Injection moulding machine tonnage selection: largest projected area per clamp, with a 10% reserve (a starting margin, not a rule)
| Clamp | Factor 2 US t/in² | Factor 3 US t/in² | Factor 4 US t/in² | Factor 5 US t/in² |
|---|---|---|---|---|
| 75 t | 242 cm² | 162 cm² | 121 cm² | 97 cm² |
| 100 t | 323 cm² | 215 cm² | 162 cm² | 129 cm² |
| 125 t | 404 cm² | 269 cm² | 202 cm² | 162 cm² |
| 150 t | 485 cm² | 323 cm² | 242 cm² | 194 cm² |
| 180 t | 582 cm² | 388 cm² | 291 cm² | 233 cm² |
Injection moulding machine tonnage selection starts with one sum: the projected area inside the mould times the resin's tonnage factor. The table shows the largest area each clamp from 75 to 180 t can hold. Find your area and factor, read across, and the smallest clamp that clears it is your starting point.
Shot volume, mould size and opening stroke then confirm or overturn that choice. Factors are in US tons per square inch, the unit most published guides use, and 1 US t/in² is about 0.141 metric t/cm².
Key Takeaways
- Clamp needed is projected area, minus through-holes, plus any cold runner, times the tonnage factor. Add a reserve and choose the smallest clamp that clears the result.
- Resin can move a part by two classes: a 162 cm² part needs about 75 t at a factor of 3 but about 125 t at a factor of 5.
- Clamp is one of four checks. The shot must suit the barrel, the mould must fit between the tie bars, and the machine must open far enough to eject the part.
- Toys sold in India need the ISI mark under the Toys (Quality Control) Order, so settle the toy design and resin before you size the machine.
How Does Injection Moulding Machine Tonnage Selection Work?
Work out the force trying to open the mould, then choose a clamp that holds it with a reserve. That force is projected area times a tonnage factor. Projected area is the part's shadow on the parting line, counted for every cavity, plus any cold runner.
A Plastics Technology column on clamp tonnage by RJG trainer Jeremy Williams sets out the steps. Take one part's projected area, subtract cored-out through-holes, multiply by the cavity count, add the full cold-runner area, then multiply by the factor. He gives typical factors of 2 to 10 US t/in² and suggests 3 as a starting point.
Williams lists what pushes the factor up or down: melt flow, flow length, wall thickness, gate location and size, the number of gates and the volume of resin. A long flow in a thin wall needs more pressure to fill, and that pressure pushes harder on the mould faces.
Hot runners change the sum. Only the cavities count, because the melt in a hot manifold stays inside the mould plates. On a tool with many cavities, that alone can move you down a class, as the multi-cavity mould article in this series shows.
Which Resin Moves Your Part Up a Tonnage Class?
Stiffer, more viscous resins need more clamp for the same area. SPE's Plastics Engineering magazine places tonnage factors between 2 and 8 US t/in², with polyethylene at the low end, PP at 2 to 3 and polycarbonate toward the top. A resin change can change the machine.
ABS sits in the middle. For Lustran ABS 248, which INEOS names for small appliance housings and toys, the datasheet gives 2 to 4 US t/in². On a 125 t clamp, that range moves the largest ABS part from about 404 cm² down to about 202 cm².
Do not compare resin families by melt flow index. Williams notes that PC behaves as a high-viscosity resin in the mould whether its MFI reads 2 or 25 g/10 min. MFI is a fair guide only between grades of the same polymer.
For toy makers, resin choice comes with rules. BIS says the Toys (Quality Control) Order came into force on 1 January 2021 and calls up seven Indian Standards, including IS 9873 Part 1 for mechanical and physical safety such as small parts and sharp edges. Thicker walls or stronger snap fits added to pass those tests add area and shot weight, so size the machine after the design is frozen.
What Else Must Fit Besides Clamp Force?
Three more checks decide the machine. The shot must suit the screw and barrel, the mould must fit between the tie bars and on the platen, and the machine must open far enough to eject the part. A clamp that passes but fails any one of these is still wrong.
For the shot, keep it inside the working share of barrel capacity explained in the screw diameter article in this series, and size it in volume rather than grams.
For the mould, compare its width and height with the tie-bar spacing, and its shut height with the machine's minimum and maximum mould thickness. Then add the opening stroke the part needs to clear the core. Deep parts can reach the stroke limit well before they reach the clamp limit.
Our view is firm: never buy on clamp force alone. A machine that holds your part's force but cannot take the mould between its tie bars, or cannot open far enough to drop the part, is the wrong machine whatever its tonnage.
Extra clamp is not free insurance either. Excess force can crush vents and crack mould blocks, as the machine upgrade article in this series describes.
Four checks before you fix the tonnage
| Check | What you compare | Typical sign of a mismatch |
|---|---|---|
| Clamp | Projected area × factor, plus reserve, against rated clamp | Flash at the parting line, or clamp set at the machine maximum |
| Shot | Shot volume plus allowance against the barrel's swept volume | Unmelt or long recovery on large shots; degraded melt on small ones |
| Mould size | Mould width and height against tie-bar spacing; shut height against mould thickness range | Mould will not pass between the tie bars or close fully |
| Opening | Part depth and core height against opening stroke and daylight | Part cannot drop clear, or needs hand removal every cycle |
What Should You Send for a Tonnage Check?
Send the part drawing or a sample, the resin grade, cavity count, runner type, the mould's outside size and shut height, and your monthly quantity. With those, clamp, shot and mould fit can be checked before any machine is named. See the 75 to 180 ton horizontal injection moulding machine page.
Frequently Asked Questions About Injection Moulding Machine Tonnage Selection
Should I size the clamp for my largest mould or my usual one?
Size it for the moulds you will run most hours, then check that the largest planned mould still fits on clamp, tie bars and opening. Buying for one rare, oversized tool means paying for capacity that sits idle on every other job, and setting a large clamp on small moulds needs care.
What tonnage factor should I use if I do not know my resin's figure?
Ask the resin supplier first. Failing that, Williams suggests 3 US t/in² as a starting point, and SPE's Plastics Engineering gives 400 bar of cavity pressure as a guide value. Both are starting points, so confirm the figure on a trial or a filling study before the purchase is fixed.
How do I measure projected area from a drawing?
Look at the part from the direction the mould opens and take the area of its outline, as if tracing its shadow. Subtract through-holes that pass right through, multiply by the cavities and add the cold runner. CAD software can report this area directly from the part model.
Does part weight tell me the tonnage I need?
No. Weight decides the shot size and the screw, while projected area and resin decide the clamp. A deep, heavy container can need modest clamp, and a wide, thin lid of the same weight can need far more. Treat weight and area as two separate checks.
Can I use a factor in kg/cm² instead of US tons per square inch?
Yes, if you convert carefully. One US t/in² is about 0.141 t/cm², or about 141 kg/cm². Multiply your projected area in cm² by the converted factor to get metric tonnes. Mixing the two units can put you a whole tonnage class out.
Related Articles
- Screw Diameter Choice: Shot Weight vs Injection Pressure on 75 to 180 Ton Machines
- Getting the Most From Multi-Cavity Moulds on 75 to 180 Ton Machines
- Injection Moulding Machine Upgrade: When to Move to a Container or Houseware Machine
Sources & References
- Plastics Technology, J. Williams (RJG): Clamp Tonnage: More Is Better...Right?
- Plastics Engineering (SPE), A. Urbina: Mastering Clamping Force Calculation
- INEOS ABS via UL IDES: Lustran ABS 248 technical data sheet
- Bureau of Indian Standards (BIS Blog): Indian Standards on Toys: Ensuring your child's safety
Technical Disclaimer
Disclaimer: The information provided in this document is for general informational and educational purposes only. It is based on technical references and industry information available at the time of preparation and may be subject to change. It does not constitute engineering, technical, commercial or professional advice, nor does it constitute a quotation, specification, warranty or contractual commitment by Pratishna Engineers.
Actual injection moulding machine and process requirements vary according to the component design, material, mould, production volume, machine configuration and operating conditions. The information, examples, calculations and recommendations provided should be independently evaluated and verified for the specific application before making any equipment, process or production decision. Information obtained from third-party sources remains subject to the accuracy and limitations of those sources.
Limitation of Liability: Pratishna Engineers makes no representation or warranty, express or implied, regarding the completeness, accuracy or suitability of the information for any particular application. To the extent permitted by applicable law, Pratishna Engineers shall not be liable for any loss, damage, cost or consequence arising from reliance on or use of the information contained in this document. Final machine selection, configuration and application suitability should be confirmed with Pratishna Engineers based on complete and accurate project requirements.
