Make Your Own Die Castings

with this Pressure Molding Machine

by Walter G. Hulett

(Click on the image for a full size view.)
While this little piece of equipment is not intended for heavy production work, several thousand castings can be turned out at very slight cost. If you had a regular professional die made for use in a commercial machine, it might cost a hundred dollars or so for a simple part. (Editor's note: This is in 1937 dollars, folks. Your mileage may vary!)

The whole secret of success in building and using this machine is accuracy. Every point covered in its construction must be perfect.

Remember that you will be using molten metal at 1,800 degrees F. under high pressure. A flaw in construction may mean damage to equipment, a fire, or serious injury.

Careful workmanship will prevent all these things.

Before beginning actual construction, let us see how our outfit works. First, the liquid metal is poured into the sprue hole until it is full, the lever is pulled toward the die, and metal is forced into it (the die). Simple, isn't it? Of course, there are details of operation, but enough for now. An examination of the sketches shows that details appear in some views, and not in others; this is to avoid confusion. For instance, the lever shown in the top view is omitted in the others, to make more important details clearer.

(Click on the image for a full size view.)
To begin construction, obtain a piece of cold rolled steel large enough to make two pieces 2.25" by 1.125" by 3.5". Dimensions are really of little importance, since this machine can be made larger if desired, and the original was made of whatever came to hand, so aside from the fitting and machine work, little else counts. Clamp together to form a square block, 2.25" by 2.25" by 3 .5". Saw out enough metal to leave a 1" square with a 5/8" flange all around one end.

Drill for dowel pins at X. This may be done by drilling through one piece, to just take a piece of drill rod, and going on through with a size smaller drill. Press the short piece of rod into the smaller hole, and ease off the projecting end into a nice sliding fit; you may want to take this apart. After firmly dowelling, drill and tap 10-32 for the flat head clamping screws. Set these screws up tight.

Drill lengthwise, clear through, and ream for a scant .250" Make it .245", if possible. Be sure this hole is smooth. You Live Steamers and power boatmen know how a steam cylinder should look. We want a glass-smooth finish on this bore, even if we have to keep reaming up to .275". Now turn down a piece of drill rod and polish it to a perfect fit. By that I mean that the plunger should be fairly easy sliding, yet should create a vacuum when the thumb is placed over the hole. Get these parts perfect or the whole project will be a failure.

Face off the flange to as true a flat as possible. Slip plunger in hole so that one end is flush with the flange face, cut the other end to 1" projection, slot 3/16" by 3/8" for the lever, and the two parts are done after you drill and countersink for the sprue hole.

(Click on the images for a full size view.)
Cut the lever bracket of cold rolled steel 1/8" thick as per sketch, securing it with round head machine screws to one side of the body only. If this thing goes haywire, we want to be able to take it apart in as few pieces as possible, so leave one side of the body with nothing attached to it except perhaps the die. Cut handle from 3/16" steel, and pivot it to bracket. Mine was cut 15" over all, with a wooden hand grip. With 3" between pivot and plunger, this gives a four to one advantage. Note lever and plunger are not linked directly. Instead, the lever rests in the slot of the plunger and is secured by a yoke. As originally built, a link was used, but rivets have a bad habit of breaking off. For legs, use 1/8" steel straps, making them so as to hold the machine as far as possible from the bench, but not so far that operation of the lever can move it.

The flame is located in the sketch as in approximate location, experience being the determining factor in final placement. That portion of the body between sprue hole and die should be kept at almost the melting point of the metal used, which in my case is an alloy known by the trade name of Zamak No. 2, this being of low melting point (about 1800 degrees F.) and giving a finished casting of high tensile strength, good finish and easy machinability. Of course you can use white metal, lead, type metal, or the softer aluminum and zinc alloys. The die should be about 300 degrees cooler than the machine. In other words, have it hot enough to keep the metal from solidifying too fast, but not so hot that it keeps the casting too hot to hold its shape upon removal. You will perhaps find that an oil hole located between the sprue and lever and filled with cylinder oil will prevent sticking of the plunger.

As to dies, I can only give you a few pointers, as this article is hardly the place for a treatise on die-making which is a highly specialized art. Of course a die could be made for , say, a locomotive cylinder casting which would have all the ports and bores ready cast, requiring only cleaning to be used as-is, but such a die could only be used in a commercial machine, and would cost more than your second-hand auto, so we'll stick to simple ideas.

Be sure the two halves of the die are in proper alignment and that the face adjacent to the machine is as nearly flat as possible. This joint, A, on the sketch is a danger spot as metal is apt to spurt if the joint is not perfect. In one half of the die, file air escape groves, D, only wide and deep enough (about .001" deep by 1/8" wide) to allow the escape of air, not metal, although a slight fin of metal on casting at these points is no objection.

Although the die recess is shown centered, keep it as close to the edge as possible. In other words, keep the gate, F, as short as is possible. Use at least two dowel pins, E, to keep cope and drag in line. I have shown some clamps used on die and between die and machine, although machine screws are just as satisfactory, but slower to operate. Vertical dotted lines in side views show steel pins to be used to knock out the casting if it sticks. Put a couple of permanent dowel pins in the face of the flange and accurately drill all dies to receive them, thus keeping the plunger bore aligned with gates. Fasten flange to drag of die with machine screws, but use a clamp on the cope if you desire.

You may find that a longer bore between sprue and gate is necessary, or that a larger bore is desirable. Make these changes if you like, for a larger casting may require them. You will find that you can cast bushings of steel or bronze into your wheels, etc., but not tires, as the metal will tend to shrink away from the rim.

Now a word about operation:

Above all, be careful. Remember that this metal is hot, it is under high pressure, and is liquid. Wear leather gloves, for molten metal penetrates woven materials more readily than does water. It might be well to wear goggles, and some sort of light sheet metal shield in front of the machine is a good precaution. Be sure your machine is hot enough; better the first shot of metal in a too hot machine than in a cool one. If the bore does plug with metal, take out the four screws, pull off one side, remove the chilled metal, reassemble, and try again. After removing each casting, give each half of the die a squirt of light mineral oil from an atomizer.

Don't try to build this outfit from brass. Brass is a zinc alloy, and has an affinity for lead, pot metal, etc., so that grief is sure to follow. As for stainless steel, I don't know; stick to cold rolled or cast iron; the latter makes a good material for the machine body.

You may, if you like, use longer rods for dowels in the die and slide the cope up and down them, it may be easier. I could give dozens of suggestions, but you fellows who build this are probably better machinists than I am, so use your own heads.

From the pages of The Model Craftsman Magazine, March 1939.

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