Can crushers need a hard, solid surface against which the user can apply pressure. Depending on the model of the can crusher and how it exerts pressure---through direct pressure, a lever or hydraulics---the manufacturer may use materials like steel, alloy metals or even hard plastic for the striking surface.
The design of the can crusher must allow the striking surface to remain fixed while a lever or other crushing mechanism extends into it; while designs vary from device to device, manufacturers typically attach the striking surface near the opening where the can is inserted, secured to the rest of the device with a parallel pair of metal connectors.
Can crushers must have some way to amplify a user's force and apply the amplified pressure to the can. Designs vary from device to device, though typical inexpensive units use simple machines.
When you pull a lever, it revolves around a pivot point to rotate a smaller extension onto the top of the can. This basic physics principle, a long lever rotating around a pivot, magnifies the user's force input to produce enough pressure to crush the can against the striking surface.
Other, less expensive models simply use a sliding striker to apply direct pressure to the can, crushing it against the striking surface.
More luxurious models use electricity to compact cans. These models employ electric or hydraulic motors.
The user places a can in an opening on the device; this opening allows the can to lie horizontally, with a pressure plate on one end and a striking surface on the other. When the user turns the device on, the motor pulls the pressure plate inward toward the striking surface.
As the pressure plate moves toward the striking surface, it exerts the motor's force on the can, crushing it.
Inexpensive can crushers generally allow the can to simply fall to the floor. More advanced models have an attached container into which the crushed can falls.
When the user has crushed enough cans, he simply removes and empties the container.