I had a little bit of time today and the weather was relatively nice so I thought I'd try making the greensand mold for the taillight housing. The first thing that was apparent is that my flasks are just a bit too small for this. Originally, I was sizing them specifically for this part but my last minute changes of adding a strip of molding (too better hold the sand), plus some minor tweaks to avoid wasting wood, led to the working area being a little smaller than I intended. I always seem to do that.
There are a couple of things that I immediately know contributed to this failure. Number 1 is that before making the pattern, I knew nothing about draft angles. For the those like me who do not know, the pattern is supposed to have a slight angle (less than 5 degrees) on any surface that contacts the sand and is not parallel to the parting line. I.e. if the pattern has to rub against sand when being removed, it should have a slight angle to reduce friction, making removal easier. As I said, I did not know this and built the pattern with the walls perpendicular to the back and rim.
The other item, and which further compounded the lack of draft angle was the interior surface of the pattern. Since the pattern was cut in layers using the laser cutter and then glued together, there are slight irregularities between layers. On the exterior, I was careful to sand and fill them until perfectly smooth but on the interior, I wasn't as thorough. This led to an uneven surface (albeit slight) which gave the interior sand places to grip.
My plan is to rework the pattern to clean up the irregularities while keeping in mind draft angles. If that doesn't make the mold come out any better, I'll most likely switch from greensand to the lost-wax method of casting.
It seems like a jump but I'm already working with the wax method on another casting project I'm doing. I haven't made any posts about it though because it's a test for someone else. If it turns out okay (and I get their permission to share), I'll do a write up on my process.