PhD. Student, Dept. of Technologies and Materials, Faculty of Mechanical Engineering, Technical University of Kosice, Slovakia

Transcription

1 Research Paper APPLICATION OF CA SYSTEMS AT DESIGN AND SIMULATION OF PLASTIC MOLDED PARTS B. Duleba 1*, F. Greškovič 2 Address for Correspondence 1 PhD. Student, Dept. of Technologies and Materials, Faculty of Mechanical Engineering, Technical University of Kosice, Slovakia 2 Professor, Dept. of Technologies and Materials, Faculty of Mechanical Engineering, Technical University of Kosice, Slovakia ABSTRACT Due to heavy demand in plastic products, plastic industries are growing in a fastest rate. This contributing deals about possibility of using CA software at molding process. Injection molding is complicated process, which accuracy intensive dependent on input data. First part of this paper describes injection molding process and simulation at load of real part. Next design and technological changes were made to achieve required rigidity of selected part. Main part of article is focused on moldflow plastics simulation in AMA software with described parts of simulations. Verification using simulation requires much less time to achieve a quality result, and with no material costs, as compared with the conventional trial-and-error methods on the production floor KEYWORDS: injection molding, molding process, moldflow simulation, plastics simulation 1. INTRODUCTION The main concept of plastic moulding is placing a polymer in a molten state into the mould cavity so that the polymer can take the required shape with the help of varying temperature and pressure. There are different ways of moulding a plastic some of them are blow moulding, Injection moulding, rotational moulding and compression moulding. Each technique has its own advantages in the manufacturing of specific item. Among these various plastic production technologies, injection moulding takes up approximately 32%, because of its ability in producing complex parts with low cost and high productivity. 2. THE INJECTION MOLDING PROCESS The injection moulding process stages starts with the feeding of a polymer through hopper to barrel which is then heated with the sufficient temperature to make it flow, then the molten plastic which was melted will be injected under high pressure into the mould the process is commonly known as Injection, After injection pressure will be applied to both platens of the injection moulding machine(moving and fixed platens) in order to hold the mould tool together afterwards the product is set to cool which helps it in the solidification process. After the product gets its shape the two platens will move away from each other in order to separate the mould tool which is known as mould opening and finally the moulded product is ejected or removed from the mould. And the process will repeat itself. [1] The moulding cycle starts with the retraction of the ejector plate, followed by closing of the mould. The injection unit melts the polymer resin and injects the polymer melt into the mould. The ram fed injection moulding machine uses a hydraulically operated plunger to push the plastic through a heated region. The melt converges at a nozzle and is injected into the mould. The melt is forced into the mould in two or three stages: Stage 1: Fill stage During this stage, the mould cavities are filled with molten resin. As the material is forced forward, it passes over a spreader, or torpedo, within the barrel, which causes mixing. This stage is determined by an injection velocity (rate), a pressure, and a time. Injection velocity is the rate at which the plunger moves forward. Stage 2: Pack stage As the melt enters the mould, it cools and introduces shrinkage. The pack stage is necessary to force more melt into the mould to compensate for shrinkage. Stage 3: Hold stage When no more material can be forced into the mould, melt can still leak back through the gate. The hold stage applies forces against the material in the cavity until the gate freezes to prevent leaking of the melt. In some machines, pack and hold are combined into a single second or holding stage. [2], [3] Fig. 1 Injection moulding cycle Moldflow is the one of developer s software solutions that enhances the design, analysis, and manufacture of injection moulded plastic parts. The commercial success of each of these products often relies heavily upon reducing the time to bring new products to market, reducing engineering and manufacturing costs, and improving product quality and design. The Moldflow software is useful in all aspects of the injection moulded plastic parts manufacturing process, including part designers, mould designers, manufacturing engineers and machine operators. It enables to speed products to market, decrease manufacturing costs and reduce costly design and manufacturing errors by [5]: assisting part designers in the selection of a plastic material, determining the strength, rigidity and ease of manufacturing of a given part design, predicting the amount a plastic part will shrink or warp during production, optimizing production conditions such as machine temperatures, injection,

2 speeds, cooling times and the locations in a mould to inject the plastic, identifying and providing optimized solutions for adverse variations during production, providing features which facilitate collaboration over shared media, such as the internet. AUTODESK MOLDFLOW ADVISER 2011 Autodesk moldflow adviser (AMA) series provides part and mould designers with applications that permit them to quickly check the ultimate manufacturability of their designs at an early stage in the design process. AMA is designed to input its results directly into MPX to enhance the efficiency of machine set-up. 3. EXPERIMENTAL WORK Part chosen for simulation is plastic holder for cables and coolant tube found in Honda Civic EJ9. Part in real due to vibrations and stress always has problems with cranks and deformations after time. At first, part was designed in SolidWorks 2012 with real dimensions. Next using Solid Works Simulations deformation of holder was analysed by using real values. Maximum experimentally measured force at end of the holder was 30N. Fig. 5 Displacement of stress Next factor of safety (FOS) was set to value of 3. Simulation showed small area of possible deformations and cranks as shown on Fig.6. Next step was the analysis of the shape of part due to technological conditions of manufacturing. One rib at bottom side of holder was added, chamfer and fillets were changed due to stress conditions. All changes are in state of negative cut, so they can be done to finished form. Addition of rib and final shape is shown on Fig. 7. Fig. 6 Possible failures with FOS 3 Fig.2 Final design of part Fig. 7 Final shape of redesigned part Fig.3 Renderation with high gloss surface Next using Solid Works Simulations deformation of holder was analysed by using real values. Maximum experimentally measured force at end of the holder was 30N. Material was chosen to DuPont Engineering Polymers, Zytel 73G30, PA6 30% Glass Fiber Filled. By simulation maximum von Misses stress was calculated to Nm 2 while Yield strength of selected material is Nm 2, showed on Figure 4. Displacement of deformation is showed on Figure 5, when maximum URES was calculated to 3,357mm. Fig. 8 Possible failures of redesigned part with FOS 3 Next simulation of stress was done again and deformation was improved. Von Misses stress was calculated to Nm 2 in comparison to Nm 2 it is improvement by 37%. Maximum deflection was calculated to 2,227mm, what is almost 50% improvement. By analysis of deformations with Factor of safety set to 3 there were no calculated areas that can be possible areas of cranks. Stress, deflection and FOS analysis is shown on Fig 8,9 and 10. Next performed simulations were injection simulations using Autodesk Moldflow. Fig. 4 Displacement of von Misses stress Fig. 9 Displacement of von Misses stress

3 Fig. 10 Displacement of stress The first step in analysing is importing the design into the moldflow software using IGES Format. After importing the design the next step is to mesh the product design using moldflow software. In next part of paper, the main analyses of AMA Material for simulations was set to DuPont Engineering Polymers, Zytel 73G30, PA6 30% Glass Fibre Filled, same as material at stress simulations with properties obtained from material list. Nominal wall thickness The Nominal wall thickness result displays thickness variations relative to the wall thickness of the part. Nominal wall thickness was analyzed to 2,292mm, end of nominal range 20% with end of low deviation range set to 50 %. Variation of nominal wall thickness of analyzed part is shown in Fig. 11. Draft Angle The Draft Angle result displays draft variations for the part. Part features cut into the surface of the mold perpendicular to the parting line require taper or draft to permit proper ejection. This draft allows the part to break free by creating a clearance as soon as the mold starts to open. Since thermoplastics shrink as they cool they grip to cores or male forms in the mold making normal ejection difficult if draft is not included in the design. If careful consideration is given to the amount of draft and shutoff in the mold it is often possible to eliminate side actions and save on tool and maintenance costs. Fig. 12 shows draft analysis of selected part, zero draft is displayed red, needed draft up to 0.5 degrees is displayed yellow, draft from 0.5 to 1 is displayed green and draft with value more than 1 degree is displayed in tones of blue. Undercut analysis The Undercut result displays areas where undercuts are located. When a feature casts a shadow on another feature of the mold, the feature causing the shadow is called an undercut. The algorithm assumes the XY-plane is the parting plane and the positive Z- direction points towards the nozzle of the machine. Therefore the shadow feature is located at a higher Z- value than the undercut feature. Sometimes, if the undercut feature is narrow and the material is very flexible, it is possible to eject the part without special tooling components, like in the case of some snap-fits. However, when possible, undercuts should be avoided in order to keep the tool design simpler. The less complex the tool is, the less expensive it will be to run and maintain. Undercut analysis of analyzed part is shown in Fig. 13. A red surface represents undercut features and a blue surface represents shadow features. Flow resistance indicator The Flow resistance indicator result shows the resistance at the flow front from the gates. This result is output from a Gate Location analysis, using the Advanced Gate Locator algorithm. It represents the flow resistance at the flow front from the gate location(s), normalized to show the highest flow resistance, through to the lowest flow resistance. Flow resistance indicator result is used in conjunction with Fill analysis results and the Gating suitability result to determine the most appropriate gate location(s). Areas that have a high relative flow resistance that are surrounded by areas of low relative flow resistance may cause defects or filling problems. Results from flow resistance indicator analysis are shown in Fig.5, where FRI was calculated to value of 1, where areas with lowest resistance are colored in tones of blue and areas with highest resistance are colored in red- Fig.14. Fig. 13 Undercut analysis Fig. 11 Nominal wall thickness analysis Fig. 12 Draftangle analysis Fig. 14Flow resistance indicator analysis Gating suitability The Gating suitability result rates each place on the model for its suitability for an injection location. The Gating suitability result is produced by the Gate Location analysis when Advanced Gate Locator

4 algorithm is used. The Advanced Gate Locator algorithm minimizes the flow resistance when determining the best gate position for the first and only injection location. If no prohibited gate regions are defined on the model, then the Gate Location analysis rates the gating suitability throughout the entire part. The suitable areas shown in this result are worth pursuing as potential injection locations. The best areas shown on the result coloured in tones of blue Fig. 15 do not necessarily represent a good solution (high quality part, or high confidence of fill), but rather the best one for the case at hand using the selected material. Areas with the same colour represent equally suitable positions. If the injection location is not suitable due to design constraints, consider the other blue regions in the Gating suitability result when placing a different injection location. Fill analysis is than need to check the suitability of these injection locations. Molding analysis The Molding Window display is a plot over the ranges of the melt temperature and the injection time for a given mold temperature. The three colors (rednot feasible, yellow and green- preferred) represent how good a particular combination of processing conditions is.melt temperature for analysed material was set to 275 o C and analysed injection time was calculated to 0,6598s and mould temperature set to 100 o C. The contours are evenly spaced. The contour spacing indicates the speed at which the polymer is flowing. Widely-spaced contours indicate rapid flow, while narrow contours indicate that the part is filling slowly. After the analysis, the fill time of part was calculated to 0,3976s and the variance of fill is displayed in Fig.8. Confidence of fill The Confidence of fill result displays the probability of plastic filling a region within the cavity under conventional injection moulding conditions. This result is derived from the pressure and temperature results. Simulation had shown that part has highest possible confidence of fill (100%) with entered material properties and moulding parameters. Fig. 17 Confidence of fill analysis Fig. 15 Gating suitability analysis results Fig. 16 Moulding analysis Fill time analysis The Fill time result shows the position of the flow front at regular intervals as the cavity fills. In the following figure, the contour colors represent the flow of plastic into the part. All regions with the same color filled at the same time. In a part with a good fill time result, the flow pattern is balanced, meaning: All flow paths finish at the same time. All flow fronts should reach the edges of the model simultaneously. This means that in the illustration, each flow path should end with red contours. Fig. 18 Confidence of fill Quality prediction The Quality prediction result is used to estimate the quality of the mechanical properties and appearance of the part. This result is derived from the pressure, temperature and other results. Pressure at end of fill The Pressure at end of fill result shows the maximum injection pressure value obtained during the whole duration of the filling phase. This result is output by a Fill analysis and is displayed on the surface only. At the beginning of filling, the pressure is zero (or 1 atm, in the absolute pressure scale) throughout the mould. The pressure at a specific location starts to increase only after the melt front reaches that location. The pressure continues to increase as the melt front moves past, due to the increasing flow length between this specific location and the melt front. The pressure difference from one location to another is the force that pushes the polymer melt to flow during filling. The pressure gradient is the pressure difference divided by the distance between two locations. Polymer always moves in the direction of the negative pressure gradient, from higher pressure to lower pressure. (This is analogous to water flowing from higher elevations to lower elevations). Thus, the maximum pressure always occurs at the polymer injection locations and the minimum pressure occurs at the melt front during the filling stage. The

5 magnitude of the pressure (or pressure gradient) depends on the resistance of the polymer in the mould, because polymer with high viscosity requires more pressure to fill the cavity. Restricted areas in the mould, such as thin sections or small runners, and long flow lengths also require a larger pressure gradient and thus higher pressure to fill. Pressure should be zero at the extremities of each flow path at the end of filling. During the filling stage, large variations in the pressure distribution, indicated by closely-spaced contours, should be avoided. During packing, pressure variations affect the volumetric shrinkage. Pressure at end of fill was calculated to maximum value of 17,54Mpa and distribution of pressure is shown in Fig.19. Fig. 19 Pressure at end of fill Fig. 20 Time to reach ejection temperature Time to reach ejection temperature The Time to reach ejection temperature result shows the amount of time required to reach the ejection temperature, measured from the start of fill. If the part has not frozen by the end of the cycle time provided, a projected time to freeze is displayed in the result. For a Dual Domain analysis, the value displayed is the time taken for 100% of the local thickness to reach ejection temperature. For 3D flow, the value displayed is the maximum time over the local thickness, mapped from the interior to the surface of the part. This result takes into account the dynamics of the packing phase, and where new hot material enters the cavity. This new hot material affects the cooling time. Ideally, the part should freeze uniformly. Areas of the part that take longer to freeze may indicate thicker areas of the part or areas of shear heat during filling and/or packing. If a long Time to reach ejection temperature is caused by thick areas of the part, consider re-designing the part. Long times due to shear may be difficult to address. To reduce the shear, the Time to reach ejection temperatures may adversely affect the volumetric shrinkage and warpage in the part. By simulation time to reach ejection temperature for part only was calculated to 30,95s and the variance is displayed in Fig.20. Air traps An air trap occurs where the melt traps and compresses a bubble of air or gas between two or more converging flow fronts, or between the flow front and the cavity wall. Typically, the result is a small hole or a blemish on the surface of the part. In extreme cases, the compression increases the temperature to a level that causes the plastic to degrade or burn. Air traps are often caused by converging flow fronts which occur as a result of racetrack or hesitation effects, or from non-uniform or non-linear fill patterns. Even when the part has balanced flow paths, air traps can occur at the end of the flow paths, as a result of inadequate venting. For a Dual Domain model, the Air traps result shows a thin, continuous line wherever an air trap is likely to occur. For 3D models, it displays a solid coloured area wherever an air trap is likely to occur- Fig.21. Weld lines The Weld Lines result displays the angle of convergence as two flow fronts meet. The presence of weld lines may indicate a structural weakness and/or a surface blemish. The term weld line is often used to mean both weld and meld lines. The only difference between them is the angle at which they are formed. Weld lines form at lower angles than meld lines. They can cause structural problems and make the part visually unacceptable. Weld lines are unavoidable when the flow front splits and comes together around a hole, or has multiple gates. Consider the processing conditions and position of the weld lines to determine whether the weld lines will be high quality. Weld and meld lines should be avoided; particularly avoid weld lines in areas that require strength or a smooth appearance. Processing conditions help to determine the quality of weld or meld lines. Weld line strength is influenced by the temperature at which the weld line is formed and the pressure exerted on the weld until the part freezes; pressure is 0 at the weld line. Typically a good weld will occur if the temperature of the melt at the weld line as it forms is no more than 20 C below the injection temperature. Fig. 21 Possible air traps in mould Fig. 22 Possible weld lines in mould

6 Setting the injection moulding runners system Sprue type was selected to cold type with circular tapered shape. Start diameter was set to 2mm and end diameter was calculated to 5mm from recommendation tables from norm and with the injection point in the middle of mould. Runner was set to circular shape with various diameter calculated by runner advisor simulation. Gate was due the shape of designed part selected as circular tapered shape with start diameter of 3mm and end diameter 1mm at injection point. Orientation was set to Horizontal by length with horizontal length of 10mm. Setting the Cooling channels system Cooling system was designed as circular channel system with diameter 8mm in layout along X-axis. Specified number of channels was set to X and offset from parts boundary was calculated to 10mm due to design of part. Ends of drilled holes are connected with hoses with 10mm length. Fill time analysis The Fill time result shows the position of the flow front at regular intervals as the cavity fills. In the following figure, the contour colors represent the flow of plastic into the part. All regions with the same color filled at the same time. Fill time with calculated parameters of injection molding was performed after simulation of molding parameters. Fill time with set injection parameters was calculated to 0,8113swith 100% confidence of fill and the progress is shown in Fig. 23. Pressure at end of fill The Pressure at end of fill result shows the maximum injection pressure value obtained during the whole duration of the filling phase. This result is output by a Fill analysis and is displayed on the surface only. Pressure at end of fill was calculated to 123,9 MPa. Temperature at flow front Temperature at flow front shows the temperature of the polymer when the flow front reached a specified point, in the center of the plastic cross-section. This result is output by a Fill analysis. Maximum temperature at flow front was calculated to 277 o C and progress is shown in Fig.24. Temperature variance 7 and maximum variance measured between centre of part and edge was only 2,798 o C. After these simulations, time to reach ejection temperature was calculated to 23,56s, only few air traps were discovered and minimum weld lines showed up in the part. Confidence of fill was rated with maximum possible value of 99,96%. Runner adviser analysis showed, that engaged system met all requirements and fulfil the criterions. Cooling system adviser calculated maximum flow rate to 10lit/min with distilled water as cooling medium. Fig. 23Results of fill time analysis Fig. 24Temperature at flow front results Circuit pressure result The Circuit pressure result is generated from a Cool analysis and shows the distribution of pressure along a cooling circuit, averaged over the cycle. The pressure inside the cooling circuits should remain evenly distributed from the inlet circuit pressure to the outlet circuit pressure. Calculated value at simulation was 281,7KPa and progress is shown in Fig.25. Variance between inlet and outlet was calculated to 2,74 o C. Minimum Raynolds number of turbulent flow was calculated to and maximum to Fig. 25 Circuit pressure result Fig. 25 Temperature variance Temperature, part result The Temperature, part result shows the average temperature at the part boundary (part side of the part/mold interface) over the duration of the cycle. This result is used to find localized hot or cold spots, and determine whether they will affect cycle time and part warpage. As show in Fig. 26, temperature variance at part was minimal due to the design of part and construction of cooling system. Time to reach ejection temperature of part was calculated to 13,01s. At warpage simulations maximum nominal deflection was calculated to 1,125mm, what is acceptable value. This value can be reduced by leaving the part for longer time in mold cavity for better cooling or choosing another type of material. 4. RESULTS Fill Part can be filled easily with acceptable quality using the current injection locations. Calculated results are listed in Table 1.

7 Table 1 Calculated fill values Cycle time breakdown Cycle time was calculated to 35s, with portions displayed in Fig.26. Longest time was associated to cooling stage (19,19s). This time can be improved by using thermal conductivity plates or pipes or by redesigning the cooling system with less space between mold and cooling plane. Cavity volume was calculated to 51,294 cm 3, Scrap rate to 7.34 % and runner volume 2,208 cm 3. Fig. 26Cycle time breakdown Table 2 Calculated pack values 5. CONCLUSIONS Mold filling simulation is a very helpful tool for the injection mold designer and plastic part designer. The process uses software to virtually simulate the filling, packing and cooling of a molded plastic part. It allows the mold/part designer to make critical decisions about the design before the injection mold is manufactured, when design changes are significantly less expensive. When considering part and mold design, gate location and part filling are initial concerns. The simulation will provide a visual representation of how the mold will fill. This information is valuable in showing potential problems, such as areas of the part that will not completely fill with plastic. With this type of initial information, the designer can then simulate different gate locations that can improve these and other situations. The software can calculate the pressure inside the filling plastic, which could point towards potential problems. Other evaluations such as material sheering, temperature and pressure can also be determined. This and other data is not only helpful for designing the part, but can also be used to decide if a specific material will work with a given part design. In more advanced simulations, the effects of cooling lines in a potential mold design can be analysed. The heating and cooling of the mold can be simulated through the entire molding cycle, over multiple cycles, and can help in determining the size and location of cooling lines. Since cooling lines affect how or if the part warps, this also allows for the potential warpage of the part to be viewed, analysed, and adjusted for. Areas of the mold where the temperature is not as controllable will be visible through the simulation as well. Although a completed mold is more expensive to change than one that is still in the design stage, mold filling simulation may point to a solution that would otherwise require trial and error on the bench. ACKNOWLEDGMENTS This paper is the result of the project implementation: PIRSES-GA supported by The international project realized in range of Seventh Frame Programme of European Union (FP7), Marie Curie Actions, PEOPLE, International Research Staff Exchange Scheme (IRSES). REFERENCES [1] GREŠKOVIČ, F., et.al : Nástrojena spracovanie plastov Vstrekovacie formy, ISBN , TU Košice [2] KAMAL, R.: Injection molding technology and Fundamentals. 1st ed. Munchen, Germany: Hanser Publications, 2009, ISBN: [3] DULEBOVÁ, Ľ. :Vplyvmnožstvaregranulátunazmrštenievýstrekov z PBT. In: Plasty a kaučuk. Vol. 48, no (2011), p ISSN [4] OSSWALD,A.: Injection molding handbook, HanserVerlag, Munich, 2008, ISBN , [5] REES, H: Mold Engineering, Hanser, Ontario, Canada, 2002, 678s, ISBN [6] ROSATO, D., DONALD, D.: Injection molding handbook, Norwell Massachusetts, Kluwer Academic Publishers, 2000, ISBN-10: [7] Shoemaker, J.: Moldflow Design Guide: A Resource for Plastics Engineers, Hanser Publishing, Massachusetts, 2006, ISBN: [8] Pattnaik, S.R., Karunakar, D.B., Jha P.K.: APPLICATION OF COMPUTER SIMULATION FOR FINDING OPTIMUM GATE LOCATION IN PLASTIC INJECTIONMOULDING PROCESS, International Journal of Advanced Engineering Research and Studies, E-ISSN , ISSUE II JANUARY-MARCH 2012 [9] Sadeghi, B.H.M. A BP-neural network predictor model for plastic injection moulding process. Journal ofmaterials Processing Technology, 2000, 103, [10] Sahputra, I.H. Comparison of two flow analysis software for injection moulding tool design, Proceedings of the International Conference on Industrial Engineering and Engineering Management,2-4 Dec. 2007, Singapore.