Linear contraction of grey iron sand castings

Abstract

A study has been made on the linear contraction of grey iron castings in greensand and silicate-bonded sands. Square cross sections were used to establish the effect of size, for free, as well as constrained contraction, while circular and triangular sections were used to study the effects of shape. A range of sizes of end flange was used to study the effect of constraint.

It was found that free linear contraction for both greensand and silicate-bonded sands is inversely related to modulus. For thin section castings the contraction is close to 1.20%. For greensand the contraction decreased to 0.70% and for silicate-bonded sand it decreased to about 0.9% when the modulus was increased to 12.5 mm. For any given values of modulus the silicate-bonded sand had higher contraction values. The effect of shape appeared to relate to the difference in modulus values.

An evaluation of constrained contraction of bars indicated that contraction was a function of mechanical constraint, C_m, defined as a product of (flange area/casting cross section area) and the third power of the ratio (casting modulus/constraining flange modulus). The latter ratio is a measure of the difference in cooling rates of the two sections. However, a limit was observed where further increase in C_m did not further reduce contraction values in either of the two moulding media.

The study indicates a pattern of contraction values for simple shaped castings. This is the first step towards the prediction at the design stage of contraction values for complex castings.

Keywords:

• linear contraction
• sand castings
• grey iron
• contraction values