Metal Shaping Processes. Vukota Boljanovic
and incorporating a gating system using a molding machine.
Pouring. Pouring the molten compound metal into the mold.
Cooling. Cooling and solidifying the metal in the mold to form a desired shape.
Sand removal. Removing sand and scales from the surface of a separated workpiece, and removal of risers and gates.
Inspection. Performing in-process and preshipment inspection in accordance with standards.
Silica sand (SO2) is the molding aggregate most widely used by the foundry industry. Silica’s high fusion point, 1760°C (3200°F) and low rate of thermal expansion produce stable cores and molds compatible with all pouring temperatures and alloy systems. Its chemical purity also helps prevent it from interacting with catalysts or with the curing rate of chemical binders. There are two basic types of silica sand that are commercially available as a mold material. The first type is a round-grain silica sand containing roughly 99% or higher silica with minimal amounts of trace materials. The second type is lake sand. This sand contains approximately 94% silica, with the balance containing iron oxide, lime, magnesia, and alumina. Since impurities are removed, round-grain sands possess higher refractoriness than the lake sands. However, because of this much higher refractoriness, round-grain sands may have a higher propensity for veining and metal penetration defects. Lake sand has lower refractoriness but also has a lower tendency for casting defects. For proper functioning, molding sand must be able to withstand the high temperatures of molten metals, hold the shape of the mold when moist (usually with the aid of a bonding agent such as clay), be permeable enough to release gases, have sufficient strength to support the weight of the metal, and be of a fine enough texture to result in a smooth casting.
A pattern is used to make a cavity in the sand mold into which molten metal is poured. The pattern is a full-size model of the part, enlarged to account for shrinking and machining allowances in the final casting. The selection of the material used to make the pattern depends on the size and shape of the casting, the dimensional accutance and the quantity of castings required, and the molding process. Generally, material used to make patterns include wood, plastics, and metals. Patterns may be made of a combination of materials to reduce wear in critical regions, and they usually are coated with a patting agent – a liquid used over a patterns that leaves a slick film–to facilitate the removal of the casting from the molds.
There are four types of patterns: solid patterns, split patterns, match-plate patterns, and cope-and-drag patterns.
a) Solid Patterns
Figure 2.1 shows a solid pattern, also called loose pattern, made of one piece, used for simple shape and low-quantity production; its geometry is the same as the casting, adjusted in dimensions for shrinking and machining. Generally, it is made from wood and is inexpensive. However, determining the location of the parting line between two halves of the mold and positioning the gate system can be a problem.
Fig. 2.1 Solid pattern.
b) Split Patterns
A more complex pattern (Fig. 2.2) for round or irregular-shaped workpieces made in two or more parts is called split pattern. The two pattern halves usually predetermine the parting line of the mold. Split patterns are used for complex shape of the workpieces and moderate production quantities.
Fig. 2.2 Split pattern.
c) Match Plate Patterns
Match plate patterns are split patterns assembled on opposite sides of wooden or metal plates known as match plates (Fig. 2.3). Holes in the plate allow the top (core) and bottom (drag) sections of the mold to be aligned accurately. For long runs and top quality, match plate patterns, or cope-and-drag plate, are used. The major advantage to this is that a single machine can make both cope and drag molds from one pattern.
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