Backward Extrusion

Backward extrusion is a cold forming operation in which a punch is forced into a confined blank, causing the metal to flow in the opposite direction to punch travel — backward and upward around the punch. The result is a hollow cavity: a cup, a deep hole, or a tubular section formed without any material removal.

How It Works

The blank is placed in a die cavity that constrains its outer diameter. As the punch descends and penetrates the blank, the metal cannot flow forward (the die base blocks it) or outward (the die walls contain it). The only direction available is backward — upward, around the punch.

The punch geometry determines the shape of the cavity:

• A flat-nosed punch creates a flat-bottomed hole
• A pointed punch creates a conical bottom
• A punch with a radius produces a radius bottom

The extrusion punch determines the inner diameter of the hole; the die insert controls the outer diameter of the part.

Key Parameter: Wall Thickness

Wall thickness is critical in backward extrusion. Too thin a wall risks collapse or fracture; too thick wastes material and increases forming load. Wall thickness is defined by:

Wall = (Outer Ø − Inner Ø) / 2

The ratio between punch diameter and die diameter determines how the metal flows and what force is required. A typical guideline is that the minimum wall thickness should be at least 15–20% of the outer diameter, though this varies by material.

Typical Applications

Backward extrusion is used to produce:

• Socket head cap screws (the hex socket cavity)
• Hollow rivets and tubular fasteners
• Deep holes and blind bores in formed parts
• Cup-shaped preforms for subsequent operations

Challenges

Backward extrusion demands precise alignment between punch and die. Any lateral offset produces an off-center hole, eccentric wall thickness, and rapid punch wear. The punch is subjected to very high compressive stress at its tip — especially when the hole is deep — making material selection and surface treatment of the punch critical.

Chevron defects — internal bursting along the centreline of the part — can occur in backward extrusion when too many extrusion passes are applied, area reduction per pass is too high, or the die angle geometry is unfavourable.

Relationship to Manzoni Technology

The Manzoni Extra Deep Forming® system was specifically developed to extend the forming depth achievable on a standard progressive header. The transfer fingers perform an unusually wide and quick movement that allows the punch to penetrate deeper into the die, enabling the production of parts with extra-deep holes that would normally require a larger, more expensive machine.