- Email: [email protected]
Rapid fabrication of surface-relief plastic diffusers by ultrasonic embossing
ARTICLE IN PRESS Optics & Laser Technology 42 (2010) 794–798
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Rapid fabrication of surface-relief plastic diffusers by ultrasonic embossing Shih-Jung Liu a,, Yu-Chin Huang a, Sen-Yeu Yang b, Kuo-Huang Hsieh c a b c
Department of Mechanical Engineering, Chang Gung University, 259, Wen-Hwa 1st Road, Kwei-San, Tao-Yuan 333, Taiwan Department of Mechanical Engineering, National Taiwan University, Taipei 106, Taiwan Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan
a r t i c l e in f o
a b s t r a c t
Article history: Received 23 July 2009 Received in revised form 17 November 2009 Accepted 10 December 2009 Available online 6 January 2010
This paper discusses an innovative and effective ultrasonic embossing process, which enables the rapid fabrication of surface-relief plastic diffusers. The metallic mold bearing the microstructures is fabricated using a tungsten carbide turning machine. A 1500-W ultrasonic vibrator with an output frequency of 20 kHz was used to replicate the microstructure onto 1-mm-thick PMMA plates in the experiments. During ultrasonic embossing, the ultrasonic energy is converted into heat through intermolecular friction at the master mold/plastic plate interface due to asperities to melt the thermoplastic at the interface and thereby to replicate the microstructure. Under the proper processing conditions, highperformance plastic diffusers have been successfully fabricated. The cycle time required to successfully fabricate a diffuser is less than 2 s. The experimental results suggest that ultrasonic embossing could provide an effective way of fabricating high-performance plastic diffusers with a high throughput. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Optical diffusers Microstructure replications Ultrasonic embossing
1. Introduction Plastic diffusers have been widely used in applications such as LCD-TVs and monitors, signs, lighting systems, etc., for beam shaping, brightness homogenizing, and light scattering. The diffusers alter the angular divergence of incident light, thereby reducing the sensitivity of a detection system to slight positional or angular changes in the incoming beam. This allows for directed intensity light patterns with high efficiency. In general, diffusers can be classified into two types: the particle-diffusing type diffuser, which relies on transparent beads inside a plastic film or plate to scatter light, and the surface-relief type diffuser, which relies on microstructures on the surface of a plastic film or plate to scatter light. Many methods have been developed to fabricate surface-relief diffusers by replicating microstructures onto the surface of plastic films, including PDMS replica molding [1], silver halide sensitized gelatin method [2], holographic recording [3], 3D diffuser lithograph [4], photofabrication [5,6], hot embossing [7], and roller extrusion [8], etc. However, most methods employ complex processes and require expensive equipment. Among them, hot embossing is a relatively low-cost replication method for fabrication plastic diffusers. During the embossing step, the original pattern is directly transferred onto a thermoplastic, which acts as resistance. When heated above its glass transition temperature, the polymer becomes viscous and conforms exactly
to the embossing shim by filling the cavities of the surface relief. After it has cooled down, the replica is demolded from the master. The heating and cooling processes in hot embossing are, however, time-consuming, and the cores of the plates are unnecessarily softened. The long cycle time caused by such heating and cooling systems makes the hot embossing an inefficient method for mass production. The goal of this report is to develop an efficient process for fabricating the plastic diffusers. An innovative ultrasonic embossing process for directly replicating microstructures onto plastic films or sheets is employed in this paper. The process adopts high-frequency mechanical vibrations, which result in cyclical deformation of the parts and of any surface roughness. In ultrasonic vibration of thermoplastics, the plastic plates vibrate in phase with the horn, and the energy is transferred to the plate/mold interface by vibration to hot emboss the plates. The ultrasonic energy is converted into heat through intermolecular friction within the thermoplastics. The generated heat, which is highest at the surface between the master mold and the plate due to asperities, is sufficiently high to melt thermoplastics at the surfaces and cause the melt to flow and fill the microstructures. Plastic diffusers made of 1-mm-thick PMMA plates can be successfully fabricated. The cycle time required is under 2 s. The uniformity, profiles and optical properties of the fabricated diffusers are verified with microscope, surface profiler, and haze meter.
2. Experimental setup Corresponding author. Tel.: + 886 3 2118166; fax: +886 3 2118558.
E-mail addresses: [email protected], [email protected] (S.-J. Liu). 0030-3992/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2009.12.005
The plastic plates used in this study were polymethylmethacrylate (PMMA) with a thickness of 1 mm. A 1500-W ultrasonic
ARTICLE IN PRESS S.-J. Liu et al. / Optics & Laser Technology 42 (2010) 794–798
vibrator was used for all the experiments. The output frequency of the machine was 20 kHz. A booster horn with gain of 1:2.5 was used to emboss the parts. The horn was made of 90% Al 10% Ti alloys, in consideration of the alloy’s good wave transmission and ease of manufacturing. Fig. 1 shows the ultrasonic embossing facility. To emboss microstructures on the plastic plates, a master mold containing microstructures was first manufactured using a turning process. For the metal mold material, mild steel was used. A
795
tungsten carbide turning tool was used to machine the microstructures. The dimensions of the microstructure are shown in Fig. 2a, while the shape, height and width of the microstructures on the mold were measured and inspected using optical microscopy (Optimas SZ-PT, Japan), surface profiler (Alpha-Step 500, TENCOR, USA), and scanning electronic microscopy (Hitachi S-3000N, Japan). Figs. 2b and c show the images of fabricated microstructures on the mold. The measured width and depth of microstructure are 525 and 136 mm, respectively. During ultrasonic embossing, the underside of the horn comes into contact with the plastic plate, and the plastic vibrates in phase with the horn to transfer the energy. When the piezoelectric devices begin to vibrate, the horn transfers and enlarges the amplitude of the waves. The plastic materials in contact with the mold surface’s microstructures absorb the vibration energy and are heated. As long as the plastic/mold interface’s temperatures reach above Tg, the molten plastic flows and fills the microstructures. After the vibration stops, the horn holds the plastic plates against the master mold for some time for cooling. Once the plastic is cooled down, the horn is released and the plastic diffusers with surface-relief microstructures are fabricated.
3. Results and discussion 3.1. Effects of processing parameters on the replication quality of fabricated diffuser Fig. 1. Schematic diagram and photograph showing the ultrasonic embossing facility.
To ensure successful embossing, the temperatures at the master mold/part interface must be higher than the glass
Fig. 2. (a) Dimensions of the master mold, (b) optical microscopy, and (c) SEM images of the microstructures on the master mold.
ARTICLE IN PRESS 796
S.-J. Liu et al. / Optics & Laser Technology 42 (2010) 794–798
transition temperature of the polymer. During the ultrasonic vibration of thermoplastics such as PMMA, heat is generated when the plastic is subjected to cyclic strain. The power dissipated depends upon the loss modulus of the polymer and the cyclic strain amplitude [9]: 00
Q ¼ oe2o E =2 E ¼ E0 þiE
ð1Þ
00
ð2Þ
where Q is the average power dissipated, o is the frequency, and eo is the strain amplitude. En, E0 and E00 are the complex, storage and loss modulus, respectively. In the experiments, the PMMA material had a high stiffness and therefore a higher storage modulus E0 and a lower loss modulus E00 . It vibrated in phase with the horn, and the energy was thus transferred to the mold/part interface by vibration to emboss the microstructure onto the parts.
Various processing parameters were studied in terms of their influence on the replicability of ultrasonically embossed plates: embossing pressure (EP), vibration time (VT), and hold time (HT). The conditions of EP, VT, and HT used are 1.5 and 3 bar, 1 and 1.5 s, and 2 and 4 s, respectively. Table 1 shows that the replicated heights at central area are higher than those at the sides. This might be mainly caused by the non-uniform embossing pressure distribution across the PMMA plate, with a higher pressure at the center and a lower value near the edge. When a force is applied at the center by the horn, the materials at the center experience the highest pressure. The pressure gradually diminishes from the center to the edge. Molded microstructures near the edge thus exhibit inferior replicability to that at the center. This problem can be overcome by adopting an appropriate processing condition. The results in Table 1 show that the proper embossing pressure is 3 bar. By applying a higher embossing pressure, the plastic plates
Table 1 Influence of processing parameters on the quality of replicated microstructures. Parameters
Embossing pressure (bar)
Vibration time (s)
Hold time (s)
Hs (mm)
Hc (mm)
Deviation (%)
1 2 3 4 5 6 7 8
1.5 1.5 1.5 1.5 3 3 3 3
1 1 1.5 1.5 1 1 1.5 1.5
2 4 2 4 2 4 2 4
53.05 84.21 78.80 72.26 107.27 124.15 107.05 103.62
82.97 90.81 99.30 105.6 134.1 132.9 123.6 121.5
22.00% 3.77% 11.51% 18.75% 11.12% 3.40% 7.18% 7.94%
Hs: height of microstructure at side area; Hc: height of microstructure at central area. Deviation= [(Hc Hs)/(Hc +Hs)] 100%.
Fig. 3. (a) Digital camera, (b) optical microscopy, (c) SEM, and (d) surface profiler images of the microstructures on the fabricated PMMA diffusers.
ARTICLE IN PRESS S.-J. Liu et al. / Optics & Laser Technology 42 (2010) 794–798
can be made to vibrate in phase with the horn and transfer the energy to the mold/plate interface to replicate the microstructure. Furthermore, the conformability of the part to the mold’s microstructures is the major concern for the embossed plates. During vibration, the embossing pressure is also applied to the samples to cause the molten polymer to flow and to fill the embossing interface. Increasing the embossing pressure should therefore increase the replicability of the embossed plates. On the other hand, the proper vibration time is 1 s. Increasing the vibration time increases energy dissipation and is expected to increase the replicability. However, when the energy input is too high, the plates may over-melt. The embossed depths may be above the optimal values and the replicability decreases accordingly. As far as the hold time is concerned, the proper hold time is 4 s. After the vibration is completed, the plate is held against the master mold for some time for the purpose of microstructure conformity. Increasing the hold time thus increases the replicability of the embossed plates. A combination of processing conditions, including an embossing pressure of 3 bar, a vibration time of 1 s and a hold time of 4 s, is adopted to fabricate PMMA diffusers with microstructures. 3.2. Shape and uniformity of fabricated diffuser Fig. 3 shows the SEM image and surface profiler images of a randomly selected area of diffuser fabricated under an embossing pressure of 3 bar, 1 s vibration time, and 4 s hold time. The microstructures on the master mold were successfully replicated onto the PMMA plate. The microstructure embossed on the PMMA diffusers was measured. It has a height of 133.5 mm and a width of Table 2 Measured optical properties of the diffuser.
Flat PMMA plate PMMA plate with surface microstructure
Tt (%)
Td (%)
Haze (%)
92.4 99.1
0.8 72.9
91.6 73.5
797
488.4 mm. The calculated deviations of the height and the width of the embossed PMMA microstructures from the mild steel master mold are 2.5 mm (1.83%) and 36.6 mm (6.97%), respectively. The small deviation shows that good transcription of microstructures has been achieved. To verify the uniformity of the replicated microstructures, the height and width of microstructures were further calculated from ten randomly selected replicated microstructures. The average height is 131.2 mm with a standard deviation of 4.8 mm (3.53%), while the average width is 496.3 mm with a standard deviation of 28.7 mm (5.46%). The small standard deviations of height and width reveal high uniformity of ultrasonic embossed microstructures. This confirms the replicating capability by the ultrasonic embossing technique proposed in this study. 3.3. Optical properties of the fabricated diffusers To further verify and inspect the optical properties of the fabricated diffusers, an automatic haze meter (TC-HIII DPK, Denshoku, Japan) was employed according to the ASTM D1003 standard, which is a standard test method for haze and luminous transmittance of transparent plastics. The haze meter has a measuring area of 10 mm in diameter and consists of an integrated sphere, a condenser, a lens, a photo detector and an ultraviolet C-range light source. The total transmittance (Tt), diffuse transmittance (Td), and haze of the diffusers are measured at 99.1%, 72.9%, and 73.5%, respectively, for the fabricated diffusers. Table 2 compares the optical properties of a flat PMMA plate and the PMMA diffuser with microstructure. The overall performance of the plate with surface microstructures is very much improved. Furthermore, to inspect the diffusion capacity of the plastic plates, an optical system consisting of a 633 nm wavelength laser light source, and object holder, and a camera is used. Fig. 4 shows the images observed through the flat PMMA plate and the PMMA plate with microstructures. As can be observed, the PMMA plate with microstructures displays better diffusing efficiency than the flat plate. The results demonstrate that the fabricated diffusers can diffuse the light effectively.
Fig. 4. The images of a laser light source observed behind (a) a flat PMMA plate and (b) a PMMA plate with surface microstructures.
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Finally, it should be noted that the minimum size of the microstructure that can be successfully fabricated by the current ultrasonic embossing method is limited by the availability of the master mold. The method can be used to fabricate surface structure of smaller size as long as the steel master mold is available. On the other hand, the plastic plate used in this study is commercially available and low cost. Despite the ultrasonic embossing process consists of two steps (film casting of PMMA resins into plastic plates followed by ultrasonic embossing of the plates), due to the fast heating of the ultrasonic oscillation, the overall cycle time of ultrasonic embossing process is still shorter than that of one-step process such as injection molding, whose cycle time is approximately 30–60 s. This would provide significant advantages in terms of reduced fabrication cost and improved product quality.
4. Conclusions This paper discussed an innovative and effective ultrasonic embossing method for the rapid fabrication of surface-relief plastic diffusers. The metallic mold bearing the microstructures is fabricated using a turning machine. In this study, an ultrasonic embossing process, which consists of an ultrasonic vibrator equipped with a booster horn, is employed to replicate the microstructures. The effects of processing parameters on the replication quality of surface microstructures of fabricated diffuser were investigated. Under the proper processing conditions, the high-performance plastic diffusers have been successfully fabricated. The cycle time required to successfully fabricate a diffuser is less than 2 s. The uniformity, profiles and optical properties of the fabricated plastic diffuser have been characterized and verified. The experimental results in this study suggest that ultrasonic embossing could provide an effective way
of fabricating high-performance, low-cost plastic diffusers with high throughput.
Acknowledgements The National Science Council of Taiwan, R.O.C. under the Grant NSC 96-2628-E-182-001-MY3, has supported this work financially. The haze measurement and technical support from the Polymer Physical Analysis Laboratory of the Department of Polymer Engineering at National Taiwan University of Science and Technology is gratefully acknowledged. References [1] Shih TK, Chen CF, Ho JR, Chuang FT. Fabrication of PDMS (polydimethylsiloxane) microlens and diffuser using replica molding. Microelectron Eng 2006;83:2499–503. [2] Kim SI, Choi YS, Ham YN, Park CY, Kim JM. Holographic diffuser by use of a silver halide sensitized gelatin process. Appl Opt 2003;42:2482–91. [3] Sakai D, Harada K, Kamemaru SI, El-Morsy MA, Itoh M, Yatagai T. Direct fabrication of surface relief holographic diffusers in azobenzene polymer films. Opt Rev 2005;12:383–6. [4] Chang SI, Yoon JB, Kim HK, Kim JJ, Lee BK, Shin DH. Microlens array diffuser for a light-emitting diode backlight system. Opt Lett 2006;31:3016–8. [5] Me´ndez ER, Garcı´a-Guerrero EE, Escamilla HM, Maradudin AA, Leskova TA, Shchegrov AV. Photofabrication of random achromatic optical diffusers for uniform illumination. Appl Opt 2001;40:1098–108. [6] Garcı´a-Guerrero EE, Me´ndez ER, Escamilla HM, Leskova TA, Maradudin AA. Design and fabrication of random phase diffusers for extending the depth of focus. Opt Express 2007;15:910–23. [7] Parikka M, Kaikuranta T, Laakkonen P, Lautanen J, Tervo J, Honkanen M, et al. Deterministic diffractive diffusers for displays. Appl Opt 2001;40:2239–46. [8] Huang TC, Ciou JR, Huang PH, Hsieh KH, Yang SY. Fast fabrication of integrated surface-relief and particle-diffusing plastic diffuser by use of a hybrid extrusion roller embossing process. Opt Express 2008;16:440–7. [9] Benatar A, Cheng Z. Ultrasonic welding of thermoplastics in the far-field. Polym Eng Sci 1989;29:1699–704.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Rapid fabrication of surface-relief plastic diffusers by ultrasonic embossing Shih-Jung Liu a,, Yu-Chin Huang a, Sen-Yeu Yang b, Kuo-Huang Hsieh c a b c
Department of Mechanical Engineering, Chang Gung University, 259, Wen-Hwa 1st Road, Kwei-San, Tao-Yuan 333, Taiwan Department of Mechanical Engineering, National Taiwan University, Taipei 106, Taiwan Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan
a r t i c l e in f o
a b s t r a c t
Article history: Received 23 July 2009 Received in revised form 17 November 2009 Accepted 10 December 2009 Available online 6 January 2010
This paper discusses an innovative and effective ultrasonic embossing process, which enables the rapid fabrication of surface-relief plastic diffusers. The metallic mold bearing the microstructures is fabricated using a tungsten carbide turning machine. A 1500-W ultrasonic vibrator with an output frequency of 20 kHz was used to replicate the microstructure onto 1-mm-thick PMMA plates in the experiments. During ultrasonic embossing, the ultrasonic energy is converted into heat through intermolecular friction at the master mold/plastic plate interface due to asperities to melt the thermoplastic at the interface and thereby to replicate the microstructure. Under the proper processing conditions, highperformance plastic diffusers have been successfully fabricated. The cycle time required to successfully fabricate a diffuser is less than 2 s. The experimental results suggest that ultrasonic embossing could provide an effective way of fabricating high-performance plastic diffusers with a high throughput. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Optical diffusers Microstructure replications Ultrasonic embossing
1. Introduction Plastic diffusers have been widely used in applications such as LCD-TVs and monitors, signs, lighting systems, etc., for beam shaping, brightness homogenizing, and light scattering. The diffusers alter the angular divergence of incident light, thereby reducing the sensitivity of a detection system to slight positional or angular changes in the incoming beam. This allows for directed intensity light patterns with high efficiency. In general, diffusers can be classified into two types: the particle-diffusing type diffuser, which relies on transparent beads inside a plastic film or plate to scatter light, and the surface-relief type diffuser, which relies on microstructures on the surface of a plastic film or plate to scatter light. Many methods have been developed to fabricate surface-relief diffusers by replicating microstructures onto the surface of plastic films, including PDMS replica molding [1], silver halide sensitized gelatin method [2], holographic recording [3], 3D diffuser lithograph [4], photofabrication [5,6], hot embossing [7], and roller extrusion [8], etc. However, most methods employ complex processes and require expensive equipment. Among them, hot embossing is a relatively low-cost replication method for fabrication plastic diffusers. During the embossing step, the original pattern is directly transferred onto a thermoplastic, which acts as resistance. When heated above its glass transition temperature, the polymer becomes viscous and conforms exactly
to the embossing shim by filling the cavities of the surface relief. After it has cooled down, the replica is demolded from the master. The heating and cooling processes in hot embossing are, however, time-consuming, and the cores of the plates are unnecessarily softened. The long cycle time caused by such heating and cooling systems makes the hot embossing an inefficient method for mass production. The goal of this report is to develop an efficient process for fabricating the plastic diffusers. An innovative ultrasonic embossing process for directly replicating microstructures onto plastic films or sheets is employed in this paper. The process adopts high-frequency mechanical vibrations, which result in cyclical deformation of the parts and of any surface roughness. In ultrasonic vibration of thermoplastics, the plastic plates vibrate in phase with the horn, and the energy is transferred to the plate/mold interface by vibration to hot emboss the plates. The ultrasonic energy is converted into heat through intermolecular friction within the thermoplastics. The generated heat, which is highest at the surface between the master mold and the plate due to asperities, is sufficiently high to melt thermoplastics at the surfaces and cause the melt to flow and fill the microstructures. Plastic diffusers made of 1-mm-thick PMMA plates can be successfully fabricated. The cycle time required is under 2 s. The uniformity, profiles and optical properties of the fabricated diffusers are verified with microscope, surface profiler, and haze meter.
2. Experimental setup Corresponding author. Tel.: + 886 3 2118166; fax: +886 3 2118558.
E-mail addresses: [email protected], [email protected] (S.-J. Liu). 0030-3992/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2009.12.005
The plastic plates used in this study were polymethylmethacrylate (PMMA) with a thickness of 1 mm. A 1500-W ultrasonic
ARTICLE IN PRESS S.-J. Liu et al. / Optics & Laser Technology 42 (2010) 794–798
vibrator was used for all the experiments. The output frequency of the machine was 20 kHz. A booster horn with gain of 1:2.5 was used to emboss the parts. The horn was made of 90% Al 10% Ti alloys, in consideration of the alloy’s good wave transmission and ease of manufacturing. Fig. 1 shows the ultrasonic embossing facility. To emboss microstructures on the plastic plates, a master mold containing microstructures was first manufactured using a turning process. For the metal mold material, mild steel was used. A
795
tungsten carbide turning tool was used to machine the microstructures. The dimensions of the microstructure are shown in Fig. 2a, while the shape, height and width of the microstructures on the mold were measured and inspected using optical microscopy (Optimas SZ-PT, Japan), surface profiler (Alpha-Step 500, TENCOR, USA), and scanning electronic microscopy (Hitachi S-3000N, Japan). Figs. 2b and c show the images of fabricated microstructures on the mold. The measured width and depth of microstructure are 525 and 136 mm, respectively. During ultrasonic embossing, the underside of the horn comes into contact with the plastic plate, and the plastic vibrates in phase with the horn to transfer the energy. When the piezoelectric devices begin to vibrate, the horn transfers and enlarges the amplitude of the waves. The plastic materials in contact with the mold surface’s microstructures absorb the vibration energy and are heated. As long as the plastic/mold interface’s temperatures reach above Tg, the molten plastic flows and fills the microstructures. After the vibration stops, the horn holds the plastic plates against the master mold for some time for cooling. Once the plastic is cooled down, the horn is released and the plastic diffusers with surface-relief microstructures are fabricated.
3. Results and discussion 3.1. Effects of processing parameters on the replication quality of fabricated diffuser Fig. 1. Schematic diagram and photograph showing the ultrasonic embossing facility.
To ensure successful embossing, the temperatures at the master mold/part interface must be higher than the glass
Fig. 2. (a) Dimensions of the master mold, (b) optical microscopy, and (c) SEM images of the microstructures on the master mold.
ARTICLE IN PRESS 796
S.-J. Liu et al. / Optics & Laser Technology 42 (2010) 794–798
transition temperature of the polymer. During the ultrasonic vibration of thermoplastics such as PMMA, heat is generated when the plastic is subjected to cyclic strain. The power dissipated depends upon the loss modulus of the polymer and the cyclic strain amplitude [9]: 00
Q ¼ oe2o E =2 E ¼ E0 þiE
ð1Þ
00
ð2Þ
where Q is the average power dissipated, o is the frequency, and eo is the strain amplitude. En, E0 and E00 are the complex, storage and loss modulus, respectively. In the experiments, the PMMA material had a high stiffness and therefore a higher storage modulus E0 and a lower loss modulus E00 . It vibrated in phase with the horn, and the energy was thus transferred to the mold/part interface by vibration to emboss the microstructure onto the parts.
Various processing parameters were studied in terms of their influence on the replicability of ultrasonically embossed plates: embossing pressure (EP), vibration time (VT), and hold time (HT). The conditions of EP, VT, and HT used are 1.5 and 3 bar, 1 and 1.5 s, and 2 and 4 s, respectively. Table 1 shows that the replicated heights at central area are higher than those at the sides. This might be mainly caused by the non-uniform embossing pressure distribution across the PMMA plate, with a higher pressure at the center and a lower value near the edge. When a force is applied at the center by the horn, the materials at the center experience the highest pressure. The pressure gradually diminishes from the center to the edge. Molded microstructures near the edge thus exhibit inferior replicability to that at the center. This problem can be overcome by adopting an appropriate processing condition. The results in Table 1 show that the proper embossing pressure is 3 bar. By applying a higher embossing pressure, the plastic plates
Table 1 Influence of processing parameters on the quality of replicated microstructures. Parameters
Embossing pressure (bar)
Vibration time (s)
Hold time (s)
Hs (mm)
Hc (mm)
Deviation (%)
1 2 3 4 5 6 7 8
1.5 1.5 1.5 1.5 3 3 3 3
1 1 1.5 1.5 1 1 1.5 1.5
2 4 2 4 2 4 2 4
53.05 84.21 78.80 72.26 107.27 124.15 107.05 103.62
82.97 90.81 99.30 105.6 134.1 132.9 123.6 121.5
22.00% 3.77% 11.51% 18.75% 11.12% 3.40% 7.18% 7.94%
Hs: height of microstructure at side area; Hc: height of microstructure at central area. Deviation= [(Hc Hs)/(Hc +Hs)] 100%.
Fig. 3. (a) Digital camera, (b) optical microscopy, (c) SEM, and (d) surface profiler images of the microstructures on the fabricated PMMA diffusers.
ARTICLE IN PRESS S.-J. Liu et al. / Optics & Laser Technology 42 (2010) 794–798
can be made to vibrate in phase with the horn and transfer the energy to the mold/plate interface to replicate the microstructure. Furthermore, the conformability of the part to the mold’s microstructures is the major concern for the embossed plates. During vibration, the embossing pressure is also applied to the samples to cause the molten polymer to flow and to fill the embossing interface. Increasing the embossing pressure should therefore increase the replicability of the embossed plates. On the other hand, the proper vibration time is 1 s. Increasing the vibration time increases energy dissipation and is expected to increase the replicability. However, when the energy input is too high, the plates may over-melt. The embossed depths may be above the optimal values and the replicability decreases accordingly. As far as the hold time is concerned, the proper hold time is 4 s. After the vibration is completed, the plate is held against the master mold for some time for the purpose of microstructure conformity. Increasing the hold time thus increases the replicability of the embossed plates. A combination of processing conditions, including an embossing pressure of 3 bar, a vibration time of 1 s and a hold time of 4 s, is adopted to fabricate PMMA diffusers with microstructures. 3.2. Shape and uniformity of fabricated diffuser Fig. 3 shows the SEM image and surface profiler images of a randomly selected area of diffuser fabricated under an embossing pressure of 3 bar, 1 s vibration time, and 4 s hold time. The microstructures on the master mold were successfully replicated onto the PMMA plate. The microstructure embossed on the PMMA diffusers was measured. It has a height of 133.5 mm and a width of Table 2 Measured optical properties of the diffuser.
Flat PMMA plate PMMA plate with surface microstructure
Tt (%)
Td (%)
Haze (%)
92.4 99.1
0.8 72.9
91.6 73.5
797
488.4 mm. The calculated deviations of the height and the width of the embossed PMMA microstructures from the mild steel master mold are 2.5 mm (1.83%) and 36.6 mm (6.97%), respectively. The small deviation shows that good transcription of microstructures has been achieved. To verify the uniformity of the replicated microstructures, the height and width of microstructures were further calculated from ten randomly selected replicated microstructures. The average height is 131.2 mm with a standard deviation of 4.8 mm (3.53%), while the average width is 496.3 mm with a standard deviation of 28.7 mm (5.46%). The small standard deviations of height and width reveal high uniformity of ultrasonic embossed microstructures. This confirms the replicating capability by the ultrasonic embossing technique proposed in this study. 3.3. Optical properties of the fabricated diffusers To further verify and inspect the optical properties of the fabricated diffusers, an automatic haze meter (TC-HIII DPK, Denshoku, Japan) was employed according to the ASTM D1003 standard, which is a standard test method for haze and luminous transmittance of transparent plastics. The haze meter has a measuring area of 10 mm in diameter and consists of an integrated sphere, a condenser, a lens, a photo detector and an ultraviolet C-range light source. The total transmittance (Tt), diffuse transmittance (Td), and haze of the diffusers are measured at 99.1%, 72.9%, and 73.5%, respectively, for the fabricated diffusers. Table 2 compares the optical properties of a flat PMMA plate and the PMMA diffuser with microstructure. The overall performance of the plate with surface microstructures is very much improved. Furthermore, to inspect the diffusion capacity of the plastic plates, an optical system consisting of a 633 nm wavelength laser light source, and object holder, and a camera is used. Fig. 4 shows the images observed through the flat PMMA plate and the PMMA plate with microstructures. As can be observed, the PMMA plate with microstructures displays better diffusing efficiency than the flat plate. The results demonstrate that the fabricated diffusers can diffuse the light effectively.
Fig. 4. The images of a laser light source observed behind (a) a flat PMMA plate and (b) a PMMA plate with surface microstructures.
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Finally, it should be noted that the minimum size of the microstructure that can be successfully fabricated by the current ultrasonic embossing method is limited by the availability of the master mold. The method can be used to fabricate surface structure of smaller size as long as the steel master mold is available. On the other hand, the plastic plate used in this study is commercially available and low cost. Despite the ultrasonic embossing process consists of two steps (film casting of PMMA resins into plastic plates followed by ultrasonic embossing of the plates), due to the fast heating of the ultrasonic oscillation, the overall cycle time of ultrasonic embossing process is still shorter than that of one-step process such as injection molding, whose cycle time is approximately 30–60 s. This would provide significant advantages in terms of reduced fabrication cost and improved product quality.
4. Conclusions This paper discussed an innovative and effective ultrasonic embossing method for the rapid fabrication of surface-relief plastic diffusers. The metallic mold bearing the microstructures is fabricated using a turning machine. In this study, an ultrasonic embossing process, which consists of an ultrasonic vibrator equipped with a booster horn, is employed to replicate the microstructures. The effects of processing parameters on the replication quality of surface microstructures of fabricated diffuser were investigated. Under the proper processing conditions, the high-performance plastic diffusers have been successfully fabricated. The cycle time required to successfully fabricate a diffuser is less than 2 s. The uniformity, profiles and optical properties of the fabricated plastic diffuser have been characterized and verified. The experimental results in this study suggest that ultrasonic embossing could provide an effective way
of fabricating high-performance, low-cost plastic diffusers with high throughput.
Acknowledgements The National Science Council of Taiwan, R.O.C. under the Grant NSC 96-2628-E-182-001-MY3, has supported this work financially. The haze measurement and technical support from the Polymer Physical Analysis Laboratory of the Department of Polymer Engineering at National Taiwan University of Science and Technology is gratefully acknowledged. References [1] Shih TK, Chen CF, Ho JR, Chuang FT. Fabrication of PDMS (polydimethylsiloxane) microlens and diffuser using replica molding. Microelectron Eng 2006;83:2499–503. [2] Kim SI, Choi YS, Ham YN, Park CY, Kim JM. Holographic diffuser by use of a silver halide sensitized gelatin process. Appl Opt 2003;42:2482–91. [3] Sakai D, Harada K, Kamemaru SI, El-Morsy MA, Itoh M, Yatagai T. Direct fabrication of surface relief holographic diffusers in azobenzene polymer films. Opt Rev 2005;12:383–6. [4] Chang SI, Yoon JB, Kim HK, Kim JJ, Lee BK, Shin DH. Microlens array diffuser for a light-emitting diode backlight system. Opt Lett 2006;31:3016–8. [5] Me´ndez ER, Garcı´a-Guerrero EE, Escamilla HM, Maradudin AA, Leskova TA, Shchegrov AV. Photofabrication of random achromatic optical diffusers for uniform illumination. Appl Opt 2001;40:1098–108. [6] Garcı´a-Guerrero EE, Me´ndez ER, Escamilla HM, Leskova TA, Maradudin AA. Design and fabrication of random phase diffusers for extending the depth of focus. Opt Express 2007;15:910–23. [7] Parikka M, Kaikuranta T, Laakkonen P, Lautanen J, Tervo J, Honkanen M, et al. Deterministic diffractive diffusers for displays. Appl Opt 2001;40:2239–46. [8] Huang TC, Ciou JR, Huang PH, Hsieh KH, Yang SY. Fast fabrication of integrated surface-relief and particle-diffusing plastic diffuser by use of a hybrid extrusion roller embossing process. Opt Express 2008;16:440–7. [9] Benatar A, Cheng Z. Ultrasonic welding of thermoplastics in the far-field. Polym Eng Sci 1989;29:1699–704.