Making a concrete mold, am I on the right track?

[Metalhead781]

Thanks for the insight... When you say that the abrasive nature of the concrete deteriorated the mold, were these deep draws, that shrunk down around the Delrin?... what i am considering, is something that would have more of a flat surface to it, maybe more like the concrete stamps just make it look like a natural rock surface. It would be perhaps .8 of depth, because that is what our machines will do in one shot. The sides of the blocks, would be formed with plywood, and just cap the bottom with this decorative carved face.

Thoughts?....

PS, it probably would be a very good time to buy stock in Dawn, there are going to be a lot of greasy birds that will get cleaned up!...

Basically what happens with the delrin is it gets scratched up. Those scratches will hold material no matter how slick the plastic is. This will make the mold stick to the concrete. With that, even though it's hardened it's not fully cured and can still break depending on the mix. It just needs to be greased properly even with that kind of slick material. The mold sounds good. If you're planning on using the ply more then a few times. Put a few thick coats of poly on the ply. They'll last a lot longer.

[Metalhead781]

Check out my post: http://forum.carvewright.com/showthr...highlight=mold

This is exactly what i was talking about. The extruded polystyrene machines well. We used to do alot of hand tooling on that and 2-3lb styrofoam (expanded polystyrene). Usually lowes carries the pink stuff and Depot caries the blue but it's the same material. You can machine styrofoam with the same if not better results however it needs to be heavier then the insulation you can buy or the packing styrofoam you can also get. It needs to be at least 1.5 pounds per square inch rated for it to machine decent. We received large blocks from a local supply company 4ft x 6ft x 12ft in both 2 and 3 lb foam. We had a cnc hotwire machine we used to cut basic shapes from. The hot wire was basically 2d with a rotary table in the middle. X and Z axis only. Then the part was rotated and a second profile was cut into the shape.

[HighTechOkie]

For production concrete molds, it appears silicone rubber is the best material. Delrin would probably be good for the master to make the mold from. As the mold wears out, easy enough to cast another mold. I know that most of the cast concrete wall panels for freeways and fencing are made using rubber molds, I'm guessing they have already figured out that rubber lasts the longest for repeated use.

Rob

[djarboe]

I think the Delrin idea is a great idea, but I have no experience using it as a form a material.

One thing to keep in mind when choosing a form release agent is how you intend to finish the final casting.

For example if you plan to stain the concrete the use of petroleum based release agents can cause blotchy stain, or if your painting it can create adhesion problems.

In my case, and being a rank amateur, I was planning on just putting color in the concrete while mixing it.

The Delrin idea makes sense. Being basically cheap, I may try wood first (high gloss varnished and then coated with mineral oil before putting in the concrete), and then when that fails, move to Delrin.

[Shanine]

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