A Review on Cooling System Design for Performance Enhancement of Injection Molding Machine

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1 A Review on Cooling System Design for Performance Enhancement of Injection Molding Machine Rupali P. Patil M.E (Machine Design), SST s College of Engg., Jalgaon Navneet K. Patil Asso. Professor, Department of Mechanical Engg. SST s College of Engg., Jalgaon Jagruti R. Surange Asst. Professor, Department of Mechanical Engg. SST s College of Engg., Jalgaon ABSTRACT Injection molding is one of the most flexible and important operation for mass production of plastic parts. In injection molding process, cooling system design is very important as it largely affects the cycle time. A good quality cooling system design can reduce cycle time and attain dimensional stability of the part. In order to reduce the cycle time, and control the even distribution of temperature, it is required to create conformal cooling channels, which match to shape of cavity of the mold and core. This paper presents a simulation study of various types of cooling channels in an injection molded plastic part and compares the results in terms of time to ejection temperature, temperature profile, contraction and part warpage to determining the configuration which is more suitable to provide uniform cooling with minimum cycle time. The Autodesk Mold Flow Insight (AMI) simulation software is used to study the results of the cooling channels performance. KEYWORDS: Injection Molding, conformal cooling, mold flow, cycle time, simulation I. INTRODUCTION: Plastic injection molding process is the most common process for economically mass producing of various complex geometries and shapes plastic parts [1]. In injection molding the molten plastic is injected into the mold cavity and cooled subsequently to form shape of the mold. Most commonly used materials for injection molding process are Thermosetting and Thermoplastic Polymers. Injection molding machine consist of a hopper through which granules of the plastic material enter a screw-type plunger. The screw-type plunger is operated by prime mover or any hydraulic system. Heaters are provided on the exterior side of this plunger which gradually heats the plastic granules as it move forward through the screw. The molten plastic is injected at the nozzle end through a small opening in the mold cavity. The mold core clamps to the cavity and mold area is formed where the filling of molten plastic takes place. The mold is cooled by water channels around the mold cavity. The three main phases in injection molding are filling; cooling and ejection. Out of these, cooling process takes about 50-75% of cycle time. In conventional Injection Molding process, straight holes are drilled in the mold cavity through which water passes for cooling the mold. A fine surface finish on the mold can be obtained by using uniform cooling. This increases the cycle time of product, which is undesirable. On the other hand if cycle time of product reduced, chances of getting improper surface finish are higher. It is therefore important to obtain a solution in which reducing cooling time also reduces cycle time. It also shows that cycle time of product is dependent on cooling of the mold. An efficient cooling circuit design reduces the cooling time, which in turn increases overall productivity. [10] Moreover, uniform cooling improves part quality by reducing residual stresses and maintaining dimensional accuracy and stability (see Figure-2). Some of the major problems facing injection molders today are reduce cycle time, part warpage and shrinkage alongside lowest costs. Nowadays it is possible to design and simulate different cooling configuration of the plastic part and investigate the proper cooling system for the plastic part by using computer simulation software AMI. ISSN: Page 129

2 Figure-1: Mold cooling accounts for more than two-thirds of the total cycle time [10] Figure-2: Proper and efficient cooling improves part quality and productivity [10] II. Components of Mold Cooling System A mold cooling system typically consists of the following items: Temperature controlling unit Pump Supply manifold Hoses Cooling channels in the mold Collection manifold The mold itself can be act as a heat exchanger, with heat from the hot polymer melt taken away by the circulating coolant. III. Design of Part and Cooling Channels In this study, a plastic part has been designed using CATIA V5 as shown in figure 3. Part is of 2 mm in thickness and 182 mm in height. The IGES (Initial Graphics Exchange Specification) CAD model of the plastic part has been imported to AMI to analyze the component. Polypropylene has been assigned as a material for plastic part. Melt temperature and mold surface temperature have been maintained at 225 C and 38 C respectively. The best gate location analysis has been done and it is found to be located in the middle of the base of the plastic part. Water was selected as the coolant liquid for all analysis because it has high cooling property, economically viable and also environmentally friendly. Figure 4 shows four types of cooling channels considered in this analysis. All cooling channels had the same diameter of 10 mm. The depth of the cooling channels from cavity surface and distance between cooling channels centers were 25 mm and 40 mm respectively. The coolant Reynolds number was calculated to be 9750 ISSN: Page 130

3 and temperature at coolant inlet was 20 C, which indicates that flow of water was fully turbulent. Figure 3: CAD Model of the plastic part (c) (d) Figure 4:Cooling channel types: Normal, conformal combination with baffle, (c) conventional combination with conformal and (d) conformal IV. Plastic Simulation Study: Autodesk Mold flow Insight has been used to design the cooling channels and perform Cool, Flow, Pack and Warp analysis. Dual Domain Meshing (Fusion mesh) was used with number of elements being The quality of the mesh can be improved with manual meshing techniques. The maximum aspect ratio was improved significantly from 20 to 10. Similarly match percentage has improved from 81.4% to 91.6% as well as reciprocal percentage has improved from 7.2% to 88.3%. Molding Windows is quick analysis to evaluate the best material conditions that are needed to produce part. Molding Windows is used to determine the best processing conditions and evaluate the sensitivity of the part to the molding process. It also evaluates gate locations and gains a measure of part design quality to improve manufacturability. It also helps to determine variation in mold temperature, melt temperature and injection to produce high quality of the part. Figure 5shows the zone plot for Polypropylene material Purell HP570R. The X-axis is melt temperature and injection time. The Y-axis mold temperature. The zone plot has three areas, which are green, yellow and red. It is clear from the fig.5 that the zone plot selected is within green area (Preferred). This means we are able to mold the part and the quality will be high and hence the part will be acceptable by customers after producing. ISSN: Page 131

4 Figure 5: Molding Windows V. Results and Discussion The simulation results in terms of time to reach part ejection temperature (time to freeze) indicates that normal cooling channels took around 75 seconds to reach ejection temperature for most of the plastic parts as shown by blue colors in figure 6. It shows that this cooling channel is the slowest cooling system as it requires more time than other systems. This is because of its structure that consists of straight drilled holes in the mold. These holes have limitations in terms of geometric complexity, non uniform cooling between the surfaces of the part and cooling fluid mobility within the injection mold [1]. However, time to reach part ejection temperature decreased to around 44 seconds with the use of conformal cooling channel combination with baffle showing significant improvement as can be seen in blue shades in Figure 6 and faster by nearly 43% in comparison with that of normal straight channels for same pitch distance and the same channel diameter. With the use of conventional cooling in combination with conformal cooling channel in the core, the value of time to reach part ejection temperature reduces to around 43 seconds as can be seen in blue in figure7. The cooling time then reduced to around 37 seconds for the fully conformal cooling system as can be seen in blue figure7 thus cooling the product the fastest, by 49.6% faster than normal cooling channel, and up to 12% faster than conformal cooling channel in combination with baffle, that is the lowest value when compared to the rest of the cooling channel used in the analysis. Figure 6: Time to reach part ejection temperature (Time to freeze part) with normal, conformal combination with baffle, cooling channels [1] ISSN: Page 132

5 Figure 7: Time to reach part ejection temperature with conventional combination with conformal, conformal, cooling channel [1] This is due to conformal cooling lines follow the part geometry in the mold and because of optimum placement resulting in uniform mold temperatures. In conformal cooling, results show the predicted temperature difference across the part thickness is drastically reduced and becomes more uniform than other cooling channels. In addition, in a conformal cooling channel, the mold surface temperature is fairly uniform around the part. Figure8 shows temperature profile for normal cooling channel, to demonstrate the temperature distribution. Since mold surface temperature is 38 C in all types of cooling channels, as a result, the normal cooling channel has non-uniformity of temperature distribution within the range from C to C as shown in figure8, which means that lowest value (42.46 C) is more than mold surface temperature which is 38 C. In contrast, small temperature variation from C C is with the use of conformal cooling combination with baffle as can be seen in figure8. Figure 8: Temperature profile of part with normal, conformal combination with baffle, cooling channels [1] In conventional cooling in combination with conformal, temperature distribution lies within the range from C to C as shown in figure 9. These cooling channels cannot provide the uniformity for all portions of part. On other hand, the temperature is uniformity distributed for all portions of part C with the use of conformal cooling channel as described in figure 9. Also the highest temperature in the whole analysis is reduced by more than 36%. Figure 9: Temperature profile of part with conventional combination with conformal, fully conformal cooling channels Therefore, it can be concluded that this result gives clear evidence that conformal cooling channel provides better temperature consistency and uniformity than other cooling channels even in complex part. Figure 10 summarize the results obtained from Autodesk Mold flow Insight analysis. Figure 10 shows the comparison for time to freeze and maximum temperature profile. ISSN: Page 133

6 Figure 10: Comparison of time to reach ejection temperature and maximum temperature [1] Volumetric shrinkage is inherent in the injection molding process, where the molded plastic part shrinks with cooling and solidifying of material. Shrinkage can sometime be to the extent of 20 percent by volume. Figure 11 shows that in normal cooling channel, volumetric shrinkage is 17.01%, which is highest among all types of channels. This rapid increase of volumetric shrinkage and its spread around the part center is due to premature pressure removal [1]. This high value of shrinkage can be reduced by changing the sharp corners to radius and improve some part features. On the other hand, the value of shrinkage has been decreased to 16.72% with the use of a conformal cooling combination with baffle as shown in figure 11. It shows that by adding baffles to the conformal cooling channel, volumetric shrinkage can be reduced and made more uniform. In case of conventional cooling combination with conformal in figure 12, the result shows that volumetric shrinkage has been reduced slightly to %. In the conformal cooling channel, the value of volumetric shrinkage has been decreased to 15.72% that is the lowest percentage among all cooling channels as shown in figure 12. It points out that volumetric shrinkages are distributed more consistently and show more uniformity in conformal cooling channels [1]. Figure 11: Volumetric shrinkage of part with normal, conformal combination with baffle, cooling channels Figure 12: Volumetric shrinkage of part with conventional combination with conformal, conformal, cooling channels ISSN: Page 134

7 The results of warpage analysis and sink analysis of different cooling channel design are shown in the following chart. Figure 13: Comparison of volumetric shrinkage, sink mark and warpage VII. CONCLUSION: It is observed from the above study that conformal cooling channel is the most suitable cooling system for the plastic part as compared to other cooling channels. It gives better cooling properties due to lower volumetric shrinkage and the lower sink mark percentage. It also shows the lowest time to reach the ejection temperature, which leads to lower cooling time and reduced overall cycle time. The conformal cooling channel shows more uniform cooling which makes it most favorable cooling system. The conformal cooling channels are mostly preferred because the cooling lines are located to follow the part geometry in the mold. The injection molding analysis software provides helpful information regarding cycle time, shrinkage and warpage for plastic product and design of mold to reduce time and cost of production especially for complex parts. REFERENCES [1] Omar A. Mohamed, S.H. Masood, Abul Saifullah, A Simulation Study of Conformal Cooling Channels in Plastic Injection Molding, International Journal of Engineering Research, Volume No.2, Issue No. 5, pp: [2] Aditya Shirish Joshi, Rahul Radhesham Jayaswal, A Survey on Methods used for Optimization of Injection Molding, (IIJME) Volume 2, Issue 9, September 2014 [3] A B M Saifullah, S.H. Masood and Igor Sbarski, New Cooling Channel Design for Injection Moulding,World Congress on Engineering 2009 Available at:< p pdf> [4] Q. Niu, X. H. Zhou, W. Liu, Research and Development of Intelligent Cooling Channels Design System, International Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering Vol: 8, No: 12, 2014 [5] B.G. Thomas, J. Jiang1, D. Lorento, Optimization Of Water Channel Design In Beam-Blank Molds 5th European Continuous Casting Conference June 2005, Available at:< r_v95_rev.pdf> [6] E. Mathey, L. Penazzi, F.M. Schmidt and F. Rondé- Oustau, Automatic Optimization of the Cooling of Injection Mold Based on the Boundary Element Method Available at :< albi.fr/~mathey/publi/mathey_numiform04.pdf> [7] Konsulova-Bakalov, Vassilev G. Application of Solidworks Simulation for Design of Cooling System for Injection Molding, Available at :< NG.pdf > [8] Vojtech Senkerik, Michal Stanek, Miroslav Manas, David Manas, Adam Skrobak, Jan Navratil, Optimization of Gate Location and Cooling System, Recent Researches in Circuits and Systems,Available at:< YS/CIRSYS-47.pdf> [9] Mrozek K., Staniek R., Szostak, Research On Injection Mold Cooling System Efficency And Uniformity For Electrical Connector Casing Machines, Technologies, Materials, ISSUE 12/2013,Available at:< [10] Jay Shoemaker, Moldflow Design Guide Available at :< mfdg_low-res_proof_part1.pdf> [11] Hong-Seok Park and Xuan-Phuong Dang, Design and Simulation-Based Optimization of Cooling Channels for Plastic Injection Mold, New Technologies Trends, Innovations and Research available at :< > [12] Rahul Vashisht, Arjun Kapila, A Comparative Study of Coolants Based on the Cooling Time of Injection Molding, International Journal of Emerging Technology ISSN: Page 135

8 and Advanced Engineering Website: Volume 4, Issue 6, June 2014 [13] Hyeyoung Shin and Eun-Soo Park, Analysis of Incomplete Filling Defect for Injection-Molded Air Cleaner Cover Using Moldflow Simulation, Journal of Polymers, Hindawi Publishing Corporation, Volume 2013, Article ID [14] Altaf K., V. R. Raghavan, A. M. A. Rani, Comparative Thermal Analisys of Circular and Profiled Cooling Channels for Injection Mold Tools, Journal of Applied Sciences 11: , 2011, ISSN ISSN: Page 136