Metal Shaping Processes. Vukota Boljanovic
bottom board are inverted; and the pattern is withdrawn leaving the appropriate cavity.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.
An expandable mold is formed of refractory materials. The thermal conductivity of the mold is a property of the material selected. It is also a function of particle size and distribution. The thermal conductivity influences the rate of transfer through the mold and therefore the rate of solidification which, in turn, influences the metallurgical integrity of the casting.
When choosing the mold material, one should be chosen that is sufficiently refractory to withstand the pouring temperature of the particular metal being cast without melting or softening. As long as the material is pure, the melting point value is a good guide to refractoriness. However, the melting point can be reduced dramatically by adding very small amounts of alkali metal salts or iron oxide.
All of these involve the use of temporary and not reusable molds, and they need gravity to help force the molten fluid into the casting cavities. In this process the mold is used only once.
Shell molding is a foundry process in which the molds are made in the form of thin shells. This technique is also called the “C” process or Croning; the Croning process was developed in Germany after World War II and patented by Johannes C. A. Croning.
Shell molds are made in the following sequence of operations:
1.Initially preparing a match-plate pattern or cope-and-drag pattern. In this process, the pattern is made of ferrous metal or aluminum.
2.Mixing fine silica sand with 3 to 6 % thermosetting resin binder. Common shell molding binders include phenol formaldehyde resins, furan, or phenolic resins and baking oils similar to those used in cores.
3.Heating the pattern, usually to between 230 and 280°C (446 to 536°F) and placing it over a dump box containing sand mixed with binder.
4.Inverting the damp box (the sand is at one end of a box and the pattern at the other) so that sand and resin binder fall onto the hot pattern and form a shell of the mixture to partially cure on the surface to form a hard shell. The box is inverted for a time determined by the desired thickness of the shell. In this way, the shell mold can be formed with the required strength and rigidity to hold the weight of the molten metal. The shells are light and thin, usually 6 to 10 mm (0.2 to 0.4 in.) in thickness.
5.Repositioning the drag box and pattern so that loose uncured particles drop away.
6.Heating the shell with the pattern in an oven for several minutes to complete curing.
7.Removing the shell mold from the pattern.
8.Repeating for the other half of the shell.
9.Joining the two mold halves together and supporting shell mold by sand or metal shot in the flask, and pouring the molten metal.
10.Removing the casting, cleaning, and trimming.
The steps in shell molding are illustrated in Fig. 2.7.
There are many advantages to the shell-mold process:
•Rigidly bonded sand provides great reproducibility and produces castings nearer to net shape with intricate detail and high dimensional accuracy of ±0.25 mm (±0.010 in).
Fig. 2.7 Steps in shell molding: a) pattern heated and clamped over a box; b) inverted box; c) repositioned box; d) shell with pattern is heated in oven; e) half shell is stripped from the pattern; f) shell mold supported by metal shot in flask and poured melted metal; g) finished casting.
•Castings can range from 30 g to 12 kg (1 oz to 25 lb).
•There is a virtual absence of moisture, resulting in a lack of moisture-related defects. In fact, the burning resin provides a favorable anti-oxidizing atmosphere in the casting surface.
•Because mold shells are thin, permeability for gas escape is excellent, allowing the use of finer sands.