Week 7: Molding and Casting

This week's task was to mill wax mold to produce a silicone mold and cast the object in a material of our choice.

3D Object

Before milling the object, we needed a 3D model. I modeled an Oloid in Rhino, an object I have 3D printed and cast in aluminum in the past. I had never milled wax before or created a silicone mold, so I was curious to try producing the same geometry I had produced in the past with this new process.

To prepare a two-part mold we need to split the model in half and give the geometry a way to be filled (the sprue) and for the two halves to attach to eachother without leaks. Our section TA suggested using spheres to lock to two-part mold in place. These aren't ideal because the molds can still slip, but I went ahead with their suggestion anyway.

Milling

In MasterCam, I set up a milling gcode file with 3 passes. Jen provided a template for this. I had milled using the Onsrud and Prototrak machines before in N51, but had never used the Intelitek in RPL, which is rarely used and was available.

The machine should be zeroed manually with each endmill before starting each cut.

The safety guard should be put on the machine as milling begins.

The first pass seemed to be going well until the machine started making loud and unusual noises! I hit the emergency stop and saw that my wax piece, which I had hotglued to the base plate had come loose.

I tried again with a new block. About 75% of the way through the milling job, the job suddenly stopped. I wasn't sure what happened, everything seemed fine on Mastercam, but the computer running the g-code read an error "I Is Not Valid". After unsucessfully trying to start the job again from the same place several times, I recorded the place in the task and left emailed Jen for help, but I would need to wait till the next morning.

It turns out that the intelitek doesn't understand "arc" movements that Mastercam generates by default. I recreated the g-code file, while turning off the "arc" setting in mastercam and tried again. This also was a good moment to make sure the milling process was optimized for speed, completeing one side of the two-part mold before movoing on to the other side, instead of going from side to side, which takes a lot longer.

Things seemed to be going well. I changed endmills and started the second pass.

When completed I vaccumed the wax away (wax waste can be reused, but RPL doesn't produce enough wax for there to be a system in place for it to be recycled.

To my dissapointment, it looked like the endmill cut way too deep into the endmill and destroyed my wax mold. It turns out the endmill has incorrectly calibrated in Mastercam...a mistake I could not have foreseen.

It took a few more days, but ultimately, Jen helped fix the error and I got the succesfully wax mold. I didn't get the chance to cast with the mold as I had to move on to the next assignment and work for other classes, but this was good practice in milling -- I feel totally comfortable with the casting part of the assignment.

Bonus: Casting Seaweed Bioplastic

Concurrently with this course's weekly assignments, I had been developing a bioplastic material made from seaweed and starch. This is a tricky process, producing a material that has its own structural integrity and consistency from cast to cast.

Two key tips I learned from this class proved particularly useful. 1) Silicone is an excellent non-stick and temperature resistant material for casting, and 2) a little heat can accelerate the curing process of a cast material.