Lecture 7 1. Conventional Molding Processes Green

The basic steps of the investment casting process Production of heat-disposable wax, plastic, or polystyrene patterns 2- Assembly of these patterns onto a gating system 3-Investing,” or covering the pattern assembly with refractory slurry<br>

4-Melting the pattern assembly to remove the pattern material 5- Firing the mold to remove the last traces of the pattern material 6-Pouring 7- Cut off and finishing.<br>

The basic difference in investment casting is that in the investment casting the wax pattern is immersed in a refractory aggregate before dew axing whereas, in ceramic shell investment casting a ceramic shell is built around a tree assembly by repeatedly dipping a pattern into a slurry (refractory material such as zircon with binder). After each dipping and stuccoing is completed, the assembly is allowed to thoroughly dry before the next coating is applied. Thus, a shell is built up around the assembly. The thickness of this shell is dependent on the size of the castings and temperature of the metal to be poured. 4-b. Ceramic Shell Investment Casting Process<br>

After the ceramic shell is completed, the entire assembly is placed into an autoclave or flash fire furnace at a high temperature. The shell is heated to about 982 Co to burn out any residual wax and to develop a high-temperature bond in the shell. The shell molds can then be stored for future use or molten metal can be poured into them immediately. If the shell molds are stored, they have to be preheated before molten metal is poured into them<br>

Advantages excellent surface finish
tight dimensional tolerances
machining can be reduced or completely eliminated<br>

The expanded polystyrene casting process uses a mold of sand packed around a poly-styrene foam pattern that vaporizes when the molten metal is poured into the mold. The process and variations of it are known by other names, including lost-foam process, lost-pattern process, evaporative-foam process, and full-mold process (the last being a trade name). The foam pattern includes the sprue, risers, and gating system, and it may also contain internal cores (if needed), thus eliminating the need for a separate core in The mold . Also, since the foam pattern itself becomes the cavity in the mold, considerations of draft and parting lines can be ignored 4-c. Expanded Polystyrene Process /
Evaporative Pattern Casting Process<br>

The mold does not have to be opened into cope and drag sections. The sequence in this casting process is illustrated and described in Figure below. Various methods for making the pattern can be used, depending on the quantities of castings to be produced. For one-of-a-kind castings, the foam is manually cut from large strips and assembled to form the pattern. For large production runs, an automated molding operation can be set up to mold the patterns prior to making the molds for casting.<br>

The pattern is normally coated with a refractory compound to provide a smoother surface on the pattern and to improve its high temperature resistance.

Molding sands usually include bonding agents. However, dry sand is used in certain processes in this group, which aids recovery and reuse.<br>

A significant advantage for this process is that
-- The pattern need not be removed from the mold.
-- This simplifies and
-- Expedites mold making.
In a conventional green-sand mold, two halves are required with proper parting lines, draft allowances must be provided in the mold design, cores must be inserted, and the gating and riser system must be added. With the expanded polystyrene process, these steps are built into the pattern itself. /, so the economics of the expanded polystyrene casting process depend largely on the cost of producing the patterns. The process has been applied to mass produce castings for automobiles engines. Automated production systems are installed to mold the polystyrene foam patterns for these applications<br>

(1) pattern of polystyrene is coated with refractory compound; The expanded polystyrene casting Procedure (2) foam pattern is placed in mold box, and sand is compacted around the pattern; and<br>

(3) molten metal is poured into the portion of the pattern that forms the pouring cup and sprue.
As the metal enters the mold, the polystyrene foam is vaporized ahead of the advancing liquid, thus allowing the resulting mold cavity to be filled.<br>

4-d. Vacuum Sealed Molding Process It is a process of making molds utilizing dry sand, plastic film and a physical means of binding using negative pressure or vacuum. V-process was developed in Japan in 1971. Since then it has gained considerable importance due to its capability to produce dimensionally accurate and smooth castings. The basic difference between the V-process and other sand molding processes is the manner in which sand is bounded to form the mold cavity. In V-process vacuum, of the order of 250 – 450 mm Hg, is imposed to bind the dry free flowing sand encapsulated in between two plastic films. The technique involves the formation of a mold cavity by vacuum forming of a plastic film over the pattern, backed by unbounded sand, which is compacted by vibration and held rigidly in place by applying vacuum. When the metal is poured into the molds, the plastic film first melts and then gets sucked just inside the sand voids due to imposed vacuum where it condenses and forms a shell-like layer. The vacuum must be maintained until the metal solidifies, after which the vacuum is released allowing the sand to drop away leaving a casting with a smooth surface. No shakeout equipment is required and the same sand can be cooled and reused without further treatment. The steps of the process are explained in Figure<br>

Sequence of Producing V-Process Molds The Pattern is set on the Pattern Plate of Pattern Box. The Pattern as well as the Pattern Plate has Numerous Small Holes. These Holes Help the Plastic Film to Adhere Closely on Pattern When Vacuum is Applied.


A Heater is used to Soften the Plastic Film<br>

The Softened Plastic Film Drapes over the Pattern. The Vacuum Suction Acts through the Vents (Pattern and Pattern Plate) to draw it so that it adheres closely to the Pattern.

. The Molding Box is Set on the Film Coated Pattern The Molding Box is filled with Dry Sand. Slight Vibration Compacts the Sand<br>

Level the Mold. Cover the Top of Molding Box with Plastic Film. Vacuum Suction Stiffens the Mold.
Release the Vacuum on the Pattern Box and Mold Strips Easily.
Cope and Drag are assembled and Metal is poured. During Pouring the Mold is Kept under Vacuum
After Cooling, the Vacuum is released. Free Flowing Sand Drops Away, Leaving a Clean Casting<br>

Advantages Exceptionally Good Dimensional Accuracy
Good Surface Finish
Longer Pattern Life
Consistent Reproducibility
Low Cleaning / Finishing Cost<br>