Forming: Using Robots for Pressure Casting
Pressure casting was invented in 1982 and, along with isostatic pressing, has provided a revolutionary way to produce high quality ceramic shapes. In pressure casting, water is removed from the slip at high speeds using pressure to extract the water through the porous mold. When the mold is opened, the body has not only taken on the shape of the mold but has also reduced its water content to an average of 19% (depending on the body formulation), making it easier and faster to dry and fire.
The second half of the 1980s saw refinements to pressure casting, and it wasn’t long before manufacturers using this method could almost guarantee the reliability of mold and machine. However, with most pressure casting operations, the demolding and finishing stages still had to be done by hand, limiting the speed of the overall process.
In 1991, researchers began investigating the production of ceramic articles using pressure-casting technology with a double-headed machine.* The machine was attached to a robot, which carried out the demolding stage from the single cavity mold and finished (fettled) the piece. Over the past decade, numerous advances have been made to this technology, and today’s ceramic plants can now rely on robots to quickly and accurately handle every stage of the forming process for flatware—and beyond.
What is a Robot?According to the Old English Dictionary, a robot is “a machine capable of carrying out a complex series of actions automatically.” The name comes from the Czech word “robota” meaning “forced labor.”
Robots are classified into six categories depending on their mechanical structure (cartesian, cylindrical, spherical, scara, articulated and parallel). While a simple handling device can be described as a robot, today’s usage generally restricts the term to machines that have multiple articulations and can operate with five or six axles.
Modern robots rely on the latest technology, such as brush-free, alternating current motors and slack-free, reversing gears, and they use integrated braking systems on all axles. They require sophisticated electronic control systems so that their actions can be quite gentle despite sudden, very high speeds of movement. Most are able to achieve a position and follow a trajectory that varies only about 0.05 mm, making them suitable for even extremely precise operations. And all of these compact systems, which are usually situated in the body of the robot, are designed with such precision that they often do not require any maintenance until they have been operating for at least seven or eight years.
Robotic Workstations for FlatwareThe first robotic workstation with automatic demolding and finishing capabilities was installed in a flatware facility 1992. The workstation consists of a single-head machine fitted with a four-cavity mold. The machine unloads the pieces together at the end of each casting cycle and places them on a rotating table for further dewatering. The machine then picks each piece back up individually for fettling, automatically removing any marks made when the slip was injected, as well as any seam lines. The workstation can produce four pieces per minute that are ready to be dried white-hard and then fired.
Despite the numerous advantages of this type of system, ceramic manufacturers did not immediately welcome the idea of a completely automated process. Many were concerned that a machine would not be as reliable as a human worker, especially in the demolding stage, and others were reluctant to invest in technology that might be too complicated to operate successfully.
Suppliers responded to these concerns by enhancing the reliability of the demolding process with features such as a cleaning stage that helps ensure that the molds retain their porosity between each cycle, and by making the machines more user friendly. Some of the main robotic workstations supplied today use a main program (an essential feature of programming) that enables trained operators to produce a “reference” path where, for instance, the rest position of a robot occurs at a reference point (the center) of the platen. The operator can then define any sub-trajectories required to produce a specific way of dealing with each type of piece simply by choosing from a drop-down menu.
Unlike labor-intensive operations, where frequent rest breaks are required and human error is often a factor, a robot maintains the same precision and speed regardless of how long it has been in operation. In plants where the molds are large and heavy, robots provide an even greater benefit by removing the potential for human injury from the equation. Some of today’s most advanced robots can handle as much as 135 metric tons (~149 tons) of closing pressure, enabling manufacturers to produce multi-cavity molds for bulkier products. Using today’s technology, manufacturers can make four 12-in. plates using a single-headed machine and one casting cycle.
Other ApplicationsBased on the great strides that have been made in robotic technology and on successful installations in numerous facilities, robotic workstations have quickly begun gaining popularity throughout the ceramic industry. In addition to forming, systems have been installed for automatic inspection with video cameras, as well as glaze dipping and spraying.
Recently, new technology has been introduced that is capable of accurately removing the seams on cups produced with pressure casting in a four-part mold. Unlike conventional technologies, the new system delivers the complete cup—including the body, handle and, if required, foot—in a single operation with exacting precision.
These and other advances in robotic technology are making it possible to increase the speed and quality of ceramic production while reducing labor requirements.
*the CSP2, a trademark of Elmeceram