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Concrete Stamp Made from Urethane Rubber

February 14, 2018

Urethane rubber has many uses because of its strength, durability, and flexibility. It is often used to pot or encapsulate underwater electronics, make shoe insoles, make molds for concrete bricks, blocks, and statues. It also makes an excellent stamp for concrete, where it is pressed into fresh, wet concrete to create a pattern. In this post, I’ll show you how to make a urethane rubber concrete stamp.

Materials needed:

Your work area:

  • Clean, level work surface, covered in paper or plastic for easier clean-up.
  • All materials comfortably within reach.

Project steps:

  • For this project, I found interestingly shaped tiles that I want to use to make a concrete stamp. Since I’m not doing a driveway or other large scale concrete stamping application, I won’t make a large 2ft x 2ft original with tiles thin-set and grouted into place. Instead, I’m making a smaller concrete stamp as a decorative accent.  I will use silicone to reverse the tiles, so that when I make the concrete stamp, I’ll be pressing the shape of the tile into the concrete, rather than pressing the space between the tiles into the concrete. When using a concrete rubber stamp to mimic a tile or brick pattern over a large area, you will do the opposite, so that the tile/brick pattern looks like the real thing.
  • First, I cut out the cardboard and glued it together with hot glue to make a mold box.
  • Next, I glued my tiles down and mixed up some AeroMarine Products AM128 Pourable silicone. I picked a corner and poured the silicone slowly, letting it flow around the tiles. After 24 hours, the silicone cured. I removed it from the mold box. Again, if you are making a stamp to mimic brick/tile work for a large area, make the mold box around your tile/brick work, but don’t pour silicone into it. You would apply mold release to the tiles and pour the 75A Urethane into instead. I go in to more detail on mixing and pouring the urethane in the next few steps below.

 

  • For the next step, I need to construct another mold box around the silicone. However, this is the box into which I will be pouring the 75A Urethane Rubber.  Urethane is moisture sensitive before it is cured. Things like paper, wood and cardboard can retain moisture from the atmosphere and cause bubbles in the urethane. So, I am going to cut out my cardboard for the mold box and then seal it with a clear acrylic spray. You can also just paint it using any old can of spray paint you have around.

  • Once the sealant is dry (read the directions on the can), I used my hot glue gun to build the box around the silicone piece. This is easier than building the box and then trying to put the silicone piece in it.

  • Before I mix and pour the 75A Urethane Rubber, I need to apply some mold release to the sealed cardboard. This 75A Urethane Rubber kit comes with a silicone based mold release paste that I am going to apply with a clean rag.

  • Now I’m ready to mix my 75A Urethane Rubber. The mix ratio on this product is 1:1 by volume. I mixed a pint of part “A” and a pint of part “B” vigorously by hand for about 60 seconds and then poured it into the mold box. This product has a short pot life, at about 3 minutes at  70 F, so mix quickly and thoroughly, taking care to scrape the sides and bottom of the mixing container.  The full cure takes 12 hours, so I’ll come back to de-mold tomorrow.

  • Fully cured now, my concrete stamp is ready to go!

 

  • Because the urethane rubber is pretty stiff, I poured it thinner than I did the silicone. The AM128 Silicone has a Shore A hardness of 28, which is a little firmer than a rubber band. The 75A Urethane Rubber has a Shore A hardness of 75, which is about the firmness of a car tire. So I always want my silicone to be about 1 inch thick, but the urethane rubber can be 1/4 to 1/2 inch thick.
  • The finished stamp can now be pressed into concrete!

If you have any questions about this project or your own projects, give us a call at 877-342-8860 or drop us an email at [email protected]

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