DIY slab mold

[cascarral]

Hi! For those of you that have made their own slab molds, I'd really appretiate your advice!

I made this wooden mold for 44 bars and line it with a 5 mm acrylic and a 0.5 mm plastic but I've found the size of my bars isn't right and it's making them weight less than they'r supposed to.

I made a batch yesterday and before unmolding today I realized the acrylics bend, reducing the wide of the bars. I'm not sure what's happening but it looks like the wood shrinks and makes the acrylic bend.

I'd love to hear if anyone has seen this happen in wooden slab molds and has advice on how to solve it.
I've considered making a new mold with some other material... any experience on this around here? Maybe HDEP plastic or something like that? Any tips on what I should look for?

It's hard to see on the pictures but all the acrylic is bend towards the middle, making a gap between it and the wood.

 

[Peachy Clean Soap]

Great Q. Looking forward to answers

 

[glendam]

Wood does shrink. While I have not noticed that with my molds (I assume the wood was already dried and shrunk by the time I made them). I know it happens just from a conversation about wood panels for fences. Could you remove one of the shorter ends of the mold to keep the acrylic from bowing? Perhaps move it out some and readjust the mold screws (or whatever is keeping it together)?

 

[peterswoj]

I cant tell from the picture, but i would recommend at least an 1/8 - 3/16" of play with the acrylic inside the wood box. As that acrylic heats up it will expands. A lot. If it is tight it has nowhere to expand too, so it just bows.

 

[cascarral]

Thank you both! I will try your suggestions!