Simple Solutions That Work! Issue 11

called spring-back in the sand. Under these conditions the sand actually springs back after the squeeze, and control over the casting dimensional tolerances is lost. Since there is not enough space between the sand grains for normal thermal expansion, there will be expansion defects in the castings and other defects due to cracking of the mold. There are newer technology improvements of machines that work with gravity to fill the flask. Sand is riddled evenly onto the pattern through an aerator, and the flask & pattern is vibrated during the fill. The mold is then squeezed and drawn according to a computer controlled recipe. Sand in the flask is still highly flowable and can normally be compacted to mold hardness in excess of 90 with a squeeze pressure of only about 100psi. As previously mentioned, any large projections or obstructions on the pattern plate make it dif- ficult to get an adequately dense fill of the mold on the side of the mold opposite the blow slot. This causes what is commonly known as the “shadow effect” which is a soft spot in the mold. These soft spots prevent the mold from being uniformly dense and are a troublesome source of casting defects and scrap. This is why blow type machines are generally limited to doing smaller castings without any deep or difficult pat- tern shapes. Large, deep or asymmetrical castings often require the use of patterns with an offset parting line. This offset parting will cre- ate an additional obstacle to the sand being blown in at 90º to the surface of the pattern plate. The obstruction is at least equal to the amount of offset on the parting line of the pattern. Since anything sticking up from the pattern can exacerbate these problems, the blow process also severely limits where pouring basins, down sprues and risers to feed the casting can be located. This can be a serious problem when large risers are required or the pattern plate is crowded. The pouring cup and down sprue locations are further restrict- ed due to the squeeze cylinder location and other mechanisms on the back side of the squeeze head. This restriction can cause a problem with efficient pattern layout, and the number of parts or pieces you can get per mold. It also dictates which way any given pattern must be oriented when the job is run on the ma- chine which can cause difficul- ties for efficient core setting or pouring. Patterns with deeper green sand pockets require venting to run on blow-fill machines. These vents are used in an attempt to get sand to flow into deeper pock- ets. This increases the cost of the pattern and later becomes a maintenance issue. While vents in the flasks are necessary for venting the mold, they are not very effective for filling pockets. If both halves of the mold are blown simultaneously, the blow pressure on both halves of the mold is equal. Then, how can venting through the pattern to fill a pocket on one side make any difference? Some blow-fill machines have the ability to blow the cope first and then the drag, or the reverse, depending on which side has the deepest pockets. This is called a “stag-gered-blow.” The theory is that the air will escape through the pattern vents and in the process carry sand with it into the pockets. The realities may be 33 and are more suited to deep and complicated pattern shapes. This has been a well known weakness of the side blow ma- chines since were invented 50 years ago. Even though these newer machines tip the flasks and patterns up on edge to blow, it is still essentially a “side-blow” relative to the pattern. Blowing a mold at the typical 40 or 50 psi pre-compacts the sand before it is squeezed. This makes it necessary for blow-fill machines to squeeze harder in attempt to force the sand into pockets on the pattern. This often results in a mold that is harder than desirable on the flat areas and the parting line, but too soft in deeper pockets and mold cavities. These higher squeeze pressures require thicker and more expensive patterns to avoid pattern flexing and break- age. If a green sand pocket is not completely filled during the blow cycle, it is virtually impossible to squeeze hard enough to correct this problem. Aluminum foundries do not want to make a mold that is too hard because of gas problems caused by the reduced permeability of harder molds. Blow-fill machines sometimes have problems in this area due to the fact that they must squeeze harder to get an acceptable mold density in deeper pockets. The important point is that mold hardness alone is not necessarily good; uniform den-sity and repeatability is! Squeezing a mold too hard can cause green sand molds to break off rather than draw properly. Research has proven that any increase in mold hard- ness at squeeze pressures above 140 psi is almost negligible. Further-more, higher squeeze pressures cause a phenomenon Continued on next page MAKING YOUR INSTALLATION ASUCCESS