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I've been making decorative concrete paving stones for use in the garden recently and everything's been going fine until I tried one in a new mold I purchased and I can't for the life of me get the slab out of he mold! (Can't believe I'm actually having to ask this!)

The mold in question is slightly different from others I've used or built before and is Polyurethane and while it's got some flex (I have been able to flex the individual sides of the mold so I can tell it's not adheared to the mold's 'walls') the overall structure is much more rigid compared with the silicone or flimsier plastics I've used before and the slab is staying put against the bottom of the mold.

The catch is that I need to extract the slab without damaging or destroying the mold as it has a custom design created by an artist friend of mine using hot glue on the base of the mold (I've used this technique successfully on other molds before so I don't believe the glue is the culprit and the glue had fully cured before the pour), here's what I've tried so far:

  • Turning it upside-down (duh! first thing I tried and has worked with others in the past

  • Flexing the side(s) of the mold ) can see the sides of the mold coming away from the but that's it.

  • Gently shaking the mold up and down while holding it upside down (over a suitable cushioned surface to catch the slab of course)

  • Same but more vigourously

  • Same but side to side

  • Negotiating with it

  • Bribing it

  • Threatening it

  • Orbital palm sander (without sandpaper of course) against the various sides of the mold to vibrate it out

  • Applying firm, progressively built pressure to the under side of the mold (have tried both the center and the corners to see if one produced more flex than the other)

  • Light taps on the underside with a hammer (I was getting desperate)

And nothing - when shaking vigourously in an up-down motion I thought I felt the weight of the slab shift but could see no visual indication so possibly I imagined that.

The concrete in question is is Blue Circle Master Crete mixed with sharp sand and aggregates according to the standard proportioning on the instructions (same mix I've used in every other pour), it was pulsed with the palm sander to remove air pockets and it was cured for ~96 hours in a reasonably cool but humid environment, every other casting I did in the same pour recieved the same treatment and has come out of their molds perfectly so I don't think there is an issue with the pour or the curing.

I've googled for tips and tricks but the only suggestion I've come across that I haven't already tried and that doesn't involve destroying the mold is to put it in the freezer to try and make the mold contract but I haven't been able to try this as it is too big for my freezer.

How can I remove this slab without damaging the slab or the mold?

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    I'm not confident enough in this idea to post it as an answer, but it might be time to consider chemistry. In particular, you might (cautiously! with gloves and ventilation!) try some muriatic acid, which etches/dissolves concrete. Pry away an edge of the mold where you can, pour some acid in, and try to dissolve the cement closest to the mold. Pry more and repeat. This may destroy or damage the slab. You might want to break up the slab a bit to help distribute the acid. Also test the acid with the mold materials you're using - I'm not sure if it could damage the poly/glue you've used. – Shimon Rura Oct 5 '18 at 16:35
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    I'm also not confident enough in this idea to post it as an answer. If you have an air compressor, try sticking an air blow gun as far as you can between the concrete and the mold. Work your way around the perimeter a few times and see if you can use the air pressure to make it release from the mold. – mrog Oct 5 '18 at 20:21
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Thanks to all who provided suggestions in comments/answers - I've actually succeeded in getting it out before having chances to try them. They'll definitely come in handy in the future I think though - and the muriatic mention from Shimon will definitely be useful as I wasn't aware of it and the ability to etch concrete could be very handy! Plus it would have allowed me to save a casting I lost earlier where excess concrete in just the wrong place caused it to be nigh-on impossible to de-mold without destroying it.

Ultimately I used a very low-tech solution of inverting the mold on the floor and standing on it, shifting my weight to flex the "bottom" of the mold and this caused the slab to drop enough (the slab wasn't flush with the top of the mold so there was a few cm of room for it to drop) that I could get a butter knife in the edge and pry the slab loose.

  • Good question, good answer. Thanks, and keep 'em coming. – Daniel Griscom Oct 8 '18 at 13:15
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It sounds like the top of the concrete is flush with the top of the mold edges. How about you make a wooden frame that, if the mold and concrete are inverted, supports the edges and not the contents? Then, put the frame on a hard surface, and drop the mold and concrete some (small) number of inches onto the frame, so that the frame blocks the edges from moving further down, but not the concrete.

It's worth trying...

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