Take a mould with 170 cm² of projected area. At a starting factor of 0.42 t/cm², it needs about 71 t of clamp, or 79 t with a 10% reserve, which is a starting margin rather than a rule. That one sum settles most 75 ton vs 100 ton injection moulding machine decisions: below roughly 68 t, the smaller machine is enough; above it, you want the larger one.
The clamp is only the first check, though. Mould fit and shot size can overrule it, and so can the moulds you plan to buy next year.
Key Takeaways
- Size by the calculated clamp need plus a reserve. Up to about 68 t suits a 75 t machine, and about 68 to 90 t points to 100 t.
- The tonnage factor moves the answer a lot. The same 75 t clamp holds about 240 cm² at a low factor and under 100 cm² at a high one.
- Compare tie-bar spacing, mould height range, opening stroke and rated shot on both quotations rather than assuming the bigger machine has more of each.
- If you expect to add cavities within a year, size the machine for the future mould, not the first one.
What Is the Real Difference in a 75 Ton vs 100 Ton Injection Moulding Machine?
The headline difference is 25 t of clamp, which buys about 54 cm² more projected area at a typical starting factor. Mould space, opening stroke and barrel size may also step up, but these vary between machine makers, so read them from the specification sheet.
The tonnage factor is the number that moves everything. RJG and Plastics Technology give a range of 2 to 10 US tons per square inch, and suggest 3 as a starting point. One US ton per square inch is about 0.14 t/cm², so the range runs from about 0.28 to 1.4 t/cm².
Projected area each clamp can hold with a 10% reserve
| Tonnage factor | 75 t clamp | 100 t clamp |
|---|---|---|
| 0.28 t/cm² (2 US t/in²) | about 243 cm² | about 325 cm² |
| 0.42 t/cm² (3 US t/in²) | about 162 cm² | about 216 cm² |
| 0.70 t/cm² (5 US t/in²) | about 97 cm² | about 130 cm² |
Read the table across a row, not down a column. If your material and wall thickness put you at the higher factor, even the 100 t clamp holds less area than the 75 t clamp does at the starting factor.
Pick one factor and use it for both machines, or the comparison means nothing. Your resin supplier or mould maker can often suggest a figure for your grade and wall thickness, and a trial on a similar mould is better still. Write the factor down with the quotation so everyone works from the same number.
Thin walls and long flow paths push the factor up. RJG's guide is that a flow length to wall thickness ratio under 150:1 carries a low risk of short shots and sinks, so short, chunky parts sit at the low end.
How Do You Work Out Which Tonnage Your Mould Needs?
Add up the projected area of every cavity and the runner, multiply by the tonnage factor for your material and wall, and add a reserve of about 10%. Compare the result with 68 t and 90 t, which are the comfortable limits of 75 t and 100 t clamps.
Switch plates make a good test case, because they are flat, fairly large for their weight and common in Indian electrical work under IS 3854.
Now change one input. Make the plates 80 mm square and the total falls to about 65 t with the reserve, which a 75 t machine handles comfortably. Make them 95 mm square and it rises to about 92 t, past what even the 100 t clamp holds comfortably. Small changes in part size move the answer fast.
A result that lands on the line is the hardest case. A trial on the actual mould and grade, watching for flash at the parting line, is the best way to settle it. A mould flow simulation with the real runner is the next best.
Can Mould Fit or Shot Size Decide It Instead of the Clamp?
Yes, often. A mould that will not pass between the tie bars, sits outside the mould height range or needs more opening stroke than the machine has cannot run, whatever the clamp says. A shot that needs more than about 65% of the barrel also argues for the larger machine.
MoldMaking Technology advises that the mould should ideally fit inside the tie bars, and at minimum the projected area of the parts must lie fully within them. It suggests an opening stroke of at least 2.5 times the part height.
The AIM Institute adds checks that buyers often skip: the ejector pattern, the ejector stroke and the platen bolt pattern. A mould built for one machine can need a new ejector plate or clamping slots to run on another.
For shot size, keep within about 25% to 65% of barrel capacity, converted with melt density; the applications article in this series walks through that conversion.
The easiest way to compare is side by side. Ask both suppliers for tie-bar spacing, minimum and maximum mould height, opening stroke, ejector stroke, rated shot and injection pressure for each screw option, and connected load. Lay them next to your mould drawing and shot calculation, and the right size is usually obvious within a few minutes.
Is the Bigger Machine Worth It If the Smaller One Is Enough?
Not by default. A small mould in an oversized machine can suffer from too much clamp and too small a shot. Buy 100 t when your main mould calculates above about 68 t, needs more mould space, or you have firm plans for a larger mould.
Too much clamp damages moulds. RJG describes one mould clamped at four times its required tonnage that separated its cavity block in two places, and a vertical press set at 100 t for a mould that needed about 10 t, whose frame rails eventually broke. Too small a shot leaves melt in the barrel for many cycles, which degrades heat-sensitive grades.
Here is the flat opinion. If the mould that sits on the line is the product your business depends on, buy the 100 t machine. A clamp running at its limit on your main job leaves you no room for a tougher grade or a worn mould.
Investment class matters too. Plant and machinery count toward your MSME classification, so plan the second and third machine with the micro enterprise limit in view; the small-unit article in this series gives the current figures.
What Should You Send to Decide Between 75 t and 100 t?
Send the part drawing, resin grade, wall thickness, cavities, runner type, mould dimensions and any larger moulds you plan to add. With these, the clamp, shot and mould fit can be checked for both sizes. See the 75 ton horizontal injection moulding machine page.
Frequently Asked Questions About Choosing Between 75 t and 100 t
Can a 100 t machine run a mould built for a 75 t machine?
Often, if the mould fits the tie-bar spacing and sits within the minimum and maximum mould height, and the locating ring, ejector pattern and platen bolt holes match. Set the clamp force to what the mould needs rather than the machine maximum, and confirm the shot uses at least about 25% of the barrel.
Is a 10% clamp reserve enough?
It is a starting margin, not a rule. Thin walls, long flow paths, stiff grades and worn parting lines all push the real need up. If your calculation lands within a few tonnes of the clamp limit, run a trial with the actual mould and grade, or a flow simulation, before you buy.
Will a 100 t machine give a faster cycle than a 75 t machine?
Not by itself. On most small parts the cycle is set by cooling, which depends on wall thickness, mould temperature and water flow. A larger clamp only helps if the bigger machine lets you run more cavities, and that depends on projected area, shot size and mould fit.
Should I set the clamp to maximum to stop flash?
No. RJG lists burns, short shots and gloss changes among the defects excess clamp force can cause, and can damage the mould. Calculate the force your mould needs, add a reserve, and find out why the mould flashes, such as worn parting lines or too much injection pressure.
How do I convert US tons per square inch to t/cm²?
Multiply by about 0.14. One US ton is about 907 kg and one square inch is about 6.45 cm², so 1 US ton per square inch is about 141 kg/cm², or 0.14 t/cm². RJG's starting factor of 3 US tons per square inch is therefore about 0.42 t/cm².
Related Articles
- What Can You Make on a 75 Ton Injection Moulding Machine?
- Moulding Caps and Small Electrical Parts on a 75 Ton Machine
- Starting a Small Moulding Unit With a 75 Ton Machine
Sources & References
- Bureau of Indian Standards: Standards for Plugs-Socket-Outlets and Switches (BIS blog)
- RJG / Plastics Technology: Clamp Tonnage: More Is Better...Right?
- Plastics Technology (J. Bozzelli): A Simpler Way to Calculate Shot Size vs. Barrel Capacity
- MoldMaking Technology: Machine Sizing and Specification for Optimal Mold Performance, Part 1
- AIM Institute: Mold vs. Machine: Evaluation Considerations
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.
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