Why machine size is decided by clamp force
Most people asking for a machine quote start with tonnage, because that is how machines are listed. But tonnage is an outcome, not a starting point. The thing you actually control is the part — its projected area and its material. Clamp force follows from those.
Clamp force exists for one reason: to hold the two mould halves together against the pressure the molten plastic exerts while filling. That pressure is roughly constant across almost all thermoplastics — around 1400 bar. So the force required scales directly with the area of the part being filled, called the projected area.
Two consequences follow, and both are useful:
- A small part in a thick mould does not need more force than a large part in a thin mould — only the projected area matters.
- Adding a cavity multiplies the projected area by the cavity count. This is why a four-cavity thin-wall mould can need more clamp force than a one-cavity heavy part.
The clamp force formula
Here is the calculation in full. Three steps, all of which you can do with a calculator:
Two corrections matter here, and skipping them is where most people go wrong.
1. Shrinkage correction
The mould is made from the finished part size. But plastic shrinks as it cools, so the mould must be larger than the finished part. If your part's projected area is 120 cm² finished, and PP shrinks about 1.5%, the mould cavity has to be 120 ÷ 0.985 = 121.8 cm². The clamp force is calculated on that 121.8, not on 120.
Shrinkage rates worth knowing: PP 1.0–2.0%, PE 1.5–3.0%, ABS 0.4–0.9%, PC 0.5–0.7%, PA6 0.8–2.0%, POM 1.5–3.0%. These vary with grade, temperature and processing history — use your resin supplier's figure rather than a general table when you have it.
2. Pressure varies with material
1400 bar suits unfilled thermoplastics. Filled and reinforced resins are stiffer and more difficult to fill, so they need more pressure:
| Material | Typical shrinkage | Relative pressure needed |
|---|---|---|
| PP (unfilled) | 1.0–2.0% | Standard, ~1400 bar |
| PE (unfilled) | 1.5–3.0% | Standard, ~1400 bar |
| ABS (unfilled) | 0.4–0.9% | Standard, ~1400 bar |
| PC (unfilled) | 0.5–0.7% | Standard, ~1400 bar |
| PA6 / PA66 | 0.8–2.0% | Slightly higher, ~1500 bar |
| Glass-filled PP / ABS (15–30%) | 0.3–0.8% | Higher, ~1800 bar |
| Carbon-fibre reinforced | 0.1–0.5% | Much higher, ~2100 bar or above |
Worked example: a 120 cm² part
Say you want to mould a household container in PP. The finished part is 120 cm² projected area.
So the part needs about 1,740 kN of clamp force, which is a machine in the 180-ton class. Applying a safety factor of 1.15 — reasonable for a stable production part — gives about 2,000 kN, so a 200-ton machine.
Now check the injection unit. If the part weighs 300 g finished, the shot must be larger than that to allow for runner and sprue. Say the runner is 30 g, so shot weight = 330 g. Adding 15% cushion gives 380 g required shot capacity. Compare that against the 200-ton machine's rated shot weight — for most machines in that class it will be in the 700–900 g range, comfortably sufficient. That is a correctly balanced machine: clamp force is the limiting factor, not shot capacity.
Sizing the injection unit
Clamp force is only half the sizing problem. The injection unit must deliver the shot weight and plasticise fast enough to hold the cycle time you need.
Shot weight
The calculation above: part weight + runner weight, then divided by (1 − cushion ratio). Cushion is the melt reserve that keeps the screw from bottoming out. Typical cushion is 5–15% of the shot; 15% is on the generous side but gives more tolerance for resin density variation. Use 10% as a sensible default.
Screw diameter
This is the decision buyers most often get wrong. The general rule is that the ideal shot is between 30% and 70% of the screw's maximum shot capacity.
- Below 30%: poor plasticising, because melt is not being mixed and sheared enough. Recover slowly and the result is inconsistent.
- Above 70%: the shot is close to maximum, cushion becomes small, and any density variation in the resin causes cushion loss and unstable shot weight.
- The best balance for general work is around 50% of machine capacity, which leaves good plasticising and a comfortable cushion.
Plasticising capacity
Plasticising capacity is measured in kg/h and must exceed your shot weight multiplied by shots per hour. If you need 380 g every 25 seconds, that is 136.8 kg/h. A machine with 150 kg/h plasticising capacity works, but the margin is thin — aim for at least 1.2× your requirement so cycle time does not drift upward when the material is at the low end of its density specification.
Three sizing mistakes that cost money
Mistake 1: sizing by tonnage habit
Ordering "a 200 ton machine" without a calculation. The number means nothing on its own — 200 tons of clamp force could be right for a 100 cm² part and badly undersized for a 300 cm² one. Every serious quotation should include the calculation. If you receive a quote with no projected area in it, ask for the calculation.
Mistake 2: forgetting the cavity count
Four thin-wall cavities at 40 cm² each is 160 cm² of projected area — the clamp force requirement is that of a single 160 cm² part. Small-looking cavities in high-count tools are frequently the reason a machine runs at 70% clamp capacity and the part still flashes. Check the total projected area across all cavities, not per cavity.
Mistake 3: buying tonnage headroom you will never use
This is the most expensive and least visible mistake. An oversized machine costs more to buy, takes more floor space, and — this is the part people forget — draws more energy for the same part, because the oil heating and driving force scale with machine size, not with output. It also stretches the cycle, because recovery and cushion are harder to optimise when running well below capacity.
The legitimate reason to buy headroom is a known future product that needs more tonnage on the same machine. The illegitimate reason is "so we have room to grow" without a specific product in mind. If you are not sure, a smaller machine now and a second one later is usually cheaper than paying for capacity you never use.
The information you need before asking for a quote
With these six items, any competent supplier can quote a properly sized machine the same day:
- Part drawing or 3D file — or the finished part weight and projected area if you can measure them
- Resin and grade, including whether it is glass- or mineral-filled and the percentage
- Annual quantity — this decides cavity count and whether new or used makes sense
- Target cycle time, or the current cycle if you are replacing an existing machine
- Cavity count, if a mould already exists or you have decided it
- Destination port and any voltage or certification requirement in that country
How we quote
Every machine quotation we send includes the clamp force calculation, the injection unit sizing and the resulting machine model — so you can check the working, not just the price. If your part will run comfortably in a smaller machine than the one we quote, we will tell you why and reduce it. If you send the six items above, expect a properly sized machine the same day.
Common questions
How do I calculate injection molding clamp force?
Divide the part's projected area in cm² by the resin's shrinkage factor, multiply by the clamping pressure the material needs (about 1400 bar for unfilled thermoplastics), then divide by 98.1 to get tonnage. A 120 cm² PP part gives roughly 121.8 ÷ 0.985 = 121.8 cm² corrected, × 1400 = 170,520, ÷ 98.1 ≈ 1,738 kgf, so a 180–200 ton class machine.
What is the rule of thumb for clamp force per square centimeter?
About 1400 bar of clamping pressure per cm² of projected area — roughly 70 kgf per cm². That suits unfilled PP, PE, ABS and PC. Glass-filled material typically needs 1.3 to 1.5× more.
Is tonnage the same as clamping force?
Yes, measured two ways. 1 ton is about 9.81 kN, so a 100-ton machine provides roughly 981 kN. Sellers quote tonnage because buyers speak it, but the calculation is done in force — so ask suppliers for kN and rated shot weight in grams, not just "tons".
Should I buy a bigger machine than I need?
Usually not. It costs more, takes more space, and draws more energy for the same part. The exception is a genuine future product that needs the tonnage, or keeping one machine for several products instead of buying a second.
What if I do not have a drawing yet?
Send the finished part weight, the resin and your target cycle. That is enough for an initial machine recommendation and price. Drawings can follow before you commit, and will only tighten the calculation.