Deep drawing
Deep drawing (also called Ronde, foil, plate, sheet or board ) to DIN 8584 the tensile compressive forming of a sheet metal blank into a hollow body open at one end or early hollow body in such a reduced cross section without deliberate alteration of the sheet thickness. A round cut is also called Ronde.
Deep drawing is one of the most important sheet metal forming and is used both in mass production and in small batches, such as in the packaging and automotive industries as well as in aircraft.
Thermoforming of plastics is often known colloquially as deep drawing, however, can not be compared with the deep drawing of metals. In deep drawing slips on the material, a wall thickness change is not provided. Changes in wall thickness is only for Abstrecktiefziehen, here the bottom is thicker than the wall, as the first deep drawn cup is then pulled through ironing rings. The product is thinner and higher. (Example: Preparation of beverage cans ).
The deep drawing can be divided into three areas:
- Deep Drawing with shape tools ( drawing ring, punch and blank holder )
- Deep drawing with active media (gases, liquids)
- Deep drawing with active energy (eg high-speed forming)
Deep drawing with tools
The classic and preferred method is deep drawing with rigid tools from the tool shop. For deep drawing presses are used here. Which is necessary for forming the press force by means of a plunger on the bottom of the deep- drawn part to be produced on the base curve and the side portion (flange) in the actual forming region directed ( the range between the drawing ring and retainer). The transformation is performed by radial tensile stress and thus caused tangential compressive stresses. Due to the compressive stresses there is a reduction in diameter, such as a round plate. Due to the radial tensile stresses in the forming a sheet thickening is avoided. The hold is intended to prevent wrinkling by the upsetting.
With the stamp of the sheet metal blank is determined by the pull ring (also called a template ) button. The blank holder prevents the formation of wrinkles on the drawn part. There are also drawing strips and Ziehwülste / drawbeads used to improve the effectiveness of hold-down.
The edges of the punch and pull ring must be rounded, otherwise the plate would snap. If the curves are too big, the sheet can be no longer held by the hold-down from the train at the end. The result is wrinkling. The positive radius of the stamper must be less than the negative radius of the die, because the punch will cut.
When the final drawing depth by a single train can not be reached, then pulled in several stages.
Deep drawing with active media
A modification of the classical drawing process is the hydromechanical deep drawing. A pressure- regulated water cushion replaces the template. The sinking stamp of the upper die presses the sheet metal blank to a water cushion, pulls them during immersion with himself and brings so exactly the desired geometry on the drawing portion.
Due to the distributed compression of the sheet to the punch by the active medium, the position of the critical pull-in range of the workpiece toward the bottom moves to the drawing radius. Therefore, higher draw ratios than the classical drawing process can be realized, all at a lower cost due to the relatively small installation space. However, the achievable compression forces are lower than in conventional systems, which is why only a limited range of sheet metal parts can be finished with this technology.
Effects in deep drawing
In deep drawing the workpiece shows different effects. The molecules are substantially shifted from each other, which leads to resistance changes. Anisotropic ( direction-dependent ) material properties affect the device performance. This can be demonstrated by measuring the various forces that are needed to rip a wall piece from a deep-drawn part (eg yoghurt pots ): Plastics are stretched in the direction of elongation, the macromolecules of which depend partly parallel to each other in the direction of the force from. For semi-crystalline polymers wherein the degree of crystallinity is increased.
A partially desired effect is the establishment of latent stress in the workpiece ( work hardening in metals ).
Examples of classic deep-drawn parts are automotive body parts, the body part almost always comes a combination of the classical deep drawing and stretch forming are used. We call this combination therefore also body dragging.