Bench (Conveyor) Plaster Mold Casting. Bench plaster mold casting is still used for a number of reasons. For some companies, bench casting is the way casting has always been done, and they haven’t considered changing their processes. Plaster is inexpensive, and the employees all understand the mold making technology. For other companies, this method simply works best for them. Modifications to molds can be made with relative ease, small numbers of pieces can be cast efficiently, and the slip making technology is well established.
However, this method poses several challenges. The process is labor-intensive, the wage rate for a skilled caster is fairly high, and a long training period is required to ensure skilled casters. Additionally, significant space is required to produce the ware, making the process capital-intensive. Other restrictions include a short mold life and a limitation on the number of pieces cast per day.
Companies that use bench plaster mold casting have two main requirements for their slurries: cost control and consistency (with regard to particle size, surface area, casting rate, viscosity and gel structure formation). Slurry producers have tried to address these needs in several ways. Improved statistical methods of analysis have been designed to answer consistency concerns, and increased-solids slurries have been developed to address both consistency and total cost requirements. In some cases, the slurry producer may recommend a traditional approach to slip adjustment (see sidebar: Improving Casting Performance). New chemical systems have also been proposed to improve slurry consistency.
Battery (Gang) Casting in Plaster Molds. Battery casting offers greater productivity and a relatively low mold cost. Additionally, more pieces can be produced in the space available, which reduces the capital required for this method compared to bench casting methods. The skills required to produce ware by this method are also lower than bench casting.
However, this method also holds challenges. Mold life is short, and larger runs of the same items are usually required because the speed with which the molds are turned over naturally produce more ware. Additionally, even though the molds are plaster, the mold making methods are different than traditional methods, requiring the interconnection of several mold parts into one larger assembly group. (The molds are not interconnected with bench casting) Lavatories tend to be the most favored piece for gang casting because they are more easily gang-cast as a group.
Slurry requirements for manufacturers using gang casting methods are generally the same as with traditional bench casting, and many of the same solutions have been implemented. Manufacturers that use this technique also wish to increase their casting rate. Coarser, custom-blended slurries have been developed that can allow sanitaryware manufacturers to increase the performance of their ball clay and kaolin slurries to meet the requirements of their specific casting method.
Capillary Casting in Plaster Molds. With capillary casting, more turns can be made per day on the same mold, which reduces the capital investment compared to bench and battery casting. The molds are still made from an inexpensive material, and the skill level required is again lower than with bench casting. Excellent recovery can be achieved with this method.
However, capillary casting also has its drawbacks. Mold life is limited, and different mold making skills are required because the molds are more complex. While the skill levels needed are not as high as with bench casting, the process still requires significant training. Additionally, the number of turns per shift is still limited to only four to six casts per 24-hour day, and the potential excellent recovery is not always easily achieved.
The demands placed on ball clay and kaolin producers for capillary casting are the same as with traditional bench casting. In addition, faster casting and firmer casts are also required. The methods used to address these needs can be defined as similar to those of battery casting. Custom slurry products seem to offer the potential to improve performance in this area.
Pressure Casting in Polymer Molds. Sanitaryware manufacturers are increasingly turning to pressure casting in polymer molds. High productivity is one benefit of this method—more pieces can be cast per day. Some plants that produce tanks have established operating parameters that allow them to turn the casts over at about five times per hour.
The number of pieces produced per square foot of manufacturing floor space is also greater, leading to potential savings in overall capital spending. Excellent mold life and excellent recovery are two additional benefits—40,000 casts per mold are not unusual, and A-grade recovery over 92% from cast to boxed has been accomplished in several instances.
Yet another advantage of this casting method is that a lower caster skill level is required. For many manufacturers, the competition for quality workers is heavy. With this method of casting, newly hired workers can quickly become proficient in performing the work.
Still, despite the numerous benefits, even this method is not without difficulties. The equipment is expensive—often the main deterrent to switching to this type of casting. Because the equipment is much more complicated than the other systems, the support and maintenance personnel must be more skilled. Some of the pressure casting equipment suppliers limit the chemistry used in the flocculation-deflocculation system on the slip, making it difficult to find a slip that works correctly with the molds.
The molds are also expensive compared to plaster molds. While the cost can easily be offset by the high number of casts per mold, the mold making technology is secretive, making it necessary to rely on the mold manufacturer rather than internal employees to modify the molds.
As with the other casting methods, high slurry consistency is required. The casting rate potential of the clays used must be increased, and the slip produced should be highly permeable.
In many cases, ball clay and kaolin producers have addressed these requirements by turning to traditional sodium silicate deflocculated slurries. Slurry producers have also provided increased technical support to the sanitaryware producer by studying the pressure casting process. Studies have shown that many of the traditional control parameters (specific gravity, rheology, etc.) can be modified to work with pressure casting. It has also been necessary to provide significantly coarser slurries and customized blends for this application.
Regardless of the casting method used, careful selection of raw materials and slurries is required to optimize the casting process. Materials are available that meet today’s needs, and more advances in materials will be made as new technologies are introduced. A solid partnership between sanitaryware manufacturers and raw material suppliers can ensure that new slurry solutions are continuously developed to meet the needs of sanitaryware producers in the future.
While these methods all improve casting performance, there is a tradeoff in that they also affect some of the other properties of the sanitaryware. For instance, adjusting the ball clay-to-kaolin ratio can influence the plasticity and rheology targets. Plastic-to-non-plastic adjustments might also affect plasticity and rheology, as well as firing ranges. And rheology adjustments can change drain properties. These property changes sometimes result in less than desirable recovery rates.
These approaches have generally been used because of the small differences traditionally observed between material types. An example of this can be seen in Table 1, which compares the casting rate of several traditional ball clay slurries. The slurries were tested for casting rates at 350 cps using a #3 spindle on a DV-III Brookfield Viscometer. This is true for all clay casting rate comparisons. In this article, all of the casting rates are determined using this viscosity.
As can be seen from the data, there is not much difference from one of these clays to the other. The difference is less than 10% from the average value. There simply is not enough range to adjust casting rate by a ball clay for ball clay exchange, so the traditional approach is required instead.
Today, however, this is no longer the case, as can be seen from the pressure casting rate comparison in Table 2, where several current ball clay slurries were tested. As can be seen from the data, it is now possible to change the casting rate and water retention by custom-blending the ball clay slurries using the ball clay for ball clay approach.
The same can be demonstrated with kaolin, as seen in Table 3. The retained moistures seen in this portion of the work comes from the very high casting rate. In practice, where a certain cast thickness is preferred over total casting rate, this range could be reduced. Again, as with the ball clays, a potentially broad range of performance can be derived from manipulating the mineral type.
In today’s highly cost-competitive marketplace, sanitaryware manufacturers must improve recoveries and operate more efficiently or fail to produce a profit. Achieving these goals requires an increased investment in production innovation, both now and in the future, and custom-blended slurries offer a way to meet this challenge.