An operator places two brass terminals in a switch base mould, closes the guard and gets a short shot with one terminal pushed sideways. The next shot is fine, and the one after cracks a week later in the store. Metal insert moulding works when the insert is held, located and heated before the plastic arrives, when the part around it can carry the shrinkage stress, and when the machine makes that repeatable on every shot.
This article covers why you would mould metal in, what goes wrong, the design rules and what the machine has to provide.
Key Takeaways
- DuPont guidance gives three main reasons to mould metal in: serviceable threads, close tolerances on female threads and a permanent joint between high-load parts.
- Delayed cracking from hoop stress is the most common complaint; a rough estimate is mould shrinkage times flexural modulus.
- Design rules include a boss about 1.5 times the insert diameter, rounded knurling, an undercut for pull-out and preheated inserts.
- The machine must hold inserts in place, close gently on them and stop the cycle if an insert is missing or out of position.
Why Mould Metal Inserts In Instead of Fitting Them Later?
Moulding in gives threads that survive repeated assembly, close tolerances on female threads and a permanent joint between high-load parts. It also removes a later pressing step. The price is a slower, more demanding cycle, so compare it with press-fit, snap-fit or ultrasonic insertion first.
Those three reasons come from DuPont's Alan Miller, in a paper summarised by Injection Molding Magazine and now hosted by PlasticsToday. The same summary notes that most inserts can instead be fixed into a premoulded part by pressing, snap-fitting or ultrasonic insertion, and that moulding metal in is often treated as a last resort.
Electrical accessories are a common Indian use. A BIS blog on IS 1293 (plugs and socket-outlets) and IS 3854 (switches) explains that these standards test shock protection, resistance to abnormal heat and fire, mechanical strength and normal operation, with a plug inserted and withdrawn 10,000 times. A terminal that loosens in its boss can fail on any of these.
What Goes Wrong in Metal Insert Moulding?
Inserts can float or move in the mould and damage it, plastic can flash into internal threads, loading can lengthen the cycle, and a rejected part wastes its insert. The most common complaint is delayed cracking around the insert, caused by hoop stress as the plastic shrinks onto the metal.
Metal insert moulding faults, causes and countermeasures
| Fault | Likely cause | Countermeasure |
|---|---|---|
| Insert moves or floats | Not held firmly; melt pushes it | Locating pin, pocket or magnet; gate away from the insert |
| Flash in internal threads | Plastic enters the bore | Close-fitting mould pin or shut-off; check insert tolerance |
| Cracking around the insert | Hoop stress from shrinkage | Lower-shrink or toughened grade, adequate boss, preheated insert |
| Sink under the insert | Too little plastic beneath it | Depth beneath at least one-sixth of the insert diameter |
| Weak weld line beside the insert | Melt fronts meet behind the insert | Preheat the insert; add a rib at the weld line |
| Damaged mould | Missing or misplaced insert when closing | Insert presence check and low-pressure mould protection |
Material choice changes the stress. The summary compares a nylon with mould shrinkage of 0.016 mm/mm and an acetal at 0.025 mm/mm; the acetal's estimated stress is about 52.4 MPa, roughly 75% of its tensile strength. Glass- and mineral-filled grades, typically 0.005 to 0.010 mm/mm, have been used with success, though heavily filled weld lines may keep only about 60% of the unfilled strength.
Insert shape matters as much as resin. The same guidance says inserts should be rounded or have rounded knurling with no sharp corners, carry an undercut for pull-out strength and protrude at least 0.4 mm into the cavity. Keep the insert small relative to the plastic around it.
Over-clamping adds risk, because broken inserts are among its listed effects, as the special purpose vs standard article in this series explains.
What Does the Machine Need for Metal Insert Moulding?
It needs a way to hold inserts in place before the mould closes, a slow and protected close, sensing that stops the cycle if an insert is missing, and enough time or a second station for loading. A vertical clamp helps, because gravity keeps loose inserts in their pockets.
Horizontal machines can run inserts too, but each insert must be held against gravity by a pin, pocket, magnet or a robot that places and holds it. Loose inserts that only sit in a pocket are easier on a vertical clamp with a flat lower mould half.
Mould protection matters more than usual. The close should slow down and run at low pressure over the last part of the stroke, so a misplaced insert stops the platen instead of being crushed into the steel. Insert presence can be checked by sensors in the mould or by a camera; the moulding cell article in this series covers in-line checking.
Preheating belongs in the process, not as an extra. The DuPont guidance says preheated inserts reduce post-mould shrinkage, pre-expand the insert and improve weld line strength. That needs a heater near the loading point and a steady time from heater to mould.
Loading time sets the cycle when inserts go in by hand. A second station lets you load one lower mould half while the other is moulding, which the shuttle and rotary table article in this series explains.
Our view: if an operator has to force an insert into place, the pocket is wrong, not the operator's technique. Fix the pocket before buying automation.
What Should You Check Before Starting Metal Insert Production?
List each insert with its material, size and tolerance, how it is held in the mould and how it will be preheated. Run the stress estimate below for your resin, then test prototypes with temperature cycling, as the DuPont guidance advises. See the 75 ton special purpose injection moulding machine page.
Frequently Asked Questions About Metal Insert Moulding
Which plastics work best for metal insert moulding?
Most engineering plastics can be used. Lower-shrinkage grades have an advantage because they build less hoop stress around the insert, and glass- or mineral-filled grades shrink less again. Toughened grades, with higher elongation, resist cracking better. Check the grade's shrinkage and modulus before choosing it for a part with moulded-in metal.
Should inserts be preheated before moulding?
In most cases, yes. Preheating expands the insert before the plastic arrives, reduces post-mould shrinkage stress and improves weld line strength. Keep the heating temperature and the time from heater to mould steady, because an insert that cools by a different amount on each shot gives parts that vary.
Can a horizontal machine be used for insert moulding?
Yes, if every insert can be held firmly in the mould half against gravity, using pins, pockets, magnets or a robot that places and holds it. Loose inserts that only sit in a pocket are easier on a vertical clamp. The choice depends on the insert, the mould and how loading will be done.
How do I stop plastic flashing into a threaded insert?
Seal the thread with a mould pin or shut-off that fits the insert bore closely, and make sure the insert seats fully before the mould closes. Flash in threads often points to a loose fit, worn pins or inserts outside tolerance, so inspect incoming inserts as well as the mould itself.
What happens to the insert when a part is rejected?
It is usually lost or costly to recover, since the plastic must be cut or ground away. That makes scrap on insert parts dearer than on plain parts. Process stability, insert presence checks and careful first-off approval matter more here, because each rejected shot wastes metal as well as resin.
Related Articles
- Special Purpose vs Standard Injection Moulding Machine: How to Decide
- Shuttle Table vs Rotary Table: When Two Stations Pay Off in Insert Moulding
- Building an Injection Moulding Cell: Robot, Sorting and In-Line Checking
Sources & References
- PlasticsToday (Injection Molding Magazine), summarising A. Miller, DuPont: Design and material advice from the pros, Part 3: Insert molding essentials
- BIS Blog, N. Agarwal: Standards for Plugs-Socket-Outlets and Switches
- Reliance Industries: Repol H110MAS Technical Data Sheet
- LyondellBasell: Technical Tip: Mold Shrinkage
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.
