In this article we will discuss about the meaning and benefits of flex forming of metals.
Meaning of Flex-Forming:
High pressure sheet metal forming with flexible tools process, called as flex-forming is widely used in the aero-space and automotive industries, as well as in other industries where sheet metal parts are produced. Parts with complex shapes can be flexing formed.
This process is economically competitive with traditional forming processes because it requires only one rigid tool half. It also improves the quality of the end product. Flex-forming processes require only one rigid tool half. This may be a male block or a cavity die, either of which can be quickly manufactured from inexpensive materials.
The other tool half is a flexible rubber diaphragm, which forms the sheet metal part with high precision under the action of pressurized hydraulic fluid under pressure of upto 2000 kg/cm2 (Refer Fig. 6.18). Special presses with large volume enclosures are used to build up the required pressure and maintain it over a certain period of time.
ADVERTISEMENTS:
All parts of the press exposed to high stresses are wound with prestressed wire. The prestressing ensures that these parts of the press will only be subjected to pressure forces and will maintain their original shape. This allows light, compact presses to be built, which at the same time are notable for high operational reliability, safety and outstanding fatigue properties.
Flex-forming is a genuinely flexible manufacturing system for sheet metal forming. A possible production concept may include CAD/CAM manufacturing of tool halves, either directly on an NC milling machine or by casting from a model, followed by forming in a press and, finally trimming with laser equipment controlled by the CAD computer.
This concept allows economical sheet metal forming of small or medium parts series. The trimming may also take place in the press for certain production series.
ADVERTISEMENTS:
With flex forming, the stresses which are built up in the sheet during hand forming of prototype parts are largely avoided. And parts produced by flex forming resemble more closely those produced later on with the tools used in series production. Only a short lead time is necessary for preparing and testing the rigid tool half. Unlike traditional processes, on joint testing and matching of tool halves is necessary.
The rigid tool half is generally made from inexpensive materials such as plastics or low melting alloys, as the tool is backed up by hydrostatic pressure from the diaphragm. The choice of tool material depends mainly on the number of parts to be formed, their radii and the quality of the blank material.
The support given by the diaphragm over the blank’s entire surface ensures excellent control of the material flow, thus minimizing the risk of wrinkle formation. The blank surface in contact with the diaphragm will be completely free from scratches or other marks. Very narrow tolerances can be achieved, since draw clearances between tools are unnecessary.
The production time and the material costs for the tools may prove to be the decisive factors when choosing the optimum forming process for small or medium-scale series production. Same tool can be used to process sheets of different thickness.
ADVERTISEMENTS:
Rubber diaphragms ensure a uniform pressure distribution over the entire tool. As a result, deeper and more complex parts can be formed with greater accuracy using either block, cavity or expansion forming.
The flexible diaphragm is located in a horizontal cylinder. Natural rubber has proved to have a long life and outstanding elongation properties, permitting deep, complex parts to be formed in a few steps, and frequently in just one.
The diaphragm is mounted in a special frame, which can be easily exchanged in a few hours. If the diaphragm is damaged, for example by a sharp object, it is easy to repair it on site. To form several parts at the same time different rigid tool halves and blanks are placed on a rectangular press tray, which is then transported into the press.
A vertical deep-drawing press is used for deeper can- shaped or box-shaped parts. This press has a telescopic ram system carrying a moving punch. The flexible tool half is a diaphragm, located in the circular fluid form pressure unit.
ADVERTISEMENTS:
A similar flexible die forming process, but without a diaphragm, is hydro mechanical drawing, in which the blank acts as the seal against the pressure fluid. This process is more suitable for punch forming of complex parts. The production of parts with the flex-forming process involves two different radii, namely the radius formed by the right tool half and that formed by the flexible diaphragm.
When the blank is drawn over the rigid tool half, it is formed exactly according to the radius of the tool. The larger radius formed by the diaphragm which it presses the blank into the tool half is called the free forming radius. The minimum free forming radius determines the pressure needed to form the part.
The radius and deep drawing requirements of the automotive industry for today’s steel qualities and gauges may be met by selecting a forming pressure of up to 1400 kg/ cm2. Pressures of up to 2000 kg/cm2 offer advantages when forming parts from special high-strength alloys etc.
ADVERTISEMENTS:
Fig. 6.21 shows a press consisting of a horizontal cylinder, one or two rectangular press trays or tables, and electrohydraulic equipment. The latter may be installed in a separate, soundproofed room above the press cylinder. For pressure levels of between 1000 and 2000 kg/cm2 pressure intensifiers are used to raise the pressure from the 200 kg/ cm2 of the basic system.
The operator loads and unloads blanks and formed parts from either side of the press. The increase the production capacity of the press, tools and blanks, can be placed on pallets in advance. The pallets can then be transferred as required to the press, where they are automatically loaded and unloaded.
To make maximum use of the press capacity several tools are normally placed on each tray or pallet. Several parts can then be formed simultaneously during each press cycle. The cycle time is normally 1 to 3 minutes, depending on the forming pressure required, the volume of the pressure medium pumped into the diaphragm, and the installed pump capacity.
When the operator has placed the blanks on the topic, the press tray is run into the press, where the other tool half—the flexible rubber diaphragm with fluid back-up—is situated. Fluid is pumped into the press and forces the diaphragm to wrap the blanks around the rigid tool halves.
During the forming cycle the hydraulic pressure is gradually raised, being uniformly distributed over the surface of the blanks. As the pressure rises, the banks are slowly forced against the rigid tool half and assume its shape with close tolerances.
When the set pressure level is reached and the blanks have assumed their final shape, decompression takes place. The diaphragm returns to its initial position and the tray is moved out of the press for unloading of the parts. New blanks can then be placed over the tools.
The deep-drawing press consists of a vertical press frame made up of columns and yokes, a fluid form unit containing the flexible diaphragm, and electro-hydraulic equipment. Normally, only one part is formed during each press cycle. Press cycle times vary between 10 and 120 sec depending on the draw depth, the selected forming cycle, the daylight opening of the press and the installed pump capacity.
After the operator has placed the blank on the single rigid tool half, which rests on a telescopic ram system, the press is closed. When punch forming is used, a preset pressure level may be applied in the fluid form unit, the forming, pressure exerted by the flexible diaphragm being carefully controlled for the duration of the process.
This allows 50 per cent higher draw ratios and 50 per cent less material thinning than with conventional deep drawing using only rigid tooling. Parabolic shapes and the like are also easily formed in one step using this process.
The high uniform forming pressure achieved with the flex forming process improves the inherent formability of blank material. New metal alloys, including those used in the aerospace industry, demand ever-higher forming pressures. If the tool has loose pieces, undercuts can be formed simultaneously.
Benefits of Flex-Forming:
The benefits of flex-forming over conventional sheet metal forming processes include the following:
i. There is no need to match the tool halves with one another, as the flexible diaphragm fully adapts itself to the forming tool half.
ii. No aligning of the tools in the press in needed. The tool halves can be placed anywhere on the press tray.
iii. Parts can be formed with the same tool half from blanks of different thicknesses. For example, a tool intended for forming a steel part of a certain thickness may also be used to form the same part from a thicker aluminium blank. The resulting strength is the same.
iv. Several parts may be formed simultaneously with different tools during the same press cycle. Only the size of the press tray limits the number of tools which can be used.
v. Tools can be made from inexpensive materials such as hard-wood, epoxy resin, polyurethane, low melting alloys, etc. The reason for this is that the rigid tool half is subjected to a uniformly distributed hydrostatic pressure, exerted by the flexible diaphragm. This substantially reduces local stresses in the tool compared with conventional forming.
vi. The choice of material for the tool is determined by the number of parts to be produced, their complexity, the blank material and the thickness.
vii. Tools can be easily modified to accommodate design changes, since only one tool half has to be replaced and no matching with the other tool half is necessary. Minor modification can be made directly on the press tray.
viii. The diaphragm supports the blank over its entire surface during forming, which leads to better control of the material flow and minimizes the risk of wrinkle formation on the part.
ix. As one side of the part will never come into contact with the rigid tool half, scars, drawing scratches, tool marks, etc. are eliminated completely on this side.
x. The formed part will assume the exact shape of the tool and can be produced with narrower tolerances than is possible with conventional processes, as the blank is passed in or drawn over a tool half under high pressure. What is more, conventional processes require a gap between the tool halves of 15- 20% of the thickness of the blank to permit variations in this thickness.
xi. Undercuts are pressed directly, since the diaphragm can assume any shape whatsoever and then return to its original shape.
xii. Sheet metal parts can be trimmed directly in the fluid cell press with the rigid tool half provided this is fitted with cutting edges.