Why tonnage decides the machine, and the budget
Clamping force is the first specification that narrows an injection moulding purchase. Too little and the mold flashes, scrap rises, and the tool wears at the parting line. Too much and the buyer pays for a larger platen, a larger footprint, a bigger drive, and higher energy consumption for the life of the machine. Sizing the job before requesting quotations keeps the comparison honest, because two suppliers quoting different tonnages are not quoting the same machine.
Once the estimate is known, the practical next step is to check where it falls against real machine sizes. Machines are sold in steps, so a job estimated at 138 tonnes is normally quoted on a 150 tonne class machine rather than anything smaller.
Frequently asked
Is projected area the same as part surface area?
No. Projected area is the shadow the part casts on the parting line, not the total surface. A deep box and a flat lid with the same footprint have very different surface areas but similar projected areas.
Do runners really change the answer?
Yes, and they are the most commonly forgotten term. In multi-cavity tools the runner system frequently adds 10-20% to the projected area, which moves the requirement into the next machine size.
Which cavity pressure should I use?
If the mold maker has given a figure, use it. Without one, 300-500 kgf/cm² is the usual planning window; move higher for thin walls, long flow paths, and high-viscosity grades.
Does this replace a mold flow analysis?
No. It is a sizing and budgeting estimate. A mold flow analysis accounts for gate position, fill balance, cooling, and pressure loss along the flow path, and should be used before the tool is cut.
Use the estimate to shortlist machines in the right tonnage class, then request a quotation with the part, resin, cavity count, and annual volume attached.