Plastic is everywhere in modern manufactured products — but plastic alone cannot hold a reliable, re-usable thread. Strip a moulded boss once, and it is gone. That is the problem threaded inserts solve: they create a permanent, metal-quality thread in a plastic host — a thread that can be assembled and disassembled hundreds of times without degradation.
The challenge is that there are now at least eight distinct insert families, each using a different retention mechanism, each suited to a different combination of host material, wall thickness, production volume and performance requirement. Picking the wrong type does not just reduce performance — it can cause inserts to spin out under vibration, pull out under axial load, or fail entirely when a technician attempts to remove a fastener in the field.
This guide covers every major insert type, how it works mechanically, the plastics it suits, and the production conditions that make it the right or wrong choice.
There is no single "best" insert type. The correct choice depends on four factors: host material hardness, available wall section, installation method on the production line, and required pull-out / torque-out performance. Every type in this guide is optimal for the right application.
1. Self-Tapping Threaded Inserts
How it works
The self-tapping insert has a coarse external thread — often combined with two or three longitudinal cutting flutes along the body. When driven into a pre-drilled or moulded pilot hole using a torque-controlled driver (engaging the internal metric thread), the cutting flutes slice into the host material, generating a matching external thread in the plastic. The insert is mechanically locked by this thread engagement — no adhesive, heat or press required.
The three-flute variant is the most capable: the flutes remove material cleanly rather than just displacing it, which reduces hoop stress on the boss and makes the design suitable for brittle thermosets and filled engineering plastics as well as softer thermoplastics. A two-flute or single-slot design is suitable for softer, more resilient grades.
Best for:- High-volume automated assembly — bowl-fed drivers or FlexiArm presses drive in under controlled torque
- Thermosets, phenolics, composites and glass-filled grades where heat/ultrasonic equipment cannot be used
- Pull-through applications with a stepped hole design
- Situations where installation torque must be monitored for quality assurance (each driven insert is automatically thread-verified)
- Double-ended variant: no orientation required on the line — faster cycle times
Avoid when: Host material is so soft that thread stripping occurs before full insertion (use barbed press-in instead). Wall thickness is too thin for a full boss depth.
2. Ultrasonic and Heat-Insertion Inserts
How it works
These inserts share a common design principle: the external form has a knurled, diamond-knurl, ribbed or multi-faceted profile — not a thread. During installation, the surrounding thermoplastic is briefly melted (either by a heated press tip pressed onto the insert, or by a vibrating ultrasonic horn creating frictional heat). As the insert is pressed to full depth, the molten plastic flows into every groove, undercut and knurl on the external surface. When it re-solidifies on cooling, the insert is mechanically locked in all axes — pull-out, push-out, and rotation.
The result is the highest possible retention performance for a post-mould insert. The interface is essentially a plastic/metal mechanical interlock that can match or exceed the tensile strength of the surrounding boss material. These inserts are sometimes called "heat-set inserts" or, in ultrasonic installation, simply "ultrasonic inserts" — the insert design is identical; only the energy source differs.
Best for:- Hard, semi-rigid thermoplastics: ABS, PC, PA (nylon), POM (acetal), PEEK, PEI (Ultem)
- Structural joints requiring maximum pull-out and torque-out resistance
- High-value products (medical devices, aerospace interiors, precision instruments) where joint reliability is non-negotiable
- Ultrasonic installation: fastest cycle times, highest bond integrity, ideal for high-volume automated lines
- Heat installation: more controllable for prototyping and low-volume assembly
Avoid when: Host material is a thermoset (epoxy, phenolic, polyester resin) — it cannot be re-melted. Also unsuitable for foam-grade or low-density substrates where the melted zone collapses. Ultrasonic installation is not recommended for thin-flange or single-thickness headed designs where the ultrasonic energy transfers unpredictably.
Ultrasonic vs. Heat: Which to Choose?
Ultrasonic installation is faster (typically <1 second per insert vs. 5–15 seconds for heat) and produces more consistent results in production, but requires a capital investment in ultrasonic welding equipment. Heat insertion is lower-cost to set up and allows better process control for difficult geometries or delicate assemblies. For prototype and low-volume work, a temperature-controlled soldering station is often sufficient for heat insertion.
3. Barbed Press-In Inserts
How it works
The barbed press-in insert uses a distinctive external profile: rather than a thread or knurl, it has axially-oriented barbs — pointed projections that angle backward from the direction of insertion. A plain pilot spigot at the leading end ensures the insert enters squarely before the barbs engage.
The barbs create a high pull-out resistance once the insert is seated — the geometry makes it mechanically impossible to withdraw the insert along its axis without destroying the surrounding material. A plain top ring above the barbs provides a clean, flat bearing face for the assembled component.
Crucially, the same insert can be installed by three different methods: cold pressing into flexible thermoplastics (the barbs simply pierce their way in), heat insertion (molten plastic flows around and behind the barbs), or ultrasonic insertion. This installation flexibility makes barbed inserts extremely versatile across different production environments. Some variants have a self-locking rotation-resistance feature built into the head geometry.
Variants — four head configurations:- Unheaded: Flush or sub-flush on both faces. Pull-through with stepped hole. M2–M12.
- Single-thickness head: Thin collar for jack-out prevention and EMC/eyelet contact surface. M2–M8.
- Double-thickness head: Maximum pull-through resistance and standoff applications. M1.6–M10.
- Microbarb (short length): Thin-section variant for vacuum-formed, rotationally moulded, or thin-wall components. M2–M10.
Best for: Flexible thermoplastics, PP, PE, TPU where heat/ultrasonic is impractical; also excellent for hard grades when heat-inserted. PCB mounting, enclosures, electrical connectors.
4. Expansion Inserts (Barrel / Rampa Type)
How it works
The expansion insert — also widely known as the Rampa insert (after a major supplier) or barrel insert — works on a completely different principle from all other post-mould inserts. The insert is dropped into an oversized pilot hole. When a bolt is threaded into the insert, a central driving pin or tapered mandrel is drawn upward, forcing the slotted or segmented barrel of the insert outward. This outward expansion presses the insert body against the bore wall, generating radial retention.
The key advantage: no installation tool whatsoever is needed. The assembler inserts the barrel into the hole, places the component, and the act of fastening the assembly bolt simultaneously installs the insert. Removal is also tool-free — removing the bolt allows the insert to contract and be extracted.
This is the dominant insert type in furniture, cabinetry, MDF products and modular assembly systems. It is also widely used in consumer electronics housing and in educational or exhibition display systems that need frequent assembly and disassembly.
Best for:- Furniture and cabinet manufacturing — flat-pack assembly systems
- MDF, chipboard, plywood and particle board (expansion provides high grip in grain materials)
- Soft plastics and low-density foams where threaded or barbed inserts cannot achieve grip
- Field-assembled products where installation tools cannot be guaranteed
- Products requiring frequent disassembly without insert replacement
Avoid when: Host material is hard and rigid plastic with thin walls — expansion force can crack the boss. Not recommended where eccentric or vibration loading is high, as the radial grip can relax over time.
5. Helical Wire Inserts (Coil / Helicoil Type)
How it works
The helical insert — universally known by the trade name Helicoil, though that is one brand among several — is a coil of diamond-section stainless steel or phosphor bronze wire, wound to a precise pitch that matches ISO metric or UNF/UNC thread standards. Unlike all other inserts in this guide, it does not create the thread itself — it is installed into a tapped thread cut into the host material.
The hole is first drilled to an oversize diameter, then tapped with a special Helicoil tap (which creates a thread slightly larger than the nominal size). The helical insert is then wound into this tapped hole using an installation driver tool that engages a tang on the wire. Once fully installed, the coil expands slightly against the tapped thread walls, locking in place. A bolt threaded through the insert engages the inner diameter of the coil — which presents a perfect ISO metric thread to the fastener.
In plastic and soft-metal applications, the helical insert distributes fastener load over the full thread length of the coil, dramatically increasing pull-out strength versus a direct tapped thread. It is also the standard method for repairing stripped threads in any material.
Best for:- Repairing stripped threads in plastics, aluminium, and soft alloys
- High-performance engineering plastics (PEEK, PTFE, Delrin) where direct tapping gives insufficient thread engagement
- Aerospace and defence applications where the coil provides load distribution and resistance to vibration-induced loosening
- Applications requiring a corrosion-resistant thread in a less resistant host
- High-temperature environments — stainless steel coil retains performance above the temperature limits of most structural inserts
6. Mould-In Inserts (DIN 16903)
How it works
Mould-in inserts are placed in the mould tool before injection, and the plastic is moulded directly around them. The external form — typically knurled, grooved, serrated or undercut in multiple directions as defined by DIN 16903 Forms A through U — is encapsulated by the surrounding plastic as it solidifies. There is no post-mould installation step.
The result is the absolute maximum pull-out and torque-out performance achievable, because the insert is fully encapsulated: the plastic bonds to every surface feature simultaneously as it shrinks onto the insert body during cooling. Retention values can be three to five times higher than equivalent post-mould inserts of the same thread size.
The trade-off is process complexity: inserts must be loaded into the mould before each shot, either manually (adding cycle time and risk of mislocation) or robotically (adding capital cost). Any misloaded insert scraps the moulding. Changing insert specifications requires tooling modifications.
Best for:- Structural joints requiring the absolute maximum retention — automotive under-bonnet components, medical equipment, power tools
- High-volume production where robotic insert loading eliminates cycle-time penalty
- Thin-section mouldings where there is insufficient boss depth for a post-mould insert
- Applications where post-mould operations on the assembly line must be eliminated entirely
7. Foam-Grade Inserts
How it works
Foam-grade inserts are a specialised variant of the self-tapping family, but engineered for substrates that conventional inserts cannot grip: structural foam (PU, PS, EPP), laminated chipboard, MDF, and certain glass-reinforced plastic foams. The distinguishing feature is the external thread form — substantially coarser and larger-diameter than standard self-tapping inserts of the same nominal thread size, with a pitch typically 1.5–2.5 times that of the equivalent equivalent insert.
This coarser thread profile distributes axial load over a larger contact area against the cellular walls of foam materials, compensating for the much lower compressive and tensile strength of the substrate. Three cutting flutes are retained for controlled installation torque. Both double-ended (no orientation) and reduced-headed variants are available.
Best for: Structural foam mouldings, furniture using particle board or MDF, marine foam cores, automotive interior foam panels, packaging with integrated fastening points.8. Knife-Thread (Cutting Thread) Inserts
How it works
Knife-thread inserts have a thread form with a sharper, more aggressive flank angle than the standard 60° ISO metric profile — typically closer to a 30° or asymmetric profile. This sharper "knife" geometry cuts into the host material more aggressively, requiring less axial force during installation while achieving high retention in softer thermoplastics. They do not rely on cutting flutes — the thread itself does the work of displacing or cutting the host material.
These are most common in polyolefin hosts (PP, PE, HDPE) where the low modulus makes standard thread forms less effective, and in very thin-walled sections where a full cutting-flute design would generate excessive hoop stress. Some designs incorporate a slotted head for direct screwdriver installation without requiring the internal thread as a drive feature.
Best for: PP, PE, HDPE, TPE — soft, resilient thermoplastics. Thin-wall sections. Products where the assembler cannot use a torque-controlled driver. Low-volume or service/repair installations.9. Press-Fit (Interference Fit) Inserts
How it works
The simplest category: a plain cylindrical or slightly tapered insert body with a smooth or lightly knurled exterior, pressed into a hole that is marginally undersized relative to the insert OD. Retention is provided purely by radial interference — the compressed plastic exerts a constant inward pressure on the insert body. There are no threads, barbs or flutes on the exterior.
Pull-out strength is moderate and highly dependent on exact hole tolerance and material stiffness. Rotation resistance is low without additional features (a flat, slot or anti-rotation spline). These are most commonly used where push-out forces are low and rotation is prevented by the assembly geometry rather than the insert itself — spacers, bushings, and pivot pins in light-duty applications.
Best for: Bushings, pivot bearings, spacer applications, and non-structural fastening points in soft thermoplastics. Not suitable for high-torque or high-pull-out joints.10. Solid Shoulder / Flanged Inserts
How it works
Flanged or shouldered inserts combine any of the above external retention mechanisms with a precision shoulder or flange that acts as a datum — controlling the insert's axial position relative to the moulded surface with a tolerance that the moulded hole itself cannot achieve. The flange sits flush or proud of the surface, providing a large bearing area for washers or mating components, and can serve as a precision spacer in stacked PCB or panel assemblies.
The shoulder length can be specified to create a defined standoff distance between clamped components — allowing the fastener to clamp the stack while the shoulder prevents over-compression of the plastic. This function overlaps with the role of hex standoffs in PCB assemblies.
Best for: Stacked assemblies requiring precise standoff, PCB mounting, panel assemblies, applications where the moulded boss face is a critical datum.Quick Selection Guide
| Insert Type | Host Material | Installation | Retention Level | Typical Use |
|---|---|---|---|---|
| Self-Tapping | Thermoplastics, thermosets, composites | Torque driver — cold | Medium–High | Electronics, appliances, automotive trim |
| Ultrasonic / Heat-set | Thermoplastics only | Ultrasonic horn or heat press | Very High | Medical, instruments, high-value housings |
| Barbed Press-In | Thermoplastics (all) | Cold press, heat or ultrasonic | Medium–High | Enclosures, PCB mounting, connectors |
| Expansion (Rampa) | MDF, chipboard, soft plastics, foam | None — bolt-driven | Medium | Furniture, flat-pack, display systems |
| Helical Wire (Helicoil) | Any — tapped hole | Special tap + driver tool | Very High | Thread repair, aerospace, high-load joints |
| Mould-In (DIN 16903) | Any injected thermoplastic | Pre-mould placement | Highest | Automotive, power tools, structural joints |
| Foam Grade | Structural foam, MDF, chipboard | Torque driver — cold | Medium (for substrate) | Foam panels, furniture board, marine |
| Knife-Thread | PP, PE, HDPE, TPE | Screwdriver or driver | Medium | Polyolefin mouldings, soft goods |
| Press-Fit | Rigid thermoplastics | Cold press | Low–Medium | Bushings, spacers, non-structural |
Choosing the Right Material: Why Brass?
The vast majority of threaded inserts across all these categories are manufactured from brass — specifically free-machining grades such as CW614N (CuZn39Pb3) or CW617N (CuZn40Pb2). Brass is the standard for four reasons:
- Machinability: CW614N achieves a machinability index of 95–100% (referenced to CW614N itself as 100%) — meaning very high production rates, low tool wear, and tight tolerances are achievable at low unit cost.
- Coefficient of thermal expansion: Brass expands and contracts at a rate close enough to most engineering thermoplastics to avoid loosening the insert during thermal cycling.
- Corrosion resistance: Adequate for the majority of interior environments. For aggressive chemical exposure, stainless steel inserts are available.
- Cost: Brass inserts typically cost significantly less than stainless steel or titanium equivalents while meeting the mechanical requirements of most plastic assemblies.
Stainless steel inserts (typically A2 or 316) are specified where the assembly is exposed to moisture, salt spray, or corrosive chemicals — food processing equipment, marine electronics, outdoor structures. Plastic (typically nylon 66 or POM) inserts are used where electrical isolation is required or metal contamination of the host product is unacceptable — medical devices, food contact equipment, and certain electronics.
Brassland: Brass Insert Supplier, Jamnagar
Brassland manufactures self-tapping, barbed press-in, foam-grade, and mould-in (DIN 16903) inserts from CW614N and CW617N brass at our Jamnagar facility. Made-to-order in any M-thread size, RoHS/REACH compliant, with full dimensional traceability. Contact our sales team for specifications and pricing.
Common Specification Mistakes to Avoid
- Under-specifying boss diameter. Every insert type publishes a minimum boss outer diameter. An undersized boss cracks during installation or fails under torque. Always design the boss to the insert supplier's recommendations, not to general DFM rules.
- Ignoring host material hardness. Self-tapping inserts specified for ABS will under-perform in glass-filled nylon — the harder material requires a higher installation torque and a different flute geometry. Always confirm material suitability with the insert supplier.
- Mixing ultrasonic and thermoset in the same BOM. Heat/ultrasonic inserts absolutely cannot be used in thermosets (epoxy, BMC, SMC). This is the single most common cross-specification error.
- Using the same insert for foam and solid plastic. Standard self-tapping inserts achieve very low pull-out in structural foam — the coarser foam-grade pitch is non-negotiable for low-density substrates.
- Specifying by brand without a generic equivalent. If you specify a single-source branded insert (a single branded part) without a dimensional alternative, you risk supply chain disruption. Always specify to the dimensional standard and confirm that your supplier can meet it.
Brassland vs. branded inserts and C-parts distributors
How a factory-direct manufacturer compares with catalogue distributors and branded insert specialists when you need brass threaded inserts:
| Aspect | Brassland (manufacturer) | C-parts distributors | Branded specialists |
|---|---|---|---|
| Customisation | Made to your drawing — any M-size, knurl, flange or feature | Catalogue parts; custom via their own supplier | Catalogue range plus some custom |
| Cost basis | Factory-direct — no distributor margin | Includes distribution / logistics margin | Brand premium |
| MOQ & flexibility | Set by run economics; flexible for OEM volumes | Catalogue pack sizes | Catalogue pack sizes |
| Traceability | EN 10204 mill certificate, per batch | Varies with the source | Manufacturer data sheet |
| Alloy choice | CW614N / CW617N, or your specified grade | As stocked | As stocked |
If your drawing calls out a branded insert, specify it to the dimensional standard — DIN 16903 for moulding inserts, or the insert's published OD, length and thread — and Brassland will produce a drop-in equivalent, removing single-source supply risk. See our self-tapping & press-in insert range and DIN 16903 moulding inserts.
FAQ: threaded inserts for plastics
Which brass alloy is used for threaded inserts?
Can I get a dimensional equivalent to a branded insert?
What is the difference between heat-set and self-tapping inserts?
Are brass threaded inserts RoHS and REACH compliant?
What is the minimum order for custom brass inserts?
Sources & references
Authoritative references for this guide:
Last reviewed: June 2026. Standards and regulatory references are checked at each review.
Need Threaded Inserts? We Supply All Major Types.
Self-tapping, barbed press-in, foam-grade and mould-in inserts — manufactured from CW614N brass in Jamnagar, India. Made to order, RoHS compliant, available in M1.6 through M12.
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