Four round lids, 70 mm across, give 153.9 cm² of projected area. In a normal PP wall, that needs about 48 to 71 t of clamp. Make the same lids thin-wall and the figure jumps to about 119 to 190 t. Same mould face, nearly triple the clamp, because of one detail: wall thickness against flow length. That is why a special purpose machine specification checklist starts with the part, not the machine. Miss one detail and the machine is sized for the wrong job. The worked example near the end shows the arithmetic.
Key Takeaways
- Share wall thickness and flow length with the drawing. They decide whether clamp and injection follow conventional or thin-wall rules.
- Name the exact resin grade, not just the polymer. Melt flow and grade change the clamp factor and the injection unit you need.
- Give full mould data: size, shut height, opening needed, ejector pattern, locating ring, weight, core pulls and hot runner zones.
- Add the output target, insert or automation steps, and plant supplies such as cooling water, air and power.
What goes on a special purpose machine specification checklist?
Four groups of details: the part, the resin, the mould and the production plan. Together they fix clamp force, shot size, injection pressure and speed, opening stroke, tie-bar gap, tables and automation. Leave one out and the builder has to guess.
Special purpose machine specification checklist
| Detail | Why it matters | How to give it |
|---|---|---|
| Part drawing or 3D model | Projected area, depth, wall, flow length | Drawing with wall thickness marked |
| Resin grade | Melt flow, clamp factor, screw and barrel needs | Supplier and grade name, with data sheet |
| Cavities and runner | Total projected area and shot weight | Cavity count, cold or hot runner, runner area |
| Mould size and shut height | Tie-bar gap, minimum and maximum shut height | Width, height, thickness in mm |
| Opening and ejection | Stroke, daylight, ejector travel | Part depth, ejector stroke, core pulls |
| Mounting | Platen fit | Ejector pattern, locating ring diameter, mould weight |
| Production plan | Cycle, tables, automation | Parts per hour, inserts, shifts |
Part details come first. Projected area, the share of it that is cored out, and the runner set the clamp. Wall thickness and flow length decide which clamp factor applies. Gate location, gate size and the number of gates also move the factor.
Accurate part data also stops over-sizing. More clamp is not a free safety margin: over-clamping can crush vents, roll the parting line, break inserts and crack mould blocks. In one published case, a mould needing about 100 t was clamped at a 400 t machine maximum, and the cavity block split in two places.
Name the grade, not the polymer. Melt flow index cannot be compared across resin types; polycarbonate stays viscous in moulding at an MFI of 2 or 25. Within one polymer, the grade still matters. Reliance lists Repol H110MAS, a nucleated PP homopolymer, at an MFR of 11 g/10 min for rigid packaging, furniture and houseware, while its thin-wall grade runs at 25. The two need different injection effort for the same part.
Which mould details must the machine match?
Mould width and height against the tie-bar gap, mould thickness against minimum and maximum shut height, part depth against opening stroke, and the mounting and ejector pattern. Add mould weight, locating ring size, core-pull circuits and hot runner zones, because each one is a fixed machine feature.
Ideally the mould fits within the tie bars, and at the least the projected area of the parts must sit fully inside them. 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 part height.
Mounting needs the same care. Check the mould mounting and ejector patterns against the platens; a mismatch may need an ejector box. Most moulds use a 101.6 mm or 120 mm locating ring. Give the mould weight so it can be checked against both platens. For level and parallelism, 0.025 to 0.05 mm is generally acceptable on machines under 500 t, so ask how the platens are checked once the special build is assembled.
Then list what moves: core pulls, unscrewing cores, valve gates and hot runner heater zones. Each needs a hydraulic, pneumatic or electrical circuit on the machine, and adding one later can cost more than building it in.
Which production and plant details complete the picture?
Parts per hour, shifts per day, and every step outside moulding: insert loading, part removal, inspection, labelling or assembly. Add the cooling water, compressed air and power supply available at the machine. These decide tables, automation, pump and motor size, and how the cell is laid out.
For insert jobs, add the insert count, insert material, how each insert is fed and placed, and the time each placement takes on your shop floor. Loading time can decide whether the machine needs a shuttle or rotary table, so a measured figure is worth more than an estimate.
Dry cycle time sets a floor under the cycle. Guidance puts it at about 1 to 2.5 s for thin-wall machines and 3 to 5 s otherwise, so state your cycle target and let the builder check it against the clamp movement.
Our view: if you can send only one thing, send the mould drawing. A special build lives or dies on mould fit, and the mould drawing answers more of this checklist than any description of the part.
How do you send these details for a special machine enquiry?
Put the part drawing, resin data sheet, mould drawing and production plan in one enquiry, and mark anything not yet fixed. A sample part helps where the mould is still being designed. Then review the special purpose injection moulding machine range.
Frequently Asked Questions About Specifying a Special Purpose Machine
Why does wall thickness change the clamp so much?
Thin walls need much higher injection pressure to fill before the melt freezes, and that pressure acts on the whole projected area. Thin-wall guidance uses 5 to 8 US tons/in² against 3 to 5 for conventional parts, so the same lid can need nearly three times the clamp.
What is shut height on an injection moulding machine?
Shut height is the mould thickness the clamp can close on, given as a minimum and a maximum. A mould thinner than the minimum cannot be clamped without spacers, and one thicker than the maximum will not fit. Give the exact mould thickness in your enquiry.
What is a locating ring and why does its size matter?
The locating ring centres the mould on the fixed platen so the sprue lines up with the nozzle. Most moulds use a 101.6 mm (4 inch) or 120 mm ring. If the ring and platen bore do not match, an adapter is needed before the mould can run.
Should I share the runner area in my enquiry?
Yes. With a cold runner, the full runner area adds to the projected area the clamp must hold, and it adds to the shot weight. With a hot runner, only the cavities count toward projected area, but the hot runner zones then need controller channels.
What if the mould is not designed yet?
Share the part drawing, resin grade, cavity target and output, and say the mould is open. The machine and mould can then be sized together, which avoids designing a mould that needs more stroke, tie-bar gap or clamp than the planned machine offers.
Related Articles
- What Is a Special Purpose Injection Moulding Machine?
- When Does a Moulder Need a Special Purpose Machine?
- Special Purpose vs Standard Injection Moulding Machines
- Applications of Special Purpose Injection Moulding Machines
Sources & References
- PlasticsToday, G. Tremblay: Never too thin (Oct 2007)
- Plastics Technology, J. Williams (RJG): Clamp Tonnage: More Is Better...Right? (May 2017)
- MoldMaking Technology, B. Catoen: Machine Sizing and Specification for Optimal Mold Performance, Part 1 (Aug 2025)
- Reliance Industries: Repol H110MAS 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.
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.
