CustomPartNet
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August 25, 2026
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Updated August 25, 2026
Before you request quotes, it helps to know roughly what kind of machine your part needs and how much raw material a production run will actually consume. Both numbers affect which suppliers are even a fit for your job, since a shop running mostly 100 to 300 ton presses isn't the right match for a part that calls for 500 tons, and a supplier quoting material cost without accounting for scrap and defect rate is going to underprice the job. Browse our supplier network on the right to compare qualified injection molding and die casting suppliers, check their equipment ranges, and request quotes once you have a sense of what your part requires.
During injection, molten plastic enters the mold cavity under significant pressure, and that pressure pushes outward against the two halves of the mold. Clamping force is the counteracting force applied by the molding machine to keep the mold shut against that pressure. If the clamping force is too low, the mold separates slightly during injection and material escapes at the parting line, showing up as flash. If it's too high, the extra tonnage adds wear to the mold and machine without improving the part.
The required clamping force is calculated from the cavity pressure inside the mold and the projected area the pressure is acting on, which includes the part and any runners in the shot. The resulting tonnage figure is what determines which size of molding machine can run the job.
At its core, the calculation is:
Clamping Force = Cavity Pressure × Projected Area
Cavity pressure is the pressure the melted plastic exerts against the mold walls. For standard commodity resins like PP, PE, and ABS, cavity pressure is generally in the range of 4,000 to 7,000 psi (roughly 300 to 500 bar), with more viscous engineering resins like PA and PC and thinner-walled or longer flow-path parts pushing that figure higher.
It is important to note when talking with a supplier that injection pressure is different from cavity pressure. Injection pressure is the pressure the plastic is injected with at the nozzle, and there is a significant pressure drop when the plastic melt reaches the mold cavity. Cavity pressure can range from 30% to 60% of the injection pressure depending on mold geometry and material, so the two should not be conflated.
Projected area is the area of the part and runner system as viewed along the direction the mold opens, not the total surface area of the part.
Because real production introduces variability, such as resin viscosity shifts, mold wear, and process drift, suppliers add a safety margin on top of the calculated figure before selecting a machine. For injection molding, that margin is commonly 10 to 20 percent (a multiplier of about 1.1 to 1.2), which is consistent across most published tonnage guides.
Say you're quoting an ABS electronics enclosure with a projected area, including the runner system, of 45 square inches. Using a cavity pressure of 6,000 psi, which sits squarely in the typical range and matches real-world sensor data for average cavity pressure:
Separating force = 6,000 psi × 45 in² = 270,000 lbf
Convert to tons = 270,000 ÷ 2,000 = 135 tons
Apply a 15 percent safety margin = 135 × 1.15 = about 155 tons
A supplier would round up from there and quote the job on a 160 ton press. Undersizing to a 150 ton machine would leave almost no cushion for process variation, while jumping to a 250 ton machine would be paying for tonnage the part doesn't need.
Knowing the machine size gets the part running, but it doesn't tell you how much raw resin to order for the full production run. That's a separate calculation, and it depends on the shot weight, the number of cavities, and the defect rate you expect over the course of the run.
In most manufacturing processes, some material never ends up in a shipped part. Runners get trimmed off, and a percentage of parts don't pass inspection. The material usage calculation starts from the part weight and works out:
Shot weight, the total material injected per cycle, equal to (cavities × part weight) plus runner weight
Material yield per cycle, the weight of good plastic that actually becomes parts
Scrap weight, covering both rejected parts and runner material
Total raw material required to deliver the full order after accounting for the defect rate
Continuing with the same enclosure, assume a 4 cavity mold, a part weight of 25 grams, a runner weight of 15 grams per shot, an expected defect rate of 4 percent, and an order for 20,000 good parts.
Shot weight = (4 × 25g) + 15g = 115 grams per cycle
Parts needed to produce, accounting for defects = 20,000 ÷ (1 - 0.04) = about 20,834 parts
Cycles required = 20,834 ÷ 4 cavities = about 5,209 cycles
Total raw material = 5,209 × 115g = roughly 599 kg (about 1,320 lb)
Of that, good parts account for 500 kg, and scrap (rejected parts plus runner material) accounts for the remaining 99 kg
That 599 kg figure, not the 500 kg the finished parts alone would weigh, is what a supplier should be quoting material cost against. Skipping the defect rate and runner allowance is a common way material cost gets underestimated on a quote.
Metal die casting sizes a machine the same way, by multiplying cavity pressure (often called specific injection pressure in die casting) by the projected area of the part, runners, and overflow wells. The material usage side of the calculation also carries over directly, since die cast parts have their own shot weight, scrap rate, and runner system to account for when ordering ingot.
The difference is the safety margin applied to the clamping force calculation. Where injection molding typically works with a 10 to 20 percent cushion, die casting generally starts higher, commonly 10 to 30 percent as a baseline (a multiplier of roughly 1.1 to 1.3), rising to 30 to 50 percent (1.3 to 1.5) for thin-walled or structurally critical castings. The reason comes down to how the material fills the die. Molten metal is injected at gate velocities commonly in the range of 30 to 60 meters per second for aluminum, with die erosion becoming a concern above about 80 meters per second, and that momentum creates a brief, dynamic spike in cavity pressure at the moment of fill that exceeds the steady-state pressure used in the base calculation. Injection pressure itself also spans a wide range in die casting, roughly 10 to 175 MPa depending on machine type, alloy, and part requirements, with most production aluminum work landing somewhere in the tens up to around 100 MPa or more, generally higher than the pressures typical of plastic injection molding. That's part of why die casting machines are commonly rated in the hundreds to thousands of tons rather than the tens to low hundreds typical of injection molding.
For a buyer comparing quotes across both processes, this means two suppliers using the exact same formula and the exact same projected area will still land on different machine sizes for a plastic part versus a metal casting, and a die casting supplier quoting with only a 10 percent margin on a thin-walled part may be cutting it closer than the process really allows.
Injection molding clamping force and material usage calculations are used across:
Consumer electronics housings and enclosures
Automotive interior trim and connector components
Appliance control panels and housings
Medical device enclosures
Packaging components and closures
Die casting clamping force calculations, with their larger safety margin, are especially relevant for:
Automotive structural brackets and housings
Powertrain and transmission components
Aerospace and industrial equipment housings
Thin-walled, high-volume aluminum and zinc parts
The mold separates slightly during injection, letting material escape at the parting line. This shows up as flash, and in more severe cases can lead to short shots or inconsistent part dimensions.
Oversized clamping force adds unnecessary wear to the mold and machine, increases energy consumption, and can push a job onto a larger, more expensive machine than the part actually requires.
Runner material is typically not part of the finished product, but it still needs to be purchased, melted, and processed, so it belongs in the material usage calculation even though it doesn't ship with the order. Its projected area also needs to be accounted for when calculating clamping force since it will exert force against the mold too.
The calculator accounts for scrap weight from runners and rejected parts. Whether that scrap can be reground and reused depends on the material and the supplier's process, and can meaningfully offset raw material cost when it's an option.
Molten metal is injected into the die at much higher velocity than molten plastic fills a mold cavity, and that speed creates a momentary dynamic pressure spike beyond the steady-state pressure used in the base calculation. The larger margin builds in room for that spike, especially on thin-walled or structurally demanding parts.
Yes. Both calculators work from part weight, projected area, and basic process parameters, which makes them useful for early budgetary estimates before final tooling drawings are complete.
Use CustomPartNet's Clamping Force and Material Usage calculators to get a realistic machine size and material budget before you reach out to suppliers, then get matched with qualified manufacturers experienced with your material and process.
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