Machine Design #96: Inserts in Plastic Parts — Design the Small Joint for Heat and Force
Plastic insert must preserve function, manufacturability, inspectability, cost, and serviceability when the boundary changes.
Function before a number
Write the input, expected result, acceptance limit, failure symptom, and measurement method before choosing a thickness, tolerance, datum, or hole pattern. Assign an owner to every value and change.
Core checks
- Function and load direction of the insert: record value, source, method, owner, and pass/fail evidence.
- Material pairing and thermal expansion: record value, source, method, owner, and pass/fail evidence.
- Knurl, undercut, depth, and retention: record value, source, method, owner, and pass/fail evidence.
- Molding, placement, and cooling: record value, source, method, owner, and pass/fail evidence.
- Torque, pull-out, and damage limit: record value, source, method, owner, and pass/fail evidence.
- Inspection, supplier process, and replacement: record value, source, method, owner, and pass/fail evidence.
| Failure mode | Symptom | Verification |
|---|
| Function not defined | Over-tight or weak design | Rebuild the functional stack |
| Datum not real | Inspection and assembly disagree | Try the real fixture |
| Documents out of sync | Correct number, wrong revision | Baseline every reference |
Three ways to place an insert, and what each one costs
A metal threaded insert in a plastic part solves a real problem: a thread cut directly into plastic survives very few assembly cycles. But the way the insert is placed decides both the strength and the cost.
| Method | How it works | Gains | Costs |
|---|
| Moulded in | The insert is placed in the mould before injection | The best bond, no secondary operation | Longer cycle, more complex mould, risk of a misplaced insert |
| Heat staking | The insert is heated and pressed into a moulded hole | The plastic flows around the knurls and grips very well; well controlled | Needs equipment and its own cycle time |
| Press fitting cold | The insert is pressed into a slightly undersized hole | Fastest and cheapest | The weakest bond, prone to residual stress and later cracking |
For parts that are opened repeatedly or that see vibration, heat staking is usually the good balance: nearly as strong as moulding in, without complicating the mould or lengthening the moulding cycle.
Two load directions, two different retention mechanisms
An insert has to resist two different things, and two different geometric features do the work:
| Load | Retention feature | How it fails if missing |
|---|
| Pull-out (the insert being drawn out) | Circumferential knurls, a flange, a flared end | The insert pulls out of the hole as the screw is tightened |
| Torque (the insert turning in the hole) | Axial knurls, a roughened surface, a non-round shape | The insert spins and the screw can no longer be tightened |
An insert with only circumferential knurls resists pull-out but still spins; with only axial knurls it is the opposite. An insert for a threaded fastener needs both, which is why purpose-made inserts carry both knurl patterns and not for decoration.
The surrounding plastic has to be thick enough
An insert is a solid metal body sitting in plastic, so it creates exactly the problems of a thick region: the plastic around it thickens, cools slowly, produces a sink mark on the opposite face, and leaves residual stress around the insert.
Three things to do:
- Provide enough plastic wall around it — too thin and it cracks when the insert goes in or when the
screw is first tightened; the minimum values come from the insert manufacturer's catalogue by size.
- Treat the thick region as a boss — make a hollow boss around the insert, fillet the root, tie it to
the wall with support ribs.
- Look at the opposite face and check whether it is a visible surface, because that is where the sink
mark will appear.
Metal and plastic expand very differently
This is the point many designs overlook. Engineering plastics expand several times more than metals with temperature — for some grades by an order of magnitude. The consequences in an assembly containing inserts:
- As the assembly warms, the plastic expands more than the insert. If the part is clamped rigidly between two
metal faces, the plastic is compressed and can creep over time, so the joint loosens even though nobody has opened it.
- With several inserts spread across one part, the centre distance between them changes with temperature more
than on the mating metal plate — the mating holes need clearance or slots to absorb the difference.
Expansion coefficients for the common engineering plastics are in Materials #08.
How much an insert holds: knurl, boss wall and thermal expansion
An insert carries two different kinds of load, and its knurl pattern decides which it resists better:
- Torque: when a screw is driven into the insert, the insert must not spin in the plastic. A straight
(axial) knurl resists rotation well.
- Pull-out: when the insert is pulled out of its hole. A transverse knurl or a ring groove resists
pull-out well.
- Many inserts use a diamond or combined knurl to take both; choose by the main load the joint sees.
The boss wall around the insert must carry hoop stress
When an insert is pressed or heat-set in, the plastic around it takes a hoop (expanding) stress. Too thin a boss cracks. A common mark: boss outside diameter ≈ 2 × insert diameter, to give enough plastic ring to carry the expanding load and the later tightening.
| Insert method | Holds | Note |
|---|
| Moulded-in | Very strong | Insert must survive mould heat/pressure; misaligns if poorly located |
| Heat / ultrasonic into a pilot hole | Strong, well controlled | Melt flows around the knurl; needs a correctly sized pilot hole |
| Cold press-in | Moderate | Leaves residual stress in the boss, can creep over time |
Thermal expansion: metal and plastic do not move together
Plastic expands with heat several times more than metal. An assembly that runs hot or cycles in temperature loosens around the insert over time. For hot assemblies, choose an insert with good pull-out knurl and check the holding torque after thermal cycling, not only at fresh assembly.
Put a minimum torque-out and pull-out figure on the drawing or the acceptance sheet, so the insert setter knows the target to hit, instead of just "fit an insert".
MINATA release checklist
- [ ] Function, boundary, and failure symptom are written.
- [ ] Datums, ownership, process, and mistake-proofing are clear.
- [ ] Six topic checks have evidence and pass/fail limits.
- [ ] Manufacturing, assembly, inspection, and maintenance were tried.
- [ ] Revision, supplier, material, and configuration records agree.
Good engineering is an explicit chain that survives manufacture, operation, maintenance, and change. For plastic insert, evidence that function and variation remain reliable is the MINATA standard.
Frequently asked questions
Can a thread be cut directly into plastic?
Yes for parts that are hardly ever opened, lightly loaded and free of vibration. But a thread cut directly into plastic survives very few assembly cycles — after a handful it strips. Anywhere opened periodically needs an insert.
Moulded-in insert or heat staking?
Moulding in gives the best bond but complicates the mould and lengthens the cycle. Heat staking is usually the good balance: nearly as strong, and it does not touch the mould. Cold press fitting is fastest and cheapest but weakest, and it leaves residual stress.
Why does an insert spin in its hole?
Because the anti-torque feature is missing. Circumferential knurls resist pull-out but not rotation; resisting rotation needs axial knurls, a roughened surface or a non-round shape. An insert for a threaded fastener needs both patterns.
How thick does the plastic around an insert have to be?
Take it from the insert manufacturer's catalogue by size — the value depends on the plastic and the insert size. Too thin and it cracks as the insert goes in or at the first tightening.
What to watch for when an assembly with inserts runs at varying temperature?
Plastic expands several times more than metal. If the part is clamped rigidly between two metal faces, the plastic is compressed and can creep over time, loosening the joint. With several inserts spread apart, the mating metal part should have clearance or slots to absorb the difference in expansion.
Frequently asked questions, continued
What is the difference between an axial and a transverse knurl?
An axial (straight) knurl resists rotation, i.e. the torque of driving a screw. A transverse knurl or ring groove resists pull-out. Choose by the main load; many inserts use a diamond knurl to take both.
How thick should the boss around an insert be?
An outside diameter of about 2 × the insert diameter is the common mark, so the plastic ring carries the hoop stress of setting and the later tightening. Too thin a boss cracks when the insert is pressed in.
Why does a hot assembly loosen the insert over time?
Because plastic expands with heat several times more than metal, so over repeated thermal cycles the plastic around the insert creeps and loosens. For hot assemblies, choose a good pull-out knurl and check holding torque after thermal cycling.
Conclusion
Inserts in Plastic Parts is not paperwork done to make a file look tidy. It is how intent becomes a result that can be manufactured, assembled and measured repeatedly. A good drawing does not need the designer standing beside it to explain it; the structure of the information has to do that work.
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