Screwing a metal bolt straight into printed plastic can work for a lightly loaded, infrequently opened joint. But repeated assembly may wear the plastic thread, especially if the hole was printed undersized or the screw is overtightened. A heat-set brass insert offers a more durable machine thread inside a thermoplastic part.
On this page
- Heat-set versus the alternatives
- How a heat-set insert holds
- Design the boss before printing
- Installation equipment
- Step-by-step insertion
- What causes inserts to pull out or spin?
- PLA, PETG, ABS and nylon: can they all use inserts?
- What should you buy?
- How we would compare inserts fairly
- The practical conclusion
The insert is a small metal cylinder with an internal thread and a textured outside. Heat softens the surrounding plastic as it is pressed into a prepared hole; after cooling, the plastic surrounds the exterior knurling. The aim is a secure insert sitting squarely at the correct depth without splitting or bulging the printed boss.
This technique is useful for electronics enclosures, removable panels, equipment mounts and repeatedly opened prototypes. It is not an automatic solution for every joint, nor does it turn an ordinary PLA print into a certified structural component.
Heat-set versus the alternatives
| Fixing method | When it makes sense | Typical limitation |
|---|---|---|
| Screw driven directly into plastic | Light loads, simple single-use or occasional assembly | Threads can wear or strip with repeated removal |
| Captive hex nut in a printed pocket | Higher torque potential, easy replacement, no heat setting | Requires pocket access and space to retain the nut |
| Heat-set threaded insert | Compact, repeatable metal thread in a thermoplastic boss | Requires controlled heat, hole design and wall thickness |
| Through-bolt and nut | Accessible joints, high clamping area and serviceability | Needs rear access and can protrude |
| Bonded stud or insert | Materials or geometry unsuitable for heat setting | Adhesive qualification and cure are separate concerns |
Choose the joint design first. An insert cannot compensate for a wall so thin that the surrounding plastic has no material to grip. A bolt may have a very high tensile rating while the printed boss fails at a much lower load.
How a heat-set insert holds
The outer ridges or knurls displace softened plastic as the insert moves into its pilot hole. Upon cooling, the plastic mechanically locks around those details. An effective joint depends on enough surrounding material, correct pilot-hole geometry, insertion alignment and suitable processing temperature.
Prusa's heat-set insert product guidance describes the intended metal-thread benefit, using heat from a soldering iron and sizing the receiving hole to allow material to move around the insert.[1] The right pilot diameter is not simply the nominal metric screw size. An M3 bolt specifies the internal thread, but the insert's exterior size varies among manufacturers and designs.
Always use the *actual insert supplier's drawing* for its recommended hole diameter and depth. A generic M3 chart can be misleading because a short insert and a long insert, or two different knurl designs, may need different geometry.
Design the boss before printing
A good insert boss needs sufficient plastic around the hole and below its base. A narrow, unsupported cylinder may split or pull away from the main part when the insert is pushed in or the screw tightened. Add fillets at the boss root and keep loads aligned with the part's stronger directions where possible.
Leave clearance below the insert so the bolt does not bottom out and lever it out of the plastic. If a through-hole is intended, ensure the screw can pass cleanly through without hitting an unexpected internal wall. The hole should be long enough for the chosen insert but not so deep that it falls through and loses support.
Consider print orientation. Layer lines can make a boss more vulnerable to splitting along one direction, and a tall thin post can wobble under the soldering iron. Printing several pilot-diameter test coupons before the final enclosure is often more valuable than guessing a number and ruining a large component.
Installation equipment
At minimum, use a temperature-controlled soldering iron and an insert-compatible installation tip or a tip that seats the insert squarely without sticking. A small positioning jig or press can improve alignment. You also need a stable, heat-resistant work area, eye protection, ventilation suitable for working with warm plastic and a way to leave the part undisturbed while cooling.
Do not use a soldering tip coated with flux or solder residue to install hardware in a print. The joint is made by the thermoplastic; foreign residue may interfere with fit and cleanliness. Keep cables, flammable material and fingers clear of hot metal.
Prusa provides temperature guidance for its own inserts related to the printing temperature of the plastic.[1] That is product-specific guidance, not a universal safe setting for every polymer, iron or insert. Consult the insert maker's instructions and filament limits. Excessive heat or pressure can collapse the boss, soften a large region or distort adjacent dimensions.
Step-by-step insertion
- Confirm the hardware: Match the screw thread, insert outer dimensions, height and the manufacturer's pilot-hole recommendation.
- Inspect the print: The receiving hole should be round enough, clear of support debris and surrounded by sound walls.
- Stabilise the part: Support the boss so it will not bend under light insertion pressure.
- Heat the tool to the approved temperature: Allow it to stabilise; avoid improvised open flame methods.
- Place the insert squarely: Keep its axis perpendicular to the intended mating surface.
- Apply controlled, gentle pressure: Let the heated insert soften the plastic; do not force it cold or plunge it too quickly.
- Stop at the specified depth: Flush or slightly recessed is common, but follow the design and insert guidance.
- Withdraw vertically and allow full cooling: Do not thread a screw into it while the plastic is still soft.
- Inspect alignment and retention: Check for raised lips, cracks, distorted walls and correct screw engagement.
If material gathers above the insert, do not automatically grind the boss flat while it is still warm. Wait for cooling and assess whether the joint remains serviceable. An obviously misaligned or loose insert is a repair problem, not something to conceal with extra tightening torque.
What causes inserts to pull out or spin?
Hole too large: Insufficient plastic flows into the knurls, leaving little mechanical lock.
Hole too small or boss too thin: Installation pushes too much plastic outward, causing bulging, splitting or deformation.
Insufficient heat: The insert may crush rather than melt its path, producing poor local contact and damage.
Excessive heat or pressure: A deep softened region can distort the boss or leave the insert at an angle.
Bolt bottoming out: A screw longer than the available thread depth can push against the base of a blind hole and jack the insert out.
Uncontrolled tightening torque: The machine thread may be sound, but the knurled insert can rotate in the plastic or pull free if the joint is overloaded. The advertised strength of the bolt does not establish the strength of the printed assembly.
For a repeated production component, test both pull-out and rotational resistance on printed samples from the actual material and build orientation. An insert that feels secure by hand is not a measured load rating.
PLA, PETG, ABS and nylon: can they all use inserts?
Heat setting is mainly a thermoplastic technique. PLA, PETG and ABS are often used, but their softening behaviour and long-term heat response differ. Nylon grades vary considerably, and composite-filled filaments may change flow and resistance around knurling. Select the insert design and heat process for the material, not merely for the common screw size.
PLA is easy to install into but can creep under sustained stress and soften in a warm environment. PETG may behave more ductilely; ABS and other engineering materials have different temperature and enclosure needs when printed. Insert performance remains a property of the *combination* of material, geometry, printer orientation and installation.
For fibre-filled materials, an insert does not make the local resin matrix unbreakable. Verify any recommended pilot-hole adjustment in the insert maker's documentation; do not extrapolate from unfilled PLA.
What should you buy?
Start with the screw sizes you actually need, then choose inserts with a published dimensioned drawing and installation instructions. Check external diameter, overall length, head or flange style, knurl design, thread quality and compatible process. A kit offering many sizes is convenient but only if the inserts are dimensionally consistent and the labels accurate.
The soldering iron should have stable temperature control and an appropriate installation tip. A small test coupon is inexpensive insurance before committing to a large finished enclosure. For joints with access on both sides, a conventional bolt and nut can be easier and more robust than embedding a metal thread.
How we would compare inserts fairly
We would use printed coupons with documented pilot diameters, material and layer orientation, install several samples at one controlled tool temperature, then measure pull-out force, torque to spin and the number of screw assembly cycles. The test would also record cracking, boss distortion and any change after thermal exposure.
We have not completed those mechanical tests for this article and do not rank brands by imagined strength. A supplier's product description is not a substitute for a tested joint design.
The practical conclusion
Heat-set inserts make sense when a printed thermoplastic part needs a compact, reusable machine thread. Their success depends less on the headline insert size than on precise hole design, surrounding plastic, correct installation heat and bolt length. Use published dimensions, test a coupon and treat heavily loaded joints as engineering problems requiring their own validation.
Sources
[1] Prusa — Heat-set threaded inserts: product and installation notes