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Plastic Case and Screw Post Repair

From RetroTechCollection
Screw boss and heat-set insert diagram
Left: a moulded screw boss holding a thread-forming screw, with the proportions fastener makers recommend and the split that too much outward pressure causes. Right: the same boss rebuilt and fitted with a heat-set brass insert, with dimensions for an M3 insert. Drawn for this wiki from the sources cited below.

Plastic case and screw post repair covers the breakages in the moulded cases of old computers, consoles and handhelds: cracks, broken clips and hinges, and the screw posts, or bosses, that split, strip or snap off. Enclosures for electronic equipment, computer cases among them, are a common use of ABS. ABS is an amorphous thermoplastic with a glass transition temperature of about 105 °C, it dissolves in ketones such as acetone, and ultraviolet light or heat makes it yellow and brittle through photo-oxidation.[1] The repairs are solvent welding or heat welding for cracks, epoxy putty to rebuild what is missing, and heat-set threaded inserts to give a damaged boss a metal thread.

When to use it

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  • A screw turns without tightening or without coming out. On the PowerBook Duo, Apple warns that over-tightening the bezel screws strips their inserts.[2]
  • Posts or bosses snap when an old case is opened, or a broken piece rattles inside. The Calculator Store, which repairs HP-41 calculators, warns that after 30 years their plastic is brittle and a post breaks or its thread strips easily.[3]
  • The case cracks round a screw or hinge. Apple notes that over-tightened bezel screws crack the PowerBook 1400's bezel.[4]
  • The halves no longer close tightly because the posts that pulled them together are cracked.[3]

Safety

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  • Acetone is highly flammable, with a flash point of −20 °C, and its vapour can flow along surfaces to an ignition source some distance away and flash back. Keep it away from a hot soldering iron.[5]
  • Keep acetone and hydrogen peroxide apart, including waste from Retrobrite treatment: oxidising acetone with hydrogen peroxide can form acetone peroxide, a highly unstable explosive.[5]
  • Do not overheat ABS. The most toxic products of its combustion or pyrolysis are carbon monoxide and hydrogen cyanide.[1]
  • Read the safety data sheet for any epoxy or adhesive; J-B Weld publishes one for PlasticWeld.[6]
  • A heat-set insert and the iron that drives it are hot enough to burn; let the insert cool before touching it.[7]

Tools and materials

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  • Screwdrivers that fit the screws, used by hand.[8]
  • An acetone-based solvent for welding ABS.[9]
  • Epoxy putty made for plastics, abrasive paper, and acetone to clean the surfaces.[6]
  • Brass heat-set inserts, a drill of the size the insert maker specifies, and a temperature-controlled soldering iron, ideally with an insert tip.[7][10]
  • Masking tape and small clamps or clothes pegs.[3]
Choosing a repair
Damage Repair Limits in the sources
Clean crack in ABS Solvent weld with an acetone-based solvent Acetone is for ABS; other plastics need other solvents[9]
Crack that needs strength, or a gap Heat weld with a filler rod of the same plastic, or epoxy putty Filler must match the plastic; epoxy will not bond polyethylene or polypropylene[9][6]
Boss broken off or missing Rebuild in epoxy putty, then drill, or set an insert in the putty Do not leave screw torque on a small glue line alone[6][3]
Stripped or split boss with enough plastic left Heat-set brass insert and a matching machine screw The boss wall must be thick enough round the insert[7][11]

Procedure

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Before starting

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  1. Take every screw out slowly and gently, by hand.[3]
  2. Keep the original screws. Thread-forming screws made for plastic have a narrower thread flank angle (30 to 48°) than standard screws (60°), which puts less outward force on the boss.[8]
  3. Work out what the damaged part is made of. Acetone welds ABS but not every plastic, and epoxy will not hold on polyethylene or polypropylene; if unsure, test on a hidden area.[9][6]
  4. Mask nearby contacts, holes and keyboard parts with tape before using any glue, solvent or putty.[3]

Solvent-weld a crack in ABS

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  1. Apply a small amount of acetone-based solvent to the faces. The solvent softens the surface layer of each part so that the plastic of both sides can mingle.[9]
  2. Press the faces together and hold them with tape, clamps or clothes pegs.[3]
  3. Leave the joint until the solvent has evaporated out of it; the weld is formed as the solvent leaves and the plastic sets solid again.[9]

A heat weld does the same job with heat: the surfaces are melted together, and a thin rod of ABS can be melted into the joint as reinforcement.[1][9]

Rebuild a boss with epoxy putty

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  1. Sand or scuff the surfaces the putty will touch and clean them with acetone.[6]
  2. Cut off the amount of putty needed and knead it until the colour is even.[6]
  3. Press it firmly onto the stump of the boss and shape it. J-B Weld PlasticWeld gives 20 to 25 minutes to work before it sets.[6]
  4. Make the new boss thick enough. Fastener guides size a boss at two to three times the screw's thread diameter (Assembly magazine) or 2.5 to 3 times the pilot hole (TR Fastenings).[8][12]
  5. Leave it to cure: PlasticWeld reaches a functional cure in two to three hours.[6]
  6. Drill the new boss for the screw, or for a brass insert set into it. Where clean tapped threads are needed, J-B Weld recommends its PlasticBonder, which machines better.[6]

Spread the load. The Calculator Store found that a threaded cylinder glued into a drilled-out post separated under screw torque every time; its replacement part is also bonded to a wide surface nearby so that the glue line does not carry the torque on its own.[3]

Fit a heat-set insert

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Heat-set insert tips on a soldering iron (Adafruit)
A soldering iron fitted with a heat-set insert tip, with spare tips and brass inserts. Photo: Adafruit Industries, CC BY-SA 3.0.

A heat-set insert is a knurled brass sleeve with a machine thread inside. The iron heats it, the plastic round it softens and flows into the knurls, and it locks in place as it cools; the screw then bears on a metal thread that can be undone many times.[10][13] Inserts are made for thermoplastics such as ABS.[13][11]

  1. Choose the insert and its screw together: the insert has a metric machine thread, so the part is then held by a machine screw of that thread in place of the original thread-forming screw.[13]
  2. Check that the boss is thick enough. The minimum wall round an M3 insert is 0.95 mm in Albany County Fasteners' table and 1.8 mm in Bulten's data, and Bulten notes that it depends on the plastic.[7][11]
  3. Drill a straight hole of the size the insert maker gives: 4.0 mm for the M3 inserts of Bulten, Accu and Albany County Fasteners. Do not chamfer the top of the hole.[11][13][7] For tapered inserts, McMaster-Carr tapers the top half of the straight hole.[10]
  4. Set the iron. Albany County Fasteners gives 400 to 450 °F (about 204 to 232 °C) for ABS, 2 to 3 seconds to insert, and 30 seconds to cool. Practise on scrap first.[7]
  5. Put the insert on the hole, hold the iron square to the surface and press steadily until the insert is flush.[7] A flat-shouldered insert tip makes it easier to keep the insert square and to lift the iron off without pulling the insert back out.[14]
  6. If the insert sinks too deep, the iron is too hot; if it will not go in, it is too cold.[7]
  7. Let it cool before fitting the screw.[7]

Refit the screws

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  1. Drive each screw by hand. Assembly magazine warns against power drivers above about 500 rpm, because friction heat melts the plastic and weakens the joint.[8]
  2. Stop as soon as the screw is snug. Overtightening is the usual mistake with plastic assemblies, and Apple's service manuals warn of stripped inserts and cracked bezels from it.[8][2][4] Larry Pina's advice for the Macintosh mouse is to hold the screwdriver between thumb and forefinger and stop as soon as the screws are snug, because the covers are breakable plastic.[15]

Checking the repair

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  • The screw pulls up and stays tight without the boss turning or lifting.[3]
  • The case halves close evenly, with no gap at the repaired post.[3]
  • No new crack has started round the boss after the screw is tightened. Bosses split from the outward pressure of the screw.[8]
  • An insert sits flush and square, and does not turn when the screw is tightened.[7][10]

Common mistakes

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  • Fitting longer screws into a cracked post. The Calculator Store finds this helps only for a while, because the crack carries on down the post.[3]
  • Gluing a broken post back with no other support. Glued posts, and threaded cylinders glued into posts, did not last in The Calculator Store's repairs.[3]
  • Overtightening, or driving screws with a fast power driver.[8]
  • Setting an insert into a boss with too thin a wall, or into a chamfered or bell-mouthed hole.[7][11]
  • Using epoxy on polyethylene or polypropylene, which it will not bond.[6]
  • Leaving glue or putty in nearby key or contact holes.[3]
  • Storing acetone near hydrogen peroxide or peroxide waste.[5]

Worked examples on this wiki

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References

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  1. ↑ 1.0 1.1 1.2 Wikipedia, "Acrylonitrile butadiene styrene" (retrieved 2026-10-25): enclosures for electrical and electronic assemblies such as computer cases among its uses; an amorphous solid with no definite melting point and a glass transition temperature of about 105 °C; soluble in esters, ketones (such as acetone), chloroform and ethylene dichloride, with poor resistance to chlorinated solvents, alcohols and aldehydes; yellowing and embrittlement from UV light or excessive heat by photo-oxidation; joints made by heating the surfaces until they melt, reinforced by melting in a thin ABS rod, or by solvents; carbon monoxide and hydrogen cyanide as the most toxic products of its combustion or pyrolysis.
  2. ↑ 2.0 2.1 Apple Computer, PowerBook 200 Series Service Source, display housing take-apart (PDF pp. 145-153): not to overtighten the T-6 bezel screws, which strip their inserts. Hosted as File:PowerBook Duo 200 Series Service Source.pdf; see PowerBook Duo General Maintenance.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 The Calculator Store, "Broken upper screw posts", HP-41 repair guide (retrieved 2026-10-25): turning the screws slowly and gently because the plastic is brittle after 30 years; cracked posts no longer clamp the halves together; longer screws help only briefly because the cracks continue downwards; gluing posts or tying them with fine copper wire does not last; a glued-in threaded cylinder separated from the post every time under screw torque; their repair part is drilled into the post and also glued to a wide surface so the glue line does not take the torque; masking nearby contacts and holes with tape, clamping with clothes pegs, E6000 glue cured overnight.
  4. ↑ 4.0 4.1 Apple Computer, PowerBook 1400 Service Source, bezel and display assembly (PDF pp. 146-155): over-tightened bezel screws crack the bezel, which shows as creaking when the lid moves. Hosted as File:PowerBook 1400 Service Source.pdf; see PowerBook 1400 General Maintenance.
  5. ↑ 5.0 5.1 5.2 Wikipedia, "Acetone", section "Safety" (retrieved 2026-10-25): extreme flammability; flash point −20 °C; vapour flowing along surfaces to distant ignition sources and flashing back; acetone peroxide, a highly unstable explosive, can form when acetone is oxidised with hydrogen peroxide, for example when waste peroxide is poured into waste solvents.
  6. ↑ 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 J-B Weld, "PlasticWeld Epoxy Putty" (product 8237), product page and FAQ (retrieved 2026-10-25): hand-mixed epoxy putty for most plastics, used to build up or recreate missing pieces; 25-minute work time (20 to 25 minutes in the FAQ) and functional cure in 3 hours (two to three hours in the FAQ); 700 PSI; rated to 300 °F; can be sawn, drilled, carved, sanded and painted once cured; bonds to PVC, ABS, polycarbonate, acetal, polystyrene, polyurethane and fibreglass but not to polyethylene or polypropylene; sand or scuff the surface and clean it with acetone; test adhesion on a hidden area if the plastic is unknown; cut, knead to a uniform colour and press firmly onto the surface; for clean tapped threads J-B Weld recommends its PlasticBonder instead; safety data sheet linked from the page.
  7. ↑ 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 7.10 Albany County Fasteners, "Heat-Set Insert Specifications", 2025 (retrieved 2026-10-25), a reseller's guide aimed at 3D-printed parts: hole diameter, insert outside diameter and length, minimum boss diameter and wall thickness by thread size (M3: 4.0 mm hole, 4.6 mm insert, 6.5 mm boss, 0.95 mm wall); iron temperature, insertion time and cooling time by plastic (ABS 400 to 450 °F, 2 to 3 seconds, 30 seconds); a temperature-controlled iron or heat-set tool; practise on scrap; hold the insert perpendicular; steady, even pressure; an insert that sinks too deep means too hot, one that will not go in means too cold.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 8.5 8.6 Assembly magazine, "Beware the boss" (retrieved 2026-10-25): thread-forming screws for plastics have a 30 to 48° flank angle against 60° for standard screws, putting less outward force on the boss and reducing cracking; boss outside diameter two to three times the screw thread diameter; a relief bore equal to the screw's outside diameter makes the boss less likely to crack; assemblers often overtighten plastic assemblies; screws should not be driven into plastic faster than about 500 rpm, because friction heat melts the plastic and weakens the joint.
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 9.6 Wikipedia, "Plastic welding" (retrieved 2026-10-25), sections "Solvent welding", "Hot gas welding" and "Speed tip welding": a solvent temporarily dissolves the polymer so that chains from both parts mingle, then permeates out leaving an entangled solid weld; ABS is typically welded with acetone-based solvents, and polycarbonate and PMMA with dichloromethane; hot gas welding softens the parts and a filler rod of the same or a very similar plastic; a speed-tip welder resembles a soldering iron with a broad, flat tip.
  10. ↑ 10.0 10.1 10.2 10.3 McMaster-Carr, "Heat-Set Insert Tips" (retrieved 2026-10-25): installation tips fit a soldering iron and carry its heat into the insert, softening the plastic, which sets round the knurls as it cools; inserts go into straight holes drilled, printed or moulded in the part; tapered inserts go into a straight hole whose top half is tapered.
  11. ↑ 11.0 11.1 11.2 11.3 11.4 Bulten, Heat-Lok Inserts & Studs, data sheet, via bulten.com (retrieved 2026-10-25): inserts with rounded knurls for notch-sensitive amorphous thermoplastics, installed by heat; moulded holes preferred, with 0.5° inclusive taper; the hole top not chamfered or counterbored and not bell-mouthed; hole tolerance −0.00/+0.10 mm; minimum wall thickness varies with the plastic; M3 insert 5.8 mm long, 4.4 mm diameter, recommended hole 4.0 mm, minimum wall 1.8 mm; M2 3.2 mm hole, 1.4 mm wall; M4 5.6 mm hole, 2.4 mm wall.
  12. ↑ TR Fastenings, "Plas-Fix 45 installation", knowledge base (retrieved 2026-10-25): chamfer pilot holes to the screw's major diameter to avoid cracking; no gap between the top of the boss and the clamped part; boss diameter 2.5 to 3 times the pilot hole; thread engagement at least twice the screw diameter.
  13. ↑ 13.0 13.1 13.2 13.3 Accu, "M3 (3mm) Heat-Fix Threaded Inserts - Brass" (HTBI-M3-BR), product page (retrieved 2026-10-25): a metric thread through the centre and diamond knurls outside; 4.4 mm diameter over the knurls, 5.8 mm long, recommended hole 4 mm (−0/+0.1 mm), minimum wall 1.8 mm, for thermoplastics, heat-press installation; brass inserts give thermoplastic parts a metal thread that can be tightened and removed many times.
  14. ↑ Adafruit Industries, "Heat-Set Insert For Soldering Irons - #4-40 / M3 Inserts", product 4239 (retrieved 2026-10-25): a turned tip used in place of the soldering tip on irons that take Hakko-style tips; ordinary tips can catch on the rim of the insert and pull it back out, while this tip's vertical wall lets the iron pull straight out and its square shoulder makes it easier to tell whether the insert is square.
  15. ↑ Larry Pina, Macintosh Repair & Upgrade Secrets (Hayden Books, 1990), ISBN 0-672-48452-8, pp. 228-229: reassembling the M0100 mouse, tightening the cover screws only until snug because the covers are breakable plastic. Hosted as File:Macintosh Repair & Upgrade Secrets.pdf; see Apple Mouse Button Switch Replacement.
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