Finding the right pressure from speed (viscosity curve and V/P)

What is this calculation used for?

Determines by measurement the injection speed to enter on the machine and the pressure that goes with it. At the first trial of a new mold, or when the material or mold changes, it answers "at what speed and with how much pressure will I fill?"; it gives the pressure limit and a starting range for the holding pressure.

How do I take the data on the machine?

• Set the part to be filled about 95–99 %, set holding pressure and time to 0, and keep the V/P changeover position constant.
• Take shots at 5–6 different speeds (e.g. 10, 20, 40, 60, 80, 100 mm/s); make sure the machine is running steadily before each shot.
• At each speed write down the fill time and the peak hydraulic (or cavity) pressure; use the same type of measurement for all.
• The calculation finds the plateau where the curve levels off; choose the speed from this region.
• Enter the resulting pressure limit and the starting holding range on the machine; determine holding time and gate freeze time separately by testing.

How is it calculated?

• Relative viscosity = fill time (s) × peak pressure (bar)
• Plateau = speeds within 5 % of the lowest relative viscosity
• Injection pressure limit = peak pressure at the selected speed × (1 + margin)
• Starting holding pressure = 50–70 % of the peak pressure (the exact value is set with the gate freeze test)

Results are general engineering estimates; the machine catalog, material data sheet and trial run take precedence.