Epoxy / Adhesive advice - Structural repair of plastic

(Topic ID: 218882)

Epoxy / Adhesive advice - Structural repair of plastic


By Zitt

5 months ago



Topic Stats

  • 20 posts
  • 7 Pinsiders participating
  • Latest reply 5 months ago by cottonm4
  • Topic is favorited by 4 Pinsiders

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    Scope (resized).PNG
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    #1 5 months ago

    I recently bought a "parts" SMT microscope for my new Electrical Engineering Workbench. This microscope has structural damage on a ~1/2" hinge feature at the rear of the scope where it attaches to the arm. The company (Vision Engineering) designed this scope so the hinge is part of the plastic housing of the scope - 1/2 of hinge is on left and the other is on right housing. I searched google images to try and find an example picture of the hinge; but they must know it has issues or no one has ever needed to take a picture of it. Emails with the local sales rep as only resulted in "you should send the scope in for service"... which when asked for a "ball park" repair cost... I got no response. I suspect the repair would probably cost near the amount of a new scope which defeats the purpose of buying a "for parts" scope to begin with.

    I'll snap a picture tomorrow after work... after the latest JBWeld has cured.

    I went thru a rather extensive "rebuild" of the hinge using some JBweld I had in the drawer. I basically re-enforced it by filling the hinge with JB weld and some #4-40 bolts to provide a clamping force. The repair worked ok for about 3-4weeks of very light usage; but came apart during use tonight. The jbweld basically didn't bond well to the hinge's plastic and couldn't handle the stress of the hinge (my guess). I've re-jbwelded the hinge tonight but honestly don't expect it to hold up very well for long.

    I'd like to solicit the wisdom of my Pinball buddies ... to see what they might suggest as a rebuild advice.

    At this point; JBWeld has failed me... mainly because it doesn't do a very good job of bonding the to plastic housing which I assume is made out of ABS or similar. Some people swear by JBWeld; but I haven't had a lot of success with it.

    The only item turned up was a youtube video which seems to recommend 3M's EC2216 Epoxy... but that appears to be specific to the carbon fiber application.

    I think some of the key limiting factors of this specific "application" are as follows:

    1. Scope/Housing cannot be disassembled. No service manual exists; to returning the optics to proper alignment would likely be an issue.

    2. Adhesive may need to bond to plastic housing
    3. Epoxy should be flexible while maintaining strength to plastic bond.
    4. May need to have fibers to aid in strength of hinge; ideally perpendicular to the hinge "crack".

    My current thought was to try and do a carbon fiber re-enforcement type job on the hinge. Maybe cut some deep groves into the hinge and fill those groves with some fiber reinforced epoxy. But alas, I can't seem to find such a beast during a quick search on amazon or google. I could make a carbon fiber epoxy by mixing it into a 2 part epoxy; but wanted to see what real-world advice could be shared by our community.

    #2 5 months ago

    Loctite epoxy plastic bonder

    #3 5 months ago

    I had a plastic part that was damaged by a group of 11 year old boys (Boy Scouts). It was a lamp assembly on the troop trailer. No matter how many times it was repaired it kept breaking....because they are 11 years old boys with the delicate nature of The Hulk. Two part epoxy always failed after a single outing but what finally did it was to reinforce the joint with thin layers of strong fibers.... used drier sheets. The fibers were super thin... far thinner than fiberglass tape and crazy strong. I added a thin layer of epoxy, then a sheet, then epoxy, then a sheet.

    Three-four layers with a topper of epoxy to give it a shinny candy coating.

    The collective "Hulk" has not broken it for over six years.

    Hack-tas-tic for sure.... and YMMV but there it is.

    Best of luck!
    faz

    #4 5 months ago

    I used to work with carbon fiber. I did repair work and parts installation work on the Beech Premier business jet.

    The two main adhesives we used are what we called 9309 and 956. I never completely learned the engineering properties of each type but for the most part we used 9309 to bond metal parts to carbon fiber parts while we used 956 to bond carbon to carbon.

    The manufacturer for these two adhesives is Henkel. Henkel also owns Loctite products.

    I don't know how the company would be towards an individual doing a onesie-twosie project but you might give them a call. The worst they can do is hang up on you. The trouble is that some plastics are just not bondable.

    http://www.henkel-adhesives.com/henkel-adhesives.htm

    http://www.henkel-adhesives.com/adhesives/contact-dialog-46520.htm

    #5 5 months ago

    Good info so far. I like the dryer sheet + epoxy idea.
    Last night I ordered some 3M EC2216 Epoxy and some fiberglass sheet for two day delivery.
    I should have it on Wed.

    I still plan on grabbing some pictures tonight.

    #6 5 months ago

    I been using 30min epoxy and carbon fiber for years on my rc models, till I started my pinball collection, what to do with the plastic ramps, how to repair them, a good sanding it's important then some super glue to maintain shape and finally fiberglass cloth then carbon fiber layer, cure for couple of days and that's it, never had a problem with my repairs.

    Gustavo

    #7 5 months ago

    Hard to tell without photos so I'll cover several options. If the two parts fit together snugly I would go with a plastic welder like sci-grip (they will have solutions for different types of plastics). Plastex is a great choice when you need to recreate and attach a plastic part (like the mounting tab on a ramp that's broken and missing). If you go the epoxy route make sure the epoxy is specifically made for plastic and use 3M AC77 Adhesive Primer. Fiberglass cloth helps make a much stronger joint, but only if you have good adhesion to begin with.

    #8 5 months ago

    Thing is I don't know what type of Plastic the housing is made out of. I assume it's ABS; but its just that... an assumption.
    I'll look into some AC77 to see how it works. Would be nice to get a small sample bottle for a reasonable cost somewhere.

    #9 5 months ago

    Here's a picture of the hinge after an overnight JBWeld cure:
    complete hinge - these two features hold all the weigth
    P_20180611_192355 (resized).jpg
    Basically; the hinge supports most of the weight of the scope head on this hinge.

    The two "top" screws are my addition in order to aid in clamp force during the first repair. The #4-40 screws mate with some blind #4-40 nuts on the inside of the main hinge feature. The nuts are basically JBWelded to the plastic just below the crack which is now hidden by JBWeld.

    ?When?/IF the weld breaks again; I'll take some better pictures of the internal repair attempt.

    #10 5 months ago

    I pictured something completely different. Wow... that's 1/2"?

    That is a pretty basic, isolated shape.
    Does that shoe like structure separate from the dome-like super structure??

    Have you considered having the part cast?
    faz

    EDIT: I see now. I did some image searches... The dome is the back of the viewer on top and this hinge is the top of the arm. Yea, big stress point.

    #11 5 months ago
    Quoted from Zitt:

    Here's a picture of the hinge after an overnight JBWeld cure:

    Basically; the hinge supports most of the weight of the scope head on this hinge.
    The two "top" screws are my addition in order to aid in clamp force during the first repair. The #4-40 screws mate with some blind #4-40 nuts on the inside of the main hinge feature. The nuts are basically JBWelded to the plastic just below the crack which is now hidden by JBWeld.
    ?When?/IF the weld breaks again; I'll take some better pictures of the internal repair attempt.

    Could you install longer #4-40 screws that would travel farther down and seat against the base and perhaps provide some extra support? Or could you make some sort of web to bond in between without causing clearance issues?

    Or maybe you could make an "L" shaped strap that would locate under the two 4-40 screws and screw into the lower part of the base and give some extra support?

    Or can you drill an extra hole at a right angle and between the two 4-40 screws, then tap the hole and run a piece of threaded rod all the way through?

    #12 5 months ago
    Quoted from pinball_faz:

    Does that shoe like structure separate from the dome-like super structure??

    I don't have an exploded BOM or diagram. I asked the sales rep for one so I could identify the parts I needed... and he responded that they don't have anything like that. :screw loose:

    So; based upon my examination early on... as far as I can figure; the entire head is a single injected molded piece in two halves. IE and left and a right half. The hinge is part of that piece. It doesn't look like you can remove the hinge; 1/2 of it is on the right... the other half on the left.
    To make matters worse; the lenses/optics appear to be either taped or screwed to the halves. My fear is that once it comes apart; it won't go back together properly without a major re-alignment.

    Quoted from cottonm4:

    Could you install longer #4-40 screws that would travel farther down and seat against the base and perhaps provide some extra support?

    That's what I sorta did with #4-40 screws. I stopped short of going into the void behind the parabolic mirror for several reasons.
    1) I don't want to risk damaging that mirror.
    2) I don't want to risk adding dust to the scope by drilling thru the housings.

    Quoted from cottonm4:

    Or could you make some sort of web to bond in between without causing clearance issues?

    Or maybe you could make an "L" shaped strap that would locate under the two 4-40 screws and screw into the lower part of the base and give some extra support?

    Or can you drill an extra hole at a right angle and between the two 4-40 screws, then tap the hole and run a piece of threaded rod all the way through?

    I was thinking along the same lines.
    I think when this JBWeld breaks... notice I said when. I'm going to try and do several different things.
    1) Using a dremel; remove all traces of jbweld from the current repair.
    2) Use AC77 or AC79 as suggested by terryb above to prime the plastic for epoxies.
    3) Use 3M EC2216 Epoxy to repair the hinge structurally.
    4) then use Fiberglass weave with 3M EC2216 to create a reinforced structure around the hinge.

    JBWeld is strong; but I think I need flexible in this application. I think the JBWeld is "shattering" instead of flexing and it's also separating from the plastic of the housing. Hopefully the Primer will help the new flexible epoxy to adhere to the housing plastic.

    #13 5 months ago

    J.B. Weld is strong but not as a support structure. Same thing with fiberglass. Fiberglass is strong but the strength comes from the resin "binding" up with the fiberglass sheet. Put some resin on some wax paper and after it sets up you can shatter it; Just like Walter and Jesse shattered their blue glass.

    Can you show a picture of the other part that goes with what you are attempting to repair? A google search brings up nothing that looks like what you have. I am having a hard time visualizing what all fits together.

    Also, It appears to be discontinued. Good thing you did not fall the salesman's trap and sent that part in. You would never get it back. Good call on your part.

    #14 5 months ago

    Best I can do until it breaks again

    1528851924394-1642079149 (resized).jpg
    #16 5 months ago

    I'm having a "not easy" time of seeing how it all hooks together, but I think you are on the right track with what your plans are in post #12. Then add step #5 and make a metal strap that bolts to your 4-40 screws, bend it down towards the base and drill two more holes and attach the strap on the opposite end with a pair of sheet metal screws.

    It sure would be nice to know what kind of plastic you are working with, though.

    One more possibility: If you had the clearance and could grind 1/8" of material off of each side, then you could make two pieces of metal with the same profile and screw the two pieces to the sides with 3 or 4 or 5 screws. They would need to be flat head screws so you would have to countersink all of the holes.

    #17 5 months ago
    Quoted from cottonm4:

    I'm having a "not easy" time of seeing how it all hooks together

    This is the arm I bought.
    ebay.com link
    If you look at the listing; the third picture shows the arm's "mating" side.
    Hinge w/o Head

    Basically in my picture; the philips head screw is a screw I picked up at Lowes to resolve the missing screw. The philips screw basically provide a gravity hinge point to the curved part the head hinge. The bottom "stock" bolt secures the head to the arm. When both screws are installed; the plastic hinge is "secure" in the arm.

    Quoted from cottonm4:

    If you had the clearance and could grind 1/8" of material off of each side, then you could make two pieces of metal with the same profile

    An interesting idea. Hopefully it won't get that complicated. I feel like changing to the new epoxy with primer will probably get me close enough.

    #18 5 months ago
    Quoted from Zitt:

    Would be nice to get a small sample bottle for a reasonable cost somewhere.

    Smallest, cheapest bottle I've seen. Also commonly used to glue game inserts so you could probably sell it if you don't need it.

    Added 155 days ago:

    Forgot the link: http://www.all-spec.com/Catalog/Adhesives-Sealants-Tapes/Adhesive-Primers-Promoters/Adhesive-Primers/00048011627281

    #19 5 months ago

    I don't expect the bond to hold for a long time. The epoxy is sticking to the plastic, not welding it... right, isn't that what happened before?

    The bolt is pressing on the same stress point.

    If you can wrap the outside of the plastic with epoxy soaked fiberglass I think it would spread the grabbing power of the epoxy over a broader surface area. It should also help to hold the plastic together to keep the epoxy from stretching apart at the glue joint.... but this feels like another effort to delay the fracture again.

    What about a completely different approach... strap the head from around the front and directly to the arm. It won't look pretty but I can't imagine it would move.

    I'm guessing this is what your model looks like.
    faz

    Scope (resized).PNG

    #20 5 months ago
    Quoted from Zitt:

    This is the arm I bought.
    ebay.com link
    If you look at the listing; the third picture shows the arm's "mating" side.

    Basically in my picture; the philips head screw is a screw I picked up at Lowes to resolve the missing screw. The philips screw basically provide a gravity hinge point to the curved part the head hinge. The bottom "stock" bolt secures the head to the arm. When both screws are installed; the plastic hinge is "secure" in the arm.

    An interesting idea. Hopefully it won't get that complicated. I feel like changing to the new epoxy with primer will probably get me close enough.

    Even with your added pics I still have some trouble visualizing how it all hooks together. I half imagine that I am looking at an X-Ray of a shattered bone that the doctor has screwed back together with screws pointed in all different directions and that one or two of those screws will interfere and cause clearance and fitting issues after the broken area has been bulletproofed.

    It looks like "not a cheap" part.

    I wish you well.

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