ARTICLE IN PRESS
Optics & Laser Technology 42 (2010) 794–798
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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: shihjung@mail.cgu.edu.tw, journal.emailbox@gmail.com
(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.
ID
732957
Title
Rapidfabricationofsurface-reliefplasticdiffusersbyultrasonicembossing
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