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Release time：2026-10-06 Visits：9
Every molding quote hides one number that decides the price of every part you will ever buy: the press size. Injection molding machine tonnage is the clamping force that holds a mold shut while molten plastic fills the cavity. Size it too low and you get flash, short shots and a worn parting line. Size it too high and you pay for capacity you never use, on every order. This guide shows B2B buyers how to size tonnage from projected area and resin, where the common mistakes hide, and what to confirm before you approve a supplier's machine selection.
Molten plastic enters a cavity at 30 to 100 MPa of pressure — tens of times the pressure in a car tire. If the clamp cannot hold, the mold cracks open by a fraction of a millimeter. Plastic escapes along the parting line and you get flash: a thin fin of plastic that operators must trim off, part by part. Flash also hammers the steel faces. After a few thousand shots the damage is permanent, and every future part inherits it.
An undersized press causes quieter defects too. Without enough clamp, the cavity never reaches full packing pressure, so parts come out short, heavy-lined or off dimension. Oversizing causes the opposite problem: the molder books a bigger, slower and more expensive machine than the job needs, and your unit price carries that cost for the life of the program.
Start from one number: projected area. Multiply the projected area of all cavities plus the runner by the cavity pressure your resin needs. In metric units:
Clamping force (kN) = projected area (cm²) × cavity pressure (MPa) ÷ 10
In imperial shorthand: tons = projected area (in²) × tons-per-in² for the resin.
Worked example: a 200 × 150 mm housing projects 300 cm². In ABS, at roughly 50 MPa, the job needs 300 × 50 ÷ 10 = 1,500 kN, about 150 tons. Add a 10–20% safety margin and the right press is 170–180 tons. If you run four cavities, multiply the cavity area by four — and count the runner, which projects area of its own.
Typical cavity pressures by resin family:
| Resin family | Typical cavity pressure | Rule of thumb (tons per in² of projected area) |
|---|---|---|
| PP, PE (commodity) | 30–45 MPa | 2–3 |
| ABS, SAN, PMMA | 45–60 MPa | 3–4 |
| PA (nylon), POM | 55–70 MPa | 4–5 |
| PC, PC/ABS | 60–80 MPa | 4–6 |
| PEEK, PPSU (high-temp) | 70–100 MPa | 5–8 |
| Glass-filled grades | add 20–30% on top | — |
Values are industry rule-of-thumb parameters — engineering review required before publication.
Projected area is the silhouette the mold presents to the clamping force — part plus runner, seen from the direction the clamp pushes. It is not the surface area of the part. A flat, wide flange projects far more area than a compact deep shell of the same weight, so two parts that weigh the same can differ by a factor of five in tonnage. Deep cores, side actions and unscrewing mechanisms do not add projected area, but they raise mold height and daylight requirements, which also constrain machine selection. The practical rule for buyers: tonnage is calculated per part, never per kilogram of resin. Any quote that sizes the press from part weight alone deserves a second look.
The area formula is a starting point, not the finish line. Three factors push the requirement higher:
If your part combines thin walls with a long flow path, ask the supplier for the fill simulation instead of a bigger machine. Simulation answers the question in hours; a wrongly sized trial costs weeks.
Our molding floor runs 50 presses from 140 to 500 tons. That span is not a marketing line — it decides which jobs run economically on which machine. A press that is too large for the part wastes energy and machine-hour rate on every cycle; one that is too small cannot run at all. Typical fit:
| Press class | Typical projected area (ABS, ~2.5 mm wall) | Example parts |
|---|---|---|
| 140–160 T | up to ~200 cm² | mouse housings, caps, small connector bodies |
| 200–250 T | 200–350 cm² | keyboard frames, medium housings, overmolded grips |
| 320–400 T | 350–650 cm² | appliance panels, power tool housings |
| 500 T | up to ~900 cm² | large automotive trim, aviation seat components |
When you compare quotes from different suppliers, ask which press class each quote assumes. A quote built on a 500-ton press for a part that fits a 250-ton machine is usually a sign of lazy sizing — or of a floor that lacks the right press.
Before you approve a supplier's machine selection, get these in writing:
Send us a 3D model and a resin grade. Our engineers return DFM feedback in 1–3 working days and a quotation in 1–3 working days, with the selected tonnage, cavity count and cycle estimate written into the quote — see our injection molding capability page. Tooling is built in-house, from mold development and customization through trial molding and small-batch production, so your tool is proven on the same class of press that will run mass production. Quality is tracked through our quality management capability system, from resin screening to final inspection.
1. Can I just ask for the biggest machine available?
You can, but you will pay for it on every order. Oversized presses cost more per machine-hour, cycle slower, and often hold thin-wall parts less accurately. Correct sizing is cheaper and produces better parts.
2. Does two-color or overmolding change the tonnage?
Yes. The clamp must cover the projected area of both shots, and two-color tools need rotating or stacked platens. Size the press from the sum of both areas, not the larger one alone.
3. Is tonnage the same as shot capacity?
No. Tonnage is clamping force; shot capacity is how much melt the barrel can inject per cycle. A part can fit the clamp and still exceed the shot window, so check both.
4. How much tonnage do glass-filled or PEEK parts need?
Glass-filled grades add 20–30% clamp demand, and PEEK or PPSU typically need 5–8 tons per in² of projected area plus high-temperature barrel capability. These resins reward suppliers with real high-temp experience.
5. Does tonnage affect cycle time and price?
Indirectly, yes. A right-sized press cycles faster and books cheaper machine time. Oversized selections raise the hourly rate and the unit price; undersized ones cause scrap and rework that cost far more than the price gap.
Injection molding machine tonnage is not a detail for the supplier to guess. A five-minute check — projected area, resin, cavity count — tells you whether a quote is sized honestly, and it protects you from flash, scrap and inflated piece prices for the life of the program. Send your 3D model to Dongguan Xiangqin Plastic: Request a Quote and our engineers will return the tonnage calculation with your DFM feedback, or ask for a free sample after the review. Thirty machines in the tool room, fifty presses on the floor — one of them fits your part exactly.
