Last updated: 2026-09-02 · 5 min read

Injection Molding Machine Tonnage — How Do You Calculate the Right Clamp Force for Your Mold?

Clamp tonnage is calculated by multiplying the projected area of the part (including runner) by the cavity pressure — typically 3–5 tons per square inch of projected area for standard resins, and 5–8 tons per square inch for engineering resins. Selecting the right machine tonnage matters: too little clamp force causes flash; too much clamp force wastes energy, crushes mold vents and slows cycle time. The correct machine is the smallest one that holds the mold closed without flash — not the largest one available.

Process parameter — A controllable variable in the injection molding process: melt temperature, injection speed, holding pressure, cooling time, mold temperature. Each parameter affects part quality — deviations cause defects. Parameters are set during T1 and locked for production. See also related guide.

Key Facts

  • Projected area rule: the projected area is the part footprint viewed from the parting line direction — including the runner system. A 10 sq.in. projected area at 4 tons/sq.in. needs 40 tons of clamp force.
  • Cavity pressure varies by resin: PP/PE at 2–3 tons/sq.in.; ABS/PS at 3–4 tons/sq.in.; PC/PA/POM at 4–6 tons/sq.in.; glass-filled resins at 5–8 tons/sq.in. Higher viscosity resins require more clamp force.
  • Machine utilization: the mold should use 20–80% of the machine’s clamp capacity. Below 20%, the platens do not close evenly; above 80%, there is no safety margin for process variation.

    • Process parameter — A controllable variable in the injection molding process: melt temperature, injection speed, holding pressure, cooling time, mold temperature. Each parameter affects part quality — deviations cause defects. Parameters are set during T1 and locked for production. The related guide covers adjacent topics.

    Tonnage Calculation by Resin

    Resin Family Tons/sq.in. Example (10 sq.in. part)
    PP, PE (commodity) 2–3 20–30 tons
    ABS, PS, HIPS 3–4 30–40 tons
    PC, PA, POM 4–6 40–60 tons
    Glass-filled PC, PA, PBT 5–8 50–80 tons
    PEEK, PPS, PEI (high-temp) 6–10 60–100 tons

    Machine Selection Rules

  • Platen size: the mold must fit between the tie bars with clearance for clamp movement — the mold dimensions should be 70–90% of the platen size. A mold too large for the platen cannot be mounted; a mold too small wastes platen area and energy.
  • Shot size: the mold’s total shot weight (part + runner) should be 20–80% of the machine’s barrel capacity. Below 20%, the material stays in the barrel too long and degrades; above 80%, the machine cannot maintain consistent shot-to-shot volume.
  • Daylight and stroke: the machine’s daylight (max platen opening) must exceed the mold height plus the ejection stroke plus clearance for part removal. A machine with insufficient daylight cannot eject the part.

    • What This Means for Your Quotation

    The quotation should specify the machine tonnage and shot size required for the mold — so the per-part rate reflects the actual machine cost. A 50-ton machine runs at $30–50/hour; a 500-ton machine runs at $80–150/hour. The machine selection affects the per-part cost directly, and the quotation should document the machine size so the buyer understands the cost basis.

    Frequently Asked Questions

    Can a mold run on a larger machine than required?

    Yes, but it wastes energy and may cause problems. A machine at less than 20% of its clamp capacity does not close the platens evenly, causing uneven parting line pressure and flash. The injection unit may also be too large for the shot weight, causing material residence time that degrades heat-sensitive resins. The correct machine is the smallest one that fits the mold’s clamp, shot size and platen requirements.

    How does machine tonnage affect cycle time?

    Machine tonnage does not directly affect cooling time, but higher clamp force crushes mold vents (reducing venting efficiency) and increases platen deflection (affecting part flatness). The optimal clamp force is the minimum that prevents flash — typically 10–20% above the calculated requirement. Excess clamp force adds no quality benefit and increases energy consumption.

    What is the difference between hydraulic and electric machines?

    Hydraulic machines use hydraulic pumps for clamp and injection — less expensive but consume more energy (especially at idle). Electric machines use servo motors — more precise repeatability, 30–50% less energy consumption, and faster cycle times, but cost 20–40% more than hydraulic machines. Electric machines are preferred for tight-tolerance and thin-wall parts; hydraulic machines are adequate for general-purpose molding.

    Key Takeaways

    1. Clamp tonnage = projected area × cavity pressure (2–3 tons/sq.in. for PP, 4–6 for PC, 5–8 for glass-filled) — the correct machine is the smallest one that holds the mold closed without flash.
    2. Machine selection must also check platen size (mold at 70–90% of platen), shot size (20–80% of barrel capacity) and daylight (must exceed mold height + ejection stroke).
    3. The quotation should specify the machine tonnage and shot size — so the per-part rate reflects the actual machine cost, from $30–50/hour for small machines to $80–150/hour for large ones.

    Send aCatMould the part drawing and resin specification, and the quotation specifies the machine tonnage, shot size and per-part rate — so the machine selection is based on the calculated clamp force and projected area, not default practice.

    Sources
    Plastics Technology — clamp force calculation and machine selection
    SPE — injection molding machine sizing guidelines
    MoldMaking Technology — machine selection for mold design

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet