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Picture of Save water and reduce erosion
Getting grass to grow is hard sometimes but is necessary to keep your dirt in place so it doesn't wash away. With this porous paver you not only reduce the amount of grass you need to grow but drastically reduce the amount of water needed to keep that grass growing. On top of this, the pavers hold the dirt together reducing erosion even when the grass has not grown in. The pavers also allow for a large amount of weight to move over them, once installed, without the ground being as damaged as it would normally.

While my design is inspired by something similar, some very important changes have been made which focus on the molding process and make this unique. The parts have been made smaller. The reduction in overall height and size of the core blocks makes for less material used, and less digging when installing the paver. Likewise, the connectors are smaller, further reducing the size. The angles on the blocks and the connectors are not as steep and more uniform, making the end product easier to remove from the mold. The tamps on the end of the paver have been created with a negative angle - when upright - in order to facilitate precise spacing when installing the pavers to give a uniform look once the grass grows in. Most importantly, no wire mesh is needed to hold the cast piece together because of the material used to make the end product. This illustrates the true benefit of building your own model and making the mold by hand.
 
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Step 1: Materials list

1 Rolling cart
1 Piece of plywood to go on the rolling cart as a work surface.
Wax paper to cover the plywood work area
1 Roll 2 inch blue painters tape
2 Sheets of two inch 4' X 8' extruded polystyrene insulation
1 Tube of foam board adhesive (with a caulk gun to apply it)
1 Gallon of isopropyl alcohol (you'll need quite a bit as it is used for a lot of things)
1 Bottle of Joy lemon scented dish washing liquid
1 Utility knife
Mixing sticks - get about 30. Paint mixing sticks from Lowes or Home Depot work well.
1 Can of Ease Release 200 from Smooth-On
2 Gallon kits of FlexFoam-iT V by Smooth-On
2 Gallon kits of Rebound 25 made by Smooth-On
1 Bottle silicone pigment (whichever color you prefer)
1 Carton of glass microspheres from Fiber Glast
2 Gallon kits of Smooth Cast 327 made by Smooth-On
1 Bottle Smooth-On silicone thinner
1 Bottle Thi-Vex silicone thickener
1 Cordless drill
1 Kreg pocket hole jig with Kreg pocket hole screws
1 Set of fostner drill bits
1 carpenter's saw with a long blade (as long a possible)
1 Japanese saw with a flexible blade
1 Glue gun with 20 glue sticks
2 2X8 10' long pine boards
5 small boxes of molding clay (kleen-klay is a good brand name to use)
Graduated measuring cups ranging in size from a few ounces to 5 quarts (I recommend buying a box or two with an assortment inside of 30 or so cups)
30 1.5" and 2" bristle paint brushes (cheap ones will work)
1 Table saw (optional - this can be done by hand)
1 Band saw (optional - this can be done by hand)
1 Miter box (you'll need this if you are not using a table or band saw)
1 Hammer
1 Wood chisel
1 3-in-1 painters tools
1 Big bottle of steel BBs
Latex/Nitrile gloves (many many)
Eye protection - goggles/glasses
Filtration mask if you plan on using the glass beads - these things will wreck your lungs if you breathe enough. Check the rating on the mask to be sure it will work for the size of the glass beads you order.

It is imperative to use gloves, eye, and respiration protection when following this instructable. The vast majority of the things used here have chemicals in them and you need to avoid contact with your skin, eyes and lungs for your own protection. If you are not skilled in the use of a table saw or band saw, stick to cutting by hand. These tools can cause serious bodily harm or death.

Step 2: Cutting model out of foam

The safety features on the tables saw have been removed to better illustrate the cut. Do not operate a table saw without the proper manufacturer's safety features/products in place - like a writhing blade and blade cover. Likewise, use eye protection and respiration protection while cutting the foam board on the table saw.

Cut one of the sheets of extruded polystyrene in half. Glue the two halves together with the foam board adhesive - be genreous with the glue as some of the board are cupped. The corners of the halves need to line up as square as possible. It should now be 4 inches thick. Trim the 4 inch piece on a table/band saw to square them up, if available, but you can cut them by hand with a hand saw. Once this is done, start cutting 4 inch strips in order to then cut them down into 4 inch blocks. The blocks will then be 4 inch square blocks. Also cut strips 2 inches wide from the other sheet of extruded polystyrene. Keep track of how much you're cutting so you get all the necessary pieces from the 2 boards.

Mark out a "slant" upwards on the 4 inch blocks and cut them so all four sides of each block are slanted. You will need 16 of these blocks. I cut my blocks so that there was a 0.65 inch difference difference between the top and the bottom. All four sides are beveled the same when you're done. Next, trim the bottom of the blocks in order to get them all to 3.5 inches high. The blocks will likely be uneven before you do this and this will create a uniform height for all the blocks.

Cut a similar bevel on the 2 inch strip. You will cut up the 2 inch strip in blocks to form supports between the larger blocks. You will need 32 support pieces. I cut them so that they were 1.75 inches long on the bottom thus providing 1.75 inches between each of my blocks. The support pieces have to all be cut exactly the same or else the model will be almost impossible to construct. The bevel angle on the edge of the support pieces should match the bevel angle on the larger blocks. This will make assembly a lot easier.

The bevels are made on the block to avoid a mechanical lock of the cast piece inside the mold.

Once the blocks are done, core out the middle with the fostner bit in order to fill it with steel BBs (or whatever else you want to use for weight). This is done to keep the model from "floating" and avoid having the molding making material run under you model.
When the hole is drilled, fill it with BBs and cover the hole with modeling clay. Be sure to get the clay in quite well so that if you drop a block the BBs don't come flying out.

Step 3: Staging

Use a rolling cart or something you can move around easily as it will make the project go a lot smoother. Wrap a piece of plywood in wax paper to build your model and mold on as it will make getting it up easier. The mold material will stick to almost anything.

I used toothpicks to hold my model pieces together. In retrospect, hot glue would have worked much better. Be sure to test any glue you might use to hold your model together to be sure it will not melt the foam chemically or thermally.

Once the model is fully constructed, you need to fill in the voids between the blocks and the support pieces with modeling clay. I put the clay in a plastic cup and heat it up for 30 - 50 seconds in a microwave so it is easier to use. This is another reason hot glue would work better as the squeeze out of the glue will eliminate most of these voids. This part of building the model is known as staging and is crucial. The more time taken while staging, the better the mold will be. Small tools, like dental tools or ice cream sticks, help to get the clay into the voids easier. Your finger (in a glove) will work though. You can smooth out the clay with isopropyl alcohol.

Now you need to build a form to go around your model with the 2X8 pine boards. I used a Kreg pocket hole jig and screws to hold my form together. They work well and can be taken apart easily. You can use whatever you like to hold it together but you will be taking the wooden frame apart and reassembling it a few times. Be sure to leave about 2 inches space all around the model within your mold. This is done so there is a lip on the mold.

Once you have the frame correctly positioned around the model, use hot glue to secure the outside of the frame to your work surface. Be very generous with the hot glue. When this is done, use modeling clay around the bottom of the inside of the frame to ensure your molding liquid does not run out.

Now we need to "release" the model in order to pour the first layer of the mold. Releasing is done in order to ensure the mold and the model come apart easily. You can use aerosol spray release for the mold material you are using, if you wish. I used a 50/50 mix of Joy lemon scented dish soap and isopropyl alcohol - be sure to stir it. It is a phenomenal release agent, is a lot safer than the spray releases, and it leaves a nice lemony scent afterwards. Use a low temp heat gun or work lights to cause the alcohol to evaporate faster if you need to. The entire model and inside of the frame need to be released. I brush on the Joy soap/alcohol mix with a cheap bristle brush but it can be sprayed on too with a spray bottle. If you use an aerosol can release agent, do so in a well ventilated area.

Brush the mixture on several times if necessary to ensure complete coverage. Run your finger over the model and you should feel the soap residue left behind once the alcohol has evaporated.

Step 4: First coat of glove mold

Cut along the seem of the bottle (if you have smaller containers) to open it as it will eliminate waste. I used these smaller bottles as I needed to use them up. If you are using gallon containers, just pour straight from the container into your mixing container. Mix the components according to the mixing instructions supplied and be cognizant of pot life for the temperature you are working at. Pot life is how much time you have to mix the components before they start setting up (solidifying).

Mark your graduated mixing cup at the lines with tape so you can see where to stop while looking inside the cup. Once you have parts A and B in the cup, use a mixing stick to mix them. Do not stir the mixture. Rather scrap from the outside in continuously as it will produce a better mix with less air bubbles. You can use the silicone thinning agent to make mixing and pouring easier. Be sure to use the correct amount by following the manufacturer's instructions.

You can see how well the silicone is mixed by looking at the bottom of your mixing cup. If the color is not uniform then you need to keep mixing.

Once you have a good mix, pour the silicone over the tops of the blocks and continue to spread with a bristle brush. Turning on work lights to provide heat will help the silicone run but with also speed up the setting time (i.e. it will reduce the pot life). You can achieve the same effect with a heat gun. Be careful not to burn the silicone. Using the heat gun will also cause smaller bubbles in the silicone to pop.

Once you have completely covered the model with a thin coat of silicone, and the silicone is starting to setup, you can carefully pop larger bubbles with a pointy object. Do not worry about some bubbles in the silicone. They are unavoidable unless you are degassing the silicone before pouring it. If the entire inside of the frame is not covered on the first pour, it is not a problem. The aim is to cover the entire model with a thin coat of silicone.

Step 5: Second coat of glove mold

Before the second pour, remove any large bubbles which are between the first coat and the model. You will be able to see them if they are there. The second pour will cover the hole/s left by removing them. Small bubbles visible in the surface are not a problem.

When mixing the silicone for the second pour, add a little pigment to change the color of the silicone. This will allow you to see where the mold may still be thin and how even your coverage is. Use just a little bit of pigment as a little goes a long way!

Once the silicone is evenly mixed, pour it over the model. I poured the silicone from a height in order to remove as much air from it as possible. Pouring it from a height like this reduces the number of air bubbles a lot. Again, use a bristle brush to spread the silicone out. Use the heat gun again to make the silicone easier to spread and remove air bubbles.

Be sure to fill the entire frame this time. Spread the silicone out between the voids but don't worry if they are not completely evenly filled.

Step 6: Thixotropic layer

A thixotropic layer is added to reinforce weak areas in the mold, like sharp corners.

The better way to do this it to add a thickening agent to your silicone and brush it on. I used glass beads to achieve the same effect. BE SURE TO USE THE CORRECT RESPIRATORY PROTECTIVE EQUIPMENT WHEN USING GLASS BEADS. The dust from this stuff is glass, don't breathe it in!

Mix the glass beads into the silicone while mixing parts A and B. If you are using silicone thickener then add some pigment to give you contrast as in the last pour. I did not use pigment and the glass beads changed the color enough to cause the same effect.

Apply the thickened silicone at all the thin/weak areas of your mold with a bristle brush. Once again, use the heat gun to get the thickened silicone to run a little and remove any topical air bubbles.

Step 7: Pouring the mother shell

Now that we have our silicone mold, we need support for it so that it does not deform when we pour our molding material into it. Therefore we are going to create a mother shell out of flexible foam. We use flexible foam as it will be a lot easier to remove the mold from it once we have cast a part in it. I used FlexFoam-iT V from Smooth-On.

We will do this in several pours as the foam generates a lot of heat and we don't want to damage our mold or start a fire.

Be sure to release your silicone mold and the sides of the frame with the Joy soap/alcohol mix. Otherwise the foam and your mold will become one. Go over the mold and sides several times to ensure full coverage. Allow the alcohol to flash off. You can speed this up with work lights or a heat gun.

Mix parts A and B of the foam as recommended by the manufacturer. Observe the pot life and this stuff tends to react quickly. Be sure to do a rough calculation for volume as the foam expands. The foam I used expanded 10 times its original amount. Pour the foam across the mold. Try to pour it as evenly across the mold as possible to avoid having very thick and very thin areas. You want the foam to expand as evenly as possible across the surface of the entire mold. Don't worry if it over flows from the frame, you want it to be above the height of the frame.

Once the foam has cured, cut the excess on top off with a long carpenter's hand saw. Trim the subsequent uneven areas with a Japanese hand saw (because it has a very flexible blade) and even it out as much as possible. Once you have it as even as possible use the 3-in-1 tool to start to loosen the sides from the wood. Be careful not to crush the foam by pressing down on it too hard.

Step 8: Unboxing the mold and mother shell

You'll notice the table and the foam look a little different in these steps. I had a bad batch of foam - it was shot due to age (expired) - and had to redo the foam step, hence the in the pictures SNAFU but I have accommodated for it.

Cut the hot glue on the outside of frame in order to be able to move the entire box. Stand the box up on it's end and remove the wax paper, or whatever you may have put down on your work surface. Lay the box so that you can see the model. You might notice some flashing around the model pieces. This can be trimmed off but needs to be done very carefully in order not to damage the mold.

Once the sides of the foam have been loosened from the frame, remove the sides of the frame. This is where the Kreg pocket holes and screws are particularly useful. Use a chisel and hammer to part any sides where hot glue may be holding the frame together. Use the 3-in-1 tool to loosen any left over foam still stuck to the frame. Do not just pull or the foam may tear.

Once the frame has been completely removed, carefully remove the model pieces from the mold. If all has gone well you can throw the model away now. The mold may start to come out of the mother shell (foam), this is ok. Once the model has been completely removed, number the corresponding sides of the foam and mold so that they go back together properly when you start to do your cast.

Once the model is completely removed and the sides have been numbered, lay the mold back in the mother shell and begin to clean the mold. There will be clay in the mold. You will also be able to clean up the flash on the mold at this time. Carefully scrap the left over clay out of the mold with a dull instrument. You can use dykes to clip off the flash but take your time and be very careful. Make sure the dykes are sharp. Wipe out the mold with a paper towel moistened with alcohol. It will get the clay in the hard to reach places. I vacuumed my mold out the following day to remove anything that was stuck because it was wet.

Once you have cleaned the mold out well, set it off to the side some place safe. Resize the wooden frame, by cutting it, so that the height of the frame is equal with the height of the mother shell (foam). The resized wooden frame should press firmly against the sides of the mother shell. You will most likely need to resize the lengths of the sides too. The mold should be able to lie in the mother shell and have it's edges hanging onto the wooden frame. Once this is done, reassemble the frame around the mother shell. Lay the mold back in the mother shell and be sure the sides of the mold are supported by the frame.

Check the firmness of the mold around the edge of the frame. If there are any voids, they will need to be filled in with modeling clay.

Step 9: Casting

Be sure the mold is clean and free of debris. For the cast I used an aerosol mold release - Ease Release 200. Be sure to hold the can the correct distance away as shown in the picture. Spray it in a well ventilated area. I sprayed the mold fives times to be sure I got all the areas covered.

Mix the casting material (Smooth Cast 327) the same way as the silicone, from the outside in. I added glass spheres to the Smooth Cast 327 to give to strength. I did three pours as the casting material gives off a lot of heat and I did not want to damage my mold or start a fire.

Follow the manufacturer's instructions for times between pours and final curing.

Step 10:

Once the casting material has cured, remove the frame. Once the frame has been completely removed, stand the mold and mother shell on end and carefully remove the mother shell. Once the mother shell has been removed, begin to remove the mold from the cast piece. The Rebound 25 is great in that it has great elasticity but retains it's shape. Be careful as the edges of the cast piece can be razor sharp. Press firmly but slowly on the pieces in the voids to remove the mold from the cast piece. Once half is done it is easier to rotate the piece and start at the top again.

Once the cast piece is completely removed from the mold, clean the mold with Joy dish soap and warm water and store flat in the mother shell.

And now you have a paver through which grass can grow. You can cast this piece out of pretty much any material. If you plan on using something like concrete and want to get more life out of the mold, add two more coats of silicone to the mold.