Some of the misfires are included in this post. Notice in the photo of the ingot, that the metal actually expanded rather than contracted. This is due to overheating the metal. We were also trying to pour a thin shell, and went overboard. Whoops. It was right at 2150degrees which is the textbook upper limit; perhaps this is not so!

The centralized pour cup with radiating runners allowed hot metal to fill into all areas of the mold cavity at once, and then allow these gates to freeze off first due to their relatively small diameter. This sets up nice directional solidification from the thin sections or tips of the casting towards the base, and from the center of the base towards the thick filleted sections, isolating the rise location. The risers fed these thick sections nearly perfectly, which are those black holes shown. The mistake was not having the risers on top of the casting rather than the side, which left small shrinkage depressions next to the risers on their topsides. We’ll move the risers from the sides to the top sections next time and should have a good gating design for better castings.

The surface quality and chunky flashing was the result of the pattern’s poor releasing, and the thickness of the plaster of paris we used on top of the styrofoam. The mold actually broke when we closed it because the pattern flexed when we rammed the sand mold. These things really hurt the surface continuity and texture, but it is only cosmetic. The part should fit just fine and polish up nicely, it just adds a lot of work chasing the casting. Milling the sand molds in the future will eliminate these issues!

A couple of things to notice about the casting ::
~This mold would have worked perfectly if we had opened those risers as they were intended, meaning the gating design was correct.

~Because the mold surfaces fit together so nicely, there is very minimal flashing at the parting line – this equates to less work chasing the casting which is great.
~The surface quality pretty much directly reflects the path of the milling tool, so if they are tightened up, we’ll have relatively smooth surfaces.
~The risers actually fully solidified and forced the casting to feed them, indicating that they were not large enough to feed sideways, and were of course not vented as is necessary for a ‘blind’ riser or a riser that doesn’t open to the sky.
~The flashing is minimized towards the ends of the arms, indicating that either the mold was better pushed together at those points (which is possible because the straps were located near there), or that the metal had lost heat and become more viscus by the time it reach the extents of the arms. This is an indication that we were probably pushing the limits of how far the bronze can travel with only two ingates in a bottom-gating scenario.

All and all a good casting experience, and affirmation of our sand-milling process!

The process has worked absolutely fantastically well so far. The resolution seems to be entirely acceptable for castings of this scale, and is a function of the amount of machine time or tightness of the toolpaths one would like to invest. I’ll probably tighten up the toolpaths a bit in the future to minimize the amount of finishwork necessary to get them smooth, in addition to the experimenting with finer mesh sands in order to tighten the overall grain of the mold surface after milling. Because the sand is not rammed against a pattern, it tends to be more porous for better or worse. As shown this is a 120mesh sand equivalent Olivine sand, which is much safer to have airborne than the silica sands which are proven to cause silicosis after prolonged exposure. Ick.

It looks like we’ve figured out a really efficient and cost effective way to produce these floor molds with a minimum of waste. I’m in the process of redesigning the arms of the machine to limit exposure to the sand flying around, and covering up the electronics and computers to keep the sand from getting into everything. The other big challenge is lifting huge molds from the foundry downstairs up to the cnc machine, but it looks like we have some solutions on hand for that as well. What great promise!

The only caveat is that you only get one chance to cast it right :: if you get the gating wrong and the casting has defects, you have to mill another mold!
Styrofoam molds tend to be one-shot also however, and it would probably take 3 or 4 mis-casts to make it a wash with the material and labor costs of producing a traditional wood pattern…

Mike’s friends had brought a giant bull ladle up from San Diego specifically for the task of pouring this mold. We needed about 400-500# of liquid hot iron to fill it’s cavity, and the cupola furnace can only produce 120-150# of metal per tap. This means the only way to fill up the mold is to keep tapping the furnace and pouring it in the giant ladle kept hot with a giant propane torch. A couple of things happened during that thunderstorm – we were low on propane and so had to take the first 4 taps out of the furnace which tend to be colder, and we were having trouble with the torch at one point… and together our efforts were not enough to keep the metal hot. As we poured the metal it was so cold it froze up almost immediately, preventing the cavity from filling properly, and entirely. Testament to this was a giant slug of solid metal at the bottom of the ladle, which had solidified prior to ever having tipped the lip. It was hard to see what was going on through the rain, sweat, and chaos; it was a great loss.

Mike and Tom called the pour off after seeing a few of the extension cords runnign theunderwater and the yard ponds had combined into one big lake.

Special thanks to Eleanor Pries for taking these awesome photos.

What are the implications for Shireen the woodenboat and making stuff in general? I’m not sure I know the complete answer, but to start: all the work and materials required for those last two blogs posts – styrofoams, trupan, shellacing/plastering, hand sanding, parting agents agents, molding processes, etc., – potentially zillions of hours of work to complete the ongoing floor project, could be forgotten. Draft angles would no longer be relevant. Anything the machine could mill we could cast. Molds with complex parting lines or many parts will register with machine precision. All off the geometry will be machine tolerance. It is awesome to consider, and a pretty exciting day in the shop. Lots to think about here. I’ve been sort-of tiptoeing around sharing this because it is just so good, so impossibly good…

“Could this really work???” – Susan U.