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AN APPLICATION OF COMPUTER AIDED ENGINEERING IN RUBBER INJECTION MOLD WITH A COLD RUNNER SYSTEM DESIGN AND MANUFACTURING Supasit Rodkwan Center of Excellence in Rubber Mould, Research and Development Institute of Industrial Production Technology and Department of Mechanical Engineering, Kasetsart University, Bangkok Thailand. Rungtham Panyawipart Center of Excellence in Rubber Mould, Research and Development Institute of Industrial Production Technology and Department of Mechanical Engineering, Kasetsart University, Bangkok Thailand. Chana Raksiri Center of Excellence in Rubber Mould, Research and Development Institute of Industrial Production Technology and Department of Industrial Engineering, Kasetsart University, Bangkok Thailand. Kunnayut Eiamsa-ard Center of Excellence in Rubber Mould, Research and Development Institute of Industrial Production Technology and Department of Mechanical Engineering, Kasetsart University, Bangkok Thailand. ABSTRACT With a recent growth in the demand of the rubber products globally, the latest technology is adopted to improve the design and manufacturing of those rubber products in term of part quality and production lead time and cost. The cold runner system is one of the technologies which can assist in unfilling part problem and raw material saving. Nevertheless, with the lack of numerical tool with an ability to predict the behavior of rubber during the injection molding process, designers still need to use their experience and trial-and-error method to design the mold and the cold runner system. Therefore, in this research, the use of CAE and a cold runner system is applied to the design and manufacturing of rubber injection molding process with a gasket mold made of SBR as a case study. The empirical and simulated results agree well and the use of raw material in the actual system is decreased by 12% shot weight which can lead to the reduced cost of products. Finally, it can be seen that the use of CAE can assist the mold designers and manufacturers to get better understanding of flow pattern and behavior of rubber during the injection process so the better part quality can be obtained. INTRODUCTION Currently, there is significant growth in the demand of the rubber products worldwide. Latest computing technology and advanced engineering have been used to improve the design and manufacturing of those products. This can lead to the better part quality and the reduction of production lead time and cost. The cold runner system is one of the technologies which can assist in raw material saving, especially for the system having long gate and runner paths and the system for mass production. However, with the lack of numerical tool with an ability to predict the behavior of rubber during the injection molding process, operators in the shop still need to use their experience and trial-and-error to design and to manufacture the mold and the cold runner system. In addition, research work in the area of mold design and manufacturing using the Computer Aided Engineering (CAE) with a comparison of empirical work has not been found. Consequently, in this paper, the CAE, one of the powerful techniques used nowadays to analyze and to predict the injection behavior during the rubber injection process. The cold runner system is applied with a mold of rubber gasket used in the rubber injection process so the effective mold and better quality of parts can be achieved. MATERIALS AND METHODS In this industrial research-based work, a case study of the rubber gasket made of Styrene Butadiene Rubber (SBR) as shown in Figure 1 is chosen. The ENGEL Model ES2700/250 V EL, with a capacity of 250 tons, injection machine is used in this study. The machine stroke is varied to investigate flow pattern, temperature and injection time at various stages. The cold runner system with sixteen cavities with a mold size of 496546240 mm. is proposed to replace the current hot runner system, as shown in Figure 2, used in the gasket production line in the rubber product manufacturing plant. The cold runner system is designed to effectively use the rubber raw material in the runner system of the rubber injection machine similar to the utilization of the hot runner system in the 1Copyright 2007 by ASME Proceedings of IMECE2007 2007 ASME International Mechanical Engineering Congress and Exposition November 11-15, 2007, Seattle, Washington, USA IMECE2007-43851 Downloaded From: / on 04/10/2013 Terms of Use: /terms thermoplastic injection machine. In addition, the defect of part such as unfilled portion of rubber part can be decreased. The schematic of the cold runner system is shown in Figure 3. Temperature of rubber compound in the runner system is adjusted to the suitable low level to maintain rubber compound in the uncured stage. The mold temperature of the cold runner block (1) is controlled by a cooling system (2) and an insulator plate (3) which can protect the heat from the heater in an upper mold (4) which can lead to the vulcanization process. Subsequently, the rubber is injected through the cold runner to the cold runner nozzle (5) that is insulated by the air gap between the nozzle and the mold. Finally, the rubber is flown into the mold cavity. Figure 1. A Rubber Gasket. Figure 2. A Gasket Mold with a Hot Runner System. Figure 3. A Cold Runner System Rubber Injection Mold. 1 The governing equations for flow of rubber during injection process can be found from 2: 0)(=+ v t r (1) where is the fluid density and v r is the fluid velocity. )( vvg t vrrr r += (2) where g r is the total body force per unit mass and is the stress tensor. vppv t p TTv t T cP rrr + + = + )():(Tkv+ r (3) where P c is the constant pressure, is the coefficient of volume expansion, p is the pressure and k is the thermal conductivity. In the simulation work, the Computer Aided design (CAD) is used to construct the gasket part geometry as shown in Figure 4 with sixteen mold cavities as well as the gate and runner systems. Then, an analysis of rubber injection process is carried out using 3D-SIGMA, a rubber injection simulation software, to obtain the optimized current runner system design. Figure 4. A Gasket CAD File. In a simulation process of rubber injection molding, the gate dimension is firstly determined. Using the preliminary simulation for a flow pattern depicted in Figure 5, it is found that the appropriate rectangular gate sizes are 0.505.00 mm. and 0.705.00 mm. for each opposite side of gasket cavities, respectively. The design lead to the equal flow rate through each gate into each gasket cavities as shown in Figure 6. Then, a completed mold with the cold runner system design as shown in Figure 7 can be obtained for subsequent analysis. The temperature in the injection process is set up as illustrated in Figure 8. Additionally, the results are collected after ten cycles of injection process for a steady state condition. Figure 5. The Rubber Flow Pattern of Gasket. 2Copyright 2007 by ASME Downloaded From: / on 04/10/2013 Terms of Use: /terms Figure 6. The Flow Rate through each Gate. Figure 7. A Completed Mold with a Cold Runner System. Figure 8. Injection Molding Process Temperature. RESULTS Using the cold runner system design, the gasket mold is manufactured as shown in Figure 9. and Figure 10. The comparison of empirical tests and simulation work of product shape at each stage of stroke distance from 30 mm. to 38 mm. are shown in Table 1 4, respectively. It can be seen that prediction from a simulation work correlates well with empirical product shape obtained using an injection machine. However, the mismatch of shape can be occurred due to geometric tolerance error during the mold production. Table 1. The Comparison of an Empirical and a Simulated Injection of Product Shape at a Stroke Distance of 30 mm. 30 mm. of Stroke Table 2. The Comparison of an Empirical and a Simulated Injection of Product Shape at a Stroke Distance of 32 mm. 32 mm. of Stroke Table 3. The Comparison of an Empirical and a Simulated Injection of Product Shape at a Stroke Distance of 34 mm. 34 mm. of Stroke 180 oC 180 oC 75 oC 75oC 75oC Upper mold Lower mold Cold runner block Barrel Cooling in Cooling out 100 oC 3Copyright 2007 by ASME Downloaded From: / on 04/10/2013 Terms of Use: /terms Table 4. The Comparison of an Empirical and a Simulated Injection of Product Shape at a Stroke Distance of 38 mm. 38 mm. of Stroke In addition, the temperature at a specific point on the mold is measured and is simulated for comparison as shown in Table 5. The discrepancy arises due to the heat loss during actual off- line measurement. Table 5. The comparison of an Empirical and a simulated injection temperature. Simulation Injection Try Out 174.10 oC 140.60 oC 171.10 oC 143.10 oC 69.90 oC 71.20 oC Figure 11 shows the rubber-air contact time in various area of the part. The high number in a specific area can refer to large possibility occurrence of the air trap and the unfilling. The filled time, which can show the position of rubber flow in each stage of injection, is illustrated in Figure 12. The overcured rubber can be identified through material age as shown in Figure 13. Figures 14 and 15 show the temperature of mold and cold runner nozzle at the cure time of 180 seconds. It can be seen from the temperature distribution that the cold runner system can cause the rubber material to the uncured stage while it is in the runner system while over 90% rubber portion in each cavities is cured as shown in Figure 16. In addition, using the mold with a cold runner system in this work, the experiment reveals that the raw rubber material in each injection cycle is empirically reduced by 12.42% shot weight compared with the hot runner system mold. Figure 9. The Gasket Cold Runner System. Figure 10. A Gasket Mold. Figure 11. The Air Contact Time between Rubber and Air. A Possibility of Air Trap 4Copyright 2007 by ASME Downloaded From: / on 04/10/2013 Terms of Use: /terms Figure 12. The Filling Time during Injection Process. Figure 13. The Rubber Age of the Part. Figure 14. Mold Temperature at the Cure Ti