An insert is the one part of a plastic housing that gets chosen after the boss is designed. When the boss matches a stock insert, the insert costs cents. When it does not — a hole drilled for a discontinued part, a wall too thin for the stock knurl, a thread the series skips — the answer is to change the insert, not the mould.
EKINSUN LTD is a custom parts manufacturer that turns threaded inserts for plastic to drawing: the length, outside diameter, knurl pattern, thread and material your boss needs, in brass CW614N, 303 or 316L stainless, from 1 piece. The stock ranges below come from a 2026 catalogue and cover the common cases; the table after them shows which insert type suits which boss, and the last table shows where the catalogue stops.
"The housing was tooled ten years ago for an insert nobody makes any more. The hole is 5.6 mm and every M4 insert I can buy is 5.4 or 6.0."
That enquiry is typical: the geometry is fixed by an old mould, and only the insert can move. The rest of this page is written for that buyer.
| Type | How it goes in | Knurl and features | Stock range | Typical use |
|---|---|---|---|---|
| Heat-set insert, through hole | heated tip or ultrasonic horn melts the plastic around it after moulding; also pressed hot into 3D-printed bosses | diamond (double-twill) knurl bands with an undercut groove | M1–M10 brass; M2–M10 in 304 stainless | ABS, PC, PA, PP housings; printed prototypes |
| Heat-set insert, blind | as above, closed end | diamond knurl, closed bottom | on request | a screw that must not break through the boss; sealed bosses |
| Flanged heat-set insert | heated; the flange seats on the boss face | diamond knurl plus a flange | M2–M8 brass and stainless | axial load on the flange, a flat seat for the screw head |
| Straight-knurl press-in insert | pressed cold into a moulded or drilled hole; press, ultrasonic or moulded in | straight knurl, single band | M1–M10 through; M2–M10 blind | softer plastics, thin walls, low-volume assembly |
| Double-band moulded-in insert | placed in the cavity before the shot; plastic flows around two knurl bands and the groove between them | two straight-knurl bands, undercut | M3–M8 through and blind | highest pull-out and torque-out; high-volume moulding |
| Micro insert | heat-set or pressed | diamond knurl | M1.4–M2 | connectors, wearables, small electronics |
| Imperial insert | heat-set, press-in or moulded-in | straight or diamond | 1/4 in | US products; other UNC and UNF sizes made to order |
| Self-tapping thread insert | screwed into a drilled hole and cuts its own thread; slotted, three-hole and 302-type drives | external coarse cutting thread | M2–M18 | thermosets, glass-filled parts, wood, soft alloys, thread repair |
| Wire thread insert | tapped hole, insert wound in | diamond-section spring wire | M2–M24 (DIN 8140) | thread repair and reinforcement in aluminium and magnesium |
| Stamped insert bushing | placed in the mould; reinforces a screw hole in a connector body | thick stamped ring or sleeve | to drawing | connector and charging-socket housings — see stamped parts |
Ranges are the stock catalogue's; every type is also made outside its range. Knurl names: diamond = double-twill, straight = axial.
| Your situation | Use this insert | Why |
|---|---|---|
| Part already moulded in a thermoplastic, hole exists | heat-set, diamond knurl | melts in after the fact; no mould change; the groove holds against pull-out once the plastic re-freezes |
| 3D-printed housing (PLA, PETG, ABS, nylon) | heat-set with a soldering-iron tip | the same insert, installed with a hand tool; the printed hole is sized slightly under the insert |
| High volume, the mould can be modified | double-band moulded-in | strongest hold: plastic fills both knurl bands and the undercut |
| Thermoset or heavily glass-filled part that will not flow | self-tapping insert, or press-in with adhesive | heat-setting needs a plastic that remelts; these do not |
| Stripped thread in an aluminium casting or a machined 6061 bracket | wire thread insert | restores the original screw size in a tapped, oversize hole |
| Boss must carry axial load or give the screw head a flat seat | flanged insert | the flange spreads the load over the boss face instead of the knurl |
| Screw must not exit the back of the boss | blind insert | closed end, no witness hole, sealed against dust and moisture |
| A boss wall that is too thin for the stock knurl | custom insert with a smaller outside diameter | the usual design rule is a boss about twice the insert diameter; when the wall cannot give, the insert gives |
Every custom insert we make starts from the same six numbers. Send them and the drawing is ours to build.
Thread depth matters more than most drawings show: a heat-set insert holds by its knurl, so a screw that runs out of thread and bears on the plastic below the insert will pull it. The screw length should stop inside the insert. For the pitch reading itself, the same routine as for any thread applies: ten crests on a rule, divided by ten.
| What you need | In the catalogue? | What we make |
|---|---|---|
| A heat-set or press-in insert M2–M8 in brass, standard length, for ABS / PC / PA | Yes | Buy it from stock. No one-off competes with the price of a catalogue insert, and the datasheet gives the hole and boss sizes. |
| The same insert in 304 stainless, M2–M10 | Yes | Stocked; buy it. For 316L / 1.4404 or 303 free-machining stainless we turn it. |
| An insert one or two millimetres longer or shorter than the stock length, to suit your boss | No | Turned from brass bar with the same knurl, from 1 piece; length held to the drawing. |
| An outside diameter sized to a hole that was moulded for a discontinued insert | No | The hole is the given; we cut the knurl diameter to suit it and keep the thread standard. |
| A thread the series skips — M2.5 fine, M3×0.35, left-hand, UNF, or any UNC size other than 1/4-20 | No | Tapped or single-pointed to the thread you specify; UNC and UNF from 2-56 upward. |
| A hex, a flat or a cross-hole for anti-rotation in a keyed boss | No | Milled or drilled on the turned insert. |
| An insert in aluminium 6061 or 7075 | Partly | Aluminium inserts are made on request for weight-critical parts; the thread wears faster than brass under repeated assembly, and we say so. |
| A stamped insert bushing for a connector or charging-socket housing | No | Stamped and pressed to drawing — see custom stamped parts. |
| Material | Designation | Where it fits here |
|---|---|---|
| Brass | CW614N (CuZn39Pb3) or C36000 free-machining brass | the default: machines cleanly, conducts, does not rust; tin or nickel plating on request |
| Stainless 304 | 1.4301 / A2 | stocked heat-set inserts M2–M10; hygiene and outdoor housings |
| Stainless 316L | 1.4404 / A4 | chemical and medical housings; turned to order |
| Free-machining stainless | 303 / 1.4305 | custom turned inserts with knurls and flats |
| Aluminium 6061 | AlMg1SiCu | on request for light inserts; also the alloy of the machined brackets and housings the inserts go into |
| Aluminium 7075 | AlZn5.5MgCu | not an insert material; listed because we machine 7075 housings and bosses that take wire thread inserts |
| Steel, zinc plated | low-carbon | self-tapping inserts for wood and soft alloys |
A heat-set insert has a diamond knurl and an undercut groove; it is installed hot so the plastic melts around the knurl and re-freezes in the groove, which gives high pull-out in thermoplastics. A press-in insert has a straight knurl and is pressed cold into a slightly undersize hole; it suits softer plastics, thin walls and low volumes, and it can also be placed in the mould.
It depends on the insert's outside diameter and on the plastic, so the insert datasheet gives the hole and the boss diameter. The usual design rule is a boss about twice the insert's outside diameter. When we make the insert to an existing hole, the hole is the given and we cut the knurl diameter to suit it.
Yes. Heat-set inserts are the standard answer for PLA, PETG, ABS and printed nylon: the hole is printed slightly under the insert diameter and the insert is pushed in with a heated tip. Flanged inserts help where the printed boss face is rough.
Yes; that is the most common custom insert we make. Send the hole diameter, the boss depth, the plastic and the thread you need, or the old insert itself, and the new insert is turned to that hole with the thread kept standard, from 1 piece.
EKINSUN LTD is a custom parts manufacturer that turns heat-set, press-in and moulded-in inserts to drawing in brass CW614N, 303 and 316L stainless and, on request, aluminium 6061, in any thread from M1.4 to M20 and in UNC / UNF, from 1 piece, with a quote within 12 hours.
There is no list price and no minimum: one piece is a quote. The price is driven by the material, the knurl and any milled flats or holes, and by quantity; a plain brass insert in a batch of a few hundred sits close to catalogue money, a single stainless insert with a hex and a fine thread does not, and the quote states the drivers.
Thread and pitch, insert length, outside diameter or the hole it must fit, boss depth, the plastic, and how it goes in. The old insert or a photo of the boss works too. From 1 piece, quote within 12 hours.
Length, outside diameter, knurl, thread and material to your boss — from 1 piece, quote in 12 hours.