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Int J Plast Technol (December 2016) 20(2):249–264 263 DOI 10.1007/s12588-016-9153-4 Elliptical cross sectional shape of runner system in injection mold design Mehdi Moayyedian1 ? Kazem Abhary1 ? Romeo Marian1 Received: 3 June 2015 / Accepted: 21 July 2016 / Published online: 27 July 2016 ? Central Institute of Plastics Engineering & Technology 2016 Abstract This paper presents a new cross sectional shape of the runner system in the mold design of the injection molding process. The aim of the new geometry is to reduce scrap, cycle time and ease the ejection of runner system from mold tools. An elliptical cross sectional shape of runner with different ratios was proposed for two circular flat plates with thickness 1 mm. Finite element method (FEM) is employed in SolidWorks Plastic for simulation of the injected part. Short shot defect in the plastic part during the injection molding process is analyzed by SolidWorks Plastic to validate the new proposed geometry. An experimental study of the injection molding process of polypropylene circular flat plates is conducted for the new geometry. The input machine parameters selected are filling time, melt temperature, mold temperature, pressure holding time, and pure cooling time. The research outcomes show no short shot defect associated with the new geometry and also significant 25 and 2.5 % reduction in scrap and cooling time respectively compared to round cross sections. Reduction in contact surface of the runner system with mold walls improved the ease of ejection of runner system out of the cavity as well. The contribution of this study is to design a new geometry of a cold runner system to reduce scrap, cycle time and also provide easy ejection of runner system in the injection molding. Keywords Injection molding process · Mold design · Runner geometry · Short shot defects & Mehdi Moayyedian mehdi.moayyedian@unisa.edu.au 1 School of Engineering, University of South Australia, Mawson Lakes Campus, Adelaide, SA 5095, Australia Introduction The past century has observed the rapid increase of plastics and their proliferation into all markets. According to world consumption of raw materials by weight, plastic is the highest in comparison with other old materials such as aluminum, steel, rubber, copper, and zinc. It has resulted from specific properties and lower production cost of plastics [1, 2]. Injection molding is one of the most significant processes for manufacturing of plastic products and approximately one-third of all plastics are converted into parts via injection molding [3]. The application of the injection molding process is increasing significantly in many industries like packaging, aerospace and aviation, building and construction, automotive parts, household articles and so on [1, 3, 4]. The quality of the injection moldings depends on material characteristics, mold design and process conditions [4–7]. Three fundamental operations in injection molding are: (1) plastic granules are converted into a melt; (2) molten plastic is injected into the mold cavity or cavities under pressure via sprue, runner and gate systems and (3) mold tools are opened to eject the part out of cavity [1, 8, 9]. One of the factors which will determine the final quality of injected part is the runner system which is a connection line between sprue and gates [10]. The main purpose of the runner system is to transfer molten plastic from sprue to gates [11, 12]. In the cold runner system, the main source of scrap is the scrap from runner and gate system after de-gating. Hence, different rules are evaluated for runner system design to demonstrate the significance of runner systems in injection round semicircular square rectangular Trapezoidal Modi?ed Trapezoidal Polygon Fig. 1 Different runner cross sectional shapes molding such as (a) smaller runner size to minimize the scrap; (b) easy ejection from mold tools and removal from molded part; (c) filling the cavity quickly with minimum sink mark and weld lines [13–16]. Three fundamental factors in the runner system design are cross sectional shape, diameter and cavity layout [13]. Seven types of cross sectional shapes are available for the runner system for different applications [13, 14, 17] (Fig. 1). Depending on the requirements, different types of runner cross sections are selected [18]. The contribution of this paper is to define elliptical or semi-elliptical geometry for runner systems as an effective cross sectional shape aiming at smaller runner size to minimize the scrap, in comparison with round shape, to reduce the total cycle time of injection and to eject the part from mold tools more easily. Furthermore, in this research remarkable phenomena related to process parameters and new geometry of runner systems have been detected that will be presented in another paper. The design criteria of elliptical cross sectional shape for runner systems are introduced herein, and a comparison between round shape and semi-elliptical shape of runner system is considered. To the authors best of knowledge, there are many papers studying process parameters and material characteristics of injection molding a few of which include runner, gate, and sprue but, to the authors’ best of knowledge, there is no reference analyzing and simulating the elliptical cross sectional shape of runner system. The design of runner and gate systems is conducted herein based on the size and geometry of injected parts. Then, the injected part with runner and gate system is designed via SolidWorks. For accurate result of simulation, finite element method (FEM) in SolidWorks Plastic is employed. Finally, to validate the model, experimental method is conducted for two circular injected plates Cross sectional shape of runner system The main purpose of a runner system is to transfer the molten plastic from sprue to all cavities via the gate. There are different cross sectional shapes for runner systems and each of them have different applications [11, 17] (Fig. 1). The designer should evaluate different factors for selecting the right geometry of the runner system for a specific product. The most popular shape of runner systems for two-plate mold tools, which is also of the highest efficiency, is round shape. For three-plate mold tools, the trapezoidal and modified trapezoidal are the best options if the runner is to be manufactured only in one half of the mold, but still they are not acceptable be- cause the gate cannot be positioned in line with the central flow stream [14]. Ejecting a runner system out of cavity with rectangular, square, and polygon shape is challenging due to sharp corners. If a designer cannot determine the appropriate cross sectional shape of the required runner system and their dimensions, pressure drops and leads to incomplete filling of cavities and high level of heat transfer to mold walls [13, 17, 19]. Hence, various cross-sectional area of a runner system can be considered to regulate the flow leading to a better injected part. Finally, the shape and the length of the channel are significant for achieving the optimal flow and consequently the best product with less defects [20]. Runner systems with elliptical cross sectional shape In injection molding, the most common cross sectional shape for runner system is round shape. In selecting the round shape for specific part design, three main elements are (a) smaller runner size to minimize the scrap; (b) easy ejection from mold tools; (c) filling the cavity quickly with minimum sink mark, weld lines and no short shot [13–15]. The aim herein is to investigate a runner system of new geometry which can lead to minimal scrap, be positioned in line with the central flow stream of gate, properly fill the cavities, and facilitate the easy eject the part from mold tools. For this purpose elliptical or semi-elliptical cross sectional shape has been taken under investigation and accurately compared with runner systems of round cross sectional shape. To demonstrate the significance of elliptical cross sectional shape of runners, the evaluation of other geometries of runner systems is necessary. The best existing comparison of these two is rectangular and square shape. Rectangle is a kind of square with different width. There are three different ratios in designing the dimension of rectangular runner system in comparison with square ones in terms of width [17] (Fig. 2). According to different applications, rectangular runner system with different ratios of width is chosen. The advantages of rectangular shape over square ones are less scrap of runner system and easier ejection from mold tools. Pressure drop is one of the disadvantages of this geometry which happens by decreasing the width of the square [17]. The comparison between circle and ellipse is similar to that of square and rectangle. As shown in Fig. 3, D is the diameter of circle, a is major axis length, and b is minor axis length of ellipse. Major axis length is fixed and the minor axis length is of different rates depending on different industrial applications (Fig. 3). As it leads to further reduction in scrap, easier ejection of part out of cavity, and further reduction in cycle time. For different parts, this factor will be changed. Hence, square shape rectangular shape with width ratio 1/2 rectangular shape with width ratio 1/4 rectangular shape with width ratio 1/6 Fig. 2 Comparison between square and rectangular shape of runner system circular shape elliptical shape with b=0.9a elliptical shape with b=0.8a elliptical shape with b=0.7a Fig. 3 Comparison between round and elliptical shape of runner system proposing different ratio of b depends on many factors of part design such as size and thickness. Advantages of an elliptical runner system over a round one are as follows: 1. Reduction in scrap: the size and volume of runner and gate system are the root cause of product scrap. Hence an elliptical runner leads to less scrap compared to the round runner. 2. Easier ejection of part from cavity: elliptical runner system, after cooling process compared to round shape has less contact surface with mold walls which leads to easier ejection of the injected part from the cavity. 3. Cycle time reduction: the elliptical runner requires less amount of molten plastic; hence the cycle time which includes the injection and cooling phase time will be reduced. 4. Central flow stream of gate with runner system. Elliptical runner has central flow stream with most of the gate designs which decrease the turbulences of molten plastic to the cavities. Simulation After designing two circular parts as two samples for this application, the next step is to simulate the part via SolidWorks Plastic. For the simulation, defining the injection system is needed. Hence, designing the sprue, runner and gate system with consideration of prior calculations should be considered (Fig. 4). The ratio for designing elliptical cross sectional shape is 0.7b. To make sure that the analysis results are sufficiently accurate, FEM will play a significant role in simulation. According to the geometry of samples, the triangle shape for FEM will be selected (Fig. 5). The selected material for this simulation is polypropylene (PP). Different sizes were evaluated for the surface mesh and from different triangle size of surface mesh, the triangle size of 1 mm is chosen for the injected part. For the injection system which includes sprue, runner and gate, smaller sizes are considered. It has resulted from the sensitivity of the injection system as a critical area of this simulation. Hence, triangle sizes of 0.3 mm for sprue and runner and triangle 0.2 mm for gate are selected for both elliptical and round cross sectional shape of runner. The accuracy of the mesh is determined through a mesh refinement study. The runner and gate length in total is 28 mm for two circular parts with diameter of 100 mm. Also, the sprue has 60 mm length with draft angle 1.5°. Fig. 4 Samples of injection with sprue, runner and gate system Fig. 5 FEA for elliptical cross sectional shape of runner Fig. 6 Easy filling of injected part with elliptical cross The next stage is to set up appropriate process parameters. According to the selected material and injection machine for this simulation, filling time is 0.59 s, melt temperature is 230 °C, mold temperature is 50 °C, pressure holding time is 2.04 s, and pure cooling time is 3.9 s. As mentioned before, the geometry and size of the injection system which includes sprue, runner and gate, have significant effects on operation cycle time, cooling time, and different defects such as sink marks, short shot etc. [25]. After running the simulation, the new runner system is checked for acceptability in terms of new geometry and size. The main factors checked are ease of fill, filling time analysis, sink mark analysis; and injection pressure at the end of injection. As shown in Fig. 6, ease of fill for the elliptical cross section is the green area which is in the most acceptable level. One common defect in injection molding is short shot which will happen on thin walls or far from the gate if there are long flow distances [26]. According to the simulation results, this part can be successfully filled and even the filling time for an elliptical cross section as shown in Fig. 7a is lower than that of for a round cross sectional shape of runner (Fig. 7b). Fig. 7 a Filling time for elliptical cross section, b Filling time for round cross section Fig. 8 a Flow front central temperature for elliptical cross section, b Flow front central temperature for round cross section Another factor to prevent short shot for the injected part is to evaluate the flow front central temperature which represents the flow front temperature at every region of the injected part. Based on the simulation results, the flow front central temperature in every region of the injected part is 230.15 °C for the elliptical cross sectional shape of runner (Fig. 8a). The simulation result for a round cross sectional shape of runner is the same (Fig. 8b). It means that the possibility of short shot in the cavities for an elliptical cross section shape of runner is low. One of the most significant factors which are necessary to evaluate for the determination of the right size of the runner and gate system is the injection pressure. According to the simulation, this part can be successfully filled with injection pressure 42.1 MPa. The injection pressure is less than 66 % of the maximum injection pressure limit which is satisfactory (Fig. 9). The injection pressure for a round cross section is 39.6 MPa which is close to an elliptical cross section. Experimental set-up A commercial injection molding granule polypropylene (PP) is employed to produce two circular plates which have 100 mm diameter and 1 mm thickness. The polymer-material parameters of selected material are listed in Table 2. Fig. 9 Injection pressure for both round and elliptical cross section shape of runner system Table 2 Material properties of PP Melt temperature 230 ○C Max melt temperature 280 ○C Min melt temperature 200 ○C Mod temperature 50 ○C Melt flow rate 20 cm3/10 min Max shear stress 250,000 pa The machines used to fabricate the mold tools are drilling machine, CNC milling machine and grinding machine. Fully electric horizontal-plastic-injection machine—Poolad-Bch series—is employed for the experiments Mold design There are different design concepts in fabrication of mold tools. In this study, a two-plate mold which has one parting line with double cavities with a feeding system and without an ejector pin is selected. The mold tools are made of steel— CK45—with surface hardness 56 HRC. The runner with an elliptical cross section, gate system, and sprue bush are allocated in the cavity plate after grinding (Fig. 10a). Also the cavity plate with guide bars before grinding is demonstrated (Fig. 10b). In designing the mold tools, another element is the cooling system which leads to the solidification of plastic part. Based on the geometry of plastic part, the design for the cooling system is vary. Hence, the circular geometry for the cooling system of cavity plate is selected (Fig. 11). Another factor to consider in fabrication of mold tools is the air vents. Their function is to release the air from the cavity after closing the mold tools; otherwise short shot will happen if air is trapped inside the mold. Both cavities have separate air vents at the left and right side of the cavity plate (Fig. 12). Fig. 10 a Cavity plate with elliptical cross section of runner after grinding, b Cavity plate with elliptical cross section of runner before grinding Fig. 11 Cooling system in cavity plate for solidification of injected part Fig. 12 Air vents to avoid the air trap in injected parts Experimental results After setting up the mold tools and injection machine based on different process parameters, the evaluation of the new cross sectional shape of the runner system from different aspects in the manufacturing process is the target of this experiment. To ensure the effectiveness of an elliptical cross section of runner in this study, the test for significance of filling the cavities and injection process based on the different process parameters need to be implemented. The result of short shot analysis (Fig. 13) shows that two cavities with the new cross sectional shape of runner are filled properly. When the injection pressure is higher than the maximum inlet pressure and filling time is higher than the input of the injection machine, short shot will happen. The most significant part of these experiments is that the cavities filled properly even with lower inlet pressure and filling time in comparison with simulation results as shown in Fig. 14. The comparison of the simulation and experimental result is shown in Table 4. Percentage change for predicted and actual results of inlet pressure and filling time are 7.36 and 3.38 % respectively which demonstrates the robustness of new geometry of runner system. The novelty of this research by defining the new geometry of the runner system is to reduce scrap and cooling time and achieve easier ejection of final injected part from the cavity. Hence, the comparison between a round and an elliptical cross section in terms of scrap rate and cooling time is necessary. Table 5 demonstrates the scrap rate and cooling time of a round and an elliptical cross section of runner system for 100,000 injected parts. The cooling time for a round cross section is 4 s per injection and for an elliptical cross section 3.9 s per injection. An elliptical cross section in comparison with round cross section has 25 % reduction in scrap and 2.5 % in cooling time for the injected parts. Fig. 13 Final injected part with elliptical cross sectional shape of runner Fig. 14 Injected part with lower level of each factor for an elliptical runner Table 4 Comparison of simulation and experimental result based on process parameters Process parameter Simulation result Experimental result Inlet pressure 42.1 MPa 39 MPa Filling time 0.59 s 0.57 s Table 5 Scrap rate and cycle time for the round and elliptical cross section Factor Round Elliptical Scrap rate of runner (g) 8000 6000 Cooling time (h) 111.11 108.33 Conclusion The main reason for scrap in injection molding for cold runner system is the feeding system which consists of sprue, runner and gate system. The runner has different cross sections for different applications. This paper presents the successful development of a new geometry of a runner sys