INVESTIGATION OF THE EFFECT OF PROCESS PARAMETERS IN CO 2 LASER CUTTING OF PMMA MATERIAL BY RESPONSE SURFACE METHOD

: Recently, the studies on lightweighting have gained significant importance due to requirements such as energy efficiency and sustainability. Polymer materials are a group of materials that are frequently used for this purpose. This study investigated the effects of laser power, cutting speed and focal point on the kerf width, which are the most effective process parameters in laser cutting of Polymethylmethacrylate, a low-cost polymer material, using Response Surface Methodology and ANOVA. The results obtained from the data analysis show that the kerf width is seriously affected by the focal point depth as well as the speed and power.


INTRODUCTION
With the development of technology, the strength/weight ratio of the material has gained great importance in all kinds of designs in recent years, depending on the useful load. The use of lightweight materials such as thermoplastic polymers is increasing to reduce the weight of the material without reducing its strength and also to allow recycling. Thermoplastics are usually presented in granular form, softening and melting when heated and become fluid. Thanks to this feature, the material's physical properties are recycled without much deterioration. Polymethylmethacrylate (PMMA) is a low-cost thermoplastic polymer with properties such as high visual transparency, low density and high absorbency (Cavdar ve Tanrisever  Changing and developing technology has also changed material processing methods. Laser cutting technologies used to cut polymer materials provide many advantages over traditional cutting techniques (Khoshaim et al. 2021). CO2 laser cutting is a method used to cut both metalbased materials and non-metallic materials. As a working principle; It consists of the movement of the focused laser beam perpendicular to the plane of the surface to be cut. The heat generated by the focused beam creates a hole in the material and the material is cut by the moving beam (Aniszewska et al. 2020, Masoud et al. 2020, Yuce 2019). PMMA is very suitable for CO2 laser cutting as it has a high wavelength absorbance of 95%. CO2 laser cutting is used to cut highly complex shapes, reducing shape cutting limitations and increasing corner strengths by reducing stress on material edges. Because PMMA has high wavelength absorption, it produces high cut quality and minimal microcracks in the material (Dudala et al. 2020, Khamar ve Prakash 2020, Prakash ve Kumar 2015).
In the literature review, it has been observed that more research is needed on the optimization of laser cutting parameters of PMMA materials laser cut with CO2 laser beam. In this study, the effects of laser power, cutting speed and focal point, which are the most effective process parameters in laser cutting with CO2 laser beam, on the kerf width (Kw) were investigated with the help of Response Surface Methodology (RSM). In this context, the singular and interactive effects of the process parameters were determined by analysis of variance (ANOVA). With the desirability function approach, optimum process parameters were determined to minimize the kerf width.
This study investigated the effects of cutting speed, laser power and focal point position, which are CO2 laser cutting parameters, on the kerf width of PMMA material . It is aimed to determine the effect of parameters on kerf width values using Response Surface Methodology (RSM). The general trend of the effect of these variable parameters on the cutting surface properties was observed. ANOVA was used to analyze and understand the kerf width on the cut surface of different parameters.

Materials and Method
The physical, mechanical and thermal properties of the used PMMA material were given in Table 1. The dimensions of the produced PMMA specimens were 90 mm in length x 25 mm in width and 3 mm in thickness. Unilaser branded LT-1080 CCD model 1000 mm x 800 mm single head laser cutting device was used for this study (Fig. 1). This device has peak power of 120W, 0°C -45°C working environment, servo and step motor motion system, 2200W air suction system, 5000 Chiller cooling technology, water cooling system and honeycomb table type.

Figure 1: Laser cutting device
The laser cutting direction and specimen geometry made in RDWorks are transferred to the laser cutting device via USB (Fig. 2). The cutting process was finished 2 mm after the end of the specimens. The aim is to remove the effect of slowing down the insert on the kerf width at the end of the process.

Figure 2: Laser cutting direction and specimen geometry
Factors affecting the kerf width of PMMA material cut with CO2 laser cutting, cutting speed, laser power and focal point position.
As shown in Figure 3, various parameters affecting the kerf width were tested with preliminary tests. The cutting parameters giving minimum and maximum heat input were determined by preliminary experiments.  Table 2. In this study, Celestron Handheld Digital Microscope Pro was used for measuring kerf width values. This device has a 20x to 200x magnification range, 5MP sensor size, 1.75 μm pixel size and CMOS Sensor in lieu of eyepiece. First, the cross-sectional images of samples were captured and then, kerf width values were measured using WebPlotDigitizer. The kerf width of the specimen shown in Figure 4a (S10F1P60) was measured as 1.23 mm, and the kerf width of the specimen given in 4b (S10F9P80) was measured as 0.46 mm. All measured kerf width values are tabulated in Table 3 with nomenclature of samples and relevant parameter levels.

RESULT AND DISCUSSION
Experiments were performed as per design of experimental to analyze the impact of the focal point upon kerf width. This methodology helps study the complex relationships between input parameters. The design of experiment prepared by using Minitab 17 is shown in Table 3. The impacts of the input parameters (laser power, cutting speed, focal point) on Kerf Width were obtained by applying ANOVA to data obtained during experimentation. ANOVA results are shown in Table 3. The confidence level of ANOVA for kerf width was taken as 95%. This model is statistically acceptable and determines the significant input parameters and their effect on kerf width. The R2 value of the model is 98.27%. This shows that there are significant relations between input parameters and obtained kerf width values. Source, degrees of freedom (DF), adjusted sum of squares (Adj SS), adjusted mean squares (Adj MS), F-values and percentage contribution (% Contribution) of each parameter and various interactions are as shown in Table  4. The contribution of individual parameter and their interactions indicates the significance of each parameter. The regression equation of kerf width including linear, square and nonlinear terms is shown in second-order polynomial equation (1), representing the relationship between kerf width and the other parameters. The ANOVA results and their coded coefficients for the equation (1) were shown in Table 5.   The normal probability plot of the residuals of kerf width is as shown in Fig. 5. The marked dots in the graph show the obtained values. The closeness of these points to the linear line indicates that the regression model well fitted with the observed data and the errors were distributed consistently.

Figure 5: Normal probability plot for Kw
The main effect plot for kerf width is shown in Fig. 6. The increase in cutting speed decreases the kerf width. The kerf width also increases with increasing laser power. This effect is attributed to increase of heat input. Both increase in laser power and decrease in cutting speed increases the heat input as shown in Eq 2 (Son ve Lee 2020). Evolume is volume energy, Plaser is the laser power [W], Vs is the cutting speed [mm/min], and A is the spot area of the laser beam [mm].

= × ( 3 ) ⁄
(2) Kerf width initially decreases by an increase in focal point and further increases by an increase in focal point. The heat input localization is determined by focal point parameter. So, kerf width is seriously affected by focus depth as seen in Fig. 6.

Figure 6: Main effect plot for Kw
The main effect plot shows the effect of only one parameter, while the interaction plot shows how the response is affected due to the interaction of the two parameters. In Fig. 7, interaction plot is given. For example, upper right plot (Focal Point * Power) shows us how kerf width value changes while focal point increasing. In this plot, different curves with different color and line style are attributed to levels of Power parameter. Similar to the results in Figure 6, the interaction plot also showed the significant effect of Focal point on kerf width.

Figure 7: Interaction plot for Kw
As shown in Fig. 8, when the focal point is hold values at 5 mm, the kerf width increased as the laser power was increased and the cutting speed decreased. When laser power constant cutting speed is decreased, kerf width increased. As shown in Fig. 9, when the cutting speed is hold values at 15 mm/s, when the focal point is hold values at 5mm, the kerf width increased as the laser power was increased and the focal point decreased. When focal point kept constant, kerf width increases almost linearly with laser power.   As shown in Fig. 11, when the laser power is 80%, speed is 15 mm/s and focal point hold values at 5 mm, from the speed* power graph, decreasing speed and increasing power will increase the kerf width. From focal point*power graph, the kerf width increases with decreasing focus and increasing power. From focal point*speed graph, the kerf width increases with decreasing focus and speed. In addition, the power remains constant in the focus* power graph and the focal point decreases, and the speed remains constant in the focus* speed graph and the focus point decreases, causing the kerf width to increase.

Figure 11: Contour plot for Kw
The desirability function approach, which is a mostly used method for optimizing response surfaces, was used to determine the optimum process parameters minimizing kerf width. Figure  12 shows individual desirability plot for minimizing the kerf width. As can be seen in the figure, the parameter set giving the minimum kerf width of 0.36 mm was obtained as %70.10, 10mm/s and 8.19mm for laser power, cutting speed and focal point, respectively.

CONCLUSIONS
In this investigation, the effects of laser cutting parameters on kerf width values were evaluated. PMMA sheets were cut in accordance with the parameter sets obtained by the Box Behnken response surface method. The resulting kerf widths were imaged and measured using microscope. The obtained main conclusions can be summarized as follows:  Confidence level of ANOVA for kerf width was found as 95%. This model determines the significant input parameters and their effect on kerf width. This the R 2 value of the model is 98.27% and is statistically acceptable.  The results obtained from the data analysis show that the kerf width is seriously affected by the focal point depth as well as the speed and power.  It has been found that when focal point kept constant, kerf width increases almost linearly with the laser power and laser power is the second significantly parameter.
 It has been found that when the laser power hold values at 80%, the kerf width increases with decreasing focus and increasing power, when speed hold values at 15 mm/s, the kerf width increases with decreasing focus and increasing power and when focal point hold values at 5 mm, decreasing speed and increasing power will increase the kerf width.