Plastic screw cap restoration

[Rich C]

Having had no luck finding a replacement screw cap for my vintage Century, I decided to restore my original.  Luckily it was cracked only at the top horizontally and not vertically through the threads, but still screwing it down would put pressure on the repaired area so it had to be a secure, strong repair.  I first tried gel super glue which worked at first, but the first time the lamp heated up it let loose.  I decided to then use a super strong epoxy I had from repairing metal and plastic model cars.  I've used it successfully to repair various kinds of automotive parts in stressful locations and wide variations of hot and cold.

Anyway, here is what a started with, a cap with a crack about 180 degrees around the top of the cap.  I had already cleaned out all the leftover super glue residue.  It does not stick well to the bakealite plastic the cap is made from

I wanted the epoxy to get a good grip on the plastic so I sanded the surrounding area to clean and slightly rough up the area to be glued.  The cap had sat cracked but screwed down for a couple of decades so the crack was permanently spread open at this point so I was able to also clean up inside the crack itself which was critical to getting a good bond.  I scuffed the rest of the cap knowing it would help with future paint adhesion.

Here I have applied the epoxy.  I made sure to spatula the adhesive into the crack and the surrounding area.  I tried to get it into critical areas but didn't worry about making it pretty since I knew I'd be sanding it smooth again anyway.  A little too much applied at this point would be good and help build a solid repair.

I first tried to gently clamp it in my vice out in my garage to help close the crack and allow the adhesive to cure the crack closed.  Once clamped I smoothed out the epoxy that was squeezed out of the crack.  I let it sit over night in the vice but it had not cured at that point probably due to it being so cold out in the garage over night.

It then dawned on me to just use a small hand clamp and place it near a heat register so it would fully cure sometime in my lifetime.  I wanted to clamp it really hard to make sure the crack was held closed, but knowing the plastic is somewhat brittle I clamped it just tight enough to close the crack an no more.

Here it is all cured and sanded smooth.  The epoxy cures to a hardness very similar to the plastic of the cap so sanding was quick and easy to it get smooth and even.

Instead of shooting it with a coat of primer prior to painting it, I first sprayed it with a light coat of adhesion promoter which is used to help paint adhere to plastics.  The promoter has the side benefit of being self leveling so it helped fill any tiny remaining imperfections.  It cured pretty fast so I then gave it two light coats of gold paint and it turned out great.  The gold paint I had doesn't exactly match the original Lava used but it really close.  It is a touch more shiny than the original paint but I am thoroughly pleased with the results.  The ultimate test will come when I reinstall it on the lamp and run it through a couple of on/off cycles.  I don't plan to crank the lid down like the factory did, just to snug it enough to make an air tight seal.  I think that's why so many of these crack, the factory really tightened them down way too hard.

I'm going to let it cure for another day or two before I reinstall it just to insure the epoxy and paint have ample time to fully cure.

[Todd Miller]

Awesome job !  Looks Nice !

[Critter]

Very nice work.  I've done similar too, I used modeler's putty to fill in chips and cracks.  Your "new cap" looks great man!

[goober jack]

Great job!!!

[Alfred]

Impressive.

[Erin]

That is the most impressive cap restoration I've ever seen. Amazing job! 

[Kirk]

Nice job, and thanks for posting!

[Tim Gill]

Nice job, a real labour of love for a cap - looks better then before the crack

[Rich C]

Grumble grumble grumble!  Cap split again.  Contrary to my earlier statement I apparently tightened it down too much and combined that with the heat and a small gluing surface, it decided to split again.  I cleaned it all up and moved up to a much stronger epoxy that can withstand 550 degrees F of heat.  I built up the epoxy in the high stress areas and plan to sand it smooth on the outside but to also leave a thicker layer of epoxy over the cracked area.  Should be painted and back in place by the weekend. This time, it will only be snugged down, not super tight!

[Jus]

Wow, that restoration looks great!  

The times I've used epoxy to heal cracks, I've had the best luck when I'm able to completely encircle the damaged item with a layer of epoxy.  I don't know if an outside layer on your cap is feasible, but perhaps an inside encircling layer would help!  Maybe if you repeated again the fix you've discussed here, then immediately coated the inside with a layer of epoxy, that might help!  I'm no physicist 4 shore, but perhaps another issue causing problems is heating/cooling & it's affect on the cap's material...maybe U could figure out a way to fix it at one temperature so that when it went the other temperature direction, it would  compress rather than force apart the crack...jus thoughts in my head....Good luck! 

DISCLAIMER: All my repair attempts have involved an outer layer of epoxy; I have no idea if a sub-surface epoxy layer would help at all or even be advisable. In fact, I've no idea whatsoever what the inside of your cap even looks like!  Oh dear... LOL

In case I'm not making myself clear, these pics might help...

These photos show a ring of epoxy which has given my .99 cent mini-tripod AT LEAST nine more lives, and counting...

[Rich C]

That is sort of what I've done this time around.  That solution would work great for vertical cracks, unfortunately mine is a horizontal one.  When the cap is screwed down, the force of trying to make it seal against the rubber ring is also trying to force the crack open again.  This time around I've laid on the epoxy inside and out much thicker while smoothing it into round curves.  The idea is to provide additional thickness strength and additional surface area to which the epoxy can grab onto.

Your suggestion has me thinking I could smooth additional epoxy from the side, over the crack and onto the top of the cap to provide similar support as you show in the photos.  I kind of started this concept, but it might not hurt to carry it further and then just sand it to a pleasing shape.  The paint will hide the additional thickness.  I really don't want to be doing this again!  Thanks for the input.

I think ultimately on the first go 'round I was just too dainty with the epoxy to save myself some sanding and shaping efforts. 

[Rich C]

When I got home tonight, the epoxy had full set and is hard as a rock.  This batch will be much harder to sand.  FYI, I used JB Weld this time around.  Anyway, I figured I run the lamp through a few cycles with the cap on before I go to all the trouble of sanding and painting to make sure it will hold up.

[Jus]

The crack being horizontal versus vertical problem hadn't even occurred to me.  Sounds like you're on your way to solving your cracked cap issue now!  Good luck-