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Hole qualities in laser trepanning of polymeric materials
Optics and Lasers in Engineering 50 (2012) 1297–1305
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Hole qualities in laser trepanning of polymeric materials I.A. Choudhury n, W.C. Chong, G. Vahid Department of Engineering Design and Manufacture, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e i n f o
abstract
Article history: Received 22 September 2011 Received in revised form 23 February 2012 Accepted 29 February 2012 Available online 29 March 2012
The present study focuses the effect of four input controllable laser cutting variables on the hole taper and hole circularity in laser trepan drilling of polymeric materials. Experiments have been conducted on acrylonitrile butadiene styrene (ABS) and polymethyl methacrylate (PMMA) polymer sheets. Laser power, assist gas pressure, cutting speed and stand-off distance were selected as independent process variables. Three different holes of diameters 2 mm, 4 mm and 6 mm were drilled in these work materials of 5 mm thickness. A Taguchi L9 orthogonal array with four factors and three levels of each factor was used to plan and conduct the experiments in order to obtain required information with reduced number of experiments. The process performance was ascertained in terms of hole taper and hole circularity. Initial analysis involved in determining the effect of the four process variables on hole taper and circularity for these two polymers at three different hole diameters. From ANOVA analysis, the optimum levels of the four process variables with respect to materials and hole diameters were evaluated. As it was found that the optimum levels of four process variables were different for different hole size and materials, additional analysis was conducted to incorporate the effect of material and hole diameter on the hole taper. From the analysis, the optimum combinations were obtained at compressed air pressure of 2.0 bar, laser power of 500 W, cutting speed of 0.6 m/min, stand-off distance of 5.0 mm, hole diameter of 2.0 mm and material of PMMA. These combinations produced the minimum taper in the hole. The circularity of the hole was more at the entrance than the exit when ABS polymer was laser drilled while in PMMA, the hole was more circular at the exit than the entrance. & 2012 Elsevier Ltd. All rights reserved.
Keywords: CO2 laser Hole taper Laser drilling Design of experiment Polymeric materials
1. Introduction The laser drilling process is one of the most widely used thermal energy based non-contact type advance machining process which can be applied across a wide range of materials. Nowadays laser drilling is finding increasingly widespread application in the industries. Laser beam machining is based on the conversion of electrical energy into light energy and then into thermal energy to remove the material from work piece. The material removal process is by focusing laser beam onto the work material for melting and vaporizing the unwanted material to create a hole. CO2 laser drilling has been widely used in industries because of its high production rate and abilities on rapidly varying holes size, drilling holes at shallow angle, and drilling hard-to-work material such as high strength materials, ceramic and composite. Laser drilled holes are inherently associated with a number of defects. Circularity of hole, spatter thickness, and hole taper are some defects associated with laser drilling. As a result, the quality
n
Corresponding author. Tel.: þ60 379675384, fax: þ60 379675330. E-mail addresses: [email protected] (I.A. Choudhury), [email protected] (W.C. Chong), [email protected] (G. Vahid). 0143-8166/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlaseng.2012.02.017
of the drilled holes is the main issue in the laser drilling process. There are two types of laser drilling: trepan drilling and percussion drilling. Trepan drilling involves cutting around the circumference of the hole to be generated, whereas percussion drilling is carried out by utilizing a focused laser spot to heat, melt and vaporize the target material such that a desired hole is formed through the work piece with no relative movement of the laser or work material [1,2]. Fig. 1 shows a schematic of laser beam drilling [2]. Ghoreishi et al. [3] investigated the relationships and parameter interactions between laser peak power, laser pulse width, pulse frequency, number of pulses, assist gas pressure and focal plane position on the hole taper and circularity in laser percussion drilling of stainless steel and mild steel. The central composite design was employed to plan the experiments in order to obtain required information. The process performance was evaluated in terms of equivalent entrance diameter, hole taper and hole entrance circularity. They found that the pulse frequency had a significant effect on the hole entrance diameter and hole circularity in drilling stainless steel unlike the drilling of mild steel, where the pulse frequency had no significant effect on the hole characteristics. Eltawahni et al. [4] investigated the effect of laser power, cutting speed, and focal point distance on the cutting edge quality parameters in CO2 laser cutting of ultra-high performance
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Laser Head
Laser Beam
Laser Beam Workpiece Workpiece
End Start Laser Path Hole Diameter
Phase Change (Vaporization) Phase Change (Melting)
Plasma Formation Erosion Front Molten Layer
Fig. 1. Schematic of laser beam: (a) trepan drilling and (b) percussion drilling [1].
polyethylene of various thickness from 6 mm to 10 mm. They conducted the experiments by implementing the Box–Behnken design and measured upper kerf, lower kerf, ratio of upper kerf to lower kerf and cut edge surface roughness. They developed model equations relating the input processing parameters with the output or response they had measured. They found that the upper kerf decreased as the focal position and the cutting speed increased, and it increased as the laser power increased. The roughness decreased as the focal point increased from its lowest level till its central level and then it increased as the focal distance increased above its central level. The roughness decreased as the laser power increased and it increased as the cutting speed increased. Higher cutting speed did not always improve the efficiency of the laser cutting process. Benyounis and Olabi [5] did a comprehensive literature review of the applications of design of experiments, evolutionary algorithms and computational networks on the optimization of different welding processes through mathematical models. According to their review of various literatures, they were of the opinion that there was considerable interest among the researchers in the adaption of response surface methodology (RSM) and artificial neural network (ANN) to predict responses in the welding process. For a smaller number of experimental runs, they noted that RSM was better than ANN and genetic algorithm (GA) in the case of low order non-linear behavior of the response data. In the case of highly non-linear behavior of the response data, ANN was better than other techniques. They also observed that the Taguchi approach of SN ratio might lead to non-optimal solutions with less flexibility and the conducting of needless experiments. Yilbas [6] studied the effect of the laser parameters and the material properties on the hole quality in laser hole drilling. A statistical approach, referred to as factorial design, was used to test the significance level of the factors that affect the hole quality. Three materials, stainless steel, nickel and titanium, were considered. The experimental study yields tables of significance of each factor on the aspects that determine the quality of the holes. The parameter which was found to be very significant in most cases was the workpiece thickness. The first-order interaction of pulse length–thickness was the most significant, whilst pulse length–focus setting was significant for all of the materials examined. For the second-order interactions, only pulse length– focus setting–thickness was found to be significant. Yilbas and Kar [7] experimentally conducted thermal and efficiency analysis of the CO2 laser cutting process to cut mild-steel samples of 0.8–2.0 mm thicknesses. They also considered the effects of
momentum and gas–liquid interface shear stress due to an assisting gas jet. The approximate magnitude of the heat absorbed was estimated and the thickness of the melting layer was predicted. The theoretical predictions were compared with the experimental findings. The liquid-layer thickness measured experimentally was found to be in good agreement with the theoretical predictions. The variation of first and second law efficiencies with jet velocity for different cutting speeds showed increasing trend with increasing jet velocities and cutting speeds. Yilbas [8] examined the laser gas assisted cutting process. A statistical method based on factorial analysis was used to identify the influence of cutting parameters on the resulting cut quality. It was found that increasing laser beam scanning speed reduces the kerf width while with the increase of laser power, kerf width increases. The main effects of all the parameters employed have significant influence on the resulting cutting quality. Choudhury and Shirley [9] investigated laser cutting qualities of polypropylene (PP), polycarbonate (PC) and polymethyl methacrylate (PMMA) with a view to evaluate the effect of the main input laser cutting parameters (laser power, cutting speed and compressed air pressure) and develop model equations relating input process parameters with the output. The experiments were carried out according to the central composite first-order design based on response surface methodology. The output quality characteristics examined were heat affected zone (HAZ), surface roughness and dimensional accuracy. It was found that the response was well modeled by a linear function of the input parameters. From the analysis, it has been observed that PMMA has less HAZ, followed by PC and PP. For surface roughness, PMMA has better cut edge surface quality than PP and PC. However, all three polymeric materials showed similar diameter errors tendency in spite of different material properties. French et al. [10] investigated the effects of seventeen factors at two levels and their first-order and second-order interactions on the hole taper and circularity in Nd:YAG laser percussion drilling. They found that pulse shape, energy, peak power, focal position, gas pressure and Nd:YAG laser rod were the most effective factors affecting the hole taper and circularity. Kamalu and Byrd [11] studied the effects of focal length of lens, position of the focal plane with respect to the material surface and laser energy on laser drilling performance. They measured drilling performance by diameter of laser-drilled holes. The three factors were studied at two levels and two sets of 23 factorial designs were analyzed. Negarestani et al. [12] developed a 3D model for simulating the heat flow and material removal rate in laser machining of
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carbon-fiber-reinforced (CFRP) composites on a heterogeneous fiber-matrix mesh. They validated the model with experimental results and found the simulation results to be in agreement with the experiments. HAZ was predicted to be more sensitive to speeds in the lower range of 50–200 mm/s as compared with higher range of 200–800 mm/s. Some recent attempts have been made to control the laser drilled hole taper through the development of drilling techniques [13,14]. Ng and Li [15] assessed the effect of laser peak power and pulse width on the hole geometry repeatability in Nd:YAG laser percussion drilling of 2 mm thick mild steel sheets. Thirty-five holes were drilled and analyzed for each set of identical laser parameters. They found that higher peak power and shorter pulse width gave better hole geometry repeatability. The circularity of the entrance hole ranged from 0.94 to 0.87, and was found to correlate with repeatability. Murray and Tyrer [16] conducted an experimental investigation using CO2 laser in drilling of ceramic samples. They optimized the process to provide the best hole geometry CO2 laser, which exhibited low level of micro-cracking and low taper. Besides, experimental investigations have been carried out to find the optimum ranges of parameters for the drilling of polymers using a CO2 laser. Most of the previous works related to hole drilling used the percussion drilling process where with intense laser burst, the hole size was the size of the beam that was varied by focusing. The present study focuses on the alternative trepan drilling. This experimental investigation of laser drilling of 2 mm, 4 mm, and 6 mm holes on two polymeric materials lead to the assessment of the hole quality in terms of hole taper and hole circularity. This has been done to provide the potential future application of laser drilling in the industries with valuable information on optimum controlled parameters to improve the quality of the laser drilling parts. The use of laser in drilling holes is justified only if the quality of the end product is decisively better and if the process becomes more reliable. The Ishikawa cause-and-effect diagram illustrating the relationship of the process parameters with the quality of hole produced is shown in Fig. 2. Preliminary experiments indicated that, out of several process parameters, laser power, cutting speed, stand-off distance, and assist gas pressure greatly influence the quality of the holes. The Taguchi design of experiment has been adopted to conduct experiments as it involves reducing the variation in a process through robust design of experiments. The overall objective of the method is to produce high quality products at low cost to the manufacturer. The experimental design uses orthogonal arrays to organize the parameters affecting the process and the levels at which they should be varied. Instead of having to test all possible combinations like the factorial design, the Taguchi method tests pairs of combinations. This allows for the collection of the necessary data to determine which factors most affect product quality with a
Cutting speed
Materials
Laser Type
Thickness
Laser type Power
Hole diameter Flow rate Humidity Environment
Gas type
Standoff distance
Pressure Assist gas
Size
Quality of cut: • Hole circularity • Hole taper
Nozzle
Fig. 2. Cause-and-effect-diagram of laser drilling process affecting the quality of cut.
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minimum number of experimentation, thus saving time and resources. The Taguchi method is best used when there is intermediate number of variables (3–50), few interactions between variables, and when only a few variables contribute significantly. As the preliminary investigations suggested that four input variables affect the hole quality in terms of taper and circularity, the use of Taguchi method is justified in the present investigation.
2. Experimental work 2.1. Experimental setup All experiments were conducted in a Zech Laser Cutting Machine. The machine had a continuous, constant and steady CO2 laser power of 500 W (CW mode) with 10.6 mm wavelength. The system consisted of two parts, which were the laser cutting workstation ZL1010, and the laser beam generation system ZLX5. The laser machine was connected to a computer system as a controlling mode of operation. It consisted of three softwares, namely: (i) AutoCAD Release 14 software to design a 2D drawing of the specimen for the project, (ii) C-Cut software which was the laser cutting software for machine’s numerical control and (iii) Zech Laser software as an automatic machining mode software. A three axes CNC-controlled table with work volume 1 m 1 m 0.1 m was used for drilling. 2.2. Work material Acrylonitrile Butadiene Styrene (ABS) is a common thermoplastic used to make light, rigid, and molded products. The most important mechanical properties of ABS are impact resistance and toughness. Generally, it would have useful characteristics within a temperature range from40 1C to 100 1C. Acrylonitrile contributes heat and chemical resistance and toughness, butadiene provides impact strength and low property retention, and styrene offers surface gloss, rigidity, and ease of processing. The major use is for pipe and fittings, automotive parts, refrigerator door liners, computer housings and covers, telephone housings, and electrical conduit. Polymethyl methacrylate (PMMA) is a transparent material often used as a light or shatter-resistant alternative to glass. It is a hard and rigid transparent thermoplastic that has good outdoor weatherability and is more impact resistant than glass. It has good tensile and flexural strengths. The impact resistance is ten times higher than that of glass. PMMA has the highest surface hardness of all common thermoplastics. It is used for glazing aircrafts and boats, skylights, exterior lighting, safety shields, protective goggles and knobs and handles [17]. 2.3. Preliminary experiments The input laser processing parameters like laser power, standoff distance, assists gas pressure and cutting speed have an effect on the hole quality and hole taper. In order to determine the range of these variables affecting the taper, some preliminary experimental investigations were conducted. The data of these preliminary experiments are not reported here. Having obtained the range of variables and the factors affecting most the hole taper from these experiments, a design based on the Taguchi method has been employed. 2.4. Taguchi design of experiments A well planned set of experiments, in which all parameters of interest are varied over a specified range, is a much better approach to obtain systematic data. Mathematically speaking,
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such a complete set of experiments ought to give desired results. Usually the number of experiments and resources (materials and time) required are prohibitively large. In this study, the effects of four input process parameters (laser power, standoff distance, assist gas pressure and cutting speed) at three levels on the hole taper in laser trepan drilling of 2, 4, and 6 mm holes on two different polymers were investigated. The values of each level were chosen based on the preliminary experiments. The chosen laser powers were 200 W, 300 W and 500 W. The laser beam was focused down vertically along z-axis direction onto the workpiece with a standoff distance of 1.0 mm, 3.0 mm and 5.0 mm. The assist gas pressures were 2.0 bar, 2.5 bar and 3.0 bar. The cutting speeds chosen were 0.6 m/mm, 0.8 m/mm and 1.0 m/mm. The four input parameters of the laser trepan drilling with three levels are illustrated in Table 1. The Taguchi L9 orthogonal array design was used to conduct the experiments with four factors each at three levels and the measured data on hole taper for both the polymers is presented in Table 2.1 while the data on hole circularity of ABS polymer is presented in Table 2.2 Taguchi’s orthogonal arrays provide an alternative to standard factorial designs. The Taguchi method determines the optimum level of input process variables based on the statistical analysis of experimental results, making the process insensitive to the variations due to noise or uncontrollable factors such as ambient temperature and humidity of air surrounding the process. Hole taper in degree is calculated on the basis of the following equation: d d ð1Þ y ¼ tan1 entrance exit 2t where t is the thickness of the material, dentrance is the hole diameter at the entrance and dexit is the hole diameter at the exit and the dimensions are in mm. Fig. 3 shows measurements of hole taper while Fig. 4 shows drilled holes on two polymers. The hole diameters at the entrance and exit were measured at six different orientations at an interval of 301.
Table 1 Control factors and their levels used in the Taguchi design. Factors
Process parameter
Level 1
Level 2
Level 3
A B C D
Assist gas pressure (bar) Laser power (W) Cutting speed (m/min) Stand-off distance (mm)
2.0 200 0.6 1.0
2.5 300 0.8 3.0
3.0 500 1.0 5.0
Table 2.2 Taguchi design of experiment and response data hole circularity. Run Factor number
Hole circularity ABS hole (mm)
A B C D 2
1 2 3 4 5 6 7 8 9
1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3
1 2 3 2 3 1 3 1 2
1 2 3 3 1 2 2 3 1
Run number
Factor
Hole taper (deg.) ABS (mm)
1 2 3 4 5 6 7 8 9
PMMA (mm)
A
B
C
D
2
4
6
2
4
6
1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3
1 2 3 2 3 1 3 1 2
1 2 3 3 1 2 2 3 1
1.624 1.375 0.917 2.454 1.346 1.442 1.251 0.917 1.041
2.387 1.260 0.745 1.757 2.913 1.557 2.511 0.430 2.006
2.693 0.974 1.051 2.798 2.464 1.117 2.454 0.793 2.273
1.060 0.621 0.420 0.774 1.031 0.602 0.697 0.458 1.127
1.337 1.050 0.898 0.831 1.509 0.392 0.821 0.573 1.318
1.461 1.623 0.888 1.347 1.595 0.955 1.328 1.012 1.643
6
Entrance Exit
Entrance Exit
Entrance Exit
0.962 0.952 0.958 0.921 0.919 0.952 0.927 0.958 0.930
0.961 0.964 0.950 0.955 0.953 0.971 0.953 0.944 0.953
0.978 0.978 0.954 0.966 0.970 0.977 0.963 0.954 0.970
0.859 0.823 0.959 0.887 0.899 0.839 0.880 0.879 0.86
0.913 0.932 0.951 0.923 0.935 0.909 0.951 0.957 0.934
0.958 0.928 0.939 0.968 0.947 0.962 0.958 0.965 0.941
Fig. 3. Measurement of hole taper.
3. Experimental results and discussions The analysis is based on two approaches namely the (i) noise performance measure (NPM) and (ii) target performance measure (TPM). The NPM determines the effect of each variable on the output variability and in this case the output is the hole taper. To determine the effect of each variable on the output, the signal-tonoise (SN) ratio, or the SN number is calculated for each experiment conducted. For the case of minimizing the performance characteristics (hole taper in this case) the definition of SN ratio is SN ratio ¼ 10 log
Table 2.1 Taguchi design of experiment and response data on taper.
4
Ni X y2u N u¼1 i
ð2Þ
where i is the experiment number, u is the trial number and Ni is the number of trials for experiment i. The signal represents the desirable value while the noise represents the undesirable value and SN ratio expresses the scatter around the desired value. The TPM on the contrary measures mean response or output and identifies control factors that largely affect the mean response and not the variance. These factors are called target control factors that can be used to adjust the mean response to the target. The mean response is given by x¼
X xi Ni
ð3Þ
where xi is the response (either hole taper or hole circularity) for ith experiment and Ni is the number of trials for experiment i.
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Fig. 4. Drilled holes on (a) ABS and (b) PMMA polymers.
3.1. Hole taper on ABS polymer 3.1.1. Hole diameter equal to 2 mm Analysis of variance (ANOVA) was carried out and subsequently the effect of each control factor on the hole taper was ascertained. The MINITAB 14 software has been used to evaluate ANOVA. The contribution of each variable on the hole taper based on NPM and TPM is shown in Table 3. The optimum levels selected by two different approaches are not identical. From the ANOVA analysis, the percentage contributions of different control factors according to TPM and NPM in decreasing order are laser power (41.4%), compressed air pressure (39.9%), cutting speed (17.8%), stand-off distance (0.8%) and compressed air pressure (40.5%), laser power (36.2%), cutting speed (21.7%), stand-off distance (1.6%) respectively. The optimum level according to TPM analysis is A3B3C3D1 and according to NPM, it is A3B3C3D3. Fig. 5 shows the mean response at different levels on the basis of TPM measure. Thus according to TPM analysis, higher assist gas pressure, higher laser power, higher cutting speed, and lower stand-off distance produce more cylindrical holes or minimize hole taper. According to NPM analysis, higher air pressure, higher laser power, higher cutting speed, and higher standoff distance produce more cylindrical holes or minimize hole taper.
Table 3 TPM and NPM analyses with percentage contribution to the hole taper for 2 mm hole size drilled on ABS. TPM
A
B
C
D
Level 1 2 3 % contribution Selected level
1.305 1.748 1.070 39.96 3
1.776 1.213 1.133 41.45 3
1.327 1.624 1.171 17.79 3
1.337 1.356 1.429 0.80 1
NPM
A
B
C
D
Level 1 2 3 % contribution Selected level Optimum level
2.748 5.352 1.776 40.49 3 3
5.234 2.803 1.839 36.24 3 3
3.456 4.555 1.865 21.66 3 3
3.498 3.513 2.866 1.61 3 3
Assist gas pressure
1.7
Laser power
3.1.2. Hole diameter equal to 4 mm The contribution of each variable on the hole taper as obtained from the ANOVA analysis for TPM and NPM in decreasing order are stand-off distance (58.8%), laser power (20.2%), compressed air pressure (10.9%), cutting speed (10.2%), and stand-off distance (61.7%), laser power (18.4%), compressed air pressure (12.5%), cutting speed (7.4%), respectively. 3.1.3. Hole diameter equal to 6 mm The contribution of each variable on the hole taper as obtained from the ANOVA analysis for TPM and NPM in decreasing order are laser power (51.9%), stand-off distance (32.2%), compressed air pressure (8.3%), cutting speed (7.8%), and laser power (41.7%), stand-off distance (36.5%), cutting speed (13.4%), compressed air pressure (8.4%), respectively. The hole taper depends on the exit and entrance hole diameters and the thickness of the work material. The important finding here is that the stand-off distance has significant effect on the hole taper if the hole size is more than 2 mm. As the stand-off distance is related to the focal length of the laser beam, the effect of beam divergence is more pronounced when the hole size is larger. The present findings are in agreement with Ghoreishi et al. [3], who observed the effect of stand-off distance or equivalently the focal point position as the most significant on the hole taper in laser drilling of stainless steel 304 sheet of 2.5 mm thick. Irrespective of hole diameters, the laser power,
Mean response
1.6
Cutting speed
1.5
Stand-off distance
1.4 1.3 1.2 1.1 1
0
1
2
3
Factor level Fig. 5. TPM mean response at different levels for 2 mm hole drilled on ABS polymer.
assist gas pressure and cutting speed have significant effect on the hole taper. So to minimize taper, these three parameters have to be regulated and optimized. Similar observations were made by Mustafa et al. [18] in their investigation of CO2 laser cutting of Teflon (PTTE) and Delrin (POM). 3.2. Hole taper on PMMA polymer 3.2.1. Hole diameter equal to 2 mm The contribution of each variable on the hole taper based on NPM and TPM is shown in Table 4. The optimum levels selected
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Table 4 TPM and NPM analyses with percentage contribution to the hole taper for 2 mm hole size drilled on PMMA. TPM
A
B
C
Table 5 Optimum levels of four control factors. Hole size (mm)
Analysis
ABS polymer
PMMA polymer
2
TPM NPM TPM NPM TPM NPM
A3B3C3D1 A3B3C3D3 A1B3C1D3 A1B3C1D3 A1B2C1D2 A1B2C1D3
A1B2C1D3 A1B3C1D3 A3B3C1D2 A2B3C1D2 A2B3C1D3 A2B3C1D3
D
Level 1 2 3 % contribution Selected level
0.700 0.802 0.761 2.84 1
0.844 0.703 0.716 6.52 2
0.707 0.840 0.716 6.03 1
1.073 0.640 0.551 84.61 3
NPM
A
B
C
D
Level 1 2 3 % contribution Selected level
2.352 1.331 1.842 2.57 1
0.704 2.139 2.681 10.31 3
2.322 0.952 2.251 5.88 1
1.420 2.983 3.961 81.23 3
by two different approaches again are not identical. From the ANOVA analysis, the percentage contributions of different control factors according to TPM and NPM in decreasing order are standoff distance (84.6%), laser power (6.5%), cutting speed (6%), compressed air pressure (2.8%), and stand-off distance (81.2%), laser power (10.3%), cutting speed (5.9%), compressed air pressure (2.6%), respectively. The optimum level according to TPM analysis is A1B2C1D3 and according to NPM, it is A1B3C1D3. To produce more cylindrical holes, according to TPM analysis, lower air pressure, moderate laser power, lower cutting speed and higher stand-off distance are required. On the basis of NPM analysis, lower air pressure, higher laser power, lower cutting speed and higher stand-off distance are required. 3.2.2. Hole diameter equal to 4 mm The optimum level selected by two different approaches again is not identical. From the ANOVA analysis, the percentage contributions of different control factors according to TPM and NPM in decreasing order are stand-off distance (72.10%), cutting speed (16.94%), compressed air pressure (6.47%), laser power (4.50%), and stand-off distance (60.97%), cutting speed (25.25%), laser power (7.65%), compressed air pressure (6.13%), respectively.
4 6
A: Assist gas pressure; B: Laser power; C: Cutting speed; and D: Stand-off distance.
Table 6 ANOVA analysis on TPM measure. TPM
Factors
Single factor
Material Hole diameter A B C D SSEa SSTrb SSTc
a b c
DOF
% contribution
5.47 2.42 0.71 2.69 0.89 4.16
1 2 2 2 2 2
23.61 10.44 3.05 11.63 3.85 17.96
6.82 16.33 23.15
42 11 53
29.46 70.54 100.0
DOF
% contribution
Sum of squares of errors. Sum of squares of six factors. SSE þ SSTr.
Table 7 ANOVA analysis on NPM measure. NPM
Factors
SS
Single factor
Material Hole diameter A B C D
185.95 114.58 20.76 79.54 48.58 201.18
1 2 2 2 2 2
SSEa SSTrb SSTc
228.64 650.59 879.23
42 11 53
a
3.2.3. Hole diameter equal to 6 mm The contributions of each variable on the hole taper as obtained from the ANOVA analysis for TPM and NPM in decreasing order are stand-off distance (49.84%), cutting speed (34.46%), laser power (15.50%), compressed air pressure (0.21%), and standoff distance (39.92%), cutting speed (31.80%), laser power (26.10%), compressed air pressure (2.19%), respectively. Stand-off distance and cutting speed are the most significant parameters contributing about 80% that affect the hole taper. Hole taper arises as a result of varying diameter from the entrance hole to the exit hole. The effect of stand-off distance on taper increases as the hole diameter decreases. The observed values of the effect of stand-off distance are 84.6% for 2 mm hole, 72.1% for 4 mm hole and 49.8% for 6 mm hole. Again these results are in agreement with previous findings [3,19]. With the same heat input, considering that the volume of material removal rate remains the same, there will be more evaporation of material along the radial direction of the hole and this has caused increased taper for lower diameter. The optimum levels based on the two performance measures for different hole sizes and different materials are given in Table 5. Since for different drilling diameters and materials the optimum levels are different, the types of material and drilling diameter are expected to have some effect on the hole taper.
SS
b c
21.15 13.03 2.36 9.05 5.53 22.88 26.0 74.0 100.0
Sum of squares of errors. Sum of squares of six factors. SSE þ SSTr.
The next analysis known as blocking is conducted to ascertain the effect of material and drilling diameters on the hole taper. In this investigation the concept of blocking from design of experiment is combined with the orthogonal array. Having analyzed the data, the contributions of four control factors along with the drilling diameter and materials have been determined from ANOVA analysis and are presented in Tables 6 and 7. These tables show the contribution of the factors on hole taper on the basis of two performance measures. From the TPM analysis, material has the most significant effect (23.61%) followed by stand-off distance (17.96%), laser power (10.44%), and hole diameter (10.44%). From the NPM analysis, stand-off distance has the most significant effect (22.88%), followed by material (21.15%), hole diameter (13.03%), and laser power (9.05%). ABS polymer has good impact and mechanical strength and is resistant to heat distortion. The material can be considered as a blend of glassy copolymer and rubbery polymer or copolymer. PMMA is a hard and rigid thermoplastic with outstanding light transmission properties. Material properties can affect the hole size both at the entrance and exit. The absorption and transmission of
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factors at different levels on quality characteristics like SN ratio and mean response are presented in Figs. 7 and 8 respectively.
laser beam and thus heating and evaporation of the work material will vary. As PMMA possesses outstanding light transmissibility, the effect of laser power on taper is less compared to that in ABS polymer. Fig. 6 shows percentage contribution of different factors on the hole taper. Interestingly, the hole taper is material dependent more than other factors. Among the four control factors, stand-off distance and laser power have significant effect on the taper. The hole diameter also has effect on the hole taper. The stand-off distance is related to the focal length of the laser beam and its position relative to the hole entry has an effect on the hole taper. On the basis of TPM analysis given in Table 8, ranking of various factors influencing the hole taper in decreasing order is material, stand-off distance, laser power, hole diameter, cutting speed, and assist gas pressure. While on the basis of NPM analysis given in Table 9, ranking of various factors influencing the hole taper in decreasing order is stand-off distance, material, hole diameter, laser power, cutting speed, and assist gas pressure. In either case, assist gas pressure is the least significant on hole taper. From these two optimum levels, a combined optimum level is determined and is shown in Table 10. The least taper is obtained from PMMA, 2 mm hole diameter, assist gas pressure of 2.0 bar, laser power of 500 W, cutting speed of 1.0 mm/s and stand-off distance of 5.0 mm. The graphical representations of the effect of
3.3. Hole circularity 3.3.1. ABS polymer The ratio of minimum to maximum hole diameter either at the exit or at the entrance is used to measure the hole circularity. Both the entrance and exit holes are reasonably circular for all diameters investigated. As the circularity is defined as the ratio of the minimum diameter to maximum diameter, an ideal condition of circularity would be 1.0 or 100% for a truly circular hole. However, the range of circularity achieved is between 85% and 96% for various experimental conditions. From the plots of hole circularity (Fig. 9), it is observed that the entrance hole is more circular than the exit hole in the range of investigated parameters. As the laser beam diverges from the focal point and the distance of entry plane of the work material is always more than focal length of the beam, the divergence is even more at the hole exit Table 10 Combined optimum level for different factors. Factor
25.00 TPM
% contribution
20.00
1303
Material Diameter A B C D
NPM
15.00
NPM
TPM
Optimum level
% contribution
Level
C (%)
Level
21.15 13.03 2.36 9.05 5.53 22.88
2 1 3 3 1 3
23.61 10.44 3.05 11.63 3.85 17.96
2 1 1 3 1 3
2 1 1 3 1 3
10.00 5.00
0 Material Diameter Assist gas pressure Laser power Cutting speed Stand-off distance
0.00 Material
D
Diameter
B
C
-1
A
Control factor -2 SN ratio
Fig. 6. Effect of different control factors on hole taper.
Table 8 Levels selection based on TPM.
-3
-4 Levels
Material
Diameter
A
B
C
D
1 2 3 SS % contribution Rank
1.65 1.014 5.4653 23.61 1
1.064 1.35 1.581 2.4166 10.44 4
1.244 1.493 1.258 0.7073 3.05 6
1.644 1.219 1.133 2.6917 11.63 3
1.156 1.459 1.38 0.8912 3.85 5
1.713 1.224 1.059 4.1589 17.96 2
Selected level
2
1
1
3
1
3
-5
-6 0
1
2 Factor level
3
4
Fig. 7. SN ratios of various factors at three levels.
Table 9 Levels selection based on NPM. Levels
Material
Diameter
A
B
C
D
1 2 3 SS % contribution Rank
4.2751 0.5638 185.95 21.15 2
0.7254 2.2514 4.2816 114.579 13.03 3
2.0454 3.2934 1.9196 20.763 2.36 6
4.0889 1.93 1.2395 79.542 9.05 4
1.2137 3.5314 2.5132 48.582 5.53 5
4.9752 1.9719 0.3113 201.18 22.88 1
Selected level
2
1
3
3
1
3
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2
Entrance circularity
1.8 1.6
Exit circularity
Mean response
1.4 1.2 Material
1
Diameter
0.8
Assist gas pressure
0.6
Cutting speed
0.2
Stand-off distance
0
Material: ABS
Laser power
0.4
0
1
2
3
2 mm hole
4
Factor level Fig. 8. Mean response of various factors at three levels.
than at the entrance. This resulted in a more uneven hole at the exit. Generally, decreasing the assist gas pressure and stand-off distance increases the hole circularity. Masmati and Philip [19] observed a similar behavior where with the decrease of stand-off distance and assist gas pressure, circularity increased at the hole entry. At the entrance, assist gas pressure and cutting speed significantly affect the entrance hole circularity while the effects of laser power and stand-off distance are not significant. The effect of stand-off distance is more significant on the exit circularity. It is also noticed that with increase of the cutting speed, the circularity of hole increases at the entrance but decreases at the exit hole. As the hole diameter increases from 2 mm to 6 mm, the holes become more circular both at the entrance and exit. 3.3.2. PMMA polymer Fig. 10 shows the plots of hole circularity at three levels of laser parameters and at different diameters. It is seen that the hole is more circular at the exit than at the entrance and this agrees with findings made by Ghoreishi et al. [3]. The energy of laser beam was high at the entrance than at the exit. This trend is quite opposite of that observed in ABS polymer, where the entrance hole was more circular. ABS is more resistant to heat distortion compared to PMMA and as PMMA has better transmissivity, beam energy was lower at the exit, resulting in more circularity at the exit. Compressed air pressure and laser power have highly significant effects. At higher laser power, energy concentration is higher, resulting in high heat and poor circularity both at the entrance and exit. Excessive energy will lead to poor circularity as the laser beam breaks through the work piece and polymer melt exiting from the hole exit. On the other hand, higher compressed air pressure is needed to remove the molten polymer from the hole and provides more circularity to the hole. Again with the increase of hole diameter, the holes at the entrance and exit are more circular. Compared to ABS polymer, PMMA produced holes of lower circularity.
Entrance circularity
Material: ABS
Entrance circularity
Material: ABS
In drilling ABS polymer, the effect of stand-off distance and laser power on the hole taper was found to be the most significant for larger hole sizes (4 mm and 6 mm). These two
4 mm hole
Exit circularity
6 mm hole
Fig. 9. Hole circularity at the entrance and exit in ABS sheet for various hole diameters: (a) 2 mm, (b) 4 mm, and (c) 6 mm.
4. Conclusions On the basis of experimental results, following conclusions are drawn:
Exit circularity
input variables contributed 80% towards the taper while the two other variables (cutting speed and assist gas pressure) contributed 20% towards the taper. However when the hole size was 2 mm, assist gas pressure and laser power contributed about 80% towards the taper. Different drilling diameters and materials resulted in different optimum combinations of control factors. When a 2 mm diameter hole was drilled on ABS polymer, the optimum combination of control factors based on TPM analysis was air pressure¼ 3 bar, laser power¼500 W, cutting speed¼1 m/min, and stand-off distance¼1 mm, and that on NPM analysis was air
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Exit circularity
Entrance circularity
Material: PMMA
2 mm hole
Exit circularity
4 mm hole
Exit circularity Entrance circularity
Material: PMMA
6 mm hole
Fig. 10. Hole circularity at the entrance and exit in PMMA sheet for various hole diameters: (a) 2 mm, (b) 4 mm, and (c) 6 mm.
TPM analysis was air pressure¼2 bar, laser power¼ 300 W, cutting speed ¼0.6 m/min, and stand-off distance¼5 mm, and that on NPM analysis was air pressure ¼3 bar, laser power¼ 500 W, cutting speed¼0.6 m/min, and stand-off distance¼5 mm. Considering the four control factors along with the variability in material and hole size, the TPM results showed that material has 23.6% effect on the hole taper, followed by stand-off distance (17.9%), laser power (11.6%), and hole diameter (10.4%). On the contrary, the NPM results showed that stand-off distance has 22.9% effect on the hole taper followed by material (21.1%), hole diameter (13%), and laser power (9.1%). In drilling PMMA polymer, the hole appears to be more circular at the exit than at the entrance. While in drilling ABS polymer, the holes were found to be more circular at the entrance rather than at the exit. Overall, the holes were more circular in ABS sheet (85–96%) compared to PMMA sheet (80%).
References
Entrance circularity
Material: PMMA
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pressure¼3 bar, laser power¼ 500 W, cutting speed¼1 m/min, and stand-off distance¼5 mm. In drilling PMMA polymer, when the hole size was 2 mm, the effect of stand-off distance on the hole taper was about 80% while in drilling holes of 4 mm and 6 mm, the effects of cutting speed and laser power on the taper were almost identical. The stand-off distance alone contributed about 60% towards the taper. The optimum combination of control factors based on
[1] Dubey AK, Yadava V. Laser beam machining—a review. Int J of Mach Tools Manuf 2008;48:609–28. [2] Rajurkar KP, Levy G, Malshe A, Sundaram MM, McGeough J, Hu X, Resnick R, DeSilva A. Micro- and nano-machining by electro-physical and chemical processes. Ann CIRP 2002;55(2):643–66. [3] Ghoreishi M, Low DKY, Li L. Comparative statistical analysis of hole taper and circularity in laser percussion drilling. Int J Mach Tools Manuf 2002;42(9):985–95. [4] Eltawahni HA, Olabi AG, Benyounis KY. Effect of process parameters and optimization of CO2 laser cutting of ultra high-performance polyethylene. Mater Des 2010;31:4029–38. [5] Benyounis KY, Olabi AG. Optimization of different welding processes using statistical and numerical approaches—a reference guide. Adv Eng Software 2008;39:483–96. [6] Yilbas BS. Parametric study to improve laser hole drilling process. J Mater Process Technol 1997;70:264–73. [7] Yilbas BS, Kar A. Thermal and efficiency analysis of the CO2 laser cutting process. J Opt Lasers Eng 1998;30:93–106. [8] Yilbas BS. Laser cutting quality assessment and thermal efficiency analysis. J Mater Process Technol 2004;155–156:106–2115. [9] Choudhury IA, Shirley S. Laser cutting of polymeric materials: an experimental investigation. J Opt Laser Technol 2010;42:503–8. [10] French PW, Hand DP, Peters C, Shannon GJ, Byrd P, Watkins K, Steen WM. Investigation of Nd:YAG laser percussion drilling process using factorial experimental design. In: Proceedings of the ICALEO 99, Section C, 1999, p. 51–60. [11] Kamalu J, Byrd JP. Statistical design of laser drilling experiments. In: Proceedings of the ICALEO 98, Section B 1998. p. 264–273. [12] Negarestani R, Sundar M, Sheikh MA, Mativenga P, Li L, Li ZL, Chu PL, Khin CC, Zheng HY, Lim G C. Numerical simulation of laser machining of carbon-fibrereinforced composites. Proc IMechE Part B: J Eng Manuf 2010;224:1017–27. [13] Low DKY, Li L, Byrd PJ. Taper formation and control during laser drilling in Nimonic 263 alloy. In: Proceedings of the 33rd international MATADOR conference, Manchester, 2000. p. 461–6. [14] Low DKY, Li L, Byrd PJ. Influence of temporal pulse train modulation on material ejection related process. J Opt Lasers Eng 2001;35(2001):149–64. [15] Ng GKL, Li L. The effect of laser peak power and pulse width on the hole geometry repeatability in laser percussion drilling. J Opt Laser Technol 2001;33:393–402. [16] Murray AJ, Tyrer JR. Low taper laser drilling of 8:26 mm thick ZrO2. J Lasers Eng 1998;6:273–89. [17] Smith WF. Foundations of material science and engineering. 3rd ed.McGraw Hill; 2004 p. 318–321. [18] Mustafa K, Yusuf K, Bagci E, Demirer H, Mahir K. Dimensional analyses and surface quality of the laser cutting process for engineering plastics. Int J Adv Manuf Technol 2009;41:259–67. [19] Masmati N, Philip PK. Investigations on laser percussion drilling of some thermoplastic polymers. J Mater Process Technol 2007;185:198–203.
Contents lists available at SciVerse ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Hole qualities in laser trepanning of polymeric materials I.A. Choudhury n, W.C. Chong, G. Vahid Department of Engineering Design and Manufacture, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e i n f o
abstract
Article history: Received 22 September 2011 Received in revised form 23 February 2012 Accepted 29 February 2012 Available online 29 March 2012
The present study focuses the effect of four input controllable laser cutting variables on the hole taper and hole circularity in laser trepan drilling of polymeric materials. Experiments have been conducted on acrylonitrile butadiene styrene (ABS) and polymethyl methacrylate (PMMA) polymer sheets. Laser power, assist gas pressure, cutting speed and stand-off distance were selected as independent process variables. Three different holes of diameters 2 mm, 4 mm and 6 mm were drilled in these work materials of 5 mm thickness. A Taguchi L9 orthogonal array with four factors and three levels of each factor was used to plan and conduct the experiments in order to obtain required information with reduced number of experiments. The process performance was ascertained in terms of hole taper and hole circularity. Initial analysis involved in determining the effect of the four process variables on hole taper and circularity for these two polymers at three different hole diameters. From ANOVA analysis, the optimum levels of the four process variables with respect to materials and hole diameters were evaluated. As it was found that the optimum levels of four process variables were different for different hole size and materials, additional analysis was conducted to incorporate the effect of material and hole diameter on the hole taper. From the analysis, the optimum combinations were obtained at compressed air pressure of 2.0 bar, laser power of 500 W, cutting speed of 0.6 m/min, stand-off distance of 5.0 mm, hole diameter of 2.0 mm and material of PMMA. These combinations produced the minimum taper in the hole. The circularity of the hole was more at the entrance than the exit when ABS polymer was laser drilled while in PMMA, the hole was more circular at the exit than the entrance. & 2012 Elsevier Ltd. All rights reserved.
Keywords: CO2 laser Hole taper Laser drilling Design of experiment Polymeric materials
1. Introduction The laser drilling process is one of the most widely used thermal energy based non-contact type advance machining process which can be applied across a wide range of materials. Nowadays laser drilling is finding increasingly widespread application in the industries. Laser beam machining is based on the conversion of electrical energy into light energy and then into thermal energy to remove the material from work piece. The material removal process is by focusing laser beam onto the work material for melting and vaporizing the unwanted material to create a hole. CO2 laser drilling has been widely used in industries because of its high production rate and abilities on rapidly varying holes size, drilling holes at shallow angle, and drilling hard-to-work material such as high strength materials, ceramic and composite. Laser drilled holes are inherently associated with a number of defects. Circularity of hole, spatter thickness, and hole taper are some defects associated with laser drilling. As a result, the quality
n
Corresponding author. Tel.: þ60 379675384, fax: þ60 379675330. E-mail addresses: [email protected] (I.A. Choudhury), [email protected] (W.C. Chong), [email protected] (G. Vahid). 0143-8166/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlaseng.2012.02.017
of the drilled holes is the main issue in the laser drilling process. There are two types of laser drilling: trepan drilling and percussion drilling. Trepan drilling involves cutting around the circumference of the hole to be generated, whereas percussion drilling is carried out by utilizing a focused laser spot to heat, melt and vaporize the target material such that a desired hole is formed through the work piece with no relative movement of the laser or work material [1,2]. Fig. 1 shows a schematic of laser beam drilling [2]. Ghoreishi et al. [3] investigated the relationships and parameter interactions between laser peak power, laser pulse width, pulse frequency, number of pulses, assist gas pressure and focal plane position on the hole taper and circularity in laser percussion drilling of stainless steel and mild steel. The central composite design was employed to plan the experiments in order to obtain required information. The process performance was evaluated in terms of equivalent entrance diameter, hole taper and hole entrance circularity. They found that the pulse frequency had a significant effect on the hole entrance diameter and hole circularity in drilling stainless steel unlike the drilling of mild steel, where the pulse frequency had no significant effect on the hole characteristics. Eltawahni et al. [4] investigated the effect of laser power, cutting speed, and focal point distance on the cutting edge quality parameters in CO2 laser cutting of ultra-high performance
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Laser Head
Laser Beam
Laser Beam Workpiece Workpiece
End Start Laser Path Hole Diameter
Phase Change (Vaporization) Phase Change (Melting)
Plasma Formation Erosion Front Molten Layer
Fig. 1. Schematic of laser beam: (a) trepan drilling and (b) percussion drilling [1].
polyethylene of various thickness from 6 mm to 10 mm. They conducted the experiments by implementing the Box–Behnken design and measured upper kerf, lower kerf, ratio of upper kerf to lower kerf and cut edge surface roughness. They developed model equations relating the input processing parameters with the output or response they had measured. They found that the upper kerf decreased as the focal position and the cutting speed increased, and it increased as the laser power increased. The roughness decreased as the focal point increased from its lowest level till its central level and then it increased as the focal distance increased above its central level. The roughness decreased as the laser power increased and it increased as the cutting speed increased. Higher cutting speed did not always improve the efficiency of the laser cutting process. Benyounis and Olabi [5] did a comprehensive literature review of the applications of design of experiments, evolutionary algorithms and computational networks on the optimization of different welding processes through mathematical models. According to their review of various literatures, they were of the opinion that there was considerable interest among the researchers in the adaption of response surface methodology (RSM) and artificial neural network (ANN) to predict responses in the welding process. For a smaller number of experimental runs, they noted that RSM was better than ANN and genetic algorithm (GA) in the case of low order non-linear behavior of the response data. In the case of highly non-linear behavior of the response data, ANN was better than other techniques. They also observed that the Taguchi approach of SN ratio might lead to non-optimal solutions with less flexibility and the conducting of needless experiments. Yilbas [6] studied the effect of the laser parameters and the material properties on the hole quality in laser hole drilling. A statistical approach, referred to as factorial design, was used to test the significance level of the factors that affect the hole quality. Three materials, stainless steel, nickel and titanium, were considered. The experimental study yields tables of significance of each factor on the aspects that determine the quality of the holes. The parameter which was found to be very significant in most cases was the workpiece thickness. The first-order interaction of pulse length–thickness was the most significant, whilst pulse length–focus setting was significant for all of the materials examined. For the second-order interactions, only pulse length– focus setting–thickness was found to be significant. Yilbas and Kar [7] experimentally conducted thermal and efficiency analysis of the CO2 laser cutting process to cut mild-steel samples of 0.8–2.0 mm thicknesses. They also considered the effects of
momentum and gas–liquid interface shear stress due to an assisting gas jet. The approximate magnitude of the heat absorbed was estimated and the thickness of the melting layer was predicted. The theoretical predictions were compared with the experimental findings. The liquid-layer thickness measured experimentally was found to be in good agreement with the theoretical predictions. The variation of first and second law efficiencies with jet velocity for different cutting speeds showed increasing trend with increasing jet velocities and cutting speeds. Yilbas [8] examined the laser gas assisted cutting process. A statistical method based on factorial analysis was used to identify the influence of cutting parameters on the resulting cut quality. It was found that increasing laser beam scanning speed reduces the kerf width while with the increase of laser power, kerf width increases. The main effects of all the parameters employed have significant influence on the resulting cutting quality. Choudhury and Shirley [9] investigated laser cutting qualities of polypropylene (PP), polycarbonate (PC) and polymethyl methacrylate (PMMA) with a view to evaluate the effect of the main input laser cutting parameters (laser power, cutting speed and compressed air pressure) and develop model equations relating input process parameters with the output. The experiments were carried out according to the central composite first-order design based on response surface methodology. The output quality characteristics examined were heat affected zone (HAZ), surface roughness and dimensional accuracy. It was found that the response was well modeled by a linear function of the input parameters. From the analysis, it has been observed that PMMA has less HAZ, followed by PC and PP. For surface roughness, PMMA has better cut edge surface quality than PP and PC. However, all three polymeric materials showed similar diameter errors tendency in spite of different material properties. French et al. [10] investigated the effects of seventeen factors at two levels and their first-order and second-order interactions on the hole taper and circularity in Nd:YAG laser percussion drilling. They found that pulse shape, energy, peak power, focal position, gas pressure and Nd:YAG laser rod were the most effective factors affecting the hole taper and circularity. Kamalu and Byrd [11] studied the effects of focal length of lens, position of the focal plane with respect to the material surface and laser energy on laser drilling performance. They measured drilling performance by diameter of laser-drilled holes. The three factors were studied at two levels and two sets of 23 factorial designs were analyzed. Negarestani et al. [12] developed a 3D model for simulating the heat flow and material removal rate in laser machining of
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carbon-fiber-reinforced (CFRP) composites on a heterogeneous fiber-matrix mesh. They validated the model with experimental results and found the simulation results to be in agreement with the experiments. HAZ was predicted to be more sensitive to speeds in the lower range of 50–200 mm/s as compared with higher range of 200–800 mm/s. Some recent attempts have been made to control the laser drilled hole taper through the development of drilling techniques [13,14]. Ng and Li [15] assessed the effect of laser peak power and pulse width on the hole geometry repeatability in Nd:YAG laser percussion drilling of 2 mm thick mild steel sheets. Thirty-five holes were drilled and analyzed for each set of identical laser parameters. They found that higher peak power and shorter pulse width gave better hole geometry repeatability. The circularity of the entrance hole ranged from 0.94 to 0.87, and was found to correlate with repeatability. Murray and Tyrer [16] conducted an experimental investigation using CO2 laser in drilling of ceramic samples. They optimized the process to provide the best hole geometry CO2 laser, which exhibited low level of micro-cracking and low taper. Besides, experimental investigations have been carried out to find the optimum ranges of parameters for the drilling of polymers using a CO2 laser. Most of the previous works related to hole drilling used the percussion drilling process where with intense laser burst, the hole size was the size of the beam that was varied by focusing. The present study focuses on the alternative trepan drilling. This experimental investigation of laser drilling of 2 mm, 4 mm, and 6 mm holes on two polymeric materials lead to the assessment of the hole quality in terms of hole taper and hole circularity. This has been done to provide the potential future application of laser drilling in the industries with valuable information on optimum controlled parameters to improve the quality of the laser drilling parts. The use of laser in drilling holes is justified only if the quality of the end product is decisively better and if the process becomes more reliable. The Ishikawa cause-and-effect diagram illustrating the relationship of the process parameters with the quality of hole produced is shown in Fig. 2. Preliminary experiments indicated that, out of several process parameters, laser power, cutting speed, stand-off distance, and assist gas pressure greatly influence the quality of the holes. The Taguchi design of experiment has been adopted to conduct experiments as it involves reducing the variation in a process through robust design of experiments. The overall objective of the method is to produce high quality products at low cost to the manufacturer. The experimental design uses orthogonal arrays to organize the parameters affecting the process and the levels at which they should be varied. Instead of having to test all possible combinations like the factorial design, the Taguchi method tests pairs of combinations. This allows for the collection of the necessary data to determine which factors most affect product quality with a
Cutting speed
Materials
Laser Type
Thickness
Laser type Power
Hole diameter Flow rate Humidity Environment
Gas type
Standoff distance
Pressure Assist gas
Size
Quality of cut: • Hole circularity • Hole taper
Nozzle
Fig. 2. Cause-and-effect-diagram of laser drilling process affecting the quality of cut.
1299
minimum number of experimentation, thus saving time and resources. The Taguchi method is best used when there is intermediate number of variables (3–50), few interactions between variables, and when only a few variables contribute significantly. As the preliminary investigations suggested that four input variables affect the hole quality in terms of taper and circularity, the use of Taguchi method is justified in the present investigation.
2. Experimental work 2.1. Experimental setup All experiments were conducted in a Zech Laser Cutting Machine. The machine had a continuous, constant and steady CO2 laser power of 500 W (CW mode) with 10.6 mm wavelength. The system consisted of two parts, which were the laser cutting workstation ZL1010, and the laser beam generation system ZLX5. The laser machine was connected to a computer system as a controlling mode of operation. It consisted of three softwares, namely: (i) AutoCAD Release 14 software to design a 2D drawing of the specimen for the project, (ii) C-Cut software which was the laser cutting software for machine’s numerical control and (iii) Zech Laser software as an automatic machining mode software. A three axes CNC-controlled table with work volume 1 m 1 m 0.1 m was used for drilling. 2.2. Work material Acrylonitrile Butadiene Styrene (ABS) is a common thermoplastic used to make light, rigid, and molded products. The most important mechanical properties of ABS are impact resistance and toughness. Generally, it would have useful characteristics within a temperature range from40 1C to 100 1C. Acrylonitrile contributes heat and chemical resistance and toughness, butadiene provides impact strength and low property retention, and styrene offers surface gloss, rigidity, and ease of processing. The major use is for pipe and fittings, automotive parts, refrigerator door liners, computer housings and covers, telephone housings, and electrical conduit. Polymethyl methacrylate (PMMA) is a transparent material often used as a light or shatter-resistant alternative to glass. It is a hard and rigid transparent thermoplastic that has good outdoor weatherability and is more impact resistant than glass. It has good tensile and flexural strengths. The impact resistance is ten times higher than that of glass. PMMA has the highest surface hardness of all common thermoplastics. It is used for glazing aircrafts and boats, skylights, exterior lighting, safety shields, protective goggles and knobs and handles [17]. 2.3. Preliminary experiments The input laser processing parameters like laser power, standoff distance, assists gas pressure and cutting speed have an effect on the hole quality and hole taper. In order to determine the range of these variables affecting the taper, some preliminary experimental investigations were conducted. The data of these preliminary experiments are not reported here. Having obtained the range of variables and the factors affecting most the hole taper from these experiments, a design based on the Taguchi method has been employed. 2.4. Taguchi design of experiments A well planned set of experiments, in which all parameters of interest are varied over a specified range, is a much better approach to obtain systematic data. Mathematically speaking,
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such a complete set of experiments ought to give desired results. Usually the number of experiments and resources (materials and time) required are prohibitively large. In this study, the effects of four input process parameters (laser power, standoff distance, assist gas pressure and cutting speed) at three levels on the hole taper in laser trepan drilling of 2, 4, and 6 mm holes on two different polymers were investigated. The values of each level were chosen based on the preliminary experiments. The chosen laser powers were 200 W, 300 W and 500 W. The laser beam was focused down vertically along z-axis direction onto the workpiece with a standoff distance of 1.0 mm, 3.0 mm and 5.0 mm. The assist gas pressures were 2.0 bar, 2.5 bar and 3.0 bar. The cutting speeds chosen were 0.6 m/mm, 0.8 m/mm and 1.0 m/mm. The four input parameters of the laser trepan drilling with three levels are illustrated in Table 1. The Taguchi L9 orthogonal array design was used to conduct the experiments with four factors each at three levels and the measured data on hole taper for both the polymers is presented in Table 2.1 while the data on hole circularity of ABS polymer is presented in Table 2.2 Taguchi’s orthogonal arrays provide an alternative to standard factorial designs. The Taguchi method determines the optimum level of input process variables based on the statistical analysis of experimental results, making the process insensitive to the variations due to noise or uncontrollable factors such as ambient temperature and humidity of air surrounding the process. Hole taper in degree is calculated on the basis of the following equation: d d ð1Þ y ¼ tan1 entrance exit 2t where t is the thickness of the material, dentrance is the hole diameter at the entrance and dexit is the hole diameter at the exit and the dimensions are in mm. Fig. 3 shows measurements of hole taper while Fig. 4 shows drilled holes on two polymers. The hole diameters at the entrance and exit were measured at six different orientations at an interval of 301.
Table 1 Control factors and their levels used in the Taguchi design. Factors
Process parameter
Level 1
Level 2
Level 3
A B C D
Assist gas pressure (bar) Laser power (W) Cutting speed (m/min) Stand-off distance (mm)
2.0 200 0.6 1.0
2.5 300 0.8 3.0
3.0 500 1.0 5.0
Table 2.2 Taguchi design of experiment and response data hole circularity. Run Factor number
Hole circularity ABS hole (mm)
A B C D 2
1 2 3 4 5 6 7 8 9
1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3
1 2 3 2 3 1 3 1 2
1 2 3 3 1 2 2 3 1
Run number
Factor
Hole taper (deg.) ABS (mm)
1 2 3 4 5 6 7 8 9
PMMA (mm)
A
B
C
D
2
4
6
2
4
6
1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3
1 2 3 2 3 1 3 1 2
1 2 3 3 1 2 2 3 1
1.624 1.375 0.917 2.454 1.346 1.442 1.251 0.917 1.041
2.387 1.260 0.745 1.757 2.913 1.557 2.511 0.430 2.006
2.693 0.974 1.051 2.798 2.464 1.117 2.454 0.793 2.273
1.060 0.621 0.420 0.774 1.031 0.602 0.697 0.458 1.127
1.337 1.050 0.898 0.831 1.509 0.392 0.821 0.573 1.318
1.461 1.623 0.888 1.347 1.595 0.955 1.328 1.012 1.643
6
Entrance Exit
Entrance Exit
Entrance Exit
0.962 0.952 0.958 0.921 0.919 0.952 0.927 0.958 0.930
0.961 0.964 0.950 0.955 0.953 0.971 0.953 0.944 0.953
0.978 0.978 0.954 0.966 0.970 0.977 0.963 0.954 0.970
0.859 0.823 0.959 0.887 0.899 0.839 0.880 0.879 0.86
0.913 0.932 0.951 0.923 0.935 0.909 0.951 0.957 0.934
0.958 0.928 0.939 0.968 0.947 0.962 0.958 0.965 0.941
Fig. 3. Measurement of hole taper.
3. Experimental results and discussions The analysis is based on two approaches namely the (i) noise performance measure (NPM) and (ii) target performance measure (TPM). The NPM determines the effect of each variable on the output variability and in this case the output is the hole taper. To determine the effect of each variable on the output, the signal-tonoise (SN) ratio, or the SN number is calculated for each experiment conducted. For the case of minimizing the performance characteristics (hole taper in this case) the definition of SN ratio is SN ratio ¼ 10 log
Table 2.1 Taguchi design of experiment and response data on taper.
4
Ni X y2u N u¼1 i
ð2Þ
where i is the experiment number, u is the trial number and Ni is the number of trials for experiment i. The signal represents the desirable value while the noise represents the undesirable value and SN ratio expresses the scatter around the desired value. The TPM on the contrary measures mean response or output and identifies control factors that largely affect the mean response and not the variance. These factors are called target control factors that can be used to adjust the mean response to the target. The mean response is given by x¼
X xi Ni
ð3Þ
where xi is the response (either hole taper or hole circularity) for ith experiment and Ni is the number of trials for experiment i.
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1301
Fig. 4. Drilled holes on (a) ABS and (b) PMMA polymers.
3.1. Hole taper on ABS polymer 3.1.1. Hole diameter equal to 2 mm Analysis of variance (ANOVA) was carried out and subsequently the effect of each control factor on the hole taper was ascertained. The MINITAB 14 software has been used to evaluate ANOVA. The contribution of each variable on the hole taper based on NPM and TPM is shown in Table 3. The optimum levels selected by two different approaches are not identical. From the ANOVA analysis, the percentage contributions of different control factors according to TPM and NPM in decreasing order are laser power (41.4%), compressed air pressure (39.9%), cutting speed (17.8%), stand-off distance (0.8%) and compressed air pressure (40.5%), laser power (36.2%), cutting speed (21.7%), stand-off distance (1.6%) respectively. The optimum level according to TPM analysis is A3B3C3D1 and according to NPM, it is A3B3C3D3. Fig. 5 shows the mean response at different levels on the basis of TPM measure. Thus according to TPM analysis, higher assist gas pressure, higher laser power, higher cutting speed, and lower stand-off distance produce more cylindrical holes or minimize hole taper. According to NPM analysis, higher air pressure, higher laser power, higher cutting speed, and higher standoff distance produce more cylindrical holes or minimize hole taper.
Table 3 TPM and NPM analyses with percentage contribution to the hole taper for 2 mm hole size drilled on ABS. TPM
A
B
C
D
Level 1 2 3 % contribution Selected level
1.305 1.748 1.070 39.96 3
1.776 1.213 1.133 41.45 3
1.327 1.624 1.171 17.79 3
1.337 1.356 1.429 0.80 1
NPM
A
B
C
D
Level 1 2 3 % contribution Selected level Optimum level
2.748 5.352 1.776 40.49 3 3
5.234 2.803 1.839 36.24 3 3
3.456 4.555 1.865 21.66 3 3
3.498 3.513 2.866 1.61 3 3
Assist gas pressure
1.7
Laser power
3.1.2. Hole diameter equal to 4 mm The contribution of each variable on the hole taper as obtained from the ANOVA analysis for TPM and NPM in decreasing order are stand-off distance (58.8%), laser power (20.2%), compressed air pressure (10.9%), cutting speed (10.2%), and stand-off distance (61.7%), laser power (18.4%), compressed air pressure (12.5%), cutting speed (7.4%), respectively. 3.1.3. Hole diameter equal to 6 mm The contribution of each variable on the hole taper as obtained from the ANOVA analysis for TPM and NPM in decreasing order are laser power (51.9%), stand-off distance (32.2%), compressed air pressure (8.3%), cutting speed (7.8%), and laser power (41.7%), stand-off distance (36.5%), cutting speed (13.4%), compressed air pressure (8.4%), respectively. The hole taper depends on the exit and entrance hole diameters and the thickness of the work material. The important finding here is that the stand-off distance has significant effect on the hole taper if the hole size is more than 2 mm. As the stand-off distance is related to the focal length of the laser beam, the effect of beam divergence is more pronounced when the hole size is larger. The present findings are in agreement with Ghoreishi et al. [3], who observed the effect of stand-off distance or equivalently the focal point position as the most significant on the hole taper in laser drilling of stainless steel 304 sheet of 2.5 mm thick. Irrespective of hole diameters, the laser power,
Mean response
1.6
Cutting speed
1.5
Stand-off distance
1.4 1.3 1.2 1.1 1
0
1
2
3
Factor level Fig. 5. TPM mean response at different levels for 2 mm hole drilled on ABS polymer.
assist gas pressure and cutting speed have significant effect on the hole taper. So to minimize taper, these three parameters have to be regulated and optimized. Similar observations were made by Mustafa et al. [18] in their investigation of CO2 laser cutting of Teflon (PTTE) and Delrin (POM). 3.2. Hole taper on PMMA polymer 3.2.1. Hole diameter equal to 2 mm The contribution of each variable on the hole taper based on NPM and TPM is shown in Table 4. The optimum levels selected
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Table 4 TPM and NPM analyses with percentage contribution to the hole taper for 2 mm hole size drilled on PMMA. TPM
A
B
C
Table 5 Optimum levels of four control factors. Hole size (mm)
Analysis
ABS polymer
PMMA polymer
2
TPM NPM TPM NPM TPM NPM
A3B3C3D1 A3B3C3D3 A1B3C1D3 A1B3C1D3 A1B2C1D2 A1B2C1D3
A1B2C1D3 A1B3C1D3 A3B3C1D2 A2B3C1D2 A2B3C1D3 A2B3C1D3
D
Level 1 2 3 % contribution Selected level
0.700 0.802 0.761 2.84 1
0.844 0.703 0.716 6.52 2
0.707 0.840 0.716 6.03 1
1.073 0.640 0.551 84.61 3
NPM
A
B
C
D
Level 1 2 3 % contribution Selected level
2.352 1.331 1.842 2.57 1
0.704 2.139 2.681 10.31 3
2.322 0.952 2.251 5.88 1
1.420 2.983 3.961 81.23 3
by two different approaches again are not identical. From the ANOVA analysis, the percentage contributions of different control factors according to TPM and NPM in decreasing order are standoff distance (84.6%), laser power (6.5%), cutting speed (6%), compressed air pressure (2.8%), and stand-off distance (81.2%), laser power (10.3%), cutting speed (5.9%), compressed air pressure (2.6%), respectively. The optimum level according to TPM analysis is A1B2C1D3 and according to NPM, it is A1B3C1D3. To produce more cylindrical holes, according to TPM analysis, lower air pressure, moderate laser power, lower cutting speed and higher stand-off distance are required. On the basis of NPM analysis, lower air pressure, higher laser power, lower cutting speed and higher stand-off distance are required. 3.2.2. Hole diameter equal to 4 mm The optimum level selected by two different approaches again is not identical. From the ANOVA analysis, the percentage contributions of different control factors according to TPM and NPM in decreasing order are stand-off distance (72.10%), cutting speed (16.94%), compressed air pressure (6.47%), laser power (4.50%), and stand-off distance (60.97%), cutting speed (25.25%), laser power (7.65%), compressed air pressure (6.13%), respectively.
4 6
A: Assist gas pressure; B: Laser power; C: Cutting speed; and D: Stand-off distance.
Table 6 ANOVA analysis on TPM measure. TPM
Factors
Single factor
Material Hole diameter A B C D SSEa SSTrb SSTc
a b c
DOF
% contribution
5.47 2.42 0.71 2.69 0.89 4.16
1 2 2 2 2 2
23.61 10.44 3.05 11.63 3.85 17.96
6.82 16.33 23.15
42 11 53
29.46 70.54 100.0
DOF
% contribution
Sum of squares of errors. Sum of squares of six factors. SSE þ SSTr.
Table 7 ANOVA analysis on NPM measure. NPM
Factors
SS
Single factor
Material Hole diameter A B C D
185.95 114.58 20.76 79.54 48.58 201.18
1 2 2 2 2 2
SSEa SSTrb SSTc
228.64 650.59 879.23
42 11 53
a
3.2.3. Hole diameter equal to 6 mm The contributions of each variable on the hole taper as obtained from the ANOVA analysis for TPM and NPM in decreasing order are stand-off distance (49.84%), cutting speed (34.46%), laser power (15.50%), compressed air pressure (0.21%), and standoff distance (39.92%), cutting speed (31.80%), laser power (26.10%), compressed air pressure (2.19%), respectively. Stand-off distance and cutting speed are the most significant parameters contributing about 80% that affect the hole taper. Hole taper arises as a result of varying diameter from the entrance hole to the exit hole. The effect of stand-off distance on taper increases as the hole diameter decreases. The observed values of the effect of stand-off distance are 84.6% for 2 mm hole, 72.1% for 4 mm hole and 49.8% for 6 mm hole. Again these results are in agreement with previous findings [3,19]. With the same heat input, considering that the volume of material removal rate remains the same, there will be more evaporation of material along the radial direction of the hole and this has caused increased taper for lower diameter. The optimum levels based on the two performance measures for different hole sizes and different materials are given in Table 5. Since for different drilling diameters and materials the optimum levels are different, the types of material and drilling diameter are expected to have some effect on the hole taper.
SS
b c
21.15 13.03 2.36 9.05 5.53 22.88 26.0 74.0 100.0
Sum of squares of errors. Sum of squares of six factors. SSE þ SSTr.
The next analysis known as blocking is conducted to ascertain the effect of material and drilling diameters on the hole taper. In this investigation the concept of blocking from design of experiment is combined with the orthogonal array. Having analyzed the data, the contributions of four control factors along with the drilling diameter and materials have been determined from ANOVA analysis and are presented in Tables 6 and 7. These tables show the contribution of the factors on hole taper on the basis of two performance measures. From the TPM analysis, material has the most significant effect (23.61%) followed by stand-off distance (17.96%), laser power (10.44%), and hole diameter (10.44%). From the NPM analysis, stand-off distance has the most significant effect (22.88%), followed by material (21.15%), hole diameter (13.03%), and laser power (9.05%). ABS polymer has good impact and mechanical strength and is resistant to heat distortion. The material can be considered as a blend of glassy copolymer and rubbery polymer or copolymer. PMMA is a hard and rigid thermoplastic with outstanding light transmission properties. Material properties can affect the hole size both at the entrance and exit. The absorption and transmission of
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factors at different levels on quality characteristics like SN ratio and mean response are presented in Figs. 7 and 8 respectively.
laser beam and thus heating and evaporation of the work material will vary. As PMMA possesses outstanding light transmissibility, the effect of laser power on taper is less compared to that in ABS polymer. Fig. 6 shows percentage contribution of different factors on the hole taper. Interestingly, the hole taper is material dependent more than other factors. Among the four control factors, stand-off distance and laser power have significant effect on the taper. The hole diameter also has effect on the hole taper. The stand-off distance is related to the focal length of the laser beam and its position relative to the hole entry has an effect on the hole taper. On the basis of TPM analysis given in Table 8, ranking of various factors influencing the hole taper in decreasing order is material, stand-off distance, laser power, hole diameter, cutting speed, and assist gas pressure. While on the basis of NPM analysis given in Table 9, ranking of various factors influencing the hole taper in decreasing order is stand-off distance, material, hole diameter, laser power, cutting speed, and assist gas pressure. In either case, assist gas pressure is the least significant on hole taper. From these two optimum levels, a combined optimum level is determined and is shown in Table 10. The least taper is obtained from PMMA, 2 mm hole diameter, assist gas pressure of 2.0 bar, laser power of 500 W, cutting speed of 1.0 mm/s and stand-off distance of 5.0 mm. The graphical representations of the effect of
3.3. Hole circularity 3.3.1. ABS polymer The ratio of minimum to maximum hole diameter either at the exit or at the entrance is used to measure the hole circularity. Both the entrance and exit holes are reasonably circular for all diameters investigated. As the circularity is defined as the ratio of the minimum diameter to maximum diameter, an ideal condition of circularity would be 1.0 or 100% for a truly circular hole. However, the range of circularity achieved is between 85% and 96% for various experimental conditions. From the plots of hole circularity (Fig. 9), it is observed that the entrance hole is more circular than the exit hole in the range of investigated parameters. As the laser beam diverges from the focal point and the distance of entry plane of the work material is always more than focal length of the beam, the divergence is even more at the hole exit Table 10 Combined optimum level for different factors. Factor
25.00 TPM
% contribution
20.00
1303
Material Diameter A B C D
NPM
15.00
NPM
TPM
Optimum level
% contribution
Level
C (%)
Level
21.15 13.03 2.36 9.05 5.53 22.88
2 1 3 3 1 3
23.61 10.44 3.05 11.63 3.85 17.96
2 1 1 3 1 3
2 1 1 3 1 3
10.00 5.00
0 Material Diameter Assist gas pressure Laser power Cutting speed Stand-off distance
0.00 Material
D
Diameter
B
C
-1
A
Control factor -2 SN ratio
Fig. 6. Effect of different control factors on hole taper.
Table 8 Levels selection based on TPM.
-3
-4 Levels
Material
Diameter
A
B
C
D
1 2 3 SS % contribution Rank
1.65 1.014 5.4653 23.61 1
1.064 1.35 1.581 2.4166 10.44 4
1.244 1.493 1.258 0.7073 3.05 6
1.644 1.219 1.133 2.6917 11.63 3
1.156 1.459 1.38 0.8912 3.85 5
1.713 1.224 1.059 4.1589 17.96 2
Selected level
2
1
1
3
1
3
-5
-6 0
1
2 Factor level
3
4
Fig. 7. SN ratios of various factors at three levels.
Table 9 Levels selection based on NPM. Levels
Material
Diameter
A
B
C
D
1 2 3 SS % contribution Rank
4.2751 0.5638 185.95 21.15 2
0.7254 2.2514 4.2816 114.579 13.03 3
2.0454 3.2934 1.9196 20.763 2.36 6
4.0889 1.93 1.2395 79.542 9.05 4
1.2137 3.5314 2.5132 48.582 5.53 5
4.9752 1.9719 0.3113 201.18 22.88 1
Selected level
2
1
3
3
1
3
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2
Entrance circularity
1.8 1.6
Exit circularity
Mean response
1.4 1.2 Material
1
Diameter
0.8
Assist gas pressure
0.6
Cutting speed
0.2
Stand-off distance
0
Material: ABS
Laser power
0.4
0
1
2
3
2 mm hole
4
Factor level Fig. 8. Mean response of various factors at three levels.
than at the entrance. This resulted in a more uneven hole at the exit. Generally, decreasing the assist gas pressure and stand-off distance increases the hole circularity. Masmati and Philip [19] observed a similar behavior where with the decrease of stand-off distance and assist gas pressure, circularity increased at the hole entry. At the entrance, assist gas pressure and cutting speed significantly affect the entrance hole circularity while the effects of laser power and stand-off distance are not significant. The effect of stand-off distance is more significant on the exit circularity. It is also noticed that with increase of the cutting speed, the circularity of hole increases at the entrance but decreases at the exit hole. As the hole diameter increases from 2 mm to 6 mm, the holes become more circular both at the entrance and exit. 3.3.2. PMMA polymer Fig. 10 shows the plots of hole circularity at three levels of laser parameters and at different diameters. It is seen that the hole is more circular at the exit than at the entrance and this agrees with findings made by Ghoreishi et al. [3]. The energy of laser beam was high at the entrance than at the exit. This trend is quite opposite of that observed in ABS polymer, where the entrance hole was more circular. ABS is more resistant to heat distortion compared to PMMA and as PMMA has better transmissivity, beam energy was lower at the exit, resulting in more circularity at the exit. Compressed air pressure and laser power have highly significant effects. At higher laser power, energy concentration is higher, resulting in high heat and poor circularity both at the entrance and exit. Excessive energy will lead to poor circularity as the laser beam breaks through the work piece and polymer melt exiting from the hole exit. On the other hand, higher compressed air pressure is needed to remove the molten polymer from the hole and provides more circularity to the hole. Again with the increase of hole diameter, the holes at the entrance and exit are more circular. Compared to ABS polymer, PMMA produced holes of lower circularity.
Entrance circularity
Material: ABS
Entrance circularity
Material: ABS
In drilling ABS polymer, the effect of stand-off distance and laser power on the hole taper was found to be the most significant for larger hole sizes (4 mm and 6 mm). These two
4 mm hole
Exit circularity
6 mm hole
Fig. 9. Hole circularity at the entrance and exit in ABS sheet for various hole diameters: (a) 2 mm, (b) 4 mm, and (c) 6 mm.
4. Conclusions On the basis of experimental results, following conclusions are drawn:
Exit circularity
input variables contributed 80% towards the taper while the two other variables (cutting speed and assist gas pressure) contributed 20% towards the taper. However when the hole size was 2 mm, assist gas pressure and laser power contributed about 80% towards the taper. Different drilling diameters and materials resulted in different optimum combinations of control factors. When a 2 mm diameter hole was drilled on ABS polymer, the optimum combination of control factors based on TPM analysis was air pressure¼ 3 bar, laser power¼500 W, cutting speed¼1 m/min, and stand-off distance¼1 mm, and that on NPM analysis was air
I.A. Choudhury et al. / Optics and Lasers in Engineering 50 (2012) 1297–1305
Exit circularity
Entrance circularity
Material: PMMA
2 mm hole
Exit circularity
4 mm hole
Exit circularity Entrance circularity
Material: PMMA
6 mm hole
Fig. 10. Hole circularity at the entrance and exit in PMMA sheet for various hole diameters: (a) 2 mm, (b) 4 mm, and (c) 6 mm.
TPM analysis was air pressure¼2 bar, laser power¼ 300 W, cutting speed ¼0.6 m/min, and stand-off distance¼5 mm, and that on NPM analysis was air pressure ¼3 bar, laser power¼ 500 W, cutting speed¼0.6 m/min, and stand-off distance¼5 mm. Considering the four control factors along with the variability in material and hole size, the TPM results showed that material has 23.6% effect on the hole taper, followed by stand-off distance (17.9%), laser power (11.6%), and hole diameter (10.4%). On the contrary, the NPM results showed that stand-off distance has 22.9% effect on the hole taper followed by material (21.1%), hole diameter (13%), and laser power (9.1%). In drilling PMMA polymer, the hole appears to be more circular at the exit than at the entrance. While in drilling ABS polymer, the holes were found to be more circular at the entrance rather than at the exit. Overall, the holes were more circular in ABS sheet (85–96%) compared to PMMA sheet (80%).
References
Entrance circularity
Material: PMMA
1305
pressure¼3 bar, laser power¼ 500 W, cutting speed¼1 m/min, and stand-off distance¼5 mm. In drilling PMMA polymer, when the hole size was 2 mm, the effect of stand-off distance on the hole taper was about 80% while in drilling holes of 4 mm and 6 mm, the effects of cutting speed and laser power on the taper were almost identical. The stand-off distance alone contributed about 60% towards the taper. The optimum combination of control factors based on
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