Select injection molding machine tonnage based on clamping force, not shot weight. The core formula is: Clamping Force (tons) = Projected Area (cm²) × Cavity Pressure (bar) ÷ 1000, plus a 10–20% safety margin. Cavity pressure is determined by the material (PP/PE: approx. 300–400 bar; ABS: 400–500; PC: 500–700; glass-fiber reinforced: 700+). Shot weight is used only to determine injection capacity—calculate the projected area first, then select the tonnage.
I.
Core Formula
Clamping Force (tons) = Projected Area (cm²) × Cavity Pressure (bar) ÷ 1000 (industry-standard formula). Projected Area: The projection of the part and runner system onto the parting line (the “shadow” area viewed along the mold opening/closing axis). Multi-cavity: Sum of the areas of all cavities plus the cold runner system. Safety Margin: Add 10–20% to the theoretical value (industry consensus)—the formula yields the minimum requirement (in practice, select the next size up).
II. Cavity Pressure by Material
PP/PE: Approx. 300–400 bar (low viscosity, good flow); ABS/HIPS: Approx. 400–500 bar; PC/PC-ABS: Approx. 500–700 bar (high viscosity); Glass-fiber reinforced materials: 700 bar+ (poor flow, high pressure); Thin-walled parts: Use the upper limit of the pressure range (long flow paths/thin walls require higher clamping force)—the more viscous the material, the higher the cavity pressure (for the same surface area, PC requires >50% more tonnage than PP).
III.
Quick Empirical Calculation
Imperial rule of thumb: 1–2 tons/in² for thick-walled rigid plastic parts, 3–4 tons/in² for thin-walled parts (Tat Ming); Estimation: Rectangular parts = Length × Width (extract projected area directly from CAD); Example: PP part with 500 cm² projected area (350 bar) ≈ 500 × 350 ÷ 1000 = 175 tons + 20% ≈ 210 tons (select a 250-ton machine); Misconception regarding shot weight: Shot weight only determines injection capacity (large, thin-walled parts might require low tonnage but high shot volume)—quick calculation yields tonnage in 10 seconds (stop asking only about shot weight).
IV.
Consider Platen Dimensions
Tie-bar spacing: Mold dimensions must fit within the tie-bar spacing (must physically fit); Platen dimensions: Mold mounting dimensions must match (locating ring/ejector holes); Shot capacity: Plasticizing capacity must exceed the weight of the part plus the runner system (ensure sufficient shot weight); Injection pressure: High-pressure thin-walled parts require a machine capable of high injection pressure; Mold opening stroke: Sufficient ejection distance (for deep-cavity parts)—sufficient tonnage does not guarantee the mold will fit (verify platen size, tie-bar spacing, and stroke).
V. Selection Steps
First, calculate the projected area (extract from CAD or use length × width + runner system); second, determine the cavity pressure (consult material-specific tables); third, apply the formula plus a safety factor (to obtain the theoretical tonnage); fourth, round up to the nearest standard tonnage (e.g., 160, 250, 320, 450); and fifth, verify mold dimensions (tie-bar spacing, platen size, and shot capacity). Regarding injection molding machine tonnage: Clamping Force = (Projected Area × Cavity Pressure ÷ 1000) + 20%.
Follow the calculation formula, select based on standard tonnage, and verify platen dimensions (do not select the machine based solely on shot weight).
