Rivets popping thru paint 1

[HAMILTONM]

Hi Folks,

Longtime lurker here, finally in need of your help.

We make a composite cover that we rivet a 20 inch long hinge to the inside edge of. We then send it out to be filled and painted. All of a sudden the rivet heads are randomly cracking the paint.

We eliminated the paint as a problem because the covers are painted by 2 different shops and both are cracking through. The riveting process is unchanged, as this is a process we've done for many years.

We used to put a shim under the hinge "as required", which I thought might be a problem. Without the shim, there's no stress on the rivets, but the rivets cracked through the paint with or without the shim.

Do any of you have an idea of where I should look next? Boeing has shut down the line waiting for us to fix this.

Thanks,
Mark

 

[wes616]

Hi Mark,
I'm guessing you are using solid, flush head rivits (alum?). Have you changed the rivet type recently. I'd check to see if they have been seated properly (you may have a new mechannic bucking the rivets). Also you may want to check the bit used to drill out. It may be old, and driving the hole oversized, where vib may cause the paint to crack.

nothing you've probablly not already thought of.

Wes C.
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[kontiki99]

Something is causing the rivets to move after the part is painted.

If it's a part that's not operating with loads applied, then somehow it's not dimensionally stable when it's painted.

The fact that it's random seems to indicate some aspect of the manufacturing process isn't repeatable.

Maybe consider some sort of artificial aging process to stabilize the assembly before painting.

Perhaps time in an oven, cycling it thermally through some schedule of non-destructive hot and cold cycles?

 

[AeroCompositesME]

You may also check the thickness of the composite material as well. I'm assuming the hinge is a standard thickness, therefore not variable, but even different pre-pregs of the same specification can vary from one vendor to the next. Also, if the composite material isn't cured well enough, it could flex.
Finally, I would ensure that you are using the same type / brand rivet consistently. Again, even the same specification rivet can vary slightly from one manufacturer to the next. I've recently seen a slight change in cherry rivets, in regards to the OD rather than length though.

 

[AeroCompositesME]

Oh yeah, and with a final note, check to see if you have the option to do a wet install on the rivet. If so, I'd opt for the highest temp rated adhesive allowed by the spec. This will bond the rivet head in place as well.

 

[HAMILTONM]

Thanks for the reply guys.

I'm looking into a few things. My feeling was that they are painting before the "bondo" is cured, as the bondo shrinks then it's causing the cracks.

Plus, we're stuck with a Boeing-supplied rivet which is hard as tool steel. LOL

Once you squeeze them, they're set. So if anything goes wrong there you have problems.

What gets me though is that none of our processes have changed, which leads me to think it's a painting problem. This is a job we've been doing for years, but our painting contractors have changed.

Thanks all,
Mark

 

[WKTaylor]

Hamiltonm...

Ask Boeing about paint adhesion problems on rivet heads... they have experrienced it before. Turned-out to be a problem with the coating system. In this case Your TiCb could be dirty [microscopically contaminated or possibly lubricated.

Try applying an epoxy primer to the rivet head, 100-deg taper surface and shank... then let it cure fully cure [various conditions/temperatures affect cure]... then do a wet-tape adhesion test to each part. Repeat this test with Your composite surfaces and machined countersinks./.. they too could have some form of contamination.

You might want to find-out who is handling Your parts and fasteners. Their hands may be contaminated with lotions, un-approved hand-cleaners containing lotions, silicones, etc. For that matter You may want to insure that no-one has introduced any new cleaners or lubricants for shop maintenance... those can also affect thing severely.

Regards, Wil Taylor

 

[rerig]

Sounds like the "bondo" isn't being overfilled, then cured, then sanded flush prior to painting.
Trying to save time on the prep and paint process usually causes this problem.

I'll bet the filler is being applied, dried to the touch, prepped and painted. Letting the filler cure after paint will crack the paint every time.

"Cure in Transit" or "Cure after Installation" does have its drawbacks.

Really. Research the possibility of Filler material change.
If the filler has been changed, but they are curing to the old filler spec., that might be your answer.

Example: We used to be able to mix sealant "Hot" for a faster cure. For the past 10 years that has not been possible due to reformulation. If an Old Rule of Thumb is being used to ship the part quicker, someone might be trying to mix the filler "Hot". The filler will then crack.

 

[randi]

When the end result is not what you want, then you should look at the process control of the new vendor and compare it with all the features of the old vendor, what ever you know.

You can also make a few controlled test samples and test with varying known conditions and test the results, to arrive at an acceptable procedure.

Is there any stress relieving involved any where ?

Once you know what works for you - you must have a vendor control by not only looking at the finished product but an evaluation of the vendor process control and also an on-going assurance that he is sticking with the plan and is not taking any short cuts as a cost saving measure.

 

[kenvlach]

wktaylor is right about Boeing knowing of a paint adhesion problem on rivet heads. May not apply to this situation, but Boeing tests show Sol-gel/Boegel-EPII (AC-131) may be a better pretreatment than Alodine (especially, aged Alodine).
See Rivet Rash, pages 28-32 of 'Status of Sol-Gel Technology for Aerospace Applications'

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