Estimated Tonnage Required

Clamp force (clamping force or tonnage) is the force the machine applies to hold the mold halves shut against the pressure of the melt during injection and pack. If it is lower than the force trying to push the mold open, the parting line separates and the part flashes — that is why sizing it correctly is a first-step machine-selection decision. ## How to calculate clamp tonnage The standard estimate: Clamp force = projected area × clamp factor - projected area is the part-plus-runner area seen along the mold-opening direction (in² or cm²). - The clamp factor (see tonnage factor) is an empirical pressure in tons per in² (or bar of cavity pressure). Example: a 50 in² projected area at 3 tons/in² needs 50 × 3 = 150 US tons; add ~10 % margin and pick a press rated near 165–200 tons. ## Typical clamp factors | Resin / situation | Clamp factor (tons/in²) | |---|---| | Easy-flow commodity (PE, PP) | 2–3 | | General engineering (ABS, PA, PC) | 3–5 | | Thin-wall, long-flow, glass-filled | 5–8 | Metric rule of thumb: clamp force in kN ≈ projected area (cm²) × cavity pressure (bar) ÷ 10. ## Why it matters - Too little: flash, dimensional drift and the mold opening during pack — see flash. - Too much: crushed vents and shutoffs, faster mold wear, wasted energy and ruling out otherwise-suitable presses. Most shops size the press from the estimated tonnage required plus a safety margin, without overspending on an oversized machine. ## Related terms - See also: projected area, tonnage factor, estimated tonnage required, flash, injection molding machine imm ## What is clamp force in injection molding? It is the force that keeps the mold closed against injection pressure, quoted in tons (or kN). Too little lets the mold open and the part flashes. ## How do you calculate clamp tonnage? Multiply the projected area by a clamp factor (tons/in²) and add ~10 % margin. A 50 in² part at 3 tons/in² needs about 150 tons, so you would choose a press around 165–200 tons. ## What happens if clamp force is too low? Melt pressure forces the parting line open, giving flash, heavier and dimensionally unstable parts, and eventually damage to the shutoff faces of the mold.

Projected area is the sum of the flat areas that a part —and the runners in cold-runner molds— occupies when projected onto the parting plane. It is the key input for calculating the clamp tonnage of the injection press. ## What it is used for The required clamp force comes from multiplying projected area by the specific cavity pressure (tonnage factor) of each resin: > Tonnage (t) = Projected area (cm²) × Tonnage factor (t/cm²) A safety margin of 10 – 20 % is applied to avoid flash at end of fill or due to imbalance between cavities. ## Typical tonnage factor by resin - PE / PP: 2.5 – 4 t/cm² - PS / ABS: 3 – 5 t/cm² - PA / PC: 4 – 6 t/cm² - POM: 4 – 6 t/cm² - Fiber-filled grades: +20 – 50 % - Thin walls (<1 mm) or long flow: +50 – 100 % ## How to measure projected area - In CAD: project the 3D model onto the mold's XY plane, export as a sketch and sum areas. - In 2D: planimetry on the parting plane. - In flow analysis (Moldflow, Moldex3D, Cadmould): automatic calculation with the resin factor. ## Common mistakes Forgetting to include runners in cold-runner molds (underestimates tonnage by 5 – 15 %), using the flat area instead of the projected one on parts with sloped walls, and changing resin without recomputing the factor.

Tonnage Factor is the specific cavity pressure needed to keep the mold closed during injection, expressed as tons per square centimeter of projected area. It is the constant that links clamp force to part geometry and resin choice. ## Basic formula > Tonnage (t) = Projected area (cm²) × Tonnage factor (t/cm²) Apply a 10 – 20 % safety margin for process variation and cavity imbalance. ## Typical tonnage factor by resin - LDPE, HDPE: 2.0 – 3.5 t/cm² - PP: 2.5 – 3.5 t/cm² - PS: 3.0 – 4.5 t/cm² - ABS, SAN: 3.0 – 5.0 t/cm² - PA, PC: 4.0 – 6.0 t/cm² - POM, PBT: 4.5 – 6.0 t/cm² - PEEK, PPS: 5.0 – 7.5 t/cm² - Fiber-reinforced: +20 – 50 % vs. unfilled ## Factor modifiers - Thin wall (<1 mm): +50 – 100 % - Very long flow (L/T >150): +30 – 80 % - Low mold temperature: higher viscosity → higher factor - High injection velocity: shear thinning may lower the factor - Hot runner vs. cold runner: cold runner adds to total projected area ## How it is determined - Supplier data (technical data sheets) - Flow analysis software (Moldflow, Moldex3D, Cadmould) computes actual cavity pressure - Experience with similar parts - Cavity-pressure sensors in instrumented molds ## Common mistakes - Using a generic factor without adjusting for thickness or flow length - Forgetting to include runners in cold-runner molds - Not accounting for fiber reinforcement when switching grades - Confusing the factor with injection pressure (they are different)