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Metal Casting Technologies : September 2005
Fig.3 shows an intermediate stage of molding. It may be noted in the Figure that a paper pipe of 5 mm dia. is attached to the outlet of vent in pattern so that gases generated by burning of EPS pattern can come out directly to atmosphere without resistance from loose sand. It also prevents from passing of loose sand inside vent in pattern that would otherwise definitely result in sand inclusion in casting. A pouring cup made of sodium silicate -- CO2 sand was placed over sprue of each pattern assembly to minimize inclusion of loose sand during pouring. Aluminium alloy (LM6) melted and degassed with 0.3 wt% hexachloroethane was poured into the molds at around 720˚C. The photographs of pouring stage are shown in Fig.4. OBSERVATIONS AND DISCUSSION The basic objective of providing vents in pattern was to escape decomposition products of EPS. Fig.4 shows that the vents provided in the pattern was quite effective to escape decomposition gaseous products, as dense white fumes were found to come out vigorously through the exhaust pipes connected to vents. Castings were reasonably good (Fig.5). Early stage of pouring was more convenient in case of vented patterns due to the fact that the gases formed escaped through vents. Without vents in patterns, pouring had to be stopped for some time at the early stage of pouring, as gases came back through sprue causing vigorous bubbling in poured liquid metal. Earlier studies have confirmed that higher permeability of coating over pattern increases metal velocity, as back pressure of decomposition gases reduces in case of higher permeability coating. Quick filling of liquid metal reduces the risk of mold collapse, since less time is available to collapse the mold. It also reduces the chances of misrun defect formation. However higher metal velocity increases the tendency to form fold defect, which are formed due to accumulation of liquid polystyrene over coating surface during filling of casting. Faster metal filling with higher permeability coating limits the scope of decomposition of liquid polystyrene and hence are captured in the flowing metal to result in fold defect. Similar condition to high permeability coating prevails in vented patterns. Therefore fold defect is more likely to occur in FMC with vented pattern. CONCLUSIONS Venting in EPS pattern of FMC facilitates escape of decomposition products. The study shows that the decomposition products of EPS can be thrown outside shop to reduce pollution or these can be collected for further utilization. Further work is needed to confirm whether it reduces the chances mold collapse and misrun defect and increases the risk of fold defect. 4 5 TECHNICAL FEATURE REFERENCES 1. Dieter H. B., Paoli A. J., AFS Transactions, Vol.75, pp.147-160 (1967). 2. Tseng C., Askeland D. R., AFS Transactions, Vol.99, pp.455-464 (1991). 3. Littleton H. E., Miller B., Sheldon D., Bates C. E., Foundry Management and Technology, pp.41-45, Feb. 1997. 4. Bennett S., Moody T., Vrieze A., Jackson M., Askeland D. R., Ramsay C. W., AFS Transactions, Vol.107, pp.795-803 (1999). 5. W. L. Sun, H. E. Littleton, C. E. Bates, AFS Transactions, Vol.110, pp.1347-1356 (2002). 6. M. A. Tschopp Jr., Q. G. Wand, M. J. DeWyse, AFS Transactions, Vol.110, pp.1371-1386 (2002). To subscribe please complete the subscription form on page 84 and fax back to Melinda on +61 2 9555 1496 casting TECHNOLOGIES 58 www.metals.rala.com.au