Engineering Wiki

Extrusion Die Swell & DIN 16941 Tolerances

Understanding the Barus effect and managing dimensional accuracy in continuous plastic profile extrusion.

Unlike injection molding, where molten plastic is forced into a closed steel cavity, plastic profile extrusion is a continuous process where the polymer is shaped by passing through a 2D die and cooling in open air or a water bath. The greatest challenge in this process is managing Die Swell.

The Physics of Die Swell (The Barus Effect)

When a polymer melt is forced through the narrow land of an extrusion die, its long molecular chains are subjected to extreme shear stress. They stretch and align parallel to the direction of flow. The moment the plastic exits the die and enters atmospheric pressure, that shear stress is removed.

The polymer chains immediately attempt to return to their natural, coiled state. This causes the profile to contract in length (the direction of flow) and expand in cross-sectional thickness. This expansion is called die swell.

Engineering Compensation: If a client requires a 10mm thick Rigid UPVC profile, the toolmaker cannot simply cut a 10mm hole in the die. Depending on the melt temperature, extrusion speed, and polymer viscosity, the die opening might need to be cut at 8.5mm to allow the plastic to swell exactly to the target 10mm dimension.

DIN 16941 Tolerance Standards

Because extrusion relies on managing die swell and pull-rate tension rather than a closed cavity, it cannot hold the same tight tolerances as CNC machining or injection molding. The global standard for extruded profiles is DIN 16941.

DIN 16941-2A (Rigid Profiles - e.g., UPVC, ABS, PC)

DIN 16941-2B (Flexible Profiles - e.g., Flexible PVC, TPE)

Flexible materials experience greater die swell and are highly sensitive to puller tension. Consequently, their standard tolerances are significantly looser.