Injection Molding Relative Viscosity Curve Calculator

Build a controlled relative-viscosity curve from fill time and peak plastic pressure measurements.

Formula and method

Relative shear-rate index = 1 / fill time. Relative-viscosity index = peak plastic pressure × fill time. These are comparative in-mold indices, not rheometer viscosity.

Inputs and units

Record fill time and peak plastic pressure from the same machine display for each velocity step. Convert hydraulic pressure to plastic pressure using the verified machine intensification ratio before entry.

Worked example

At 1.0 s fill time and 125 MPa peak plastic pressure, the relative-viscosity index is 125 MPa·s.

When to use the study

Use during a controlled fill-only study after melt temperature, shot size, transfer position and pressure limit are stabilized.

How to interpret the result

Compare the curve shape and repeatable low-index region. The lowest index is not automatically the production setting; confirm appearance, dimensions, shear sensitivity and machine velocity control.

Assumptions and limitations

The index is mold-, material-, temperature- and machine-specific. It is not absolute viscosity and cannot compare unrelated studies.

Validation guidance

Preserve machine, mold, material lot, melt/mold temperature, cycle state, units, instrument and sample order with the result. Repeat after any material, machine or tooling change.

FAQ

Can two molds share the same relative-viscosity index?

No. Compare points collected within the same controlled mold, resin, temperature and machine study.

Related workflow

Process Validation & Stability hub · Scientific molding validation workflow · Weight measurement guide · Gate seal and pressure-drop setup

Sources

PlasticsToday: Scientific Molding, Part 1 · M. Holland: Monitoring Pressure Loss · BASF: Injection-Molding Problems · NIST: Measures of Scale · NIST: Coefficient of Variation · PMC: Filling Imbalance in Multi-cavity Molds · Hingtung: Cavity Balance Study