Engineering tools

Gate shear-rate calculator — is your gate burning the material?

Everything the machine pushes must pass through the gate, and small gates at fast fills shear the melt violently. The shear rate through a round gate is γ̇ = 32Q / (πd³) — flow rate over diameter cubed — which is why a fraction of a millimetre on gate size changes the answer enormously. Exceed the material’s critical shear rate and you get burnt streaks, splay, broken fibres in glass-filled grades, and degraded properties that no downstream setting can repair.

Reading the result

Shear rate scales with the cube of gate diameter, so small changes act like levers: going from a 1.5 mm to a 2.0 mm gate cuts shear by more than half. It also scales directly with flow rate, so a slower fill helps — at the price of possible short shots and weld-line quality on thin walls. Heat-sensitive materials (rigid PVC, POM, flame-retardant grades) deserve the most respect here; glass-filled grades add a second concern, fibre breakage, which degrades stiffness even when the polymer itself survives.

Gate size is a system decision

The gate must freeze at the right moment too: too large and the holding phase runs long and the gate scars; too small and packing is cut off before sinks are fed — the working band is commonly 0.4–0.7× the local wall thickness. Shear rate, freeze time, and vestige location get balanced together in mould-flow analysis, which we run before cutting steel on every tool. Pair this tool with the shot-size and cycle-time calculators.