Well, Poo

This is another really awkward story from the build files.

I had the idea of doing a resin and wood turning based on some bark and oak burl scraps that I had lying around in the shop.

This ought to be straightforward, right? I cut a plywood disc then glued the wood to it and hunted up a silicone bake pan to use as a mold for the whole thing.

It took around 600ml of resin to fill the mold, then I lowered it carefully into the pressure pot (it just fit!) and brought it up to 60psi to sit overnight. Since I’m worried about heat buildup in the pot, I thought it would be a good idea to put it out on the porch, where the sweet spring evening breezes would keep it nice and cool.

That was “a thought too far” – apparently, as I moved it, a small(ish) amount of resin slopped over the edge of the mold and now my pressure pot is a permanent sculpture.

It’s barely possible to slip a crowbar down the side into the gap between the wood and the steel; I was thinking that perhaps I could get the resin to release from the silicone bake pan and at least I’d be able to remove the center-piece. But… Nooooooo. The silicone fit tightly and the wood appears to have forced it to seal against the edges – then the wood is epoxied down on top of that. It’s solidly bonded. It appears to be a job for high explosives (but that would compromise the integrity of the pot).

I suppose I can still use the pressure pot, by putting objects on that nice plywood platform epoxied into the bottom. It’s a “spacer” – that’s the ticket – it’s there to raise any future projects a bit higher so they will be easier to reach until something sticks to that plywood and I’ve added another layer.

Well, I needed another pressure pot, anyway.

Pressure pot conversions are fun: you need to remove the bubbler tube that goes down into the paint, and disassemble and cap off the stirring rod holder. After that, it’s a matter of coming up with the correct mix of ball valves and adapters to fit your pressure pump. For those of you who are not into extreme resin casting, these pressure pots are designed to hold paint for pressure roller systems. So, there’s a pressure-fit agitator that lets you stir the paint while it’s under pressure, and a siphon tube that collects the paint that’s being forced up into the hose that leads to the roller. The good news about this is that the pressure pots are not super expensive, since they make a lot of them for paint systems and that drives the price down. The bad news is that they’re not designed to take high(er) pressures and are often listed as “80psi” but when you get them the maximum pressure listed right on the outside is 40psi. I’ve been tempted to wrap mine with some fiberglass mat and paint it down with resin, but I suspect that if anything “pops” it will be the lid going straight up, so I don’t ever get in the path of the lid while I am pressurizing the can. I replaced the safety valve with a 60psi blow-point and that is good enough. When I pressurize it, I hook up the hose and stand on the other side of a cinderblock wall while I start the pump. I can only imagine the god-awful mess that would be created if the pot blows out while there’s pigmented liquid resin in it.

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This explains why some of the resin-workers I’ve seen always put a poly bag around everything before it goes in the pot. I’m also thinking, now, that I need to be extra careful not to get any epoxy resin on the lip of the pot, before I clamp it down nice and tight with the screw-clamps.


  1. MattP (must mock his crappy brain) says

    If you’ve got some decent thickness metal scrap and a length threaded rod, you could try pulling the block out. Several wood screws into the plywood+resin (in areas that are likely to be turned into waste on the lathe) to attach to some scrap metal with the threaded rod attached at the center of the metal. Place another piece of scrap wood or metal across the top of the pot with a clearance hole in the center for the threaded rod. Carefully tighten a nut on the threaded rod to pull on the block of wood+resin out of the pot. Might cause the pot edges to collapse, but lower chance of damage than prying or cutting the resin out.

    It might even be enough just to twist the wood+resin block inside the pot to break the bond. That depends on the resin’s bond-strength to metals and how tightly the silicone is wedged against the pot. If it were the epoxy I’m used to, then any air gap you manage to make between the epoxy and smooth metal will rapidly propagate to release everything.

  2. says

    Heat? It’s the usual way to release epoxied things when circumstances allow.

    I was thinking about that. I could blowtorch the bottom of the can but would it weaken it?

  3. says

    Place another piece of scrap wood or metal across the top of the pot with a clearance hole in the center for the threaded rod. Carefully tighten a nut on the threaded rod to pull on the block of wood+resin out of the pot.

    I like that. Basically, a gear puller. I’ve got all the requisite parts, too. There should be a great big shear-line where the silicone pan separates layers of epoxy. I may wind up with a partial removal, which is still better than totalling the whole thing.

    So it is written, so it shall be done. I will report back eventually.

  4. komarov says

    This sort of thing is the reason why I stick to simple wood glue. And it’s the water-soluble kind just to be sure.

  5. Jazzlet says

    I had an old fashioned stove top pressure cooker in college, inherited from my grandmother it was the kind you put weights on up to 15 psi. A friend had seen me using it and being impressed with the time saving asked for instructions. I was out the first time she used it, when I returned I noticed the pressure cooker on the stove, and a clean patch on the 10′ high ceiling above it. My friend had only remembered part of my instructions about how to cool the pressure cooker down if you were in a hurry – put the cooker under the cold tap until it stops hissing, then use the tip of a knife to nudge the weights, if the valve hisses run more cold water over the pressure cooker, repeat until there is no more hissing. Yes, she had nudged the weights while the pressure cooker was hot, the pressure had lifted the weights off and sent a stream of boiling hot chicken stew slush through the valve with enough force to reach the ceiling. That was the only time she used the pressure cooker, she refused to even be in the kitchen when I used it after that. Pressure vessels, don’t mess around with them!

    Seriously Marcus, what is the temperature at which the resin will start to be liquid and what is the temperature the steel would start to be deformable? You’d have more idea than me whether that is a reasonabbly safe margin.

  6. MattP (must mock his crappy brain) says

    The point of thermoset polymers is that they will not really melt after they have set/crosslinked. They get softer and deform/creep with enough heat and force, but are more likely to decompose/combust than melt when heated. Things generally tend to get softer when heated and harder when cooled, but whether the adhesive bond strength increases with heating is dependent on the chemistry of the adhesive and the materials being bonded.

    If the pot has a smaller thermal expansion coefficient than the resin, then sticking the pot in a pan filled with dry ice and alcohol/antifreeze will cause the resin to shrink more than the pot and peel off. Cooling may also weaken the bond generally, and will almost certainly make the bond less resistant to impact (cool several minutes then carefully pound/bounce the pot upside-down on a wood block on a table; let the weight of the resin+wood do the work).

    If the pot is stainless steel, then it is not very likely to have been given any hardening/strengthening treatments, so probably not weakened long-term just by short-term local heating without pressure. The resin is likely to be damaged by the heat long before the metal pot.

  7. lochaber says

    what about pouring a bunch of Acetone in there, putting a lid on, and let it sit for a few days?

  8. dangerousbeans says

    you have an angle grinder and welder, chop the bottom off and weld on a new one :P
    Acetone and/or makeship puller is probably the more sensible option

  9. rrutis1 says

    Along with what MattP said, you could try a combination of heat and cooling. Stick the whole thing in an ice bath for a few hours so everything is the same (low ) temperature, then remove and stick in a bath of boiling hot water. The metal should expand much quicker than the resin because I am guessing the heat conductivity is much higher in the metal. Have your puller ready to go too.

    We used to do a variation of this in the Navy on pump shaft/impellers to get the impeller off without applying a million psi to the end of the shaft with a puller. The stainless shaft would often deform before releasing the impeller otherwise.

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