Force is pressure times shadow
The melt in a filled cavity is under pressure and it presses outward on everything, including the two faces of the mould. Only the component along the opening direction tries to part them, and that component acts across the projected area — the outline of the cavity seen from the direction the mould opens, filled in solid.
Take the default tool. Four cavities at 4.2 square inches each is 16.8, plus 2.6 square inches for the runner and sprue, which are pressurised the same way, giving 19.4 square inches. At a cavity pressure of 6,000 psi that is 116,400 pounds of force, or 58.2 US tons. Add the 10 percent margin in the form and it comes to 64 tons, 58.1 tonnes, or 570 kN.
Shadow, not surface
This is where the arithmetic goes wrong most often. A 3 by 1.4 inch tray casts 4.2 square inches of shadow whether the walls are 5 mm deep or 50 mm deep. The melt in those walls is pushing sideways into steel, not against the platens, and it contributes nothing to the clamp load. Its surface area could easily be four times the projected area, and using that figure would put the same job at 256 tons instead of 64.
The other half of the same mistake is counting overlapping features twice. If a boss sits inside the outline of the part, its shadow is already inside the outline and adding it again inflates the area. Trace the outline once.
Cavity pressure is not injection pressure
The gauge on the machine reads what the screw is pushing at, and most of that is spent moving melt through the nozzle, the sprue, the runner and the gate before any of it reaches the cavity. What is left inside the cavity is the number this calculation wants, and it is commonly a fraction of the gauge reading. Putting an injection pressure of 18,000 psi into the field instead of a 6,000 psi cavity pressure triples the answer and would have you looking at presses three times the size.
Where the real figure comes from is a filling analysis or from a moulder who has run the resin at that wall and that flow length. It is not a resin property, and it is not something a web page can supply, which is why the field says so.
What the ratio at the bottom is and is not
If you enter a machine rating, the page divides one number you gave it by another and prints the percentage. On the default job that is 64 tons against a 150 ton press, or 42.7 percent, and the rating divides out to ten cavities once the runner is taken off the top.
That is arithmetic, not an assessment. Whether a given press will actually run a given tool turns on platen size, tie bar spacing, daylight, the condition of the machine and how the process is set, and none of those are visible from a force. Take the number to whoever owns the press. Hazards here are not subtle: the clamp closes with the force being measured, and melt escaping a parting line that is flashing is at nozzle temperature.
Questions people ask
How do I calculate clamp tonnage for injection molding?
Multiply the projected area on the parting line, in square inches, by the cavity pressure in psi. That gives pounds of force; divide by 2,000 for US tons. Include every cavity and the runner and sprue, because the runner is pressurised too. On the default tool, 19.4 square inches at 6,000 psi is 116,400 lbf, or 58.2 tons before any margin is added.
What is projected area?
The outline of the part seen along the direction the mould opens, filled in solid. It is a shadow, not a surface. Deep side walls add nothing to it because the melt in them pushes sideways into steel rather than against the platens, and features that sit inside the outline are already counted by the outline. This is the single most common place the calculation goes wrong, and it always goes wrong in the direction that understates the force.
What cavity pressure should I use?
Whatever your filling analysis or your moulder gives you for that resin at that wall thickness over that flow length. It is not a property of the resin, it is not on a datasheet, and it is not the injection pressure the machine gauge shows — most of that is spent getting melt to the cavity in the first place. This page has no value for it and deliberately refuses to suggest one.
Is tons per square inch a useful rule?
It is shorthand for a cavity pressure, expressed in different units. Three tons per square inch is 6,000 psi, so quoting the rule and quoting the pressure are the same statement. The reason it gets quoted as a rule is that it is quick, and the reason it misleads is that the right figure moves with the resin, the wall and the flow length, so a single number that works for one job is wrong for the next.
Does the calculator tell me if my machine is big enough?
No. If you enter a rating it will divide your force by your rating and print the percentage, and that is all it does. Platen size, tie bar spacing, daylight, the state of the machine and how the process is set all decide whether a tool runs on a press, and a force ratio sees none of them. The number is there to take to whoever owns the machine, not to answer the question on its own.