Here is when a 180 ton PET machine is enough: every mould you plan to run passes four checks, namely clamp for its cavity count, mould size between the tie bars, opening stroke and shot against the barrel. Move to 220 or 260 t when one of those checks fails for a mould you will actually buy. A bigger frame bought for a mould that may never come costs you energy and floor space on every job it runs.
This article shows how far the clamp stretches at 180, 220 and 260 t, why the mould often runs out of room first, when moving up pays and what an oversized machine costs to run.
Key Takeaways
- Clamp, tie-bar fit, opening stroke and shot are separate checks; any one can rule out 180 t for a given mould.
- On a 400 bar guide pressure with a 10% reserve, 180 t covers about 401 cm² of projected area, 220 t about 490 cm² and 260 t about 579 cm².
- A preform mould's cavities must sit inside the tie bars, so a wide high-cavity mould can need a larger frame before it needs more clamp.
- A standard hydraulic machine's base load can be 75% or more of its energy, so a machine too large for the job costs money every hour.
How Do You Know When a 180 Ton PET Machine Is Enough?
Run four checks for every mould in your plan: clamp, mould size, stroke and shot. If all pass with a reserve, 180 t is enough. Clamp is easiest to work out. Divide usable clamp by pressure per square centimetre to find the largest projected area.
Largest total projected area and cavity count by clamp, with a 10% reserve (a starting margin, not a rule)
| Basis | 180 t | 220 t | 260 t |
|---|---|---|---|
| 400 bar guide pressure (0.408 t/cm²) | 401 cm² | 490 cm² | 579 cm² |
| Cautious 5 US t/in² (0.703 t/cm², an assumption) | 233 cm² | 285 cm² | 336 cm² |
| 36 mm ledge cavities at 400 bar (10.18 cm² each) | 39 | 48 | 56 |
| 36 mm ledge cavities at 5 US t/in² | 22 | 27 | 33 |
The 400 bar figure is SPE Plastics Engineering's guide value where no data exist, and the same article puts tonnage factors between 2 and 8 US tons per square inch depending on material. The cautious row uses 5, an assumption in the upper middle of that range. How to take a preform's projected area is set out in the applications article in this series.
Notice how far apart the two rows sit. The cavity pressure your mould will really see, from the mould maker's filling analysis or a trial, decides which row applies.
Why Does Mould Size Often Decide Before Clamp?
Because preform cavities are small in area but need room around them for cores, neck splits, cooling and hot-runner nozzles. The mould grows wider faster than its projected area grows, so it can reach the tie bars while the clamp still has spare force.
Bruce Catoen's machine sizing guide in MoldMaking Technology says the mould should ideally fit within the tie bars, and at minimum the parts' projected area must sit wholly inside them, because platens can pull in near the bars and wear the parting line.
The same guide sets three more checks. The mould must be thick enough for the minimum shut height and thin enough for the maximum. Opening stroke should be at least 2.5 times the part height. Both platens must carry the mould weight, which grows quickly on high-cavity preform tools.
Ask the mould maker for the mould's width, height, thickness and weight early. A cavity layout that misses the tie-bar gap by a few millimetres can force a larger frame or a lower cavity count.
When Does Moving to 220 or 260 t Pay Off?
When a mould you will buy fails a check at 180 t and passing it earns more than the extra cost. Moving from 180 to 220 t adds about 22% of clamp; 260 t adds about 44%. A larger frame can also bring more tie-bar space and a larger screw.
The gain is real where a buyer's volume justifies more cavities on a light, high-volume preform, or where a heavy preform needs a larger barrel. In those cases a larger machine can replace two smaller ones and share one dryer, one chiller and one operator.
Our view: size the machine for the moulds on your order book and the next one you are confident of, not for the largest mould you might someday buy. A machine sized for a possible mould runs every actual mould at a disadvantage.
What Does an Oversized Machine Cost You?
Energy, mainly. The energy saving 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.
The same guide puts all-electric and hybrid base loads at about 10% to 20%, mostly barrel heating, and says ten-year energy cost exceeds purchase cost in almost every case. That is why the running cost of an oversized machine matters more than its price difference.
In Maharashtra, MSEDCL's kVAh billing FAQ says demand charges are on kVA and energy charges on kVAh, and MERC intended this billing for HT consumers and LT consumers above 20 kW from April 2020. Larger motors and heaters can raise the demand you pay for, and at a power factor of 0.85, 15% of the power does no useful work.
There is a quality cost too. A light preform on a large screw spends longer in the melt, which raises acetaldehyde, as the heavy preforms article in this series explains.
What Should You Send to Decide Between 180, 220 and 260 t?
Send every mould you plan to run, with cavity count, ledge diameter, preform weight and length, and the mould maker's width, height, thickness and weight. Add the monthly volumes. With those, each check can be run at all three sizes. See the 180 ton PET injection moulding machine page.
Frequently Asked Questions About Choosing 180, 220 or 260 t
Should I buy a 220 t machine now to allow for future growth?
Only if a specific mould on your plan needs it. A larger machine costs more to buy and, with a high base load, more to run on today's moulds. If growth is likely but not certain, a second machine later can add capacity and also gives you a backup when one machine is down.
Can a mould built for a 220 t machine run on a 180 t machine?
Only if it passes every check on the smaller machine: clamp for its cavities, width and height between the tie bars, thickness within the shut height range, stroke, mould weight, locating ring and ejector pattern. A mould that fits on paper can still fail on the tie-bar gap.
What is tie-bar spacing and why does it matter for preform moulds?
It is the clear distance between the machine's tie bars, horizontally and vertically. The mould has to pass between them to be mounted, and its cavities should sit inside them under clamp. Wide preform moulds with many cavities in a row reach this limit early.
How do I find the real cavity pressure for my preform mould?
Ask the mould maker for a mould-filling analysis based on your preform design and resin grade, or measure it during a trial with cavity pressure sensors. Either gives a better basis than a guide value. Then size clamp from that figure plus your chosen reserve.
Does a larger machine give a faster cycle on the same mould?
Not by itself. On a preform mould the wall thickness sets most of the cycle through cooling. A larger machine may move its platens more slowly over a longer stroke, so dry cycle and mould movement can even take longer. Faster cycles come from the mould and cooling.
Related Articles
- What PET Products Can You Make on a 180 Ton Machine?
- Heavy Preforms for Large-Volume Bottles on a 180 Ton Machine
- Planning Preform Output: Cavities, Cycle Time and Drying Capacity
Sources & References
- SPE Plastics Engineering, A. Urbina: Mastering Clamping Force Calculation
- MoldMaking Technology, B. Catoen: Machine Sizing and Specification for Optimal Mold Performance, Part 1
- ExxonMobil, AMI and Tangram Technology: An Energy Saving Guide for Injection Moulders
- Maharashtra State Electricity Distribution Co. Ltd (MSEDCL): kVAh Billing: Frequently Asked Questions
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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