Aluminum shear strength 1

[ChrisGill]

For the last three years I have been working on 6061 Al molds coated with Ni and Teflon. It's a similar process to mold injection. Recently we've been experiencing some issues with the integrity of the Al.

We have 8 counter-sunk through holes on the mold. 1/4-20 bolts are used. Four 3" bolts and four 2" bolts. The bolts are shearing the coated Al. Obviously the torque on our drills are set too high. I'm trying to calculate what the appropriate torque setting should be. The molds are subjected to 300 F heat and 50 F cooling, but for now, I'd like to ignore this variable. I just need a starting point.

The shear strength of Al is 30000 psi. I need to figure out how that translates to inch-lbs, so I know where to set our drill setting. The little bit of investigating I did told me that this can't be done. I suspect otherwise.

 

[Tmoose]

"Obviously the torque on our drills are set too high."
- Are the "drills" tightening the 1/4-20 bolts?
- When the bolts are tightened is the joint fully compressed metal-to-metal, with no gap?
- Are the failures happening as the bolts are being tightened when the mold is being assembled?
- Or, are the failures happening sometime during the production process, perhaps when the mold halves being forced apart by injected material?
- When you say "countersunk" are you following this convention?

https://upload.wikimedia.org/wikipe...sunk_and_counterbored_holes_cross-section.png

Some drawings and photos would save a bunch of posted replies.

Right now I'm picturing the holes are counterbored deeply, and the socket head bolt heads are pulling through the thin shelf remaining, similar to the attached image. My Confidence level in this assumption ~10%
I guess a countersink in thin material could do something similar.

 

[ChrisGill]

We are joining 3-4 coated plates with the bolts. They are assembled and disassembled after each cycle. Zero seam is what we are shooting for.

Counter bored holes are as deep as the bolt head. Which, by the way, are socket bolts. There is anywhere from a 1/4" to 2" of metal underneath the bolt head. No matter how deep that hole is, the metal will start to mushroom from the top. We have to drill these holes out occasionally for clearance. Eventually, we remove too much metal and the counterbored hole becomes a through hole. I'm trying to get the most torque without shearing any metal.

The drill does drive these into the threads. The threads have SS locking key inserts.

 

[EdStainless]

As you heat the Al expands much more than the steep bolts, greatly increasing the clamping force, and the Al gets softer reducing its strength.
You may need to torque lightly when cold and then re-torque at temperature.

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P.E. Metallurgy, Plymouth Tube

 

[BrianE22]

Ed -

At 300 Deg F, doesn't the aluminum permanently lose some off its strength?

 

[ChrisGill]

No way to tighten after heat. We use hydraulic presses to heat.

I'm intrigued by your "losing its strength" idea. I've been asking the same question. Since Al doesn't anneal until 600 or 700 F (don't remember exactly), the theory was that the Al properties don't change. I've suggested that the repeated heating/cooling has an effect on the Al integrity. But the senior engineer keeps shooting that idea down.

 

[tbuelna]

I read thru the posts but it is still not clear exactly how or where the aluminum mold material is failing. Can you provide a photo or sketch?

From the written description, it seems possible the limited under head contact area of the cap screws is producing high enough bearing stress levels to yield the aluminum surface. So you might consider installing steel hat bushings in the counter bored holes.

 

[ChrisGill]

Unfortunately, I do not have pictures available. Even if I did, I doubt I'd be able to capture the failure well enough for you to get anything out of it. Tbuelna, the way you describe it is correct.

When the counterbored holes fail and become through holes, we fabricate SS bushings to fix. In some cases we started with the bushings before putting the mold into production. These bushings are failing as well. Obviously at a much slower rate.

I don't want to lose site of my initial question. Is there a way to convert the 30000 psi of Al shear strength to an inch-lbs. or foot-lbs. number? I'm aware that I can run a DOE on this and figure it out that way, but I'd like to be able to calculate a starting point. Or save myself a 4 hours of testing (the boss isn't very generous with the machine downtime).


Furthermore, I highly suspect the root cause has more to do with the thousands of healing/cooling cycles than the torque from the drill. If I can calculate the optimum torque, then I can move on and investigate other avenues.

 

[EdStainless]

You have them too tight. period. And you have too little bearing area.
Of course I misspelled steel, but you go the idea.
Honestly I would go to 1/3 of your torque and see how it works.
The yield of 6061 at 300F will be about 70% of the RT value.

Lets see, a 4" long steel bolt heated to 300F will expand from 4.000" to 4.006", while 4" of Al will go from 4.000" to 4.012". Guess which will give.
In effect, even with zero compression cold you are overloading the Al

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P.E. Metallurgy, Plymouth Tube

 

[ChrisGill]

So, I was actually thinking the heat was making the Al weaker, but, as simple an idea as it is, I hadn't considered the expansion.

Looks like I'll be doing some trials today. Thanks for the thermodynamic insight. It's something I hadn't considered.

 

[ChrisGill]

I remember now why I hadn't considered expansion...

The mold is in a 20 ton hydraulic press set at 1400 psi while it is being heated. I'm guessing that under that type of stress, the Al isn't going to expand the same.

 

[EdStainless]

Then why do you bolt it at all? Why not just use alignment pins and let the press hold it together?
But trust me, you press load isn't enough to cancel out the differential thermal expansion issues.

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P.E. Metallurgy, Plymouth Tube

 

[John2025]

We often have the same issue as yours. The aluminum basically extrudes into the clearance hole at the bottom of the counterbore, making the clearance hole too tight. It's not a big issue for us, we just re-drill the clearance holes occasionally. Maybe a washer between the shoulder bolt and the aluminum plate would help spread out the load or else a larger diameter bolt to start with.

 

[ChrisGill]

I gave you the wrong numbers. It's a 125 ton press. Don't know if that changes anything. So, you're saying the the metal will expand at the same rate, even with that much force on it?

We've tried clamping and magnets, both to no avail. We vibrate the molds before putting them in the press. If we don't make it as seamless as possible, it creates a huge mess.

Think powder metallurgy, but with plastic.

 

[John2025]

Maybe I misunderstood your problem. I'm picturing a scenario where each time you tighten the bolts they set into the base material a little bit. This pushes the material in towards the center. The next time they go to assemble it, there isn't enough clearance in the holes to allow the plates to line up so they re-drill the clearance holes. Each time they go through this process the clearance holes gets a few thousands deeper. Eventually there isn't any material left and they have clearance holes all the way through. This is different than actual shear failure in the plates that TMoose and others are talking about. If it's what I'm describing then I think the fundamental issue is that socket head cap screws are designed more for steel than aluminum. I don't think the small heads of the screws are going to spread the load enough to use the torque that a mechanic is used to using, especially if the clearance holes are large to start with. Do the plates really need to be that tightly clamped? If so, I still think washers, larger/ more screws or maybe button head socket screws, as they have slightly bigger diameter heads.

 

[ChrisGill]

We've just recently started installing SS bushing where the holes fail. That works better, but we've had them fail as well. The issue is getting the torque as low as possible and still keep the powder from going inbetween plates. I thought I might be able to calculate this torque.

I think your idea of using button heads need to be considered. The area that the current bolt contacts the mold is minimal.

By the way John, you are spot on with your description. I assumed this was the same as shear.

 

[EdStainless]

Have you looked at flange head fasteners? To spread the load more.

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P.E. Metallurgy, Plymouth Tube

 

[CoryPad]

Faq725-536

 

[ChrisGill]

Exactly what I was looking for. Thanks Corypad