I spent the morning working on casting the bomb pieces. The castings look a bit like short run model parts (i.e. need a lot of cleanup), but they should work out okay.
I started out by making the master. Initially, I tried modifying an Airfix bomb from my Hampden kit, but that wasn't cutting it. So I changed my approach and found a piece of sprue rod that was near the correct diameter of the bomb at it's widest part. I kept the sprue longer than needed and then chucked it up in my motor tool. Running the tool on low RPM and taking my time to avoid overheating the part, I shaped it to match the template I drew out yesterday.
Then I started working on the mold. I had originally thought about using thermoplastic which is ideal for a single pressing, but because it cools too fast, would not allow me to make multiple pressings of the master. I wanted to make a mold of five halves connected together by a runner in order to make it easier to handle and to flatten the bonding surfaces of the bombs by sanding. So I used some modeling clay I have and placed it in a small receptacle made from upside down Lego blocks.
The reason for the Lego (and the clamps) was to give the clay some support and keep it at a consistent height when pressing the master into the mold. The Lego blocks also allowed me to add height adjustment runners so that I could press each mold to the same depth.
To hold the master while pressing, I took a piece of acrylic rod and flattened one edge so that it would rest on the runners at a square angle. Then I drilled a small hole in the master and inserted a straight piece of stiff brass wire. A slightly larger hole was drilled through the acrylic rod and the wire was insert through this hole but left loose initially.
I took some measurements of the master diameter and the distance between the bottom side of the acrylic rod and the top of the mold. Halving the diameter of the master gave me the measurement I needed to set the distance between the master and the acrylic rod. I used spacers set to half the height of the master to raise the acrylic rod and let it rest on the mold surface. The wire was then super glued to hold it at that position.
Removing the spacers allowed me to start pressing the molds. First I used a piece of .040 inch plastic to press a runner in the mold, then used my master tool to press five pressings of the master, giving me 5 half molds.
Once I was satisfied with the molds I took some low viscosity UV cure resin and tinted it using Tamiya clear blue (any color would have worked). I used a small pipette to add resin to the mold pieces (and the runner) and then cured it with the UV lamp.
Removing the parts from the clay didn't work so well as the clay is quite sticky, but I did get the parts out without damaging the parts, but the molds were no good. I had to clean off clay form the molded parts using IPA.
I remade the mold after fixing the clay and molded a second set. These were also cleaned up. I then sanded both castings on a flat sheet of sandpaper on the bonding surface. This flattens the bonding surface and made it easier to remove flash on the bomb part sides.
At this point, I am ready to do the final clean up of the parts, remove them from the runners, assemble the halves and then work on the seams. Afterwards I can focus on the bomb fins and fuses. I'll post that when I get to it.
