stripper plate

A stripper plate is a hardened steel component mounted directly above or below the punch in a stamping or punching die. Its sole function is to hold the workpiece or scrap material firmly against the die table while the punch retracts, preventing the strip from rising with the punch and jamming or tearing. As the punch moves upward on its return stroke, the plate and its springs push back, releasing the material so the press operator or automated feed system can advance the next blank into position.

The plate itself is typically a flat, rectangular piece of tool steel, hardened to 48-52 HRC, with precisely drilled or reamed holes that allow the punch posts to pass through freely. Around these posts sit compression springs (usually chrome vanadium steel) that keep tension on the plate throughout the press cycle. The springs must be matched in stiffness and rated for millions of cycles without loss of force, or the plate will sit too low and bind against the punch or too high and fail to strip properly.

Stripper plates are essential in high-speed progressive dies where multiple operations occur in sequence on a single strip of material. A weak or worn plate causes material to stick to the punch, creating scrap, tool damage, and downtime. Conversely, a plate under excessive spring pressure wastes energy and accelerates die wear. In compound dies where a single station does multiple operations in one stroke, the stripper may have stepped recesses to accommodate different punch heights or shapes.

The term "stripper" comes from the action of stripping away, though the plate does not itself pull anything apart; rather, it strips the material away from the punch by maintaining contact with the workpiece while the punch withdraws. Some shops call this component a "knockout plate" when its springs are particularly stiff, though that term more properly refers to the ejector on the reverse side of a plastic injection mold.

Wear and maintenance of stripper plates are routine concerns in tool shops. Spring fatigue, loss of hardness near edges, and scoring from burrs on punch faces all reduce effectiveness. Replacement or refurbishment is typically done during die maintenance intervals, and proper shim adjustment when reassembling is critical to die function.