Week 11: Molding and Casting¶
This was an exciting week, as it presented an opportunity to experiment with 3D milling operations, mold-making, and casting, none of which I had ever attempted before.
Apr 14¶
Neil gave an overview of molding and casting techniques, all of which were interesting, but I decided that I wanted to focus on working toward a goal that I have had for a long time: fabricating the enclosure of my pill case using injection molding. This is a very lofty goal, only made possible by my access to a few key machines: a Tormach PCNC 440 CNC mill capable of milling aluminum, and a Galomb Inventor desktop injection molding machine. In order to achieve this goal, there are several steps I would have to take before I could manufacture the finished part. First, I would gain experience with the Tormach and its software by milling a one-part wax mold. Next, I would mill a small test part on a scrap piece of aluminum extrusion. Then, I would replicate this design on a pre-made aluminum injection mold blank, like the one pictured below.
I could then use the desktop injection molder to test the injection molding process and the mold design. Once all of these steps are completed, I could mill a mold of my pill case enclosure on the Tormach and use the Galomb to create an injection molded replica of the part!
Apr 15¶
Green Labs Recycling is a company that collects and processes pipette tip boxes from biological labs in the Boston area. On April 8th, I had a virtual meeting with David Waterman and Serena Monteiro from Green Labs to discuss their company, my project, and how I could go about processing pill bottles to be injection molded into pill cases. I was particularly interested in meeting with them, as pipette tip boxes and pill bottles are both made of polypropylene, so the they have similar considerations for recycling. On April 15th, they dropped off 8 lbs of shredded pipette tip boxes at Dassault Fab Lab. This material will be very useful to test the injection molding process.
This bucket of plastic was immediately useful during our local mold-making tutorial, as I was able to fill the test mold cavity with plastic shred, then pour the shred into a graduated cup to determine the amount of silicone required to fill the mold.
The mold-making tutorial used Smoth-On Oomoo 25. After the demo, I reviewed the Material Safety Data Sheet. This data sheet stated the obvious: do not get in eyes, do not ingest. It also sais to store the product in a cool, dry, well-ventilated place. The material should be handled with gloves and eye protection, but a respirator is not required. The material is not carcinognic.
After reviewing the MSDS, I set about to get the Tormach PCNC 440 set up to begin cutting my mold.
The machine has an emergency stop button mounted to an enclosure that’s attached to the machine by a thick cable.
The power switch to supply power to the spindle and motion table is relatively hidden from sight on the right side of the machine.
The machine also has a pendant that allows the user to control the motion of all three axes with gross and fine adjustment dials.
On top of the machine, the spindle has a swinging arm that can be engaged to lock the rotation of the spindle so the user can untighten a nut to release the tool and collet.
Apr 18¶
I designed a relatively simple mold that I would cut out of machining wax, then cast with silicone. I decided to make my first attempt at a “maker coin” by utilizing some work that I had done previously. I took the STL file I generated from scanning my head in Week 4 into Fusion 360, flattened it, and positioned it on top of a two-inch disc.
I then modeled the block of machining wax I would be starting with, and modeled the negative of the silicone mold that I would be using to cast the maker coin.
The CAD and CAM for the part can be found here.
Apr 19¶
I followed a basic Fusion 360 CAM tutorial from Lars Christensen’s YouTube channel. I downloaded the Tormach tool library from the Autodesk CAM site. I then modified the feeds and speed for the 1/8” square end mill based on information from Bantam Tools
Once I dialed in these settings, I generated the roughing and finish tool paths for milling the maker coin.
I loaded the block of machining wax into the Tormach.
I used the height gauge to set the height of the 1/8” end mill in the Tormach software.
I then input this value to the Tool Offset value for the corresponding tool number in the Tormach software.
I then used the pendant to zero the axes of the mill.
After I zeroed each axis, I typed in the g-code command “G0 X0 Y0 Z0” to bring the tip of the tool to the origin and ensure that the machine was properly calibrated.
Success.
I then inserted my USB drive into the computer and loaded the g-code into Tormach
The g-code file must be moved from the USB drive to a folder on the computer.
I then pressed “Cycle Start” on the machine.
The roughing pass went smoothly… or should I say… roughly.
The finishing pass also went without a hitch. I was impressed by the surface finish of the part.
I then poured Smooth-On Oomoo 25 into the finished wax mold.
I placed the mixed silicone into the degassing chamber.
Once it was degassed and allowed to cure for about 90 minutes, I removed the silicone mold from the wax mold.
Apr 20¶
I received epoxy resin and mica powder pigment in the mail so I could pour a casting into the silicone mold.
I mixed the two parts of the resin in the paper cup with the popsicle stick, and mixed in a pinch of pigment. I poured the mixture into the silicone mold.
After 12 hours, I removed the casting from the mold, and held my first resin coin!
