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Dimensionally accurate, lightweight castings immediate goal of auto industry.

The driving force for dimensional accuracy in the automotive manufacturing industry is the marketplace's demand for higher performance, efficient, reliable, compact and lighter car engines. This demand translates into a challenge for the casting producer to raise and maintain the standard for dimensional integrity and casting surface quality.

The challenge to achieve dimensional stability and accuracy in gray iron castings appears overwhelming when one considers the variables of the total system (Fig. 1). Represented are three very broad areas interacting and/or influencing each other with respect to dimensional casting accuracy. The pattern, tooling and equipment category illustrates a few of the many variables influencing dimensional stability and repeatability. The gray iron casting design function also plays a major role in accomplishing the dimensional control objective of delivering repetitive dimensional tolerances.

The casting process area is categorized into three main technology subdivisions: sand, metal and molding/core (Fig. 2). On closer examination of the green sand casting process variables, the complexity and the need for absolute control and continual monitoring to achieve consistent dimensional accuracy are readily evident.

For example, the sand type, its grain size and distribution, and the sand mixture formulation for cores and/or green sand are significant variables affecting the dimensional capability of the casting process. The metal composition, solidification characteristics and metal treatment practice also have been shown to influence casting dimensions.

Recent molding and core technology developments, both in binder systems and production procedures, have contributed significantly to improved dimensional control of castings. For instance, the impact molding process achieves mold hardnesses consistently above 90 on all surfaces, a factor critical to dimensional control.

Automotive engine blocks today are being designed to very close tolerances with reduced wall thickness requirements that are pushing the limits of the green sand casting process. The Ford five litre V8 engine block, which has been in production for more than 20 years, represents a technology with much more liberal tolerances in both machined and cast surface contours. The casting technology originally was not pressured for dimensional near-net shape capability like it is today.

In production, the block is produced basically in an all green sand mold (minimal core application) in which the loose cores are set manually into a mold-setting fixture. This practice has an inherent limitation for getting consistent dimensional accuracy. Break with Tradition

The first break with traditional cylinder block casting production was introduced with the 1.9 litre CVH, four-cylinder engine block. This casting was produced with less than 0.05% scrap and with exceptionally high dimensional tolerance control. This dimensional accuracy and control can be credited to the core design and interlocking assembly technique. The crankcase and barrel cores are blown in one piece and locked into a base core. The water jacket core is locked in between a slab core and the crankcase core to minimize core movement.

The other break with traditional engine block core practice was pre-assembling the core package and securely banding it to assure dimensional integrity during the casting process. The resulting accuracy in the block casting proved the concept and method to be a gain for increased productivity. The most recent generation engine block casting designs encouraged the foundry to take even greater advantage of coremaking and pre-assembly technology with yet another level of improvement in dimensional control and accuracy. In far too many foundries today, the belief is prevalent that the correct application of the shrink rule can solve dimensional accuracy problems. It is recognized, however, that a much more comprehensive approach is required. In Fig. 3, the effect of metal shrinkage and mold enlargement on final casting dimensions is illustrated. For cast iron, the whole process is further complicated by an expansion during solidification due to graphite formation. The impact of graphitization is not nearly as much in a rigid dry sand mold as it is in a green sand mold. For large castings, expansion due to graphite formation in green sand molds can be as high as 15%. The mechanism of green sand mold enlargement appears to be related to the water content in the molding sand. A water-rich layer is created due to migration of the water molecules as a result of molten metal causing water vaporization at the mold-metal interface. Because the compressive strength of the water-rich sand layer is considerably lower, it will collapse under the metallostatic pressure and result in mold cavity enlargement. (See Fig. 4). The influence on expansion due to the solidification mechanism and graphite shape is dramatic as it goes from flake graphite to compacted graphite and, finally, spheriodal graphite as seen in ductile iron. Spheroidal graphite solidification expansion is about four times higher than flake graphite. Solidification Studied The gray and ductile iron solidification expansion characteristics and graphite growth mechanism were studied using hard and rigid sodium silicate-bonded CO2 molds. Expansion for gray iron occurs primarily during the eutectic solidification. Graphite growth, which is responsible for expansion, occurs in the later stages of the eutectic solidification. For ductile iron, solidification expansion is preceded by some contraction first, and then expansion toward the end of eutectic solidification. This difference in mold dilation behavior is due to ductile iron solidification starting with austenite dendrite formation. This difference in solidification mechanism is further illustrated in Fig. 5, where gray iron solidifies into its characteristic pattern with the eutectic grains forming a shell next to the mold. This shell increases in thickness by growing inward and i decreases the transfer of metal pressure to the mold. Ductile iron also solidifies in a widely different pattern with dendrites nucleating throughout the entire melt. Since no shell is formed, the solidification expansion pressure is transferred totally to the mold. The mold cavity expands and, thus, cancels the benefits of graphitization dilation. There is demonstrated data for gray iron cast in green sand molds that, as the amount of graphite increases, mold expansion also increases while the defect volume or shrink decreases. Figure 6 shows data that illustrates the effect of inoculation efficiency resulting in higher expansion or growth of the casting. Apparent is the significant difference in casting growth between green and core sand molds. The type of sand used in mold preparation will influence casting and mold expansion. Lower solidification contraction in castings was observed for a hypoeutectic ductile iron cast in zircon sand as compared with silica sand. This is explained by the lower expansion coefficient and lack of phase transformation in the zircon sand as compared with the silica sand.

The type and amount of green sand molding additive affect the dimensional variation in hypoeutectic gray iron. It appears that while sea coal, sea coal substitute, sodium silicate and fuel oil were effective in controlling mold cavity enlargement, ground anthracite was not.

Molding sands with sea coal and sea coal substitute exhibited greater high temperature strength than the base sand or the sand with hard coal. The mechanism responsible for decreased mold enlargement in the presence of some of these additives seems to be related to the volatilization of the heavier coal tars from the sand layer next to the hot iron casting. As these vapors move away from the mold/metal interface, their temperature decreases and the vapors condense again. This results in increased mold strength in the highly volatile layer and in the dry sand layer adjacent to the casting. This condition tends to have a decreasing effect on mold cavity expansion.

One of the most important factors controlling green sand mold cavity enlargement is mold compaction, or hardness. Expansion during the graphite growth stage is particularly dramatic in softer molds. The effect of graphitization on mold expansion and the influence of mold hardness are illustrated convincingly with iron and steel in Fig. 7 and 8.
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Author:Hetke, Al
Publication:Modern Casting
Date:Apr 1, 1990
Words:1288
Previous Article:Testing procedures significant to casting quality.
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