OPTIMIZATION PROCESS PARAMETER OF INJECTION MOLDING. Mohd Aswadi Bin Muhamad. Politeknik Tuanku Sultanah Bahiyah,

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1 e- Proceeding National Innovation and Invention Competition Through Exhibition 2017 OPTIMIZATION PROCESS PARAMETER OF INJECTION MOLDING Mohd Aswadi Bin Muhamad Department of Mechanical Engineering, Politeknik Tuanku Sultanah Bahiyah, Kulim Hi-Tech Park, Kulim, Kedah, Malaysia Phone : ; Fax ; aswadi.poli@1govuc.gov.my Muhammad Amin Bin Harun Department of Mechanical Engineering, Politeknik Sultan Haji Ahmad Shah Semambu, Kuantan, Pahang, Malaysia Phone : ; Fax ; amin.harun.poli@1govuc.gov.my Hafiz Reza Bin Haron Department of Mechanical Engineering, Politeknik Tuanku Sultanah Bahiyah, Kulim Hi-Tech Park, Kulim, Kedah, Malaysia Phone : ; Fax ; hafizreza.poli@1govuc.gov.my Abstract Based on factors influencing the product quality Injection molding such as melt temperature, screw speed and injection speed. This study is to optimize injection molding parameters for a product based on Polypropylene (PP) by using design of experimentalfull factorial (DOE) approach. Demag injection molding machines were used in the process to produce the tagging product. Three input parameters were evaluated namely, the holding pressure time, injection pressure and screw back pressure. The analysis showed the best process parameter setting at the prediction of weight value, one is to set 0.18 oz. for holding time at 3 seconds, the screw back pressure to be set at 3 bars and injection pressure to be set at 44 bars.the finding of the study indicated that the most significant parameters affecting the weight and quality characteristic is holding pressure time. Keywords: Injection moulding; Process parameter; Design of experiment 1. Introduction The injection molding is one of the most efficient processes where mass production through automation is feasible and products with complex geometry are easily attained (Kuo-Ming Tsai et. al., 2009). Injection molding is used to create many things such as wire spools, packaging, glass, bottle caps, automotive dashboards, pocket combs, and most other plastic

2 products available today. Parts produced by the process are also becoming commonplace in less obvious applications. The advantages of injection molding are high production rates, repeatable high tolerances, the ability to use a wide range of materials, low labor cost, minimal scrap losses, and little need to finish parts after molding. Injection molding, a key polymer processing technique, transforms plastic granules into various plastic parts. The requirements of quality injection molded part have been significantly increased in the past decade (Ho Yin Wong et. al., 2008). Although the molding processes are quite simple, the rheological behaviors of polymers are complicated. Therefore quality characteristics of injected products are highly unpredictable. The injection molding process is characterized by melt temperature, screw speed, injection speed, injection pressure, screw speed, screw backpressure, holding time, cooling time, and mould temperature. The Injection molding is a complex technology with possible production problems. The types of problem such as blister, burn marks, delamination, flash, sink marks, short-shot, weld line, shrinkage, flow marks, and others. There are four majors categories, which is material, machine, method and mould that affecting the product defects (Shaik Mohamed et. al., 2004). 2. Injection Molding Process Injection molding is a process in which a plastic material is heated until it becomes soft enough to force into a closed mold, at which point the material cools to solidify and form a specific product (Nuraida, 2007). Generally, injection molding is used in an industry because it has good production rate for complex shaped product resulting in good dimensional accuracy and surface finishing. The process cover five stages: (1) mold closing, (2) filling, (3) packing holding, (4) cooling and (5) mold opening which proceeded repeatedly, as illustrated in Figure 1. Figure 1. (a) Simplified diagram and (b) Schematic flow diagram illustrating the injection molding process. (Ho Yin Wong et al., 2008).

3 Based on Figure 1, the first stage in injection molding cycle starts when the mold is closed. Then the second stage is the filling stage which is the screw moves forward and forces the polymer to melt into the mold cavity under a desired velocity. The third stage process is the packing holding stage until the cavity is completely filled with plastic material. An additional materials are packed into the cavity under a certain pressure to compensate for the part shrinkage. In the cooling stage, the plastic product continues to cool down. Simultaneously, the polymer inside the barrel undergoes plastication so as to prepare a specific amount of melting for next cycle. Eventually, once it becomes sufficiently solidified and rigid the molded part is ejected (Ho Yin Wong et al., 2008). The interaction between the material variables, operation variables, mold and machine designs, process variables and end-product quality is, however, complex and non-linear, as demonstrated in Figure 2. Figure 2. Factors influencing the product quality (X. Chen et al., 2004) Recently, the DOE procedure has been used to systematically investigate process variables or product variables that influence the quality of products. It is possible to identify the process conditions and product components that influence product quality and costs, which in turn enhance the product manufacturability, quality, reliability, and productivity. The DOE procedure consists of the following four steps : (J. Krottmaier, 1993) i. Planning. Definition of the problem and the objective, and development of an experimental plan. ii. iii. iv. Screening Reduction of the number of variables by identifying the key variables that affect product quality. Optimization Determination of the optimal values for various experimental factors. Verification

4 Performing a follow-up experiment at the predicted best processing conditions to confirm the optimization results. 3. METHODOLOGY The next stage is preparing the materials for injection molding. The materials used are Polypropylene and the binder are color filler. Then the machine are prepared to run the experiment and the product for this research is tagging. A designed experiment is an observation process where tests are conducted in a rigorous, systematic manner. For each test, important outputs are measured. Analysis of the resultant data is used to characterize, optimize, or troubleshoot an injection molding process (Shahril, 2010). Based on literature review of past case study, several factors that can affect the filling of the mould have been identified. The factors and response of the experiment are shown in Figure 3. Among the factors selected are holding pressure time, injection pressure and screw back pressure. It is then followed by the response of this experiment which is quality characteristic and weight scale. Based on (Shahril, 2010) research, selecting either the appropriate response or quality characteristic to measure is critical to successful experimentation. Factors (Input) Holding Pressure Time Injection Pressure Screw Back Pressure Response (Output) Quality Characteristic Weight Scale Figure 3. Factors and response selected In this study the Design of Experiment (DOE) method, there are eight runs that need to be completed. They evaluate the three variables as stated in Table 3.1, which consists of high and low value, as determined from the literature review. Table 1. The range of parameters used for optimization of the processing condition. No. Variables Low value (-) High value (+) Unit A Holding pressure 1 3 Second time B Injection pressure kg/cm2 C Screw back pressure 1 3 kg/cm2 The experiment also included the constant parameters which affected the product quality less significantly. Table 2 shows the constant parameters that are used during the molding process. The constant parameters are setup based on the manual of a manufacturer.

5 Table 2. The constant parameter used for the injection molding. Constant parameters Value Unit Holding pressure 60 kg/cm2 Injection speed 45 cm3/sec Holding pressure 60 kg/cm2 Cooling time 25 Second Nozzle Temp. 235 ºC Front Zone Temp 230 ºC Middle Zone 220 ºC Temp Rear Zone Temp 195 ºC Product cycle time 36 second Design of experiment is an observation process where tests are conducted in a rigorous, systematic manner. As such, for each test, important outputs are measured. Analysis of the resultant data is used to characterize, optimize, or troubleshoot an injection molding process (Shahril, 2010)., The experimental matrix for the full-factorial design of experiment is shown in Table 3. From the table, it s shown that, there exist the varying range of the controlled factors which have the upper and lower limits of the interference of environment. Table 3. The design of 2 3 Full-Factorial experiments (M.S. Huang et al., 2008). Experiment No. A B C When one is performing an experiment, varying the level of the factors simultaneously rather than one at a time is more efficient in terms of time and cost. And it allows for the study of interactions between the factors (Shahril, 2010). Polypropylene are selected as raw materials because their advantages in terms of properties when compared to other materials to provide high quality product. Table 4 shows the melting temperature for Polypropylene material which is ranges from 130 ºC to 168 ºC. The processing temperature for Polypropylene material also between 202 ºC to 252 ºC.

6 Table 4. The thermal stability polyethylene and polypropylene (Brandrup and Immergut, 1989) Polyethylene Polypropylene Melting temperature ( C) Maximum service temperature, Air ( C) Processing temperature ( C) RESULTS AND DISCUSSION Table 5 shows the definition of quality characteristic index scale. The scale index number of 1 represents product which has exhibited sink and warpage, while index number of 2 is for product that exhibits sink or warpage, and index number of 3 indicates no defect on the product. Table 5. Response Quality Index Index Number Sink + Warpage Sink or Warpage No defect The injection molding process was investigated using full factorial design. This design is used to identify the significant factors that affect the injection molding responses that are weight and prod uct quality. Design expert software version was employed and the experimental results are given in Table 6. Table 6. Results of the Experiment Std Run Block Factor 1 A: Pressure (bar) Factor 2 B : Screw (bar) Factor 3 C: Holding (sec) Response 1 Weight (Oz) Response 2 Product Quality 10 1 Block Block Block Block Block Block Block Block Block Block Block

7 4.1 Effect of Process Parameters To Weight Based on Table 7, two parameters are able to show the significant main effect to the weight value. The parameter were injection pressure (A) and holding pressure time (C). The most significant main effect is holding pressure time and followed by injection pressure. This is so because the holding pressure time has high F Value of The main effect can be seen from Figure 4 below, where the graph has a higher slope than the injection pressure graph. Based on Shahril (2010) research, the greater the slope, the more important the effect. It s can be concluded that, when the holding time increases then the weight value also increase steadily. Table 7. ANNOVA (analysis of variance) tables for weight Std. Dev E-004 R-Squared Mean 0.17 Adj R-Squared C.V Pred R-Squared N/A PRESS N/A Adeq Precision Then, the second significant main effect is injection pressure (A), with F Value of and also gives effect to weight value. Figure 5 shows that the slope of the graph is lower than that of the holding pressure time graph. The effect of injection pressure to the physical of plastic product is supported by Nagahanumaiah et al.( 2009) research which is, injection speed and melt temperature have significant influence on part weight and shrinkage.

8 Figure 4. The main effect of holding pressure time Figure 5. The main effect of injection pressure 4.2 Effect of Process Parameters to Quality Characteristic Based on Table 8, two parameters show the significant main effect to the weight value. They are screw back pressure (B) and holding pressure time (C). The most significant main effect is holding pressure time and followed by screw back pressure. This is because holding pressure time has high F Value which is 75. The main effect can been seen from Figure 6 below, which show that the graph has higher slope than screw back pressure. It is supported by Adam Kramschuster et al.(2005) where, the holding pressure and holding pressure time have the most significant effect on the shrinkage and warpage in conventional injection molding. It can be concluded that, when the holding pressure time increases then the quality characteristic value also increases steadily. For this situation, the holding pressure time should be set to high level to get high quality of product.

9 Table 8. ANNOVA (analysis of variance) tables for weight Std. Dev E-004 R-Squared Mean 0.17 Adj R-Squared C.V Pred R-Squared N/A PRESS N/A Adeq Precision Then, the second significant main effect is screw back pressure (B), with F Value of 27. Figure 6 shows that the slope of the graph is lower than the holding pressure time graph. According to Sheik Mohamed Mohamed Yusoff et al.(2004) the significant main effects identified were screw back pressure and manifold temperature. Figure 7 shows that the slope of the graph is lower than the holding pressure time graph. Figure 6. The main effect of screw backpressure

10 Figure 7. The main effect of holding pressure time 4.3 The Best Injection Molding Parameters Table 9 shows the optimization process based on the weight and quality characteristics response after optimization by using Design Expert software. The factor variable goal for injection pressure is in range, screw back pressure is in range and holding pressure time also is in range. The response goal for weight is maximum and quality characteristic is maximum. The using of Design Expert software to determine the optimum process parameters was supported by Keun Park et al.(2004) research, performed an experiment for various process conditions with additional DOE scheduling in order to determine optimal process parameters. As a result, the quality and productivity of the product have been improved. In Table 9, there are 10 data for optimum process parameters to be considered the best process parameters and the process parameter data number one is selected as the best, because it has the highest value for weight response which is oz and the high scale value for quality characteristic which is The data for the best process parameter for this experiment can be found in Table 10. The best process parameters for injection pressure is bar, screw back pressure 3 bar and holding pressure time 3 sec. The scale value shows the maximum number of index scale which has no defect. The desirability thus shows that the maximum value is and equivalent to 1.

11 Table 9. Optimization process parameter Table 10. The best process parameter selected Factors Unit Value A-Injection Pressure Bar B-Screw Back Pressure Bar 3 C- Holding Pressure Time Sec 3 Figure 8 shows the contour plot for weight response. From the graph, to get the best process parameter setting at the prediction of weight value, one is to set 0.18 oz. for holding time at 3 seconds, the screw back pressure to be set at 3 bars and injection pressure to be set at 44 bars. Figure 9 show the 3D surface graph for weight prediction. The 3D graph surface is produced by contour plot of weight.

12 Figure 8. Contour plot graph for weight Figure 9. 3D surface graph for weight Figure 10 shows the contour plot for quality characteristic response. From the graph, for one to get the best process parameter setting at the prediction of weight value of 2.87 scale index at holding time of 3 seconds, the screw back pressure to be set at 3 bars and the injection pressure to be set at 44 bars. Figure 11 shows the 3D surface graph for quality characteristic prediction. The 3D graph surface is produced by contour plot of quality characteristic.

13 Figure 10. Contour plot graph for quality characteristic Figure 11. 3D surface graph for quality characteristic

14 5. CONCLUSIONS This research successfully utilizes full factorial DOE approach in optimizing injection molding process parameters. The results showed us that using DOE in injection molding process parameters able to predict the weight and quality characteristics for various combination of processing parameters. Several conclusions that can be drawn from this research are as follow: i. The objective of identifying the main effects of injection molding parameters to the response was achieved. From two responses measured, the experiment shows the most significant main effect process parameters is holding pressure time. The objective to obtain the best injection molding parameters for Polypropylene is also achieved. The best process parameters predictions are identified based on a combination of the two responses. The parameter for injection pressure is bar, screw back pressure is 3 bar and holding pressure time is 3 sec. The prediction response for weight is oz and for quality characteristic scale index is ii. iii. The parameters that significantly affect the weight of the product are holding pressure time and injection pressure. It shows that, if the holding pressure time and injection pressure increase then the weight value will also be increased. The parameters that significantly affect the quality characteristic response are holding pressure time and screw back pressure. It shows that, holding pressure and screw back pressure should be set at a higher level so as to increase the quality characteristic of the product.

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