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The Art Of Die Making
By Richard Pelleriti, Executive Vice President, RAM Products, Inc., Columbus, Ohio, USA. Reprinted from Ceramic Industry magazine May, 1998 issue.
"Mystique" is a word often associated with mold or die making. Many a master craftsperson has gone to the grave with secrets for making a perfect mold. For years, apprentices have learned their trade from the masters, picking up secrets by paying close attention. Even in today's highly technological environment there are many who cling to the theory that mold or die making is an art, or a black science at best.
Certain aspects of mold making, however, are truly scientific. For example, anyone who has cast a mold will confirm that the temperatures of the air, mold and slip in casting, or clay in RAM pressing, have a definite impact on the results. By deduction it can also be assumed that temperature affects the slurry in mold making, as well as the finished product.
In reality, mold makers produce scientific results by trial and error. This is basic science, but instead of working in a research-and-development environment, these craftspeople experiment in the real world of production. Results are not necessarily documented for others to read but are stored in the mind and are passed to apprentices through example and as finished procedures. With advanced forming processes, the idea of mold and die making as a science is starting to gain more acceptance.
MOLDS vs. DIES
First let's define some of the differences between a mold and a die. The mold is usually made from a very soft, porous plaster. The die is made from plasters/cements that have a very high crushing strength (15,000 psi or more). The die is poured into and supported by a steel or cast-iron die case; the mold is not.
Leveling the die is an important step. The back of a die and its case must be flat and parallel, or it will be subject to cracking and may yield parts that are thick in some areas and thin in others. Extreme differences in parallelism can also keep parts from releasing from the die.
The die has an internal air system which, when activated on the press, releases the ware from the die. This is referred to by many as an air-release die. The release is actually caused by interaction between air and water within the die. In most cases, a mold does not have such an air-release system, though there has been recent testing using RAM air systems in molds to decrease drying time and to achieve additional turns of each mold.
Mold making and die making are similar in purpose. Molds are used to form articles that are poured from slip. The mold absorbs moisture from the slip and the result is a cast piece. Dies serve the same purpose but within a medium of plastic clay instead of slip, and less water needs to be absorbed.
There are, of course, many variations to the casting mold. Casts can be made in simple open molds, two-part open molds or two-part closed molds for making solid cast items. Complex multi-part molds are used to manufacture sanitaryware and hollowware. This article will discuss the die used in the RAM process.
The bar used to scrape the back of the die is made from 1/2-inch tool steel, which has been ground flat and parallel to a tolerance within 0.0003 inch.
Die making is becoming more of a science because so many more potters and potteries have now adopted the RAM process as a method of forming. With this acceptance, many potters have provided suggestions for improving the RAM process, leading to significant enhancements.
There are aspects of mold and die making that are in fact art. The graceful curve of an area for clay flow in a die (known in pressing as the "contour") can be artfully accomplished. But spending time to make this area of the die pleasing to the eye is probably a waste of time if production is to be maximized and labor minimized. The flow area needs to be clean and without intrusions so that the clay will properly flow.
Gutters and their design are also extremely important to the die. By definition, a gutter is simply a trough that restricts clay flow. A gutter that is too deep wastes clay and precious die making time, and may detract from the die's intended purpose. The flowing clay should be trapped by the gutter and create a back pressure on the part being pressed.
A gutter that is too shallow also negates its purpose to restrict clay flow and help fill the cavity in which the part is formed. A proper gutter helps fill the cavity evenly and with relatively the same pressure across the piece. Good back pressure helps produce a piece with similar density throughout.
The benefits of uniform density are many. The finished piece has a better chance to dry without cracking or warping and fire without breaking. Additionally, glazes and color adhere to surfaces better because of the uniform structure of the body.
As previously mentioned, the die's air-release system is one of the specific features that differentiates the die from the standard slip casting mold. Placement of this air system in the die, also once thought to be an art, can be quantified and scientifically defined so that the results are the same, or nearly the same every time.
A master tool is ready to accept the air system followed by pouring of the working die.
The basic construction of a good air system places the special tubing exactly 1 inch from the surface to be released. The air system must also follow the contour of the part, and there must be a minimum of two inches of material from the back of the die to the air system.
The layout of the die requires consideration as well, since it will affect the release of the finished part. If the air does not uniformly cover the pressed item, release can be delayed or, worse, it may occur sooner on one part of the item and later on another, thus causing the pressed part to move or bend upon release. This type of movement can cause the part to warp during drying. Jumpers are often used to eliminate this problem on larger and deeper items that are more susceptible.
The air system is ready to be placed into the master tool. This relatively simple air system will be suspended exactly one inch from the surface of the master. The spacing between air lines is also at one inch intervals.
The air tubing, known as Molduct Tubing, is also critical. After many years of experimentation, it has been determined that the best possible material to use for this tubing is a combination of cotton and paper. The specially constructed paper serves as the base upon which to weave the cotton. The specific number of cotton strands combined with the paper base, as well as the tightness of the weave, determine the amount of air that is finally released into the die.
The tightness of the weave also determines whether the material used to pour the die will be absorbed into the air channels, thus closing paths of air release for a given part. Often, lack of uniform air release results in an inadequate die, which in turn results in defective parts.
Shown is a very complex air system for a 156 cavity die.
The steel casings that hold and support the die on the press are another important factor. These castings must be flat, parallel and strong enough to withstand deflection. Plasters and cements tolerate some deflection and will therefore compensate for some errors in die case construction, but after the point of deflection is reached, the material will crack. The press and die cases must be designed and constructed to react to these points of deflection so that the dies will last and produce finished parts in sufficient quantities to offset the costs of manufacturing dies. The deeper and larger the item being pressed, the more critical deflection becomes because it will take more overall pressure to close the press on these shapes.
The air system shown here is an example of a very simple system that may not do its job. Note the lack of uniformity between the air lines. The air system is also loosely tied to the die case and will probably float when the plaster is poured. It is unlikely that the final result will place the air system one inch from the surface of the finished part.
If the plaster/cement being used as a die material can deflect up to 0.003 inch before breaking, then the press and die casings must not deflect under load more than 0.003 inch, or the die will crack. And if the plaster/cement has a crushing strength of 14,000 psi, the press should not exert more pressure than this, or the die will also break.
In addition to deflection and crushing strength, another primary factor affecting die cracking is fatigue. Fatigue results after the die has opened and closed under pressure many times. Mixing times and consistency--the ratio between water and plaster--affect the strength of the die and its point of fatigue. Air and water temperature will affect the die set-up time, and along with the timing of air injection during curing, there will be an effect on the fatigue factor. Fatigue causes a die to crack; however, deflection is the most common cause of breakage.
Until such engineering principles were applied to the art of die making, it was almost impossible to press large and/or deep shapes with the consistency required by production-oriented manufacturers. Today, many manufacturers can press bowls and planters as deep as 9 inches. Very large planters and other similar items are being pressed as well.
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