Over the past few months I have created over a hundred wax models for silver rings. I have not cast them yet as I have recently upgraded to a Neutec J2R-CE casting machine that has not yet arrived. As I was attaching a runner to a model I had a moment of panic, “Am I using the right gauge wax?” I have been using 4.1mm (6 ga. wax wire).
Most of my designs are not very bulky and are connected to the tree with only 1 runner. My question is: what is the ‘standard runner size’ that you reach for when sprueing up a ring model? I understand that “it depends” on the size and design of the model, but what is your ‘base wax-wire size?’ and… should I be using a runner with a reservoir as a matter of standard practice on all models?
ideally, the gate should have the same cross-sectional area as the piece, and the piece itself should have pretty much the same cross-sectional area all the way around. That’s not always possible, so when there are thicker and thinner areas, I always gate to the thickest part of the piece with a gate that has as close to the same cross section as the point I’m gating it at.
The idea is to avoid forcing the molten metal to pass through thinner sections to fill thicker sections. That causes porosity. In other words, ideally, the metal should have the same size hole to pass through all the way from the button to the farthest point on the casting, even if that hole has different shapes as it goes around. If that’s impossible, the metal should always go from thicker to thinner.
I will make vulcanized molds of some of my designs which will determine the gate location for all subsequent waxes. I want so badly to place the gate in a location that is easy to cut, file and finish which is often the shank. Unfortunately the shank is often not the thickest part. Is it common in mold making (with production in mind) to make a few different molds, experimenting with different gate locations until you find the best solution?
In watching youtube videos for some of the large trees made in the big casting houses, it looks like most rings are simply gated at the shank. How do they get away with it?
HI Steve,
The “lolly-pop” sprue on production rings is pretty traditional, because you can get away with it, not because it’s the best from a casting standpoint. The only real advantage to it is it’s quick. Sometimes you get porosity, sometimes you don’t.
My trick when I was doing a lot of ring molds in vulcanized rubber was to get some 1/8" brass rod, and depending on the nature of the master, either solder it on, or epoxy it on, in whatever manner I wanted the final ring to be sprued. Then lead that back to the injection button. Sometimes I’d use 3/16", or whatever seemed right in terms of the thickness and cross section of the piece. A more ‘deluxe’ version would be to use silver rod. Brass reacts with vulcanized rubber, and gives a rough brown surface to the mold. Silver doesn’t, so silver molded sprues are smoother, which does actually make a difference. The other advantage to molding your sprues in is that way you can focus on getting the joints at the model, and in the spure (if there are any) clean, so they cast smooth and fast. Do it once, and you’re done forever. Just shoot and tree.
It’s also handy in that the sprues you need for metal are going to be much more than you really need for wax, so the wax will shoot easier too.
To be honest, I just used heavy silver rod because I had it, and didn’t have heavy stainless. Stainless would work just as well, but be harder to bend. I’d probably soft solder it in place. (stay-brite, tix, whatever. Epoxy as a last resort.)
What solder you use depends on what your models are. Some of mine were silver, some copper, some plated brass. Adjust to taste.
The important part is to make smooth bends, and clean, smooth joints. Following the sprue in from the button as you cut makes things easier in the sense of knowing exactly where you’re going, but harder in the sense that you frequently end up with multi-level cuts. (As in: it’s not just a flat book cut, the sprue ducks underneath, and has to be cut clear, and you need to clear the top of the part too, which leads to some pretty interesting cutting. Make sure you think about that when you build the sprues.)
Make sure you have a strategy for how you’ll cut it before you pack it. These days, I’d take a couple of pictures on my phone while packing, just so I remember exactly how it all sits while I’m cutting.
I’m out of that part of the game these days, thankfully.
The big casting houses can get away with it primarily because the equipment they use is very high tech. The metal temperature at the instant of casting is very accurately controlled, the metal is melted in an oxygen free environment, etc. They also tend to follow the same rules that I described by hollowing out the top portion of the ring or otherwise making the cross section approximately the same all the way around the ring. The shape isn’t important, the uniform distribution of weight is. You will also likely find that if you send them a wax that’s heavy on the top, they will gate it it the thickest place.
Brian’s advice is spot on. A little time spent preparing the model with the proper sprue size and position will pay major dividends over time. It shouldn’t really affect the cutting of the mold too much. Mold cutting is an engineering art form of it’s very own. Physically cutting a mold is easy, doing it properly isn’t. That’s another reason the casting houses can get away with orienting waxes the way they do, for the most part they have very well engineered masters and molds to work from.
Not to steal Brian’s thunder, I don’t think epoxy is a good idea for a vulcanized mold. It’ll work fine with RTV rubber, but I think the forces and temperatures involved in the vulcanized mold making process would break the joint. On the other hand, you never know until you try. If you do try it, let us know how it works.
Epoxy does work for vulcanized molds. I used to do a lot of it, once upon a time. All my magnesium masters were epoxied together. I used 5 minute epoxy.
The epoxy is pretty well dead by the time the mold’s finished cooking, but it holds together long enough for everything to stay in the proper place. It’s actually a good way to put things together that you want to come apart after molding. Also makes it handy to be able to take the master out of the mold in pieces. Cutting an empty sprue channel is a whole lot easier than trying not to blunt your scalpel tip while cutting along a piece of existing metal.
It’s also a good quick way to ‘butter’ in some extra thickness onto a metal master. Some of my masters had hollowish backs, and I wanted a fillet in the rear corners. Pop a little epoxy in there, and it makes a decent little fillet all by itself. So it’s right useful, actually.