The Formula Is Short; the Inputs Are Not

Injected plastic pushes the mold open. Clamping force pushes back. When the press wins by enough, you get a clean part. When the melt wins, you get flash. The calculation formalizes that fight.

The metric formula reads: tonnage equals projected area in square centimeters times cavity pressure in kgf/cm², divided by 1000. One documented example: a 270 cm² cavity filled at 220 kgf/cm² needs 59.4 tons before any margin. The metric formula and the 2–5 tons-per-square-inch rule are the same physics in different units.

American shops use the thumb rule on the injection molding process floor every day. Easy-flow plastics like PP sit near 2 tons per square inch. Stiff engineering plastics like PC sit near 5. A 36-square-inch PC part at 5 tons/in² needs 180 tons. Add a 10% margin and you land at 198, so it buys a 200-ton press.

Both formulas look trivial. Neither saves you when the pressure number is a guess. That is why the next two sections matter more than this one.

Projected Area: the Measurement Buyers Get Wrong

Projected area is the shadow the cavities and the runner cast on the parting line. It is not the surface area of the part. Only the footprint pressing against the clamp direction counts.

Two mistakes dominate. Buyers forget the runner, and buyers forget extra cavities. A cold runner is usually the thickest steel in the mold, and its shadow counts like any cavity. In a four-cavity mold, you add all four footprints plus the runner. Count the shadow on the parting line — every cavity plus the runner — not the surface area of the part.

A hot runner system shrinks this number. With no cold runner, there is no runner shadow, so the same part needs less tonnage in a hot-runner mold. The runner also freezes last, so a cold runner often dictates both cycle time and tonnage.

Draft angles of 1–3° change the footprint by less than 2–3%. Ignore that correction. The pressure assumption swings the answer ten times harder.

Cavity Pressure: Where Material and Geometry Decide

Cavity pressure is the pressure inside the cavity during filling. Nozzle pressure is higher; pressure is lost along the runner and through the gate. The clamp only feels what reaches the cavity.

The table below lists typical cavity pressure ranges by resin, published for this exact calculation. Use them as starting points, then adjust for your wall thickness and flow length.

Geometry multiplies these ranges. The flow ratio — longest flow length divided by thinnest wall — sets how hard the melt must be pushed. A documented PC lamp holder case ran at a 105:1 flow ratio with a 1.9 mm wall: base pressure 160 bar times a 1.9 viscosity factor gave 304 bar cavity pressure, and 380 cm² of projected area turned into 115.5 tons before margin. At a 105:1 flow ratio, one PC lamp holder needed 304 bar of cavity pressure — before any margin.

Trapped air adds its own spike. Vents sized to the resin keep that spike down, which is one more reason venting belongs in the tonnage conversation.

Clamping Force Calculation: Margin and Machine Size

Multiply the raw number by 1.1 to 1.2. That 10–20% margin absorbs pressure spikes, resin batch changes, and the gap between a handbook pressure and the real one. Then round up to the nearest standard press size. The 115.5-ton lamp holder from the last section buys a 120-ton machine, not a 100-ton one.

Too little tonnage lets the melt push the mold open mid-shot. You get flash, parting-line leakage, and short shots. Too much tonnage buys clamp wear, higher energy use, and a larger hourly rate than the job needs. A mold that calculates to 115.5 tons runs on a 120-ton press; rounding down buys flash.

The margin is not a fudge factor for sloppy math. It covers the difference between a calculation and a process, and processes always drift.

Tonnage Is One Line on the Machine Spec

Tonnage gets the headline. Four more numbers decide whether the mold actually fits and runs well.

• Tie-bar spacing: the mold’s diagonal must clear the bars. A high-tonnage press with narrow spacing still cannot hold a wide, shallow mold.
• Mold stack height and platen size: the mold must bolt onto the platens and fit between the machine’s minimum and maximum daylight.
• Shot capacity: the screw must hold the full shot plus a cushion of unpurged melt. An oversized screw for a small shot hurts melt residence time.
• Injection pressure reserve: if filling already uses 80% of the machine’s rated pressure, no process window remains for tuning.

Tonnage gets the quote line; tie-bar spacing gets the mold onto the machine. Check all five numbers before you approve a press for your tool.

Four Questions for Your Supplier Before You Approve

Ask these before the tool steel is cut. The answers tell you whether the tonnage is engineered or guessed.

1. What cavity pressure did you assume for my resin, and what flow ratio drove it?
2. Is the runner area — and every cavity — included in the projected area total?
3. What safety margin did you apply, and which machine model will actually run this job?
4. What happens to the number if we double the cavities later?

If a supplier cannot show the pressure assumption behind the tonnage, the tonnage is a guess. A solid answer also covers gate and runner strategy, because gate size and runner layout feed straight back into cavity pressure.

Run the Numbers Yourself

Plug in your projected area and resin. The calculator below applies the same formula, margin included. It points to the standard press size that fits your numbers.