Mold (casting)

“ Nonexpendable mold casting, on the other hand, relies less on the conditions of gravity and pressurized atmosphere. The molds tend to last for a greater number of runs. The main disadvantages are that (1) production devices tend to be large, on the order of tons, and (2) the processes are more complicated than for expendable mold casting. ”

“ There are two methods of preventing air from being trapped in die-casting molds; either by constructing the dies so that the air may be46 exhausted from the mold cavity before admitting the metal, or by venting the die so that the air may be forced out by the inrushing metal. In the first of these methods it is necessary that the joints in the mold be made as close as possible, otherwise it will be impossible to produce anything like a vacuum in the mold cavity. ”

“ WHEN THE PLASTER MOLD IS READY TO REMOVE, a table knife slightly inserted between partitions and carefully pryed will cause the parts to separate. A most important part of casting molds and the using of the plaster molds is the oiling. ”

“ CASTING IN THE ROUND is more difficult than flat casting or bas-relief work and while the making of plaster molds for vases and bowls is given particular attention in the chapter on Cement Bowls and Vases, directions for casting objects in the round will be given here.
AS A GOOD TEST PROBLEM take any small object or toy and, if it is wood, oil it well. If it is porcelain or glass it will not need to be oiled. A simple form, animal or bird, can be made in modeling wax or clay to be used as the original or model from which to make a mold. ”

“ Plaster Mold.—Nearly all fine grades of metals can be cast in plaster molds, provided only a few pieces of the castings are wanted. Dental plaster should be used, with about one-half of short asbestos. Mix the two well together, and when the mold is complete let it dry in a warm place for several days, or until all the moisture is excluded. If the mold is of considerable thickness it will answer the purpose better. When ready for casting, the plaster mold should be warmed, and smoked over a gas light; then the metal should be poured in, in as cool a state as it will run. ”

“ In making the molds it is necessary to remember that aluminum has a large contraction in cooling, and also that at certain temperatures it is very weak and tears readily, while all metals shrink away from the mold when this is wholly outside the casting, but they shrink on to cores or portions of the mold partly inclosed by metal. ”

“ In the center of this metal pot is a rod with a spring attachment which, at each operation of the machine, acts as a plunger to force a small stream of hot metal through a side aperture into the jet-hole of the mold. After the casting, the two parts of the mold separate slightly, the matrix is drawn away from the face of the type, and the cast is moved out; then the mold and matrix close together again and the operation is repeated. Cold water or air is circulated near the mold to keep an even temperature. ”

“ Molds.—I.—Brass molds for the casting of soft metal ornaments out of britannia, pewter, spelter, etc., should be made out of brass that contains enough zinc to produce a light-colored brass. While this hard brass is more difficult for the mold maker to cut, the superiority over the dark red copper-colored brass is that it will stand more heat and rougher usage and thereby offset the extra labor of cutting the hard brass. The mold should be heavy enough to retain sufficient heat while the worker is removing a finished casting from the mold so that the next pouring will come full. ”

“ This method is very useful, indeed, indispensable in the manufacture of such wares commercially, when the same designs are to be repeated many times, but it is hardly to be recommended for artistic work, except for conventional designs, borders, etc. This method of casting the forms in a mold may be useful, however, in the reproduction of leaves of intricate outline, which can be made of various sizes, and, from the plasticity [Pg 75] of the clay, can be infinitely varied in position according to the taste of the worker. ”

“ If the back knife does not trim the bottom of the slug perfectly the metal will adhere to the back of the mold, making high lines (i. e., lines over .919, as explained under Fig. 39) , and also prevent good contact between the mouthpiece and mold, which would, of course, cause metal to squirt back of the mold disk.
To prevent this, the mouthpiece should be faced up true with back of mold, as follows: First send a line through the machine, stopping at casting point; then screw up the nut on end of pot lever eyebolt until it touches the pot lever. ”

“ The mold disk is held in position for casting and trimming the slug by means of pins attached to the vise, called mold disk locking studs. The studs are located in relation to the mold disk locking bushings at the factory so as to give perfect alignment of the matrices. They are dowelled in place and should never be changed. ”

“ With a sharp knife trim off all ragged edges and superabundant plaster, leaving your mold small as possible, and lighter to handle. These leaf molds are much better for all uses, even for sheeted wax flowers, than those metal molds that cut the wax, and never give the fibrous look needed for a natural looking leaf. The lily of the valley needs a wooden mold, the flower is so delicate a plaster mold cannot be made. ”

“ The plaster process requires carbon, a relatively rare substance in nonterrestrial materials, for the gypsum binder. Once used and cracked away, normal plaster cannot easily be recast. The water used in mold production may be recycled during the baking process. Plaster casting is normally used for nonferrous metals such as aluminum-, zinc-, or copper-based alloys. It cannot be used to cast ferrous material because sulfur in gypsum slowly reacts with iron. Also, the plaster process requires gravity or centrifugal injection of casting fluid into the mold. ”

“ A mold can be made in two pieces of a round object like a ball and if each piece is exactly one-half, it will draw, because there is no point under which the plaster will hold. Any hollows or projections will form "under cuts" necessitating making the mold in a number of pieces that it may relieve or be lifted off the cast. Molds of heads from which to cast paper forms are often wanted and are easily made. With the skinned head of a fox, let us say, on the table, the lower part is embedded in fine sand or clay about on a line with the mouth. ”

“ By allowing the work to stand a while, however, [132] and by slightly jarring the mold from time to time with the ball of the thumb the piece can generally be removed without damage.
In using a new mold it is customary to make what is called a "waste filling." The mold is filled with slip and at once emptied. After standing a few minutes it is forcibly opened and the thin layer of clay inside is picked out with a ball of plastic clay pressed against it. A tool should never be used as this will damage the face of the mold. ”

“ Molds feed upon all cakes or breads, upon meat, cheese, and many raw vegetables. They are almost sure to grow upon flour if it is allowed to get damp. Moisture seems necessary for their growth. Jelly is a substance particularly favorable to molds for this reason. Shoes, leather, cloth, paper, or even moist wood will give food enough to support their growth. At least one troublesome disease, ringworm, is due to the growth of molds in the skin.
What Mold does to Foods.—Mold usually changes the taste of the material it grows upon, rendering it "musty" and sometimes unfit to eat. ”

“ Button molds make tops. The big wooden molds that the tailor uses for coats are best to make into tops. The hole in the center must be enlarged to admit of a sharpened end of a meat skewer being inserted. These button mold tops may be painted, too, and a splendid game can be played with them on the nursery table. ”

“ Care should be taken not to become too anxious and force the sections apart before the cast has dried or they will cause it to break. If it refuses to come apart easily when completely dry, a little wooden wedge tapped into the crevices in one or two places will cause it to part easily. Sometimes plaster flows over the sides of the mold and binds the edges together. On removing the mold and cast from the retaining bars, examine it to see if any plaster is binding it and if so remove it. ”

“ The mold is filled with molten metal, and when the operator thinks the desired thickness has cooled next to the walls, he pours out the balance. An experienced man can make hollow castings in this way, and make the walls of any thickness.
Casket hardware trimmings, which are so extensively used on coffins, especially the handles, are nearly all cast out of tin and antimony, and in brass molds. The metal used is brittle, and requires strengthening at the weak portions, and this is mostly done with wood filling or with iron rods, which are secured in the molds before the metal is poured in. ”

“ The first step in the process is the preparation of the mold. The substance originally used for the construction of this was plaster of Paris. This substance is, however, porous and must be rendered impermeable. The materials most commonly used of later years are stearine, wax, marine glue, gelatin, india rubber, and fusible alloys. With hollow molds it is a good plan to arrange an internal skeleton of platinum, for ultimate connection with the anodes, in order to secure a good electrical contact with all parts of the mold. ”

“ Shellac the surface of the top mold, cleaning away any small bits of plaster around the edges. Trim down the box so that the top mold projects over it about 3⁄8 in., making it easy to drop the top mold into place over the bolts. To use the mold, make certain that it is clean inside and set the top into place. Fasten down the wing nuts at the washers. Be very careful that the mold is dry, as hot metal poured on a wet surface may cause a dangerous splash. ”

“ If mold and clay are in good condition the vase will fall forward to be supported on the fingers of the right hand and steadied by the thumb. The half mold is now laid down and the vase taken in both hands, set gently on a plaster bat and put aside to dry. It often happens that the vase leaves the mold with reluctance. If the slip be very new, or the mold either damp or hard or worn out there will be some difficulty in effecting a separation. ”

“ The molds are made in part the same as when of brass, and out of tin that contains as much hardening as possible. The hardening consists of antimony and copper. This metal mold must be painted over several times with Spanish red, which tends to prevent the metal from melting. The metal must not be used too hot, otherwise it will melt the mold. ”

“ The upright threads with the balls at the end contain many tiny bodies called spores. These spores have been formed by the division of the protoplasm making up the fruiting bodies into many separate cells. When grown under favorable conditions, the spores will produce more mycelia, which in turn bear fruiting bodies.
Physiology of the Growth of Mold.—Molds, in order to grow rapidly, need oxygen, moisture, and moderate heat. They seem to prefer dark, damp places where there is not a free circulation of air, for if the bell jar is removed from growing mold for even a short time, the mold wilts. ”

“ Mold can grow on virtually any organic material as long as moisture and oxygen are present. There are molds that grow on wood, paper, carpet, food, and insulation. Because mold eats or digests what it is growing on, it can damage a building and its furnishings. If left unchecked, mold eventually can cause structural damage to building materials. ”

“ The netting, or fabric, having been spread as evenly as possible over the inside surface of the mold hole, the upper edges were allowed to lie out upon the ground. The soft clay being now pressed evenly upon the fabric to the required thickness, the sandy surface of the mold hole easily gave it shape and gave the potter no anxiety about the outside surface. ”

“ A hand would be missing, a portion of a shoulder, a bit of the squat body or there would be a flaw where the running metal had not filled the mold.
“I'm hanged,” he cried, “if the beggars are not right about it. The thing can't be done! I've tried it in all sorts of dimensions. You will see some of the big figures in the garden. I've used a ton of metal and every sort of mold.”
Then he flung his hand out toward the bookcase.
“I've studied the art of molding in soft metal. I have all the books on it, and I've turned the boathouse into a sort of shop. ”

“ Settle a board, cut to approximate body outline but much smaller, into the unset plaster and press the flap edges of the skin back together over the board, molding edge of back and belly to round back away from trimmed edge of mold. This must be all done with accuracy before the plaster sets, but you will find it gives enough time. Do not work in a strung-up, nervous way.
When the plaster is set hard, remove fish from mold, hold it upon palm of left hand and tack edges of skin to back-board. (For general details of this method see Fig. 21) . ”

“ For preparing the mold, which consists of two parts put together with their concave surfaces, the workman first powders a kind of rough plastic clay, and mixes this with levigated powder of a blackish ‘tuff-stone’ and a little charcoal powder and water, till the paste is plastic and suitable for being molded. ”