Adding an attachment point to a plastic material 2

[WCarpt]

I don't know if there is a better place to address my issue but I thought I would start here.

I need to add two more attachment points or "ears' to a plastic material on a cab of a tractor. Since the manufacturer did not properly space the bolts that attach an access cover to the cab, the thin plastic access cover ~0.01 inches thick bows substantially between the bolts which are spaced 16.5 inches apart, as a result of the pressure of the sealing gasket which then creates a sealing issue. The plastic material at the front of the access hole is 0.150 inches and the plastic thickness at the rear of the access hole is 0.137 inches thick. See attachments.

NOTE:I tried to upload several photos and a drawing but in the preview, it only shows one attachment, the last.) After posting, I found out that only the last attachment was attached so I will try to attach the photos. I found out I cannot add any more attachments! I will try the image function above.

What is a feasible way to do this? Try to form a piece of metal to conform to the area at the edge of the hole which would be difficult and cement it to the plastic for an additional attachment point or perhaps there is better way.

Thanks

 

[SnTMan]

WCarpt, without knowing what kind of plastic it is, about impossible to say anything definitive, but...

I have found regular old Gorilla Glue can bond to difficult plastics, and that cyanoacrylates (superglue) with a proper primer can as well. Surface prep is all important.

HDPE can be bonded with a flame treatment of the surface, see attached.

You kind of need to know what you got...

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[SnTMan]

Maybe more practical to fab a metal back-up plate with added attachment point?

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[WCarpt]

A problem is how do I fab a metal back-up plate that conforms very closely to the curved surfaces so that the glue would hold sufficiently to the plastic? Around the perimeter of the access hole, toward the inside, there is nothing directly below the area to screw a bolt into. Away from the perimeter, more toward the inside,there is a cover for the HVAC box but it is just thin sheet metal. Yes, it would be nice to know what type of plastic it is. I wasn't sure about Gorilla Glue but concerning a cement to use, after extensive searching, and contacting J-B Weld, this suggested this: Link.

Thanks

 

[SnTMan]

WCarpt, I was just spitballing on the metal backup, I really can't tell enough about the physical situation to offer more detail. The intent was that it be mechanically attached, not bonded.

The JB product may well work, or then again, maybe not. Whatever you fabbed your tabs from would have to bond with it as well.

You'd just have to try it. That goes for any adhesive system unless the substrates can be identified.

Best of luck.

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[Compositepro]

Part of your problem appears to be poor seal design or application. The gasket may be too thick or too hard. It looks like at the screws the gasket is over compressed (smashed flat). There should be a metal spacer at these points to prevent over-compression due to over-tightening. Rather than stiffening the cover, the solution may be to reduce the clamping loads required to get a seal.

 

[WCarpt]

I very much agree that this is a poor design but now I have to somehow address it so as to make sure it doesn't leak. I don't want to have a problem with water getting into the top of the cab.

I am not trying to stiffen the cover but rather would like to provide a more uniform clamping force between the cover and the cab top between the bolts that are spaced too far apart. The OEM neoprene rubber gasket sealing strip is 3/4 W x 1/2 H and is what is called ultra soft which is the softest material I can get. Because of the large bolt spacing on the long side of the cover and the cover being thinner than the cab plastic material, the cover bows significantly up when the bolts are tightened down. A concern of mains is that if I were to somehow stiffen the cover, the lower cab plastic would probably bend downward and potentially the force on the two plastic tabs or ears with the nutserts on the long side may have too much force applied and break.

 

[3DDave]

It looks like an entirely sh** design. Those are hard to fix. It forms a trough that traps water all the way around and there is no clear runoff path even if the seal is good. The close-up photos remove all context so there's no telling how any of this is sloped and how it's exposed to water, but it looks like that doesn't matter as all alternative attachment avenues are removed anyway.

I would probably try a sheet of mylar and double face tape to glue it all the way around but the cover prevents that and the screws punch holes into the otherwise sealed area.

Normally one could just add hold-down clamps to the base structure but, again, that just makes for openings directly into the volume you want to remain dry.

There are pawl-latch clamps; again, those will require holes in the cover that will leak, as the current fasteners probably do now.

https://www.mscdirect.com/product/details/32812364

They turn the pawl out of the way when they disengage and then turn the pawl under the mating part when tightened, but it looks like they made sure that could not work. They are also expensive from any of the several suppliers, probably $100 worth to get what you want; Not sure why because similar items come on electrical boxes that cost a few dollars.

I would take the current situation, put on the cover with the gasket and fill the entire outside with sealant. If I'm in a happy mood I'd put some mold-release down first.

 

[EdStainless]

I would put double stick tape on both sides of the gasket.
Put it in place and then just tighten the screws enough to make contact.
I was thinking like Dave of using dogs to catch the edge, but putting more holes in the sealed side just seems wrong.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Compositepro]

You could also switch to plumber's putty for sealant. It makes access a little more difficult and messy but it will fix your problem.

 

[MintJulep]

I'd toss the OEM cover and make a replacement out of something much stiffer. 3/16" aluminum sheet might be a good starting point.

Add a few clip nuts around the edge of the opening where you can.

 

[SnTMan]

Form in place gasket, maybe?

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[WCarpt]

It seems like there is no good way to resolve this very poor design. I do like the Signature Line of SnTMan which is "The problem with sloppy work is that the supply FAR EXCEEDS the demand." As was mentioned, mold release would be a very good idea to put on the gasket---if the OEM had put some on, then I wouldn't have ruined the existing gasket upon removal of the cover and then have to buy another which cost about $60. (I tried to get another sealing strip from New Holland but it was too wide which would cause several issues, much firmer, but substantially thinner so I returned it and then researched and found an equivalent to the OEM sealing gasket.)

It was suggested that the OEM cover be tossed and make it out something much stiffer. If this were done, what type of plastic would someone suggest and how thick? The overall approx. length and width is 34.5 x 11.5 inches.

Some people thrive on challenges but I would rather not have to deal with a very poor design.

Thanks for your input so far.

 

[SnTMan]

How do I know this?

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[Ron]

Increase the gasket thickness



A Great Place For Engineers to Help Engineers

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[1gibson]

Easy way: Use thicker, self adhesive neoprene foam weather stripping as a gasket?

Right way: Make a rigid spacer that is permanently glued/sealed to the cab. Through holes for the existing bolts, new tapped holes in the spacer for new bolts, drill corresponding through holes in the cover plate. Gasket between rigid spacer and cover plate.

Fast way: Put the plate on as-is and pick your favorite tape to go all around the outside of it. That black silicone plumbing tape holds up pretty well over time to heat/weather/UV.

 

[WKTaylor]

Maybe off-topic or possibly too-high level... for what its worth, subject of fastening Plastics...

Found these references useful for plastics...

MIL-HDBK-727 DESIGN GUIDANCE FOR PRODUCIBILITY

MIL-HDBK-755 PLASTIC MATERIAL PROPERTIES FOR ENGINEERING DESIGN

MIL-HDBK-797 POLYAMIDE (NYLON) PLASTICS: PROPERTIES, PROCESSING, PERFORMANCE, AND MILITARY APPLICATIONS

Books...
Plastic Design Handbook SAE R-415

Mechanical Fastening of Plastics: An Engineering Handbook ISBN 9780824770785

USAF T.O. 1-1A-12 ENGINEERING HANDBOOK SERIES FOR AIRCRAFT REPAIR - FABRICATION, MAINTENANCE AND REPAIR OF TRANSPARENT PLASTICS

NOTE. When drilling 'hard' plastics, a special drill-bit point is required for clean penetration and minimal burrs/chips. I think these drill-bits typically have a very sharp point angle of 60-Deg

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
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[WCarpt]

Thanks. To prevent the bowing up of the cover, I need to devise some way to provide a clamping force between the too widely spaced existing bolts. If I use a fairly thin piece of metal, it needs to be formed to match the curved surface of the edge of the cab top. Since I'm but not a metal fabricator, it is a problem. If I were to obtain a suitable block of plastic, I was thinking I could fairly easily grind it down to match the curved shape of the cab edge. I would either have to glue the piece to the cab top or use the piece as a turn pawl latch but this appears to not be easy to implement---there is no easy way to resolve this.

 

[1gibson]

You could glue strong magnets on both sides for extra clamping force (sorry, but there is always an "easy way" it just might not be very professional.)

 

[MikeHalloran]

It's like an old Chevy SB rocker cover.

I'd vote for FIP RTV, with very little torque on the bolts, just enough to make the goo squeeze out nicely.



Mike Halloran
Corinth, NY, USA