Clamp Force / Tonnage

Clamp close is the stage of the molding cycle where the clamp advances the moving platen to bring the mold halves together and lock them up before injection. It is the first motion of every cycle. ## The three speed phases 1. Fast approach: the platen moves quickly across most of the stroke to save cycle time. 2. Slow / mold protection: near touch it slows to a low-pressure creep so the controller can detect an obstruction — a stuck part or misplaced insert — before metal hits metal. This is "mold protect". 3. High-pressure lock-up: the toggle or ram builds full tonnage, applying the clamp force tonnage that holds the mold shut against injection pressure. ## Why it matters - Mold protection prevents expensive damage: if a part did not eject, the low-pressure phase senses the resistance and stops instead of crushing the tool. - The speed profile trades cycle time against safety — too aggressive a close risks the mold, too slow wastes time. - Only after lock-up do the injection stages begin. ## Related terms - See also: clamp, clamp force tonnage, molding cycle, injection stages, part ejection ## What is clamp close in injection molding? It is the cycle stage that closes and locks the mold before injection, in three phases — fast approach, slow mold-protection, and high-pressure lock-up to full tonnage. ## What is mold protection during clamp close? A low-pressure, slow phase just before the mold touches, so the machine can sense an obstruction (an unejected part or insert) and stop before damaging the tool. ## What happens after clamp close? Once full clamp tonnage is reached, the injection stages begin — first-stage fill, then pack and hold.

Estimated tonnage required is the clamp tonnage a given part needs to keep the mold shut during injection — the number you calculate before selecting a machine. It is estimated, not measured: you compute it from the part geometry, then choose a press with margin above it. ## How it is estimated Estimated tonnage = projected area × tonnage factor - projected area: the part-plus-runner area seen along the mold-opening direction (in² or cm²). - tonnage factor: an empirical pressure per unit area (tons/in²) that depends on the resin and the wall thickness / flow length. Example: 50 in² × 3 tons/in² ≈ 150 US tons; add ~10 % margin → choose a press of ~165–200 t. ## How it is used It drives machine selection: pick an injection molding machine imm whose rated clamp force tonnage comfortably exceeds the estimate. Too little tonnage and the part flashes; too much wastes energy and rules out otherwise-suitable presses. Confirm on the press, since real cavity pressure and venting shift the true requirement. ## Why it matters Getting this number right up front avoids quoting a job onto the wrong machine. It is the planning side of clamp force tonnage (the force itself) and feeds capacity and cost estimates. ## Related terms - See also: projected area, tonnage factor, clamp force tonnage, injection molding machine imm, flash ## What is estimated tonnage required in injection molding? It is the clamp tonnage a part needs, estimated as projected area × tonnage factor, used to pick a machine before running the job. ## How do you estimate required tonnage? Multiply the projected area by the resin's tonnage factor and add ~10 % margin; e.g. 50 in² × 3 t/in² ≈ 150 t → choose ~165–200 t. ## Is estimated tonnage the same as clamp force? It is the same quantity (tons of clamp force), but framed as the required value for machine selection; the running clamp force should comfortably exceed it.

Flash is the molding defect in which material escapes along the parting line, vents, ejector-pin clearances or between inserts, forming a thin film stuck to the part. It signals that internal mold pressure exceeded the clamp force locally or that sealing is poor. ## Common causes - Insufficient clamp force (actual tonnage < required tonnage) - Excessive injection or hold pressure - Injection velocity too high at end of fill - High melt temperature (resin too fluid) - Mechanical clearance: worn parting line, vents too deep - Misaligned mold or platen flatness out of tolerance ## How to detect - Visual inspection of part, especially around the parting line - Typical flash thickness: 0.03 – 0.3 mm - In multi-cavity tools, only some cavities may flash → imbalance ## Systematic fix 1. Verify actual clamp force (tie-bar sensor) 2. Reduce pressure / transfer velocity 3. Lower melt temperature 5 – 10 °C 4. If it persists: repair mold mechanics (re-grind plates, adjust vents) ## Cost of flash - Manual deflashing secondary op: $0.01 – 0.05 per part - Scrap if flash falls in a critical area - Accelerated mold wear due to poor sealing - Safety risk from sharp edges on technical parts

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)