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Metals
Punching in metal fabrication is the process of using a machine to press a shape through a sheet of metal and into a die to create that shape in the metal. This is most commonly done by use of a turret, a computer numerical controlled machine that houses tools and their corresponding dies in a revolving indexed turret. These machines use hydraulic, pneumatic, or electrical power to press the shape with enough force to cut the metal.
A misconception about punching is that the shape does the cutting, when in fact the shape presses the material into a die that cuts the metal. The die is also given a tolerance that is measured in thousands of an inch.
Punching can be better understand as pressing the material against a die with a huge force,this force pushes the material into the die and shears off the waste material.
Plastics
Punching in plastics fabrication usually refers to the removal of scrap plastic from the desired article. For example, in extrusion blow molding it is common to use punching dies to remove tails, flash (scrap plastic) and handle slugs from bottles or other molded containers.
In shuttle machinery, the containers are usually trimmed in the machines, and finished containers leave the blow molding machine. Other blow molding equipment, such as rotary wheel machinery, requires the use of downstream trimming. Types of downstream trimming equipment include detabbers for tail removal, rotary or reciprocating punch trimmers, and spin trimmers.
"Punching"