This paper reports a simple and effective ultrasonic embossing method which enables the rapid fabrication of dual-side surface-relief plastic diffusers with various cross angles. Metallic master molds bearing microstructures are fabricated using a tungsten carbide turning machine. A 1500-Watt ultrasonic vibrator with an output frequency of 20 kHz was used to replicate the microstructure onto 1 mm thick PMMA and PET films 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, high-performance dual-sided plastic diffusers of various cross-angles can be successfully fabricated. The proposed method shows great potential for fast fabrication of micro-optical components due to its simplicity and versatility.
©2009 Optical Society of America
Plastic diffusers have been widely used in various 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 an incoming beam. This allows for directed intensity light patterns with high efficiency. In general, the diffusers can be classified into two types: the particle-diffusing type diffuser which relies on the transparent beads inside the plastic films or plates to scatter light, and the surface-relief type diffuser which relies on the microstructures on the surface of the plastic films or plates to scatter light.
Many methods have been developed to fabricate the surface-relief diffusers by replicating the microstructures onto the surface of plastic films, including PDMS replica molding , silver halide sensitized gelatin method , holographic recording , 3D diffuser lithograph , photofabrication [5,6], hot embossing , and roller extrusion , 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. This goal of this report is to develop an efficient process for fabricating dual-side surface-relief plastic diffusers with different cross angles.
This study reports a simple and effective ultrasonic embossing process for directly replicating microstructures onto both sides of plastic films. The materials used included a semi-crystalline PET and an amorphous PMMA films. Master molds bearing different V-shaped microstructures were employed to emboss the parts. By changing the orientation of the bottom master mold, dual-sided plastic diffusers with various cross-angles can be successfully manufactured. The uniformity, profiles and optical properties of the fabricated diffusers are verified with microscope, surface profiler, and haze meter.
2. Experimental setup
The plastic films used in this study included polyethylene terephthalate (PET) and polymethylmethacrylate (PMMA) with a thickness of 1 mm. A 1500-Watt ultrasonic vibrator was used for all the experiments. The output frequency of the machine was 20 kHz. Figure 1 shows the setup of the ultrasonic embossing facility.
To emboss microstructures on two sides of the plastic films, master molds containing microstructures were first manufactured using a turning process. A tungsten carbide turning tool was used to machine the microstructures. Microstructures of two different sizes, i.e., “greater” and “smaller” ones, were adopted in this study. The top “greater” microstructure was directly machined on the vibration horn which is made of Al-Ti alloy, while the bottom microstructures (including a “greater” and a “smaller” ones) were made onto two mild steel molds. The shape, height and width of the microstructures on the molds were measured and inspected using surface profiler (Alpha-Step 500, TENCOR, USA), and scanning electronic microscopy (Hitachi S-3000N, Japan). Figures 2a and 2b show the measured profiles and SEM images of fabricated greater and smaller microstructures on the molds. The average depths of the greater and smaller microstructures are 150 μm and 20 μm, respectively.
During ultrasonic embossing, the microstructures on the top vibration horn and the bottom master mold come into contact with the plastic plate, and the plastic films vibrate in phase with the horn to transfer the energy. The ultrasonic energy is converted into heat through intermolecular friction within the thermoplastics. The generated heat, which is highest at the surface between the horn (or the master mold) and the plastic films due to asperities, is sufficiently high to melt thermoplastics at the interfaces and cause the melt to flow and fill the microstructures. After the vibration stops, the horn holds the plastic films against the master mold for some time for cooling. Once the plastic is cooled down, the horn is released and the plastic diffusers with dual-sided surface-relief microstructures of different sizes (Figs. 3a and 3b) are fabricated.
By shifting the bottom mold horizontally (see Fig. 1), diffusers with offset microstructures (i.e., with the top-bottom microstructures of peak-to-valley as shown in Fig. 3c) can be fabricated. Furthermore, to manufacture diffusers that have top and bottom microstructures of different cross-angles, the bottom master mold used in this study can be rotated. By adjusting the orientation of the bottom mold, diffuser of various cross-angles can be obtained (Figs. 3d and 3e).
3. Results and discussion
3.1 Effects of processing parameters on the replication quality of microstructure
Various processing parameters were studied in terms of their influence on the replicability of ultrasonically embossed plates: embossing pressure, vibration time, and hold time. The mold bearing the greater microstructure was selected as the bottom master mold to manufacture single side diffusers. After ultrasonic embossing, the microstructures embossed on the plastic diffusers were measured. Tables 1 and 2 show the uniformity of replicated microstructures at the central and at side areas for PMMA and PET respectively. The results suggest that the replicated depths at central area are higher than that at side area. This might be mainly caused by the non-uniform embossing pressure distribution across the plastic films, with a higher pressure at the center and a lower value at the sides. When a force is applied at the center by the horn, the materials at the center experience the highest pressure. The pressures gradually diminish from the center to the edge. Molded microstructure near the edge thus exhibits inferior replicability to that at the center. This problem can be overcome by adopting appropriate processing conditions (the values in bold in Tables 1 and 2).
The measured results in Tables 1 and 2 show that the proper embossing pressure for both PMMA and PET is 3 bars. In the embossing process, the conformability of the part to the mold’s microstructures is the major concern for the embossed films. By applying a higher embossing pressure, the plastic films can be made to vibrate in phase with the horn and transfer the energy to the film/mold to replicate the microstructure. During vibration, applying the embossing pressure the samples also causes the molten polymer to flow and to fill the embossing interface. Increasing the embossing pressure should therefore increase the replicability of the embossed plates.
Increasing the vibration time increases energy dissipation and is expected to increase the replicability. However, when the energy input is too high, the plastic films may over-melt. The embossed depths may be greater than the optimal values and the replicability decreases accordingly. The proper vibration times are found to be 1.5 and 2.5 seconds for PMMA and PET respectively. In ultrasonic vibration of thermoplastics, 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:
An atomic force microscope (AFM, DIMENTION, DI-3100) is also used to measure the surface morphology of the diffusers. The specimen is randomly selected from a single microstructure. The actual measurement area on the diffuser’ top surface is 7 μm x 7 μm. Figure 4 shows an AFM image and surface roughness of the fabricated diffusers. The average surface roughness (Ra) is 24.0 nm. The experimental result suggests the proposed method used in this study can successfully fabricate optical diffusers with good surface qualities.
With a combination of proper processing conditions, i.e., the values in bold in Tables 1 and 2, plastic diffusers with microstructures of high dimensional uniformity (96.88% and 97.86% for PMMA and PET respectively) could be obtained. This confirms the replicating capability by the ultrasonic embossing technique proposed in this study. The optimum proper embossing conditions are thus adopted to subsequently manufacture all the diffusers.
3.2 Optical properties of the manufactured diffusers
In this study, plastic diffusers of various types, i.e., with different microstructure sizes, single-side and dual-side, different cross angles, and with peak to valley offset, were fabricated. 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 fabricated diffusers with microstructures are measured and listed in Table 3 .
The measured results suggest that the overall performance of the films with microstructures is very much improved when compared to that of the flat films (Table 3). The hazes of dual-sided diffusers are all higher than those of the single-sided diffusers except the PMMA one with 150 μm/20 μm microstructures. By examining the replicability of the films, it is not difficult to find that while the 20 μm depth microstructure could be easily replicated onto the PET films, the replicability of the microstructure onto the PMMA films was less than optimal, even with the proper processing condition. This can be explained by the fact that PMMA has higher glass transition temperature (100°C) than that of PET (70°C). During ultrasonic vibration the energy is converted into heat through intermolecular friction at the surface between the master mold and the plastic films due to asperities. A mold of 20 μm in-depth microstructure did not provide surface asperity rough enough to generate heat to melt the PMMA at the interfaces and cause the melt to flow and fill the microstructures. Embossed parts thus exhibit inferior replicability and optical properties.
Offsetting the microstructures (i.e., peak-to-valley) of the diffusers improved somewhat the haze of PMMA films, but not for the PET films. Overall the influence of microstructure offset on the optical performance was found to be limited. Among the fabricated diffusers of various cross angles, the 90° one exhibits the best capability of light diffusion. PET diffusers with 150 μm/20 μm microstructures on both sides of the surfaces exhibit better light diffusivity than those with 150 μm/150 μm. However, the PMMA diffusers did not show the same trend. Again this might be due to the fact that the 20 μm in-depth microstructure was not well replicated onto the surface of PMMA film. Its diffusing efficiency decreased accordingly.
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. Figure 5 shows the images observed through the flat PMMA films and the films with single-side microstructures. As can be observed, the plastic films with microstructures displays better diffusing efficiency than the flat film. The results demonstrate that the fabricated diffusers can diffuse the light effectively. Furthermore, Figs. 6 and 7 show the images observed through the PMMA and PET films of 150 μm/150 μm and 150 μm/20 μm dual-side microstructures, respectively. Obviously the diffusers with 90° cross angle exhibit the best capability of light diffusion.
Finally, it should be noted that the plastic films used in this study are commercially available and low cost one. Despite the ultrasonic embossing process consists of two steps (film casting of polymeric resins into plastic films followed by ultrasonic embossing of the films), 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 seconds. Furthermore, by either rotating or shifting the bottom master mold, plastic diffusers with offset microstructures and various cross angles can be easily manufactured. This would provide significant advantages in terms of reduced fabrication cost and improved product quality.
This paper discussed an innovative and effective ultrasonic embossing method for the rapid fabrication of dual-side, angle-adjustable surface-relief plastic diffusers. Metallic molds bearing microstructures are fabricated using a tungsten carbide turning machine. A 1500-Watt ultrasonic vibrator with an output frequency of 20 kHz was used to replicate the microstructure onto 1 mm thick PMMA and PET films in the experiments. Under the proper processing conditions, high-performance dual-sided plastic diffusers of various angles and different microstructures have been successfully fabricated. Among the fabricated diffusers of various cross angles, the 90° one exhibits the best capability of light diffusion. Overall the influence of microstructure offset on the optical performance was found to be limited. Furthermore, PET diffusers with greater-smaller microstructure on both sides of the surfaces exhibit better light diffusivity than those with greater-greater one. It has been shown that ultrasonic embossing could provide an effective way of fabricating high-performance, low cost plastic diffusers with high throughput.
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