Sculpted Lettering Molds for Concrete

[gerald_martin]

I've been commissioned to create lettering and a logo for a concrete headwall (coal mine entrance). I need to cut mirrored images of letters about 16" high, that the contractor will fasten to the inside of his form, then when the form is removed the lettering will be recessed into the wall.

I've done this before using plywood for the letter material. That time I beveled the edges about 3/8 inch with a 90 deg. v groove cutter.

However, the contractor had some issues with the concrete chipping around the edges when he removed the forms.

This time I may try a rounded releif carving approach, hoping for a better release.

Is there a better material to use - say, some foam that could be broken out of the concrete instead of a wood piece that comes out intact?

Will my "rounded" letters work better than flat-bottomed ones?

Ideas? has any shopbotter done this? and how can I refine this technique?

Gerald

[pappybaynes]

Gerald,
I don't have an answer for you, but it would be great if you document in pictures...it would be great to see not only how it is done, but the final outcome. Thanks.
Dick

[harryball]

I would think blue or pink construction foam would work well enough. I might paint it with a good primer, a few coats to get smoothness. After it cured and was removed you could use a pressure washer to finish the clean up getting the foam off the concrete.

Just a thought. You should probably experiment a bit first.

/RB

[Gary Campbell]

Gerald..
Another option, no matter what the material you use, is to cut the letters with a tapered ball nose bit. That should help eliminate the chipout.

Try not to use a material that is made from wood. As the concrete is poured, the heat and moisture from hydration will expand a wood based material making removal difficult, if not impossible.

For 16" high letters I would recommend 15 to 30# HDO with a couple coats of oil based or catalyzed primer and maybe even a topcoat to ease removal.

Post your results.
Gary

[bryson]

Gerald, I have made molds for this purpose with blue foam then coated with "PAM" no stick cooking spray. The more radius or chamfer the better. If you use foam, there is no need for draft in the mold since you can just break it off when finished.
Bryson

[dray]

I have done this with MDO, Blue open cell foam and even birch plywood. Make sure the letters have at least a 5 deg taper and the corners are slightly rounded.

If you are going to use a plywood, make sure it is saturated with form release agent. Should spray the form the day before, that night then before the pour. The more it is saturated the less it will expand and contract.

FOam works well, but it can be hard to get clean cuts without using a shaped hotwire..

[gerald_martin]

And here's the results. I used 13 ply Baltic Birch in the absence of better material. After we sanded these letters, we applied two coats of a primer/sealer to them.


7619


7620

The letters were created and nested in ArtCam Insigia, then toolpathed in VCP. Profiling was done with Centurion's 1/2" 30 degree cutter. (this way we could have a 15 degree edge angle the whole way to the bottom of the cut)

The Logo was created (and partly redrawn) from a low-res jpeg file in VCP. I used Centurion's 15 degree edge angle "color core" engraving bit for the engraved edge, and a 1/4 downcut to area clear the remaining material. (the color core bit cut happily in plywood)

Gerald

[dubliner]

Gerald, nice work, cant wait to see the final product. a question, did you use tabs to hold the letters together & how does the 30 degree bit affect the tabs?

[gerald_martin]

Hello Neville:

No, I did not use any tabs. I have a vacuum table, and even though the surface area of the letters is not enormous, they held down just fine. Keep in mind that I was cutting them with a v-bit, and only the tip was piercing thru to the table, therefore hardly exposing any spoilboard.

Tabbing would hardly have worked as it would have left a "bump" on the edge profile, right?

Gerald

[dubliner]

Tabbing would hardly have worked as it would have left a "bump" on the edge profile, right? Yes thats what I was thinking, but I thought you had mastered the reverse offset tabbing anti-climb milling procedure :-)Now I see that its easier than when I talked myself out of trying it - Thanks Gerald

[joe]

Gerald,

We cut letters for several concrete companies on a regular basis. Perhaps you'd be interested in our process.

I use Extira on all letters larger than 2". All are cut with V bits. Many of them custom made. Gary Beckwith, (www.beckwithdecor.com (http://www.beckwithdecor.com)) has a good assortment of these. Most have a 1.5' OR 2" cutting depth.

A couple of years ago, I realised there was a market for reverse cut, smooth, letters for the concrete casting industry. I tried many materials. Few substrates were smooth enough or would stand up to the legnthy affects of moisture in the casting process.

I quickly learned that sanding was out. All plywood materials required too much time to clean up. EPS foams were too rough and deliclate. MDF would puff up when got wet. I could use a sealer but that took extra time. So I use good old Extira.

I use tabs. Usually, three per letter is enough. I have a delta 1" belt sander. The table tilts and I buzz of the tab.

Last month we went through 14 sheets of .75" and a dozen .50".

To finish off out letters, we lightly sand the fronts, and backs. It gives them a very nice look.

I'd be glad to send you a photo or two.

Joe
www.normansignco.com (http://www.normansignco.com)

[joe]

Gerald,

We cut letters for several concrete companies on a regular basis. Perhaps you'd be interested in our process.

I use Extira on all letters larger than 2". All are cut with V bits. Many of them custom made. Gary Beckwith, (www.beckwithdecor.com (http://www.beckwithdecor.com)) has a good assortment of these. Most have a 1.5' OR 2" cutting depth.

A couple of years ago, I realised there was a market for reverse cut, smooth, letters for the concrete casting industry. I tried many materials. Few substrates were smooth enough or would stand up to the legnthy affects of moisture in the casting process.

I quickly learned that sanding was out. All plywood materials required too much time to clean up. EPS foams were too rough and deliclate. MDF would puff up when got wet. I could use a sealer but that took extra time. So I use good old Extira.

I use tabs. Usually, three per letter is enough. I have a delta 1" belt sander. The table tilts and I buzz of the tab.

Last month we went through 14 sheets of .75" and a dozen .50".

To finish off out letters, we lightly sand the fronts, and backs. It gives them a very nice look.

I'd be glad to send you a photo or two.

Joe
www.normansignco.com (http://www.normansignco.com)