A Pressing Matter

July 30, 2000
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Figure 1. The isostatic tool system.
The first isostatic press for tableware was introduced about 20 years ago. Today, approximately 300 isostatic presses for tableware are in operation worldwide. In Europe, this process has successfully replaced the conventional technology of roller forming.

Further development of the isostatic pressing process, along with advances in granulate technology, are making it possible to achieve increased product quality and economic efficiency in tableware production.

Figure 2. The technology triangle.

Isostatic Pressing Tool

At the heart of isostatic pressing technology is the isostatic pressing tool. Figure 1 shows a schematic of such a tool. The cavity of the tool is between the firm polyurethane coating on the male punch and the polyurethane membrane on the female punch. The membrane isostatically transfers the force with hydraulic oil.

Main Process Steps

In the positioning stage, the lower and upper punch form a defined filling gap. This gap is specified when designing the tool to achieve the desired thickness of the article bottom or layer. During press operation, the filling gap can be modified in increments of 1à10 mm to a maximum thickness of 1 mm upwards and a minimum thickness of 0.5 mm downwards. Such modification can be an advantage if varying granulate properties prevent the press operator from maintaining the desired article weight or bottom thickness. By simply adjusting controls on the computer monitor, the operator can quickly intervene and maintain the high quality standard of the ware without interrupting production.

A patented vacuum filling system* incorporates a defined gap between the upper punch plate and the filling ring insert. A special connection in the gap allows a vacuum to be used to accelerate the filling process. The vacuum also helps ensure that the granulate is uniformly distributed in the tool cavity. Any residual air in the tool cavity-which could cause cracks during subsequent isostatic compaction operations-is removed during static compaction.

After the tool is filled with granulate, the tool members are brought together into pressing position. During this phase, the granulate in the tool cavity is statically compacted to a certain degree. The percentage of static compaction depends on the design of the cavity and on the article geometry. After static compaction, the remaining percentage of isostatic compaction is fairly small. Therefore, the movement of the membrane during isostatic compaction is also fairly small. Any displacement of the membrane caused by varying hydraulic flow conditions at the rear is thereby avoided, and maximum shape consistency can be achieved.

As soon as the required closing force (holding force) has been reached, the isostatic pressing process is carried out at a pressure of 300 bar. At that point, pressure reduction begins. The process of isostatic pressure reduction can be influenced by a hydraulic throttle valve, and the pressure reduction time can be adjusted on the computer monitor.

Tool Design

As with other pressing methods, a good isostatic pressing tool is developed around three main elements-the ceramic body, pressing and firing-also known as the "technology triangle" (see Figure 2).

Body. Granulate technology is the basis of isostatic pressing and a large part of its success. However, variations in granulate properties can have a considerable influence on the quality of the pressed and fired article, often resulting in cracks in the pressed article regardless of the press or tool system. For that reason, several development programs have been conducted to improve granulate properties. Through these programs, binding agent suppliers and granulate manufacturers have been able to develop improved granulates for isostatically pressed tableware. Even for special bodies, such as bone china and vitreous china, remarkable improvements can now be achieved through isostatic pressing. The following granulate properties have proved advantageous for most bodies:

  • Moisture 2-3%

  • Green strength >100 N/cm2

  • Expansion <1%

  • Median particle diameter below 300 Km

  • Good flow properties

    Flexibility should be designed into the tooling to compensate for changing granulate properties.

    Pressing. Several pressing characteristics must also be considered in the isostatic forming of tableware articles. These include article geometry, article thickness, configuration of the rim and compensation for firing effects. The pressing tool is important and should comprise both a static and isostatic pressing/filling system. It should also be possible to set the press for decompression of isostatic pressure.

    Firing. Firing conditions are the third part of the technology triangle. Granulate characteristics, tool design and the pressing process must be optimally harmonized with the firing process to avoid poor quality ware or economically inefficient production.

    Isostatically pressed tableware that is bisque fired may incur cracks. However, these are typically due to the raw materials and binding agents used or to unsuitable heating or cooling speed, rather than the pressing method. In rapid bisque firing, the crystalline structure can be insufficiently formed, resulting in deformation of the ware bottom during glost firing. These defects can usually be overcome by adjusting the raw materials or firing schedule.

    Tool Development. Once the main dimensions of the article to be pressed have been determined, the tool calculation is carried out. The green dimensions for the isostatic pressing tool are calculated based on the known shrinkage of the material during firing. The tool components-such as supporting ring, punch core, upper punch and article geometry-are designed using CAD (computer-aided design). Once the design is complete, the production drawings and parts lists are transferred by data line to production, where the tool components are produced on modern CNC (computer numerical control) machines using CAD/CAM (computer-aided manufacturing).

    Future Progress

    The high level of development in isostatic forming technology today allows manufacturers not only to press rotationally symmetrical flatware articles, but also to press salad dishes and other tableware articles that are not uniform in shape, and as deep as 100 mm (~4 in.). While mugs and cups cannot yet be isostatically pressed, a pressure casting plant to produce cups with handles in one step is currently being tested.

    Carefully directed studies in the various technological sectors have helped advance isostatic pressing technology. But it has also become clear that not all problems are a direct result of the process-a comprehensive system is involved. In the development, design and testing of a tool, close cooperation between the tableware manufacturer and machine manufacturer is required to ensure that high economic efficiency and improved product quality are achieved.

    For More Information

    For more information about isostatic pressing technology, contact Dorst Maschinen-und Anlagenbau, MittenwalderStr. 61, P.O. Box 109, D-82431, Kochel am See/FRG, Germany; phone (49) 8851-1880; fax (49) 8851-188310; or e-mail sales@dorst.de.

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