ANALYSIS OF INJECTION MOLDING COOLING SYSTEMS AND ITS EFFECTS ON THE EJECTION TIME OF THE PART AT THERMOPLASTIC INJECTION MOLDING

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1 Journal for Technology of Plasticity, Vol. 40 (2015), Number 1 ANALYSIS OF INJECTION MOLDING COOLING SYSTEMS AND ITS EFFECTS ON THE EJECTION TIME OF THE PART AT THERMOPLASTIC INJECTION MOLDING Viktor Filiposki, Jasmina Chaloska Faculty of Mechanical Engineering, St. Cyril and Methodius University of Skopje, R. Macedonia ABSTRACT In the focus of this paper are the effects of the injection molding cooling process on the ejection time of the part, at thermoplastic injection molding. The analysis is performed using a specificaly designed 3D model of a thermoplastic product, following the Finite Element Method (FEM). Based on the shape and the dimensional characteristics of the part, three different versions of cooling systems were designed; each defined using the same input parameters in the FEA software. The obtained results are analyzed and used as an input for developing an optimal solution for the cooling system. In addition, the approach of defining the input parameters in the FEA software, which have a direct impact on the analysis of the injection molding process, is elaborated. The direct correlation between the numerical approach and the theoretical explanations for the used methods of analysis is also elaborated and explained. Kew words: mold; injection molding; thermoplastics; cooling; Finite Element Method (FEM) 1. INTRODUCTION The thermoplastic injection molding process can be divided into four separate phases: filling (injection), holding, cooling and ejection. The process of providing a thermal balance in the mold and appropriate working temperature, subject to the used thermoplastic material, is a process of injection molding tempering [1, 3]. The injection molding tempering is a critical step in such process, and has a direct influence on the mold productivity, as well as on the quality of the molded parts. In order to provide high quality parts manufacturing and also meet the economical demands of the production process, it is necessary to design a homogeny cooling system, which reduces the production cycle time, avoids appearance of warps and defects and also provides dimensional stability of the product during the whole production process [3, 4, 6]. * Corresponding author s jasmina@mf.edu.mk

2 2 The injection molding cooling systems, despite the rapid development of new and advanced manufacturing technologies, generally are produced using conventional tooling methods, like drilling and EDM. Appling advance manufacturing technologies for producing the injection mold core and cavity significantly increase the price of the mold, and therefore such technologies are used in high productivity injection molding processes, as well as in processes where products with high quality surface finishing and strong mechanical characteristics are needed to be produced. The Finite Element Method (FEM) is generally recognized from practical applications, known as Finite Element Analysis (FEA) software. FEA software are used in engineering for calculation of engineering analysis and tasks. It uses the principle of generating a mesh from polygonal elements, which approximate the geometrical domain of the analyzed model, for subdivision complex forms to smaller elements, while using Finite Element Method algorithm [3, 7]. In this paper, for the injection molding analysis the FEA software Autodesk Moldflow is used, which provides exact results combined with visual simulations, and therefore it is more efficient and compact method, compared to the conventional calculation methods which are based on theoretical definitions and equations. 2. THEORETICAL AND NUMERICAL ANALYSIS OF INJECTION MOLDING COOLING SYSTEMS The subject of this analysis is injection molding cooling system of the forming elements of the mold, i.e. core and cavity, used for producing of Bottom Part of a conceptual multimedia system, which is manufactured from thermoplastic material Acrylonitrile Butadiene Styrene (ABS), Fig.1. Fig. 1 3D model of the part that is subject of the analysis For the manufacturability analysis of this part, in the Autodesk Moldflow software, a Dual Domain (Fusion) mesh is selected for the model approximation, where the mesh of the model is contained from edges with global length of 1.4mm and tolerance of 0.1mm. With those parameters, on the model is generated mesh with geometrical elements (triangle elements), connected nodes, and the model volume is cm. The percent of appropriately generated elements regarding the 3D model of the part, is 90.1%, which is significantly greater than the minimum needed percent of 85%, Fig. 2 [3, 7].

3 3 Fig. 2 Mesh characteristics of the analyzed model For this analysis, ABS material with trade name A. Schulman, Polyman K1836 from the software material database is selected. The processing of this material requires the surfaces of the forming elements of the mold to dispose with temperature of 45 C, while the melting temperature of the material is 230 C. This material reaches maximum melting temperature at 290 C. The ejection of the part from the mold, according to the supplier suggestions, needs to be performed when the part reaches temperature of 80 C in order to avoid deformation and warping on the part. The injection mold for this part shall be manufactured with four cavities, whereby in one production cycle four parts will be produced, and it shall use a direct hot runner system. For this reason, the analysis is performed on one model, and the obtained results refer to the whole system for one working cycle. Because of the part symmetry, analysis for the gate location is not performed, and the gate is placed in the center of the model, on the bottom surface, regarding the ejection system which will be used in the mold, Fig. 3.

4 4 Fig. 3 Display of the analyzed model after applying the mesh and the gate location The material processing temperature is automatically recommended by the software and its value is 240 C, which corresponds to the material characteristics as presented above. According to steel material table, as most suitable material for producing the mold core and cavity, tool making steel DIN , with mass density of 7850g/mm, elastic modulus of N/m, thermal conductivity of 28W/(m K) and thermal expansion coefficient of 1.1е-005 /К is selected. Regarding the dimensions of the part and the number of cavities, the mold shall be produced in standard molding unit with dimensions of 696x596 mm. Because of the mold size, as most suitable injection molding machine, from the software machine database a machine with trade name Arburg Allrounder 720 S 3200 is selected, which completely correspond with the requested machine characteristics [8, 9]. 2.1 Defining possible solutions for the injection molding cooling system of the analyzed part In order to determine an optimal solution for the injection molding cooling system of the analyzed part, three different systems are designed, on which separate analysis contained of different methods are performed with the focus of providing the results of the cooling system influence on the ejection time of the part, while using the software Autodesk Moldfolw [3, 7]. The channels in all of the possible solutions of the cooling systems, are designed with circle cross section with diameter of 8mm along the whole length, where the value of the channel surface roughness is 0.05mm. As a cooling fluid water with temperature on the entry of 25 C is selected. According to the entry temperature of the water, its dynamic viscosity µ has a value of N s/m. Because of the fact that the values for the specific pressure and the specific flow rate are unknown, for this analysis as fluid control unit in the software Autodesk Moldfolw, the Reynolds Number is selected, which according to the equation 1 represents a non-dimensional quantity, i.e. function of the fluid density (kg/m ), the speed of flow (m/s), the channel diameter (m) and the fluid dynamic viscosity µ(n s/m ). Re = r u d m (1)

5 5 As a consequence, for the Reynolds number value of is assigned, which according to the expression 4 correspond to a turbulent fluid flow [2, 3, 7]. Re < 2300 laminar flow (2) 2300 <Re < 4000transient flow (3) Re > 4000 turbulent flow (4) In Fig. 4 a serial solution for the mold cooling system is displayed. This solution practically covers the whole surface of the analyzed part. Besides the cooling channels on the core and the cavity, one channel is also designed on the side forming unit, which forms the side hollow surface of the part. For mesh placement on the cooling channels, the software uses Beam geometrical elements [12, 30]. The global length of the channels edges is 4mm, with joining tolerance value of 0.1mm. With these parameters, for this cooling system solution a mesh with 435 geometrical elements, with 438 connected nodes, on 3 connection regions is generated. Fig. 4 Display of the first cooling system solution for the analyzed part In Fig. 5 a relatively simplified serial solution for the mold cooling system is displayed. Compare to the previously explained solution, the channels cover smaller surface area of the analyzed part, therefore the results are expected to be lower. The advantage of this solution is that it is simpler and easier for manufacturing, because of the number of channels and the positioning in the mold core and cavity. In this solution a cooling channel for the side hollow surface of the part is designed as well. For this solution, a mesh with 253 geometrical elements, with 258 connected nodes, on 5 connection regions is generated.

6 6 Fig. 5 - Display of the second cooling system solution for the analyzed part The simpler solution of the mold cooling system is displayed at Fig. 6. In this solution a parallel cooling system is designed, in which two channels are placed along the whole surface of the analyzed part. This solution covers least part surface area, comparing to the previously explained solutions, so according to that the results are expected to be lowest. However, the main advantage of this solution is that is the easiest for manufacturing. The generated mesh contains 229 geometrical elements, with 234 connected nodes, on 5 connection regions. As in the previous cases a cooling channel for the side hollow surface is also designed. Fig. 6 - Display of the third cooling system solution for the analyzed part

7 7 2.2 Time needed for reaching the ejection temperature of the part during an injection molding cycle The results of the time needed to reach the ejection temperature of the part, are showing the time period measured from the moment when the cavity is full with thermoplastic material, until the moment when the part has reached the ejection temperature, and is ready to be ejected out of the mold [2, 5, 7]. At the beginning of the measurement it is presumed that the cavity is completely full with thermoplastic material, with appropriate melting temperature. In ideal conditions, the part cools in homogenous cooling state. The areas on the part where longer cooling time is needed, may indicate to occurrence of hot spots, in which case longer time to reach the ejection temperature is needed. Those effects mostly occur because of the larger amount of thermoplastic material, or occur on areas with thin cross sections and walls. If the time difference between the areas on the part who are reaching the ejection temperature faster, than the ones who are reaching slower is large, it is necessary to determine if the problem is caused by areas with thicker walls, compared to the residual areas, or because of the increased mold temperature. If the wall thickness is larger on those areas, the model needs to be redesigned and the analysis to be performed again. If the problem is caused by the higher mold temperature, the cooling system of the mold needs to be modified, in order to eliminate the hot spots on the part. During the analysis of the results of the ejection time, these aspects must also be analyzed: - Whether there is homogenous cooling state along the whole surface area of the part, - The values of the Reynolds number along the whole length of the cooling channels; low values of the Reynolds number indicates to inefficient part cooling, - Whether there are areas with hot spots on the part. For the selected thermoplastic material (ABS - Polyman K1836), the cooling system needs to reach ejection temperature of 80 C. The results for the time needed for reaching the ejection temperature for the three different cooling systems solutions of the analyzed model, are displayed in table 2. Measurements and analysis of the ejection time are performed on 6 differently located spots on the part, on every cooling system separately, and the results are compared. In table 1 a display of the coordinates of the spots on which the measurements are performed as generated by the software is given. The spots are selected in order to cover the whole surface of the analyzed part. In the third column in table 1 the distances between the first analyzed spot and the next analyzed spot are given, appropriately. The analyzed spots are the same for all of the cooling system solutions. Table 1 Characteristically spots on which the analyzes are performed Spot no. - T XY coordinates of a spot Distance from the first spot [mm] 1 T T T T T T

8 8 In table 2 are displayed the results from the 6 analyzed spots, for all 3 cooling system solution for the injection mold of the analyzed part. Table 2 Results from the time needed for reaching the ejection temperature of the part Spot no. - T Time for cooling system 1 Time for cooling system 2 Time for cooling system In the first column of table 2 the analysis results for the first cooling system solution are shown. From the results, it can be noted that the time needed for reaching the ejection temperature of the part, has a highest value of 11.35s for the analyzed spot T2. The results for the ejection time of the part for the first system are obtained from the diagram displayed in Fig. 6. Fig. 6 Diagram display of the ejection time results for the first cooling system solution In the second column of table 2 the analysis results for the second cooling system solution are displayed, in which the highest value of 13.71s is obtained for the ejection time for the analyzed spot T1. The diagram for the ejection time results for the second system are displayed on Fig. 7.

9 9 Fig. 7 - Diagram display of the ejection time results for the second cooling system solution In the third column of table 2 the analysis results for the third cooling system solution are given, which are insignificantly higher compared to the second analyzed system. As in the second system, in this system the highest value of 13.82s for the ejection time is obtained for the analyzed spot T1 also. The diagram for the ejection time results for the third system are displayed in Fig. 8. Fig. 8 - Diagram display of the ejection time results for the third cooling system solution 2.3 Analysis and discussion of the results From the results can be noticed that the lowest values for the time needed to reach the ejection temperature of the part, are obtained with the first injection molding cooling system solution, mostly because of the equivalent placement of the cooling channels regarding the part, and the number of channels compared to the other two solutions. The time difference of 2.36s obtained

10 10 from the spot with the highest measured value in the first analyzed system compared to the second, and the time difference of 2.47s compared to the third system is very significant, primarily if the injection mold for the analyzed part is manufactured for high series production, in a limited time period. 3. CONCLUSION Using advanced technologies and software tools is inevitable in all industrial fields, especially if the end result improves the in-time production of high quality products, with competitive market price. The increasing of the number of thermoplastic products, has direct influence on improving the design approach of injection molded parts. Regarding that, the manufacturing of thermoplastic parts is relatively expensive and complex process, where every step in the process must be maximally optimized, and in the amount of time spent and the costs for preparing and completing the process needs to be reduced to minimum. The advanced software tools enable modification of the conventional design and analysis methods of thermoplastic products, while providing results with high level of accuracy, with preliminary preview of the production process. Using such software, the level of improvement in the process of designing and selecting a suitable cooling system solution and in the process of injection molding design was explained and elaborated trough practical examples. The provided results are used to select a cooling system before the process of mold designing, which directly affects the time spent, and the costs of the mold manufacturing also, while avoiding possible mold modification later in the process. In addition, the results obtained from the analysis of the separate cooling system solutions, in which the ejection time results are determined, have a great impact on the improvement of the injection molding process productivity, which was the main subject of analysis in this paper. REFERENCES [1] Boško Perošević, Kalupi za injekciono presovanje plastomera (Termoplasta), Naucna Knjiga, Beograd, 1988 [2] Јасмина Чалоска, Компјутерско моделирање на алати за пластични маси, МФС, Скопје, 2014 [3] S.W. Churchill, Friction factor equations span all fluid-flow regimes, 1997 [4] Jay Shoemaker, Moldflow design guide, Massachusetts, USA, 2006 [5] Lars Erik Rannar, On optimization of injection molding cooling, Trondheim, 2008 [6] Olaf Zöllner, Optimised mould temperature control in: ati 1104 d (Application Technology Information), Plastics Business Group, Bayer AG, Leverkusen, 1999 [7] Tom Kimerling, Injection Molding Cooling Time, Reduction and Thermal Stress Analysis, University of Massachusetts, Amherst, 2002 [8] [9] [10]

11 11 UTICAJ SISTEMA ZA HLADJENJE NA VREME IZBACIVANJA DELA U PROCESIMA TERMOPLASTIČNOG INJEKCIONOG PRESOVANJA Viktor Filiposki, Jasmina Chaloska Faculty of Mechanical Engineering, St. Cyril and Methodius University of Skopje, R. Macedonia REZIME Fokus istraživanja prikazanih u ovom radu je efekat hladjenja na vreme izbacivanja gotovog dela u procesima termoplastičnog injekcionog presovanja. Analiza je sprovedena korišćenjem specijalno dizajniranog 3D modela termoplastičnog dala, uz primenu metode konačnih elemenata (MKE). Bazirano na obliku i dimenzijama dela postavljene su tri različite varijante sistema za hladjenje. Pri tome su na ulazu, za potrbe MKE metode, korišćeni identični ulazni podaci. Dobijeni rezultati su analizirani i korišćeni kao input u cilju kreiranja optimalnog rešenja. Pored toga, elaboriran je i način definisanja ulaznih podataka u MKE analizu, koji imaju direktnog uticaja na dobijene rezultate. Ustanovljena je i direktna korelacija izmedju numeričkog pristupa i teretskog objašnjenja datog problema. Numeričko modeliranje bilo je realizovano korišćenjem FEA softvera Autodesk Moldflow. Ključne reči: kalup; injekciono presovanje; termoplasti;hlađenje; Metoda konačnih eleemnata)