Influence of Injection Molding Parameters on the Mechanical Properties of Injected PC/ABS Parts

This study optimized the influence of process parameters on the mechanical properties during injection molding (IM) of PC/ABS blend. The Taguchi method of design of experiments (DOE) was employed to optimize the process parameters and to increase the tensile strength and the elasticity module. Taguchi’s L9 (3) orthogonal array design was employed for the experimental plan. Process parameters of the injection molding such as material temperature, injection pressure, holding time, and mold temperature were studied with three levels. The Signal to noise (S/N) ratio for mechanical properties of PC/ABS blend using the Taguchi method was calculated. Taguchi’s results proposed two sets of optimal injection parameters conditions to achieve the best mechanical characteristics (σ, E). The (S/N) ratio results proved that the injection pressure was the more prominent than the other IM process parameters for the tensile strength, and the material temperature was the more prominent for the elasticity module.


Introduction
Plastic injection molding (IM) is one of the most important polymer processing operations in the plastic industry nowadays. The application of this process is increasing significantly in many fields, especially in the automotive parts. The most process injection molding parameters that analyze the mechanical properties are pressure, temperature, and time.
Experimental and-error approach to control the process parameters for injection molding is no longer effective. Wrong selection of these parameters induces a drop in the mechanical properties of injected molded parts [1][2][3][4][5][6].
Such problems can be solved by first developing optimization models correlating the responses and the process parameters. Optimizing these parameters requires a careful endeavor. Besides, the method of varying one parameter while keeping the others constant contains limitations such as the long time experience and the large number of experiments which raise its cost in order to get an important quality of the parts. Hence, a suitable optimization technique using design of experiment DOE by Taguchi approach is then applied to search for fine tuning of parameter values to obtain the desired responses. The Taguchi method introduces an integrated approach that is simple and efficient for finding the best range of designs for quality, performance, and computational cost [7][8][9]. Taguchi method is a useful methodology for analyzing the variation using the signal-to noise (S/N) ratio. The Taguchi analysis of the S/N ratio specifies three situations of quality characteristics, as well as the-largerthe-better (LB), the nominal-the-better (NB) and the-smaller-the-better (SB).
A great deal of research used this approach is being carried out to understand, identify critical factors and to optimize the molding process. For example, Wegrzyn et al. conducted a study to investigate the impact of injection speed and melt temperature on electrical conductivity and dynamic mechanical properties of PC/ABS-MWCNT Nano composites [10]. These authors figured out that the injected molded specimens recover conductivity. In addition, Wang et al. elaborated experimentally the compression behavior of PC/ABS during monotonic and cyclic loading over a wide-ranging of temperatures (up to 373 K) and strain rates (up to 5000 s −1 ) [11]. In parallel, Li et al. analyzed effect of process parameters namely, melt temperatures, injection speed and injection pressure to determine the influence of weld lines on appearance of PP products using Taguchi experimental design method [12]. Rafizadeh et al. showed that when fixing the mixing time and temperature, temperature and pressure of injection, mold temperature, and blend composition in mixing through the Taguchi orthogonal array, the impact strength of PC/ABS blend could be optimized [13]. Equally, Ozcelik studied the influence of the injection parameters and weld line on the mechanical properties of polypropylene (PP) during IM process via the design of experiment (DOE). Mechanical properties such as maximum tensile load, extension at break and charpy impact strength (notched) of the specimens were measured [14]. Furthermore, Kuram et al. discussed the tensile and the flexural strength during injection molding of virgin and recycled PBT/PC/ ABS blends using Taguchi experimental design. The results showed the impact of injection pressure, holding pressure, and injection temperature on the mechanical properties of the molded parts [15]. Aditya M. Darekar used the Taguchi approach and found that quality and mechanical properties parts are mainly caused due to improper selection of the processing conditions during production [16].
In this study, the effect of the injection molding process on the mechanical properties of the injected PC/ABS parts was investigated. The Taguchi method was used to find out the optimal combinations of the injection molding (IM) parameters that provide better mechanical properties. These results are considered interesting especially in the automotive industries. We intend to reduce the rejection rate caused by the surface quality of some injected PC/ABS parts.

Industrial context
SKG Company recognized by the injection and metallization of the PC/ABS automotive parts, discovered the occurrence of some defects in the surfaces of the injected parts. These defects caused a high rejection rate [17].

Material elaboration
The commercial PC/ABS blend used in this investigation was the T45 PG. This blend was proximately oven dried at 120°C for 4 h [17]. Its mechanical and thermal properties are exhibited in Table 1.

Injection machine and molded part description
As illustrated in Figure 1(a), a horizontal injection molding machine "FCSHN 125SV" was used to elaborate the PC/ABS parts. Their maximum injection pressure and clamping force were 217 MPa and 125 kN, respectively. Its screw's diameter was 34 mm.
The injection mold forms three imprints presented in Figure 1(b). Imprint A was the tensile specimen. Imprint B was the dynamic tensile specimen. Imprint C was the torsion specimen [17].
This study is an alternate to investigate the impact of the injection process on the mechanical properties of injected PC/ABS parts.

Design of Experiment (DOE): Taguchi method
The developed Taguchi method goal is to determine the set of parameters which leads to the best mechanical properties of the PC/ABS injected part. MINITAB 18 statistical software was used to find out the optimal processing conditions. A confirmation test was performed in order to guarantee the validity of the optimal experimental conditions considered from the design of the parameters.  Table 1.
Following preliminary tests carried out within the SKG Company, the choice of the factors and their levels was made as illustrated in Table 2.
The impact of the injection molding parameters on the mechanical characteristics are evaluated through the S/N ratio obtained in the Taguchi experimental plan. The S/N ratio was calculated by the larger is better quality characteristic for determining the effect of each process IM factor. The responses required are the tensile strength and the elasticity module.

Tensile tests
The tensile tests were carried out, with imposed displacement, on a machine of the LLYOD INSTRUMENT type equipped with a 5 kN load cell.
All tests were achieved at ambient temperature and for a strain rate of 5 mm/ min. The local deformation of the specimens during these tests was measured using an extensometer having an initial length of 25 mm and an elongation of ±1 mm.
The measurement was repeated at least 4 times for the samples and the average of three sample readings was taken for accurate results. The specimens used for the simple tensile test are taken from the ISO 527-2 standard. Its geometry is illustrated in Figure 2.

Experimental
The injection molding (IM) process parameters considered in this study were: material temperature (A), injection pressure (B), holding time (C) and mold temperature (D).
In order to optimize these parameters, the famous optimization approach Taguchi was adopted. One main reason for using Taguchi is the low number of experiments. The   number of experiments required could be reduced to nine, instead of 81 (3 4 ) experiments to find out the parameters that influence on the mechanical properties. As depicted in Table 3, an orthogonal array (OA) L9 was involved to investigate the effect of the process IM parameters. Three levels for each factor were studied. The DOF for the three levels was 2 (DOF = number of levels−1).
The elaboration of the injected PC/ABS parts was done within the SKG factory. Each combination was iterated 5 times (9 × 5 = 45). The tensile strength and the elasticity module of the injected PC/ABS parts were regarded as the desired responses.  Table 3.

Mechanical test results
Experimental layout using an L 9 orthogonal array. properties measured of each test performed were extracted. These values were introduced in the MINITAB 18 statistical software in order to optimize the injection parameters.

Analysis of mean
The S/N ratios were calculated for each of the nine conditions of the test. All the details: the values, the average of each parameter at different levels, and the standard deviation of each test were depicted in Table 4.
The responses studied are the tensile strength and the elasticity module. Tables 5 and 6 represented the average values of mean response relative to the four control parameters (factors) studied. Figures 4 and 5 illustrated these values.
The optimum conditions corresponded to the maximum tensile strength and the elasticity module. These conditions illustrated the levels of the highest mean responses values chosen for each IM process parameter.
The averages values of the tensile strength (σ) and the elasticity module (E) were depicted in Figures 4 and 5. Hence, the optimum condition levels corresponding to the maximum tensile strength of the injection molding process of the PC/ABS blend was A3, B3, C2, and D3. That is to say, in the third level, at the material temperature of 260°C, represented by parameter A; an injection pressure of 50 bar, represented by parameter B; a mold temperature of 60°C represented by parameter D; and in  the second level at a holding time of 8 sec, represented by parameter C. In parallel, the injection pressure was considered the more prominent than the other IM process parameters for the tensile strength.     Figure 5 shows that the material temperature represented by parameter A was more prominent than the other IM process parameters for the elasticity module. Moreover, the maximum elasticity module was recorded in the third level, at the material temperature of 260°C, represented by parameter A; a mold temperature of 60°C, represented by parameter D; and in the second level at; an injection pressure of 40 bar, represented by parameter B; a holding time of 8 sec, represented by parameter C.
The average values of S/N ratios of various parameters at different levels are shown in Tables 7 and 8. These values are plotted in Figures 6 and 7.
The S/N ratio analysis of the tensile strength plotted in Figure 6 provides in to the same levels of the parameters as depicted in Figure 4. Therefore, A3, B3, C2, and D3 were the best levels for reducing the variability of the injection molding process of the PC/ABS blend.  Table 7. The S/N ratio analysis of the elasticity module depicted in Figure 7 offers in to the same levels of the parameters as shown in Figure 5. So, A3, B2, C2, and D3 were the best levels for reducing the variability of the injection molding process of the PC/ABS blend. Table 3 shows the combinations of factor levels (3, 3, 2, 3) and (3, 2, 2, 3) were not among the obtained combinations within the tested experiment. Thus, it was necessary to verify the optimum injection molding conditions via confirmation tests of the experimental design.

Estimation of predicted mean
The optimum levels of the control parameters of the tensile strength (μ 1 ) and the elasticity module (μ 2 ) are determined as A3, B3, C2, D3 and A3, B2, C2, D3 respectively [18,19]:   time, 8 sec, 3 D is the average elasticity module at the third level of mold temperature, 60°C and E is the mean of elasticity module obtained from Table 4.

Confirmation tests
Three confirmation experiments were conducted at the optimum settings of the process parameters recommended by the investigation. The average tensile strength and the elasticity module obtained at the optimal level of the process parameters were 56.28 MPa and 1983 MPa.
Obviously, there was a difference between the computed and the experimental results. However, this difference can be considered not significant. Therefore, the confirmation tests indicate that the optimal conditions obtained above produced the best mechanical properties (σ, E) of the PC/ABS blend ( Table 9).

Conclusion
This work focused on Taguchi experimental method for investigating the influence of the injection parameters on the mechanical properties of PC/ABS during injection molding. Taguchi's results proposed two sets of optimal injection parameters conditions to achieve the best mechanical characteristics (σ, E). A first series: a material temperature of 260°C, an injection pressure of 50 bars, a holding time of 8 sec and a mold temperature of 60°C. A second series: a material temperature of 260°C, an injection pressure of 40 bar, a holding time of 8 seconds and a mold temperature of 60°C. All mechanical properties measurements matched very well with the experimental data. The most important parameter affecting the maximum tensile strength was the injection pressure. However, the material temperature was considered the most important parameter affecting the elasticity module. Consequently, it is shown clearly the above performance characteristics in the injection molding process are greatly significant through this study. So, we recommend that future works involve the impact of injection processing on the surface quality of the PC/ABS parts. Design of Experiment S/N Signal-to-Noise ratio