Four ways to fit threaded brass inserts into plastic parts
| Method | How it works | Strength | Plastic type |
|---|---|---|---|
| Moulded in | Insert placed on a mould pin each cycle, then encapsulated | Best pull-out and torque | Thermoplastic or thermoset |
| Heat staking | Heated insert pressed into a slightly undersized moulded hole | Good | Thermoplastic only |
| Ultrasonic | Vibration melts the hole wall as the insert goes in | Good | Thermoplastic only |
| Cold pressed | Insert pushed into a hole without heat | Lowest | Thermoplastic or thermoset |
Moulding threaded brass inserts in gives the highest pull-out and torque strength, because molten plastic flows into every knurl and groove and locks the insert in. The price is loading time every cycle, tight pin tolerances and a risk of cracking around the insert. For a thermoplastic part with moderate loads, heat staking after moulding can be enough.
Key Takeaways
- Pull-out and torque are the two strengths that matter: a longer insert resists pull-out, a larger diameter resists torque.
- Knurl pattern trades one against the other; grooves between knurl bands add pull-out.
- Moulded-in inserts can crack the boss through hoop stress; estimate it as shrinkage times modulus before the mould is cut.
- Through-threaded inserts must seal against both mould halves, or resin fills the thread.
When should threaded brass inserts be moulded in?
Mould inserts in when the joint must carry high loads, the thread will be undone often, or the part is a thermoset. Heat staking and ultrasonic insertion remelt the plastic, so they only work with thermoplastics. Otherwise, adding inserts after moulding can be simpler and cheaper.
Design guidance from DuPont describes moulding in as often a last resort, with pressing, snap fits or ultrasonic insertion into a premoulded part as alternatives. Its list of drawbacks, from floating inserts to flash in the threads and costly rejects, is covered in the insert moulding setup article in this series. The point here is that each drawback is paid on every shot, while a staked insert adds one fast step after the press.
Our flat view: for a thermoplastic boss, start with heat staking and move to moulding in only when tests show the staked insert is not strong enough. Heat staking is easy to control, can set several inserts at once, and is the more commonly applied method for thermoplastics, according to a Plastics Technology feature on threaded inserts. Brass remains the usual insert metal; stainless steel adds strength and corrosion resistance, while aluminium is about 70% lighter than brass.
What controls the pull-out strength of threaded brass inserts?
Pull-out strength rises with insert length, the depth of plastic around and beneath it, and knurl features that resist axial load. Torque strength rises with diameter and straight knurls. Full encapsulation, as in moulding in, gives the best of both, provided the boss does not crack.
The knurl is the insert's grip. Straight knurls resist rotation best but give less pull-out. Grooves between knurl bands add pull-out. Diagonal or helical knurls balance the two, and the diamond knurl, probably the most common, resists load in all directions.
Geometry around the insert matters just as much. The DuPont guidance asks for rounded inserts or rounded knurling with no sharp corners, an undercut to resist pull-out, at least one sixth of the insert diameter of plastic beneath it, and a boss of 1.5 times the insert diameter for inserts up to 12.9 mm.
Knurl patterns and what they resist
| Knurl type | Torque resistance | Pull-out resistance |
|---|---|---|
| Straight | Highest | Lower |
| Straight with grooves between bands | High | Raised by the grooves |
| Diagonal or helical | Balanced | Balanced |
| Diamond | Good in all directions | Good in all directions |
Why do bosses crack around moulded-in inserts?
Plastic shrinks onto the metal as it cools, but the brass does not shrink with it, so the ring of plastic around the insert is left in tension. That hoop stress can cause delayed cracking, some time after moulding. A rough estimate is mould shrinkage times flexural modulus.
The DuPont guidance names delayed cracking from hoop stress as the most common complaint with moulded-in inserts. It compares nylon at 0.016 mm/mm shrinkage with acetal at 0.025 mm/mm, and estimates the acetal stress at about 52.4 MPa, roughly 75% of its tensile strength. Glass or mineral-filled grades shrink less, 0.005 to 0.010 mm/mm, but heavily filled weld lines may keep only 60% of unfilled strength.
Preheating the inserts, toughened resin grades and rounded knurls all lower the risk. Test finished parts with temperature cycling, because a crack that is absent at the press can open after a few heat cycles in service. Where the estimate comes close to the resin's yield strength, change the boss or the grade before the steel is cut, not after the first field returns.
How do you keep resin out of the threads?
A through-threaded insert must touch both the core and cavity sides so its bore is sealed at each end. A blind insert needs contact only on the core side. In both cases the mould pin must fit the thread or bore closely, or melt runs in.
Close pin tolerance is one of the conditions the Plastics Technology feature sets for moulded-in inserts, alongside the placement time it adds. Check thread gauges on the first shots of every batch, since a new lot of inserts can sit differently on the same pins.
Clean threads matter for compliance too. IS 14772:2020, the BIS standard for boxes and enclosures for electrical accessories, requires lids, covers, knock-outs and screw fixings to protect against ingress of solids and water, and covers mechanical strength. A thread with resin in it, or a boss cracked by an overtightened screw, works against both.
What should you send to plan a moulded-in insert job?
Send the part drawing with the insert drawing and its knurl type, the resin grade, the pull-out or torque value the part must meet, the inserts per part and the cavity plan. See the vertical clamp insert moulding machines page for the machines a moulded-in insert job is matched to.
Frequently Asked Questions About Threaded Brass Inserts
Can threaded inserts be heat staked into thermoset parts?
No. Heat staking and ultrasonic insertion work by remelting the plastic around the hole, and a thermoset does not remelt once cured. For thermoset parts the options are moulding the insert in or cold pressing it into a hole, and cold pressing gives the lower strength of the two.
Which knurl pattern gives the best pull-out strength?
Grooves between knurl bands add the most pull-out resistance, while straight knurls favour torque and give less pull-out. Diagonal or helical knurls balance the two. A diamond knurl resists load in all directions and suits parts that see both twisting and pulling in service.
Are there lead-free alternatives to brass inserts?
Yes. A Plastics Technology feature on threaded inserts names stainless steel, which adds strength and corrosion resistance, and aluminium, which is about 70% lighter than brass, as lead-free alternatives. Check the insert maker's data for the exact grade, because thread strength and cost differ from brass.
How deep should a moulded-in insert sit in the boss?
DuPont guidance asks for plastic beneath the insert of at least one sixth of the insert diameter, and for the insert to stand at least 0.4 mm into the cavity so it is fully encapsulated. Deeper engagement and a longer insert both raise pull-out resistance, within the space the part allows.
Why does a boss crack long after moulding?
That is delayed cracking from hoop stress. The plastic has shrunk onto the brass and stays in tension, and heat, chemicals or screw torque in service can tip it into cracking. Lower the stress with preheated inserts, a toughened grade or a thicker boss, and test parts through temperature cycles.
Related Articles
- Insert Moulding on PVDL Vertical Clamp Machines: Setup and Loading
- Moulding Plug Tops With Brass Pins on a Vertical Machine
- Sliding Moulds on Vertical Machines: How Much Cycle Time They Save
Sources & References
- Plastics Technology (sponsored, PennEngineering): Four Ways to Tackle Threaded Inserts for Plastics (Jan 2018)
- PlasticsToday (IMM), summarising A. Miller, DuPont: Design and material advice from the pros, Part 3: Insert molding essentials (Sept 2005)
- BASF via Material Data Center (M-Base): Ultramid A3L HP datasheet
- BIS: IS 14772: 2020 Boxes and Enclosures for Electrical Accessories, summary
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