Experimental Work on Possibilities to Predict Casting Defects in LPDC Brass Castings

摘要

In water tap production 0.5 mm of material needs to be ground off from the surface of LPDC (Low Pressure Die Casting) brass castings in order to remove the defects deteriorating the quality of later applied coating. In order to minimize the amount of removed material the causes of these defects need to be discovered and properly connected with the process parameter window. At Oras Oy foundry in Finland, nearly 100 castings were produced under actual process conditions. To monitor the process seven thermocouples were inserted into the die. Thermal camera was also used for monitoring the die conditions during the open time of the die. Castings were divided into sets of ten pieces for statistical reasons. A few key process parameters were selected based on the basis of earlier knowledge and they were systematically varied during casting experiments. Each cast piece was marked and later analysed in order to find the dependencies between detected defects and process parameters. Computer simulations of the process were conducted to study the possibility to use numerical simulations for defect prediction. It was found that shrinkage defects could be reasonably well predicted and the influence of the process parameters on their formation was also apparent. The predictability of surface defects, however, was poor and only indirect conclusions could be made. Observations were made using as cast, ground and polished and cut surfaces from certain sections of the castings. It is very difficult to make any conclusions on surface defect formation based on parameter variation. One reason probably is the too narrow process window, but several promising ideas on the influence of e.g. mould shape, temperature and composition of the graphite coating on the defect formation was discovered.