A customer sends a new drawing: a thin-wall food tub, or a switch base with ten brass inserts. Your setters try it on the best machine you have. The tub short-fills at full injection speed, or the inserts take so long to load that the mould sits open half the cycle. That moment answers when to buy a special purpose machine. You need one when the part hits a physical limit of your standard machines (pressure, speed, stroke, tie-bar gap) or when a step outside moulding, such as insert loading, sets the cycle. Settings cannot fix either.
Key Takeaways
- Buy special when the limit is physical: injection pressure or speed, opening stroke, tie-bar gap or a loading step that dominates the cycle.
- Thin-wall parts with flow length to wall ratios of 100:1 to 200:1 need pressures and speeds far above conventional moulding.
- Insert moulding can spend more time with the mould open than closed. Time the loading before you size anything else.
- Try process and mould changes first. If the limit stays after a fair trial, the machine is the constraint.
Which part features push a job beyond a standard machine?
Thin walls with long flow, very deep or very wide moulds, many inserts per shot, side actions or unscrewing, and resins that attack or degrade in a general screw. Each one stresses a different system, so the feature tells you which part of the machine must change.
Thin walls are the clearest case. Thin-wall designs have flow length to wall thickness ratios of about 100:1 to 200:1. They need injection pressures typically above 30,000 psi, about 2,109 kg/cm², and injection speeds above 3 in/s (76 mm/s), with many jobs near 6 in/s (152 mm/s) and fill times under 1 s.
Clamp demand rises too: 5 to 8 US tons/in² against 3 to 5 in conventional moulding. Thin-wall machines tend to have accumulator-assisted clamps for fast movement and extra-thick platens; on better machines the tie-bar distance to platen thickness ratio is 2:1 or less. The same source calls adapting conventional machines for this work usually not a wise idea.
The injection unit changes in detail as well. Thin-wall machines often carry a smaller barrel, because the shot should be at least 40% of barrel size; too large a barrel may not give the pressure boost and lets the melt degrade. Control resolution tightens to about 1 bar of pressure and 0.01 s of time.
Resin choice follows. Thin walls need high-flow grades; Reliance lists its Repol H200MA, a PP homopolymer with an MFR of 25 g/10 min, for thin-walled containers, multicavity moulding and long flow paths. A high-flow grade helps, but it does not replace the pressure and speed the machine must deliver.
Corrosive or heat-sensitive resins are a third case. Rigid PVC, for instance, needs a streamlined melt path, a suitable screw and barrel metal and a check ring that leaves no dead spots; the applications article in this series covers it.
Deep and wide moulds hit stroke, daylight and tie-bar limits instead; the article on what a special purpose injection moulding machine is, in this series, explains those limits.
When do inserts or added steps justify a special machine?
When the time spent loading inserts or handling parts with the mould open grows close to, or beyond, the time the mould spends closed. At that point output depends on loading, not moulding, and a shuttle or rotary table, automation or a vertical clamp can pay back.
Moulding metal in has real costs. Inserts can float or damage the mould, flash can enter internal threads, preheating may be needed and rejects are costly to salvage. Design guidance often treats moulding-in as a last resort, after pressing, heat staking or ultrasonic insertion into a moulded part.
Heat staking and ultrasonic insertion remelt plastic, so they suit thermoplastics only. For thermosets, the choices are moulding in or cold pressing. If your part must have moulded-in inserts for pull-out or torque, the loading step deserves its own design.
Automation is one answer. In one published case, unloading a four-cavity mould, checking cavities for debris and loading and verifying inserts took 30 s by hand. A SCARA robot fed from a vibratory bowl took over, with vision confirming clean cavities and seated inserts before each shot. Steady mould-open times also ended swings in mould temperature.
How do you decide when to buy a special purpose machine?
Run a fair trial on your best standard machine and record what limits the job. If the limit is a setting, a mould change or an operator step, fix that first. If it is a physical machine limit that stays after the trial, a special purpose machine is the answer.
Signals from a trial and the machine change they point to
| Signal in the trial | What it means | Likely machine change |
|---|---|---|
| Short shots at full speed and pressure | Injection unit at its limit | Higher pressure and speed unit |
| Part cannot clear the mould on opening | Stroke or daylight too small | Longer stroke, more daylight |
| Mould will not fit between tie bars | Tie-bar gap too small | Wider gap or larger platens |
| Mould open longer than closed | Loading sets the cycle | Shuttle or rotary table, automation |
| Black specks, streaks, pitted screw | Resin attacking or degrading in the barrel | Special screw, barrel and check ring |
Our view: a thin-wall or heavy insert job deserves a machine built for it from the start. Where such a job will run for years, stretching a standard machine can cost more in scrap and lost hours than a special build.
What should you prepare before you ask for a special machine?
Bring the trial record: the part, the resin grade, the settings tried, what limited the job and the output you need. With the insert count and loading time, or the wall thickness and flow length, the change can be scoped quickly. See the special purpose injection moulding machine range.
Frequently Asked Questions About Needing a Special Purpose Machine
Can I run thin-wall containers on a standard machine?
Some can, if the wall and flow length are moderate. True thin-wall parts with flow ratios of 100:1 to 200:1 need injection pressures above about 2,109 kg/cm² and fill times under 1 s. One thin-wall source calls adapting conventional machines for this work usually not a wise idea.
What is a fair trial before buying a special machine?
Run the actual mould and resin on your most capable machine, with the mould maker present if possible. Record speed, pressure, fill time, cycle and defects at each step. If the job stops at the same machine limit after settings and mould fixes, the limit is physical.
Are moulded-in inserts always the best choice?
No. Inserts can also be pressed, snap-fitted, heat staked or ultrasonically inserted into a moulded part. Moulding in gives full encapsulation and the best pull-out and torque, but it adds loading time, flash risk and mould wear, so design guidance often treats it as a last resort.
What does a high flow ratio mean for a part?
Flow ratio is flow length divided by wall thickness. A 100 mm flow in a 0.5 mm wall gives 200:1. The higher the ratio, the more pressure and speed the melt needs to reach the end before it freezes, which is why thin-wall jobs push injection units hard.
Should I buy a special machine for a single customer order?
Look at the order length and the chance of repeat business. A special feature that is tied to one part may sit idle when the order ends, while a change such as a longer stroke or wider tie-bar gap can serve other jobs. Weigh both before you commit.
Related Articles
- What Is a Special Purpose Injection Moulding Machine?
- Special Purpose vs Standard Injection Moulding Machines
- Important Details to Share Before Specifying a Special Purpose Machine
- Applications of Special Purpose Injection Moulding Machines
Sources & References
- PlasticsToday, G. Tremblay: Never too thin (Oct 2007)
- PlasticsToday (IMM), A. Miller: Design and material advice from the pros, Part 3: Insert molding essentials (Sept 2005)
- Plastics Technology (PennEngineering): Four Ways to Tackle Threaded Inserts for Plastics (Jan 2018)
- Reliance Industries: Repol H200MA 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.
