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| Concrete form |
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| Eugene:
Another wild speculation ..... :scratch: in fairness I've got all the bits and bobs below to hand; you may not. If you have a woodwork type router, use it with the aid of a trammel to cut appropriate sized holes in MDF sheets of the required thickness. If you cut the biggest size first, you can use the "waste" bit for the smaller sizes, Stick each of these to a baseboard using wood glue Take a saw and cut the form in two right along the diameter. I have a Japanese pull saw that has a very thin blade and thus an equally thin kerf. You now have a split mould that can be held together with a strap wrench or one of those cheap ratchet affairs for trailer covers; the ones below are £5.99 off EBay. Paint the parts with a polyurethane type varnish to smooth them up a bit and make them waterproof. For a release agent I'd use "Renapur" shoe wax available off the 'net. It's a gobby paste of beeswax and oils that you can apply in layers as thick or as thin as you want. It's excellent in fibreglass work, so should be fine for your application. Good for yer boots too! http://www.renapur.com Eug |
| PekkaNF:
Depending on your climate, you may want to consider additive that will make the concrete frost proof (or some that kind of words). Here pretty much all pavement needs to have that additive, or it will erode and crumble away in couple of winters. I have heard that additive will actually create micropores - how this will make it winter proof I have no idea. My brother made quite a lot of these, he had a big table made out of film faced ply wood and moulds were assembled top of it. Then prepared concrete (color + other additives) was poured pretty dry (less water there is harder it will be), then rapped (minimum possible, water separation on top is allready bad thing), leveled, sprayed with water, covered with plastic sheet and let to rest until next morning. Fresh concrete is pretty weak and brittle. In few hours the surface can be brushed with wirebrush to make it bit more rough. Film coated plywood needed very little oil. There are special oils, but I think he used normal vegetable cooking oil, because these are next to herbs and flowers. He said that economy goes down on drain, unless you use your own sand, work, concrete mixer and need special size/color/shape or cause any waste. He made one or two mixes at the time on concrete mixer, used it all on moulds. Took few practice runs. He made a lot of long/narrow edging pieces, those were special and those payed off. Pekka |
| vtsteam:
--- Quote from: CrazyModder on November 17, 2015, 03:08:56 AM ---Thanks for you detailed instructions, vtsteam. Would you use a quite thick layer of lard on the sheet metal, or is a thin smearing sufficient, in your experience? Speaking of the mix... the finished product will often be moved around, and while I'd like a somewhat smooth surface, it would be even more important if it were a bit robust (i.e., slightly resistant to chips flying off all the time when someone clumsily sets them down in a not-so-tenderly fashion). In the past, I have used the pure concrete directly from the bag, just mixing it up with water; I have not experiemented with additional sand. --- End quote --- You can paint it on like thin paint once warmed and that will be sufficient on clean sheet metal. Wood will generally show grain unless highly finished and be harder to remove, which is why I suggested the above together. For a really fine finish gravel is out, for obvious reasons. Bagged mix isn't usually optimal for a fine finish. Mix 3 parts clean masonry sand with 1 part Portland cement for a rich sand mix. For durability you'll want to add the mesh. Keep it clear of the exterior -- sometimes tricky to do in a mold. For small batches of sculptural quality mix it's best to hand mix with gloves on in a bucket. Concrete mold for a biogas generator project I was involved in: Castings: |
| JD:
In the past I have used cheap cooking oil for mold release, if the wood is dry you may need 2 coats. JD |
| Swarfing:
Just to add, in the past i used olive oil for the release agent and an electric sander to vibrate the mix to expel the air. |
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