Key takeaways
- A moulded-in insert is the strongest fixing option, but it adds cycle time, tool wear and a placement risk.
- The insert shape, not the plastic alone, decides pull-out and torque resistance. Knurls, grooves and a flange all do work.
- The boss wall must carry the hoop stress from thermal expansion and from the insert itself.
- Test pull-out, torque and thermal cycling on the first article, not just the insert position.
What insert moulding does
Insert moulding places a metal part in the tool before the plastic is injected. The melt flows around the insert and shrinks onto it as it cools, so the finished part has a metal thread, a bearing surface, a contact or a mounting point that is mechanically locked into the housing. The insert can be a threaded bush, a plain sleeve, a pin, a terminal or a blade.
The alternative is to mould a plain boss and install the insert afterwards by heat setting, ultrasonic insertion or a self-tapping screw. Moulded-in inserts give the highest pull-out and torque resistance and remove an assembly step, but they add cost and risk to the cycle.
Moulded-in, heat-set or ultrasonic
A moulded-in insert is placed on a core pin in the tool. It gives good resistance to pull-out and torque because the plastic shrinks onto the knurl or groove around the full circumference. The cost is cycle time, because the insert has to be loaded and the tool has to close around it, and tool wear, because every cycle brings metal against the mould.
A heat-set insert is installed after moulding. It is heated and pressed into a boss, and the plastic reflows around the knurl. It suits low volume, prototypes and repairs and does not slow the moulding cycle, but it adds an operation, a heat-affected zone and a boss designed for insertion.
Ultrasonic insertion uses high-frequency vibration to soften the plastic locally. It is fast and gives good control, but it needs a suitable boss and a process that does not overheat the surrounding wall. Self-tapping screws are the simplest option and the weakest for repeated assembly, because each removal cuts a fresh thread.
| Route | Retention method | Best fit | Main design check |
|---|---|---|---|
| Moulded-in | Shrink fit plus knurl or groove | High volume, high torque, sealed joints | Boss wall, insert support and tool wear |
| Heat-set | Reflowed plastic around a knurl | Low volume, prototypes, repairs | Boss diameter, installation depth and heat control |
| Ultrasonic | Local melt around the insert | Medium volume, controlled assembly | Horn access, energy and cycle control |
| Self-tapping screw | Thread cut into the plastic | Low load, few assembly cycles | Pilot hole, screw type and driving torque |
The retention values depend on the insert, the plastic and the boss geometry. Test the chosen combination rather than applying a generic figure.
The insert shape does the holding
A plain cylinder relies on friction and shrinkage alone. A knurled insert adds mechanical interlock, and a groove or undercut resists axial pull-out. A flange at the top spreads the load into the plastic and stops the insert being pulled through the boss. A closed end stops plastic entering the thread, and a lead-in chamfer helps the insert seat on the core pin.
The knurl pattern matters. A fine diamond knurl gives good torque resistance in a small insert, while a coarse knurl or groove gives more axial grip. Use the insert supplier's recommended hole and boss dimensions, and do not copy a boss size from a different insert family.
The insert should also be located positively in the tool. A core pin that supports it from the inside keeps it square and stops plastic flowing behind it. If the insert can float, the thread will not be concentric with the boss.
Boss wall and the hoop stress problem
The plastic around a metal insert is a ring in tension. When the insert expands more than the plastic, or when a screw is driven, the ring carries hoop stress. Too thin and it cracks; too thick and it sinks the face behind it. The working range is narrower than for a plain screw boss.
A common start is a boss outside diameter of about two to two and a half times the insert diameter, with a wall at 50 to 60% of the nominal wall. The insert data, the plastic's modulus and the assembly torque decide the final wall.
The boss should be supported by a gusset or tied into a nearby wall, because the bending moment from a tightened screw is carried at the root. A free-standing boss with a tall, thin wall is the most likely to crack. Keep the boss height down where the design allows, and radius the root rather than leaving a sharp corner.
Thermal expansion and the material pair
Metals and plastics expand at very different rates. Aluminium is roughly 23 parts per million per kelvin, brass about 19 and steel about 11 to 13. An unfilled PA66 can reach 80 to 100, and a glass-filled grade is lower but still several times the metal. The mismatch peaks when the part is hot.
The design response is to give the plastic somewhere to go. A slightly compliant boss wall, a flange that spreads the load and an insert with a rounded profile all help. A sharp-cornered insert in a stiff, glass-filled plastic is the worst combination for cracking. If the part will see thermal cycling, test the assembly through the temperature range, not just at room temperature.
Moisture adds a second movement in PA66. The boss may grow after conditioning, which can relieve or increase the stress. For a sealed or high-reliability part, condition the samples before the retention test so the result reflects the service state.
Process control on the moulding floor
The insert should be clean, dry and at a controlled temperature. Preheating reduces thermal shock and improves the shrink fit, but an insert that is too hot keeps the plastic molten and increases sink. Put the set point in the process sheet, not in the operator's judgement.
Placement is the next control. The insert must sit square on the core pin, at the correct depth, with the correct orientation. A vision check or a mechanical gauge at the tool is more reliable than a visual check after ejection. If the insert is loose, plastic can flash behind it and the thread can fill.
Tool maintenance matters more than for a plain mould. Core pins wear, insert seats erode and shut-offs lose their fit, which shows up as flash or a thread that is not concentric. Add the insert seats to the preventive maintenance schedule and check them at a defined interval.
What to test on the first article
A dimensional check confirms the insert position but not the joint. Pull-out tests apply an axial load until the insert moves or the boss fails. Torque tests tighten a screw to a defined value, hold it and check for rotation or cracking. For a sealed insert, add a leak or pressure test.
Run the tests on parts that have been conditioned to the service state, and record the failure mode. A pull-out that leaves the insert in place but cracks the boss is a different problem from one where the insert pulls out cleanly. The first points to the boss wall or the insert shape, the second to the knurl or the shrink fit.
Thermal cycling before the retention test is worth the time when load and temperature act together. A joint that passes at 23°C can fail after a few hundred cycles if the hoop stress is near the limit. State the cycle range, dwell time and acceptance load.
What to put on the drawing and the RFQ
Specify the insert by part number, material, thread size, knurl or groove and insertion depth. State the boss diameter and wall, the allowable flash and the thread concentricity. Add the pull-out and torque requirements with the test method and conditioning state.
Send the mating screw or stud, the assembly torque and the service temperature. If the housing will be potted, sealed or washed, say so, because those processes change the moisture and thermal load on the joint. Ask for a first article that includes the insert retention results and a section through the boss.
For a new design, ask for a DFM review before the tool is cut. It should cover the insert support, the boss wall, the gussets, the gate position and the sink risk on the visible face. Those decisions are expensive to change after the steel is cut.
Frequently asked questions
What is the difference between insert moulding and heat-set inserts?
Insert moulding places the metal insert in the tool before the plastic is injected, so the plastic shrinks around it. Heat-set inserts are installed after moulding by heating the insert and pressing it into a boss. Moulded-in inserts usually give higher retention; heat-set inserts suit lower volume and repairs.
How thick should the boss wall be around a moulded-in insert?
A common starting point is a boss outside diameter of about two to two and a half times the insert outside diameter, with a wall at 50 to 60% of the nominal wall. Confirm the final size with the insert supplier and a pull-out and torque test, because the knurl and the plastic both affect the result.
Why does the plastic crack around an insert?
Usually the boss wall is too thin for the hoop stress, the insert has a sharp corner, the plastic is stiff and glass-filled, or the screw is over-torqued. Thermal expansion mismatch makes it worse at high temperature. Check the section, the insert profile and the assembly torque before changing the material.
Should a moulded-in insert be preheated?
Preheating can reduce thermal shock and improve the shrink fit, but an insert that is too hot keeps the surrounding plastic molten and increases sink. The set point should come from the insert and material suppliers and be fixed in the process sheet.
Sources and further reading
- ISO 20457:2018, Plastics moulded parts, tolerances and acceptance conditions
- ISO 898-1, Mechanical properties of fasteners made of carbon steel and alloy steel
- Qumek insert moulding DFM reviews and first article inspection records
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
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