A two platen injection moulding machine holds the mould between just two platens, one fixed and one moving, with no rear platen or toggle linkage behind them. Small traverse cylinders move the moving platen, locking nuts grip the tie bars when the mould closes, and short-stroke hydraulic cylinders then build the clamping force. This two platen clamping unit can make the machine shorter than a toggle machine of similar tonnage, and it suits large, tall or heavy moulds such as crates, bins, pails and furniture.
Key Takeaways
- Two platens, four tie bars, locking nuts and short-stroke clamp cylinders replace the rear platen and toggle links.
- Clamp force is set as hydraulic pressure, so you can match it to each mould instead of running at full tonnage.
- Opening stroke falls as mould height rises, so check daylight against your tallest mould, not stroke alone.
- The design pays off on large and tall moulds; on small, shallow, fast parts a toggle can be the simpler choice.
How is a two platen clamping unit built?
A two platen clamping unit has a fixed platen that carries the injection side of the mould, a moving platen for the ejector side, four tie bars, traverse cylinders that move the platen, locking nuts that grip the tie bars, and short-stroke cylinders that apply the clamping tonnage.
The work is split into two jobs. Moving the platen needs speed and long travel but little force, so small cylinders do it. Holding the mould shut needs large force over a few millimetres, so large-bore cylinders with a very short stroke do it. A toggle machine does both jobs with one mechanism, which is why it needs a rear platen and a linkage.
Locking happens at the tie bars. Each bar carries a grooved section, and split nuts close onto those grooves once the mould halves touch. The full sequence, from fast close to pressure build-up and break-away, is covered in the hydro-mechanical clamping article in this series.
Main parts of a two platen clamping unit
| Part | What it does | What to ask the supplier |
|---|---|---|
| Fixed platen | Holds the injection half of the mould and takes the nozzle | Platen size, locating ring diameter, mounting pattern |
| Moving platen | Carries the ejector half and travels on guides | Maximum mould weight, platen guidance |
| Tie bars (four) | Join the platens and carry the clamp load | Clear distance between bars, tie-bar pull option for loading |
| Locking nuts | Grip the grooved tie bars so the platen cannot move back | Locking time, how mould height is set |
| Short-stroke clamp cylinders | Build tonnage by pulling on the tie bars | Maximum tonnage and whether it can be set per mould |
| Traverse cylinders | Open and close the mould quickly | Opening and closing speeds, maximum daylight |
How does it differ from a three platen toggle machine?
A toggle machine uses a third platen and a linkage that straightens to lock the mould, so its stroke stays the same at every mould height. A two platen machine has no linkage; its opening is the daylight left after the mould, and its clamp force comes from hydraulic pressure.
That last point changes how you size the machine. On a two platen machine, usable opening equals maximum daylight minus mould height, so a tall mould eats into the stroke you can use, while a short mould leaves a long opening. A Plastics Technology review of hybrid clamps also noted that most two platen hydro-mechanical machines reduce overall machine length compared with toggle or fully hydraulic machines.
Speed, opening force, maintenance and the full side-by-side table are in the two platen vs toggle article in this series.
Which jobs suit a two platen injection moulding machine?
Two platen machines suit jobs where the mould is large, tall or heavy: crates, bins, pails, table tops, chairs, automotive panels and appliance housings. Deep parts need long opening, wide moulds need room between the tie bars, and large projected areas need high tonnage that can be set accurately.
Material choice follows the part. For large articles, Reliance lists Repol B300MN, a high-crystalline PP impact copolymer with an MFR of 30 g/10 min, and states that its high flow and fast crystallisation make it suitable for large articles with lower cycle times. High flow helps fill long flow paths; clamp force still follows projected area, which the applications article in this series works through for crates, pails and table tops.
What are the limits of a two platen design?
A two platen machine loses opening stroke as mould height increases, because stroke and mould share the same daylight. Toggles are reported to lock slightly faster and to have greater opening force for deep-draw parts. Very small moulds on a large two platen machine also risk platen bowing and flash.
The small-mould limit applies to any clamp. A Plastics Technology mould-mounting guide gives a rule of thumb that the mould should cover at least 70% of the distance between the tie bars, because smaller moulds let the platens bow under clamp and flash in the centre of the mould. Very small moulds can cause serious platen damage, so check the minimum mould size in the machine manual.
Reported is the right word for the speed and force points: they come from supplier claims quoted in a 2010 review, so time the lock-up on your own mould before you weigh them.
My view: if your largest mould fits a toggle with stroke to spare, a two platen machine solves a problem you do not have.
What should you send before choosing a two platen machine?
Send the mould drawing with width, height and shut height, the mould weight, the part drawing with projected area, the material grade and the tallest part height. With those, a supplier can check tie-bar fit, tonnage and daylight together instead of one at a time. If you run several large moulds, list each one: the machine has to suit the tallest, the widest and the heaviest, and these are not always the same mould. See the two platen injection moulding machine range.
Frequently Asked Questions About Two Platen Machines
Is a two platen machine hydraulic, hybrid or electric?
The clamp on a two platen machine builds tonnage with hydraulic cylinders, so the clamp itself is hydraulic. The pump that feeds it can be driven by a fixed-speed motor or a servo motor, and some designs use an electric drive for the screw. Ask which drive each movement uses before you compare energy figures.
Why is a two platen machine shorter than a toggle machine?
It has no rear platen and no toggle linkage behind the moving platen. The clamp cylinders sit in or beside a platen and act over a short stroke, so the clamp end needs less length. A Plastics Technology review of hybrid clamps noted that most two platen hydro-mechanical machines reduce overall machine length.
Can you run a small mould on a large two platen machine?
You can, but check the minimum mould size in the machine manual first. A mould that covers much less than about 70% of the tie-bar gap lets the platens bow under clamp, which can flash the centre of the mould. Very small moulds can damage platens, so keep tonnage to what the job needs.
Do moulds need changes to run on a two platen machine?
The cavity and core need no change, but the mounting must match. Check the platen bolt pattern, ejector pattern and locating ring size, since a mismatch may need an adapter plate or ejector box. Then confirm the mould height sits between the minimum and maximum, and that the remaining daylight still gives enough opening.
Are two platen machines only built in very large sizes?
No. The design is linked with large moulds, but a 2010 Plastics Technology review reported two platen hybrid clamps from about 55 tons upward. It makes most sense where mould size, opening or tonnage control matters more than the fastest possible lock-up on small, shallow parts. Judge it on your mould, not on tonnage alone.
Related Articles
- How Does Hydro-Mechanical Clamping Work?
- Two Platen vs Toggle Injection Moulding Machines: Key Differences
- Important Specifications to Consider Before Buying a Two Platen Machine
- Applications of Two Platen Injection Moulding Machines
Sources & References
- Plastics Technology, M. Knights: Hybrid Injection Clamps: The Best of Both Worlds? (June 2010)
- Plastics Technology, P. Witaszek, J. Klees, J. Fattori, E. Di Siro, J. Bozzelli: How to Mount an Injection Mold (June 2017)
- MoldMaking Technology, B. Catoen: Machine Sizing and Specification for Optimal Mold Performance, Part 1 (Aug 2025)
- Reliance Industries: Repol B300MN Technical Data Sheet (July 2024)
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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