Keynote Papers

Development and Manufacture of Dies for Car Body Production

Klaus Siegert, Stuttgart, Germany (2) T. Altan, Columbus, OH, USA (1) T. Nakagawa, Tokyo, Japan (1)

Abstract

The costs for development and production of draw dies for car outer panels are extremely high and should be reduced.

Furthermore it is necessary to reduce the time for developing, designing and producing the dies for the production of

parts. This paper discussed new press techniques, die designs and milling techniques. The trend goes to single acting

presses with CNCcontrolled multipoint cushion systems in the press table and to special designed dies. These systems

lead to a more robust and reproduable forming process with improved product qualtty.

Kevwords: Deep Drawing, Cushion System, Binder Design

Press shop people ask for more robust ,sheet metal

forming processes and hope the designers will come up

with stamping geometries that are easier to form.

We can certainly expect that efforts will be made to

design parts with easier to form contours. Nevertheless,

the automotive and stamping industries will continue to

work towards achieving the following objectives:

0 Introducing aluminum sheet metal

0 Increasing the use of'HSLA-steel with higher yield

strength

0 Increasing the use of zinc coated steel

0 Reducing the number of parts per car

0 Reducing the number of dies per part

0 Production of parts with zero defects

0 Production of outer panels with excellent surface, so

that no handwork is needed in the body in white shop

0 Reduction of the production costs

In the USA a so called 'Near Zero Stamping' - program

is conducted for achieving precision and agility in sheet

metal stamping. This program is camed out jointly by

twenty-two Auto Body Consortium companies in

cooperation with Chrysler, Ford and General Motors and

We research institutions.

The objectives of the 'Near Zero Stamping' program is

to improve the accuracy of stamped sheet metal parts

and to reduce the time currently required for sheet metal

die design, try out and production. This is only an

Annals of the ClRP Vol. 46/2/1997

example that shows the need for improvements in the

press shop.

These goals are not only important in the USA, but they

are important for every press shop in the world.

Furthermore we have to realize that the time from

developing and designing the die to the first production

of parts will be reduced from about 18 months to 12

months. We must also remember that stamping dies

needed to produce one passenger car costs between

200 and 300 million dollars and that a press shop for the

produdion of all the sheet metal parts for about 1000

cars per day costs about $ 500 million. Thus, the

investments in press shops are very high. Thus, it is

worthwhile to make significant improvements in the

press shop. In the future the problems associated with

stamping will increase and not decrease because of ever

increasing demands of the automotive industry.

In this paper the present situation, future trends and the

goals for development and manufacture of dies for car

body production are discussed.

In doing this the focus is on draw dies.

3. Draw Process

Drawing sheet metal parts for the autobody is in principle

a combination of deep drawing and stretch forming.

During the drawing of non-axisymmetric sheet metal

parts, the flow of the material between upper and lower

binder should be controlled, so that neither wrinkles nor

tears appear in the drawn part.

To control the material flow between the binders one or

a combination of the following possibilities can be used:

535

a0 Shape of the blank Larger blanks lead to higher

deformation- and frictional-forces which hinder

material flow. Smaller blank sizes favour the material

flow.

bO Draw beads: Both friction- and bending-forces

induced by draw beads provide a hindrance to the

material flow.

c 0 Lockbeads: They prevent the material flow between

the binder surfaces entirely.

dOFriction between the upper binder and the sheet

metal and between the sheet metal and the lower

binder. Higher friction can hinder the material now,

lower friction can favour the material flow.

The friction can be influenced by

the type of lubricant and the amount of lubrication

adjusting the gap between the upper and lower

binder. A larger gap results in less pressure and less

friction, a smaller gap results in higher pressure and

more friction.

last but not least, the friction that can be influenced

by elastic deformation of the blankholder (This is the

upper binder of dies for double action presses and

the lower binder of dies for single action presses with

a cushion system in the press table) deformed by the

blankholder forces.

3. Realization of the Blankholder Forces

3.1 Double Action Press with a 818nkholder Ram

Fig. 1 left, shows a draw die for a double action press.

These presses have the possibility to adjust the

blankholder forces at four pressure points of the

blankholder ram. It is possible to adjust constant binder

forces over the stroke differently for "front left", "front

right", "rear left" and "rear right" (2), (4).

For mechanical presses normally the blankholder forces

is constant over the stroke at each of the four points. For

double acting mechanical presses the possibility to run

at each pressure point different force cuwes over the

stroke, is still in the developmental stage (5). This is

easy to achieve in a hydraulic double action press.

w m , -*dnm,r.~- --p-d.nlbwlo ( h r Q n M a h m s h . r W D m C u d h ~ h * . h r M

0

h

aJ Rmn d ) u R y 1 6 0) QI.lia h s n n r a r w c r * a n w b ) l H h d d r R m e ) L o r h d u h J C U l k n C W #J Amh 3h.r-

Fig.1: Draw dies in double action presses and in single

action presses with cushion systems in the press

table.

3.2 Single Action P m with a Pneumatic Cushion System in the Press Table

To control the material flow by deforming the blankholder

elastically the following must be considered:

Fig. 1 middle. s h o k the draw die for a single acting

press. In this type of mechanical hydraulical press the

female die is clamped to the ram and the male die is - the manner in which the load is applied from the

press to the die

- the elastic behavior of the die positioned on the press table. The blankholder is the

lower binder. It is supported by a cushion system in the

- the distance between the upper and the lower binder. press table or by nitrogen cylinder systems in the dies

(9).

Thus the blankholder forces are introduced to the lower

binder (blankholder) by nitrogen cylinders in the die or by

a cushion system in the press table. Most of the

536

Keynote Papers

presently used cushion systems are pneumatic systems.

Here a number of cushion pins are positioned between

the lower binder and a cushion plate which is supported

by a pneumatic cylinder.

The blankholder force is introduced to the lower binder

by the cushion pins when the ram (upper die) forces the

lower binder downwards with the force of the cushion

system. The total cushion force is given by the

pneumatic pressure multiplied with the cross sectional

areas of the pneumatic cylinders (9).

There are a number of disadvantages of the pneumatic

cushion system:

High peak load on the blankholder at the moment when the upper binder contacts the blank on the

lower binder

Dependence of the blankholder force on the number

of strokes per minute

The cushion plate tips by eccentric loading

The cushion pins, after some time, have slightly

different heights because of to high localized

pressure. This effects the elastic deflection of the

binder.

As a result of all these disadvantages, the pneumatic

cushion does not provide a reproducable forming

process. Furthermore it should be noted that pneumatic

or hydraulic central cylinders do not allow to adjust the

force at the corner points of the blankholder, as it is

possible with a double action press.

3.2 Single Action Presses with a Hydraulic 4 Point Cushion System in the Press Table

during the stroke for each cylinder separately by servo or

proportional valves.

When the upper binder, which is attaches to the ram,

contacts the blank placed on the lower binder a pressure

peak occurs on the blankholder force. This peak can be

reduced by appropriate programming of the hydraulic

cylinders, Figure 2 (2).

The pressure peak caused by contacting the blank on

the lower binder by the upper binder which is fixed to the

ram can be reduced below the level of the constant

blankholder force by programming an ascending ramp

(Fig. 2) (2).

I !

Fig. 2: Reduction of the pressure peak by programming

an ascending ramp.

The direct support of the blankholder by 4 hydraulic

cylinders in the comer points results in a well defined

method of the blankholderforce-transmission. It allows

the control the force of every cylinder induvidually during

the stroke. It was shown in practice and by experimental

investigations that mechanical single action presses with

hydraulic 4-point cushion systems in the press table give

better and 'reproducible draw results than mechanical

double action presses.

A big step forward was made by the development of a We can state that:

hydraulic four comer point cushion system. This is seen

in Fig. 1 right (4), (8) ,Mechanical single action presses with an excellent.

hydraulic cushion system are preferable to This system is mainly a passive hydraulic system

because the pressure in the 4 cylinders increases by

compression when the lower binder is forced

downwards. So it is possible to control the pressure

mechanical double action presses."

537

3.3 Single Action Press with Multipoint Cushion System in the Res8 Table

The direct support of the lower binder by 4 hydraulic

cylinders seems to be excellent for dies up to 1OOOmm x

1000mm. Larger dies have to be supported by more

than 4 hydraulic cylinders (Fig. 3). If every cylinder has

its own sew0 valve it is possible to control the pressure

between the binders and the blank in certain areas by

corresponding hydraulic cylinders (9).

F i g 3 Multipoint cushion system.

A further possibility to build up muttipoint cushion

systems is shown in Fig. 4. This system supports the

cushion plate by' four hydraulic cylinders that are

individually CNC controllable and height adjustable

during the stroke by servo valves (1 0).

Fig.4: Multipoint cushion system with 10 pins.

CNC-height-adjustable cushion pins with load cells are

pressed against the cushion plate and transfer the

blankholder forces to the lower binder. The blankholder

forces can be adjusted for every pin by adjusting the pin

height. The measurement of the axial force on each

cushion pin allows a programmable and reproducible

adjustment of the active cushion pins. Thus it is possible

to program a reproducible adjustment of the elastic

binder deflection which affects the pressures and friction

forces between the binder and the blank (10).

A tendency to transfer presses can be observed in all automotive companies. In the past 10 years huge

transfer presses for side wall panels have been built. An

example is the large transfer press, built by MClller-

Weingarten AG at the Mercedes-Benz plant in

Sindelfingen, Germany (Fig. 5). This press was installed

in 1994 for the production of side wall panels inclusive of

quarter panels, with blanks of 4200mm x 2000mm. The

total press force is 73.000kN.

Fig.5: Transfer press for sidewall panels.

The multipoint cushion system of the draw operation

works with 15 hydraulic cylinders. Each cylinder has its

own CNC controllable valve. The total blankholder force

is 6000 kN.

In summary it can be stated that:

,For automotive parts of excellent surface and

dimensional quality CNCcontrollable multipoint

cushion systems are necessary."

538

Keynote Papers

4. Draw Die%

Normally the dies are produced as 'rigif and not as

flexible dies. But we have to consider that a die can not

be entirely rigid. It will be elastically deformed by the

process forces. So it is necessary to spotin the distance

between upper and lower binder due to the elastic

behavior of the die and of the press.

This results in a specific blankholder pressure

distribution between upper binder, blank and lower

binder of a draw die in a given press.

When using a CNC-controllable multipoint cushion

system it is possible to influence by each cushion pin a

corresponding defined area of the lower binder surface.

Conventionally designed dies have ribs so located that

the die is relatively .rigi&.

But these rigid dies deflect under the blankholder forces.

Fig. 6 show for a rectangular pan the pressure between

lower binder, blank and upper binder when all pin forces

are equal (4).

Fig.7: Normal pressure of an blankholder with optimized

cushion pins lengths measured with Fuji foil.

The idea is that the press operator has a segmented

shape of the lower binder on the screen. He only has to

order certain blankholder pressures for every segment. If

he wants to have a correction of the pressure in a certain

area he only has to put new data on the specific

segment of the binder at the screen (1).

So we must have a flexible binder where the cushion

pins apply forces to specific and exactly defined areas

on the lower binder. In a first assumption the upper

binder is 'rigid' but we know that this is not the case so

that we have to correct the calculation in a second step

by calculating the elastic behavior of both the lower and

the upper binder.

Fig. 8 shows a flexible binder with ribs. Here the

blankholder forces are applied to ribcrosses. Every

blankholder force corresponds to a certain area on the

blankholder. For this the links between the blankholder

segments are ribs with less stiffness which can be

obtained by hples located in the ribs (1).

Fig.6: Normal pressure of an blankholder without

optimized cushion pins lengths measured with

Fuji foil.

Here the pressure on the long sides is not large enough.

The result is too much flow-in of the Mank so that the

side walls of the pot are curved. In other words the

quality of the product is not good enough.

By optimizing blankholder forces, as shown in Fig. 7, it is

possible to obtain a higher pressure on the long sides of

the binders, so that the material flow in results in a good

quality product (4). Fig. 8: Schematic design of an elastic blankholder.

539

Fig. 9 shows the result. It can be clearly seen that by

increasing the pressure in a certain area the other areas

are not influenced (1).

Fig.12: Normal pressure of the flexible blankholder.

Fig.9: Normal pressure of the elastic binder.

Fig. 10 shows a specially designed lower binder with

pyramidal shaped steel inserts in a special nylon

reinforced thermoplastic material (EB-RIM). Here the

plastic material is the link between defined binder

segments.

Fig.10: Flexible binder from EB-Rim with pyramidal

shaped steel inserts.

This die is a low production or a prototype die. A

production die build up in cast steel in a similar design is

shown in Fig. 11 (1).

5. Die DeveloDment

The input for the die development is the CAD-part-data.

These data should describe the outer surface (Edge A)

of the part (3).

The first step of the die development process is defining

the draw direction. This is done mostly empirically. All

part surfaces, which are visible at the car and which

have to be of excellent surface quality have to be

positioned on the punch of the draw die. We can

influence by the draw direction the first contact area of

the punch with the blank (7).

In relationship to the tipping of the part or in other words

to the draw direction we have a different progression of

the contact area between the blank and the punch over

the stroke (7).

The punch contact progression can be easily simulated

using 'commercial CAD-codes like CATIA by cutting

equidistant planes as shown in Fig. 13. Doing this is

quite helpful but the real draw process will show a

different contact progression (7).

Fig.11: Schematic design of the new elastic blankholder.

Fig. 12 shows that it is possible to increase the pressure

in one area without influencing the pressures in other

areas.

Fig.13: Evaluation of the tipping position.

For evaluating the tipping position of the part we need a

commercial CADcode that compromises a minimum of

draw depth (possible with CAD-codes similar to CATIA)

540

Keynote Papers

and a good contact progression of the blank and the

punch.

After deciding the draw direction it is necessary to unfold

all non-visible areas to drawable positions, which are as

a first approximation surfaces tangential to the visible

part surface on the punch. By doing this we get the

trimline (Fig. 14 ) (3).

Fig.14: Unbending of non visible surfaces.

For unfolding processes commercial CAPcodes are

available (i.e. Develop2).

In the next step we have to decide' the contour of the

punch which is equidistant to the punch opening line of

the binder.

A very important rule for the design of the draw die is

that the draw depth should be equidistant around the

punch. This makes it sometimes necessary to build up

addendum surfaces on the punch. By definition, the

addendum is the surface between trimline and punch

radius (Fig. 15).

-am

Fig. 15: Flange unfolding.

The addendum has to be developed in an interactive

process together with the binder design. For outer

surface-panels it is a condition that when dosing the

binders the blank is free of plastically formed wrinkles

and buckles.

So when the binder surface has been developed by the

CAD-system it is necessary to control whether this

binder design guarantees a good surface when closing

the binders. To control this, one step FEMcodes like

ISO-PUNCH are useful.

If the test shows good results, it is useful to mill a

styrofoamrnodel of the binder and the punch to

demonstrate the binder design.

From this model it is possible to laminate reinforced

shells of the binder surface. These shells, representing

the surfaces of the upper and of the lower binder can be

fixed to a binder-simulator.

When clamping the blank between these shells we can

observe weather there are wrinkles or buckles in the

blank or n d If not the binder design is 0.k. as far as

being free of buckles.

6. Die Production

After the design the next step is the manufacturing of a

prototype die. Here we have different possibilities. The

die can be made of plastic, zinc-alloy or cast gray-iron.

The trend is to use zinc-alloys which can be near net

shape cast to solid binders, male- and female-dies.

The near net shape zinc-alloy cast die is then finished by high speed milling. This method of manufacture gives

excellent surface finish and reduces the amount of

handwork needed to finish the prototype dies.

If the press shop results with these dies are o.k then the

production die can be designed and produced by using

the CAD-data of the prototype binder die surfaces.

A new trend in the machine structures can be observed:

Milling machines with parallel kinematic structures.

Fig 16 shows the ,Variaf .- milling machine of Giddings

and Lewis, WI, USA. This machine does not use a

conventional structure.

541

7- Conclusions

Fig.17: "VARIM-Virtual Axis Machine.

Supported and moved by six legs, driven by individual

ball screws and servo motors, the entire upper portion of

the Variax m'oves in six axes. This section, which holds

the spindle and cutting tool, aligns itself to the

workpiece. The Variax is a virtual axis machine, which

has control over all six degrqes of freedom. It uses the

mathematically aligned virtual axis to adjust itself to the

part, simplifying fixturing and part set up. Because of its

geometry and acceleration it contours considerably

faster than a typical machining center (Source: Giddings

and Lewis Inc.).

Machines like the Variax have .parallel kinematic

structures'. In addition to the advantage of having

reduced mass of moving parts and thus increased

acceleration capabilities, parallel kinematic structures

allow the implementation of modular construction

methods. This was pointed out by G. Pritschow and K Wurst in their ClRP 97 presentation .Systematic Design

of Hexapods and other Parallel Links Systems'.

The costs for development and production of draw dies

for car outer panels are extremely high and should be

reduced. Furthermore it is necessary to reduce the time

necessary for developing and designing the dies for the

production of parts.

The trend is in using single acting presses with CNC

controlled multipoint cushion systems in the press table.

These systems lead to a more robust and reproducible

forming process with improved product quality.

Furthermore it is possible to run flexible blankholders in

a CNC-controlled multipoint cushion system. These

flexible blankholders allow that the load points on the

blankholder apply the local forces to specific locations on

the blankholder. This makes it easier to adjust the

blankholder forces and to improve the control of the

material flow between the binders.

In developing the binder design it is necessary to assure

that the blank does not have any plastic wrinkles or

buckles when closing the binder. The binder surface can

be developed by a combination of CAD, expert-

knowledge and FEM-process-simulation. To evaluate

the performance of the binder design it is possible to run

computer simulations and to conduct trials with prototype

dies. 'Then the production dies have the same binder

design as the prototype dies.

For manufacturing production dies for stamping, in the

future castings will be machined by high speed milling.

As a result hand finishing will be reduced and lead times

for die manufacturing will be reduced.

New 6 axis milling centers with parallel kinematic

structure are in development in several countries and

are expected to be on the market in 1998.

542

Keynote Papers

References

(1) Hohnhaus, J., 1997, Blank Holder Designs using with (1 0) Siegert, K., 1991, Zieheinrichtungen a Multipoint Cushion System, Institute for Metali einfachwirkender Pressen for die Blechumformung.

Forming, Stuttgart , unpublished. DGM-Informationsgesellschaft, Oberursel.

(2) Klamser, M., 1992, Vielpunkt-Zieheinrichtungen als

Voraussetzungen far einen reproduzierbaren

Umformprozea. In: Siegert, K (Hrsg.): Neuere

Entwicklungen in der Blechumformung, DGM-

Informationsgesellschaft, Oberursel, 321-41 7.

(3) Kienzle, S., 1993, Optimierungsverfahren fur die

Rahmenanlagen-Entwicklung von Ziehwerkzeugen, . Beitrage zur Umformtechnik, Nr. 1, DGM-lnformations-

gesellschaft, Oberursel.

(4) Klamser, M., 1994, Ziehen von Blechteilen auf

einfachwirkenden Pressen mit hydraulischer

Zieheinrichtung im Pressentisch. BeitrAge zur

Umformtechnik, Nr.5, DGM-Informationsgesellschaft,

Oberursel.

(5) Klose, L, Braunlich, H.:, 1996, Vergleichende

Untersuchungen zur Mehrpunktziehtechnik. Blech Rohre

Profile 10, S. 512-514

(7) Roll, K., Harthun, S., 1996, Rechnergestutzte

Ziehrahmenkonstruktion. In: Siegert, K. (Hrsg.): Neuere

Entwicklungen in der Blechumfomung. DGM-

Informationsgesellschaft, Oberursel, S. 287-298.

181 Schneider, F. Steuerung des Kraft-Weg-Verlaufs

beim Umformen, dargestellt an der Ziehstufe einer

Groateilstufenpresse. In: Doege, E. (Hrsg.): "Ziehtechnik

auf der Groateilstufenpresse". HFF-Bericht Nr. 11, 1987,

S. 99-1 14. Hann. Forschungsinst. f. Fertigungsfragen

e.V.

(9) Siegert, K., 1997, Advances and Trends in Sheet

Metal Forming Processes. SAE Technical Papers Series

970436.

543