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Fig. 2.3 Match plate pattern.
Early match plate molding required operators to assemble a pair of removable snap-flasks together with the pattern, and then fill each side of the mold with sand, followed by a simple machine squeeze cycle to make the mold. Those early mold machines were generically called squeezer machines. After stripping the mold, the snap flasks remained at the machine for reuse, and flockless molds were delivered onto a mold handling system for pour-off and cooling prior to shakeout.
d) Cope-and-Drag Patterns
Cope-and-drag patterns (Fig. 2.4) are similar to match plate patterns except that each half of the split pattern is assembled to a separate plate so separate patterns, and possibly separate machines, are used to make the mold halves. Cope-and-drag patterns include a gating and riser system.
Fig. 2.4 Cope-and-drag pattern.
The first step in match-plate design for both match plate patterns and cope-and-drag patterns is to modify the cast part’s geometry for the sand casting process. The part is scaled to accommodate metal shrinkage during the casting and machining in finishing operations. Shrink rate varies with alloy and part geometry. After the parting line is defined, draft is applied to the part. Typically two degrees, the draft allows the pattern to be removed from the cope and the drag.
As mentioned above, match plates use split patterns. The part file is separated along the parting line and the two halves are attached to the match plate base. The half that forms the cope side of the mold is assembled to the top face of the match plate, and drag side is placed on the bottom face. Next, runners, gates, risers, and wells are added. To align the cope and drag, locations are also added to the match plate. For storage purpose this type of patterns uses a removable sprue; a mounting pad is placed where the sprue will be attached.
To define the internal shape of the casting, a core is required. A core is a full-scale model of the internal surface of the casting part, which is placed in the mold cavity to form the internal surface and removed from the finished part during shakeout and further processing. The actual size of the core must include allowance for shrinking and machining. Sand cores are made of special sands, which are mixed with binders and rammed into a core box that has been made to produce cores of the proper dimensions. Cores must be baked at carefully controlled temperatures to make them hard enough to withstand the pressure exerted by the molten material. Cores of complex shapes may be made in several sections and cemented together. Depending on the geometry of the casting, the cores may require structural supports to hold them in the proper position in the mold cavity during the pouring of the molten metal. These supports are called chaplets. On pouring and solidification, the chaplets are integrated into the casting. The portions of the chaplets protruding from the castings are cut off. Figure 2.5 schematically illustrates how a core is held in the mold cavity with and without chaplets.
Fig. 2.5 Core held in place in the mold cavity: a) core held in with chaplets; b) core held without chaplets.
Sand molds are characterized by the types of sand that compose them and by the methods used to make the molds. They are often classified as greensand, dry sand, skin-dried, and no-bake molds.
Greensand molds. Clay-bonded sands have provided the principal medium from which molds for castings have been produced for centuries. In essence, the mold material consists of sand, usually silica in a quartz form, clay, and water. The water develops the bonding characteristics of the clay, which binds the sand grains together. Under the application of pressure, the mold material can be compacted around a pattern to produce a mold having sufficient rigidity to enable the metal to be poured into it to produce a casting. When the mold is used in its moist condition, it is referred to as green and the method of producing the molds is referred to as the greensand molding process. The term greensand does not refer to color but to the fact that the raw sand and binder mixture in the mold is moist or damp while the metal is being poured into it. Greensand molding is the least expensive method of making a mold, and the sand is easily recycled for subsequent use.
The sand used for greensand molding must fulfill a number of requirements:
1.It must pack tightly around the pattern, which means that it must have flowability.
2.It should be capable of being deformed slightly without cracking, so that the pattern can be withdrawn. In other words, it must exhibit plastic deformation.
3.It must have sufficient strength to strip from the pattern and support its own weight without deforming, and to withstand the pressure of the molten metal when the mold is cast. It must therefore have green strength.
4.It must be permeable, so that gases and steam can escape from the mold during casting.
5.It must have dry strength, to prevent erosion of the mold surface by liquid metal during pouring as the surface of the mold cavity dries out.
6.It must have refractoriness, to withstand the high temperature involved in pouring without melting or fusing to the casting.
7.With the exception of refractoriness, all of these requirements are dependent on the amount of active clay present and on the water content of the mixture.
Dry sand molds. If the mold is made using oily or plastic binder and dried at a temperature just above 180°C (356°F) the majority of the free moisture will be removed. This is the principal of the dry-sand molding process. Removal of the free moisture is accompanied by a significant increase in the strength and rigidity of the mold. This enables the mold to withstand much greater pressures and so, traditionally; the dry-sand process has been used in the manufacture of large, heavy castings. A dry sand molds provides better dimensional control in the cast product compared to greensand mold.
Skin-dried molds. Skin-dried mold is made in the same way as a greensand mold, but after it is made, the inside cavity surfaces need to be sprayed with a mixture of 10% water to one part molasses or lignin sulphite. The sprayed areas are dried using torches, heating lamps, or other means to a depth of about 10 to 15 mm, (0.4 to 0.6 in.), leaving a smooth hard skin.
No-bake molds. In the no-bake mold process, a synthetic liquid resin is mixed with the sand to form a filled mold that hardens at room temperature. This type of mold has a good dimensional control in high-production applications.
Molding material is a mixture of sand and other components that serve as binders. In industry the ingredients are blended together in mulling machines. To form the mold cavity, the traditional method is to pack the molding sand in a box called a flask, around a pattern, and with a gate system (pouring basin, sprue, sprue base—wall, runner, riser, and gate). Hand ramming of sand around a pattern is rarely used today except under special circumstances
