Week 08 - Mar 7th 2012 - moulding, Casting and Composites
Weekly Assignment - Design and fabricate a moulded object
The basic brief this week was to make a moulded object. This requires three physical processes; milling wax to create a positive mould, casting latex to create a negative mould and finally casting the desired object from Hydro-Stone. The design constraints, given the equipment availability and number of people working on this task, were to create a small single-sided mould with a cutting depth of at most 20mm (the max cutting depth of the router mills we were using).
I wanted to design a small pot, comprising two identical sides that would fit together. I designed this in OpenSCAD as it was a fairly simple geometric design and I wanted to gain experience with this program.
I exported this model as an .stl file and loaded it into the PartWorks 3D software to create roughing and finishing toolpaths to cut the wax on the Shopbot as experience within the lab was showing that progress on the the Modela was very slow. Unfortunately during the process of the roughing cut I managed to break the roughing mill twice - once through an erroneous keypress while zeroing the mill and once right at the end of the roughing cut, as the mill hit one of the screws securing the wax to the mdf scrap that was secured to the cutting bed. At this point it was time to rethink my plan.
The reason the mill hit the screw was my fixing margins were too tight, however another issue is that the PartWorks 3D software only works in rectangles - a lot of wax was being cleared that wasn't necessary to remove in order to cast the latex in the next step. Playing with the software, there appeared to be no way to not mill the entire rectangular void surrounding the .stl object. Therefore I went back to OpenSCAD and redesigned my parts as cutouts within a solid rectangular block.
Loading the resulting .stl file in to PartWorks, I was able to create roughing and finishing toolpaths that only clear a small amount of material around my desired components, leaving a lot more room for the fixing screws. I will re-cut the wax and finish this assignment in the coming weeks, hopefully without any more costly breakages!
Update - task completion
My second attempt at milling wax on the shopbot was much more successful. Using the modified design above I used PartWorks 3D to create roughing (0.125" endmill, speed 3k, feed and plunge 1 inch/sec, step 50%) and finishing (0.125" endmill, speed 8k, feed and plung 1.5 inch/sec, step 5%) toolpaths - as I had no curved surfaces I could use the same endmill for both. I set the model 2mm below the surface of the of the wax to provide a well for the latex. The finished mould is shown here:
and the resulting latex mould here - I shear-mixed the latex/setting agent mixture for a long time and poured carefully from one side, resuling in a mould with no bubbles:
and the resulting products, cast in hydrostone (one small bubble, which could have been avoided by tapping the mould once the plaster had been poured in):
Doing this assignment, I gained more experience with the shopbot software and hardware operation. I also completed my first moulding exercise, and was suprised by the high fidelity of the the final product - the toolmarks from the original wax mould can clearly be seen on the sides of the final cast objects. This process would be a much more cost effective alternative to 3D printing for large runs of (fairly) simple objects.