Study of the Transcription Effects of Pressing Dies with Ultrasonic Polishing on Glass Molding

At a Glance

Metadata	Details
Publication Date	2021-11-21
Journal	Processes
Authors	Ken-Chuan Cheng, Chien-Yao Huang, Jung-Chou Hung, A-Cheng Wang, Yan-Cherng Lin
Institutions	Chien Hsin University of Science and Technology, Nan Kai University of Technology
Citations	3
Analysis	Full AI Review Included

Executive Summary

This research investigates the critical relationship between pressing die surface roughness, achieved via ultrasonic polishing (UP), and the transcription quality of Micro Lens Arrays (MLAs) fabricated through Precision Glass Molding (PGM).

Core Achievement: Successfully fabricated high-precision sine-wave MLAs capable of transforming a Gauss-distributed spotlight into a uniform straight line, demonstrating excellent optical function.
Methodology: SKD-11 pressing dies, initially cut by Wire Electrical Discharge Machining (WEDM), were finished using UP with diamond abrasives to achieve ultra-smooth surfaces.
Optimal Parameters: PGM achieved maximum transcription (99% Tr) at a working temperature of 690 °C and a pressing force of 250 N.
Surface Roughness Requirement: Only pressing dies polished to a surface roughness of 0.023 µm Ra or less could produce MLAs with sufficient transparency and precision to perform the required optical transformation.
Polishing Efficiency: Ultrasonic polishing (using #8000 mesh diamond) achieved a Roughness Improvement Ratio (RIR) of 90%, significantly higher than the 33% RIR achieved without ultrasonic vibration.
Failure Modes: High temperatures (over 690 °C) caused glass sticking, while high pressure (270 N) caused MLA rupture. Dies with Ra = 0.08 µm produced lenses with poor transparency, failing the optical test.

Technical Specifications

Parameter	Value	Unit	Context
Die Material	SKD-11	Mold Steel	Pressing Die
Glass Material	Soda Lime Glass	-	PGM Preform
Glass Diameter / Thickness	10 / 1	mm	PGM Preform
Glass Transition Temp (T_g)	564	°C	Soda Lime Glass Property
Optimal Molding Temperature	690	°C	Achieved 99% Transcription Ratio (Tr)
Optimal Pressing Force	250	N	Achieved 98% Tr without rupture
Sine Wave Amplitude (A)	100	µm	Optical Design Parameter
Initial Die Roughness (WEDM)	0.20	µm Ra	Before Ultrasonic Polishing
Required Die Roughness (Ra)	0.023	µm Ra	Required for successful optical function (99% Tr)
Poor Die Roughness (Ra)	0.08	µm Ra	Resulted in poor transparency (92% Tr)
Best Abrasive Mesh	#8000	-	Diamond Powder for Ultrasonic Polishing
Max Roughness Improvement Ratio (RIR)	90	%	Achieved with Ultrasonic Polishing
Ultrasonic Vibration Frequency	30	kHz	Polishing Tool Operation
Glass Composition (Primary)	73% SiO₂, 13% Na₂O, 10% CaO	%	Soda Lime Glass

Key Methodologies

The research followed a structured four-step process to design, fabricate, polish, and test the pressing dies and resulting MLAs:

Optical Design and Die Fabrication:
- A sine-wave lens profile (A = 100 µm) was designed using CAD/Zemax software to achieve Gauss-to-uniform straight-line transformation.
- Pressing dies (SKD-11) were initially cut using Wire Electrical Discharge Machining (WEDM).
Ultrasonic Polishing (UP) Optimization:
- UP was performed using a polymer rod with long fibers, combined with high-frequency (30 kHz) ultrasonic vibration and a CNC-controlled reciprocating path.
- Diamond slurries of various mesh sizes (#3000 to #28000) were tested.
- Optimal Polishing Recipe: 25 minutes of UP using #8000 mesh diamond abrasive reduced the surface roughness from 0.20 µm Ra to 0.023 µm Ra (90% RIR).
Precision Glass Molding (PGM) Optimization:
- Soda lime glass preforms were placed in the mold chamber, which was evacuated before heating.
- Temperature Optimization: Tested temperatures between 680 °C and 690 °C. Optimal temperature was 690 °C (99% Tr).
- Force Optimization: Tested forces between 210 N and 270 N. Optimal force was 250 N (98% Tr).
Evaluation and Verification:
- Roughness Measurement: Surface roughness (Ra) of the dies and resulting MLAs was measured.
- Transcription Ratio (Tr) Calculation: Tr was calculated as (Ra_glass / Ra_die) * 100%.
- Optical Verification: The finished MLAs were tested using a laser pen to confirm the transformation of the Gauss-distributed spotlight into a uniform straight line.

Commercial Applications

The successful fabrication of high-precision MLAs via optimized PGM and ultrasonic polishing techniques has direct relevance to several high-tech sectors:

Optical Systems and Illumination:
- Light energy distribution uniformity (e.g., in LED-based projection systems).
- Precision light source control in advanced optical setups.
Consumer Electronics:
- Fabrication of high-precision glass elements for 2.5D/3D smartphones.
- Components for Liquid Crystal Displays (LCDs), which utilize soda lime glass.
Metrology and Robotics:
- Precision measurements and calibration systems utilizing structured light.
- Robot visual systems requiring uniform illumination fields.
Manufacturing Technology:
- Establishing ultrasonic polishing as a cost-effective, high-precision alternative to traditional diamond grinding for complex mold shapes (like micro-aspheric or sine-wave dies).
- High-volume, short-cycle-time fabrication of high-accuracy glass elements (PGM).

View Original Abstract

The micro lens array (MLA) has played an important role in optical systems for the past few years, and the precision of pressing dies has dominated the quality of MLAs in glass molding. Few studies have covered the transcription effects on surface roughness of pressing dies for this technology. Therefore, this research utilized pressing dies to produce a sine-wave lens array on glass molding, to transform the Gauss-distributed spotlight into a uniform straight one and then characterize the transcription effects of these lenses. Pressing dies with a sine-wave shape were firstly cut by wire electrical discharge machining (WEDM), and then ultrasonic polishing using diamond abrasives was applied to finish the sine-wave surface with an original roughness of 0.2 μm Ra. Next, the sine-wave lens arrays were pressed by glass molding at the appropriate pressure and temperature, before evaluating the transcription effects of transforming the Gauss-distributed spotlight into a uniform straight one. The result showed that the sine-wave lens array stuck easily to the pressing die and then ruptured during glass molding due to the poor surface roughness of pressing tool. However, the diamond abrasive with appropriate sizes could establish good surface roughness on pressing dies via ultrasonic polishing, and the pressing die with a low surface roughness of 0.08 μm Ra was able to successfully perform MLA in the glass molding. However, only pressing dies with a surface roughness smaller than 0.023 μm Ra could produce precision glass lenses to transform the Gauss-distributed spotlight into a uniform straight one.

Tech Support

Original Source

DOI: https://doi.org/10.3390/pr9112083

References

2007 - Homogenized LED-illumination using microlens arrays for a pocket-sized projector [Crossref]
2020 - Thermo-viscoelastic Modeling of Nonequilibrium Material Behavior of Glass in Nonisothermal Glass Molding [Crossref]
2020 - Analyzing sustainable performance on high-precision molding process of 3D ultra-thin glass for smart phone [Crossref]
2002 - Use of micro ultrasonic vibration lapping to enhance the precision of microholes drilled by micro electro-discharge machining [Crossref]
2002 - Study of precision micro-holes in borosilicate glass using micro EDM combined with micro ultrasonic vibration machining [Crossref]
2008 - Precision treatment of silicon wafer edge utilizing ultrasonically assisted polishing technique [Crossref]
2012 - Combined ultrasonic vibration and chemical mechanical polishing of copper substrates [Crossref]