When it comes to this technique, it is worth going into some depth
There is a lot more to the term ‘deep-drawing’ than you might think. Stainless steel or materials such as Hastelloy® or titanium are very robust.
Deep-drawing them without heat, i.e. ‘cold’, poses a huge technical challenge. At Mankenberg, deep-drawing is undertaken using cup-shaped components which form complex casings for valves in combination with the turned and milled cast-iron components that we manufacture ourselves. In order to join all these components together, we use the Wolfram inert gas welding technique (WIG).
For the deep-drawing process, you need hydraulic presses which can shape the material cold using a high level of force. Up to now, the largest press we have used at Mankenberg can apply a force of 2500 kN. Or in other words: six fully loaded lorries.
But how do you get the complex valve from a flat blank?
The shape produced is basically the result of the tool used, which consists of a mould (to form the outer contour) and the punch (to form the inner contour). The space between the twon shapes is what contains the shaped cup following completion of the drawing process. A round blank which undergoes a three-dimensional shaping process produces a cup-shaped mould which is buckled on the bottom. As the material is unyielding, the blank is compressed between the blank holder and drawing ring and stretched again between the drawing ring and punch. T his means that the bottom is more or less the same thickness as the original blank although it decreases towards the bottom and increases towards the top. This process requires the utmost attention because if the tensile load is too great, the material can tear during processing.
Thimbles were deep-drawn from brass as early as 1500, rather than cast. Later, the deep-drawing process was used to manufacture cooking pans from stainless steel as part of massproduction processes. Today, deep-drawing is best known for its use in vehicle body construction in the automotive industry. Mankenberg first used this manufacturing technique in the manufacture of valves which – because of design and safety aspects – were technically regarded as pressure containers and therefore subject to particular demands. The dimensioning of the wall thicknesses of the casing was dependent on the pressure load that the valve would be required to withstand in practice.
Another difficulty was that cold froming, which encourages the disred metallurgical process of strain hardening in the material, results not only in the required higher materials strength, but also leads to increased brittleness. If several deep-drawing processes are undertaken consecutively, ‘austenisation annealing’ is used to remove the effects of strain hardening.
Thanks to the manufacture of this mass-produced material, the quality of the original blank is considerably higher than with cast materials, which may suffer from shrinkage, impurities and build-up of flaws in the mould. Deep-drawing does not have any negative impact on the high surface quality, meaning that this is exactly what our deep-drawn products are known for. Another advantage in comparison to cast iron is that stainless steel sheets (including deep-drawn) can be easily welded. This means that we can weld different components to create anything from small-scale production series to one-off special components. This makes the products highly versatile, especially when it comes to installing them in pipelines. In a nutshell, deep-drawing stainless steel is a manufacturing technique which proves its worth through entire product ranges.
A glimpse into the deep-drawing process
Deep-drawing is the use of tensile compression to form a hollow body from a sheet blank. Related techniques include flow turning or hydroforming. The deep-drawing process begins with the blank (the punching) of the so-called round blank, the thickness and diameter of which determine the geometry of the cup.
The interim result
If the right balance is struck between the punch force and the holding pressure, the fi rst draw results in the manufacture of medium-sized cups. The round blank is held in place with a pressure of around 60 bar while the punch creates the shape using a pressure of around 150 bar.
The round blank is deep-drawn to form a cup which changes the diameter and wall thickness of the base material.
The end result
The shape produced by the first draw has a diameter of approx. 210 mm. The hollow body then undergoes further deforming to create a cup with a diameter of approx. 160 mm. The ratio between the diameter and cup depth changes from approximately 2:1 to 1:1.
If the degree of deformation is too high, this can lead to stress cracks. Repeated skilful use of so-called stress relief heat treatment can be used to remedy this and enables deep cups to be manufactured.
The tools
The most important parts in the press used for deep drawing are drawing punch, the draw die and the counter holder. The drawing punch is screwed tightly to the press table. The draw die also functions as a blank holder. The counter holder, which is connected to the cushin using rods, positions the round blank centrally and also functions as a stripper plate. It is forced downwards through the draw die, while the blank fills the space between the drawing punch and draw die.
The first run
The round blank is placed in the press and the draw die is lowered to fix the blank in the place between the die and the counter holder. The holding pressure and punch force are administered precisely according to the pre-programmed values. Rge system unloaded and returned to the starting position ready for the next deep-drawing process.
Step by step
During the second draw, the shape of the draw die and force of the drawing punch act on the preformed cup. The forming time and frictional forces are now crucial to the success of this process. Mankenberg’s expertise in tool construction is invaluable when it comes to optimisation of the process parameters.
