Choosing a horizontal injection moulding machine starts with the part, not the tonnage list. First decide which job the machine will do most of its hours: small technical parts, thin-wall packaging, or large household articles. Each job is limited by something different, so each points to a different machine family. Then check the numbers inside that family: clamp from projected area and resin, shot volume against barrel size, mould size against tie-bar spacing, and opening stroke against part depth. A machine built for fast thin-wall fill is the wrong buy for a heavy bucket, and a large-shot machine wastes power on small housings.
Key Takeaways
- Start from the part and its wall. A thin wall can need well over twice the clamp of the same footprint moulded at a normal wall.
- Thin-wall packaging needs fast injection, fill times under one second and stiff platens, not only more tonnage on the nameplate.
- Large household parts are limited by shot volume, tie-bar space and opening stroke more than by clamp per square centimetre.
- Oversizing costs you twice: higher base-load power on every shot and a real risk of over-clamping and damaging the mould.
Which machine family suits your part?
Group parts by what limits them. Small parts are limited by clamp per cavity and shot accuracy. Thin-wall containers and lids are limited by injection speed, pressure and platen stiffness. Buckets, tubs and crates are limited by shot volume, mould size and opening stroke. Each points to a different family.
Clamp force comes from projected area times a tonnage factor. Take the area of one part as seen from the nozzle, multiply by the cavities, add any cold runner, then multiply by a factor for the resin. Published factors run from about 2 to 8 US tons/in². Polyethylene sits at the low end, polycarbonate toward the high end, and PP at about 2 to 3. One US ton per square inch equals 0.1406 t/cm², so a factor of 3 is 0.422 t/cm².
Technical parts in ABS or PC need more clamp per cm² than the same footprint in PP, but the areas are small. These jobs reward fine control of injection pressure and screw position more than raw size. Large household articles turn the problem round: the clamp per cm² is moderate, but the area is large, the shot is heavy and the mould is tall. The mould sizing article in this series works through tie bars, daylight and shot weight on one mould.
Matching the part to a machine family
| Part type | What limits it | What to look for |
|---|---|---|
| Small technical parts: housings, clips, fittings | Clamp per cavity, shot accuracy | Fine pressure and position control; shot well inside the barrel window |
| Thin-wall containers and lids | Injection speed and pressure, platen stiffness | Fill under 1 s, accumulator-assisted clamp, thick platens, 5 to 8 US t/in² clamp |
| Caps and closures | Cycle time and cavity count | Fast clamp movement, short dry cycle, platen space for many cavities |
| Buckets, tubs, crates, furniture | Shot volume, mould size, opening stroke | Large screw, wide tie-bar gap, long opening stroke |
What changes when choosing a horizontal injection moulding machine for thin-wall parts?
Nearly everything on the injection side. Thin-wall parts with flow-to-wall ratios of 100:1 to 200:1 typically need injection pressures above about 2,100 kg/cm², fill times under one second and clamp factors of 5 to 8 US tons/in². A machine designed for normal walls struggles on all three counts.
The clamp has to be stiff as well as strong. Thin-wall machines tend to carry accumulator-assisted clamps for fast movement and extra-thick platens. On the better designs, the distance between the tie bars is no more than twice the platen thickness, which limits platen bending under high cavity pressure.
Injection speed matters as much as pressure. Velocities above 76 mm/s are the starting point, many jobs run near 152 mm/s, and the control has to hold speed, position and pressure to fine resolution shot after shot. The resin must keep up too. A high-flow PP homopolymer such as Reliance Repol H200MA, at MFR 25 g/10 min, is listed for thin-walled containers, multicavity moulding and long flow paths.
Our view is flat: do not buy a general machine and plan to adapt it for thin-wall packaging later. Pressure, speed and platen stiffness are designed in, and the article behind these figures advises against adapting conventional machines for this work.
How do you avoid buying a machine that is too big?
Size to your largest regular job, not a mould you might buy later. An oversized hydraulic machine carries a high base load on every shot, and its full clamp can damage a small mould. Take the calculated clamp, add a reserve, and choose the nearest machine above that figure.
We use a 10% clamp reserve in this series; treat it as a starting margin, not a rule. The energy case is strong. Surveys of moulding plants found that on a standard hydraulic machine, base load can reach 75% or more of total energy, and a high base load can mean the machine is too large for the job. All-electric and hybrid machines sit at about 10 to 20%, mostly barrel heating.
Ten-year energy cost exceeds purchase cost in almost every case, so the size you choose sets your power bill for a decade. The servo pump article in this series explains where hydraulic machines lose that power.
The machine suffers from oversizing too. Excess clamp force can crack hydraulic cylinder mounting plates, deform platens and, in the worst case, fracture the machine frame. The toggle clamping article in this series covers how tonnage is set and why it should be checked with the mould hot.
What should you send before choosing a machine?
Send the part drawing with wall thicknesses, the resin grade, the mould drawing with its size and thickness, the cavity count and your target output per hour. With those, the clamp, shot and mould-fit checks can be done before anyone proposes a machine. See the horizontal injection moulding machine range.
Frequently Asked Questions About Choosing a Horizontal Machine
Can one horizontal machine run both technical parts and thin-wall containers?
It can run both, but it will be a compromise on one of them. A machine specified for thin-wall work has injection speed, pressure and platen stiffness that technical parts rarely need, while a general machine may not fill a thin wall at all. Buy for the job that fills most of your machine hours.
Does a bigger clamp make a machine safer for future moulds?
Not by itself. Clamp you do not need raises base-load power on every shot, and if the full force reaches a small mould it can crack mounting plates, deform platens or damage the tool. Size to the moulds you will run most, with a modest reserve, and plan a separate machine for the outliers.
What tonnage factor should I use for my resin?
Start from the resin datasheet if it gives one. Without data, published factors run from about 2 to 8 US tons/in², with polyethylene at the low end, polycarbonate toward the high end and PP at about 2 to 3. Thin walls and long flow lengths push the factor up, sometimes to 5 to 8.
Is a hybrid or all-electric machine worth considering for small parts?
It can be, where the machine runs long hours. Energy guides put the base load of all-electric and hybrid machines at about 10 to 20% of their energy, mostly barrel heating, against 75% or more on some standard hydraulic machines. Compare running cost over ten years, not only the purchase price.
What part details should I share before asking for a machine?
Share the part drawing with wall thicknesses, the resin grade and its datasheet, the cavity count, the mould drawing with dimensions, the target output and the planned cycle. Note any thin-wall sections or long flow paths. Those details decide the machine family before tonnage or screw size is discussed.
Related Articles
- How to Size a Horizontal Machine From Your Mould: Tie Bars, Daylight and Shot Weight
- Toggle Clamping on Horizontal Machines: Speed, Stroke and Tonnage
- Servo Pumps on Horizontal Injection Moulding Machines: Where the Power Savings Come From
Sources & References
- SPE Plastics Engineering, A. Urbina: Mastering Clamping Force Calculation (Mar 2024)
- PlasticsToday, G. Tremblay (Sabic Innovative Plastics): Never too thin (Oct 2007)
- Reliance Industries: Repol H200MA Technical Data Sheet (July 2024)
- ExxonMobil, AMI and Tangram Technology: An Energy Saving Guide for Injection Moulders
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.
