Product Realization Lab
Product Realization Lab Design Division, Mechanical Engineering Terman 501 MC 4021 Stanford University Stanford, CA 94305-4021 USA prl.stanford.edu
Carissa Carter SteamĂŠ
Carissa Carter developed Steame’ at Stanford’s
Product Realization Lab during her first year as a student in the Joint Program in Design. Conceived as a final project for ME203, Carissa explored silicone casting in order to bring her idea to life. The main take away for Carissa while making her steamer was discovering the power of an iterative design process. Her project was as much about the process as the final product, and so she spent a lot of her time in the initial phases. Carissa told us, “With each new mold I made, casting I poured, rod I milled, and tube I bent, I improved my technique and figured out the details that would make my project work.” Like many of her classmates from the Joint Program in Design, ME203 was Carissa’s first engineering class, and the PRL served as a gateway to the world of product design. “Because I used many of the PRL resources while working on Steame’ I came to see the PRL as a manufacturing and prototyping resource throughout the year. With each manufacturing challenge that I faced, it was great to be able to draw from the collective knowledge and perspectives of the TA’s and instructors.” The following case study is a transcription of a conversation we had with Carissa.
Where did you get your idea for Steamé? I decided I wanted to do something in the kitchen, so I did a mind map about things I liked about the kitchen. Broccoli came out of that. It was either that or nunchucks. I did a lot of thinking about broccoli and steaming broccoli and what you could do with it. I started doing lots of sketches exploring form and function. I gave myself permission to do lots of random thinking. Then I came across this shape with the handles. Originally I wanted to be able to make the steamer invert. The silicone would pop up and the the steamer would stand on the handles so it would be possible to cook rice underneath. It was a great concept, but it went away during development for a few reasons. How did you choose a material? One of the things which I find dissatisfying about steel flower formed steamers is that it is so hard to get the steamer out of the pot after you steam something. You have to put your hand way into the hot pot, you are in there in the steam, and you can burn yourself. This is not a very satisfying user experience. The whole premise of Steamé was to make a steamer so your hand wouldn’t get hot when you took it out. Silicone was a good answer to that.
How did you pick the type of silicone you used? I used 940 because it is food grade. Marlo Dreissigacker, a TA in the PRL, taught me about it. I did tests with both 920 and 940. The 920 probably wouldnâ€™t hurt anybody, but the 940 is technically approved. The 940 is a higher durometer and had the stiffness that I needed as well. What was your first challenge in developing the mold? To begin with, I made a smaller version of SteamĂŠ to begin testing the casting process. I made a positive and a negative out of renshape, which I turned on the wood lathe. Renshape is a great, forgiving material, so it was a pleasure to turn it. My first real challenge was developing a technique for putting holes in the silicone. I started by randomly putting various sized pieces of Sculpy modeling clay on the mold. I wanted to see how different size holes worked. I poured silicone into the negative mold, shmushed the top on, clamped it, and put the whole thing in the oven. When it came out, it worked. But I had to find a way of making regularly placed, clean holes.
I experimented with steel mini dowel pins. I drilled holes in the negative side of the mold and press fitted the pins into the form. When I pushed the two sides together, I wanted to see if the pins made contact with the positive side and left holes in the silicone. Some of the holes came out great, and some not so great. Also I learned that there was a big variation in the thickness of the silicone wall. It wasnâ€™t due to the mold geometry, per se, but happened because I didnâ€™t have a way of registering and spacing the two pieces of the mold. The real take away was that the next prototype needed a way of registering both halves exactly an indefinite number of times. I tested different materials for the pins, and I used steel dowel pins in the final version, in order to get the cleanest perforations in the silicone. I eventually did end up with a good silicone casting that had nice holes in it, but the shape was a bit off. I knew I had to make a new, clean mold and start fresh.
I designed a spacer in SolidWorks and cut it on the LaserCam to insure that the positive and negative parts of the mold would give me the right wall thickness. I used the spacer to gauge the curve on both the bottom and the top of the hemispheres, and to account for the space in between. I held it against the renshape as I turned it on the lathe in the model shop. Another thing I found out was it took a lot of practice to separate the mold correctly. Initially I didnâ€™t use mold release, because I didnâ€™t know about it. It took lots of energy to separate the piece from the mold and the mold degraded. Of course, I needed the mold to stay sound so I could use it over again. So I had three problems to solve coming out of this iteration: separating the mold, keeping consistent wall thickness, and getting the holes just right. How did you pick the color? With each test I changed color. It was sort of a two for one prototyping, iterate on it while exploring color. This one I added dobs of yellow to the blue, to see if it became more interesting. The red ended up being the cleanest looking color.
What was your next iteration? I was still primarily concerned with getting the pins right, so I focused on that. This time I used the bolt-hole pattern on the mill to make the holes for the pins. Again, Marlo told me about it. She was amazing. I wanted to make holes for the pins to mate into, to solve the problem I encountered with the pins just pressing against the side in the last prototype. I used 3/8â€? plastic dowels this time. The tricky part here was lining up the curved surfaces. I had to think about that. I planned to have two rows of holes, but I did each row on each piece separately on the mill using the bolt-hole pattern. I thought I had it fixtured just right, but the slight difference due to the re-fixturing of the part was enough to make them incompatible. I knew that in the final round it was a one shot deal. I had to be finished in one shop session, so I had to have my process down. It was in this iteration that I experimented with the two types of silicone, the 940 and the 920. 920 is softer. I knew I was going to make a bigger piece in the final version. This iteration helped me determine that 940 would give SteamĂŠ the structure it needed.
After that I was pretty much ready to go on to the final version. How did you choose which size hole to use in the final version? I ended up with two sizes of holes: quarter inch near the bottom of the strainer and for the handles to go through, and half inch near the rim of the bowl. One reason was those sizes were available from McMaster-Carr, so I knew that I could get them and finish within the time limitations of ME203. Another reason came out of making a CAD model. The CAD model made it really easy to visualize how the pattern would look in the finished product. For example, I could make the holes radiated from the center of the hemisphere, or they could all be parallel to one another. Originally I had the idea to make them come out radially, but when I did it in SolidWorks, I saw that if I made the holes straight down, the holes would give the illusion of being bigger as they got closer to the circumference. In reality, it ended not being so dramatic, but the sense of expansion is there.
This iteration was the full sized version. I started by making a full sized spacer in SolidWorks and then cutting it out of acrylic in the LaserCam. I turned two large blocks of Renshape on the wood lathe, using the spacer as my guide. I hand-turned the parts on the wood lathe, rather than doing it on the metal lathe, because I had previous experience turning wood and knew I could get the best results with the time I had to work with. It came out great. I made one version of the basket before I cut the holes in the form for the pins, just in case the whole thing imploded. In case something went wrong, I would have a basket and I could just post-cut the holes if necessary. It was getting near the end of the quarter and I needed a final product no matter what. The funny thing is, it came out wrong! I poured the silicone, put it in the oven, and let it cool. I got a bit trigger-happy and I opened the mold and it hadnâ€™t yet set, so I shoved it back together. It cured eventually, but came out really gross. I hadnâ€™t anticipated that the curing time would be so different in the larger version than the smaller prototypes.
The smaller versions cured in about an hour and they came out fine. I let this one cure for four hours, but that wasnâ€™t enough. It gave me a benchmark for doing the next one. I ended up putting the final one in the oven for eight hours, and letting it cool overnight. I didnâ€™t want to take any risks at his point. They were probably cured before that, but I wanted to be safe. I cut the first hole in the Renshpe by eye. I wanted to see where the hole would be on the actual mold, rather than how it looked in my drawings and CAD. So I eyeballed the position of the first one and then used that as my gauge for the others. I also got the mold release protocol down right by this time. It took about six times to work that out. I ended up lightly spray painting the mold first to fill the surface. Then I used mold release. It gave it a very smooth surface and the silicone pulled away very easily.
How did you design the handles? I made a little tub and caddy to see if the silicone would cure and stay on the steel handle. That was the easy part. From there I made a mold that was the shape I wanted. The tough part was shaping the steel. I did about ten iterations in order to get the handles right. I had to mill each of the joints, and that took a long time. The big deal with the bending was that the tube bender has all these pins in it, but there are no guides on it. I had to bend all these pieces of stainless steel to be exactly the same. So I had to experiment to find where I had to put the piece to get the bend to where I wanted it. All the joints had to be milled before bending, because you canâ€™t bend first and then mill, so it had to be right the first time. I made lots of measurements and ended up bending the same bends in the different pieces at the same time in order to avoid confusion.
Steame’ process overview: Approximate project time: 120 hours in a ten week period Materials used: Type 303 stainless steel rod 6’ Smooth Sil 940 Smooth Sil 920 assorted washers assorted nuts and bolts 1/2” dowel pins 1/4” dowel pins 1/8” dowel pins 1/16” dowel pins Sculpey wooden and plastic dowels Renshape Cost of materials: silicone, coloring, mold release: stainless steel: hardware: Renshape: generously donated by a design company estimated value: Other prototyping materials: Total cost of materials:
$70 $20 $40 $200 $20 $350
This book describes the iterative prototyping process I used to create Steame, a new vegetable steamer. Photography, interview, and layout...