WO2000067931A1

WO2000067931A1 - Die forming

Info

Publication number: WO2000067931A1
Application number: PCT/GB2000/001626
Authority: WO
Inventors: Neil John Pitchford; Martin Jarrett; Mark Savage; Gordon Frederick Smith
Original assignee: Bayerische Motoren Werke AG; MG Rover Group Ltd
Current assignee: Bayerische Motoren Werke AG; MG Rover Group Ltd
Priority date: 1999-05-08
Filing date: 2000-04-27
Publication date: 2000-11-16
Anticipated expiration: 2001-11-08
Also published as: AU4421600A; GB9910597D0; GB2350312A

Abstract

A method of forming a hollow workpiece (40) particularly made of metal in which the workpiece is placed between at least two parts (32, 33) of a die set; the parts are closed together so that the workpiece conforms approximately to the shape of the interior surfaces (45, 45') of the die set; a solidifying fluid (70) is then injected into the material at a greater pressure than applied outside the workpiece by the die.

Description

DIE FORMING

The present invention relates to the forming of a hollow normally metal object in a die. Such a process may be used to form a metal profile requiring a high precision such as for a structural component in an automotive engineering application. By "hollow" is meant having an at least partly enclosed cross-section such as "U" or "0" shaped and where the hollow cross-section may be curved or rectilinear.

A process for forming such metal profiles known as "hydro- stretch forming" is known from US Patent No 5,771,730 (J-P Huet - assigned to GEC Alsthom ACB) .

The GEC Alsthom process referred to hereafter as the "ACB Process", partly described in the above US patent, comprises the following steps :-

1. A hollow component is firstly stretch formed to an approximate radius (some wrinkling is tolerable) by using a preformed elastic material mandrel that is rigid enough to hold the extrusion shape whilst forming, but flexible enough to expand when a pressure is applied to the inside hollow of the mandrel. (It is possible to expand the mandrel whilst stretch forming to achieve greater control of forming deformation) .

2. The extrusion and mandrel are then inserted into a die.

3. The die is closed and the die is locked securely - because of the high pressures involved. 4. The openings of the mandrel are then sealed and a hydraulic fluid such as oil is pumped into the mandrel.

5. The mandrel expands with the fluid pressure and consequently the extrusion also expands until it is entirely restrained by the die walls. This process is termed Calibrating' .

6. The component is then removed from the die and the mandrel is removed from the component for final finishing which may include forming holes or slots.

The die also may be used to cut to length the component using pressure as the cutting medium (no moving parts) . A groove is cut into the die, and the pressure from the extrusion walls pushing against the die is enough to produce a clean cut through the metal.

An elastic material which perhaps could be polyurethane is used for the mandrel as this material would possess the required rigidity for the stretchforming operation, whilst having the quality of adequate flexibility to expand when a pressure from the fluid is applied. This method of forming a tube has the advantage that subsequent cleaning of the component is not necessary.

Whilst the ACB Process after calibration is very accurate, the time taken for this process and the cost of making the necessary tooling is such that the overall cost is considerable. In particular, inserting and removing a mandrel is time consuming and it is therefore an object of the invention to decrease the cost of hydro stretchforming whilst retaining the accuracy of known processes.

Accordingly a method of forming hollow material particularly metal according to the present invention comprises the steps of providing a die set formed of at least two die parts having a die forming space for a hollow workpiece having interior wall surfaces, placing the workpiece between the die parts, closing the die parts together to cause the workpiece to conform in an initial phase approximately to the die set, injecting a solidifying liquid under a pressure into the hollow workpiece directly or closely onto the interior wall surfaces to cause the workpiece to conform in a further phase to the die set, the fluid pressure applied to within the workpiece being greater than that without the workpiece.

The method according to the invention differs from the cited prior art in that the solidifying fluid solidifies directly or closely in contact with the interior walls of the workpiece and that a mandrel as such is not used. This is in contrast with the method adopted in US Patent No 5,771,780, where the specification points to the disadvantage of direct contact. In the present invention, the solidifying fluid used for forming may be allowed to remain with the workpiece to act as a corrosion inhibitor, acoustic damper to improve the sectional properties, for example to make a laminate or support for further working such as punching or drilling.

Preferably, the workpiece is secured at either a point adjacent one end or at a point intermediate both ends to ensure location in the die of a partly preformed workpiece such as an extrusion in a desired orientation.

Where it is convenient to inject the solidifying fluid into the workpiece at one end, the workpiece requires to be blocked at the other end either by its own conformation or else by the local design of the die. In some cases the solidifying fluid may be allowed to set to form an end plug. In other cases it may be desirable to heat the die so that the fluid remains viscous in the tube until forming is complete. In other cases it may be desirable to ensure that the fluid is allowed to solidify at one point whilst remaining fluid at another point during the forming process.

If a thin layer of plastics material is required on an inner surface of the workpiece, this can be deposited by inserting a fluid slug into the workpiece before injection of a forming gas so that the workpiece can be finally formed with composite walls. Furthermore the die used in the process can be formed with an inlet for plastics material to be directed over the outer surface of the workpiece.

Preferably, the pressure range applied to the fluid is up to 2000 bar above the ambient pressure existing around the workpiece.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: - Figure 1 is a plan view of the lower die part of a first calibration die set for carrying out the method of the invention, as seen from A in Figure 2,

Figure 2 is a cross-section of the upper and lower die parts of the calibration set of Figure 1 with a workpiece in between the parts,

Figure 3 is a cross-section of a die set comprising two die parts of a calibration set where the workpiece has been preformed.

The present invention is concerned with the accurate bending of hollow tubular or open sections, particularly of both ferrous and non-ferrous metal. For the purposes of this description, the material to be formed will be referred to as "tube" though it should be understood this may include material which may not be strictly tubular but may have a "U" shaped cross-section or between a "U" and a fully closed round or rectangular-shaped cross-section.

The present invention is moreover more concerned with the precision forming of tube sometimes termed "calibration" which may take place after bending. Stretch bending machines may be used to initially shape the "tube". In Figure 1 and Figure 2 the "tube" to be calibrated is initially straight and the final shape for calibration is shown as being only slightly radiused. In Figure 3 the "tube" to be calibrated has initially been preformed by stretch forming but requires calibration. The die set shown in Figures 1 and 2 comprises a first and upper die part 2 and second and lower die part 3 which are mounted between press plates 6 and 8 so as to close in direction B onto a tubular workpiece or "tube" 10. Tube 10 may be straight and in this case is straight and is clamped to die part 3 by means of clamping plate 12.

Clamping plate 12 not only holds tube 10 but also covers sprue duct 14 where it rises from sprue inlet 16 located substantially in the centre of plate 8 and lower die part 3.

The lower die part as shown has an initial horizontal surface 18 parallel with plates 6 and 8 and is then curved over some of the continuing surface 20 with a shallow radius of 3 metres resulting in an end deflection of 6.66 mm over a specimen length of 300 mm of the continuing surface 20 outside the clamped area under plate 12. The alloy used in the trials for the test components was aluminium alloy, British Standard 6063 (solution heat-treated T4) .

After clamping tube (in the trial case 20 mm square aluminium alloy) into the die the upper die part 2 is closed onto the lower die part and after it closes, liquid under pressure is applied to the sprue inlet 16. Depending on the metal of the tube, the degree of bending required and the liquid used for forming, the time of pressurising through inlet 16 is variable as is also any heat applied to the die.

In trials of the process, spring-back ratios of 0.99 have been achieved; spring-back ratio equals tool radius divided by radius of extrusion after forming. This ratio equates to

0.25 mm deflection from the desired shape over a metre in length. Generally accuracies of within 0.04% of the desired shape have been achieved.

The following examples using the same 6.66 mm deflection die over a 300 mm specimen length and 6063 alloy indicate the relationship between calibration pressure as applied to the solidifying fluid which in examples 2 to 4 was polypropylene injected at 50 cc/sec at 230°C with an injector shock volume of 135 mm x 452.39 mm² = 61 cc:-

Injection Pressure Spring Back (Bar) (mm)

Example 1 0 2.69

2 750 0.17

3 1000 0.13

4 1400 0.05

The holding time corresponding to the time the die was closed over the specimen and when a calibration fluid was applied was 20 seconds in each example.

The holding time may be between 15 and 30 seconds.

The pressure used for the solidifying liquid has been up to 1400 bar above ambient pressure but it is believed that up to 2000 bar may be used. It will be seen from the above examples that the reduction in spring back is nearly linear between 750 and 1400 bar.

In Figure 3 a different die is shown comprising a first part

32 and second part 33 which close over a tube 40 which may be an aluminium extrusion along a line 41. The tube 40 is allowed to fit with a slight gap 43 between it and the die's inner surfaces 45, 45' . End plugs 46, 48 are then fitted to the die end orifices 50 and 52 to seal the tube. Plug 46 has an inlet 54 for solidifying fluid which may be wax, a hot plastics melt or plastic composite material. The fluid may be warmed before injecting under pressure.

In order to maintain a desired temperature within the die during the forming stage it may be necessary to use heater 56 in die part 33.

If it is required to form a part number or wording on the tube's outside, a suitable matrix can be provided at say point 58 on the inner wall 45 of die part 32.

Furthermore a piston 60 supporting die wall element 62, normally flush with wall 45, can be adjusted outwardly to use the hydraulic pressure of the solidifying fluid to punch out a corresponding area of the tube. Alternatively, the piston 60 can support a punch which can pierce inwards against the pressure of the solidifying fluid to punch out a hole or slot or otherwise cut the tube. Also a port 64 can be -used to force a plastics coating in an initially fluid form into gap 43 to coat the outside of tube 40.

The action of the fluid under pressure and possibly heat will be to inflate the tube to fit the die cavity and hence calibrate the preformed workpiece to an accurate finished form.

After inflation the tube is removed from the die set and the solidifying fluid is melted out or retained as an inner coating or filling as required. Using the same die in a modification of the above process, a slug 70 of a composite material is injected prior to injecting a fluid which may be a gas or solidifying fluid. The slug is expanded to coat the internal surface 66 of tube 40. This can be retained as an internal lamination after calibration and could be used as an acoustic damper or corrosion inhibitor.

Claims

1. A method of forming hollow material, particularly metal, comprises the steps of:-

(a) providing a die set formed of at least two die parts having a die forming space for a hollow workpiece having internal wall surfaces,

(b) placing the workpiece between the die parts,

(c) closing the die parts together to cause the workpiece to conform in an initial phase approximately to the die set,

(d) injecting a solidifying fluid closely into the interior wall surfaces to cause the workpiece to conform in a further phase to the die set, the fluid pressure applied to within the workpiece being greater than that without the workpiece.

2. A method of forming according to claim 1 wherein the workpiece is secured in the die set at a point adjacent one end.

3. A method of forming according to claim 1 wherein the workpiece is secured in the die set at a point intermediate both ends.

4. A method of forming according to any one of claims 1 to 3 wherein the fluid pressure applied to within the workpiece is up to 2000 bar above the ambient pressure existing around the workpiece.

5. A method of forming according to any one of claims 1 to 4 wherein the solidifying liquid is a plastics material.

6. A method of forming according to claim 5 wherein the solidifying liquid is injected into the workpiece before being inflated by a further liquid, preferably a gas.

7. A method of forming as claimed in any one of claims 1 to 6 wherein the die is heated.

8. A method of forming as claimed in any one of claims 1 to 7 wherein the fluid pressure is applied to the workpiece in the die for between 15 and 20 seconds.

9. A method of forming material substantially as described with reference to the accompanying drawings.

10. A die for carrying out the method of any one of claims 1 to 9 substantially as described with reference to the accompanying drawings.