Materials for homemade molds for concrete casting

[carlfredrik]

Hello! I need to make my own mold for a small project I'm working on. I'm wondering which material is best to use for concrete casting. It will be a fairly large mold, around 50x50x50cm. What is best to use? Plastic, silicone, rubber...? It needs to be a reasonably hard material to support the concrete when it's poured. Could the mold even be made of cement if a lot of grease is applied in between?

If anyone has experience with concrete casting, I would really appreciate your help!

Thank you in advance!

 

[Drömhus2012]

Formplyfa seems like an obvious choice, and if you are going to use it multiple times, you should oil it between castings.
But it depends a bit on what you are casting, if it's feasible with a board material.
The price is a downside though.

 

[carlfredrik]

Unfortunately, I need a material that I can shape around an existing object. I was thinking of building a frame (for which formplywood is perfect) and pouring some kind of plastic or rubber solution, with the object in the middle, to create my own mold that I also want to be able to use multiple times. A bit based on the blow molding technique: http://2.bp.blogspot.com/-soigKD_FNF4/UEMj3nBbOhI/AAAAAAAAAG0/Tr-5f4-xBF8/s1600/blow.jpg (but without the high air pressure... Just two holes down to the mold itself, one for the cement to get in and one for the air to escape)

 

[Drömhus2012]

I think I read somewhere about making your own molds/casting molds from silicone.
I'll ask my wife about it since she was the one who found it.

 

[Drömhus2012]

No, she didn't remember. But she found out a lot about this through something called google she said

 

[Helioz]

Then you probably need to use silicone. This works well for concrete:
http://www.kinn.com/product.html/rtv-silikon-70-hr?category_id=28

Kinn also has release agents that you should use for the silicone mold.

 

[carlfredrik]

Silicone becomes very expensive when molding larger objects. Are there any cheaper alternatives?

 

[oleg]

Make a box out of wood, place it over what you want to cast. Fill with foam sealant, unscrew the wooden mold, take a sharp long knife, and divide the foam sealant. Voilà, you have a good mold to cast in. Don't forget to paint with some color in the mold so you can use it multiple times and spray in what you want to cast, and when you make the mold with mold release oil, it will go smoothly and easily! Good luck!

 

[AXS]

You can brush on silicone or latex, then fill it with something like foam or other cheap material. But you probably need to shape it in advance, otherwise it sticks like a rock.

I use a lot of planed wood as a mold, cheaper than form ply many times and available in standard dimensions.