Kind of Interesting Stuff You Might Want To Know That Will Make Casting Easier
Before we start this whole process, I want explain my basic understanding the mechanics of casting metal and the physics (basic natural laws) that affect the castings. If you like, you can call them "Don's Principles of Casting". I believe these will help you understand why we have to do certain things to get consistent good castings. At the bottom of the page, make sure to follow the link to instructions for building your own steam caster.
Don's First Principle of Casting
You Have To Have A Mold. In other words, you cannot usually just pour metal into the air and have it make something useful. Of course, if you were high enough, you could pour metal in to the air and it would solidify, probably into small balls before it hit the ground. The air and air pressure would actually be your mold. Or, if you could pour it in to space, without gravity, it might be really interesting and make some neat jewelry. If you pour molten metal into water, the water becomes the mold. You could just look for an interesting place (be conscious of safety), pour the metal in it or on it, and you might get an interesting casting. Whatever you pour the metal on, or in, becomes the mold. So, Don's Principle of Casting #1: You have to have a mold.
Don's Second Principle of Casting
Something Has To Force The Metal Into The Mold. In other words, metal will not jump out of whatever you melt it in and magically fill the mold. The only forces that I can think of that are used for casting art metals for jewelry and sculptures are: gravity, vacuum, centrifugal (or spinning), steam, and pressure. I would like to discuss each briefly.
Gravity is one of the oldest forces used for casting, for all the "natural reasons". We all know what gravity is, right? Well, I do not. Yes, I understand that it is a law of nature, but I truly do not think we know all there is to know about gravity and why it acts as it does. For casting, let me just say that the force of gravity pulls metal down towards the mass of the earth just as it pulls everything "down" towards the mass of the earth. This seems overstated, but if you are gravity casting, it is very difficult to make the metal run horizontally or against the force of gravity. However, when I teach sand casting, I routinely prove that you can make the metal flow up against the forces of gravity, but that may be a whole different article. Types of gravity casting are: sand, RTV, natural, water, cuttle bone, fire brick, charcoal block, and bean casting. (I plan to write a full article on each.)
Vacuum casting is a little more difficult to understand. Consequently, I divide it into two different categories based on types of vacuum casters--one that most small volume jewelers and craftspeople use, and one that high volume casters use. I will be the first to admit that I am not familiar with the latter. These casters are very expensive (in the thousands and tens of thousand of dollars). I believe that the expensive high volume casters may actually use the force of a vacuum to cast the metal, but the small vacuum casters that are under $1,000.00 do not use the force of vacuum to actually cast the metal. Now, before the manufacturers of these machines or all of you vacuum casters out there get too excited, please read my explanation carefully. I believe that in the large casting machines the mold is placed in a chamber, and an actual vacuum is created in this chamber; so a true vacuum is created in the chamber and the mold. The molten metal is then allowed to fill this vacuum, and vacuum casting occurs. However, on the small vacuum caster the mold sets on top of the caster. A vacuum is not created in the mold, it is created inside the caster. I always use the bell jar to create more vacuum. Then just as the metal is poured in to the mold, a switch is moved so that the vacuum creates a suction action that "sucks" the metal into the mold. If I am wrong about this, let me know, but please be very specific about it. I believe that it is not the force vacuum that actually casts the metal in to the mold, but the force of suction that does the casting. BUT, the real force of the suction is air pressure, not the vacuum. In other words, the small vacuum caster builds up a vacuum (void of air and air pressure) within the caster and/or the bell jar. The mold is placed on top of a small hole on top of the caster, and as the metal is poured, a lever is moved. The air pressure outside the mold tries to equalize, as it always does, so the air attempts to rush into the vacuum. It has to go through the mold (investment) as it does this. As the metal is poured into the mold, it gets in the way of the air. The air pushes it into the mold.
Am I right or am I wrong? Any physics or science teachers out there? My science teacher always taught me that if it were not for air pressure and the vacuum that we formed in our lungs, that we could not "suck" our favorite drinks through a straw.
- Special note: At high altitudes there is less air pressure there for less sucking action. This makes it more difficult to "vacuum" cast at high altitudes.
I live in Estes Park at about 8000 feet above sea level--a lot higher than Mile High Denver. This is why I say vacuum casting sucks. It sucks the metal into the mold. I also, believe that gravity has a lot to do with this kind of casting. Think about it; if it was truly the metal trying to move in to a vacuum, we could set the caster on it's side and the metal would move toward the vacuum. I want to be very careful here to make sure that everyone knows that I own two of these small vacuum casters and would not want to be without them. I use them for vacuuming my investment to remove bubbles.
Centrifugal or spinning creates a force that is somewhat hard to understand, too. I know there is a natural law about this, but I really do not understand it myself. We all know that when we were kids we could grab our little brother's legs and spin really quick, around and around, and he would spin out and fly around in a circle. Then, we could let him go and crash him in to the wall. Weren't those days fun? I miss that! Well, that is how centrifugal and spin casters work.
These are actually two different kinds of casters. A spin caster is a casting machine that is motor driven, has large rubber molds and usually is used to cast pewter. The molds are spun and the pewter is poured in to a center hole in the mold. As it enters the mold, the centrifugal force pushes (or pulls, I'm not really sure) the metal to the outer edge of the rubber molds and in to the cavities. The molds are spun until the metal cools and freezes.
A centrifugal caster (sometimes called a broken arm caster) uses this same force, but with a little angle. The mold is placed on an arm that is hinged in the middle and is cocked at a 90 degree angle. When the caster melts the metal, he releases this arm that has been wound up and is driven by a spring, and the arm begins to swing in a circular motion. As it does, the half of the arm that has the metal and the mold on it swings out straight. The centrifugal action causes this. Within the first half turn, the arm has snapped fully straight, and this action throws the metal (by centrifugal force) into the mold. The caster is left spinning for a few minutes to allow the metal to cool and freeze solid. The actual casting is done instantly. Centrifugal casting actually uses a force that is stronger than gravity. It is really important to understand this point. Remember swinging the bucket of water over your head? The centrifugal force actually was greater than gravity and kept the water in the bucket. Interestingly enough, this is how the first centrifugal casting was done. In Egypt, thousands of years ago, centrifugal casting was done just that way. A rope was tied onto a mold, the metal was poured in to the mold, and a very dumb person swung the mold around and around his head until the metal cooled and froze. Even more interesting is that most platinum is cast just like that today. Oh, not by a very dumb person, but by a vertical centrifugal casting machine.
Pressure casting is used in large companies for casting what I call industrial metals--metals like high tech alloys and metals like magnesium, so I will not discuss this method much. I really don't know that much about this method. (I do know that some of these casting machines use a powdered metal and a screw to build up the pressure. As the powder is squeezed, it is melted and then released into the mold).
The only reason I mention it here, is that now and again you will read or hear about a pressure caster built with a bicycle air pump. It always sounds like a Rube Goldberg type of machine, over engineered for what it does. Basically, the pump is used to build up air pressure in a chamber, the metal is melted or poured on top of the mold. The pressurized "caster" is then placed tightly on top of the mold, and the air pressure is released. The air pressure tries to equalize again, and it rushes out of the chamber through the mold. The metal gets in the way of this air pressure and is pushed into the mold. I am sure it works, but if you are not a machinist the caster itself can be hard to make.
Steam pressure is fairly easy to explain and even easier to use. It is definitely cheaper to use than any of the other methods. Steam is created when water, and most liquids, are heated to a temperature at which they turn in to a gas. When water turns in to a gas, it expands. I almost believe that steam might be the most powerful force in nature, after nuclear power. It is hard to beat the force of the sun and a good old atom bomb. By the way, the sun is just too far away to use its nuclear force, and even a small nuclear bomb would probably wake the neighbors. So, I ruled both out for casting purposes.
But STEAM! Man is it powerful! If you doubt this, just remember, the force of some volcanoes. Those explosions are not usually caused by some ignition of an explosive, like dynamite or gas. I believe that most, like Mount Saint Helens, is caused by the steam built up by the ground water being heated by the extreme temperatures of the magma. Again, if I am wrong, science teachers let me know. In any case, we have all heard the terrific stories of pressure cookers exploding, old steam engines blowing up, and sinking steam ships blowing up when the water hit the boilers. The best proof is that the steam engine in old locomotives created enough power, not only to pull cars full of tons of supplies and goods to every part of this country, it literally had the power to build this country. So, even though, we all know what steam pressure is, I hope to explain how a steam caster uses this force to cast metal.
It is important to know that when steam is created, it is a force that is expanding in all directions. It must be controlled. Unlike all the other forces used to cast, steam does not follow a definite direction. To use it, we must find a way to control this force and direct it into the mold, so that as the metal gets in it's way, it is forced in. For now, just understand that we will heat water to the point that it turns in to a gas, steam, in a small contained space. We only give the steam one way out of this contained space and that is through the mold. We melt the metal on top of this mold so that the steam, in its attempt to escape, pushes the metal down into the mold.
These methods all have their place in the metal working world, and later articles will expand upon some of them. For now, let us return to the principles of casting.
Don's Third Principle of Casting
All Metals Do Not Cast The Same. It is important that you know the basics of metals. I taught metals for most of my 13 years as a junior high school teacher. Then the last three years, I taught art. I teach all my classes as a metals teacher, not an art teacher. I believe that if you understand metals--how they melt, bend, react to heat and oxygen, and all that boring stuff, the better silversmith and silvercaster you will be.
The next installment of the steam casting class will be how to build your own Steam Caster.