33 CHAPTER5 5 ZERO DEFECT MANUFACTURING IN THE PRODUCTION OF IMPELLER THROUGH THE APPLICATION OF CAD / CAE 5.1 INTRODUCTION In the first place of research, CAD/CAE was applied to achieve ZERO DEFECT MANUFACTURING in the production of impeller of the set pump. The manufacturing of impeller begins with the production of is casting. In this module research, the casting process was modelled in CAD package and later analysed. The design of gating system and Risers so was achieved ZERO DEFECT MANUFACTURING in the production of the impeller of jet pump. The details of this research activities presented in this chapter. 5.2 STEPS INVOLVED IN CASTING PROCESS The impeller of a jet pump is produced by employing sand casting process. The beginning of this process the patterns are manufacturing. The patterns are negative impeller to manufacture. In order to make hollow sections core pipes are manufacture then moulds are prepared by using to moulding box namely core and drag boxes. The cores are made separately
34 using core boxes. The cores thus made are fixed in the mould to prepare hollow surfaces. Once mould is ready, the molten metal is pour. After solidification, the casting is removed. The removed casting is cleaned by apply fetching process. The sand casting process adopted for producing the impeller of jet pumps in AarVee industry is briefly described in this section. 5.3 ANALYSIS USING AUTO CAST SOFTWARE 2.1 5.3.1 Auto CAST Software Auto CAST is a software program for casting methods design, simulation and optimization developed by IIT Bombay. It uses geometric reasoning for automating the design of casting methods elements cores, mould cavity layout, feeders, feed aids and gating channels. Fast and intelligent simulation technology is employed to visualize mould filling and casting solidification in near-real time. Foundry engineers can quickly import a 3D model of cast part, create methods elements, simulate the casting, predict internal defects (like shrinkage porosity and sand inclusion), and modify the methods design to improve quality and yield. Product designers can improve the casting design for manufacturability by minor modifications, such as fillets at junctions. This enables better compatibility between part requirements and process capability, leading to zero defects. The purpose of this project is to improve the quality of impeller and to produce components with no casting defects. The main aim is to eliminate the defects by analysing the model of rough casting using Auto CAST software.
35 5.3.2 Procedure for Analysis Using Auto CAST Software Before analysis, the model of rough casting is to be produced. This is done by measuring the dimensions of Pattern-cope and drag. Photo snaps of both patterns are taken for reference while designing the rough casting model. This helps to better design the model. The model of rough casting is prepared using Pro-E software package and is compared with the photo snaps of pattern. Finally, after the preparation of correct model of rough casting, the model is saved as.stl file or.igesfile using Pro-E software package. The Auto CAST software accepts only these two file formats for analysis. Auto CAST software are The various steps involved in the analysis of rough casting using 1. Creating the model of rough casting 2. Saving the model as.stl file or.iges file 3. Importing the model into Auto CAST software 4. Selecting the mould box dimensions 5. Selecting parting line 6. Feeding the input data required for analysis a. Pouring temperature = 1475 o C b. Casting material grade = 20G cast iron c. Type of casting = sand casting d. Type of sand = green sand
36 e. Specify contact surface = cooling surface 7. Analyze the model at different sections 8. Store the result of analysis 9. Predict the result a. Brown color = good b. Yellow color = ok c. White color = high chance of shrinkage The model of rough casting developed by the above method is imported into Auto CAST software for analysis. The input parameters such as pouring temperature, casting material, type of casting etc. are fed to the software. Then the model is analyzed at various sections. The output of the analyzed model is in the form of different coloured casting. The presence of shrinkage and the soundness of casting are predicted by the colour in the analyzed model. The areas with dark brown indicate no defects and the areas with white and light yellow colour indicate a high shrinkage. From the analysis it can be seen and understood that the current gating system produces castings with shrinkage. 5.3.3 CAD Modeling using Auto cast Software. The present gating system poses chances for the core to break and create uneven casting. Hence it was decided to change the model of gating. Hence the model of rough casting, shown in Figure 5.1 was prepared using Pro/E Software Package. After the analysis it was decided to change the gating system for effectively flow of molten metal.
37 Figure 5.1 Model of rough casting prepared using Pro-E software package This model was converted to.stl file and imported into the Auto CAST software. Subsequently mould box dimensions and parting line were selected. The image shown figure 5.1 was taken whole analyzing of rough casting model using Auto CAST software. Figure 5.2 Model of rough casting after importing into Auto CAST software
38 The inputs such as pouring temperature, casting material, type of casting etc. were entered into the software for analysis. Figure 5.3 Model of rough casting after feeding input data After entering the input values the model was analyzed. The result of analysis was in the form of coloured model of rough casting. The result of analysis is interpreted by noting the coloured of model at various portions. Brown colour indicates defect-free casting, yellow indicates the part is just satisfactory and white indicates high shrinkage.
39 Figure 5.4 Model of rough casting after analysis Sample castings were prepared with the existing model and the castings were machined. During inspection, it was found that the castings contained hollow surfaces which occurred due to improper flow of molten metal. The flow was improper high shrinkage and breakage of core occurred during the entry of molten metal. Since the flow of molten metal enters with high pressure, there is a chance of core to break and form uneven surfaces. This has to be avoided by designing optimized gating system. 5.3.4 Validating the Result obtained Using Auto CAST Software The impellers are found defective only after they are machined. The presence of defects cannot be known prior to machining. The cost due to rejection is high since the impellers are rejected only after machining. An optimized gating system should be designed to produce castings with no defects. The model of existing system was analyzed and the areas of shrinkage were noted using Auto CAST software. After analysis, sample castings were prepared and machined. The impellers were found with shrinkage near the areas as analyzed in Auto CAST software. Validate the
40 result obtained using Auto CAST software. Subsequent to this validation the gating system was newly designed to minimize the casting defects and create uniform filling by molten metal in to the mould. The details of these activities are described in the next section. Figure 5.5 Existing model of impellers with shrinkage as mentioned in Auto CAST software 5.4 DESIGN OF NEW GATING SYSTEM In order to eliminate the drawbacks of the existing gating system, a model with new gating system was designed. In this new gating system, the inlet of molten metal angle was increased from 90 o to 170 o. This increase in inlet angle minimized the core damage and promoted uniform filling of the mould. A model with new gating system was prepared using Pro- E software. The number of inlets was increased from two to three in each impeller. This creates uniform filling of the mould cavity when the molten metal was poured. There will be a slight increase in the weight of gating and runner, but this will improve the quality of casting. The increase in cost due to a slight raise in the weight of gating and runner is negligible when compared to the less occurred, when the impellers are rejected. In the previous model the drag gating was exactly at the center of the impeller, but in the new design, the gating in the drag is slightly away from the center to have a better flow of molten metal when it is poured into the mould cavity.
41 This change in the gating system allows the molten metal to flow in such a way that the gases are pushed away and ensures no gas being trapped in the molten metal. Figure 5.6 3D model of New Gating system The three dimensional model of the proposed gating system with castings shows Figure 5.6. It can be seen that the number of inlets are three for each impeller and the inlet of molten metal angle is nearly 170 o.there is a small dummy piece at the end of impeller just opposite to the gating, where vent holes are provided to release the gases evolved when molten metal enters the mould cavity. The thickness of the runner was increased when compared to the existing design to have sufficient flow of molten metal. Two inlet gating were fixed at the cope side and one at the drag side. The gating at the drag provides uniform flow of molten metal at the bottom portion and directs the molten metal entering from the top to fill the mould completely. The gating should be designed in such a way that there should be a clearance of about 30 mm from the mould box so as to avoid box leak during pouring molten metal into the mould cavity.there is a minimum pressure should be applied at the core, which reduce the damage of core. The
42 inlet in the cope side at the end should be facing inside in order to reduce the entry of sand into the mould cavity. 5.4.1 Analysis of New Gating Design Using Auto CAST Software Before the new design was implemented, the model of new gating system was analyzed using Auto CAST software to find whether there was any effect on the minimization of shrinkage. The same set of steps was used to analyze the new gating design. These steps are illustrated in this section. Figure 5.7 Importing New Gating design into Auto CAST software The first step, the new gating design was integrated into the Auto CAST software. The screen showing the new gating system integrated into the Auto CAST software is shown in Figure 5.7. Then the input parameters were entered and the casting is analyzed for soundness at various cross sections. The feeding of these input parameter is shown in Figure 5.8
43 Figure 5.8 Feeding input parameters The new design was analyzed using Auto CAST software and the results were interpreted form the model. As explained earlier, each colour indicates the relative quality of casting with respect to shrinkage. From the analysis it could be seen that the major portion of the casting is of brown colour, which indicated that the casting was sound. There was no white colour. The result of this analysis is indicated in Figure 5.9. Figure 5.9 Result of New Gating design after analyzing using Auto CAST
44 5.4.2 Comparison of Existing and New Design of Gating System The exist and the new design of gating system are shown in figure 5.10. As shown the inlet gating of existing system has two inlets A1 and B1. In new design the inlet gating of new design has three inlets A1, B1 and C1. The new design has inlets greater than that of existing design. This creates a more uniform flow of molten metal when the molten metal is poured into the mould cavity. The gases are released first in new gating system. The inlets A1 and B1 are placed in the cope half and C1 is placed in the drag half. A total of sixteen vent holes are provided in the mould cavity to exit the gases into the atmosphere. Exiting Gating System New Gating System Figure 5.10 Comparison of existing and New Gating design
45 5.4.3 Preparing Sample Castings from New Gating Design After preparing new gating system, mould was created and corrections were made in case of any deviations. The mould was prepared and cores were placed and the mould box was closed using pins. Then molten metal was poured into the mould and allowed to solidify. These steps are shown in Figure 5.11. After sufficient time, the castings were removed from the mould and then fettling and shot- blasting were carried out. Finally the impellers were machined and inspected for defects. Figure 5.11 Preparation of casting from New Gating design The impellers which were produced as samples from new gating design were machined and it was found that the shrinkage has been greatly reduced. But there is a formation of blow hole in the impeller at the top side.
46 5.5 ANALYSIS OF DEFECTS IN IMPELLER AFTER DESIGN CHANGES It was observed that the shrinkage which was present in the castings have been completely eliminated, but there is formation of blow holes. These blow holes arose as a result of the gas being trapped inside the casting due to improper provision of vents for air to escape. Due to improper venting, these gases get trapped inside the casting and resulted in the formation of blow holes which could be identified only after machining. The formation of blow hole is noted in the same spot in all impellers. The blow holes were present at the junction of inlet bore and side wall. After inspection, it was found that the blow holes were not distributed and they were formed at the same location in all the impellers. This was due to the following: due to excess moisture content present in the sand, presence of excess binder in the core, less fluidity of molten metal when it enters the mould cavity and due to improper venting of mould box. The blow hole could be minimized by correcting the factors specified above. Care should be taken to ensure that the moisture content present in the moulding sand is in correct quantity. The binder added to the core should not be too high. When the molten metal reacts with the binder present in the core, it liberates excess amount of gases which gets trapped in the casting and form blow holes in the casting. In order to have a proper casting, proper venting should be done to facilitate escape of the gases present in the mould cavity when the molten metal is poured. Proper venting along with provision of chills is likely to improve the quality of casting.
47 The steps taken to minimize the formation of blow holes are discussed in the following paragraphs. The formation of blow holes results from the entrapment of gases inside the metal. The reasons are listed below: 1. Due to excess moisture present in sand, 2. Due to gases released by binder present in core when it reacts withmolten metal, 3. Due to improper venting of mould cavity. 5.5.1 Control of Binder Present in Core In order to have a defect free casting the blow holes due to the reaction of binder present in the core with molten metal needs to be controlled. To control this, the presence of excess binder should be controlled. Steps were taken to control the amount of binder present in core. With a minimized amount of binder castings were made and were found to be defective due to the breakage of core when the molten metal was poured. From this it is concluded that to have a defect free casting the amount of binder should not be reduced in order to minimize the breakage of core during casting process. So, to minimize the blow holes, alternate changes must be done. 5.5.2 Design of Riser at the Suction Bore The change in the amount of binder in core resulted in breakage of core during casting. In order to have could castings, defect free risers were designed. To minimize the formation of blow holes, a proper venting should
48 be done. This helps to escape the gases formed during filling of mould cavity by molten metal. For this purpose holes were drilled in the pattern and pins were attached for venting. The pins were prepared using copper rod of diameter 6 mm and length of 35mm. The copper will not hold sand particles and helps in easy release of mould from the pattern. The pattern with pins used for venting in shown in Figure 5.12 Figure 5.12 Pattern-cope with pins for venting After preparing the pattern care should be taken to see that the pins are straight. Otherwise the pins can damage the mould while releasing the mould. After ensuring that the pins are straight moulds are prepared and venting is done. Vent holes are provided at the places wherever necessary
49. Figure 5.13 Mould prepared from pattern with pins for venting Both the moulds were prepared and closed using pins and the molten metal was poured. The molten metals were allowed to solidify and castings venting as shown in figure 5.13. Figure 5.14 Impellers after solidification The molten metal was allowed to solidify and castings were removed from the mould cavity. The solidification casting impeller shown in figure 5.14.The castings were sent for fettling and then shot blasted. After shot blasting, the impellers were sent for machining.the impeller after machining is shown in figure 5.15
50 Figure 5.15 Impellers after machining After machining, the impellers were found to be having blow holes at the same location as found in the previous case. From the result of design change, it was concluded that the change in design has no effect in minimizing the defect in impeller. It can also be seen that the gases present in the mould cavity was getting trapped in the metal itself and was not released by providing the risers. 5.5.3 Provision of Open Riser in the Mould at the Suction Bore In the previous design the risers did not allow the sufficient releasing the gases. To have a proper release of gases in the mould cavity, it was decided to have open riser at the suction bore walls. This is likely to enhance the easy release of gases present in the mould cavity. Mould was produced as that of previous design with open risers at the position of previous riser to release the gases in the mould cavity. Mould box was prepared and molten metal was poured into the mould and allowed to solidify. Chills were used. Castings were prepared simultaneously using both open risers and using chills.
51 In this case external chills were used which improves directional solidification. The heat transfer rate of metal was greater than that of mouldingsand.this created directional solidification, which means the molten metal was attracted towards the chills and the metal near the chill got solidified further than that are placed in other portions. The castings were removed and sent for shot blasting and then machined. 5.6 CONCLUSION Castings were prepared by using both chills as well as open risers simultaneously and the results are compared. The presence of chills increased complexity of casting process. There are chances of chills to get in contact with the mould cavity during ramming, which makes the chills to get into direct contact with the molten metal. In this case, stainless steel chills are used, which may get melted and may change the composition of the casting. Chills produce spot hardness in the component, which may reduce the tool life. The impellers were subjected to machining after rough castings are produced and this spot hardness causes the tool to wear soon. In case of producing large quantity of impellers, the process becomes complex and may result in defective castings.
52 Figure 5.16 Defect free impellers after design changes with open riser in mould The impellers after machining were found to be defect-free with open riser. Nearly 50 castings were prepared and all the components were found to be free of shrinkage and blow holes. Hence 100% defect free impellers could be produced by using new design of gating system along with open riser.