Optimum Design and Analysis of Riser for Sand Casting

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1 Optimum Design and Analysis of Riser for Sand Casting C. M. Choudhari 1, B. E. Narkhede 1, S. K. Mahajan 2 1 Department of Production Engineering, V.J.T.I., Mumbai, India 2 Director, Technical Education, Maharashtra State, Mumbai, India (c.choudhari75@gmail.com) Abstract - Solidification of metals continues to be a phenomenon of great interest to physicists, metallurgists, casting engineers and software developers. It is a non-linear transient phenomenon, posing a challenge in terms of modelling and analysis. This paper attempts to study heat flow within the casting, as well as from the casting to the mould, and finally obtains the temperature history of all points inside the casting. The most important instant of time is when the hottest region inside the casting is solidifying. ANSYS software has been used to obtain the last solidifying region in the casting process by performing Transient Thermal Analysis. Location of the hot spot predicted by software simulation showed good agreement with the experimental trial. It was also observed that the simulation of casting helps in obtaining optimum design of riser. Keywords: Design, analysis, riser, sand casting I. INTRODUCTION Casting is one of the earliest metal shaping method known to human beings. It can be effectively used to make complex shaped parts which weigh less compared to parts manufactured by any other production process. Casting is one of the cheapest methods for mass production of any part. However, it gives poor dimensional accuracy and cannot take up high amount of shock loads. Also casting leaves rough surface finish which requires machining often. Since it is subjected to many defects, it is necessary to eliminate them. Many of these defects cannot be eliminated by changes to tooling and process parameters. This study mainly deals with the defect of shrinkage cavity. One of the methods to remove this defect is by attaching a riser to casting, which serves as a reservoir for the molten metal. The study also emphasizes on the optimum design of riser so that the yield of the casting is improved. Casting consists of various parts like cope, drag, pattern, sprue, runner, ingates, riser, etc. The process consists of design, solidification; shake out, finishing and heat treatment. To eliminate the defect of hot spot riser is used in casting. It helps to fill in the cavity formed inside the casting [1]. Whenever the cavity is formed inside the casting the molten metal from the riser moves to that space and fills the cavity. In order to achieve this, the dimensions of the riser should be optimized so that the metal in the riser solidifies at last and hence increases the yield of the casting [2]. Reis A. et. al. [3] modelled the shrinkage defects during solidification of long and short freezing materials. The shrinkage defects in short freezing materials tends to be internal, as porosity, while in long freezing materials these defects tend to be external in the form of surface depressions. Prabhakara Rao et. al. [4] carried out the simulation of mould filling. He concluded that the use of casting simulation software like PROCAST can eliminate the defects like shrinkage, porosity etc. in the casting. It also improves yield of the casting, optimize the gating system design and the mould filling. Ravi B. et. al. [5] worked on computer-aided casting design and simulation. This paper describes computer-aided casting design and simulation gives a much better and faster insight for optimizing the feeder and gating design of castings. Rabindra Behera et. al. [6] has suggested that the application of computer aided methoding, and casting simulation in foundries can minimize the bottlenecks and non-value added time in casting development, as it reduces the number of trial casting required on the shop floor. From the existing and recent literature citations it is found that the currently available casting solidification simulation software s have not taken all constraints and conditions required for the realistic simulation process. This matters more and influences critically on the output results. II. METHODOLOGY The formation of hot spot inside the casting is a major defect in metals like aluminum and steel. Optimum riser design will ensure removal of hot spot from the casting. Here, riser having higher value of the modulus has been design so that it should have higher solidification time compared to casting. This will ensure that metal will remain in the molten state inside the riser until solidification of the casting is completed. Initially, casting design for aluminum metal has been carried out to obtain the dimension of runner, riser, ingate etc. Later, the optimum size and location of the riser was identified based on ANSYS simulation. Finally, the experimental trial has been performed based on design calculations and simulation results which has provided defect free casting. III. THEORETICAL STUDY There are various methods for designing riser like /13/$ IEEE

2 Caine s method, modulus method, etc. Riser has a neck attached to it at the lower end. Neck facilitates easy separation of the riser from the casting after the casting is completed. Hence neck is an important part of the riser. Initially rectangular plate casting was designed with its various parts. The size of the plate is 200mm 200mm 40 mm as shown in Fig. 1. Design calculation begins with calculation for pattern allowances followed by gating system calculation and finally design of the riser [7, 8]. Surface area=πr 2 + 2πrh Freezing ratio(x) = (A c /V c )/ (A r /V r ) Where, A c = Area of casting V c = Volume of casting A r = surface area of riser V r = Volume of riser V f = Volume of feeder For aluminium metal: a=0.1 b=0.03 c=1 where a, b, c are constant The riser diameter by Caine s method is mm. The neck diameter was calculated by the formula D n = H n D r Fig. 1. Geometry of Casting Calculation for pattern allowances: For 200 mm: 2.6 mm For 40 mm: 0.52 mm Draft allowance = 1.5 Machining allowance = 2 mm on each side Tolerance = ± 1 mm Calculation for solidification time Total surface area = mm 2 Total volume = mm 3 Modulus=15.29 mm. Solidification time = min. Weight of the casting = kg For gating system: Pouring time = sec Choke area = mm Sprue bottom diameter = 12 mm Sprue top diameter = 15 mm Sprue height = 42.5 mm Total area of ingates = mm 2 The design of riser was done using Caine s method. The height of riser was assumed to be 70 mm and the height of riser neck was assumed to be 10 mm. Following formulae were used for finding the dimensions of casting. Volume of the riser = πr 2 h Neck Diameter, D n = 21 mm According to optimum design of riser [9], D n = 0.35 D r Riser Diameter, D r = 60 mm. Taking the higher value, we get diameter of riser, D r = 60 mm. Yield of feeder = (V c ) / (V c + V f + V n ) = % Yield of casting = (W c )/(W g + W f ) = % where, W c = weight of casting W g = weight of gating elements W f = weight of feeding elements Thus, it was found that the yield of the feeder was greater than yield of the casting. Hence the riser designed is of optimum dimensions and helps to increase the yield. IV. SIMULATION Simulation of casting was done to serve two main purposes. First, it was used to find the location of hot spot. Second, it was used to find the optimum dimension of riser so that hot spot shifted into the riser [10]. These studies were done using both free and mapped mesh. Here finite element ANSYS 12.0 software has been used for modelling and simulation. At the end of simulation the last solidifying region was obtained. A. Simulation using Linear Elements

3 For this study, PLANE 55 was used as the linear element. The element has four nodes with a single degree of freedom, temperature, at each node. The top view was modeled showing inside portion for casting (labeled as 1) and surrounding region for sand (labeled as 2). Then, the material properties were specified for metal and sand followed by meshing of geometries. The meshed geometries with free and mapped mesh are shown below in Fig. 2, 3. Fig. 4. Location of Hot Spot in center with Free Mesh Fig. 2. Free Mesh of Casting Fig. 5. Location of Hot Spot in center with Mapped Mesh a. Optimum Riser Dimensions Once, the location of hot spot was identified, the next objective was to find the optimum riser dimensions. For this purpose, following dimensions of riser were considered. Fig. 3. Mapped Mesh of Casting After meshing, convective load was applied on the outer boundaries of the casting along with the ambient temperature and initial temperature of molten aluminium. The simulation was run for 1 hour and the location of hotspot was obtained as shown below in Fig. 4, 5. TABLE I RISER DIMENSIONS Sr. No. Riser Diameter Riser Height Neck Diameter Neck Height 1 40 mm 70 mm 14 mm 10 mm 2 50 mm 70 mm 17.5 mm 10 mm 3 60 mm 70 mm 21 mm 10 mm The risers with different diameters as mentioned in TABLE I were modeled in ANSYS software using PLANE 55 element one after the other. The simulation of these models yielded the following results (Fig. 6-8).

4 V. EXPERIMENTAL ANALYSIS OF CASTING A. Pattern Making Fig. 6. Location of Hot Spot for Riser with Diameter 40 First step of experimental analysis was to prepare a wooden pattern with various allowances for shrinkage, draft, machining etc. Second step is to made wooden pattern for pouring basin, sprue and sprue base well, runner, ingates and as per the design dimensions for aluminum casting. Fig. 10 shows rigging system with pattern. Fig. 7. Location of Hot Spot for Riser with Diameter 50 Fig. 10. Rigging System with Pattern B. Mould Box Preparation Fig. 8. Location of Hot Spot for Riser with Diameter 60 It is clear from the Fig. 8 that the hot spot shifts into the riser for diameter of 60 mm. b. Effect of Sleeve on Riser Diameter An insulating sleeve was used around the riser to slow down the rate of heat transfer from the riser. A sleeve of 5 mm thickness was used around the riser of 50 mm diameter. The result of this simulation is shown below in Fig. 9. Silica sand was mixed with Bentonite in the ratio of 10:1. Bentonite has a property to retain moisture. It also acts a binder and holds the sand firmly when pattern is withdrawn from the mould box. A standard size mould box of dimension 300mm 300mm 160mm was used. The mould cavity was prepared in two parts, cope - the upper part and drag - the lower part as shown in Fig 11. Fig. 9. Location of Hot Spot for Riser with Sleeve As seen from Fig. 9, simulation with sleeve helps in maintaining the riser hot for a longer time. As a result, a riser of diameter 50 mm can be used instead of 60mm. Fig. 11. Cope (Left) and Drag (Right) with pattern embedded in mould box. VI. RESULTS First casting trial was carried out without riser. Fig. 12 shows shrinkage defect at the center as per ANSYS simulation study. For the experimentation purpose, two

5 different types of cope preparation methods were used. In one of the methods riser was placed along with a sleeve in the mould box and sand was rammed. While removal of pattern, only wooden riser was withdrawn and sleeve was retained in the cope. In other method only riser was placed in the cope and ramming was done. Fig. 13 shows defect free casting with riser diameter 50mm and sleeve of 5mm thickness. Fig. 12. Casting without riser Fig.13. (Left) Casting with riser, diameter 60 mm and (Right) Casting with riser, diameter 50 mm with sleeve thickness 5mm VII. CONCLUSION Simulation of the solidification process enables visualization of the progress of freezing inside a casting and identification of the last freezing regions or hot spots. This facilitated the optimized placement and design of feeders and feeding aids with improvement in yield while ensuring casting soundness without expensive and timeconsuming trial runs. Optimum location of riser based on ANSYS software has helped in minimizing the solidification related defects, thereby providing a defect free casting. Majority of small scale foundries are least concerned about optimizing the casting process. This study shows that simulation can be of great use in optimizing the riser dimensions and increasing the feeding efficiency of the casting. Simulation was carried out with casting having riser of dimensions 40 mm, 50 mm and 60 mm. Furthermore, a sleeve of thickness 5 mm was incorporated and simulation was run. Casting with riser of diameter 50 mm along with sleeve of thickness 5 mm has shown similar result when compared with the riser of diameter 60mm. Therefore, the optimized riser dimensions based on simulation were validated by carrying out actual trials in a foundry. Using sleeve as a feed aid helped in reducing riser dimensions form 60 mm to 50mm and thereby increasing the casting yield. REFERENCES [1] C.M. Choudhari, K. J. Padalkar, K. K. Dhumal, B. E. Narkhede, S. K. Mahajan, Defect free casting by using simulation software, Applied Mechanics and Materials, 2013, , [2] Richard W Heine, Carl R Loper, Philip C Rosenthal, Principal of Metal Casting, Tata McGraw Hill, [3] A.Reis, Y. Houbaert, Zhian Xu, Rob Van Tol, A.D. Santos, J.F. Duarte, A.B. Magalhaes, Modeling of shrinkage defects during solidification of long and short freezing materials, Journal of Materials Processing Technology, Volume 202, Issues 1 3, Pages ,2008. [4] Prabhakara Rao, P. Chakravarthy, G. S. Kumar, A. C. and Srinivasa Rao, Computerized simulation of sand casting process, 57th Indian Foundry Congress, Institute of India Foundrymen, Kolkata, [5] B. Ravi, Computer-aided Casting Design and Simulation, STTP, V.N.I.T. Nagpur July 21, [6] Rabindra Behera, Kayal S., and Sutradhar G., Solidification behaviour and detection of Hotspots in Aluminium Alloy castings: Computer Aided Analysis and experimental validation, International Journal of Applied Engineering Research, Dindigul Volume 1, No 4, pp , [7] P.N.Rao, Manufacturing Technology, Tata McGraw-Hill Education, New Delhi, [8] John Campbell and Richard A Harding, Solidification Defects in Casting, IRC in Materials, The University of Birmingham. [9] T. Nandi, R. Behera, S. Kayal, A. Chanda and G. Sutradhar, Optimization of Riser size of Aluminium alloy (LM6) castings by using conventional method and computer simulation technique, International Journal of Scientific & Engineering Research, Volume 2, Issue 11, November [10] Saeed Moaveni, Finite Element Analysis: Theory and Application with ANSYS, 3rd ed. Pearson Education, India, 2003.