Concrete Compression Mold

New to the site and was hoping to tap into other user’s knowledge to avoid wasting time potentially. I’m looking to make my first molds for some very simple shapes. I essentially want to make some 1″ thick disks with different ODs and IDs. I already have some aluminum baking dishes to set my ODs and have some other components to create my IDs. Without going into crazy detail, I’m struggling with my compression and was thinking about creating matching concrete disks that I can simple lay on top of the melting plastic inside my baking dishes. My basic questions are, do you think the simple weight of the concrete is enough force to create the necessary compression? Secondly, do you see any issues using concrete in general? I was a bit worried about the porous nature of concrete and if that would lead to issues separating the two apart after cooling. Any thoughts, guidance, or experience would be greatly appreciated.

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You may want something smoother between the plastic and the concrete. In your setup include some feature that will keep your press plate from tilting and creating a wedge. The other option to using the concrete is C clamps which may be easier to control.
If you are melting the plastic in your house oven, be aware of the fume issue

Very good input from all, thanks for that. In general, what kind of forces are we talking here? With the mentions of presses it makes me think my head is completely in the wrong place with the amount of compression needed. And I assume this is all to prevent voids and to create a consistent structure throughout the thickness of the part being made?

The point about adding guidance is a very valid and something I didn’t consider. To prevent the wedge effect would require some very very tight tolerances to achieve. It’s an interesting thought using clamps to try to keep things level, but it seems like you can only control one of the two between equal force and flatness. Clamps would give you more exact dimensions but not equal compressive force. A large concrete block would balance the forces but create the wedge as you mentioned. I’ll have to think through that more.

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you can get a ring roller and 20T hydraulic presses for less than 200$ from craig list; for 300$ from harbor freight. if space and budget is an issue; there are gear drives you can mount at a wall, you could lift a good bunch of bricks with that; the hardware-store should have it for 50 bucks.