While plaster works well as a mold material, it has the unfortunate properties of gradually dissolving when in contact with water and calcining (losing water of hydration) at temperatures above 40°C. For the production of sanitaryware and tableware items, these properties can lead to short mold lives and frequent mold changes as the surface deteriorates and the cast piece begins to change dimension.
Table 1 presents a summary of typical plaster properties under standard test conditions. This information suggests that plaster is best described as a stiff/brittle material, having a relatively low flexural strength but a high flexural modulus. It clearly has an open cell structure, with a degree of permeability and a mean pore size of 1 micrometer.Clearly a new mold material could benefit the manufacturing process. But what information is relevant to the development of a new material that would be suitable for replacing plaster as a casting medium? This question is not as simple as it may at first appear. Not only are the material considerations important, but the process variables are also significant.
Developing a new material first involves the evaluation of its properties, both from a scientific and a commercial viewpoint. The scientific evaluation has to differentiate between those properties that are essential and those that are merely desirable, and this analysis has to be performed for each type of material and process being considered. Typical properties that require evaluation include strength, modulus, permeability, porosity, absorption and capillarity. Each process has different requirements that the materials scientist has to consider to tailor a material with suitable properties.
For example, a material with high permeability and a relatively large mean pore size would not be readily appropriate for tableware roller molding; however, it may be applicable to pressure casting, provided that its strength and modulus are also adequate. Equally, materials with lower permeability and smaller mean pore sizes are unlikely to be suitable for pressure casting applications, but may be applicable to roller molding, capillary casting or ram pressing, again depending upon other properties such as their strength, modulus and casting ability.
Optimizing the Mold for the Process
Once the material properties have been evaluated for a particular process, and a test material that matches these properties has been produced, the next step is to determine whether the material is commercially viable. When this final obstacle has been overcome, the material is ready for trial, first in-house and then eventually at a manufacturer’s site.Through the constant development of new materials and the inherent understanding of which parameters are critical for each process, materials and production methods have been developed and refined to provide new products that redefine the material boundaries and processes involved.
The key requirements for a mold material for use in the pressure casting process are high strength, consistent permeability and low shrinkage. Casting pressures can vary from 5 bar for medium pressure sanitaryware to 40 bar for tableware pressure casting. Pressure is the driving force in this casting process; therefore, a material with natural capillarity is not required.Most materials currently available for use in the pressure casting process have a pore size around 10 microns, but these molds require frequent water flushing to prevent pore blockage and blinding. A unique fine pore material has been developed that avoids the requirement to back-flush molds with water between cycles. Careful control of manufacturing conditions is required to obtain a consistent surface permeability and dimensional control. Molds that have produced over 55,000 pieces are in operation at a number of manufacturing sites around the world.
The key properties of plaster to be reproduced in a new material for ram pressing are its fine pore size and healthy appetite for water. The driving force for de-watering between cycles in ram pressing is pressure, which means that while high absorbency is required to dry the mold surface after de-watering, the actual material affinity for water needs to be relatively low to provide fast de-watering and rapid turn-around between cycles. While this might appear to be somewhat paradoxical, a successful compromise was eventually found.The key advantage offered by synthetic ram-pressing molds over plaster is the material’s inherent strength, which can give mold lives of well over 10,000 pieces. Many tableware producers have operated exclusively with such ram pressing molds for several years, enabling them to reap the benefits of longer mold lives, reduced mold costs per piece, improved yield and the elimination of plaster mold-making costs.
A new material was recently developed* for use at casting pressures close to atmospheric (0.5 bar is adequate). The development of this material has focused on attempting to produce a plastic based material that replicates the natural casting ability of plaster, while still enabling ready de-watering between casts. This combination of properties enables the molds to be used continuously without the need for mold drying, which typically limits plaster molds to a maximum of three casts per day.
The key objectives for developing the material were as follows:
The combination of these three properties enables the material to replicate the natural capillarity found with plaster, while still enabling rapid de-watering between casts, which gives greatly enhanced productivity.Roller Molds
A plastic-based material has also been developed to successfully produce commercial quality flatware and hollowware pieces by the roller molding process. The key properties in the development of this material were its pore size, strength and high degree of dimensional reproducibility. It was also advantageous for the material to readily extract water, although not to the same extent as that required for capillary casting. This is principally because heat is the main driving force for drying the ware in this process.
The main failure mechanisms of plaster roller molds are surface erosion due to the dissolution of plaster, which leads to unacceptably rough surfaces, and mechanical failures due to manual handling operations. Unlike plaster, the plastic-based roller molds are insoluble in water, thereby negating these problems.
In addition to increased mold life, the plastic-based roller molds can also be operated at significantly higher temperatures. This enables either equivalent production output from a significantly smaller number of molds, or increased output from existing production equipment using a full round of molds.
Forming New Conclusions
The new range of mold materials developed over the last several years is revolutionizing the ceramic industry. Materials that eliminate the disadvantages and associated expenses of plaster, while providing increased output and reduced costs, have been introduced to the global market. Practically every type of casting process, from capillary to pressure casting, has been influenced by these developments, and more advancements will undoubtedly follow in the future.
For More Information
For more information about synthetic molds, contact Porvair Ceramics, Kings Lynn, Norfolk PE30 2JG, UK; (44) 1553-778910; fax (44) 1553-778929.
*Surecast®, a registered trademark of Porvair Ceramics.