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
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?