By: Ramon Luquin-Project Manager
In accordance with the level 2 System Requirement to make the body out of resin (link to system requirements), I made molds for each part of the Hexapod body. The process was a lot of trial and error, but it ultimately provided the results I was hoping for. With the molds completed, I can create and recreate any part of the Hexapod that might break, or even build another Hexapod entirely.
The silicone I used was Smooth-on Mold Max 40 (http://www.smooth-on.com/Mold-Max%3D-Perfor/c1135/index.html?catdepth=1).
Mold Max 40 comes in two parts and is mixed in a 1:10 ratio by weight. It takes about 24 hours for the silicon to completely cure. The price of the 1 gallon kit I used was $96.54. Mold Max 40 was the strongest silicon at this price range, and could be used in both a two part mold, or a single pour-in mold. I spoke with a smooth-on representative on the phone, visited A-R products which carried all Smooth-On products (http://a-rproducts.com) and was shown different samples. After explaining my purpose for the silicon to the representatives at both locations, they recommended that I use the Mold Max 40. Mold Max comes in many varieties, the number 40 represents the shore hardness of the material (Link to Substech Shore Hardness Study). To ensure that I would be able to reuse the molds without breaking them, I chose the strongest at that price range. A 40A shore hardness is roughly equivalent to a rubber phone case. It can withstand multiple molding and can stretch up to 250% of its size until it breaks (link to product description PDF).
www.Smooth-ON.com offers a large variety of products. Most of the resins are tooling quality, which allows me to drill and mount the resin without worrying about breaking. Again, with advice from the Smooth-ON representative and the sales representatives at A-R products, I was recommended Smooth-Cast 300 which sold for $85.85. This resin works well with the Mold Max 40 and its low viscosity allowed for easy pouring. The resin comes in two parts and is mixed in a 1:1 ratio by volume. Other resins available, like the 380Q, begin to harden once mixed in 20 seconds. For being my first time using the material I felt it was a better idea to use something that gave me more time to fix my mistakes and figure things out. I also looked at the Task Series resins (link to Task series resins). These resins were designed for heavy duty use: replacing cart wheels, gun props used in film and television, door knobs and handles. They would have served great but were nearly twice the price of Smooth-Cast 300 at $145.87 and were actually a little heavier. The Smooth Cast 300 has a specific gravity of 1.05 g/cc where the Task Series had a specific gravity of 1.15 g/cc. With just the body’s volume alone being 11655 cc, the weight of the Smooth-Cast 300 would be 12.237 kg, where the weight of the Task series would be 13.40 kg.
Other Important Materials
Building a mold box is important, and can save a lot of time and headache in the long run. Making a mold out of 2×4’s and composite boards is an easy way to create a box to place the pieces in and pour over silicon.
With our particular build, the 3D master pieces we used were light and weren’t dense enough to stay under the silicon while it was being cured. I had to screw in the pieces to a piece of composite wood to make sure they stayed in place. Using a power drill for this made things a lot faster. Making the wooden mold box also required a power drill.
This is used to make the mold box. Its possible to use other materials to make a mold box, such as topper wear or a bucket, but for a large piece like the body cutting the wood was necessary.
For small parts, such as our brackets, foam board can used to create a mold box. It works well, and saves time and money over building a mold box made out of wood. Foam board retails for about $2 at most hobby shops, or office supply stores.
Hot glue was used heavily in this build. It can be used to create walls for a foam board mold box and to seal any crevices that may be left when joining the walls. Make sure to have plenty of glue. I used about 20 mini sticks for this build.
A utility knife to cut off excess bits in the resin molds
Plastic cups to mix the silicon and the resin. Make sure their disposable, the chemical will ruin the cups.
A scale, I used mostly ounces to measure the ratios.
Rubber gloves. Avoid latex gloves, they will react with the silicone and can damage the mix.
Prepping the Pieces
We made the molds out of the 3D pieces we had designed. A special thanks to Santiago Landazuri from project Rover for spending the long hours printing these pieces as well as the Scan and Tilt platform we needed. Hexapod says thanks Santiago.
We tried to sand and fix any issues with the pieces before we molded them. Any tiny imperfection or any bump on the master pieces will show up in the molds afterwards. Make sure you have the pieces exactly like you want them to be in the final mold.
Making the Mold Box
After first using topper wear and hot glue to try and create molds by gluing the pieces to the bottom of the topper wear, I changed to using a mold box made out of wood. My first attempt resulted in the pieces floating to the top, and the molds were too shallow and unusable. I will be making a one piece pour over mold, so making sure the pieces don’t move while the silicon is curing is important to getting an accurate mold made.
I measured the length and width of the piece, in this case the assembled body plate, and made a mold box slightly larger. Its important to make a mold box with an area close to that of the piece. Leaving a small margin is important to making sure the silicon doesn’t tear while demolding, while making it small enough to save on silicon. I cut the 2×4 pieces and tried to make sure it was as straight as possible. The more accurate the cuts, the less likely the silicone will leak, and silicon tends to leak anywhere there is a gap. I screwed in the pieces to the box and screwed in the pieces the bottom composite board as show below. Note the got glue applied to all the edges of the box to protect from leaks. Other materials could have been used, but this was quick and easy way to do it.
Mixing the Silicone
Mold Max 40 is sold in two parts. Part A and Part B is mixed in a 10:1 ratio. It is a mint green color when fully mixed and cured. To measure the volume needed for the pour, I poured rice in the mold box, enough to completely cover the body plate with a roughly a 5mm thickness, and measured the weight in ounces. I used this weight as my estimate for Part A of the silicone. Water could have also been used, but I didn’t want to have to wait till the body plate completely dried before pouring over the mold.
Part A is heavy, thick, and pours out very slowly. It has the appearance and consistency of condensed milk. I measured the volume needed for this piece which was 20 oz, and set it aside. I poured out the 1/10th of Part B, which was 2 oz. Part B is green in color, and has the consistency of water.
When the mixture is a solid green color all the way through, its ready to be poured over the piece. The silicone has a 45 min pot life, so it will give you plenty of time before the silicon begins to cure if you need to make more and add it in to the pour.
Its recommended to place the silicone mix in a vacuum pot to remove the air from the silicone. I didn’t have access to one, so I used a high pour method. I poured the silicone in by holding the cup as high as possible and created as thin as possible stream over one corner of the box. This attempts to pour over the piece and create as few air bubbles as possible. You will still see air bubbles rise to the top while it is curing, but the mold should be free of any bubbles.
Once it has been poured and covered completely, the mold should look like this:
The mold will take 24 hours to fully cure. Leave it to set in a clean and dry environment, do not try to rush it in a refrigerator or by applying heat, it can damage the silicone.
Once the mold had been cured completely, I unscrewed the box and removed the 2×4. It took significant force to open the box, the silicon attaches itself to other surfaces, but can be pealed away with force. Avoid any sharp objects to remove the parts, silicone can be easily cut.
Carefully remove the mold from the bottom composite board and peel away the original piece.
What should result is a 1 to 1 inverse copy of your original piece. Here, because of the unions that were glued on in my 3D part, a thin line of silicon was inserted in the middle of the shape. I used some flush diagonal cutters to cut that away and leave it a smooth surface. This part is now ready for the resin pour.
Resin comes in 2 parts, and is mixed in a 1 to 1 ratio. I measured the volume again with rice, which gave me an estimate of 9.5 ounces. I mixed in 10 oz to be safe, and to allow for any residue I might leave behind in the cups.
I measured out 5 ounces of each liquid, and mixed it together. The resulting mix had the consistency of water and was clear. This resin has a short pot life, 3 min, so making sure everything is completely ready, your molds, your space, is very important. Also this liquid can attach itself to anything and make it difficult to clean. Make sure your using cups and mixing equipment you don’t expect to reuse again.
I poured this mix a little quicker than the silicon, and filled the molds as close to the top as I could. After 3 mins, the liquid begins to cure, and can no longer have any added. The chemical reaction generates heat, which is normal and the silicone was able to handle it without a problem. After 10 mins, the mixture will begin to turn white.
The mixture will completely cool down after roughly 30 mins, depending on the size, and will be ready to use. It will take another 24 hours before it can be machined. Multiple pieces can be made right after each and set to cool together. Once all the silicone molds had been completed I was able to build all the parts to he Hexapod in 4 hours, which were ready to assemble the following day.
The rest of the parts were molded and poured the same as the body plate.
Some of the holes had to be drilled and some excess was cut off using a utility knife. If I was to build this again, I would take more care to ensure that the 3D parts were designed with molding in mind. I would design sacrificial holes in the 3D parts that would allow for the parts to be screwed on to the boards which would be covered when the silicon was poured. This way the mounting holes for the servos would not be covered up and need drilling.
Overall, the process was difficult to figure out at the beginning, but once I understood how the different materials would react to each other, it would be really easy to recreate. I could have saved on a lot of silicone had I known how the 3D parts would float to the top of the silicone. I used about 100 ounces of silicone, where as had I built this again I would use less than half that amount. I used less than 1/2 a gallon of each Part A and Part B to make these pieces, and for my next build I will probably use less than that.
Tips for a Resin Mold
Be patient, pulling the molds too early may bend or break the resin. It may seem like its ready to be demolded, the material may seem rigid enough, but bending can easily occur even an hour after it was poured. Don’t try to place it in a freezer to speed it up. I did that as a trial and the piece became very brittle and broke easily.
The more pieces you can mold together, the more you can save on silicone. I was able to mold part of the bracket and the small plate together. With better planning I could have molded the femur, the small bracket plate, the leg and the bracket servo mount together. This would have saved silicone, time and money. Plan ahead.
What I created was a 1 part, pour over mold. For better detail, a two part mold could have been created. This might require the use of clay as a base for your parts, and can be tricky when parts are as thing as the ones I have been using (5-7mm). A mold release would also be needed to ensure that the silicone doesn’t stick together. I used ease release 200 (http://www.smooth-on.com/index.php?cPath=1226) in my trial runs.
Smooth-On sells trial samples of silicone, they run around $27 and can be great if you don’t know what material to use and want to try out different ones.
If you want to see some samples of each material first hand, AR-Products has different samples of silicone and resin on display. They are located in Santa Fe Springs, CA. (http://a-rproducts.com/)