GAS PRESSURE IN ALUMINUM BLOCK WATER JACKET CORES A. Starobin
Flow Science, Inc., Santa Fe, NM, USA
Currently under a Research Contract with the American Foundry Society D. Goettsch
GM Powertrain, Pontiac, MI, USA M. Walker
GM R&D Center, Warren, MI, USA D. Burch
Alchemcast LLC, Birmingham, AL, USA Copyright © 2011 American Foundry Society Abstract
Thermophysical properties of mixture gas are computed from prior laboratory pyrolysis studies of polyurethane cold-box binder (PUCB) performed by Lytle, Bertsch and McKinley.1
This gas is found to have an overall gas
constant of 230 J/kg/C. Controlled aluminum submersion tests of a PUCB bonded water jacket core yield values of gas pressure at discrete points during fill of an engine-
Introduction
In general in foundry practice it is difficult to identify the source of the gas defects in the finished casting. Possible sources of gas include atmospheric gases entrained during pour, gases dissolved in the metal, water vapor from green sand molds blown back into the casting, gases evolved from core and mold coatings and gases produced in the course of core binder pyrolysis. The physical and numerical modeling of the casting process offers an opportunity to study each source of gas independently and develop solutions to reduce the occurrence of gas defects.
This work examines the role of core binder gas in defect formation in aluminum castings and shows significant prog- ress from previous attempts to quantitatively understand core gas pressure.2,4
through pyrolysis of polyurethane cold-box Isocure®
block casting. These are found in adequate agreement with predictions of a physical model that considers details of binder pyrolysis and product gas transport in the sand core.
Keywords: thermal degradation of binders, core gas pressure prediction, aluminum casting gas defects
input, and use this input along with the detailed knowledge of core geometry and the filling sequence to make predic- tions of core gas pressure. The computed pressure values are compared against pressures obtained from the combination of foundry core submersion tests and real-time X-ray recordings of gas blow performed at the General Motors Corporation.
Parameters Affecting Core Gas Pressure
The basic estimate for peak core gas pressure goes back to Campbell’s description of binder gas generation and trans- port.6
Essentially a thin pyrolysis zone develops near the
Specific focus is on the gases generated binder
(hereafter simply referred to as a PUCB binder) under alu- minum casting conditions. The physical description and the computational approach taken are general and could be used for other binders and other metal castings if the necessary data is available.
The specific goals were to take the detailed laboratory data on the pyrolysis of the binder, 1,3
deduce the essential physical
surface of the core as the metal fills the casting cavity. The products of pyrolysis are then transported from the core sur- face to the core prints down the gas pressure gradient. Along the flow path large temperature variations are expected as the pyrolysis range of a typical resin binder is well above room temperature.
core,up
proportional to the total core area exposed to the metal, Ac
As a general rule, the peak gas pressure is proportional to the pyrolysable binder density in the core, ρb
. It is also
, and to the mean speed of the pyrolysis zone into the . This speed is a complex quantity that depends on
the binder, on the metal cast and on the shape and immer- sion history of the core.
International Journal of Metalcasting/Summer 2011
57
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