Understanding Rheology

Understanding the factors that affect rheology can enable manufacturers to change the rheology characteristics of the slip and optimize its performance in sanitaryware production.

In simple terms, rheology is defined as the flow of matter and its deformation character. This article will refer to rheology as it pertains to aqueous (water-containing) suspensions of clay and non-plastics possessing characteristics of both a viscous (thick) liquid and elastic solid.

The conditions of rheology have a strong impact on the performance quality of the slip during sanitaryware casting. Improper slip rheology can cause sticking of the cast piece in the mold, a deficient cast rate, a soft cast or distorted ware, a brittle cast or an improper trim quality, cast spots (also called soda spots or hard spots), a premature mold release, and a cessation of the dewatering process.

Influences on Rheology

From a practical viewpoint, three key factors influence the rheology of a ceramic slip cast system: the solids-to-water ratio (specific gravity) of the slip; the deflocculation level (dispersion); and the type and amount of chemical modifiers used.

Specific Gravity. The water content of a slip serves as a mobility factor for the solids and influences the thixotropy (long-term viscosity) properties of the suspension. The thixotropy quality of the slip has very definite influences on the casting characteristics of the formula in terms of the casting rate and the plasticity of the cast piece.

Dispersion. Modifying rheology through the use of chemical deflocculants or dispersants has important effects on the level of thixotropy. This, in turn, influences the plastic qualities of the slip and also the slip performance parameters.

Chemical Modifiers. Sulfate compounds have been used for years as a means of gaining deflocculation range and obtaining rates of cast from thixotropic viscosity conditions. This rheology performance feature opens the cast structure of the slip to permit the flow of water into the mold. Chemical modification of rheology became especially important once ball clays and kaolin were made available in slurry form. This product form created excessive deflocculance for the end use application. Modifying the slip with chemicals provided a means to overcome this condition simply and economically.

More recently, other slip modifiers have been made available to flocculate the fines in the system and prevent their migration to the mold/cast interface. These are particularly useful in pressure cast systems, where rheology plays an extremely important role in optimizing yield rates.

Figure 1. The influence of specific gravity on the deflocculation curve.

Changing Specific Gravity

For a number of years, the U.S. sanitaryware industry has commonly used a specific gravity in the range of 1.83. However, changing this specific gravity could provide better casting rates.

Figure 1 illustrates the effect that specific gravity has on the deflocculation curve. The specific curve is on a sanitaryware formula comprising 52% total clay content and 48% non-plastics material. The plastics portion consists of 30% ball clay and 22% kaolin. Increasing the specific gravity from 1.78 to 1.83 changes the slope of the curve to indicate less sensitivity to deflocculant demand, thus creating an expanded deflocculation range.

Figure 2. The effect of sulfate addition: 1.83 specific gravity.
In Figure 2, the effect of specific percentage additions of a gypsum compound can be observed. The progressive addition of 0.02 and 0.04% (based on total dry batch weight) shifts the curve to the right and moderates its slope. This effect substantially expands the deflocculation range of the formula, and likewise alters the formula’s casting quality. The farthest displaced curve to the right would be expected to have an increased rate of cast and perhaps a more plastic behavior due to rheology modification through these chemical means.

Figure 3. The effect of sulfate addition: 1.78 specific gravity.
Figure 3 illustrates the same composition at a 1.78 density with similarly added percentages of gypsum additions. In this curve, the sulfate increases are more dramatically observed, especially since the slip formula incorporates the clay additions in slurry form. The range is considerably expanded, and very significant increases in casting rates can be expected. The lower density (higher water content) slip has a particle configuration structure permitting the rapid movement of water into the mold, which provides the greater casting rate compared to the higher density (1.83) slip.

Figure 4. Gellation curve.

Changing Thixotropy

Up to this point, only the initial viscosity features have been emphasized. However, another type of viscosity is also very important in ceramic slip systems: the thixotropy or gellation nature of the slip. Figure 4 represents a range of gellation curves commonly found in slips throughout the sanitaryware industry. The two curves in the center of the illustration are probably more typical of slips.

The significance of this type of curve, and the rheology it represents, is the gel structure necessary for proper dewatering of the slip during the casting process. It is important to understand that the permeability of the slip is enhanced with proper deflocculation. The deflocculation builds thixotropy, which can be easily sheared to provide proper drainage and firming characteristics of the slip and cast structure. In large part, thixotropy can be used to enhance the permeability of the slip composition aside from the natural permeability of the raw materials.

Minimizing the Moisture Gradient

Specific rheology requirements will vary depending on the production system used. The objective in the slip casting process is to minimize the moisture gradient in the cast piece cross section. This is the area between the mold/slip interface and the drain face of the cast structure.

Figure 5. Water distribution in slip casting.
The concept of the moisture gradient objective is illustrated in Figure 5. In the center of the illustration, the slope of the line connecting the moisture level of the mold face to the drain face should be as low as possible.1 Proper rheology influences this slope by developing a proper cast structure, which, in turn, permits easy water migration through the cast and subsequent firming of the cast for proper handling. Once the cast is removed from the mold, rheology facilitates the drying performance of the piece by continuing the water migration process to the surface of the cast.

The Role of Rheology

As illustrated in this article, the rheology of a slip does have an effect on sanitaryware production. It is equally as important as the formula in maximizing slip cast performance. The initial viscosity characteristics and thixotropy qualities of the slip should be evaluated simultaneously to understand the rheological performance range of the formula. When used in combination with the right raw material selections and percentages, the proper rheology can help optimize production yields.

For More Information

For more information about the role of rheology in sanitaryware slip casting, contact Old Hickory Clay Co., Box 66, Hickory, KY 42051-0066; (270) 247-3042; fax (270) 247-1842; e-mail psp@oldhickoryclay.com; or visit www.oldhickoryclay.com.

Rheology Requirements - Standard Drain Cast

  • Specific Gravity Range: 1.80-1.83
  • Soluble Sulfate: 350-450 ppm
  • Deflocculation Level: Mid-range on curve
  • Important Slip Performance Features:
    -Suitable Cast Rate
    -Clean Drain Characteristics
    -Sufficient Plasticity/Low Moisture Gradient

Rheology Requirements - Standard Solid Cast

  • Typical Specific Gravity Range: 1.83-1.85
  • Soluble Sulfate: 350-500 ppm
  • Deflocculation Level: Mid to upper level on curve
  • Important Slip Performance Features
    -Suitable Cast Rate
    -High Permeability/Well Developed Particle Floccs
    -Sufficient Plasticity But Firm/Low Moisture Gradient

Rheology Requirements - Pressure Cast

  • Specific Gravity Range: 1.78-1.80
  • Soluble Sulfate: 500 ppm or more (Filtration aid chemical may be a desired option.)
  • Deflocculation Level: Mid to high range on curve
  • Important Slip Performance Features:
    -Maximum Filter Cast Rate
    -Uniform Moisture Distribution
    -Sufficient Plasticity
    -Minimum Segregation of Fine/Coarse Raw Material Components

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