I guess most of us realize that molten metal initially freezes as a thin skin and that risers shrink down as they feed metal into a casting. However, I think it is interesting when castings demonstrate those principles quite clearly. I have been noting the shrinkage pattern on an 8" iron straight edge I cast 2 at a time in green sand. The risers that work best on this casting are simple 1.25" diameter by about 1" tall pieces that I place right on the upper surface (the back of the straight edge) . The castings are somewhat blocky. I showed the pulling of one of the patterns in my recent Puppet on a String thread. The pattern seen on the top of the riser looks just like what one would see on a collapsing wet tent supported by a single center pole. The "pole" on the riser is the support provided by the vent I poke to the surface of the mold beginning in the middle of the riser. Here are a couple pics. Denis
I'm sorry , i'm not seeing what you are seeing, have you cut one of those risers open, because it does not look to have" piped" at all
The risers were molded with convex/flat smooth tops. They collapsed about 1/4” folding and wrinkling as the did so. Only the areas directly adjacent to the vents remained at their original height. I have cut these risers off. They are solid at their bases. Can’t you see how they are tented up? The original height is at the level of the junction of the vent and the cone-shaped tent. I figure this riser (corrected prior typo “vent”)donated about 1 tablespoon of iron to the casting Denis Added: 1 tablespoon of iron donated may not sound like much. But, in this casting it makes the difference between a fine casting ready to machine and an ugly casting with shrink defects that relegate it to the remelt pile. It took a few tries to figure this one out. The pattern seemed so simple that I thought it would be a cinch. Ya, not so much...
That's a rather interesting effect, I would have imagined a bar like that would shrink on the top surface and lose a bit off the top, but the cooling on all surfaces means a bit of iron has to be fed at low pressure. I thought aluminium was the only alloy to form a skin easily.
Looks like it fed a little iron to the casting to me. It is usually more of a definitive hollowing out of the riser though... what some people call piping. We played around with riser shapes and sizes alot in the old hand floor molding area and that was always fun.
I have seen riser shrinkage patterns ranging from no evident feeding to very dramatic central shrinking with deep depressions. As you say, sometimes deeply depressed in the center where the riser obviously froze off last. What struck me about these was the "wet-cloth" pattern that is so evident. On this particular pattern with these risers the pattern repeats every time. The way a very thin film of metal must just coalesce to a bright red hot and very soft but still solid state of interest to me. Sure, that is the way it should work----the limp weak film, but that effect is demonstrated in spades here. Kinda cool, that's all. These risers contributed about a tablespoon of iron to the 6 pound casting---just enough to prevent malformation of the casting. Denis
Tells me your process is very consistent. I have seen the same riser look different from the slightest change...even atmospheric changes. We normally had them down to a minimum so those changes would be more apparent on them. Pretty cool!
