Clamp Force Calculator
Estimate the clamping tonnage your part needs — before you book machine time.
Estimates the clamping force a mould needs to resist melt pressure trying to open it, using whichever pressure information you actually have at this stage of the project — from a material-based estimate before any trial exists, through to your own measured process data. Use it as a starting point for press selection — confirm against the machine spec and, where it matters, a trial.
Projected area, explained
Result
Check the highlighted field.
Machine size
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How this is calculated
This calculator offers three tiers, matched to how much pressure information you actually have at this stage of a project — not two competing methods where one is "the real calculation" and the other a rough check. Use whichever fits:
- No cavity pressure known (earliest stage) — a material-specific clamping factor stands in for a pressure figure.
- Yes, estimate (material chosen, no trial yet) — a typical published cavity pressure for that material.
- Yes, real data (trial or process data exists) — your own measured or process-set cavity pressure.
Tiers 2 and 3 — cavity pressure. Force = projected area × cavity pressure × safety factor, with a unit conversion folded in (1 bar = 10 N/cm²):
Force (N) = Projected area (cm²) × Cavity pressure (bar) × 10 × Safety factor
Force (tonnes) = Force (N) ÷ 9806.65
This is the same clamp-force formula used in RFT's own process-development tooling — sourced directly from that app for this calculator, not re-derived.
Projected area breakdown. The per-part shape (after subtracting its own holes/cored sections) repeats once per cavity; the runner system and any external slider area are added once, as a property of the whole tool layout, not multiplied by cavity count:
Total area = (Per-part area − Holes) × Cavities + Runner + Slider
Tier 1 — clamping factor. Force = a material-specific clamping-factor range (min and max, tons per in² of projected area, US short ton) × projected area × the same safety factor, giving a force range rather than a single number:
Force (US ton) = Clamping factor (ton/in²) × Projected area (in²) × Safety factor
This clamping-factor table is not sourced from RFT's process-development app — that app has no per-material clamping-factor field at all (checked twice, including a search for a field named something like clampFactor in N/mm² or N/cm², across its full ~34-material list and all 4 local copies of the app). It is sourced from a different, real reference: Colm's own legacy spreadsheet (Legecy_Calculators.xlsx, "Mold clamp force" sheet, "Projected area - clamping force calculation" block), all 39 materials, cross-checked cell by cell against that sheet's own numbers.
The chart. Plots Tier 2/3's formula across 200–800 bar at your current projected area and safety factor, with your result marked on it. For Tier 1, there's no pressure value to mark directly — the marker instead shows the cavity pressure that would give the same force as the clamping factor, for reference, clearly labelled as an equivalent, not a real entered pressure.
Machine size. Rounds the calculated force up to the smallest size on a standard metric-tonne clamp-tonnage ladder (25, 35, 50, 65, 80, 100, 125, 150, 200, 250, 320, 400, 500, 630, 800, 1000 t) — a press smaller than the calculated figure can't physically hold the mould shut against melt pressure, so this is always rounded up, never down.
Flow-ratio method (Tier 1, "method 2"). A second earliest-stage estimate, built from part geometry instead of a clamping factor:
Flow ratio = Distance from gate to end of fill ÷ Average wall thickness
That ratio reads a base cavity pressure off a piecewise-linear table (ratio 20 → 100 bar, 40 → 150, 60 → 200, 80 → 250, 100 → 300, 120 → 350 bar; interpolated between points, held flat outside 20–120), then the safety factor and a material family's flow-index multiplier are applied in sequence — base pressure × (1 + safety factor), then × the family's min/max multiplier — before converting to force the same way as Tiers 2/3 above.
Where this table comes from, honestly. It's a simplified, single-variable approximation. The real source — Figure 7, "Mold cavity pressure: wall thickness graph", The Dynisco Injection Molders Handbook, found pasted into Legecy_Calculators.xlsx's "Mold clamp force" sheet — is a two-variable nomograph: pressure depends on wall thickness and a separate family of flow-ratio curves (50:1 through 300:1), not ratio alone. The one data point in that spreadsheet traceable to a real formula (ratio 60:1, wall 1.25 mm → base 200 bar) matches this table exactly, but the rest of the table is this session's straight-line approximation between the given points, not a digitisation of the real curves. Treat results from this method as rougher than the clamping-factor method above, and prefer Tier 3 (real data) once it exists.
The material-family flow-index multipliers (PE/PP/PS 1.0; Nylon/POM 1.2–1.4; ABS/ASA/SAN/ Cellulosics 1.3–1.5; PMMA 1.5–1.7; PC/PES/PSU 1.7–1.9) and the one verified ratio/pressure data point are both read directly from that same spreadsheet. Its own "Tons" conversion used a non-standard constant (÷9.964) found nowhere else in the sheet; this calculator uses the same 9806.65 N/tonne conversion as every other result on this page instead, so figures stay comparable across tiers and methods.
Assumptions
- "Projected area" means the total shadow of all cavities, cores and the runner system on the parting plane — surface area is not the same thing and will overstate the result.
- A cavity pressure (Tiers 2/3) is a single representative value for the shot, not a pressure profile — real pressure varies through fill and pack.
- The safety factor (default 10%, entered as a percentage) is a simple margin added on top of the base figure, applied the same way in all three tiers, not a statistical confidence level.
- The recommended machine bracket is the smallest standard size at or above the calculated force — it isn't a substitute for checking the press's full spec (shot size, platen size, tie-bar spacing), only its tonnage.
- The projected-area breakdown assumes the runner system and any slider area are each a single total for the whole tool, not multiplied by cavity count — enter per-cavity sliders as their already-totalled sum if that applies to your tool.
- Moving between tiers does not re-validate your figure — each tier simply uses the kind of information appropriate to that project stage.
Limitations
This is a starting point — verify by study, and against the machine's actual rated clamp tonnage, before committing to a press. Specifically, this calculator does not yet account for:
- Glass-filled or other reinforced grades, which typically need a higher cavity pressure than their unfilled base resin.
- Flow-length-to-wall-thickness effects on required pressure.
- Rounding up to a standard machine tonnage — the figure shown is the raw calculated value.
- Any warning when a result is close to a press's rated limit.
How far apart the tiers can land. Run across the 8 materials that have both a Tier 1 clamping-factor range and a Tier 2 typical cavity pressure, at a representative area, Tier 2's point figure falls inside Tier 1's range for 5 of the 8 (ABS, NYLON 6, PE-HD, POLYCARBONATE, POM-H). For the other 3 (PMMA, PP-H, PS/GPPS), Tier 2 actually sitsabove Tier 1's upper bound — i.e. the "no pressure known" estimate would have undersized the press for those materials at this area. This isn't a sign one tier is wrong — they're built from different kinds of data — but it's a reason not to treat Tier 1's range as a hard ceiling: where you can, move to Tier 2 or 3 and compare, rather than relying on Tier 1 alone.
The valid input ranges enforced on this page (area, pressure, safety factor, and the breakdown fields) are sensible proposed defaults, not sourced engineering limits — they're still pending confirmation. (C10)
Worked example
No worked example yet. A real example needs a known-correct input/output pair confirmed against validated data — until then, one isn't published here rather than risk a plausible-looking but unverified number.
Slider opening force
A separate check from the clamp-force number above — this asks whether a given heel-block locking angle (or, for opposing sliders, the mechanical preload at a meeting face) is strong enough for the melt pressure and slider area involved. It's a tool-design decision made before steel is cut, not a machine setting, and it is not automatically covered by having enough main clamp tonnage.
Result
Check the highlighted field.
How the slider check is calculated
F_actuator = (P_injection × A_slider ÷ tan(α)) + F_friction
Melt pressure acting on the slider's projected area creates a sideways force trying to push the slider open. The heel block's locking angle (α) converts that sideways force into a mechanical advantage — a shallow angle resists the sideways push strongly (so less tends to rely on the actuator holding position during injection) but needs a larger actuator force to retract afterwards; a steep angle needs less actuator force but gives less mechanical resistance to the sideways push. Friction is added as a simple constant, not modelled from contact pressure or surface finish.
F_meeting-face = P_injection × A_slider
Each slider feels the same sideways push as the single-slider case, calculated the same way — melt pressure acting on the slider's own projected area. There's no heel or actuator holding the joint shut here: toolmakers instead build mechanical preload or interference into the meeting faces so they stay clamped together under this load. If that preload is undersized for the force involved, the faces can work loose — and unlike a single slider against a fixed heel block, that force is not always fully independent of the mould's main opening direction, so an undersized joint can end up loading the main clamp too.
How to use this calculator
Video coming soon
This spot is ready for an instructional video walking through the calculator, the projected-area breakdown, and the slider checks above. Once filmed, swap the placeholder for a YouTube/Vimeo<iframe> embed or a plain <video> tag.
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