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. Finer sand and excellent flowability produce dense mold surfaces and contribute to producing complex casting with high-quality surface finish 1.25 µm (50 µin.) RMS (root mean squere).
•Heat from burning slows the casting-cooling rate, yielding a more machinable structure.
•Resin-bond strength allows smaller draft angles, deep draws, and built-in mold locators that prevent mold shift mismatch.
Disadvantages to the shell-mold process are:
•Since the tooling requires heat to cure the mold, pattern costs and pattern wear can be higher.
•Energy costs are higher than for other processes.
•Material costs are higher than those for greensand molding.
Tooling for shell molds is generally more expensive than for other processes because it is more precise and must resist heat and abrasion. Also, heat and resin binders are necessary. For these reasons, shell molding is most suitable for medium-to high-volume parts, where the manufacturer utilizes added value.
Tooling for shell molds is generally more expensive than for other processes because it is more precise and must resist heat and abrasion. Also, heat and resin binders are necessary. For these reasons, shell molding is most suitable for medium-to high-volume parts where the manufacturer utilizes added value.
The V-process, or vacuum molding, is one of the newest casting processes; in it, unbonded sand is held in place in the mold by a vacuum. In this process, a thin plastic film of 0.7 to 2.0 mm (0.03 to 0.08 in.) is heated and placed over a pattern. The softened film drops over the pattern with 26 to 52 kPa, (3.8 to 7.6 psi) and the vacuum tightly draws the film around the pattern. The flask is placed over the plastic-coated pattern and is filled with dry, unbonded, extremely fine sand and vibrated so that the sand tightly packs the pattern. The flask walls also create a vacuum chamber with the outlet shown in Fig. 2.8 at the right side of each illustration.
Another unheated sheet of plastic film is placed over the top of the sand in the flask, and the vacuum is applied to the flask. The vacuum hardens the sand so the pattern can be withdrawn. The other half of the mold is made the same way. After cores are put in place (if needed), the mold is closed. During pouring the mold is still under vacuum but the casting cavity is not. When the metal has solidified, the vacuum is turned off and the unbonded sand runs out freely, releasing a clean casting with zero draft, high-dimensional accuracy and with a 125 to 150 RMS surface finish.
Fig. 2.8 Steps in V-process: a) the pattern is placed on a hollow carrier plate; b) a heater softens the plastic film; c) the vacuum draws the plastic film tightly around pattern; d) the flask is placed on the film-coated pattern; e) the flask is filled with dry sand; f) the back of the mold is covered with unheated plastic film; g) the vacuum is released and the mold is stripped; h) the cope and drag assembly form a plastic-lined cavity; during pouring, molds are kept under vacuum; i) the vacuum is released and as the sand flows freely, the casting is freed; j) finished part.
