It was SHRINKAGE!!!

[blacksmithn]

To steal a line from George Costanza. After a brief hiatus from casting due to the ridiculously hot weather, I got back to work and cast some scale tires. They came out pretty nice:



The only trouble I had is the shrinkage I experienced on some of these castings:



Is this a completely normal phenomenon? And is there anything I can do to either keep it from happening again or to keep it to as little shrinkage as possible?

[RageofAchilles]

Some resins don't behave well over a certain thickness, the heat of the reaction causes evaporation and results in shrinkage in an open mould and expansion in a closed mould. Another addict had a similar problem recently I think. try pouring in small amounts to build up the tyres in layers rather than one resin dump. 3 separate pours per tyre should help.

[mangozac]

Can you remind us what resin you're using?

[blacksmithn]

The Specialty Resin 2 minute resin, in this case. I had almost a quart left after my first flat car attempt and I found the quick stuff was just perfect for a small job like this. I also discovered that I can avoid the shrinkage problem if I add an additional drop of resin once I fill the individual tire mold. Just a drop. Add more and it runs all over, of course, but just a drop and the surface tension seems to keep it all together. Any protruding excess can be easily removed after setting by running the back of the tire over a piece of sand paper laid on a flat surface.

[mangozac]

You've got it sorted! Don't need us at all

It does sound like that particular resin product has a high degree of shrinkage though. The rare times I've done open top moulds I've tended to have the opposite problem - moisture tends to make the resin foam/expand somewhat...

[blacksmithn]

Well, keep in mind that I'm doing my casting outside in in Southern California, where it is absurdly warm and, at this particular point, not humid at all. So, picking up airborne moisture isn't a problem. I do have to do most of my casting in the morning, though, before the heat hits. When I've tried casting after the temps go up, my resin (which, as noted, is fast anyway) sets way too fast. How fast? Well, it's the difference between being able to pour one mold set of 8 tires and 4 dual rear tires or 2 such sets.