Casting hot liquids

Casting is a process where a substance is given a form via a mold. We could divide casting into two very rough categories; one which destroys the mold, and one which the mold is not destroyed. I’ll be talking about two basic level casting processes, the evaporative casting and sand casting, to give you a peek behind the curtains what goes on in industries at large. The two examples are a form of metal casting.

Low level casting can be easily done with sand casting sand, which is a combination of fine sand, oil and clay kind of substance. The sand is rather crumbly at first, but after giving it some pressure, like forming a fist around it, makes the sand malleable much like Playdough. With this sand you can do open casting, sand casting, evaporative casting and few other casting methods I won’t go into. It’s relatively cheap material as 95% of the sand can be used again after the casting has done. This is because the liquid metal burns the sand around the mold. The sand becomes hard, black and can’t be used again because it has lost its malleable properties.

Evaporative casting is rather inefficient method of casting. In the process a prototype is made of easily flammable and quick burning substance, which serves as a reverse mold of sorts. The details in the mold is preserved in the cast, so one should always take care that the surface of the mold is as desired.

Sand casting is similar to evaporative casting, but the prototype is made of wood or other similar substance. The key difference to evaporative casting is that the prototype is not lost and can be used again and again for molds. Sand casting is superior method evaporative casting in almost every regard, especially if one wishes to mass produce a certain shape. Sand casting preserves the details on the prototype much like evaporative casting. However, due to the nature of sand, both of these methods are usable only on objects that have no escape corners.

The both methods of casting have nearly identical preparations. The casting case comes in two parts; upper half and lower half. The lower half is first filled with sand casting sand, which is stamped down with a hammer or similar tool to ensure that it forms a tight base. Depending on the size of the prototype the caster has to leave some space to the upper level of the lower case, as the middle line of the prototype has to correspond with the joint line of the casting cases. With evaporative casting the upper part is the put on and the prototype is covered with the sand to the uppper limit. Of course, a sprue has to be left either next to the prototype or on top of the prototype for the metal to run into.
In sand casting before attaching the upper half, the prototype is removed and the surface of the sand is covered in any powdery material of sorts. Talc or the like should be enough in a hurry. Then, the prototype is put back and covered in sand again with the sprue. After stamping the sand down, the halves are separated once again so that the caster can remove the prototype from the now-formed mold. The halves then are attached to each other and fastened for casting process.
I was going to post a reference drawing, but google has so many good examples that it wouldn’t served a purpose.

The casting process itself is quick and rather safe. The desired metal, in this case aluminium (Al) is heated up enough so that it turns into a bright liquid. Personally I prefer brass and copper over aluminium due the two being much heavier and thus they fill the cavities of the mold with more confidence. Aluminium barely has weight to burn through evaporative casting, and even then the temperature of the metal is the key.

Naturally, this is a hazardous operation. Liquid metal is a toy to plaid with. The temperatures with metal casting range from 650°C all the way several thousands. It burns through clothing and can cause extremely severe burns on the skin, and can even melt Terminators. Certain safety procedures are a must in any case. Safety shoes, inflammable clothing, inflammable gloves and safety visor. Minimum. With caution there’s no danger, and the only truly random factor is the occasional splashes from the liquid metal if colder temperatures are introduced. For example, adding more material to the already heated metal can cause a splash effect in the material is not preheated. In a small kiln there are possibilities of splashes simply due to cooler air introduced to the surface.

The process of casting itself is straightforward; pour the liquid metal in. This can be divided into two parts again. The first small pour is to give the metal some leeway. In evaporative casting this first pour is to burn parts of the prototype. In sand casting it is to drive out some of the excess gases. Very soon after (in matter of one or two seconds) the rest of the liquid metal is poured into the sprue all the way to the top of the sprue. Then the wait game begins and we can start to wonder whether or not the casting was successful.

In evaporative casting the prototype is now lost and the metal has taken its place in the sand. Depending on luck, the cast can have bits and pieces of the burned prototype, bubbles or the metal hasn’t burned all the way through. In sand casting the same things may happen, except the prototype burning. However, the metal might have only gone halfway through for some reasons like pressure, underheated metal or gas pockets that have no way to escape.

The excess metal is then poured out into an open mold.

The lump at the very end of the video is mass of impurities. You’re supposed to take those out of the liquid metal before casting anything, not after

The cast is then allowed to cool down on its own, as quick temperature changes may actually break the cast metal. There are occasions where the metal may be cooled down, but it includes risks. After some 15~30 minutes or more, the cast metal is taken from the sand. The cast metal is then handled with various operations, starting with the removal of the sprue’s residual shape, cleaning, grinding, sanding, and polishing. Not in this order in some cases.

With these casting methods we can create many products from medals and plaques to basic knife shapes and rough jewellery. For example, the plaques in old buildings are sand cast or done with with similar casting method.

If you stop thinking for a second how many different casting methods there are the answer is in hundreds. Most of them are separated by the cast material or slight difference in methods. Take look around your house you’ll most likely see at least ten objects that have been manufactured in some sort of casting process, may it be actual casting or molding. The frames of your monitor, the keyboard, the mouse your hand lays on, the CD cases, game console shells and so on.
It’s literally everywhere.