Oolitization

Formation of fireclay shells around the quartz grains of a bentonite-bound molding sand caused by high thermal loads.

The extent of oolitization is influenced by the degree of precoating during mold material conditioning, thermal loads during casting and by the quantity of new, refreshing substances introduced to molding material circulation . In particular, the proportion of new sand (including the coresand supply) defines the degree of oolitization.

If only low quantities of new sand are added, a specific sand grain remains longer in the circulation. This means that a grain is repeatedly coated in binder shells that are ceramically fixed on the grain by sintering (Figures 1 and 2).

The fireclay shells have a microporous structure, their density is considerably lower (ρ = 1.80 g/cm3) als die des of quartz.

Proof of oolitization can be furnished by means of the following methods:

  • Microscopic examination of the quartz grains with the help of petrographic thin sections
  • Determination of the standard test specimen mass; progressive oolitization minimizes the mold material mass that is required for production of the test specimen; reference values are contained Table 1
  • Definition of the degree of oolitization by determining the true density using a pycnometer
  • Determining the degree of oolitization by removing the firecly shell using hydrophoric acid or phosphoric acid and differential weighing

An increasing proportion of overburned bonding clay lowers the sintering temperature. Up to an oolitization degree of 10%, the sintering temperature is only moderately reduced, i.e. the refractoriness (sintering resistance) of quartz sand is decisive for the behavior. Above an oolitization degree of 10%, the sintering temperature rapidly decreases. The surface quality of the castings demonstrates a similar behavior.

The increasing ratio of inactive fine particles almost proportionally decreases the sintering temperature and increases roughness.The increasing oolitization degree continuously decreases the absolute expansion value of the mold material; at the same time, the critical scabbing time increases. In contrast, inactive fine particles increase the absolute expansion value thus increasing the tendency towards expansion defects (s. Sand expansion defects).

The main reason is the increasing water demand of the mold materials and the higher packing density. As a result, the compressive stresses increase, the wet tensile strength is reduced. Increasing oolitization causes slight increase in permeability to gas; on the other hand peremeability to gas is considerably minimized through increasingly inactive fines.

  • Fig. 1: Structure of an oolitized sand grain in conditioned circulation sand (schematic)
  • Fig. 2: Thin section image of oolitized quartz grains
  • Table 1: Correlation between the mass of the standard test specimen and the oolitization degree (s. Figure 3)
  • Fig. 3: Schematic illustration of the composition of two recirculated molding sands at different conditioning status; left: considerably regenerated mold sand (using new sand) with a high quartz content, tendency for sand expansion defects, mold explosion or penetrationright: Sand with reduced quartz content and increased content of quartz-replacing component, no tendency for expansion defects or mold explosion
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