Controlling sand and additives.

While the practice of pouring molten metal into molds composed of sand with clay binder and various additives has been common for several centuries, today the success of the modern green sand foundry requires a much higher degree of control than ever before. Sand types, grain size, thermal expansion, surface area and sand segregation all require close attention in order to optimize the benefits of green sand molding. A variety of sands in various AFS grain fineness numbers, as well as specialty sands such as zircon, olivine and chromite are available, each with its own unique properties that must be understood if they are to be effectively utilized and controlled.

Likewise, attention must be paid to the effects and availability of various clays, primarily fireclay, and sodium and calcium bentonite. The clay/water relationship, as well as the use of water as a bond, coolant and oxidizing agent must also be factored into the processing of system sand.

Additives, primarily carbons, cellulose, cereals and dextrine, are available with high quality, consistently reproducible properties under numerous private labels. The use of these additives in bulk, custom blended pre-mixes are popular today even in smaller foundries.

Traditionally, foundries have been aware of the necessity to control green strength, active clay, permeability, moisture and mold surface hardness. Today, new emphasis has been placed on controlling additional sand properties including sand temperature, compactability and loss on ignition.

Current state-of-the-art molding sand technology has increased the sophistication of sand mixing and monitoring. Some of today's more complex compositions require more thorough mixing, and develop adequate strengths only at high compaction energy levels. Consequently, the common three ram compaction specimen does not necessarily yield indicative results of the actual strength potential of a sand used in high-pressure molding machines. Newer tests and instruments are available today that better monitor and determine specific sand properties. Some of these are discussed below.

The deformation limit, which describes the distance a compacted specimen can be deformed before it loses its cohesion, must be incorporated into high-pressure molding system sand testing to provide practical strength level differentiation. This makes the moisture level of the sand more critical than ever, as well. Moisture increases the deformation limit more than any other parameter, since it increases the plasticity of the bentonite through swelling, but only after sufficient moisture has been added.

Particle size distribution, methylene blue index and pH tests are performed by some sophisticated foundries using state-of-the-art equipment such as laser particle counters to determine the fineness and uniformity of the clay particles. The methylene blue index yields a measure of the surface area of the clay, having a direct relationship to the laser particle count. A pH meter, using a filter press technique for water extraction, should indicate an alkaline range of 8 to 9.

A differential thermal analysis test, using a thermal analysis instrument, yields a curve characteristic for each mineral and identifies foreign mineral contaminants such as feldspar in the clay sample.

An x-ray diffraction instrument identifies combustible contaminants. These instruments determine conditions at which a bentonite does not gel, will not hydrate and contains contaminants of a variety of materials, such as calcium carbonite, which destroys the bentonite bond when subjected to heat.

Sand Reconditioning

Reconditioning of system sand is one of the most difficult steps in modern molding sand technology. The constant influx of core sand and the need to replenish systems with new sand add to the difficulty because long and intensive mulling is required before used core sand assumes the same properties as molding sand.

With the recent advent of high-frequency vibrating equipment, and the current trend toward cooling shakeout return sand prior to screening, storage and mulling, a larger percentage of used core sand is finding its way by increased attrition into the system sand. Much of this used core sand has grains coated with residual isocyanate binder, and these sand grains are more difficult to recoat with bentonite than new sand. In fact, because of this substantial influx, core sand is currently being considered as an additive by AFS and should be treated as such.

Many highly mechanized sand systems in operation today were designed and installed during the peak production years of the 1970s. For many foundries the industry slump in the early 1980s was not conducive to major investments in upgrading system capacity. Consequently, to keep up with the recent higher demands, it has resulted in frequently operating sand mixing equipment at overload conditions. Batch cycles and sand retention time in these mixers has been curtailed to produce higher volumes, while sacrificing sand quality, This, together with the more sophisticated testing required today, are issues that must be addressed if foundries are to take full advantage of their green sand systems.