A knockout, also called an ejector, is the mechanism that positively pushes a formed part or a slug out of a die cavity or off a punch after the working stroke, so the press can repeat the cycle. After drawing, forming, or coining, the part tends to stick in the cavity or grip the punch from friction and the natural clamping of the formed metal. Without a means to free it, the part would ride up with the tooling, jam, or have to be removed by hand — none of which is compatible with automatic running.
Knockouts get their motion from several sources. Spring or nitrogen knockouts store energy on the downstroke and release it to eject on the upstroke. Positive knockouts in the upper die are driven by the press knockout bar (a fixed bar in the ram that trips a plunger near the top of the stroke), giving a firm, timed push regardless of friction. Cam-actuated knockouts use the press motion to drive ejectors at an angle, useful for freeing undercut or side-formed features. The choice depends on how much force is needed and exactly when in the stroke the part must release.
Timing is as important as force. The knockout must act after the forming is complete but in coordination with the strippers and pressure pads, so the part is freed cleanly and presented for removal by gravity, an air blow, or a transfer system. If a knockout fires too early it can distort the part; too weak and the part hangs up; mistimed against the stripper and the strip can buckle. On a draw die, for instance, a spring-loaded knockout in the bottom of the cavity lifts the finished cup just enough for an air jet or shuttle to clear it before the next blank arrives.
Knockouts work hand in hand with strippers, which pull the strip off the cutting punches, and pressure pads, which hold material flat during forming and help lift it afterward. Together these ejection devices keep material flowing cleanly through the tool, and their reliable operation is one of the prerequisites for unattended, high-rate stamping. Undersized springs, sticky cavities, or poor venting that creates suction under a drawn part are common causes of ejection trouble.
Sizing a knockout means estimating the force needed to break the part free and then providing a margin above it. For a drawn cup the resistance comes from friction and the slight clamping of the formed metal on the punch or in the cavity, plus any vacuum that forms under a close-fitting part; venting the cavity with a small air passage eliminates that suction and can dramatically cut the required force. Spring-driven knockouts are then chosen so their force at the ejection point exceeds the release resistance with reserve, while staying within the spring's safe deflection and fatigue life. A practical example is a vented draw cavity fitted with a heavy die spring under the cushion pad: as the ram rises, the spring lifts the finished cup just clear of the cavity, the vent breaks the vacuum so it releases cleanly, and an air blast or transfer finger sweeps it away before the next blank lands — the whole cycle repeating automatically every stroke.