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2.It should be capable of being deformed slightly without cracking, so that the pattern can be withdrawn.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 for simple casting.
To allow for easy removal of the pattern, the flask is made to separate horizontally at a parting line. When the pattern is drawn from the mold, if holes and cavities are required inside, they are made by inserts of sand (cores) before the two mold halves (the cope and drag) are reassembled; a cavity remains in the sand.
To increase production rate and improve quality of casting, a sand mixture is compacted around the pattern by a molding machine.
There are a number of techniques for doing this.
Squeeze molding machines. Squeeze-molding machines automatically insert and compact sand in a mold. The processes used are designed to produce a uniform compaction. Jolting is sometimes used to help settle the sand in a mold. These molds are made in flasks.
Sandslingers. High-speed streams of sand fill the flask uniformly and tend to pack the material effectively. Sandslingers are used to fill large flasks and are typically operated by machine.
Impact molding. A controlled explosive impulse is used to compact the sand. The mold quality with this technique is quite good.
An alternative to the traditional flask for each sand mold is flaskless molding, which refers to the use of one master flask in a mechanized system of mold production. Each sand mold is produced using the same master flask. The most frequently used include the following:
Vertical flaskless molding. In flaskless molding, the master flask is contained as an integral unit of the totally mechanized mold-producing system. Once the mold has been stripped from the integral mold-producing unit, it is held against the other half of the mold with enough pressure to allow the metal to be poured.
In the vertical flaskless systems the completely contained molding unit blows and squeezes sand against a pattern (or multiple patterns), which has been designed for a vertical gating system. Molds of this type can be produced in very high quantities per hour, and they are of high density with excellent dimensional reproducibility.
Among the disadvantage of flaskless molding are those restrictions that apply to the size of casting, the use of complicated cores and core assemblies, and the number of castings per mold. Mold handling may be more difficult.
2.2.6 The Sand Casting Operation
In order to produce a sand casting, a typical outline of the manufacturing steps that need to be followed in the sand casting operation is shown in Fig. 2.6.
1.A mechanical drawing of the part is used to generate a design of the pattern and core (if necessary). Decisions about such issues as part shrinkage, material to be used for the pattern, and draft must be built into the drawing. Core drawings need to define how to hold the core in place.
2.Patterns are made and mounted on plates equipped with pins for alignment. Core boxes produce core halves, which are pasted together.
Fig. 2.6 Outline of production steps in sand casting operation.
3.The cope half of the mold is assembled by securing the cope pattern plate to the flask with aligning pins and attaching inserts to form the gate system (sprue and risers). The flask is rammed with sand. Sand is packed about the pattern and gate systems. The half pattern and other inserts are removed. The drag half is made in the same manner with the pattern inserted. A bottom board is placed below the drag and aligned with pins.