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How to Glue/Weld Two Small Pieces of Plastic

Fleeting

authorship
this note is edited using Claude/default, governed by the note how to write a literature note
tl;dr
for ABS, the note lists cyanoacrylate, solvent welding with MEK or acetone, heat welding with an ABS rod, and recasting from an undamaged piece, which neither hook is; none of the passages cited shows any of them over a few mm².
Table of Contents

the ways to try for joining two small pieces of plastic — glue, weld or any other

rule
a way holds even when the pieces touch over only a few mm².
what
the two pieces are a snap hook and the molded part it broke off.
  • how: the hook flexes each time something clips onto it.
  • what: both of the molded part’s snap hooks broke off.
  • rule: both hooks are stuck back on.
  • rule: a hook is remade only if sticking it back on fails.
    • why: the user would rather not spend time perfecting the 3D model of the part if the hooks can be stuck back on.
what
the plastic of the pieces is unknown.
  • what: the molded part carries no recycling mark.
  • what: the user guesses ABS.

telling whether the plastic is ABS

what
for a plastic with no ID symbol, Polyvance gives the procedure below1, 2.
what
Polyvance says a rod that is not a proper match should pop off easily2.

cyanoacrylate, on ABS

what
in Henkel’s tests on one ABS grade, Loctite 414 (general purpose) and 401 (surface insensitive) cyanoacrylates bonded more strongly than the ABS itself3.
what
in the same tests, Loctite 380, a rubber-toughened cyanoacrylate, bonded ABS at 6.6 MPa4.
what
those tests bonded an overlap of 0.5 in², about 320 mm²5.
what
they loaded it in block shear, a method that minimizes peel and cleavage in the joint6.
therefore
these tests do not show how a cyanoacrylate holds over a few mm².
what
Henkel lists poor peel strength among the drawbacks of cyanoacrylates7.
  • what: Henkel says rubber-toughened cyanoacrylates offer high peel strength and impact resistance8.
what
uncured cyanoacrylate can stress-crack ABS9.
  • rule: excess cyanoacrylate is removed from the surface immediately9.

solvent welding, on ABS

what
solvent welding joins a plastic to itself or to another plastic that dissolves in the same solvent10.
what
Covestro says ABS parts are typically solvent-welded with MEK, acetone or a mix of the two11.
what
Covestro says a paste of MEK and the base resin can fill small gaps11.
what
Covestro says too much evaporation before clamping gives a poor weld12.
what
Covestro says that for critical applications needing more durability, glue is to be considered12.
what
Henkel says solvents are more likely to cause stress cracking than glues are13.

recasting the hook, on ABS

what
Polyvance’s PlastiFix kit replaces « a broken tab » by casting a new one in a mold taken from an undamaged piece14.
  • what: Polyvance speaks of « a broken tab », and its page does not say whether that covers a hook that flexes.
  • what: neither hook on the molded part is intact.
how
a FlexMold bar, softened in very hot water, is shaped over the pattern and, once cool, set in the repair area15.
how
the mold is filled with the PlastiFix powder/liquid mix, left to cure 30 minutes, then removed15.
what
Polyvance says the kit is ideal for ABS and other hard plastics, but does not work on PE, PP or TEO14.

heat welding, on ABS

what
for ABS, Polyvance lists welding with an ABS rod, by nitrogen/hot air or with an airless welder’s hot tip16.
how
the part is V-grooved halfway through, then the hot welder tip melts the rod into the groove and mixes it with the base material17.
what
its airless procedure pre-melts about two inches of plastic inside the groove17.
therefore
this procedure is not shown on a joint of a few mm².
what
Polyvance’s book comes from automotive plastic repair18.
what
in its hot-air welding, Polyvance says hot staples may also be used if needed to stabilize the crack19.
what
Polyvance says its staples can also be used to reinforce plastics20.

UHU Max Repair Extreme, which the user has

what
UHU says Max Repair Extreme repairs plastic among many materials21.
  • what: UHU says it is not suitable for PP, PE or PTFE22.
what
its data sheet gives a shear strength of 800 N/cm², that is 8 MPa23.
  • what: the data sheet does not say how, or over what area, that value was measured.
what
UHU describes it as extremely elastic24.
  • what: the same sheet’s specifications give its elasticity as limited25.
what
UHU lists it as resistant to shock and vibrations26.
how
glue goes on one side, the parts are pressed together, can be adjusted within 5 minutes, and reach full strength after about 24 hours27.
what
on the user’s hooks, the UHU did not hold at all.

epoxy, on ABS

what
the user has an epoxy resin used to smooth 3D prints, whose product is unknown.
  • what: the figures below are for Loctite epoxy adhesives.
what
in Henkel’s ABS tests, Loctite E-90FL, a flexible epoxy, bonded at 24.2 MPa, and E-20HP at 18.3 MPa28.
what
Henkel says their bonds did not break the ABS but performed exceptionally well29.
what
those tests bonded an overlap of 0.5 in², about 320 mm²30.
therefore
these tests do not show how an epoxy holds over a few mm².
what
Henkel says epoxies are typically rigid, and flexible ones need special formulation31.
what
Henkel lists long cure and fixture times among the drawbacks of epoxies32.

  1. « 1. Is the part very rigid with lots of glass fibers in the broken area? • If yes, then it’s probably SMC or Fiberglass. » « 2. Perform a melt test - try to melt the surface with a hot airless welder. Does it melt cleanly and resolidfy? • If yes, go to Step 3. • If no, then you have a thermoset material. » « 4. If none of the rods stick, then perform an adhesive repair using Method E » — Polyvance, The Book of Plastic Repair, 12th edition, 2022, p. 3, primary, from the seller of these welders and rods. ↩︎

  2. « 3. Perform a weld test - Judging from the feel and appearance of the plastic, pick some rods that match best. Clean and sand the part in an inconspicuous area. Make a spot weld by melting the rod together with the base material. Allow the weld to cool completely. Pull on or pick at the weld; if it is not a proper match, it should pop off easily. Weld with the rod that sticks best. » — Polyvance, The Book of Plastic Repair, 12th edition, 2022, p. 3, primary, from the seller of these welders and rods. ↩︎ ↩︎

  3. « Loctite® 401™ Prism® Instant Adhesive, Surface Insensitive », « Loctite® 414™ Super Bonder® Instant Adhesive, General Purpose », « Loctite® 401™ Prism®, 414™ Super Bonder® Instant Adhesives and Loctite® 4307™ Flashcure® and 3105™ Light Cure Adhesives created bonds that were stronger than the ABS substrate. » — Henkel, The Loctite Design Guide for Bonding Plastics, Volume 6, 2011, p. 21, primary, from the maker of these adhesives. ↩︎

  4. « Loctite® 380™ Black Max® Instant Adhesive, Rubber Toughened » … « 6.6 » (MPa, unfilled resin column) — Henkel, The Loctite Design Guide for Bonding Plastics, Volume 6, 2011, p. 21, primary, from the maker. ↩︎

  5. « A second test specimen was mated to the first with a 0.5" overlap (bond area = 0.5 in.2). » — Henkel, The Loctite Design Guide for Bonding Plastics, Volume 6, 2011, p. 86, primary. ↩︎

  6. « a block shear test method (ASTM D4501) was chosen. […] due to the geometry of the test specimens and the block shear fixture, peel and cleavage forces in the joint are minimized. » — Henkel, The Loctite Design Guide for Bonding Plastics, Volume 6, 2011, p. 85, primary. ↩︎

  7. « Disadvantages • Poor peel strength » — Henkel, The Loctite Design Guide for Bonding Plastics, Volume 6, 2011, p. 6, primary, from the maker. ↩︎

  8. « rubber-toughened cyanoacrylates offer high peel strength and impact resistance to complement the high shear and tensile strengths characteristic of cyanoacrylates » — Henkel, The Loctite Design Guide for Bonding Plastics, Volume 6, 2011, p. 6, primary, from the maker. ↩︎

  9. « ABS can be stress cracked by uncured cyanoacrylate adhesives, so any excess adhesive should be removed from the surface immediately. » — Henkel, The Loctite Design Guide for Bonding Plastics, Volume 6, 2011, p. 21, primary, from the maker. ↩︎ ↩︎

  10. « Solvent bonding joins one plastic to itself or another type of plastic that dissolves in the same solvent. » — Covestro, Engineering Polymers Joining Techniques Design Guide, 2015 (COV-031 10/2015), p. 29, primary. ↩︎

  11. « Parts made of ABS, SAN and ASA polymers can be solvent bonded using similar procedures and different solvents. Typically, use methylethylketone (MEK), acetone, or a mixture of the two. Additionally, a paste made of MEK and the base resin can be used to fill small gaps in a part or assembly. » — Covestro, Engineering Polymers Joining Techniques Design Guide, 2015 (COV-031 10/2015), p. 30, primary. ↩︎ ↩︎

  12. « If too much evaporation occurs before clamping, poor bonding will result. For critical applications needing more durability, consider adhesive bonding. » — Covestro, Engineering Polymers Joining Techniques Design Guide, 2015 (COV-031 10/2015), p. 30, primary. ↩︎ ↩︎

  13. « Solvents are more likely to cause stress cracking than are adhesives. » — Henkel, The Loctite Design Guide for Bonding Plastics, Volume 6, 2011, p. 13, primary, from a maker of adhesives comparing them with solvents. ↩︎

  14. « The FlexMold bar allows you to replace a broken tab by forming a mold from an undamaged piece, then casting your new part using the PlastiFix acrylic adhesive system. This system is ideal for ABS, acrylic, polycarbonate and other hard plastics, however it does not work on olefinic plastics like PE, PP, or TEO. » — Polyvance, The Book of Plastic Repair, 12th edition, 2022, p. 11, primary, from the seller of this kit. ↩︎ ↩︎

  15. « Place FlexMold bar in very hot water until it softens. » « Shape FlexMold bar over pattern. Caution! Very hot! » « When FlexMold bar cools, position in repair area. » « Fill mold with PlastiFix powder/liquid mix, allow to cure 30 min., remove mold. » — Polyvance, The Book of Plastic Repair, 12th edition, 2022, p. 11, primary, from the seller of this kit. ↩︎ ↩︎

  16. « Method A or B w/ ABS (R03 series) rod » (ABS row, p. 4); « Method A - Nitrogen / Hot Air Welding », « Method B - Airless Fusion Welding » (contents, p. 2) — Polyvance, The Book of Plastic Repair, 12th edition, 2022, primary, from the seller of these welders and rods. ↩︎

  17. « V-groove halfway through the part » … « Use the tip of the welder to pre-melt about two inches of plastic inside the v-groove. » … « go back over it with the hot welder tip and thoroughly melt the rod together with the base material. » — Polyvance, The Book of Plastic Repair, 12th edition, 2022, p. 7, primary, from the seller of these welders and rods. ↩︎ ↩︎

  18. « Polyvance has been at the forefront of automotive plastic repair technology ever since. » — Polyvance, The Book of Plastic Repair, 12th edition, 2022, p. 2, primary. ↩︎

  19. « Hot staples may also be used if needed to stabilize the crack. » — Polyvance, The Book of Plastic Repair, 12th edition, 2022, p. 6, primary, from the seller of these welders, rods and staplers. ↩︎

  20. « They can also be used to reinforce plastics. » — Polyvance, Staples for Plastic Stapler, product page published 2025-02-05, updated 2025-11-06, primary, from the seller of these staples. ↩︎

  21. « Max Repair Extreme is an extremely strong adhesive for repairing porous and non-porous materials such as wood, textile, leather, cork, metal, glass, plastic, rubber, ceramic tiles, stone, cement and many other materials. » — UHU, 37416 Max Repair Extreme data sheet, 2024-04-17, p. 1, primary, from the maker. ↩︎

  22. « Não adequada para Polipropileno (PP), Polietileno (PE) e P.T.F.E. » [not suitable for polypropylene (PP), polyethylene (PE) and PTFE] — UHU, 37416 Max Repair Extreme data sheet, 2024-04-17, p. 1, primary, from the maker. ↩︎

  23. « Resistência ao corte: 800 N/cm² » [shear strength] — UHU, 37416 Max Repair Extreme data sheet, 2024-04-17, p. 2, primary, from the maker. ↩︎

  24. « extremamente elástica » [extremely elastic] — UHU, 37416 Max Repair Extreme data sheet, 2024-04-17, p. 1, primary, from the maker. ↩︎

  25. « Elasticidade: Limitada » [elasticity: limited] — UHU, 37416 Max Repair Extreme data sheet, 2024-04-17, p. 2, primary, from the maker. ↩︎

  26. « Resistente ao choque e vibrações » [resistant to shock and vibrations] — UHU, 37416 Max Repair Extreme data sheet, 2024-04-17, p. 1, primary, from the maker. ↩︎

  27. « Apply the adhesive on one side. Press the parts against each other. Adjustments can be made within 5 minutes. The maximum final bonding strength is reached after approximately 24 hours. » — UHU, 37416 Max Repair Extreme data sheet, 2024-04-17, p. 1, primary, from the maker. ↩︎

  28. « Loctite E-90FL Hysol Epoxy Adhesive, Flexible » … « 24.2 »; « Loctite® E-20HP™ Hysol® Epoxy Adhesive, Fast Setting » … « 18.3 » (MPa, unfilled resin column) — Henkel, The Loctite Design Guide for Bonding Plastics, Volume 6, 2011, p. 21, primary, from the maker. ↩︎

  29. « The bond strengths achieved by […] Loctite® E-90FL™ and E-20HP™ Hysol® Epoxy Adhesives […] did not achieve substrate failure but performed exceptionally well. » — Henkel, The Loctite Design Guide for Bonding Plastics, Volume 6, 2011, p. 21, primary, from the maker. ↩︎

  30. « A second block shear was mated to the first with an overlap area of 0.5 in.2. » — Henkel, The Loctite Design Guide for Bonding Plastics, Volume 6, 2011, p. 86, primary. ↩︎

  31. « Epoxy adhesives are typically rigid and formulating techniques must be employed to produce flexible epoxies. » — Henkel, The Loctite Design Guide for Bonding Plastics, Volume 6, 2011, p. 7, primary, from the maker. ↩︎

  32. « Disadvantages » … « Long cure and fixture times » — Henkel, The Loctite Design Guide for Bonding Plastics, Volume 6, 2011, p. 7, primary, from the maker. ↩︎