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Metal casting is an ancient technique used for manufacturing parts of metals. This process involves the formation of metallic objects by melting metals, pouring it into a mold cavity and allowing it to solidify. This paper discus the procedures, advantages, disadvantages, defects and applications of sand casting, die casting and lost foam casting.
1. Die casting
This is a manufacturing process that is employed in the manufacture of geometrically complex parts of metals through the utilization of reusable moulds referred to as dies. In this process, the use of metals, die casting machines, die and furnace is necessary. The two types of die casting machines include cold and hot chamber machines. The cold chamber machines are used for the metals with very high melting points while the hot chamber machines are used for metals with low melting temperatures. After the molten metal is injected into the dies in both machines, it is cooled and solidifies to form the casting which is final part. (Custompart.net)
Die casting has far-reaching applications. Many production processes creating high volume components of metals benefits from the metal casting process. Several manufacturing industries such as auto, aerospace and power tools industries rely heavily on die casting processes.
1. Die casting procedure
In die casting, the molten metal is inserted into the die where it is held under very high pressure until it cools and dries. Then, the parts of the die are opened and an automatic ejection system is used to detach the casting. In order to improve the quality of the surface and assist the ejection process, the die between each casting is cleaned, preheated and lubricated. In addition, the coating material that is molten is applied to preserve the metal from the cool and conductive veneer of the mould. Finally, some cooling systems are used to maintain the required operating temperature (Industrial metal casting).
Fig 01. Diagrams showing the die casting process (Industrial metal casting).
Advantages and disadvantages of die casting
Dimensional stability and accuracy- This process produces durable parts that are dimensionally stable and which are able to maintain close tolerances. In addition, the parts produced are highly resistant to heat.
Strength and weight- The die cast parts are stronger relative to plastic injection moldings with similar dimensions. For instance, the thin wall castings are lighter and stronger than those with different casting methods. Since the parts of the die casting are not made of separate parts that are joined together, they are characterized by the alloy strength rather than the joining process strength.
Multiple finishing techniques- The die cast parts can be easily plated with minimum preparation of surface and they can be produced with textured or smooth surfaces.
Simplified assembly- The die castings can provide integral fastening elements such as studs.
The Also, the holes can be made to tap drill sizes.
High speed production- Unlike other mass production processes, die casting provides complex shapes within closer tolerances. The process requires little or no machining and it produces many identical castings before extra tools are needed (NADCA).
The disadvantages of die casting include
i. High initial costs.
ii. The weight of the casting is limited (between 30 grams and 10 kilograms).
iii. The casting must be smaller than 24 inches and thickness must be less than 0.5 inches.
iv. It requires large production volume to establish (Rehman, M).
Potential die casting defects
The possible die casting defects include flash defect which is caused by high injection and low clamp force, bubbles defect which is caused by too high injection temperature and non uniform cooling rates, hot tearing defect caused by non uniform cooling, unfilled section defect caused by slow injection and ejector marks defect which results due to low very short cooling time and too high ejection force (Custompart.net).
This is the most widely used cast process where sand molds are used to make complex parts that can be made of many alloys. This process involves the use of metal, pattern, sand mold and furnace. The furnace is used to melt the metal which is then poured into sand mold cavity that is formed by the pattern. The sand casting process gives people the possibility of forming a wide range of shapes using a wide range of materials. The main requirements of the casting process include mould cavity, melting process, pouring, solidification, removal of casting and finishing draft allowance. The patterns are made from plastics, wood, fiberglass, cast iron and aluminum. Among these materials, wood is commonly used because it can be easily made into many shapes (Sand casting). Some practical applications of sand casting include gear blanks, crankshafts, engine blocks and brake drums.
Sand casting Procedure
The sand casting processes takes place in six steps namely mold making, clamping, pouring, cooling, removal and trimming (Sand casting).
i. Mold making
In this stage, the sand mold is formed and packed around the pattern which is a replica of the casting's external shape. When the pattern is removed, the casting cavity remains. If the mold requires heating, the mold-making time is substantially increased. Often, lubrication is applied to the surfaces of the mold cavity to ease casting removal. The lubricant also improves the metal flow.
During this process, the surface of the cavity is replicated, cores positioned and the halves of the mold are clamped together. This clamping ensures that the mold halves are securely closed to avoid any material loss.
After clamping, the molten metal is ladled from the container and poured into the mold. This is done by the use of a machine or manually. The pouring time should be very short in order to avoid early solidification of some metal parts.
Once the molten metal enters the cavity, it begins to cool and solidify to form the final casting. The cooling time is set based upon the casting wall thickness and the temperature of the metal.
After solidification, the casting is removed from the mold and it is done using a vibrating machine that shakes the casting and the sand out. Shot blasting is used in removing any sand that may remain on the casting surface.
In this stage, the excess material is trimmed from the casting by cutting or by using a trimming press
Advantages sand casting
i. The casting process is performed on any metal that is in liquid state.
ii. The process is suitable for mass production.
iii. It is an easy and quick process right from design to production.
iv. It can be used to produce complex pad geometries (Sand casting).
Disadvantages sand casting
i. Poor surface finish and dimensional accuracy.
ii. Environmental problems and safety hazards to people.
iii. Mechanical properties limitation.
iv. Difficulties in removal of small and thin parts (Sand casting).
Potential sand casting defects
Some of the defects of sand casting include micro porosity which is caused by solidification shrinkage, hot tearing due to die's contraction prevention, shrinkage due to lack of riser system and misruns due to rapid solidification (Sand casting).
Lost foam casting
Lost foam casting is a metal casting process that produces castings by using the expendable foam patterns. The process utilizes the foam patterns that remain in the mold during the process of metal pouring. This foam patterns are replaced by the molten metal thus producing castings. This process has been used by people who practice the art of home hobby foundry work. This is because foam casting provides a simple and cheap way of producing the metal castings in the backyard foundry (Croucher, C).
Lost foam casting procedure
The process starts by making a foam pattern with a shape of the finished metal part. The foam patterns are made from the beads of polystyrene and more complex shapes can be made by gluing many patterns together. After sometime, the completed pattern is reinforced by dipping in a refractory material which forms a coating on the pattern and then dried. The coating also provides a good surface finish. After drying, the foam is suspended in a steel container which is vibrated to ensure that all voids are filled and the material is compacted. Then, the molten metal is poured into the mould where the foam burns and vaporizes while the molten metal replaces the pattern of the foam and duplicates the original form's features and a new pattern is formed. After solidification, the castings are subjected to some operations such as removal of risers, sprue, heat treatment, etc (Black, J. K., 2004).
Advantages of Lost foam casting
i. The properties of the foam facilitate easy carving and gluing to produce complex patterns.
ii. It has high corrosion resistance hence improving life in saline environment.
iii. The foam castings have precision casting and excellent dimensional tolerances.
iv. It is more reliable and durable relative to other casting technology types.
v. It has less failure chances due to rigidity.
vi. The castings have no size limitations (Tech Articles).
Disadvantages of Lost foam casting
The major disadvantage of the lost foam casting is the high cost of tooling which may sometimes prevent the parts produced in low volumes to take an advantage of the process capabilities.
The defects of Lost foam casting
The potential defects of lost foam casting include porosity defects which are caused by fast filling or turbulence, folds which are caused by the converging metal fronts that cannot "knit" together and cold metal misrun which is caused by thermal forces (Ross, F).
In conclusion, the metal casting processes requires specialized tools and equipments and a lot of creativity. The metal castings are important in industrial levels because of their efficiency. Also, some people may participate in metal casting for artistic endeavors and as hobbies. Finally, metal casting is an enjoyable and rewarding practice but they need some education to get started.