Simple Solutions That Work! Issue 11

34 somewhat different. Accelerated flask and pattern wear is a prob- lem due to the “sand blasting” ef- fect caused by the blowing sand. Any projections on the pattern directly in front of the blow slots will wear the fastest and experi- ence the most severe damage. Also, operating costs and down time will be higher with numerous blow seals and flask and pattern vents to maintain and replace. Squeezing the mold from both sides requires that both halves of the mold be filled with sand uni- formly, and the squeeze pressures on both cylinders be perfectly bal-anced. Otherwise, the result will be a damaged or broken pattern plate. Users of blow-type machines must be very careful to adjust the machine’s operating program during every pattern change. Attempting to make a mold that is not tall enough for a particular pattern can easily break or damage the pattern plate. High sand compactability when making a mold or a blow that is short of sand can also break the pattern. In contrast, the gravity filled ma- chine squeezes from the bottom and the pattern and flasks move up against a fixed squeeze head. In this process the pattern essen- tially floats, so that the squeezing forces on both sides of the pat- tern are naturally equal. The con- trol system also protects against over squeezing due to short filling of the flasks. Large asymmetrical castings such as cooking pots or brake drums inherently cause the biggest problems for these machines. One side with a large cavity will require significantly more sand and a longer stroke to squeeze. The other side with a large flat surface on the projection will re- quire less sand and only permit a short squeeze stroke. This is very difficult to balance and control with the blow-fill machines limit- ed ability to consistently fill and squeeze deep pockets to an ade- quate and consistent mold density. Blow-fill machines permit only small variations in sand prop- erties, especially moisture and compactability. Wet sand and/or higher clay levels will quickly plug up the machine’s blow head. Sand Segregation Most foundry workers are a are of sand segregation problems that occur when sand is transported pneumat-ically. There are indica- tions that this occurs to degree during the blow-fill molding pro- cess and that finer material ends up at the mold interface on the joint line of the mold. This further reduces the mold permeability and can contribute to gas problems in the mold. In con-trast, gravity-fill machines do not fluidize sand, so this is not an issue. Gravity ma- chines can easily adjust squeeze pressures and vibration to produce a mold of optimum and uniform hardness for even the most diffi- cult pattern configurations. Operating blow-fill machines in warm or tropical climates can be difficult. A surge tank is required near the machine to supply a large quantity of air on demand. The air must be dry and there can be no fluctuations in the air pressure at the machine. They use large amounts of compressed air to blow the sand. In general they will consume 3 to 4 times as much compressed air as a gravity fill ma- chine.Warm or hot sand combined with compressed air can cause condensation in the blow cham- ber. This in turn can cause sand to partially clog or plug up the blow chamber. A batch of sand that is too wet or too dry is very diffi-cult to dispose of once it enters the blow chamber of the machine. On the gravity filled machine the operator can easily inspect both sides of the pattern and the finished surface of the cope or drag molds before closing. With the blow machines, the operator never sees the drag side of the pattern or the cope mold. This means that the ma- chine could be producing molds with a defect in the cope mold and the operator would never know about it. Anything that is set on the pattern and /or invested in the mold is a problem for the blow machines. This includes ram- up cores, chills, filters, chap- lets, facing sand, exother-mic sleeves, insulated or even open risers, etc. The blow fill process precludes all of these opera- tions, since the sand rushing in and pattern tipping up vertical- ly during the molding process would dislodge anything placed on the pattern. As always it should be kept in mind that green sand mold quality is a function of pattern and sand quality with the third essential ingredient being high density and uniform compac- tion of the mold. Horizontal Tight Flask Molding Systems The development of high pres- sure molding using flasks, took place in the USA in the mid 195O’sh. These handling sys- tems were a natural progression away from heavy man-ual labor, and the reliance on labor to meet production targets. The IN-LINE arrangement is a single line arrangement, which runs parallel to a mould convey- or. It takes in all stages of mold manufacture from mold pick off, through punch out, flask sep- aration, molding, core placing clos-ing, and placing the com-