For a casting process to be successful, various variables will need to be controlled. These variables will range from the characteristics of the metals, or the alloys, the specific method of casting, the molten metal materials and design and other process parameters. Others like the flow of the molten metal in the cavities, the gating systems, the gases evolved, and the cooling rate of the molten metals will influence its quality. This paper is going to discuss the design consideration in casting, the selection of the material processes, selection of the casting process, inspection and problems in casting and the economic consideration in the casting process. A brief summary of the casting process is also discussed
During the casting process, a molten metal is poured a cavity or mould that will look like the shape of the finished product. Heat will then be extracted through the molten metal using sand or any refractory material and the metal will then solidify into final solid shape. There are various processes in the casting which differ by the mould materials used like metal and sand and the pouring method which can be by gravity, low pressure or vacuum among others. The widespread use of casting is informed by various advantages it offers. The properties of casting can be optimized by process control and any metal that can be melted can be cast .
Design Consideration
The first design consideration during casting is the selection of the casting process. This is mainly influenced by the quality required of the casting surface, the number of the castings required and the cost of the making the mould. The effect of the selected processes in the design of the casting process, the type of equipment and the patterns needed and the dimensional accuracy of the required castings are the major factors that will influence the casting process selected. Each selected casting process will offer advantages and disadvantages in casting .
The other design consideration in the casting process is the design of the cast parts. This can be in terms of corners, section thickness and angles in the casting process. This can be done by avoiding raised surfaces and blending of cast sections smoothly with each other. Use of large area is supposed to be avoided to avoid warping in the surface during cooling. Others in the design of the parts can be an allowance of shrinkage during cooling and solidification and dimensional tolerance and markings on the casting. The other design consideration if the location of the parting lines. The parting line should be low for a metal that is less dense. The other design consideration is the designing and locations of the gates. This can be by the use of multiple gates for a large system, avoiding curved gates and placing a fillet where the gate meets the cast .
The last design consideration is the other mould features and runner design. The designed mould should be used to achieve the required metal flow rate and help in the prevention of excessive dross formation. This design can be done by the use of filters to trap large contaminants, reducing metal velocity and chills to speed up solidification. The last design factor to be considered is the establishment of good design practices. This can be quality control like temperature, handling procedure and metal chemistry should be considered for effective casting process .
Material process selection
Choice of the correct material is very important in the casting process. Materials selected are usually from a material that has some modified characteristics like alloys. The casting processes can be classified as investment casting, sand casting and die casting. The selection of the casting processes will influence other processes like the type of tools to be used, the intensity and extent of machining, the control procedure and the heat treatment in the processes. The selection of the material for casting will then affect the labour cost, type of tools to be used and the economic quantity of the material to be used. The combination in the casting process is defined by the different range of geometric features like the thickness and the core size of the materials. The materials should also be of high quality like good surface finishing, the level of complexity required, and the mechanical properties of the materials .
In the selection of a feasible casting process, a corresponding requirement of the end system is considered. This can be the compatibility of the process capability attributes like the minimum thickness and the material to be used dimensions. Other requirements of the process will be evaluated and the requirement that meets them will be used in the materials selected. The screening phase of the casting process will consist of a selection of the type of materials where any casting process that will not cast the selected materials will be eliminated. This is shown by the fact that in die-casting, steel alloys and iron cannot be cast successfully. The second process will be the selection of the shape of the materials where the casting process that would not allow a given shape will be eliminated. This can be shown by an example of a centrifugal casting allowing symmetrical cylindrical shapes. The shape can be categorised as planar, prismatic, thin wall, constant cross section surface of revolution and the free form drape shapes .
Economic consideration
It is great to consider the economies of scale presented by the casting process. This can be presented in the quantity of materials used in the process. The quantity required will influence the selection of the casting process. The costs that will influence the selection of the casting process can be the cost of fabrication and die design. The number of the necessary parts in the casting process should be chosen to reduce these costs and make the process economical to take into account the cost of fabrication and design. The realisation of any given number of the required parts which falls out of the economy of scale region will lead it to be eliminated. Other factors that will lead to economy of scale in the casting process will be the cost of tools, the cost of labour, the cost per unit and the relative cost of the given quantity and quality . The cost of equipment will decrease as the number of casting parts increases. The costs involved in the casting process can arise also from the melting and pouring of the metals, heat treatment and inspecting and cleaning of the cast. For a scenario where the cost-per-casting is high, alternative casting processes like sand casting should be employed for the economics of scale.
Inspection and problems of Casting Process
The last stage in the casting process is the inspection of the castings. This can be a visual inspection, dimensional inspection for the case of precision casting and X-Ray radiography. The dimensions can be special features, the size of the cast and the contours of the cast. Also, use of magnetic particle inspection to locate cracks, ultrasonic testing to detect internal voids in the cast, fracture test for fluorescent dye penetration test for locating cracks in the cast. Others like the macroscopic and microscopic test and the chill test can be used to inspect the quality and composition of the given casting .
The various problems that can be found during the casting process include, shifts caused by low moisture, cracked casting, crushes, and drops. Others can be porosity, hot tears, warp-age, veining and penetration problems caused by wrong pre-blend.
In conclusion, the casting process involves various processes and design consideration to assure its optimal performance. This will lead to the consideration of the materials and process selection, the design consideration and the economy of scale in the casting process which have all been discussed.
References
A Modern Casting Staff. (2000). Identifying Molding Problems. Retrieved from www.moderncasting.com
Campbell, J. (2011). Complete Casting Handbook: Metal Casting Processes, Techniques, and Design. Elsevier Ltd.
Daws, M. K.-K. (2008). Selection of Metal Casting Processes: A Fuzzy Approach. Jordan Journal of Mechanical and Industrial Engineering, 45-52.
Engineered Casting Solutions. (2006). Guide to Casting and Molding Processes. Casting Source Directory.
iugaza.edu. (2016, April). Metal Casting: Design, Materials, and Economics. Retrieved April 16th, 2016, from site.iugaza.edu: http://www.site.iugaza.edu.ps//Ch12_Metal_Casting_Materials _and _Economics
