Die Science: Friction and heat in metal stamping

Most materials being deep drawn or stretched behave in a similar fashion. This explains why complex deep-drawn parts such as sinks, bathtubs, and wheelbarrows usually are made in slower hydraulic presses. The excessive speed of a typical crank-drive mechanical press often causes splitting because the material needs time to flow.

The point at which the punch contacts the material during deep drawing is critical. At this point in the press stroke, the material is forced to go from zero velocity to a value equivalent to the ram speed—in zero time. If there is not enough time to initiate the metal flow, the result is excessive stretch and potential splitting.

Metal cutting, on the other hand, usually is performed quickly. Think of metal cutting as metal forming to failure. In metal cutting, you want the material to fail, so excessive speeds are desirable.

The Game-Changers: Heat and Friction
Heat and friction are generated in all metal cutting and forming operations. The amount generated is a product of many variables, including the material type and thickness, the forming geometry, the tool steel type, the friction between the sheet material and the die, the forming velocity, the forming severity of the feature, and the behavior of the lubricant.

Heat. Heat can be a friend or a foe in a stamping process. An increase in heat can improve the ductility and performance of the material you are forming. This is why some dies need to warm up to perform adequately. When you want to bend a large piece of angle iron, what do you do? You heat it up, and it bends easily.

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