Week 07: Molding & Casting

This week, I learned the overall molding and casting process. More specifically, I learned setting the 3D tool path on shopbot and mastercam, and molding in Oomoo silicone and drystone.

Rhino Modeling

I am doing a project about the materials of the pyramid in another class I am taking, so I made a simplified version of the stepped pyramid in a two part mold. I designed a 6-layered stepped pyramid and then spilt it in half vertically. I then put 3 registration cylinders on each side of the mold.

Because Jen reminded us to design the wax mold around the available tools and their limitation, I then went from designing 8 layers to only 6, to ensure the lower point of the pocket is not too deep to reach.

Also, because we need to take into the consideration of the width of the tool, I put the registration marks as far as from the other vertical walls, so that the tools will not result in collision. I then BooleanUnion all of the geometry in the model to make a unified object.

For final product, I exported a 3d model of a wax block engraved with the half of the pyramid on each side.

MasterCam

I actually found the obj.txt file online, provided by Denmark National Museum. I then converted it to obj and then imported into Rhino. The mesh imported was extremely detailed, that it has the flaws on the original sculpture. I then scaled it to 40 inches tall and exported as a stl file

1. For 3D tool path, actually only need the horizontal surfaces, the various heights of the horizontal surfaces will take care of the instructions. Because I had all the vertical faces, we had to selected the horizontal ones individually.
2. For 3D tool path, it is actually easier to cut curved surfaces rather than straight surfaces, because we have a “scallop” tool path for them.
3. Whether we can cut the wax to something I planned depended on the limit of the tools we had. We had 1/4 and 1/8 end mills, which means that all of the surfaces need to have at least 1/8 separation. Also, due to the lengths of the shoulder and the flute, we made sure that it is able to reach that deep.
4. For my straight edges, we decided to use “parallel contour” tool paths and the 1/4 end mill on the model, and then adjusting the offsetting distances to closely match up with the shape.
5. For the registration holes, we used the “pocket” tool paths to make indentations. We also set the containment limits to make sure we have the right sizes.
6. To clean up all of the extrusion inside the 2 pockets, we used the 1/8 tool to go over the paths again to clean up the corners.
7. I learned how to calculate the feed rate from the formula on the arch shop website.

8. The most important step is to simulate the process, to see if there is any collision in the animation. One most common collision happens when the drill is retracting and traveling at the same time.

Shopbot

We first put up the right sacrifice table on to the shopbot, and then hot-glued the wax block onto the bed. The board was really heavy. Then we installed the drill bits using the torque helper tool, and then warm up. We first performed all of the 1/4 contours, and then 1/8 for the pockets and the cleaning up.

The process of the end mill going into the wax and get the wax bits out of the block was so satisfying. It was so real when I saw each line of the G code is executed.

During the process, an error was raised, saying that the end mill is not supposed to be used as a drill, but as we knew we were trying to make 3 small indentation, we continued the job.

For the result , we found an accidental cut between the two pockets, which was the result when the drill was moving between 2 pockets during jobs. But this accident could be fixed easily, either by filling it with the hot glue gun, or just break the silicone formed by the mold.

Casting the Silicone

Because I did not cut a funnel to pour the casting material into the mold, I used a knife to cut a small opening on one side of the mold.

After the first casting of the silicone, I realized that even though I thought I forced all go the air in the silicone out, but there were more, probably because I stirred in too much air when I was mixing the A and B. Thus the first casting of the silicone was not that successful.

However, the second casting, with much longer shaking time, produced a much cleaner result, as shown in the image. This contrast really taught me the difference from shaking the mold.

Casting the Drystone

The first attempt was made from the bubbly silicone mold, and with the funnel that is bigger than I possibly needed. And because I was not sure how much should I pour and what would result in a clean top, the result ended up being incomplete at the top. Also, because the registration marks did not hold the two sides perfectly together. Thus we can see that the two halves of the pyramids don’t match.

After the first attempt, I used the bubbly mold to cast 2 single layered mold. Shown below.

For the second attempt, I then had a cleaner silicone mold, so that the side will be definitely smoother. However, I think the funnel I cut messed up the geometry of the top of the pyramid, so the top might not be finished again. But what I learned was to tape the two sides together to make sure the two sides will match up. The photos will be uploaded once demolded.

Next Steps

I also created a 3d modeling mesh to be printed out and also use as a mold. I would love to compare the smoothness of the surfaces of the milled wax and printed PLA.

For my final project, I want to make an optical art installation. And I think I can eventually use the molding and casting process to create a small prototype of a screen. I also want to look into the transparent plastic or epoxy casting.