INTRODUCTION:

Even though sand casting enjoys wide application the needs of advance casting process is inevitable. The following discussion will give an idea of application of advanced casting process with advantages, limitations and applications in the field of engineering.

Expandable Mold with Expandable Patterns:

One major difference between the permanent pattern casting methods and the expendable pattern methods is that the expendable pattern is typically always the positive shape of the part. In contrast, permanent patterns are the negative or mirror image to be cast.

When using the expendable pattern method, the part is typically made twice: once in an expendable form of the part (which is disposable) and then as the actual functional metal form of the part. Casting with expendable molds is a very versatile metal-forming process that provides tremendous freedom of design in size, shape, and product quality. Adding expendable patterns to this equation increases the complexity and tolerance of product.

Depending upon the size and application, castings manufactured with the expendable mold process and with expendable patterns increase the tolerance from 1.5 to 3.5 times that of the permanent pattern methods. The two major expendable pattern methods are lost foam and investment casting. A hybrid of these two methods is the Replicast casting process which involves patternmaking with polystyrene (similar to lost foam) but with in a ceramic shell mold (similar to investment casting). These three methods are briefly reviewed here.

A. Investment Casting Process:

Investment casting (also known as ‘lost wax casting’ or ‘precision casting’) has been a widely used process for Centuries. As per [1] Taylor (1983), the principles can be outlined back to 5000 BC when the early man engaged this method to produce elementary tools. As per [2] Barnett (1988), the technology have a great advancement in USA during Second World War, to the need of fastidiousness components with complex geometry. [3] Eddy et al. (1974) reckoned has a different applications and advantages of investment casting process. It is commonly used to manufacturing parts ranging from turbocharger wheels to golf club heads, from electronic boxes to hip replacement implants, general engineering to aerospace engineering and defense outlets.

Kalpakjian and Schmid (2008) It explained the basic steps of an investment casting, using a ceramic shell [4]. and their presentation, in the investment casting technique, are in desired shape, its made of wax, formed by injecting molten wax into a metallic die. Then the pattern or a cluster are gated together to a central wax sprue. The sprued pattern is invested with ceramic or refractory slurry, which is solidified to build a shell around in the wax pattern. The pattern is removed from the shell by melting or combustion process, leaving a hollow void within the shell. Prior of casting, the shells are fired in an oven where intense heat burning out any remaining wax reduce. The resulting shell are hardened by heating, it filled with molten metal. After that molten metal is solidified, the shell is broken and the gates are cut off from the casting to obtain the required shape of component. Good surface finish is the major advantage of this process. No elaborate and expensive tools are involved in this process. So that shapes are difficult to produce by other casting methods and are very easily possible to be produced by this method. Thin cross sections and intricacies can be made by this processes. Finished machining is considerably reduced on this castings made by this process, making it economical in cost. The process has no metallurgical limitations.

But there is more expensive in this process because of large manual labor involved in the preparations of wax pattern and ceramic slurry. As the shells are delicate, the process is limited by the size and mass obtained. Making intricate and high quality pattern increases the process costs. Steel investment castings is for one-third of the total output by value. Among the non-ferrous alloys, there is a wide range of applications of aluminum and its alloys. Splines, holes, bosses, lettering and even some threads can be successfully cast. Very fine and thin sections can be produced by this process.

B. Full Mold Process:

H.F. Shroyer patented for metal casting on April 15, 1958. In this patent, expanded polystyrene (EPS) block were used by him to machine the pattern and during pouring, it was supported by bonded sand. In the full mold process, the pattern is usually machined from an EPS block and is used to make large kind of castings primarily. Originally this process is also known as ‘lost foam process’. The Evaporative pattern casting process (EPC) is a binder less process and no physical bonding is required to bind the sand aggregates. Foam casting techniques has been known by a variety of generic and proprietary names such as in lost foam casting, evaporative pattern casting, cavity-less casting, full mold casting and evaporative foam casting.

The full-mold process (lost foam) is a sand casting method in which polystyrene is used as pattern [5]. The more suitable polymeric materials is to manufacture the patterns are expandable polystyrene and polymethilacrilate, or combination of both [6,7]. The polymer density can vary from 16 to 24 kg/m3. The pattern is covered with refractory coating and inside of mold during metal pouring. As the metal is poured through feeding system, and the metal takes its place, reproducing the exact pattern shape. The gases from the foam burn flee through the sand, crossing the coating layer. The generated gases must travel through the sand easily.

The full-mold process has more advantages compare to other casting methods, especially for high production of difficult shape parts. The patterns are cheap and easy to manufacture, the produced parts are free of lines and exit angles. It possible to reuse the sand [8,9]. The energy consumption is low, as well as operation costs and investments. There is more flexibility for parts design [10]. The production cost cutback with respect to green sand method is around 20–25% for simple parts and 40–45% for complex parts [11].

Since every casting requires a new pattern, it is a costly process. There is an limitations on the minimum section thickness of the pattern. Quality of the casting fully depends upon the quality of the pattern. As the sand is unbounded, during pouring, because of difference evaporation rate of the metal and flow rate of the metal, sand falls down in the cavity generated. Hence, defective casting. The foam is 92% C by weight, the lost foam process is unsuitable for the majority of steel alloys.

This process is suitable for non-ferrous alloys and irons. It is used for making automotive components (cylinder heads, engine blocks, inlet manifolds, heat exchanger, and crank shaft). It is used in marine, aerospace and construction industries.

C. Ceramic Shell Casting – Replicast:

The Replicast process can be best characterized as a hybrid of the investment casting process and expanded polystyrene (EPS) as in lost foam. [12] Ashton et al. (1984) developed the ceramic shell casting process based on the foam pattern, researchers and precise casting production enterprises has already recognized its advantages over lost wax casting and lost foam casting, and have mainly employed as a solution to carburization of low-carbon steel castings produced in the lost foam process.

First of all, the foam pattern is based on the part shape is prepared as prototype, and the thin shell with fewer layers is fabricated using the shell fabrication technology and investment casting outside the foam prototype. After the foam prototype is removed, the shell is taken to be roasted. Following the boxing and modelling, the molten metal is poured and solidified under vacuum and air pressure at a level.

In this foam pattern has higher dimensional accuracy and much lighter weight, in this new process can be used to produce large precise castings. Furthermore, the shells are very thin because of the loose-sand uniting vacuum was employed to further reinforce it, thus the production cycle of shells can be significantly shortened. This new shell casting process also over in lost foam casting process. In as much as the foam prototypes are removed before pouring, the filling capacity of molten metal can be improved, especially for non-ferrous metals, and carburization of low-carbon steel castings would also be eliminated. Air emissions are easier to control than with lost foam. This application of a vacuum during casting allows improved fill-out of molding.

The support provided by the ceramic shell during casting allows large, thin shells to be easily poured. Sand inclusions and other sand mold-related defects can be virtually eliminated. As with investment and lost foam casting, there are no cores or parting lines, high dimensional accuracy, and excellent surface finish. The ceramic shell does not have a thick as for shell casting. The technique minimizes dust emissions from molding and finishing, as compared to sand molding.

In the lost wax casting process, wax can be retain in its common name of ‘precision casting process’ only for very small castings. Since the surface quality of the foam pattern is much poor when compared to that wax casting, the shell fabricated outside the foam pattern would produce a relatively higher surface roughness of the casting, which has been verified by [16] Kumar et al. (2007) and Li et al. (1998). This has consequently hundreds of application in this new shell casting process in the precise casting field. However, it was verified by Campbell (2000) and Bonilla et al. (2001) that, for large castings, the process becomes no better than low technology sand castings.

Wang et al. (2007) and Wen et al. (2009) reported that the rapid development of aerospace and automotive industry [13], the demand for complicated and thin-walled aluminium and magnesium alloy precision castings increases market due to their high strength-to-weight ratio and lightweight. [14] Liao et al. (2009) introduced vacuum and low-pressure casting process into primary ceramic shell process to produce magnesium alloy and aluminium precision castings, which could eliminating pore and shrinkage defects as present in lost foam castings. As presented by [15] Jiang et al. (2010). Table 1 compares replicast with investment casting process.

Semi-solid metal processing:

Semi solid metal processing, also known as semisolid metal casting, semisolid forming, or semisolid metal forging, is a special die casting process wherein a partially solidified metal slurry (typically, 50% liquid/50% solid instead of fully liquid metal) is injected into a die cavity to form a die-cast type of component. It was discovered that when the dendritic structure of a partially solidified Sn-15wt%Pb alloy was fragmented by shear in a Couette viscometer, it results in a globular structure.

The apparent viscosity of the globular structure was dramatically lower than that of the dendritic, and the slurry is formed has fluidity approximating that of machine oil. That semisolid material has rest held its shape like a solid; however, when a shear stress was applied, it became fluid to be injected into a die casting this property is known as thixotropy. The key to SSM processing is to generate a semisolid metal slurry that contains a globular primary phase (surrounded by the enriched liquid phase) and exhibits thixotropic behavior. That is, the viscosity of the slurry decreases continuously under shear deformation, whereas the viscosity value can be recovered once the shear action ceases. There are three major semisolid processing routes: thixocasting, rheocasting, and Thixomolding, and several variations within those.

TABLE 1 : Comparison of investment casting, Full mold process and replicast Process

Feature	Investment casting	Full mold process	Replicast process
Pattern	Softened wax is injected at high pressure into a metal tool. The wax is subject to shrinkage and deformation, and it is expensive and heavy. It is reclaimable to some degree.	Pattern is made from polystyrene foam and Injected into aluminium tool	Partially expanded EPS beads are blown into aluminium tooling and completely expanded. Finished patterns are lightweight, have high density, and provide good surface finish and excellent dimensional accuracy.
Shell	Successive coats of refractory slurry and stucco are applied. Five to ten coats are required completed shells are often heavy and difficult to handle. Firing at 1000^oC for 20 min removes the residual wax and hardens the shell.	No shell is prepared but coated with primary refractory paint	Successive coats of refractory slurry and stucco are applied. Three or four coats are required, resulting in a relatively light and easy-to-handles shell. Firing at 925–1000^oC for 5 min removes the EPS pattern and hardens the shell
Pouring	Metal is frequently poured into hot, unsupported shells, breakage is possible.	The polystyrene foam pattern left in Sand mold is decomposed by the molten metal. The metal replaces the foam pattern, exactly duplicating all of the features of the pattern	Thin ceramic shell is surrounded by loose sand vibrated to maximum bulk density, and the vacuum is applied during pouring to prevent shell breakage.
Applications	Suitable for all alloys. Less suited for heavy section components.	Suitable for non-ferrous alloys and irons (preferably aluminum) Not suitable for steel.	Suitable for all alloys. Not ideal for very thin section parts (e.g. <2mm).
Manufacturability	Medium productivity with outstanding dimensional control.	Less productivity	Very less productivity

A. Thixocasting:

Thixocasting consists of two separate stages: the production of billet feedstock having the appropriate globular structure, and reheating of billets to the semisolid temperature range, followed by the die casting operation. The thixocasting route starts from a non-dendritic solid precursor material that is specially prepared by manufacturer, using continuous casting methods. Upon reheating this material into the mushy (two-phase) zone, a thixotropic slurry is formed, which is the feedstock for the semisolid casting operation. There are three major processing methods to make thixocasting feedstock which can be reheated to the semisolid region to develop the fully globular structure for forming into SSM parts.

The advantages of thixocasting are exceptional part quality extremely because of low levels of shrinkage or gas porosity, excellent leak tightness and weldability. Excellent mechanical properties (T5 heat treated thixocasting parts are able to achieve properties typically found in T6 heat treated permanent mold castings). Fast cycle times. Long die life because of limited thermal shock to the tooling and less heat checking. Near-net shape because of low draft angles possible, reduced machining stock compared to competitive casting processes

The thixocasting machines incorporate larger injection cylinders with additional hydraulic multiplication systems, as well as thicker platens and larger-diameter tie bars to accommodate the high injection forces. Due to the extra material costs and engineering necessary to achieve the higher injection forces and velocity control, these specialized thixocasting machines are more expensive than conventional die-casting machines.

B. Rheocasting:

Rheocasting (also known as slurry-on-demand), the liquid state is the beginning point, and a thixotropic slurry is formed directly from the melt via special thermal treatment/management of the solidifying system [17]. The rheocasting method is favored over thixocasting because there is no premium added to the billet cost, and the scrap recycling issues are alleviated.

C. Thixomolding (Magnesium Pellets) :

Thixomolding process uses solid chips or pellets of conventionally solidified magnesium alloys that are fed into a heated injection system containing a reciprocating screw. Upon heating, the metal chips are converted by the shear action of the screw into a thixotropic, low-solid-content slurry (solid fraction less than 0.3), which is fed into the shot accumulator by the rotating screw. Once the accumulation chamber is filled, the slurry is injected in the mold.

A major advantage of the process is that it effectively combines both slurry making and slurry injecting into a one step process, leading to high productivity and energy savings. In addition, the process avoids the safety problems usually associated with melting, handling, and die casting molten magnesium. thixomolding is successfully implemented in some of magnesium alloys. Table 2 provides a comparison of semi-solid metal process.

No-bond sand molding process:

Sand molding processes are classified according to the way in which the sand is held (bonded). Most sand casting employs green sand molds, which are made of sand, clay, and additives. Binders are also used to strengthen the cores, which are the most fragile part of mold assembly. However, some molding processes do not use binders [18]. Instead, the sand or mold aggregates are held together during pouring by the pattern itself (as in lost-foam casting) or by the use of an applied force (as in vacuum molding and magnetic molding described here). No-bonded molding processes involve free-flowing mold particles and do not require binders, mulling equipment, or mold additives.

A. V-Process:

The vacuum-sealed molding process allows molders to make complex molds using dry, unbounded, and freely flowing sand. Molds are sealed by using plastic films along the top and bottom sand surfaces of the cope and the drag molds and then vacuum applied to the sand medium of cope and drag [19]. The plastic film along with top of the drag mold and the bottom of cope mold is softened by heating and formed on an appropriate pattern to produce the hollow cavity for the finished mold. The control factors of the V-process are may affect the quality of the castings and the molding sand, vibration frequency, vibrating time, degree of vacuum imposed, and pouring temperature.

The advantage of the V-process is that the use of vacuum to maintain the mold eliminates the requirement for a sand binder. Consequently, no sand mixing system is required, and the machinery for shakeout and sand reclamation are therefore less costly to installing and operate. Additional benefits for V-process molding include reduced requirements for sand control and lower fume and dust generation [20]. V-process mold will also retain heat longer, slowing solidification, due to presence of no moisture in the mold sand.

B. Magnetic Molding:

Based on a concept similar to the lost-foam process using an expandable polystyrene (EPS) pattern, magnetic molding was developed. In initial development of the magnetic molding process took place at the same time as the lost-foam process, but it has never achieved the same level of industrial development as the lost-foam method [21]. The magnetic molding process involves a coated EPS pattern that is surrounded by a mold material of magnetic iron or steel shot (instead of sand as in lost foam). After the EPS pattern is positioned in the flask and encased with magnetic shot particles (between 0.1 and 1.0 mm, or 0.004 and 0.04 in., in diameter), the mold is compacted further by periodic vibrating and/or tilting. The mold is then made rigid by the application of a magnetic field prior to pouring the molten metal. Evolving gases are drawn off through the base of the flask. The magnetic field is turned off after solidification and cooling, resulting in immediate shakeout. The free-flowing magnetic shot molding material is returned to its point of origin after cooling, dedusting, and metal splash removal.

Advantages of the magnetic molding, like other no-bond methods, include the absence of a chemical binder, reductions in dust and noise levels, full mechanization or automation of the process, and the elimination of normally used molding activities (such as ramming and jolting). The increased heat conductivity of the iron or steel molding material also results in a finer grain structure in the cast metal. Another advantage is that a piece mold can be produced without a joint line.

Magnetic molding using irons, carbon and low-alloy steels, high-chromium steels, and copper-base alloys are under research.

Centrifugal Casting:

Centrifugal casting is the largest casting branches in the casting industry, accounting for 15% of the total casting output of the world in terms of tonnage. In centrifugal casting the molten metal is introduced into the rotating mould. Centrifugal force tends the poured a metal to fly outwards away from the axis of rotation, this centrifugal force can thus play a part in shaping and in feeding of the molten metal into the casting. The centrifugal force produced by rotation is larger than normal hydrostatic forces. (Sand casting process utilizes hydrostatic forces) it creates a high pressure in the metal or casting while it is solidifying. This results in producing high quality casting [22]. The wall thickness of the casting is determined by the volume of metal introduced into the mold. Casting cools and solidifies from outside towards central axis thereby providing directional solidification.

The most important advantage of using centrifugal is high metallurgical quality. It can be used for cylindrical components. A further function of centrifugal force is seen in the tendency for nonmetallic inclusions to segregate towards the axis of rotation. In centrifuge, is combined with marked directional freezing and with a short path to the free surface in the bore, so that a high standard of freedom from inclusions is achieved. The pressure gradient within the casting, dissolved gases to form bubbles at the bore region, where the gas can escape readily from the casting [23].

The limitation is more for alloy component during pouring of metal under the forces of rotation and possibility of contamination of internal surface of castings with non-metallic inclusions.

The application of centrifugal castings are bearings for electric motors and industrial machinery. Cast iron pipes, alloy steel pipes and tubing. Liners for I.C. Engines. Rings, short or long pots and other annular components. Table 3 shows the difference between DeLavand process and Moore Casting process

Table 3: Difference between DeLavand process and Moore Casting process

Categories	De Lavand Process	Moore Casting Process
Mold Material	Metal	Dried sand and sand lining
Pouring Method	Long pouring spout	Poured at one end of the mold
Molten Metal Filling	Moving mold horizontally	Tilting mold to an angle
Production rate	Mass production	Low to medium production
Investment	High	Low

CONCLUSIONS:

Improvement in replicast of the surface roughness of the shell at the level of shells in the lost wax casting, The advantages and application of the foam pattern process would then be remarkably enhanced. This process become the first choice of various processes to produce an alloys casting of various shapes and sizes. In this surface roughness of the casting mainly depends on the surface quality of shells, replica ability of the molten metal and other factors such as the casting oxidation on the surface as well as damage while removing the shell. In addition, of the foam pattern outside which the shell was fabricated has great influence on the surface roughness of castings. Therefore, it is very useful to develop some methods to improve the surface roughness of foam patterns and castings. It is more expensive than the lost foam process.

SSM advantages reduced enthalpy, increase of tool life. Because the slurry is approx. 50% solid when cast, much of the sensible heat and heat of fusion has already been released before injection into a die, and the thermal load on tooling is thus greatly reduced. Reduced cycle time, the reduced energy release also affects solidification time, typically reducing it by nearly 50%. Reduced solidification shrinkage. Injecting a slurry with 50% fraction solid into a die cavity means that 50% solidification shrinkage associated with the primary phase has already taken place, thus reducing the tendency for shrinkage pores and the need for shrinkage feeding improved casting integrity and properties. Reducing solidification shrinkage and porosity ultimately improves mechanical properties also the eutectic morphology typical of semisolid processed material favors high ductility, even in less-pure alloys such as secondary material made from scrap recycling.

REFERENCES:

1. Taylor, P.R., 1983. An illustrated history of lost wax casting. In: Proceedings of the 17th Annual BICTA Conference

2. Barnett, S.O., 1988. Investment casting—the multi-process technology. Foundry Trade Journal 11 (3), pp. 33–37.

3. Eddy, W.P., Barbero, R.J., Dieters, W.I., Esarey, B.J., Frey, L., Gros, J.R., 1974. In: Lyman, T. (Ed.), Investment Casting. American Society for Metals, Ohio, pp. 237–261.

4. Kalpakjian, S., Schmid, S., 2008. Manufacturing Processes for Engineering Materials, fifth ed. Pearson Education Incorporation.

5. S. Shivkumar, X. Yao, M. Makhlouf, Polymer–melt interactions during casting formation in the lost foam process, Scripta Metall. Mater. 33 (1995) pp. 39–46 (Number 1.3946).

6. T.S. Piwonka, A comparison of lost pattern casting processes, Foundry T. J. 164 (1990) pp. 626–631.

7. R.H. Immel, Expandable polystyrene and its processing into patterns for the evaporative casting process, AFS Trans. 87 (1979) pp.545– 550.

8. A.J. Clegg, Expanded-polystyrene molding a status report, Foundry T. J. 159 (1985) pp.177–196.

9. M.R. Barone, D.A. Caulk, A foam ablation model for lost foam casting of aluminium, Int. J. Heat Mass Transfer 48 (2005) pp. 4132–4149.

10. J.R. Brown, Processos de Fundic¸ ˜ao a Poliestireno, Atual Est´agio de Desenvolvimento, 40 Conbrafund, S˜ao Paulo, 1984, pp. 129– 136.

11. R. Bailey, Understanding the evaporative pattern casting process (EPC), Mod. Cast. (1982) pp. 58–72.

12. Ashton, M.C., Sharman, S.G., Brookes, A.J., 1984. Replicast CS (ceramic shell) process. Mater. Des. 5 (2), pp. 66–67.

13. Wang,Y.C., Li, D.Y., Peng, Y.H., 2007. Numerical simulation of low pressure die casting of magnesium wheel. Int. J. Adv. Manuf. Technol. 32 (3–4), pp. 257–264.

14. Liao, D., Fan, Z., Jiang, W., 2009. Study on thin-wall and low-roughness ceramic shell mould based on EPS process. Foundry 58 (12), pp.1220–1223.

15. Jiang, W., Fan, Z., Liao, D., Zhao, Z., Dong, X., 2010. Preparation process of compound shell for lost foam casting based on foam pattern prototypes. Special Casting Nonferrous Alloys 30 (2), pp. 161–163.

16. Kumar, S., Kumar, P., Shan, H.S., 2007. Effect of evaporative pattern casting process parameters on the surface roughness of Al–7% Si alloy castings. J. Mater. Process. Technol. 182 (2–3), pp. 615–623.

17. O. Ludwig, C.-L. Martin, J.-M. Drezet, and M. Sue´ry, Rheological Behaviour of Al-Cu Alloys During Solidification: Constitutive Modelling, Experimental Identification and Numerical Study, Met. Mater. Trans., Vol 36A, 2005, pp. 1525–1535

18. M.C. Flemings, Semi-Solid Processing — The Rheocasting Story, Test Tube to Factory Floor: Implementing Technical Innovations, Proceedings of the Spring Symposium, May 22, 2002, Metal Processing Institute, Worcester Polytechnic Institute, Worcester, MA, 2003

19. Y. Kubo, K. Nakata, and P.R. Gouwens, Molding Unbonded Sand with Vacuum— The V-Process, Trans. AFS, Vol 81, 1973, pp 529–544

20. T. Miura, Casting Production by V-Process Equipment, Trans. AFS, Vol 84, 1976 p 233–236

21. P.-M. Geffroy et al., Thermal and Mechanical Behavior of Al-Si Alloy Cast Using Magnetic Molding and Lost-Foam Processes, Metall. Mater. Trans. A, Vol 37, Feb 2006, pp. 441–447

22. J.-N. Lee and G.-C. Yun, Effect of Centrifugal Force Applied During Investment Casting on the Soundness of Cast Al Alloys, J. Korean Inst. Met., Vol 15 (No. 2), 1977, pp. 155-163

23. V.I. Osinskii, M.V. Solov’ev, and A.V. Morozov, Soundness of Intricate Centrifugally Cast Investment-Mold Castings (Translation), Sov. Cast. Technol., Vol 7, 1

Received on 02.08.2017 Accepted on 15.10.2017

Research J. Engineering and Tech. 2017; 8(4): 440-446.

DOI: 10.5958/2321-581X.2017.00076.9

Thixocasting	Rheocasting	Thixomolding
Thixocasting begins with a non-dendritic solid precursor of material that is specially prepared by the manufacturer using continuous casting methods	Rheocasting starts with material in the liquid state, and the thixotropic slurry is formed directly from the melt via special thermal treatment/management of the system	Thixomolding process uses solid chips or pellets of conventionally solidified magnesium alloys that are fed into a heated injection system
Upon reheating the material into the mushy (i. e., two-phase) zone, thixotropic slurry forms and becomes the feed for the casting operation	The slurry is subsequently fed into the die cavity.	Upon heating, the metal chips are converted by the shear of the screw into a thixotropic, low-solid-content slurry which is fed into the shot accumulator by the rotating screw. Once the accumulation chamber is filled.
No prior treatment is required	It is favored in that there is no premium added to the billet cost, and the scrap recycling issues are alleviated	A major advantage of the process is that it effectively combines both slurry making and slurry injecting into a one step process, leading to high productivity and energy savings. In addition, the process avoids The safety problems usually associated with melting, handling, and die casting molten magnesium.