Most plastic diffusers are either of surface-relief or particle-diffusing types, based on different principles and fabrication methods. This paper reports an innovative extrusion roller embossing process, which enables the fabrication of diffusers with both surface-relief and particle-diffusing functions. An extruder with die is employed to fabricate the thin film of PC/bead composite; the roller micro-embossing process is used to replicate the microstructure onto the surface of PC composite film. A metallic roller mold with microstructures is fabricated using turning process. During the extrusion rolling embossing process, the extruded film of PC with diffusion beads is immediately pressed against the surface of the roller mold. Under the proper processing parameters, the plastic diffusers integrating surface-relief and particle-diffusing functions have been successfully fabricated. The shape, uniformity, and optical properties of fabricated diffuser have been verified. This method shows the great potential for continuous fabrication of high-performance plastic diffusers integrating surface-relief and particle-diffusing functions with high throughput.
©2008 Optical Society of America
In recent years, LCDs have been widely used in many applications, such as LCD-monitors and LCD-TVs. Because the liquid crystals can not emit light, a back light unit (BLU) is needed to convert the linear light source into surface light source. The diffuser, a key optical element in BLU, plays a critical role for beam-shaping, brightness homogenizing, and light scattering.
Generally, the diffusers can be classified into two types: the particle-diffusing type or the surface-relief type. Particle-diffusing diffusers rely on the transparent beads inside the plastic films or plates to scatter light. For fabricating particle-diffusing diffusers, methods such as extrusion molding [1–2], electrospray method , and commercialized coating method  have been reported. Among them, the coating method, which coats the mixture of diffusing beads, curing agent, binder, and organic solvent onto the polyethylene terephthalate (PET) film, is relatively simple and most popular. However, the distribution of diffusing beads in the diffuser is non-uniform. Non-uniform distribution of diffusing beads affects the performance of diffusing light. On the other hand, surface-relief diffusers rely on the microstructures on the surface of the plastic films or plates to scatter light. For the fabrication of surface-relief diffusers, many methods have been developed, including PDMS replica molding , silver halide sensitized gelatin (SHSG) method , holographic recording method , 3D diffuser lithography , photofabrication method [9–10], hot embossing , etc, for replicating the microstructures onto the surface of plastic films. Most replication methods involve complicated process and require expensive equipments. Among them, hot embossing is relatively inexpensive. However, the process involves heating and cooling and is a time-consuming batch-wise process. In summary, the particle-diffusing diffusers can be fabricated using coating method, while the surface-relief diffusers can be fabricated using hot embossing. This paper is devoted to developing a process for fabricating the plastic diffusers with both functions.
An innovative extrusion roller embossing process for directly fabricating microstructures onto plastic composite films or sheets is employed. This new process is composed of the film extrusion and the roller embossing processes. The film extrusion process is used to extrude plastic films containing diffusing beads, while the roller embossing process is used to fabricate microstructures onto the surface of plastic composite films. Accordingly, this new process can fabricate plastic diffusers integrating surface-relief and particle-diffusing functions. Additionally, this new process also noticeably raises the productivity of diffusers. This hybrid extrusion rolling embossing process not only can fabricate the plastic diffusers integrating surface-relief and particle-diffusing functions, but also is an efficient fabrication method.
During the extrusion roller embossing operation, the extruded composite film is immediately pressed against the surface of roller mold. Plastic diffusers with surface reliefs and diffusion beads can be successfully fabricated. The uniformity and optical properties of the fabricated diffuser are verified with microscope, surface profiler, and haze meter. The performance of integrated surface-relief and particle-diffusing diffuser is compared to those of the particle-diffusing diffusers and surface-relief diffusers.
2. Experimental setup
2.1 Theoretical background behind the die extrusion and roller embossing processes
A plasticating single screw extruder is first used to continuously generate polymer composite melt. The extruder turns solid polymer into homogeneous polymer melt and mix them with beads. Inside the screw extruder are three zones: solids conveying zone, melting zone, and metering (pumping) zone. The final metering zone is the most important section in melt extruder that relies on it to generate pressure sufficient for pumping. The pumping capacity in the metering section can be estimated by solving the equation of motion. Basically, the flow rate is the sum of drag flow and pressure-driven flow. The analytical solution for estimating the flow rate during extruding a Newtonian fluid can be obtained . The composite melt is then pumped into the die. To generate a uniform extrudate geometry at the die lip, the geometry of the manifold inside the die must be of coat-hanger shape. An analytical design equation for a coat-hanger sheeting die can be derived . Immediately, the extruded hot films or sheets are pressed against two rollers to be squeezed, and the microstructures to replicate the microstructures from the roller to the films or sheets. The roller embossing is a continuous process similar to calendering in polymer processing. Analysis of the maximum pressure in the nip region and roller separation force can be obtained .
2.2 Hybrid extrusion roller embossing system and process
As shown in Fig. 1, a hybrid extrusion roller embossing facility has been designed, constructed and used for the fabrication of diffuser in this study. This facility has also been used to fabricate plastic microlens array . This facility is composed of the film-extruding and the roller embossing unit. Extruder and die are used for plastic film extrusion. The film thickness can be adjusted by changing the die lips of coat-hanger die. Films with thickness of 1.5mm, 1mm and 0.5mm can be extruded. 0.5mm is chosen in this study. Film of polycarbonate (PC) composite is extruded. Polycarbonate composite, consisting of polycarbonate (DS3002R 7041A, Mitsubishi, Japan) mixed with less than 10 wt% of diffusion beads (cross-linked PMMA, average diameter 5µm, MBX-5, Sekisui, Japan), is used. The roller micro-embossing unit is composed of a plain driving roller and a micro-embossing roller with microstructures. Two rollers are pressed against each other by a couple of pneumatic cylinders in the ends of the shaft of the embossing roller. The pneumatic cylinders generate thrust to press the embossing roller and provide the embossing pressure between two rollers. The embossing pressure can be controlled by pneumatic cylinder pressure. The maximum average embossing pressure is about 420 Kgf/cm2. The driving roller is driven by a DC motor which has a rotation speed of 1800 rpm, and a torque of 1.62 Kg-cm. The maximum rotation speed of driving roller is 10 rpm by using reduction gear box.
The hybrid extrusion roller micro-embossing process is described as follows. First, the plastic pellets mixed with diffusion beads are fed into the extruder to be melted and mixed in the barrel. Then the melt is extruded out from a film-making die. After the PC/beads composite film is extruded, the hot plastic film directly enters into the gap between the driving roller and embossing roller. With a proper pressure applied on the shaft of embossing roller, the PC composite film is squeezed between the driving and embossing rollers. Thus the microstructures on the embossing roller are replicated onto the PC film. After cooling, the plastic films with microstructures are obtained.
2.3 Fabrication of embossing roller mold
The embossing roller mold, which produces the microstructures on the surface of the film during the extrusion roller embossing process, was directly manufactured using turning process. For the metal roller mold material, the aluminum alloy 6061 was used. A tungsten carbide turning tool with a nose radius of 400µm was used to machine the microstructures. The roller mold was first turned to a diameter of 74mm and microstructures were machined at central 50mm of length under the conditions of 1800 rpm of spindle speed and 419µm/rev of feed. The shape, height and width of the microstructures on the mold were measured and inspected using profile projector (VPS 250, 3DFAMILY, Taiwan), optical microscopy (Zoomkop), surface profiler (Alpha-Step 500, TENCOR, USA), and scanning electron microscopy (S-3000H, Hitachi, Japan). Figure 2 shows the images of fabricated microstructures on roller mold. The measured height and width of microstructure are 46.9µm and 392.3µm, respectively.
3. Results and discussion
3.1 Effects of processing parameters on the replication quality of fabricated diffuser
To study the effects of processing parameters on the replication quality of surface microstructures of fabricated diffuser, three processing parameters, i.e., die temperature (Td), rotation speed of driving roller (Rdr) and average embossing pressure (Pe) are chosen. The conditions of Td, Rdr, and Fe used are 260 and 290°C, 4.3 and 8.9 rpm, and 70 and 350 Kgf/cm2, respectively. Table 1 shows that the replicated heights at side area are higher than that of central area. This is mainly caused by the bending deflection of the roller, which is supported from two ends of its shaft. The deflection affects the uniformity of local rolling pressure. The problem can be overcome by cambering the roller, providing a slight convexity in the roller for the compensation of bending deflection. The results show that the proper die temperature is 290°C. Higher die temperature can increase replicated height of microstructures at central area. The proper rotation speed of driving roller is 4.3 rpm. If rotation speed is too high, the hot plastic film may not have enough time to completely fill the cavity of microstructures, resulting in low replicated height in the center. As far as embossing pressure is concerned, higher embossing pressure is preferred. With high embossing pressure, the embossing roller could squeeze the hot plastic film and to replicate the microstructures, resulting in high replicated height of microstructures. A combination of processing conditions including 290°C of die temperature, 4.3rpm of rotation speed, and 350 Kgf/cm2 of embossing pressure is adopted to fabricate PC composite diffusers with microstructures.
3.2 Shape and uniformity of fabricated diffuser
Figure 3 shows the SEM image and surface profiler images of a randomly selected area of diffuser fabricated under the conditions of 290°C of die temperature, 4.3 rpm of rotation speed, and 350 Kgf/cm2 of embossing pressure. The microstructures on the embossing roller were successfully replicated onto the extruded PC film. The microstructures embossed on PC diffusers were measured. It has a height of 46.27 µm and a width of 396.3 µm. The calculated deviations of the height and the width of the embossed PC microstructures from the aluminum alloy embossing roller are 0.63µm (1.34%) and 4µm (1.02%), respectively. The small deviations show 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 45.69µm with a standard deviation of 0.84µm (1.84%), while the average width is 398.06µm with a standard deviation of 17.05µm (4.28%). The small standard deviations of height and width reveal high uniformity of roller embossed microstructures.
3.3 Optical properties of the fabricated diffusers
An automatic haze meter (TC-HIII DPK, Denshoku, Tokyo) was employed, following the ASTM D1003, to further inspect and verify the optical properties of the fabricated diffuser. For ASTM D1003, it is a standard test method for haze and luminous transmittance of transparent plastics. The haze meter has a measuring area of 10 mm diameter. It is composed 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 can be measured. The Tt, Td, and haze of the fabricated PC composite diffuser with microstructures are 98%, 87.7%, and 89.5%, showing the good diffusing performance. Table 2 compares the optical properties of a flat pure PC film, a flat PC/bead composite, a pure PC film with microstructures, and the PC/bead composite film with microstructures. The overall performance of the film with surface microstructures and diffusion beads is the best among all.
To inspect their diffusing capacity, an optical system, composed of a 633 nm wavelength laser light source, an object holder, and a camera, is used. Figure 4 shows the images observed through a flat pure PC film, a pure PC film with surface microstructures, a flat PC/bead composite film, and a PC/bead composite film with microstructures. As can be observed, the PC/bead film with microstructures displays the best diffusing efficiency. The results demonstrate that the fabricated diffuser integrating particle-diffusing and surface-relief functions can scatter the light uniformly and diffuse the light effectively.
This paper reports an innovative and efficient method to fabricate plastic diffusers integrating both surface-relief and particle-diffusing functions via hybrid extrusion roller embossing process. In this study, the hybrid extrusion roller micro-embossing system combining thin film extrusion and roller micro-embossing has been employed. The effects of processing parameters on the replication quality of surface microstructures of fabricated diffuser were investigated. Under the proper processing parameters, the surface features on roller mold can be replicated onto extruded PC composite films. The plastic diffusers with diffusion beads and surface microstructures have been successfully fabricated. The uniformity, profiles and optical properties of the fabricated hybrid plastic diffuser have been characterized and verified. Compared with particle-diffusing diffusers or surface-relief diffusers, the fabricated plastic diffusers integrating both functions can scatter light uniformly and diffuses light effectively, In conclusion, this paper shows the great potential of hybrid extrusion roller embossing process for continuous fabrication of plastic hybrid diffusers with excellent optical performance, low cost, and high throughput.
This work was partially supported by the Department of Industrial Technology of Ministry of Economic Affairs of Taiwan (Under the grant of 95-EC-17-A-08-S1-015) and the Industrial Technology Research Institute of Taiwan (Under the grant of 6301XS7410). The financial and technique supports were acknowledged. The authors would like to thank the haze measurement facility and technique support from the Polymer Physical Analysis Laboratory of Department of Polymer Engineering at National Taiwan University of Science and Technology. Help and encouragement from co-workers at the Grace Laboratory for polymer processing at National Taiwan University are greatly appreciated.
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