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. The pattern, flask, and bottom board are inverted; and the pattern is withdrawn leaving the appropriate cavity. The core is set in place with drag cavity to make concave or internal features for the cast part.
4.The cope is placed on top of the drag and the assembly is secured with pins mating the mold halves. The flasks are then subjected to pressure to counteract the force of buoyancy. (Buoyancy results from the weight of the liquid metal being displaced by the core, according to Archimedes’ law.) The force tending to lift the cope is equal to the weight of the displaced liquid less the weight of the core. A mathematical expression of this situation is
| Fb = Wm − Wc | (2.1) |
where
| F b | = | force of buoyancy N, (lb) |
| W m | = | weight of molten metal displaced N, (lb) |
| W c | = | weight of the core N, (lb) |
5.Molten metal is preheated in a furnace or crucible to pouring temperature. The exact temperature may be closely controlled depending upon application. Degassing and other treatment procedures, such as removal of impurities (i.e., slag) may be done at this time.
6.The molten metal is poured slowly but continuously into the mold until the mold is full. As the molten metal solidifies and cools, the metal will shrink. As the molten metal cools, the volume will decrease. During this time, molten metal may backflow from the risers to feed the casting cavity and maintain the same shape.
7.After the metal solidifies below the eutectic point, the casting is removed from the mold with no concern for final metal properties. At this point, the sand mold is broken up and the casting removed. The bulk of the remaining sand and cores can be removed by using a vibrating table, a sand/shot blaster, hand labor, etc.
8.The sprue and risers are cut off and recycled. The casting is cleaned, finished, and heat treated (when necessary).
9.The casting is inspected using nondestructive testing (NDT) and destructive methods in accordance with standards.
The rammed graphite mold is typically used for large industrial casting for reactive metals such as titanium and zirconium. It uses graphite instead of sand in a process similar to sand casting. Traditionally, a mixture of properly size-fractioned graphite powder, pitch, corn syrup, and water is rammed against a wooden or fiberglass pattern to form a mold section. The mold sections are air-dried, baked at 175°C (350°F) and then fired in furnace for 24 hours at 1025°C (1877°F). This causes the mold to carbonize and harden. Mold ramming is a labor-intensive process that cannot be easily mechanized. The graphite mold is so hard that it must be chiseled off the cast parts. The castings are generally cleaned in an acid bath, followed if necessary by chemical milling, to remove any reaction zone, and weld-repaired, then sand-blasted for a good surface appearance.
Rammed-graphite molds need to be stored under controlled humidity and temperature.
2.3 OTHER EXPENDABLE MOLD CASTING PROCESSES
After sand-casting, the oldest expendable mold casting technology, was developed, several other expendable mold casting processes were invented to meet special needs. The differences among these methods are in the composition of the mold material, the methods by which the mold is made, or in the way the pattern is made.
