Fig. 1-2. These are the pattern layouts for the steel castings, with the step cone test that was developed to detect gas in the phenolic urethane binder system.
A conservative estimate is that a majority of all castings produced require some sort of machin- ing operation. As much as half of the machine stock added is commonly required to account for variations in the final dimensions. These final casting dimensions are affected by several issues. Conventional molding methods rely on forming casting cavities by forming sand around pattern shapes. CNC machining has much improved the dimensional accuracy from manual pattern making methods allowing tooling toler- ances in the thousandths of an inch from nominal. The molding process itself imparts a large degree of variation in transferring pattern geometry to molds and cores. These production methods inherently add as much as +/- 0.02 – 0.03 in. variation to the mold’s surface dimensions. All of these considerations fail to account for the variation of pattern dimensions to final casting dimensions. Accurately measuring the high
2
tridymite or cristobalite transformations. Tese changes in
Fig. 3. The baseline silica thermal expansion curve shows the results of expansion, with silica sand undergoing an alpha to beta transition at approximately 1,058 F (570 C).
dimensions cause changes in the volume of the cast- ing cavity as well as the individual features. Te amount of dimensional change is directly pro- portional to the amount of heat transferred from the liquid metal to the mold and the length of time before solidification occurs. Te final dimen- sions of the casting also are affected by issues such as mold wall movement or casting swell in both green sand and chemi- cally bonded molds. Computer modeling
of the casting process has given us tools to understand and predict not only the internal
temperature dimensional changes in molding materials has shed new light on additional sources of dimensional variation in final casting dimensions. Dimensional changes of 1–1.5% result from the phase transformation of silica from alpha to beta quartz. Changes over 4% are seen as a result of either
Procedure Te step cone casting test
was originally developed to detect gas in the newly developed phenolic urethane
binder system (Fig. 1-2). Since then, its use has expanded to include the testing aggregates and binders for their
susceptibility to penetration and vein- ing defects.
Tis test is conducted by pouring metal against a step cone core. Te step cone core consists of six different sections with internal diameters from 1.5-4 in. (3.81 - 10.16 cm) in 0.5 in. (1.27 cm) increments. Te different
soundness of the castings to opti- mize feeding systems, but also to understand thermal events at a higher level. Although at this time this method only considers one vari- able in a group of many, it provides a valuable and at times extremely accurate prediction of final casting dimensions.
steps represent different section thick- nesses of the metal casting and give a good understanding of the role of different cooling rates of the metal in casting quality and defects. Te flaskless mold is produced using a similar binder system, but it does not affect the veining, penetration
July 2016 MODERN CASTING | 37
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