Hole Characteristic of CO2 Laser Drilling of Poly-Methyl ...No.3(2020)/186-197.pdfduring drilling. Laser beam drilling of hol es angled at 90º to the samples. A 63 mm focal length

Journal of Mechanical Engineering Research and Developments ISSN: 1024-1752 CODEN: JERDFO Vol. 43, No. 3, pp. 186-197 Published Year 2020

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Hole Characteristic of CO2 Laser Drilling of Poly-Methyl Methacrylate PMMA

Abeer A. Shehab†*, Iman M. Naemah‡, Abbas Al-Bawee† & Athil Al-Ezzi†

†Department of Material Engineering, College of Eng., University of Diyala, Diyala, Iraq

‡Department of Mechanical Engineering. College of Eng., University of Diyala, Diyala, Iraq

*E-mail: [email protected]

ABSTRACT: The use of laser drilling process to produce high quality end product is a potential for mass production. This process shows an important role in this field of industry, particularly in the drilling of many types of material includes metals, plastic, rubber, and ceramics. In this current work, an experimental investigation of the laser drilling process is carried out on 4 mm acrylic (Polymethyl Methacrylate, PMMA) thickness using Carbon Dioxide (CO2) laser. The study investigates the effect of laser power density and exposure time on hole quality characteristics, namely aspect ratio, circularity and taper. The results revealed that both laser power density and exposure time play a significant role on uniformity, roundness and drilled holes diameter. It has been observed that aspect ratio, circularity and taper increase with decreasing laser power density and/or exposure time for the drilled open holes.

KEYWORDS: Laser Drilling, Poly-Methyl Methacrylate, Power Density, Exposure Time, Hole Quality Characteristics

INTRODUCTION

Poly-methyl methacrylate (PMMA) is usually known as acrylic which, has been used instead of glass in optic applications [1, 2]. This kind of material has many vital properties which lead to stand it among many polymeric materials. The main properties of this kind of polymers are lightweight, easy to fabricate, high chemical stability and good mechanical strength. PMMA has been utilized in many applications such as optoelectronic, microfluidic device, and vehicle industry also, in some medical applications [3-5]. Machining of PMMA is normally performed by using traditional processes, such as drilling and milling operations. These processes require special tools to produce the final required shape. However, the operating cost and service life of these tools have been reported to be the main issue that makes it limited in use to some extent. On the other hand, laser drilling able to machining different kinds of materials regardless of their physical properties such as hardness, softness, and absorptive [6-8]. Laser drilling process can be defined as a thermal processing method in which it has a veryhigh focused beam that causes a depth through the thickness of any materials. During this high energy beam, the material being processed is melted and then evaporated. Flowing that material particles are thrown out the drilling hole due to the steam pressure that generated from the process. [9] This technology has become one of the advancing drilling processes besides electrical discharge machining, ultrasound and electron beam [10, 11]. Laser processing is, therefore, an alternative method that able to cut and drill the PMMA precisely without the need for the conventional techniques. This is mostly important for obtaining high quality of drilling holes with small diameter size reached to less than 0.004 mm.

It has been documented that laser processing method is extremely effective technique for drilling and cutting different materials such as polymers, composites, ceramics and metals [12-15]. This technique is commonly used to obtain holes characteristic such as diameter percussion and repeatability. Also, holes taper, edge finishing of the samples and small heat affected zone. [16, 17]. Since the last two decades several kinds of laser sources have been used to perform the material drilling, the main point of these sources that it has a range of pulse duration and wavelengths. In fact this can proved the capability to obtain highly precision holes with small heat input to the work pieces. The main sources of laser that are widely used in the industry are CO2, Nd:YAG, excimer and copper vapour laser[18,19].

Many authors have used lasers processing technique for the polymer materials. Chen et al. [20] did an experimental investigation of the laser drilling for the different types of polymers materials, which were

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Hole Characteristic of CO2 Laser Drilling of Poly- Methyl Methacrylate PMMA

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polyethylene terephthalate and polyimide. They have found that laser drilling process has the ability to produce reasonable quality of holes in terms of smooth and clean edge. They determine also the optimum range of the energy density which was1.7 to 0.36 j/cm2 respectively for those types of material. Azare and Tokarev[21] investigated the mechanism of the micro drilling process using KrF laser source. The emphases of their study was obtaining the perfect condition of the process parameters that can provide high aspect ratio. In their work six different kinds of polymer were used. The paper has documented that when a fluence of krypton fluoride KrFe increased, maximum aspect ratio was obtained. From the study they also concluded that, the photon cost of short wavelength lasers, e.g. ultraviolet lasers, is essentially higher than that of the near-infrared lasers. Hence, the infrared lasers, such as Nd:YAG and CO2 laser are still more efficient tool for the machining of PMMA.Xia et.al.[22]have made a comparison study in order to enhance the microholes that produced by laser drilling. Thy tried to obtain high aspect ratio and high-quality microholes of the PMMA with the ambient pressure adjusting from 105 Pa (air) down to 1 Pa. They compared the laser processes with and without vacuum. The results showed that efficient energy propagation and easy ejection of ablated material/plasma were provided under a vacuum condition.

Tangwarodomnukuna and Chen [23] have used nanosecond Nd:YAG laser as a laser source for the laser grooving of 3.5 mm thick PMMA. The study aimed to reduce the thermal damage of this process, therefore, laser grooving was conducted with ethanol and water environments to avoid the wide heat effected zone. A surprise results were gained from this study in terms of HAZ size and clean groove when using this environment. Shozui Takeno et al. [24] have experimentally studied the effects of laser pulse duration and frequency on CO2 laser the thickness of the carbonized layer surrounding the drilled holes were investigated in Epoxy for pulse durations of 50-120 μs and frequencies of 20-80 Hz. It was found that carbonization takes place at frequencies higher than a certain value and that the frequency becomes lower as the pulse duration increased. Moon et al. [25] did an intensive study of producing stepwise on multilayers nano structure polymer composite. CO2 laser was utilized as percussion drilling process for the electronic applications. The study aimed to determine the perfect process conditions of the percussion drilling using a CO2laser that could supply and enhance the electrical interconnection for the composite multilayers electrodes with a thickness of 25 µm. From the study laser drilling profile was produced and scan electron microscope was used to provide more understanding for the formation of the layer by using this technique. Prakash and kumor [26] have conducted an experimental investigation of laser microchannel for the polymer material. Following this effort, the study developed an analytical model that was used to determine the micro channel depth and profile of the laser processing. The study focused on producing a process simulation can be utilized as an alternative approach to optimize the process parameters in terms of thermal profile and working temperature.

It has been seen from literatures that the laser processing can achieve a wide range of the PMAA thickness drilling. However, processing of PMAA thickness higher than 3mm was not observed besides high quality end drilled hole is needed and much efforts can be done to understand the influence of laser intensity and exposure time on hole quality of the drilling process for this particular material and thickness. Therefore, in the current work the influence of laser intensity and exposure time on the hole aspect ratio, circularity, taper uniformity, roundness and drilled holes diameters for PMAA 4mm thickness, have been investigated in order to determine the perfect conditions that can produce for the PMAA drilling process.

MATERIALS AND METHODS

Material used for the present work is acrylic polymer (C5H8O2) PMMA. The samples were cut to rectangular shapes with dimension of (4×70×15) mm; after that they were washed by water to remove surface impurities and kept free defect and smooth surface of the micro channels. The study has considered a range of processed parameters of the laser processing and their effect on the heat effected zone (HAZ) depth, width, shape and the surface roughness product of this process. In their study, a 3 mm sample thickness was used and CO2 laser was utilized as a laser source. From the study, it has been concluded that the CO2 laser can be used to produce a perfect surface finishing micro channels with less efforts in terms of economic prospective. PMMA material was used in the study of Serckovic et al. [27] in order to determine the perfect conditions of the laser cutting and drilling process. Different kinds of laser sources were used to machining a 3 mm PMMA samples. During the study, laser HAZ, cutting surface shape and surface roughness of the samples have been investigated. Following that COMSOL software was utilized to predict the thermal profile of this process. The study found that clean from dust before drilling process. The mechanical and physical properties of the polymer are presented in Table 1 [28].


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Table 1. Mechanical and physical properties of PMMA [28]

Laser drilling of polymer has been carried out using a 100 W continuous wave (CW) CO2 laser, CNC machining system, model CMA-1080 KII, China. Samples were mounted on a special clamping device to prevent vibration during drilling. Laser beam drilling of holes angled at 90º to the samples. A 63 mm focal length lens which created spots of ∼0. 6 mm size is used. Laser spot size at the surface of the workpiece was 1.2 mm. Laser beam was focused on the one-thirds of the plate thickness below its surface as used previously elsewhere [29].Laser parameters were varied such as power (12, 16, 25, 45, 65) W and (0.5 -1.5) s exposure time with 0.5 s step size, details of the process parameters which have been used to obtain the results of this study are given in Table 2. The characteristic dimensions and geometry of the holes were measured by using GENNEX optical microscope connected to a computerized camera. Magnifications provided for this microscope are (20X, 40X and 100X).

Table 2. Process parameters for the present work.

Power density (W/cm2)

Time )S(

Power )W(

Test No.

5745 1.5 65 1

3977 1.5 45 2

2209 1.5 25 3

1414 1.5 16 4

1060 1.5 12 5

5745 1 65 6

3977 1 45 7

2209 1 25 8

1414 1 16 9

1060 1 12 10

5745 0.5 65 11

3977 0.5 45 12

2209 0.5 25 13

1414 0.5 16 14

1060 0.5 12 15

Material Density

(kgm-3)

Melting

point (Co)

Thermal conductivity

Wm-1.K-1

Coef. of linear

thermal expansion

(1. K-1)

Young's modulus

(GPa)

Yield

Strength

(MPa)

Hardness

HRB

Elongation

at break

(%)

PMMA 1100 170 0.19 7 x 10-5 3.036 54 - 73 Shore M92-100

2.5 - 4


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RESULTS AND DISCUSSION

In this study, it has been observed that there is a difference between the two diameters of the samples, diameter at entry (Dent) and diameter at exit (Dexit) Figure1. Therefore, taper of the laser drilled hole is calculated by equation (1) [29]:

𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 =𝐷𝐷𝑇𝑇𝐷𝐷𝐷𝐷 − 𝐷𝐷𝑇𝑇𝐷𝐷𝐷𝐷𝐷𝐷

2𝐷𝐷 (1)

Where t is the thickness of the workpiece. The quality of the hole can also be expressed in terms of circularity. The circularity (CIR) of the hole is expressed as the ratio of minimum diameter (D1) to maximum diameter (D2) as shown in Figure 1. The circularity of the hole is expressed in equation (2) [ 30, 31], in which means when CIR value is equal to 1, then it indicates that the hole has a perfect circle.

𝐶𝐶𝐶𝐶𝐶𝐶 =𝐷𝐷1𝐷𝐷2

(2)

Figure 1. (Left) Schematic diagram for the taper of the drilled hole, (Right) Diameters of the drilled hole

The aspect ratio of the drilled hole is calculated by equation (3) [32]:

𝐴𝐴𝐴𝐴𝑇𝑇𝑇𝑇𝐴𝐴𝐷𝐷 𝑇𝑇𝑇𝑇𝐷𝐷𝐷𝐷𝑟𝑟 =𝐻𝐻𝑑𝑑𝐷𝐷𝑇𝑇𝐷𝐷𝐷𝐷

(3)

Where, Hd is the depth of the drilled hole which equals to sample thickness(t), since the drilled open holes are just considered in this study. EFFECT OF POWER OF DENSITY ON HOLE QUALITY CHARACTERISTIC

Figure 2, Figure 3 and Figure 4 show optical images for the top and bottom surfaces of holes at 0.5, 1and 1.5 s exposure times respectively. Here, a range of power density values from (1060 - 5745) (W/cm2) were used with a constant exposure time across each figure. It is observed that laser power density has a direct influence on uniformity, roundness and drilled holes diameter. The diameters at top, bottom and plastic deformation at the edges of holes, increase progressively as the power density increase (Figure 5 and Figure 6). This can be explained by the fact that when the laser beam attack the surface to be drilled, the heat imported by the beam propagates in all directions, this spread will lead to increase the width of the hole and foam a plastic flow which flows over the entire contour of the holes and deforms the vicinity of the hole.


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Figure 2. Holes top and bottom surfaces at 0.5 s exposure time and 1060, 1414, 2209, 3977 and 5745 W/cm2 power density (a, b, c, d and e respectively).

Figure 3. Holes top and bottom surfaces at 1 s exposure time and 1060, 1414, 2209, 3977 and 5745 W/cm2 power density (a, b, c, d and e respectively).

Figure 4. Holes top and bottom surfaces at 1.5 s exposure time and 1060, 1414, 2209, 3977 and 5745 W/cm2 power density (a, b, c, d and e respectively).

Figure 5. Hole size at top surface vs. power density at 0.5, 1, and 1.5 s exposure time.


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Figure 6. Hole size at bottom surface vs. power density at 0.5, 1, and 1.5 s exposure time

Figure 7 shows the effect of power density on the aspect ratio of the hole at 0.5, 1, and 1.5 exposure times. It is clearly observed that the aspect ratio of the drilled holes decreases as the power density and exposer time increase which is a fully awake with previous studies [21,22]. Since just drilled open holes were considered in this study a decrease in the aspect ratio occurred with increasing power density and/or exposure time as a result of increasing the hole diameter at top.

Figure 7. The aspect ratio of the hole vs. power density at 0.5, 1, and 1.5 s exposure times

Figure 8 shows the effect of power density on hole circularity (max size /min size) of the top surface at 0.5, 1, and 1.5 exposure times. It can be concluded from this figure that hole became more circular at lower power densities and shorter exposure times (0.5 s).


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Figure 8. Hole circularity vs. power density at 0.5, 1, and 1.5 s exposure times

Figure 9 shows the cross section of the hole at (a) 5745 (W/cm2) power density, 1s exposure time and (b) 2209 (W/cm2) power density, 0.5 sec exposure time. Formation of some structures or debris at the top surfaces of the edges was clearly observed specially at 5745 (W/cm2), where high levels of power densities lead to thermal damage along the hole create [19]. Based on the above discussion, power densities values ≥ 2209 (W/cm2) were not considered for the next sections.

Figure 9. Hole Cross section at (a) 5745 (W/cm2) power density, 1sec exposure time and (b) 2209 (W/cm2) power density, 0.5 sec exposure time

EFFECT OF EXPOSURE TIME ON HOLE QUALITY CHARACTERISTIC

The influence of drilling time on the drilling hole quality of top and bottom surfaces of PMMA is well observed in Figure 10 and Figure 11, the time was varied from 0.5 to 1.5 s for two different (1060 and 1414 W/cm2) power densities. It was noted that for a shorter exposure time (0.5 s) the shape of the hole is fairly uniform and the plastic flow of the material in the vicinity of the holes does not observed (see Figure 10a and Figure 11 a).

However beyond 0.5 s and 1414W/cm2 power density the drilled hole contour deforms and takes any shape (oval, ellipse) as a result of providing more time for heat propagation in the material surrounding the top hole surface (see parts c, d and e of the Figure 2, Figure 3 and Figure 4). Effect of exposure time on hole size at top and bottom surfaces for 1060, 1414, 2209, 3977 and 5745 W/cm2 power densities is presented in Figure 12 and Figure 13, where the size of hole increases as exposure time increase.


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Figure 10. Holes top and bottom surfaces at 1060 (W/cm2) power density and 0.5, 1 and 1.5 exposure time (a, b, and c, respectively)

Figure 11. Holes top and bottom surfaces at 1414 (W/cm2) power density and 0.5, 1 and 1.5 exposure time (a, b, and c, respectively)

Figure 12. Hole size at top surface vs. exposure time at 1060, 1414, 2209, 3977 and 5745 (W/cm2) power densities


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Figure 13. Hole size at bottom surface vs. exposure time at at 1060, 1414, 2209, 3977 and 5745 (W/cm2) power densities

BEST DRILLING PARAMETERS

After visual and optical inspection for the drilled samples, the best drilling parameters for the present work were chosen such as laser power varied as 8, 12, and 16 W (which meets 707, 1060 and 1414 W/cm2 power density respectively) and exposure time 0.2 and 0.4 s. Table 3 presents the best drilling parameters of the present work.

Table 3. Best drilling parameters of the present work.

Hole circularty Aspect ratio terdiameTop hole

)mµ (

Power density )2(W/cm

Time )s(

Power )W(

Test .No

0.99 2.99 1339 707 0.2 8 16

0.97 2.76 1450 1060 0.2 12 17

0.93 2.66 1500 1414 0.2 16 18

0.97 2.79 1430 707 0.4 8 19

0.97 2.74 1460 1060 0.4 12 20

0.91 2.62 1527 1414 0.4 16 21

Cleaner and smoother edges were obtained at power densities below 1060 (W/cm2) rather than other power densities used in this work and as seen in Figure 14a, b and Figure 15a,b respectively.

Figure 14. Holes top surface at 0.2 s exposure time and 707, 1060 and 1414 (W/cm2) power density (a, b and c respectively)


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Figure 15. Holes top surface at 0.4 s exposure time and 707, 1060 and 1414 (W/cm2) power density (a, b and c respectively)

Figure 16 shows the hole cross section at 707 (W/cm2) power density: (a) 0.2 s exposure time and (b) 0.4 s exposure time. It should be noted that the hole taper is calculated using eq.1. As power density and exposure time decrease the drilled hole became narrower (specially bottom hole diameters) and steeper, thus hole taper increase as the power density and /or exposure time decrease which showed an agreement with what has been reported by [20] and clearly observable when comparing the cross section of the drilled holes in Figure 9 with Figure 16. However, in many applications a taper-less geometry is required [32]. Hence, the entrance hole diameter is either larger or smaller than the exit hole diameter.

It is important to note that blind hole was obtained at power density and /or exposure time below 707 (W/cm2) and 0.2 s respectively where power density and /or exposure time were insufficient to melt and vaporize more PMMA material to make open hole.

Figure 16. Hole cross section at 707(W/cm2) power density: (a) 0.2 s exposure time and (b) 0.4s exposure time

CONCLUSION

In this paper, a Carbon Dioxide 100W CO2 laser beam was used as drilling tool to drill 4 mm thickness Polymethyl Methacrylate, (PMMA). The Effect of variation the power density (5745,3977,2209,1414,1060, and 707 W/cm2) and exposure time (0.2, 0.4,0.5,1 and 1.5 s) on hole quality characteristics, namely aspect ratio, circularity and taper was investigated. The following points were concluded:

1. Laser power density and exposure time have a direct influence on uniformity, roundness and drilled holes diameter. The diameters at top, bottom and plastic deformation at the edges of holes, increase progressively as the power density increase.

2. The most suitable drilling parameters were (1414, 1060, 707 W/cm2) power density vs. (0.2, 0.4 s) exposure time. Power densities and exposure times lower than these limits were not sufficient to drill open hole. Contamination of the vicinity of the hole was observed at power densities and exposure times upper than these limits.


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3. For drilled open hole, the decrease in power density and/or exposure time leads to increase in hole aspect ratio circularity and taper.

4. The material removal by this process can be controlled effectively in order to obtain smooth and clean edge of quality holes at these particular conditions.

ACKNOWLEDGMENTS

Support from the high ministry of education is acknowledged.

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Hole Characteristic of CO2 Laser Drilling of Poly-Methyl ...No.3(2020)/186-197.pdfduring drilling. Laser beam drilling of hol es angled at 90º to the samples. A 63 mm focal length