for the hydraulic pressure to be varied during the forming process. This is because it will become necessary to prevent excessive stretching by the hydraulic pressure in the initial stages of the forming process, to add preliminary stretching steps on the

In hydraulic counter-pressure deep drawing, double action hydraulic pressure press with blank holding pressure is performed. The use of a hydraulic single-motion press with cushion or mechanical single-motion press will help reduce installation cost to a great extent. This method which was first developed with this aim supplies liquid using a valve while positioning the hydraulic pressure cavity at the top as shown in Fig. 4 [3].

Occur more readily. t:nabling deep drawing to bc pcrformed more easily. Two methods as shown in Fig. 5 are being attempted, where in both cases. ;i grcatc~ blank holding force is required. Fig. 6 shows one example of applying rcvers4: deep drau ing based on this idea. A high drawing rate cLbn be obtained in one process.

As meiationed earlier. there are numerous features to hydraulic counter-pressure deep drawing and diverse and useful applications. The following introduces thcsc examples centering around actually applied products.

Fig. 1I. Shallowpanel often formed with surface distortion ventional processes.

Fig. 12.

Ti

alloy sheet which is a typical difficult-to-form

in con-

material.

Hydraulic counter-pressure deep drawing clears forming restrictions to a large extent. For example, the drawing rate of the conventional process is 2.2 while that with the hydraulic counter-pressure deepdrawing is 2.8, enabling forming up to 3.2 by the assistance of radial pressure. For this reason, only one forming process is required as shown in Fig. 7 when normally it would req!tire two. Tapered shapes can be formed in one process as body wrinkles can bc prevented. Fig. 8 shows such an example and the forming process has been shortened to a considerable extent. Furthermore, for examples such as Fig. 9. *.vherr:the forming restrictioll is severe due to the complex shape made up of the above two shapes, hydraulic counf.erpressure deep drawing proves to be most suitable. Foi, punches with hollows and protrusions at the bottom. breakage does not occur easily by performing drawing forming after reverse stretching by hydraulic pressure before the punch descends. Fig. IO shows such two examples with very complicated bottom shapes.

T. Nttktt.quwct ct d.

166

5.2. Prcvmtion

qf strrfirce

distortion

Juttwctl qf Matrrbls Protwsitt~ Tdtttology

in shctliow~ cotttpkx

Conventional

71 (1997) 160- 167 system

shcrpe forming

Surface distortion tends to develop easily in shallow drawing and complex-shape stretch-forming. In hydraulic counter-pressure deep drawing, because the sheet metal can be pressed against the punch consis-

Liquid-pressure press forming

High-speed.

Mui+direction

three-dimensional co:

laser

pressforming

cutting

Fig. 17. Small lot production system for auto-body panels using hydraulic counter-pressure deep drawing (Toyota. Amino).

by the hydraulic pressure. the generation of surface distortion can be controlled considerably. Fig. 11 shows such a panel which suits the hydraulic counter-pressure drawing.

tently

Fig. 15. Large panel of hot-rolled steel sheet in which cold rolled steel sheet was formerly used.

5.3. Fornting

of dij$cult-ro-fbrttt

tmteriuls

uttd tm*

tmtterictls

Titanium alloys and high strength aluminum alloys themselves are difficult-to-form materials. In hydraulic counter-pressure deep drawing, by clearing forming restrictions, such difficult-to-form materials can also be formed. Fig. 12 shows such an example of titanium alloy sheet. Sandwich type laminate steel sheets such as vibration dumping steel sheets and light steel sheets can also be formed without having to worry about peeling of the interfacial area. Fig. 13 shows an example of such laminate steel sheet. In addition, sheet metals coated with printing can also be formed by hydraulic counter-pressure deep drawing because the method does not cause surface scratches. Fig. 14 shows an example of coated steel. 5.4. Cotmmion

to inexpetuiw

sheet tttetal

By clearing forming restrictions, grade-down of materials can be achieved. Fig. 15 shows an example converted from the cold rolled steel sheet to hot rolled. By controlling localized thinning, thinner sheet metals can also he used, thus realizing lightweight and cost reduction of materials. 5.5. Conversion

Fig. 16. Outer and inner panels formed by using the same die and blank holder.

to itmpensive

c/k ntuterid

In hydraulic counter-pressure deep drawing, friction on the die and blank holder is reduced and this reduces the surface scoring of the die material. For punches, because static hydraulic pressure is applied, weaker tool

167

material can be used. This as a result enables gradedown of die materials. For instance. die can be converted from cast steel and tool steel to cast iron. In addition, the method also allows safe use of resin ~lnd cement punches.

The substitute of die with hydraulic pressure proves to be an enormous economical merit. In particular, this substitution can often be applied in the forming of automobile parts with complex bottom shapes. For the forming of the outer and inner panels shown in Fig. 16. using the same die and blank holder, replacement of only the punch is sufficient. Furthermore, the same forming tools can be used to form workpiece with minor differences in thickness.

Hydraulic counter-pressure deep drawing enables drawing with severe conditions to be performed. By making use of this advantage. instead of performing rim bending after trimming of the drawn product, by performing rim bending during forming and then performing trimming, the process can be shortened. In addition, the use of laser cutting instead of die trimming will also considerably reduce die costs. Furthermore, by performing the post process rim bending with a newly developed multi-bending press, die costs can also be reduced. The system shown in Fig. 17 was developed based on these ideas. Due to the slow forming cycle of each step of the process, the system is suited to small-lot production. Fig. 18 shows examples

of the product used for actual automobile

body parts.

Many of those involved in sheet metal forming are unaware of the true nature of hydraulic counter-pressure deep drawing. Most are surprised to learn that the forming process changes sharply with the application of hydraulic pressure. Indeed, the method does complicate the already complicated deep drawing mechanism. but it presents numerous interesting tasks to researchers. This report discussed some advantages of applying hydraulic pressure that can be employed. It is hoped that this report will help researchers and cnginccrs to further understand the hydrau!ic counter-pressure deep drawing method.

eferences K. Kasug;l. N. Nozaki, Bull. JSME 4- 14 (lY61) 394. K. Nakamura, T. Nakagarva, Fracture mechanism and fracture control in deep drawing with hydraulic counter-pressure. .I. JSTP 25 (284) (1984) 831-838. K. Suzuki. T. Nakagaawa. Counter-pressure drawing by using single action press, in: Proceedings 1st ICTP. 1984. pp. 769 774. K. Nakamura. T. Nakagawa. Hydraulic counter-pressure deep drawing assisted by radial pressure, in: Proceedings 1st ICTP. 1984. pp. 775 780. K. Nakamura. T. Nakagawa. Reverse deep drawing with hydraulic counter-pressure uxinf. the peripheral pushing effects. Ann. ClRP 35 (I) (1986) I73- 176. H. Amino, K. Nakamura. T. Nakagavva. Counter-pressure deep drawing and its application in the forming of automobile parts. J. Mater. Proc. Technol. 23 (IYYti) ?43- 265.