Torque Power Live Steam Models Hyattstown, Box 144-D R. F. D., Ijamsville, Maryland 21 754
Hi There Young Engineers:
I am going to discuss three types of molding processes sand molding, investment molding and permanent molding. You may end up using all three as each has its own advantages.
To do sand molding you must find a supply of molding sand. Hopefully you may have a nearby foundry from which you can get the sand and some firsthand knowledge about sand molding. The size, shape and material you wish to cast in, as individual factors which vary in combination, make it impossible to give one set of rules which will work all the time. So, this is something with which you are going to have to experiment. All I can do is give you some tips. Since aluminum is excellent to work with, I will mainly describe the molding of it.
The molding sand comes in several grades of coarseness; use the fine or medium. The fine can reproduce extreme details to the extent of being able to reproduce a fingerprint and it is the best for small castings. Be sure to get used sand as green sand is very hard with which to work. In regular foundry practice green and used sand are mixed fifty-fifty. The high temperature of iron or brass would eventually burn the green sand to the point where it would become workable. But with the low temperature of aluminum and with such little use as the sand would receive making small castings, it would take too long for green sand to get into good condition.
You will also need parting dust. The foundries use an industrial type of flour. So, any low grade flour would be good. The flour should be put in a small cloth bag that is porous enough to allow the flour to sift through it when shaken in an up and down motion over the open mold.
Now, as for the flask. The flask (Fig. 1) consists of two box-like frames pinned together so that they can be taken apart and put back together without misalignment. They can be constructed of pine or redwood. The lower portion is known as the cope. An eight-inch flask is a nice size to start out with for small castings. This measurement is taken from the inside dimension of the flask.
Patterns can be made from scrap wood or even plastic. Wood patterns should be painted with a flat black all-purpose enamel. This prevents the pattern from warping which is caused by the moisture in the sand. For some reason flat black enamel works better than any coating I have tried. Patterns of this size can be made to the exact size that the casting should be. But, remember to build up the pattern where the surface is to be machined. Often a piece of cardboard cut and glued to the surface will build up a pattern that is too small.
Now take a spoon and scrape a channel or gate (Fig. 4) leading away from the impression made by the pattern. You may have to make a tool smaller than the spoon as it depends on the scale of the piston. There should be two gates leading from the mold. One is for pouring into and the other is for a riser (Fig. 4). Carefully blow all excess sand away. Also, using a tool or your finger, compress the sand in the gateways so that none will be washed loose by the metal flowing in and cause a flaw in the casting. Now, take a piece of metal tubing (Fig. 2), the size again determined by the scale of the object you are molding, and drive the tube right through the molding sand. This is at the end of each gate. When the tube is removed it will leave a nice clean hole leading from the gate to the surface of the mold. Again, compress with a tool or your finger the loose sand around the hole where it meets the gate. One of the holes is a riser and the other is the pouring hole.
Every pattern must have a predetermined parting line (Fig. 3). This is the point at which the sand mold will separate. When you set up a mold for a piston (Fig. 4), the flat surface of the face of the piston determines the parting line. The cope is placed on a flat surface and the piston pattern is then placed face in the center of the cope. The edge of the piston pattern is slightly tapered so that it will release from the sand. Parting dust is then dusted over the pattern. Sand is sifted over the pattern until the cope is full. You must overfill the cope or drag so that when the sand is compressed by ramming the sand is level with the outer edge of the flask. The cope is then turned over and the drag is placed in position and the filling and ramming operation is then repeated. The flask, now being upside-down, is then turned upright. The mold is then pulled apart. The pattern is still in the cope. (Fig. 4) A tool like an ice pick (Fig. 2) is then stuck into the pattern. Often you have to tap it in with a metal rod not a hammer as it is too heavy. Use the metal rod to tap the pattern lightly all over and then tap horizontally around the pick where it sticks into the pattern. The pattern should be loose enough to be removed by lifting it out with the pick.
Now place the cope on its side and using a spoon scrape the opening of the pouring hole out so that it is funnel shaped. The higher the pouring and riser holes are above the part you are casting the better the casting will come out. This puts more pressure in the mold and cuts down on shrinkage in the casting.
Now, this has been a fairly simple mold to set up, but, what about a pattern with a parting line that follows a curve or is uneven. The piston pattern was only in the cope of the flask. Many patterns will be partly in the cope and partly in the drag (Fig. 5) and often they have uneven parting lines. Setting up a pattern like this requires some skill. It is easier to use a split pattern which is a pattern cut into at the line of parting.
In any case, when you put the one-half of a split pattern in the cope or even a regular pattern (Fig. 3) you must position the pattern so that it can be pulled vertically out of the mold. This may require placing a support block under the pattern in order to have it stay in position while the sand is being rammed in. When the sand is rammed in and the cope is turned over you must then take a spoon and scrape all the sand away to the parting line. The sand should be scraped from the edge of the cope gradually down to the parting line (Fig. 5). Of course, this depends on how much space there is between the pattern and the cope. When using a regular pattern, the drag is put in position, the mold is then dusted and the sand is rammed in.
The gates, riser and pouring hole are put in the same way as before. In the case of the split pattern, you must add the other half of the pattern before ramming the sand in. The split pattern is held together by small pins (Fig. 3) left loose enough so that the pattern can be easily pulled apart when the mold is pulled apart.
Tempering is the term used for the process of adding water to dampen the sand. It must be sprinkled on, not poured on. Use a soft drink bottle (the quart size or larger) with a sprinkler attachment which is used by the ladies for dampening clothing before ironing. For larger amounts of sand use a flower watering can. A small amount of water should be sprinkled on and then the sand should be turned over with a shovel. This is repeated until the sand is completely damp. You should be able to grab up some sand in your hand and squeeze it. If the sand is tempered right, it will stay together when you let loose. Store the sand in a plastic trash can and it will be ready for use at any time. You will have to retemper the sand each time you use it. If you think the sand is too wet, let the sand dry for a day or two.
When you have your mold set up and are ready to pour, be sure to skim off the slag from the melted metal before pouring. This is done with a small ladle. When pouring, keep the metal flowing at an even rate for best results.
I will have to continue this in the next issue. I hope you all are able to attend your local shows this spring and summer. Well, that is all for right now.