Projected area tonnage calculation: largest area at 125 t and 150 t, with a 10% reserve (a starting margin, not a rule)
| Tonnage factor (US t/in²) | Where it applies | Largest area at 125 t | Largest area at 150 t | Extra area at 150 t |
|---|---|---|---|---|
| 2 | Low end of the typical range | 404 cm² | 485 cm² | 81 cm² |
| 3 | Common starting point when nothing better is known | 269 cm² | 323 cm² | 54 cm² |
| 4 | Middle of the range | 202 cm² | 242 cm² | 40 cm² |
| 5 | Upper end listed for a general PC grade | 162 cm² | 194 cm² | 32 cm² |
| 8 | High factor, long flow or thin walls | 101 cm² | 121 cm² | 20 cm² |
A projected area tonnage calculation multiplies the part's projected area by the resin's tonnage factor. If the result, plus a reserve, stays under about 114 t, a 125 t machine fits. Between about 114 t and 136 t, you need 150 t. The table shows the area each clamp can hold.
The step from 125 t to 150 t adds 20% more clamp, and so 20% more area at any factor. The real question is whether your part sits in that band.
Key Takeaways
- Clamp needed is projected area, minus through-holes, times cavities, plus any cold runner, times a tonnage factor.
- Typical factors run from 2 to 10 US t/in², and 3 is a recommended starting point. One US t/in² is about 0.141 t/cm².
- Moving from 125 t to 150 t adds about 54 cm² of area at a factor of 3 and about 32 cm² at a factor of 5.
- Oversizing has costs: a higher base load on the power bill, and the risk of clamping a small mould far harder than it needs.
How Does a Projected Area Tonnage Calculation Work?
Take the area of one part as seen from the direction the mould opens, subtract through-holes, multiply by the cavity count and add the full cold runner area. Multiply that total by a tonnage factor for the resin. The answer is the force trying to push the mould open.
This is the method a Plastics Technology column by RJG trainer Jeremy Williams sets out. It gives typical factors of 2 to 10 US tons per square inch and recommends 3 as a starting point. With a hot runner, only the cavities count, because the runner melt stays inside the manifold.
Units trip people up. Published factors are in US tons per square inch, while Indian machines are rated in metric tonnes and drawings are in millimetres. One US ton is 0.9072 t and one square inch is 6.4516 cm², so 1 US t/in² is about 0.141 t/cm².
The column also lists what moves the factor: melt flow, flow length, wall thickness, gate location, gate size, number of gates and resin volume. It suggests that a flow length to wall thickness ratio below 150:1 carries a low risk of sinks, short shots or dimensional problems.
When Is 125 t Enough and When Do You Need 150 t?
If your calculated clamp, with a 10% reserve, stays under 125 t at the upper end of the resin's factor range, 125 t is enough. If it lands above 125 t at that end but under 150 t, choose 150 t. Use the upper end, not the average, to decide.
Our view: size on the factor you can defend, not the one that makes the smaller machine fit. A part that needs 120 t at the high factor will run on 125 t on a good day and flash on a bad one.
Clamp is not the only reason to step up. A larger machine can bring more tie-bar space, a longer opening stroke and a bigger injection unit, and any one of those can decide the choice. Mould outside size and shut height should be checked as early as clamp.
Resin choice can settle it too. A Bayer MaterialScience (now Covestro) datasheet for Makrolon 2558, a general-purpose PC, lists a clamp tonnage of 3.0 to 5.0 US t/in². At 5, a 125 t clamp holds about 162 cm² with a 10% reserve, while 150 t holds about 194 cm².
What Does Oversizing to 150 t Cost You?
It costs more to buy, and it can cost more to run, because a larger hydraulic machine carries a larger base load. It can also tempt operators to set clamp near the machine's maximum, which can damage a small mould. Buy 150 t because the part needs it.
An energy guide from ExxonMobil, AMI and Tangram Technology says a standard hydraulic machine's base load can be 75% or more of its total energy, and that a high base load can mean the machine is too large for the job. It also found ten-year energy cost exceeds purchase cost in almost every case.
In Maharashtra, MERC set kVAh billing for HT consumers and LT consumers above 20 kW from April 2020, with MSEDCL charging demand on kVA and energy on kVAh, so poor power factor and unneeded load both show up on the bill. Check your latest bill before you compare running costs between two machine sizes.
Too much clamp is also a mechanical risk. The Plastics Technology column describes a mould that needed about 100 t being clamped at the 400 t machine maximum, and its cavity block split in two places. Set clamp from the calculation, whatever the nameplate says.
How Should You Decide Between the Two Sizes?
Run the calculation for every mould you plan to put on the machine in its first few years, at the upper factor for each resin. If the largest stays under about 114 t, choose 125 t. If one or two land between 114 t and 136 t, compare 150 t. See the 125 ton horizontal injection moulding machine page.
Frequently Asked Questions About Projected Area Tonnage Calculation
How do I measure projected area from a 3D model?
CAD software can project a part onto a plane and report the area. Choose the plane at right angles to the mould opening direction, then subtract through-holes that run in that direction. Check side cores separately, because melt pressure on a side core loads the slide, not the main clamp.
Does cavity pressure give a better answer than tonnage factors?
It can, when you know it. Clamp force equals projected area times average cavity pressure, and a mould-filling simulation or cavity pressure sensors can give that figure. Tonnage factors are a shortcut for when pressure is unknown. Use simulation results where you have them and factors for early sizing.
Can a 150 ton machine run a mould that needs only 80 t?
Yes, if you set the clamp force to what the mould needs rather than the machine maximum, and the mould fits the platens and minimum shut height. Running far below capacity can raise energy use per part, so check whether the job could sit on a smaller machine instead.
Why do different sources give different tonnage factors?
Factors are averages that bundle flow length, wall thickness, gating and melt temperature into one number. A source writing about general moulding will quote a wider or lower range than one focused on thin walls. Use the resin supplier's figure for your grade where it exists, then adjust for your part.
Does mould temperature affect the clamp needed?
Indirectly. A hotter mould keeps the melt fluid for longer, which can lower the pressure needed to fill and so the force on the clamp. A cold mould can raise fill pressure on long flow paths. The effect tends to be smaller than flow length or wall thickness, but it can matter near the limit.
Related Articles
- What Can You Make on a 125 Ton Injection Moulding Machine?
- Multi-Cavity Closure Moulds on a 125 Ton Machine
- Moulding Automotive Clips and Fasteners on a 125 Ton Machine
Sources & References
- Plastics Technology, J. Williams (RJG): Clamp Tonnage: More Is Better...Right?
- Bayer MaterialScience (now Covestro) via IDES: Makrolon 2558 datasheet
- ExxonMobil, AMI and Tangram Technology: An Energy Saving Guide for Injection Moulders
- Maharashtra State Electricity Distribution Co. Ltd (MSEDCL): kVAh Billing: Frequently Asked Questions
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