Sheet metal pressworking - Materials for Automobile Bodies

The majority of parts comprising the bodywork of a current mass produced motor vehicle are shaped by pressworking, i.e. a sheet metal blank is made to conform to the required contour largely by a mixture of drawing and stretching within the initial main draw die. This first operation is illustrated in Fig. 5.1 which shows the cast iron main draw tool set mounted within the press frame. This is normally followed by four or five subsequent operations in tandem which in turn consolidate the features

of the panel (‘restrike’) and then progressively trim the peripheral shape and pierce holes where required.

Loading the press can be done manually or by using a destacking and feeding mechanism whereby batches of sheet are delivered to the side of the press, the sheets magnetically separated and delivered into the jaws of the press by transfer using rubber suckers or a sliding roller mechanism. Lubricant can be pre-applied electrostatically or by spraying on entry to the press.

This type of tandem line was popular 20 years ago but in more recent years massive investment has been made in enclosed progression or tri-axis presses of the type shown in Fig. 5.2. Four successive operations are carried out within a massive enclosed frame, the parts being moved from one station to another by a walking beam transfer system. Typically, a Hitachi Zosen 5000 tonne cross bar feed press operates at speeds up to 15 strokes per minute and die changes can be effected in 5 minutes, and the target running efficiency is in excess of 70 per cent. The running of such an installation1 calls for exceptional cleanliness and accuracy and effects such as ‘pimpling’ due to the impression of small particles of atmospheric debris or zinc from cut edges of coated steel blanks can cause serious problems due to the high number of panels produced before  detection (often not until the painting stage) and on general productivity.

As this is the type of press that will feature more often in the future it is worth reiterating precautions that have to be taken to run such a line efficiently:

• More efficient and uniform packaging from suppliers

• Controlled processing through blanking operations with optimized die clearances

• Stillages in stainless steel

• Environmental controls to ensure the pressure inside the press shop is maintained above that of adjoining facilities

• Washing equipment incorporating amorphous filtering capable of removing particles down to 5 micrometres in diameter

• A second washing operation as blanks are automatically fed into the installation Although most of the emphasis of this chapter is concerned with material properties and the relationship with behaviour during component manufacture and how they influence performance, it is nevertheless worth recapping the effect of lubrication during pressing as this has such a profound part to play in performance. Systems used for tandem operations mainly relied on mill-oil plus manually applied high performance paste and recent campaigns sought to replace these with selective spray application. However, this type of system can be unsatisfactory from a housekeeping viewpoint, due to drippage onto the floor of the press bay, so electrostatic or prelubricant

application of wax films was developed but even at coating weights of 2– 3 g/m2 these still tended to attract particles. It has been found that for tri-axis operation the more accurate tool location associated with this type of new installation, general uplift in washing procedures and slight enhancement of wash-oil lubricity plus chrome plating of critical tool surfaces has enabled most jobs to be run without interruption.

Careful monitoring of mechanical properties also ensures that the increasingly high levels of drawability and stretchability, now a feature of today’s steels, are fully exploited for each specific job. The more fundamental aspects of the interaction of surface with lubricity will be considered later, together with surface topography.

At the outset it should be stated that much of the extensive research carried out on the formability of sheet materials is laboratory based and therefore conclusions reached must be qualified by differences in geometric scale, punch velocity, etc. encountered under normal operating conditions in the press shop. Although facilities have improved considerably in recent years, performance will also be subject to operational changes  due to constant reworking of tools, minor changes in tool/press alignment and locational factors arising from the change in press condition/location. Thus the sheet must have a wide tolerance between wrinkling, the unacceptable condition that develops due to the material drawing in too freely at the blank edges, and  splitting/necking due to plastic instability. Above all, however, and this will be a recurrent theme in most

other chapters, is the need for product consistency. This is emphasized in a classic paper by Butler and Wallace.2 Even with moderate properties the press can be set to run continuously and maintain a high level of productivity, but if material properties fluctuate from very good to very bad then frequent changes become necessary with attendant frustrations which further add to downtime and variable quality.

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Materials for Automobile Bodies

Geoff Davies F.I.M., M.Sc. (Oxon)

AMSTERDAM

Butterworth-Heinemann

An imprint of Elsevier

Linacre House, Jordan Hill, Oxford OX2 8DP

200 Wheeler Road, Burlington MA 01803

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