How to repair a damaged plastic screw column? - Page 1
Forum section: electronics / repair

author: daisizhou
author_rank: Super Contributor
timestamp: September 26, 2025, 10:53:16 am
content: Hallo everyonePlease see the picture below. One of my instruments has a damaged fixing screw. The fault is shown in the picture below.How can I repair it? The fixing screw has a pointed thread.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 26, 2025, 11:04:12 am
content: Find the chipped pieces and fix then together with a drop of cyanoacrylate (aka Superglue).If you don't have any at hand and don't want to wait, alternatively you can hold together the chipped pieces of plastic by reinforcing them with pieces of wire (e.g. TH resistor terminals leftovers). Heat the resistor terminal with the soldering iron until the terminal starts to melt the plastic and sink into it. Orient the wire alongside the screw direction. Once hot enough and starting to sink into plastic, the wire will cover itself in melted plastic. You will probably need 3-4 such wires dipped along the plastic bolt.

author: Kean
author_rank: Unknown
timestamp: September 26, 2025, 11:19:16 am
content: FYI, this column or pillar is usually called a "screw boss" (or just "boss") in injection molding.Seehttps://www.protolabs.com/resources/design-tips/plastic-boss-design-on-molded-parts/My suggestion:Buy some ABS round rod - e.g.https://www.aliexpress.com/item/1005009297118405.htmlCut a piece of rod of the right diameter to the same length as the old bossDrill out the center - it doesn't have to be perfect, or all the way through, but if too small or shallow for the screw it may quickly break againRemove remains of old boss by cutting it away with whatever tools you have, but keep the ribs around itGlue new ABS boss in place with CA glue to the base and ribsIf possible glue on extra pieces of ABS (thin rod or sheet) to add extra support around it

author: jpanhalt
author_rank: Super Contributor
timestamp: September 26, 2025, 11:29:57 am
content: CA might work, but some plastics are made brittle by it over time. I have used filled epoxy. I use colloidal/fumed silica to make a consistency of thick ketchup. After everything is hardened, clear the hole with an appropriate sized drill bit held in a pin vise or similar. If you have a good mold release you can insert the treated screw before assembling to be sure the threads line up. I usually do the latter.When inserting those coarse threaded screws, be sure to turn them CCW (backward) to catch the thread before tightening CW (clockwise). Cross threading is one cause for such damage. You can tell when they are properly caught in the thread as they should turn easily for the first few turns.

author: ondraN
author_rank: Regular Contributor
timestamp: September 26, 2025, 12:33:52 pm
content: In such cases I proceed as Kean described. Or I drill out the rest of the post with a larger diameter drill bit and glue (with epoxy) a threaded metal post into it and use a screw with the same thread as the post. Repairs with regular cyanoacrylate or epoxy glue don't last very long and will crack again.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 26, 2025, 12:44:12 pm
content: Another filling material could be cotton (or any other fiber, e.g. hemp fibre you may already have at hand for pipe threads) mixed with cyanoacrylate. Easy to prepare in seconds, and can be drilled, cut or filed after it solidifies.Something this style, but adapted to fix the missing broken plastic:Super glue and cotton miracle! Pour glue onto cotton and admire the results321Techwood

author: kripton2035
author_rank: Super Contributor
timestamp: September 26, 2025, 12:48:42 pm
content: or this kind of plastic you melt under boiling water, that then solidifies.called "polymorph plastic"did not test it though under long times, or permanent instrument heat !https://www.aliexpress.com/item/1005006030350664.html

author: Ian.M
author_rank: Super Contributor
timestamp: September 26, 2025, 01:23:17 pm
content: First you need to determine if the case is ABS or some other plastic. If ABS, Kean's boss replacement suggestion above will work if you use solvent pipe cement for ABS pipes, orABS glue/slurry. Incidentally, reinforcing this boss might be a good use for a 'maker' 3D printing pen loaded with ABS filament.If its *NOT* ABS, a similar procedure with compatible plastic rod and solvent cement should work.If the broken fragments can be bonded back in place, then a tight whipping of linen or cotton thread round the outside of the boss extending from as deep as you can to near the boss face, then saturated in thin cyanoacrylate glue may provide sufficient strength.

author: sokoloff
author_rank: Super Contributor
timestamp: September 26, 2025, 01:45:41 pm
content: Adding to Ian's post above, nail polish remover is typically acetone.If you don't want to go out and buy crap for this one thing and have nail polish remover in the house, just test/use that...

author: Ian.M
author_rank: Super Contributor
timestamp: September 26, 2025, 01:58:36 pm
content: ... but do check it doesn't have added lanolin to condition the skin!

author: minifloat
author_rank: Contributor
timestamp: September 26, 2025, 10:12:52 pm
content: In the photos I see the bore goes deeper than the original screw. A longer screw could help here, without any further repair of the plastic. A lab assistant at work repaired a bunch of HP signal generators that way, which would've otherwise failed the yearly electrical inspection.Another Idea would be remodeling the boss with 2-component epoxy casting resin. Rough up the surface with a piece of sandpaper. Form a cylindrical mould around the stud with gaffer tape. Epoxy does not bind to the sticky side of the tape. So, fill it with epoxy resin, then drill a new hole.br, mf

author: Stray Electron
author_rank: Super Contributor
timestamp: September 26, 2025, 10:32:47 pm
content: Even if you can glue it back together you're going to have to be VERY VERY carefully when reinserting the screw! Pointed screws will tend to spread the plastic apart and they will break it. I've repaired a bunch of items with broken post like this and I always insert the screw very gently into the hole and then turn it backward (to the left) until I can feel the screw drop into the threads. Then I carefully tighten it. If you don't let the screw drop into the threads first it will try to cut a new thread into the plastic and will break it every time. GUARANTEED! Also you can just barely tighten the screw until it seats. NO MORE! Or the plastic will break.After doing this several hundred times, I had about a 70% success rate. The real problem is that the repaired plastic won't stand much twisting force or force in tension so even trying to put a tight metal sleeve over the repaired post in an effort to prevent it from splitting still won't keep it from pulling apart under tension.

author: David Hess
author_rank: Super Contributor
timestamp: September 27, 2025, 12:28:00 am
content: I have fixed a few of those using just plastic welding fluid, but all of the cracked pieces were available.They used to make a hot glue which was white and solidified quite hard and would stick very well, but I do not know where to find it now.I would probably make some polystyrene putty, coat the screw with mold release, and "putty" the screw into place. Then it could be removed after hardening.

author: Oilngas
author_rank: Contributor
timestamp: September 27, 2025, 02:19:56 am
content: I've repaired and strengthened a damaged boss by wrapping it with carbon fiber tow saturated with epoxy.