Ceramic Industry

How Low Can You Go?

March 1, 2002
A new resin system allows resin molds to be used with capillary-effect low pressure, making them cost-effective for even medium-output, complex operations.

The new microporous resin system is capable of providing high performance at low pressures and an affordable cost.
Conventional sanitaryware production uses plaster molds, which operate on the principle of capillarity—the same system that allows plants to transport water from their roots to the highest leaf without the aid of any other mechanism. The plaster works like a sponge and “sucks” the water from the slip. Plaster molds operate at low pressure and are relatively inexpensive, but they do carry several drawbacks. Forming and reuse times are quite lengthy, and the molds typically have a limited working life, often making them impractical for large-volume operations.

Porous resins have been used as an alternative mold-making material for several decades, and their use has coincided with the introduction of higher and higher casting pressures. Today, pressures of 13-15 bars are common. The resins work like filters, trapping the ceramic body particles contained in the slip while drawing off the water. Pressure casting with resin molds has resulted in shorter casting cycle times, more compact casting departments and significantly reduced labor requirements. However, the high pressures and complexity of these automatically controlled cycles have led to the development of machines that require a considerable economic investment. Hence, their use can only be justified where the output is particularly high and the product mix not too demanding.

In most cases, the resins themselves dictate the use of high pressures. Two types of resins are used for pressure casting: microporous and macroporous. The former have an average pore diameter (note: this should not be confused with pore radius) of no greater than 10 microns, while the latter normally have average pore diameters of around 20 microns. However, both resin types have a very low or nonexistent (in the case of macroporous resins) thickness-forming capacity, i.e., a very low capacity to remove the water contained in the slip, as shown in Table 1.

In the last column of the table, the thickness-forming capacity (at atmospheric pressure) for various resins is compared to that for plaster over a period of 60 minutes. The thickness formed by the plaster is defined as 100 and acts as the reference number for values assigned to the other materials. A look at the table shows that the thickness-forming capacity of microporous resins is 30-40% of plaster capacity, while macroporous resins fail to reach even the 10% mark. Consequently, if a sanitaryware article is produced using a resin mold, it is absolutely necessary to apply a certain pressure (from 6-7 bars upwards) to the slip. At lower pressures, formation times will be unacceptably long.

Pressure casting techniques have now reached levels of reliability and quality that make their application imperative where manufacturing conditions, output volumes and the product mix permit. Yet what does the technology have to offer when these conditions are unattainable or have been only partially achieved? This problem is particularly evident in the production of toilets and bidets, complex articles that can only be produced in four- or five-part molds. Such manufacturers often fail to achieve the output volume and production mix thresholds that justify the use of resin molds.

Developing a Low-Pressure Resin

For nearly 10 years, researchers have been trying to find both the ideal resin formula and the most suitable drainage system for lower-pressure casting technology. Five years ago, the first industrial attempts were made to use the resins shown in Table 2 with a specially built casting bench. However, while the system met nearly all the technical requirements, it was also very expensive, and the endeavor was eventually abandoned.

Unlike high-pressure resins, which operate by filtration, the two resins shown in Table 2 both operated on the basis of capillarity, yet they produced different results. While the first exceeded the thickness-forming capacity of plaster by up to 20-30%, it caused serious de-molding problems. The second resin required 20-30% more forming time than needed with plaster molds but released the piece more easily.

By raising the slip pressure slightly to 1.5-2 bars, researchers were able to improve the capillary action of the second resin and accelerate its thickness-forming times. However, this made the casting bench somewhat complex and therefore expensive, thus making it an investment of dubious economic advantage.

Based on these results, additional research efforts were launched to modify the relevant parameters. Recently, these efforts have resulted in the introduction of a new microporous resin system* capable of providing high performance at low pressures and an affordable cost.

A New Pressure Casting Solution

As shown in Table 3 and Figure 1, the new resin system behaves similarly to plaster (the thickness-forming capacity is about 105-110% that of plaster), thus allowing equivalent technological processes to be used. The new resin allows castings to be made at a slip pressure of just 0.3 bar and features cycle times similar to those achieved with plaster molds.

Figure 1. Comparison of a typical porous resin (a) and the new resin (b) under an electron microscope. The new resin is rather sponge-like and, in practice, works like a sponge by “sucking” the water from the slip, just like plaster. The difference is that it does so in a shorter amount of time without wearing the mold, and thus lasts longer.
To test the new technology, molds for open rim and box rim toilets produced with loose pieces were first put through long-term tests in laboratories and then underwent industrial trials at several factories. Following is a brief summary of how the results from the new resin molds compared to those obtained with plaster:
  • Mold preparation time was shorter than with plaster.
  • No differences were noted in thickness formation, emptying, consolidation and natural hardening.
  • De-molding times were slightly higher than with plaster molds. The additional time varied from just a few seconds to two or three minutes, depending on the complexity of the model.
  • The cast product had a very smooth, even surface texture. Unlike products made in plaster molds, the quality of the joins (i.e., the amount of burr on the piece) was good and remained constant over time.
To maximize the new resin technology, researchers also developed a new three- and four-module bench** capable of handling 10 of the new resin molds—with the appropriate air and water feeds. The new bench proved to be extremely simple and low-cost, thanks to the characteristics of the new resin. Where conditions allow, the new resin molds can also be used on previously installed, already-operative casting benches.

The Best of Both Worlds

With the development of the new resin system, resin molds are no longer limited to the high-output production of relatively simple pieces. Today, manufacturers producing medium quantities of complex pieces can now use resin molds while keeping costs low and guaranteeing a level of quality that is typically unattainable with plaster. The new system offers a wide range of opportunities in that output range where use of plaster molds is no longer convenient but where the use of resin molds has yet to become so.

Based on these new developments, the toilet and bidet casting rooms of the future may be very different from today’s operations. Plaster molds may be used alongside the new resin molds and pressure casting machines, with sometimes demanding production mixes. Articles may be subdivided among the various production technologies according to output levels and the complexity of each individual model. With the new resin technology, manufacturers can truly maximize the efficiency of their production operations.

For More Information

For more information about the new resin system and/or the new casting bench, contact Sacmi Whiteware, Via Selice Prov.le 17/a, 40026 Imola Bo, Italy; (39) 542-607111; fax (39) 542-642354; e-mail sacmi@sacmi.it; or visit www.sacmi.com.

*LOPREM, developed by Sacmi Whiteware
**The GVU 010, developed by Sacmi Whiteware