The Gravity casting process has several advantages. The process is suitable for mass production with better reproduction; dimensional accuracy and surface finish than conventional sand castings. The GDC process is capable of achieving 20% higher mechanical properties than that of a sand casting because of faster rate of solidification imparting better grain size. The process can be automated and can produce semi-gravity die-castings employing sand or plaster of Paris cores for production of interior details.
The process has certain disadvantages. Limitation of geometry/size is a main disadvantage, as it is difficult to cast large size highly complex shapes. Beyond a particular shape and size the process becomes uneconomical. It is difficult to attach gates and risers at all desired locations. The casting yield is low when compared to other die casting process. As the component size and complexity increases the process becomes more expensive and becomes uneconomical. It will also cause difficulty in handling the die and in extracting the casting from the die with reduction in dimensional accuracy and soundness of the casting. [1]
Current industry practices like the production of Aluminum follow ANS H35 standards are developed under approval by the Accredited Standards Committee H35 - Aluminum and Aluminum Alloys, which is an ANSI accredited standards committee. Aluminum Association (AA) standards are promulgated by the Technical Committee on Product Standards (TCPS). Most industry product standards for aluminum mill products are published in Aluminum Standards and Data available in both customary and metric editions. [2] Industries also follow ISO 8062-3:2007 Geometrical product specifications (GPS) -- Dimensional and geometrical tolerances for molded parts -- Part 3, which is the General dimensional and geometrical tolerances and machining allowances for castings, and also the ANSI/ASME Y14.8M-1996 (R2002) which is for castings and forgings. [3] These are important in order for the industry to meet machining requirements, for dimensional and geometrical tolerances also to maintain and control the casting process precisely.
Casting quality tests can be divided into two major categories: destructive and nondestructive. A destructive test can be performed only once, and if another measurement is needed, another part representative of the first must be tested and subsequently destroyed. This destruction creates several problems. The first and most deterring is that a broken or damaged casting cannot be sold. Repeatability causes a problem because a suspected erroneous measurement cannot be taken again with the same casting. In any testing operation, multiple iterations of the same test are performed and an average result is reported. Nondestructive test like AES and XRD are used in the industries.
Casting is still relevant for advanced material fabrication technology. Some current researches are dental titanium casting in China. To expand the applications of titanium alloys as dental prosthodontics materials, TiZr alloy and TAMZ alloy for dental uses were also developed. Their physical and mechanical properties are better than pure titanium and their biocompatibilities are excellent. [4] The Rene 125 nickel-based super alloy is used by Precision Castparts Corporation in investment casting of equiaxed turbine airfoils. The investment casting process results in scrap metal, also known as revert, which comes from the gates, runners, and sprues. The revert is re-melted into future master heats for cost savings. However, using more revert material has been shown to cause higher micro-porosity, which results in a lower yield. The goal for this project is to recommend compositional changes to minimize the occurrence of porosity, while maintaining high revert usage. [5] Nickel-based super alloys are the primary material used in the hot section of almost all modern jet engines and industrial gas turbines due to their high temperature mechanical properties. A production bottleneck occurs during the current aging cycle. Therefore, the aging cycle time needs to be reduced. The goal was to develop a process that achieves equivalent microstructure and mechanical properties of current Alcoa-Howmet production alloys with an aging time of 10 hours or less. [6] In this experiment, the objective was to evaluate the quality of the mold and the casted product visually.
Methodology
A pattern was created and placed on a clean flat surface using modeling clay. Once the pattern was in place, Pringles container was used to define the boundaries of the mold. Adequate amount of Plaster of Paris powder was dissolved in the container. Once acceptable viscosity was obtained, the fluid was poured carefully into the placed pattern so as not to displace its arrangement. The fluid was let to flow through the pattern and was allowed to dry. The ceramic mold was removed from the surface and the patterns and clay from the mold carefully. This served as the lower half of the mold. It was allowed to fully dry by putting in an oven for 5 minutes. The same procedure was done with the upper half of the mold, that time without the patterns but with a stick at the center to serve as gating. The center of the mold was removed. When the upper and lower parts of the mold were ready, a swab of petroleum jelly was put into the cavity. An adequate amount of resin was prepared in a beaker. Then hardener was added into the resin. The volume ratio of the hardener to the resin was 1:10. The previously prepared resin was poured into the mold and allowed to dry. Once fully dried the casted product was removed from the mold, the mold and the casted product were observed under the microscope. After that, the molds were weighed using analytical balance. Then were soaked in water for 24 hours and weighed again when soaking was done. Then porosity volume was determined.
Results and Discussion
The cast product was not as exactly the same as the pattern looked. This is because the pattern is a turtle that has marks on its shell making it intricate. The cast product did not have all the details the pattern had. Another cause of the differences might be the viscosity of the cast product.
Gas porosity defects were present in the product. These are bubbles that were seen under microscope. These defects are present because the resin held dissolved gas when it was in liquid form and also when it was poured to the mold and mixed. The porosity of the mold could also be the reason for such defects in the cast product. By determining the porosity volume of the mold, it proved that the mold has high porosity volume. The porosity volume is equal to the mass of water divided by the density of water. The mass of water is the difference between presoaked weight and the weight of the mold after it was soaked in water.
Putting a thin layer of petroleum jelly in the surface of the mold cavity prior to casting is for it to separate the mold cavity and the resin to prevent difficulty in removing the cast product after it has dried. This would prevent the product from breaking and also lessen the amount of gas defects in the cast product.
Fig.1. Micrographs of the cast product (500X)
Significant factors are also important in the selection of a casting process. When applied to castings, the term quality refers to both degree of soundness (freedom from porosity, cracking, and surface imperfections) and levels of mechanical properties (strength and ductility). However, it should be kept in mind that in die casting, although cooling rates are very high, air tends to be trapped in the casting, which gives rise to appreciable amounts of porosity at the center. Extensive research has been conducted to find ways of reducing such porosity; however, it is difficult if not impossible to eliminate completely, and die castings often are lower in strength than low-pressure or gravity-fed permanent mold castings, which are more sound in spite of slower cooling.
With die casting, it is possible to maintain close tolerances and produce good surface finishes. Die castings are best designed with uniform wall thickness. Die castings are made by injection of molten metal into metal molds under substantial pressure. Rapid injection and rapid solidification under high pressure combine to produce a dense, fine-grain surface structure, which results in excellent wear and fatigue properties. Air entrapment and shrinkage, however, may result in porosity.
Mold making and poor dimensional accuracy were some problems in this experiment. Sources of error in this experiment were the molding method, molding equipment, molding materials and casting geometry. The main causes for poor dimensional accuracy in sand casting are pattern equipment errors, pattern wear, and casting contraction uncertainty. [8]
Conclusion
The quality of the cast product is not only influenced by the raw material used but also on the quality of the mold. In this experiment, the obtained porosity volume of the mold contributed to the gas defects as seen in the micrographs.
References
[1]http://www.esi-group.com/products/casting/publications/Articles_PDF/F_GDC_Ennore.pdf [Accessed: Feb, 2012]
[2] [Accessed: Feb, 2012]
[3] [Accessed: Feb, 2012]
[4] Z. M. Yan, T. W. Guo, Y.M. Zhang and Z. C. Li 2002, Dental Titanium Casting Researches in China
[5] http://www.purduecasting.org/docs/PCC_Poster_2010.pdf [Accessed: Feb, 2012]
[6] http://www.purduecasting.org/docs/PCC_Poster_2010.pdf [Accessed: Feb, 2012]
[7] http://www.kimuragrp.co.jp/English/casting/index.html [Accessed: Feb, 2012]
[8] , and 2010, Sand Casting Dimensional Control
Appendix