How clamp tonnage is calculated
Clamping force is what holds the mold shut against the cavity pressure trying to push it open. The relation is simply clamp force = total projected area × cavity pressure, where total projected area is the part footprint times the number of cavities plus the sprue and runner footprint. Leaving the runner out is a common source of error on multi-cavity tools. US injection presses are rated in US tons of 2,000 lb, and that is the unit used here.
Cavity pressure depends on resin, wall thickness and flow length. Easy-flowing PP and PE sit near 4,350 psi, ABS and PS near 5,800 psi, PC and nylon near 7,250 psi, and glass-filled grades near 10,150 psi. Thin walls and long flow lengths push pressure higher, so a large thin-wall part should be estimated above the typical value.
A quick sanity check is tonnage per square inch. Most parts land between 2 and 5 US ton/in², so a result well outside that band usually means the pressure assumption or the area needs another look. Final machine selection also depends on daylight, tie-bar spacing, shot capacity and plasticizing rate, so confirm it with your mold designer and the press supplier.
Frequently asked questions
Adding 10 to 20% and stepping up to the next machine size is common. Too little margin lets the mold breathe during pack and hold, which produces flash, while a much larger press costs accuracy on the shot and energy efficiency.
No. Presses in Korea and Japan are rated in metric tonf of 1,000 kg while US presses use the 2,000 lb short ton, so the same mold shows a US ton number about 10% higher.