MEASUREMENT OF ELASTIC MODULUS OF PUNB BONDED SAND AS A FUNCTION OF TEMPERATURE
J. Thole and C. Beckermann University of Iowa, Iowa City, IA, USA
Copyright © 2010 American Foundry Society Abstract
The stiffness of molds and cores has a large effect on casting quality in sand foundries. Measurements of the elastic modulus of PUNB bonded silica sand are performed from room temperature to 500C (932F). The measurements are taken almost instantaneously during the heating to capture the changes in the elastic modulus throughout the temperature range. The effect of the heating rate is investigated in detail. For an intermediate heating rate of 8C/min (14.4F/min), the elastic modulus decreases steeply from a room temperature value of about 3,900 MPa to 600 MPa at 125C (257F). Above 250C (482F), it increases to 1,200 MPa at 280C (536F) and then decreases again to 900 MPa at 325C (617F). Above 350C (662F), the elastic modulus increases almost linearly with temperature until it reaches 2,400 MPa at 500C (932F). At approximately 500C (932F), the strength of the bonded sand vanishes. At elevated temperatures, the elastic modulus
Introduction
The stiffness of the bonded sand used to make molds and cores has a large effect on the quality of castings produced in sand foundries. Weak molds and cores can result in excessive casting distortion or warpage. Many casting defects, such as hot tears and veins, are also strongly affected by the stiffness of the mold and cores. In this respect, it is important to not only know the stiff- ness of the bonded sand at room temperature, but also at the elevated temperatures encountered during and after solidification of a casting.
Casting simulation software has recently made some prog- ress in the area of prediction of stresses, distortion and re- lated defects. However, significant gaps exist in the knowl- edge of the mechanical properties of the metal and mold materials. Monroe et al.1
is found to be a strong function of the heating rate and time. For example, while holding a specimen at a constant temperature of 200C (392F) for 30 min, the elastic modulus can increase from 600 MPa to 2,000 MPa. Only the steady- state values of the elastic modulus are in agreement with previous measurements. Upon cooling to room temperature, the bonded sand regains its full stiffness only for holding temperatures below about 275C (527F). These variations in the elastic modulus with temperature correlate well with the physical and chemical changes the binder undergoes during heating. Additional experiments are needed to investigate the elastic modulus variation for higher heating rates and other binder systems.
Keywords: mold properties, stiffness measurement, PUNB bonded sand, elastic modulus, casting simulation
Measurements of the elastic modulus of chemically bonded sand first appeared in the late 1950’s, where the effects of grip, shape and gauge length of tensile test specimens were investigated by Wallace and coworkers.2
In a later study3
employing a strain gauge, they verified that approximately the same value for the elastic modulus at room temperature is obtained when measured under compressive, tensile or bending loading. It was also found that for all loading meth- ods, the bonded sand shows both elastic and brittle behavior.
More recently, the elastic modulus of phenolic urethane cold-box (PUCB) bonded sand was measured at elevated temperatures using a three-point bend apparatus.4
It was found that the predicted stresses
and distortions in steel casting are particularly sensitive to the elastic modulus (stiffness) of the sand mold. The objec- tive of the present study is to accurately measure the elas- tic modulus of phenolic urethane no-bake (PUNB) bonded sand as a function of temperature, in order to provide im- proved input data for stress simulations.
found in this study that the elastic modulus decreases from about 4,300 MPa at room temperature to 2,200 MPa at 200C (392F). Above 200C (392F), the elastic modu- lus was observed to remain constant, except for a tempo- rary re-stiffening to 3,000 MPa at 300C (572F). At 400C (752F), another increase to 3,000 MPa was measured. Very recently, Thiel5
performed elevated temperature
elastic modulus measurements of chemically bonded sand using a tensile testing machine with an extensometer and an elongated dog-bone test specimen. The results were in approximate agreement with those of Reference 4. In
International Journal of Metalcasting/Fall 10
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