Introduction: Bent Concrete Side Table From a Rubber Mold.

This Instructable will demonstrate how to make a rubber mold for casting a bent concrete table. It will cover making a cardboard mockup, producing the positive, casting a rubber mold, and finally casting the finished concrete piece using a fiber-reinforced mix and bending it to shape.

This project was completed during the 3 day Advanced Fiber Reinforced Concrete Training at CHENG Concrete in Berkeley, California with guest instructor Chris Franzen from Surecrete. The unique characteristics of a fiber-reinforced concrete mix make it possible to cast a flat piece, wait until it enters a "false-set," and then bend the piece. For this we need a flexible mold, and in this Instructable we'll outline the steps needed to create a polyurethane rubber mold that will work for this technique.

Even if you aren't using a fiber-reinforced concrete system and bending concrete, the rubber mold-making steps are general enough to apply to any casting project.

Be sure to enter the Concrete & Casting Contest for a chance to win some nice prizes!
General Materials and Supplies:

Step 1: Full Size Mockup

Look for Inspiration:Constraints of this project:
  • Cut out from a single sheet of 4' x 8' melamine
  • Minimum of 2.5" width of legs (for strength)
  • Thickness of concrete will be 3/4" (same thickness as the melamine that the rubber mold is created from)
  • Shore hardness and thickness of mold rubber will determine the radius of the bends
After deciding on a direction, sketch out some ideas. Cut out small paper models to test ideas quickly. Then create a full scale mockup from something like paper, cardboard, or sheet plastic. Change proportions as you see necessary. The first mockup for this bent concrete side table didn't have enough usable area on the top, so the second version was made a few inches wider in the middle.

Take your time during this phase while it's still easy to make improvements.

Step 2: Cutting Out the Melamine Positive

Don't scratch the positive:
Any imperfections in the positive will be transferred to the rubber mold, which will then be transferred to the finished concrete piece. Take care not to scratch the positive when laying out the shape, cutting it out, or trimming the edging.

Trace the template:
If the cardboard mockup is 1:1 scale, you can use it as a template for cutting out the positive. Trace around the cardboard with a pencil, and mark a center point. By keeping this center point, you can make sure everything is of equal length (important for the legs).

Use a ruler to make sure everything is laid out correctly, and for the curves you can use a compass or just find some things around the shop that are the right size and trace around them.

Cut out the shape with a Jig Saw*:
Adjust the angle of the blade so it's running straight up and down (or at whatever angle you want). Then do a test cut. Start cutting out the shape, following the line as closely as you can, and going slowly. When it comes to making the last few cuts, make sure the positive is supported, or it can bend down and break off.
*If you have access to computer modeling software and a CNC Router, you can use those tools to accurately cut out the positive.

Sand Edges:
Sand the edges smooth with a palm sander and a sanding block.

Clean it up:
Remove any pencil marks or marker outlines from the positive with denatured alcohol.

Trace around the shape on a sheet of melamine:
After the shape is cut out and finished, place it on a sheet of melamine (or another water proof surface that is very flat). Trace around the shape and mark one side so you can put it back in the exact same place. Then trace a 1" border around the shape (or whatever thickness you want the edge of the rubber to be). I used a roll of 1" thick blue tape as a guide

Seal the edges of the positive:
The edges of the positive should be sealed before rubber is cast against them. There are a lot of ways to do this, from iron-on edge banding, to clear tape, to varnish.

Step 3: Building the Form for Casting Rubber

Attach the form walls to the base board:
These form walls are made for casting concrete countertops. They are sections of bendable foam that are stuck to the base board with double sided carpet tape and screws. Place the walls around the perimeter, spaced 1" away from the positive. The walls don't need to be perfect, but the perimeter of rubber should be about the same thickness everywhere so it will bend evenly.

If you don't have these walls, you can also use strips of 3/4" plywood, cut to the height of the finished rubber (1" in this case). For curved sections, you can use bendable plywood or wiggle board.

Some sort of liner or clear tape on the inside of the form walls will give the rubber a smooth finish around the border.

Seal the walls and drill holes in the base board:
Apply a thick bead of silicone to the perimeter of the form walls. This will keep the rubber from leaking underneath the walls.

After the walls are in place, drill a few holes to secure the positive to the base. You will be screwing up from underneath the baseboard, into the positive. Pre-drill and countersink the holes and use short screws (1") that won't go through the positive.

Silicone the positive to the base board and secure with screws:
Before placing the positive in the mold, coat the back side with silicone. This is an important step and will help keep any air that is stuck between the positive and the base board from leaking out while the rubber is being cast. Air will leak out and cause air bubbles if you skip these steps.

Place the positive in the form, line it up with the marks you made earlier, and screw it in place from underneath. Clean up the silicone that squeezes out.

Run a bead of silicone around the positive to help seal it to the surface and clean up the excess with denatured alcohol.

Allow the silicone to cure and then clean the excess up by rubbing it away with your fingers.

Clean out the form:
Blow the form out with compressed air, and clean inside the mold with denatured alcohol, mineral spirits, or acetone.

Spray Mold Release before casting:
Before casting rubber, liberally spray Pol-Ease mold release in the form.

Step 4: Calculating How Much Rubber to Mix

Calculate the volume the old fashioned way:
You want to mix up enough rubber to fill the form, but you don't want to have excess leftover because it's expensive. If you have a simple shape, calculating the volume will be easy, and there are many volume calculators online. If the mold is complex, break it down into separate shapes, calculate the volumes individually, and then add them together.

Volume = Length x Width x Height

You can also fill the form with water, figure out approximately how much water it holds, and convert that to cubic inches.

For U.S. Gallons divide by .0043290 to find the volume in cubic inches.

1.5 gallons / .004329 = 346.5 in³

Divide this number by the specific volume of the rubber being used, which is 27 cubic inches per pound for Polytek 75-60 Polyurethane Mold Rubber.

346.5 / 27 = 12.8 lbs. of rubber

Calculate the volume through computer modeling:
Modeling the rubber mold in a program like Sketchup. If the model is solid, you can right click "Entity Info" and it will give you the volume. With 1" thick walls and a 1/4" thickness on the back side, the volume of the rubber mold is 357 cubic inches.

The specific volume of Polytek 75-60 Polyurethane Mold Rubber is 27 cubic inches per pound. This number will vary depending on what type of rubber is used.

357 / 27 = 13.2 lbs. of rubber.

Assume that some rubber will be stuck to the inside of the mixing buckets, and mix up 15 - 16 lbs to be safe.

Calculate the amount of concrete it will take:
With the shape already modeled in the computer, we can also calculate the volume of the piece and use that to figure out how much concrete to mix up. You can also fill the rubber mold with water after it's been cast and use the same gallon to volume conversion listed above.

The volume of finished concrete piece is 431 cubic inches.

Convert that to cubic feet by dividing by 1728

431 / 1728 = .25 cubic feet

1 cubic foot of concrete weighs about 130 lbs, so we multiply .25 and 130 and arrive at 32.4 lbs of concrete.

Step 5: Mixing Rubber

  • Rubber Gloves
  • (2) 5-gallon Buckets
  • 1/2" Corded Drill
  • Mixing Attachment
  • Scale
  • Spatulas to scrape bucket
  • Poly Purge Dry Gas Blanket (helps extend life of unused rubber)
  • (15 lbs.) Polyurethane Rubber
Mold Rubber:
There are a lot of types of mold rubber (Polytek and Smooth-On make great stuff). For this project we used Polytek 75-60 Polyurethane Rubber. The number 60 refers to the shore hardness of the rubber, which is a mid-range hardness. A lower number means a softer rubber, and a higher number means a harder rubber. In this case, we need the rubber to bend but still hold it's shape, so a 60 series should work well.
This mold rubber is 2-part in a 1:1 ratio. Mix 1 part A to 1 part B, by weight.

Double bucket mixing:
Part A and Part B are first mixed in a 5-gallon bucket (white in the pictures). Then they are transferred to a second 5-gallon bucket (grey in the pictures). One way to ensure no unmixed material gets into the final form is to use the double-bucket mixing technique.

Weigh out the more fluid Part B:
Part B of the Polytek 75 series is more fluid than part A. Weighing it out first will make it a little easier to scrape the sides of the bucket after Part A has been added. This mold took 7.5 lbs. or Part B.

Add Part A to the same bucket:
Add 7.5 lbs. of Part A for a total of 15 lbs.

First Mix:
Blend Part A and Part B together using the corded drill. Scrape the sides and bottom of the bucket with the spatula and continue blending for a few minutes.

Second Mix:
Pour the rubber into the second 5-gallon bucket and resume blending. Scrape down the sides and bottom of the bucket just like before. Total mixing time shouldn't be longer than 7 minutes.

Working Time:
After the rubber is mixed, begin pouring it in the form. There's no time for standing around or cleaning stuff!

Clean Up:
Cleaning up the rubber is actually pretty fun because it will just peel off after a few hours.

Step 6: Pouring Rubber

Work Surface:
Make sure the table / work surface is level. If the table is not level, the rubber will settle to one side and be uneven.

Pouring Rubber:
Pour from one place and let it flow outwards. Pouring from multiple places will create more air bubbles. Use a spatula to move the rubber to the edges of the form. The whole time, you're trying not to create extra air bubbles, so try to minimize any turbulent motions. The total thickness of this mold is 1", with about 1/4" covering the positive. Look for any large air bubbles that are coming to the surface and use the spatula to drag them up and pop them.

When the rubber is at the full height, you can spray the back side with form release to knock down any bubbles on the surface.

The rubber will start to set up relatively quickly, in about 10 minutes, so be efficient when filling the form. It needs to cure at room temperature and will be ready to demold the next day. If you try to demold too quickly, the rubber can change shape because it hasn't fully cured.

Step 7: Demolding Rubber Mold

Remove the form walls first. Then slice the silicone seal with a razor blade if necessary. Loosen the rubber by pulling on it, and gently pull it off of the positive.

Step 8: Preparing Mold for Casting

Flat and Level Surface:
Make sure the casting surface is perfectly flat. Lay down a piece of plywood if necessary - any debris left under the rubber form will make a divet in the finished piece. If the surface isn't level, the piece will be uneven in thickness.

Mold Release:
If the rubber molds are fresh, and this is the first time they're being used, mold release isn't necessary as long as it was used when initially casting the rubber. After that, spray the molds with a light dusting of Pol-Ease mold release.

Step 9: Mixing Fiber Reinforced Concrete

Mixing Equipment and Supplies:
  • Rubber Gloves and Particle Mask
  • 17 or 25 gallon mixing bucket
  • Paddle Mixer
  • (1 gallon) diluted Activator
  • (1) 50lb. bag CHENG D-FRC Casting Blend (Light Base)
Add 1 gallon of diluted Activator (1 part Activator : 4 parts water) to a 17 or 25 gallon mixing bucket.

Add 1 bag of CHENG Smart Color Pigment (Indigo) and blend slowly. Scrape the sides of the bucket with a trowel and blend once more.

Concrete Mix:
Add the 50lb. bag of concrete mix and blend. Scrape the sides and bottom of the bucket with a trowel, making sure any unmixed material is blended. Add additional water if necessary, but the mix shouldn't be too wet. After the mix is completely blended, move the bucket over to the form and get ready to cast.

Refer to the How-To-Mix Concrete Instructable for more tips on mixing fiber-reinforced concrete.

Step 10: Pouring and Bending Concrete

Pour Concrete:
Pour from one spot and let the concrete flow outward, then fill the remaining areas and level it off with your hands. Final thickness of this piece is 3/4".

While filling the form, turn on the table vibrator or shake the form by hand to help chase air bubbles to the surface.

Wait 20-40 minutes:
Temperature and humidity will have an effect on how long it takes for the fiber-reinforced concrete to enter a "false-set" (the point at which it can be bent). This piece was cast at 4:05 and bent at 4:45 in a 80° environment.

Test Bend:
Lift one of the legs to check the consistency of the material. If it slumps down, it's not ready. If it cracks at the bend, you've waited too long.

Attach to frame or drape over form:
It really helps to have a few people to help you through the next step.

If you have a form made, you can drop the rubber mold down into it, or drape it over a form.

In this Instructable, we quickly cut 4 pieces of plywood and screwed them to short sections of 2x4. Each leg was held up while the plywood support was screwed in place. A screw through the rubber mold and into the support held the legs upright.

Minimize Vibration:
Any vibration will cause the concrete to slump. Screwing the supports to the base board was not ideal. Using clamps, straps, or even duct tape will work just the same.

Cover with Plastic:
Drape painter's plastic over the piece to help keep the humidity inside while it's curing.

With this concrete mix, you only need to wait 4-6 hours before demolding. On a piece like this, waiting until the next day is a good idea. The concrete will have a little more time and will be stronger the longer you can wait, but typically 24 hours is plenty of time to cure.

Step 11: Demolding

Demolding Tips:
Wear rubber gloves during demolding. The oils from your hands can leave stains on the fresh concrete.

Remove the supports first, then loosen the rubber mold from each leg and let it drop to the table. The piece will release easily from the rubber mold.

Break away excess concrete from the edges and sand with diamond hand pads. If necessary, grind the bottom of the feet so the piece sits flat.

Caring for the Mold:
Cleaning - The mold should be cleaned after each use with a soft cloth, water, and a mild soap if necessary.
Storing - Store the mold flat, indoors, out of the sun (UV light will deteriorate the rubber), and maybe with a light coat of Pol-Ease mold release.

Step 12: Sealing

The final step of this project is to seal the concrete. This step is optional, but sealing will help protect the concrete from staining and preserve the finish over time. There are a lot of concrete sealers available, but the instructions below are specific to CHENG Concrete Countertop Sealer and might work for other water-based acrylic sealers.

Sealing Supplies:
  • Rubber Gloves
  • (2) 1-Gallon Buckets (1 empty, 1 filled with water)
  • (2) Terry Cloth Rags
  • CHENG Concrete Countertop Sealer
Where To Seal:
  • Always indoors or in the shade when possible.
  • Away from moving air.
  • In a dust-free location.
  • Avoid anything that will cause the sealer to dry too quickly. If the sealer dries too quickly, this can cause streaking when it's being wiped off.
Sealing Process:
  • Make sure the piece is free of dust by wiping it down with a cloth.
  • Saturate the piece with clean water.
  • In the second bucket, mix up a sealer dilution (30% Sealer to 70% water)
  • Saturate the piece with the sealer dilution, allowing it to soak in.
  • Wring out the towel and wipe off the sealer.
  • Work quickly and methodically to obtain a streak-free finish. As the sealer dries, it will start to gel, and this can cause streaks.
  • Continue saturating the piece with sealer and wiping it off evenly.
  • Repeat these steps until the piece passes the Water Drop Test. It may take up to 10 light coats of sealer, but the entire process is quick and easy.
For more detailed information about sealing concrete, check out the Sealing Concrete Countertops Instructable.

After the piece is sealed, it can also be waxed. This will provide a sacrificial layer to help protect the sealer. Pieces that are outdoors do not need to be waxed, because the wax will break down and will need constant maintenance.

Step 13: Is It Strong?

After about a week, this piece should have reached 80% of it's 10,500 psi. compressive strength. It easily supports (8) 50lb. bags of concrete mix (400 lbs.) and will continue to get stronger. The actual weight it can support is unknown, but the mix is quite strong even at only 3/4" thick! With a flexural strength of 1500 psi. after a few days of curing, this piece will actually bend before it breaks.

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