Capillary Rheometry Data Corrections for Polymer Processing Accuracy
Correcting raw capillary rheometry data for entry losses, shear gradients, and wall slip prevents costly tooling rework and protects gross margins.
Extrusion Die Sizing
Meaning
Dimensional calibration of the exit orifice of a shaping tool ensures that the extruded polymer profile meets strict geometric tolerances after cooling. Process engineers use extrusion die sizing to calculate the required dimensions of the metal flow channels to counteract post-extrusion swell and thermal shrinkage. The technique establishes the precise geometry needed to yield a target profile at a specified production speed.
If this calculation is done incorrectly, the resulting product will fail dimensional audits and require expensive tooling recuts.
Swell Compensation
Viscoelastic recovery causes polymer melts to expand upon exiting a die orifice. Through extrusion die sizing, the tooling designer modifies the land length and shear rate to control this expansion. Long die lands help align polymer chains and reduce elastic memory.
This action reduces the discrepancy between the die shape and the final profile.
Thermal Calculation
Cooling contraction must be anticipated based on the specific thermal expansion coefficient of the thermoplastic resin. When performing extrusion die sizing, the volumetric change from the melt temperature to the ambient room temperature is factored into the channel coordinates. Semicrystalline polymers exhibit higher shrinkage rates than amorphous polymers due to molecular chain ordering.
Consequently, the sizing of the tool must be customized to the specific resin grade being run rather than a generic polymer class. This rigorous adjustment prevents the production of undersized profiles that cannot be fitted into assemblies.
Processing Limit
Operating window limits restrict the range of output rates where a single die geometry remains effective. High extrusion speeds change the shear stress and alter the swell behavior of the melt. This variation implies that a single tooling design cannot accommodate widely different output rates without causing dimensional drift.