Viscosity curve test: choose injection speed by measurement, not guesswork

The plastic melt is a fluid that thins as speed increases (shear-thinning). It behaves very viscous at low speed, and more fluid at high speed. That's why choosing injection speed is really about choosing the region where viscosity is most stable.

How is the test done?

1. Set a dosage and V/P changeover that fills the cavity to 95–99% (no holding pressure, or very low).
2. Increase injection speed step by step from low to high (e.g. 10 different speeds).
3. At each speed, record the fill time and the peak injection pressure at the moment of changeover.
4. Calculate relative viscosity: fill time × pressure (or a value normalized by volumetric flow rate).
5. Plot relative viscosity against speed.

What does the curve tell you?

• At low speeds the curve is high: the material is viscous, the frozen layer is narrowing the mold, filling is slow.
• In the middle region the curve flattens: viscosity is insensitive to speed. This is the most stable region for the process.
• At very high speeds the curve may rise again, or problems like burning, jetting or flash appear.

Choosing speed in the flat region gives a process that's resistant to variables like material batch, temperature fluctuation and machine speed sensitivity.

Things to watch out for

• The test should be done while mold temperature and melt temperature are held constant.
• If the machine's pressure limit kicks in as speed rises too much, the curve becomes artificial; keep the limit high enough.
• In thin-walled parts the flat region is narrow; in thick parts it's wide.
• Different batches of the same material can give different curves; repeat the test on batch change for critical jobs.

Speed unit and scale

You can choose speed in mm/s (screw advance), but volumetric flow rate (cm³/s) lets you compare across different screw diameters: speed × screw cross-sectional area. When moving the same mold to a different screw diameter, keep the flow rate the same.

Note: The speed obtained from the curve is a starting value; it should be evaluated together with part quality (burning, jetting, flash).

The calculation tool calculates a starting speed and flow rate for your machine and part; you can fine-tune with this test during the trial run.