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

[JDPratt]

I do a fair amount of concrete forms for sidewalk repairs, concrete pads, and even dabbled a bit in concrete counter tops. I try to stay away from the kerosene or diesel (don't like the flammable nature of it and what it tends to do to the surrounding area). I use plain-old mineral oil. As long as there is a good coating (slick to the touch but not dripping) it works great. I normally spray it on heavy with a spray bottle and then go back over it with a cheap paint brush to spread it around evenly and work it into the wood form and it seems to do the trick. Pros - nonflammable, doesn't kill grass, Cons - Wife hates it when I use up all the mineral oil which she uses to treat our butcher shop butcher block.

[spalted]

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.

[JDPratt]

I was remiss in mentioning how to remove the relaease agent. I have stained the concrete after using mineral oil, but you have to ensure all the residue is removed prior to finishing/staining. Soap and water does a good job of removing the mineral oil. Any good emulsifier that allows the oil and water to mix and then remove should work. I find a heavy mix of dish soap and water works for me. They will be using a lot of that here soon on the Gulf Coast.

[spalted]

Thanks for the tip JD,

I cast a couple hundred feet of concrete cornice caps for some parapet walls about 15 years ago. I drove by the building a few months back and was shocked to see how crappy they looked. The stain was blotchy and mottled looking. I'm assuming it had something to do with not having the release agent cleaned off well enough. I have no idea what we used anymore now.

I never received a call back about it, but I'm embarrassed to say I had anything to do with it now that I see it.

[Metalhead781]

MH,
My background is in the plastic injection mold making business, quite different background than yours, so I have limited knowledge on how the concrete will want to stick to different surfaces. I have built wood forms for sidewalks, steps, and things like that, I was always taught to spray the wood with kerosene, or diesel feul (no smoking during that stage ) and the forms come off easily.

I have pondered the thought of good, long lasting forms, for smaller items, and based on machining up prototype parts at work, i found that the Delrin (which is a plastic resin called Acetal) machines very nicely. It doesnt ball up on the cutter, and leaves a rather shiny slippery surface. I havent tried the release properties of Delrin with concrete, but have with other resins. In your experience, do you think it would want to adhere to something like this, or release easily???.. I have wanted to try this but have never took the time. (actually Lori, my wife has wanted me to try this ) If you were making dozens or hundreds of an item, I am thinking this could be the ticket to keep from having to make several foam forms... Since you have experience with the concrete forming i would love to know your thoughts?

Thanks!

Ron

.... As Iron sharpens Iron, so one Man sharpens another!

We used to use delrin for quite a few parts. Never cast into it but around it. We put cylinders on a lathe and turned what we needed to put in blast nozzles for tempering furnaces and certain ports and drains on permanent molds. After a 8 or 9 casts you could see a lot of wear on the delrin just from the abrasive texture. Any permanent molds we would build besides the ones made of steel, would need rebuilding after 10+ casts. Keep in mind one of the small units we built would retail for tens of thousands of dollars. The cost of delrin is minute considering the parts we made from it and with it. Even with it's properties you still need to clean and grease it between casts. The kind of durability you're looking for would be better made of metal. Making a few casts of a certain item foam for it's cost would be a better idea. We've even used resin that you would cast into a mold to make molds for concrete. If you can get a hold of machinable resin. Cast it and machine it. It would hold up better then Derlin because it is harder. It's just the abrasive nature of the concrete that makes "permanent" molds so hard to make and come by. On top of that, many of the systems we built were custom. Most of the large machines were all different due to certain requirements etc.

MH

[Metalhead781]

Vegetable shortening works outstandingly well even when it's warmer. To clean the concrete use plain Dawn dish soap and a bit heavier then you would to clean dishes then spray or power wash it off. After that simply stain the concrete once it's dry. We never had much cause for staining however some trough systems and machines that needed coatings applied would be cleaned before they went in the kilns. We used the Dawn brand original dish soap because it just worked the best.

On an interesting note. The kilns we used to dry and cure the concrete were top loaded with a 20ton crane. As a joke a night supervisor took all four of the smaller forklifts and put them inside the medium sized kiln to hide them. Obviously that supervisor didn't work there much longer and he had his crew do it on second shift.

MH

[rjustice]

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!...

[www.go3d.us]

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

[rjustice]

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

Nice Thread, I'm not sure how I missed that one...

Thanks for steering us to it.


Happy Carving... (or Casting),


Ron

[earlyrider]

I use the CW for cutting patterns, then mold them with silicone or urethane rubbers. The urethane rubber is very resilient to abrasion by concrete, and suitable to produce large numbers of casts. Look up Polytech mold making products.
Ron