UHMW compressibility

Hey all, I operate a compressed adobe block making machine over here, which I'm currently using to build my new shop.

This is the machine: Advanced Earthen Construction Technologies - Construction Equipment

Here are a couple of clips of the cycle, and a few pics: Shared album - Jason Atkins - Google Photos

It's a really cool technology, but the machine is getting a bit old and I have a failure in a part I don't have a backup for.

The machine forms adobe blocks at about 2000 psi, by pressing a 6"x12" piece of 1.5" plate up against another piece of plate.

That plate has a 1/4" deep recess in its 6"x12" face, which from the factory was filled with some kind of polymer that's baked in place, which serves as a release agent, because the machine presses the plate up to form the block, backs off half an inch while the carriage returns, and then pushes the block up out of the forming cavity. The factory said that when they used plain steel plate, before they tried that polymer, the suction between the block and the plate was ripping the blocks in half on that little half inch retract. (The material it's pressing is just slightly damp to the touch, it's not wet.)

Unfortunately, today the polymer let go of the plate. The factory says they don't have any success rebonding it, and is offering a new plate. That's great, but I don't have a way to get it here in a timeframe that's soon enough.

So, I'm considering a UHMW part as a bandaid.



Basically just a piece of 1/2" thick UHMW sheet, with a little inset cut into the bottom side to fill the void in the plate where the polymer was. I'll tap the plate and screw the UHMW down with countersunk hex head screws. This solution will end up being 1/4" taller than the previous solution, but I can adjust that out of the machine movement.

So, what say you about UHMW suitability for this? First, is it strong enough in compression? My suspicion is yes. Worst case I can imagine is that over time the UHMW gets compressed a little shorter, and I can adjust the machine accordingly. The larger question is the adhesion problem. UHMW is slick in terms of material sliding over its face, but that's not the motion in question here. With a slightly damp clay material compressed over the face of the UHMW, how well do you think it lets go of the clay? Or does the fact that the face of the UHMW sheet is so smooth mean it vacuums itself to the bottom of the block and adheres to it so well that when the plate retracts away, it rips the block in half on the retraction cycle?

Can you texture the UHMW so that it doesn't stick to the clay? You will of course end up with textured clay but that may be okay.

I was thinking about that, perhaps by pressing something textured against it?

Maybe something like this rust pitted spring steel?

Yes, a little texture on the bottom is acceptable.

JasonPAtkins said:

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Maybe something like this rust pitted spring steel?

Yes, a little texture on the bottom is acceptable.

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They make a clear liquid concrete release that smells just like bubblegum that would probably work if you can source it.

Don’t ask me what it is because I don’t know but we used it for years in the concrete trade to keep our textured stamps from sticking.

Murf


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Murf said:

They make a clear liquid concrete release that smells just like bubblegum that would probably work if you can source it.

Don’t ask me what it is because I don’t know but we used it for years in the concrete trade to keep our textured stamps from sticking.

Murf


Sent from my iPhone using Tapatalk

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It's called bond breaker. We use it for pouring tilt up concrete panels on existing slabs. Dayton is the manufacturer we use.

First, is it strong enough in compression?

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UHMW is good for 750 psi loading

What I would worry about in a use case like this is not the UHMW taking a compression set, but rather the edges getting nibbled over time. When you compress it, the old sheet was fully supported on the sides, right? This one is a little taller and will mushroom out under load, then the edges are going to get nibbled up, which could lead to any number of headaches. If you can get it all to fit within the original cavity then I expect you'll be alright.

Polyethylene in general is pretty middle-of-the-road when it comes to surface energy, with no polar component (directly relevant to water adhesion), so I would start with whatever kind of finish was on the original part before spending a bunch of effort to get a specific texture.

If there's a metal plate backing the UHMW plastic I don't see a problem. Assuming the 2000psi is correct, the PE compression modulus is ~ 0.7GPa or 100000 psi. So your PE's strain will be 2000/100000 or about 2% That is about 0.010 inches. Probably should leave a little relief between the space cut into the metal and the UHMW PE plate as the latter will squash a bit (Poisson ratio of 0.33).

There's a significant number of adjuvants to improve water resistance (including termite mounds- apparently the termite saliva has something in it that stabilizes the mound) and release. One could use vegetable oil for this purpose I guess. Before I textured the stuff I'd just try it as is. If it works, great. If not, try texturing or veggie oil. Be a bummer to texture the stuff, have it not work, and then have to machine a smooth plate!

There's a product out of South Africa called Vesconite. Might have better release properties. I believe that its expensive as hell though. Something to check out, if you want.

why dont you reattach the "polymer"?

I think that any sort of texturing is exactly the wrong way to go. If it was pressing against a solid surface that might make sense, but the adobe will flow under pressure and conform exactly to the surface of the plate. Texturing will only add to the surface area and make release more difficult.

Throwing some dry sand on the plate before adding the adobe should help it release - would that slow the process too much? Or a sheet of paper, maybe.

JohnMartin said:

I think that any sort of texturing is exactly the wrong way to go. If it was pressing against a solid surface that might make sense, but the adobe will flow under pressure and conform exactly to the surface of the plate. Texturing will only add to the surface area and make release more difficult.

Throwing some dry sand on the plate before adding the adobe should help it release - would that slow the process too much? Or a sheet of paper, maybe.

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Agree the texturing would add to the stickiness.

The deep recesses of my memory dredge up a CINVA ram setup that was a manual block making device. IIRC they used a sheet of newspaper for just this reason.

Of course readily available free paper might also be an issue given the location - don't know.

PDW

The machine is automated and makes a block every 15 seconds, and the forming cavity is inside a guarded area. So, adding anything manually would involve my non-technical workers putting their hands inside places I've threatened then within an inch of their lives of ever putting their fingers, and would cut the production rate at least in half.

I'll try a non textured uhmw sheet tomorrow and see how it goes.

The 2000 psi is system pressure, btw, I think it was a 3" rod and 5" bore maybe? So that'd be 39,000 pounds of force spread over half a square foot, so 77 sq inches, which would be 500 psi on the face of the plate.

Consider a system of holes in the form plate with a back plate that would form a hollow on the back side. Connect an air compressor to the cavity with perhaps a solenoid valve such that when the machine opens air from the holes will help push the block away. Common practice in molding.

Tom

Why not drill and ctsk some holes in the face of the old polymer insert so you can re-attach it?

dian said:

why dont you reattach the "polymer"?

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It's 1/4" thick and floppy like an old, thin shoe sole. It doesn't really have any structure, so now that it's not bonded to the plate I think it would take a lot of screws to hold it down.

TDegenhart said:

Consider a system of holes in the form plate with a back plate that would form a hollow on the back side. Connect an air compressor to the cavity with perhaps a solenoid valve such that when the machine opens air from the holes will help push the block away. Common practice in molding.

Tom

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I've seen that done in ceramics, but you're talking about a towable machine that's meant to be drug out into the bush and make blocks for schools and the like. An air compressor adds a lot of complexity and overhead to a system that's supposed to be easy (and is very dusty, btw). The UHMW (or something like it if that doesn't work) follows KISS.

Fortunately it appears the designer used good sense and arranged for the clay to be shoved/scraped off the polymer. So stiction should not be a problem. If you have the UHMW, I would definitley use it. For that matter, a tropical hardwood would work also. Hardwood might need to be replaced every few days, but I am pretty sure it would work and since many are somewhat oily should allow sliding of the adobe and might be surprising how well it would work. ( My dad told of using a local hardwood during WWII to replace worn brake shoes on army trucks. It worked quite well. Might not be great descending a 5000 foot mountain, but crawling through the jungle they did the job when asbestos was not available.)

Denis

dgfoster said:

Fortunately it appears the designer used good sense and arranged for the clay to be shoved/scraped off the polymer. So stiction should not be a problem. If you have the UHMW, I would definitley use it. For that matter, a tropical hardwood would work also. Hardwood might need to be replaced every few days, but I am pretty sure it would work and since many are somewhat oily should allow sliding of the adobe and might be surprising how well it would work. ( My dad told of using a local hardwood during WWII to replace worn brake shoes on army trucks. It worked quite well. Might not be great descending a 5000 foot mountain, but crawling through the junkle they did the job when asbestos was not available.)
Denis

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Fascinating story, Denis. Where was Dad stationed?

JasonPAtkins said:

The machine is automated and makes a block every 15 seconds, and the forming cavity is inside a guarded area. So, adding anything manually would involve my non-technical workers putting their hands inside places I've threatened then within an inch of their lives of ever putting their fingers, and would cut the production rate at least in half.

I'll try a non textured uhmw sheet tomorrow and see how it goes.

The 2000 psi is system pressure, btw, I think it was a 3" rod and 5" bore maybe? So that'd be 39,000 pounds of force spread over half a square foot, so 77 sq inches, which would be 500 psi on the face of the plate.

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I was wondering about that - 2000psi on the bricks would mean a 72 ton force on the ram....

So the deformation would be about 0.003" on the UHMW PE block. It will be interesting to see how this works... Good luck.