Key Engineering Materials Vol. 625

Abstract: Microlens arrays have been fabricated by 3D diffuser lithography in this study. The method mainly adopts two kinds of diffuser films with different transmittances and hazes, integrated by photolithography, polydimethylsiloxane (PDMS) molding and UV forming techniques, to get microlens arrays with different parameters and geometries. The features, such as height, geometry and fill factor of microlens arrays, are controlled by photolithography, using a photomask with circular holes and different exposure doses. The microlens arrays can also be duplicated and transferred to the surface of flexible polyethylene terephthalate (PET) substrate through PDMS molding and UV forming processes. Finally, the outcoupling efficiency of microlens arrays attached to organic light-emitting devices (OLEDs) can be measured and analyzed. More than 60% enhancement of luminous current efficiency can be obtained in experimental results.

Abstract: Reflecting lens is an important component of optical systems, such as high-resolution cameras, large space telescopes and meteorological satellites etc. Among the lens materials, Silicon Carbide (SiC) has attracted a lot of attention as an important optical material because of its excellent mechanical and physical properties. Apart from the form accuracy, the attainment of a consistently high optical quality in polishing SiC is still of a concern. There are advanced ultra-precision polishing machines that can correct geometrical errors and surface finish of the workpiece. These include surface roughness and waviness. However, the hardness of SiC material itself put an challenge for polishing process. In this paper, A computer controlled ultra-precision polishing (CCUP) method based on mechanical polishing is used to produce the SiC lens. Experiments are being designed on a 7-axis ultra precision polishing machine (Zeeko IRP200). As it is difficult to find out slurry which is harder than SiC so that the conventional polishing slurry is be used. It provides a nice consequence that it also efficient when the polish powder is softer than the machined materials. The tool pressure, polishing head speed and the feed rate are varied and optimized to obtain the best reflectivity of the lens being polished. A pilot experiment will be conducted for the corrective polishing for the form error of the optical surface made of SiC. The result from the study will provide an important means to optimize the process for machining SiC reflective lens using the CCUP process.

Abstract: There are numerous parameters and steps involved in a computer controlled ultra-precision polishing process (CCUP). The success of CCUP relies heavily on the understanding and optimization of material removal when new materials and new surfaces are polished. It is crucial to optimize the polishing parameters to enhance the effectiveness of the polishing process and to assess the impact of different process parameters on the material removal rate of particular difficult-to-machine materials such as CoCr alloys, which is commonly used in orthopedic implants. This paper aims at studying the process parameters and optimization of the parameter to enhance the material removal rate and quantify the contribution of process parameters.

Abstract: The practice shows that complex lapping motion trajectory can decrease the adverse effects caused by the lapping direction which has a great influence on the lapping of the natural single crystalline diamond cutting tool. For example, addition of a reciprocating motion can get better lapping performance and surface roughness. On some kinds of natural diamond cutting tool lapping machines a further improvement is made by designing an additional planetary spindle to provide planetary motion to the main shaft, which offers a more complex motion trajectory. With an increase in degrees of freedom of the lapping machine, it becomes difficult for technicians to master a good suit of motion process parameters. This paper proposes a set of methods to plan the lapping trajectory with respect to the machine that can perform complex lapping motion. Firstly, the proposed method designed a statistical parameter which is the variance of the direction angle distribution for the lapping velocity and its calculation method to evaluate the complexity of the trajectory. And then the multi-body kinematics model was established by using Simulink tool box in Matlab software for the purpose of model kinematics simulation and outputting data for calculation of the statistical parameter above. Finally, the genetic algorithm was used to find the best motion parameters of every shaft under certain restrictions. The advantage of the set of lapping trajectory planning method presented in this paper is that it combines the powerful multi-body simulation and signal processing ability of Simulink tool box and the characteristics of the genetic algorithm in almost no limitation on awaiting optimization function and suitable for complicated problems. So the method provides not only a way for planning complex diamond lapping trajectory but also a general approach to solve the problem on machine motion process parameters optimization.

Abstract: Single crystal sapphire is widely used as the material for precision equipments, due to its high hardness, chemical inertness and light transmission. However, it is difficult to obtain a scratch-free and damage-free sapphire surface with high-efficiency through traditional mechanical polishing or etching. We developed plasma assisted polishing (PAP) for the finishing of difficult-to-machine materials, such as silicon carbide, diamond, and sapphire. In this article, preliminary research results are showed about PAP applied to polishing of single crystal c-plane sapphire substrates. Combination of helium based atmospheric pressure water vapor plasma irradiation and silica abrasive polishing drastically increased removal rate of the sapphire c-plane. XPS measurements of the surfaces with and without irradiation of water vapor plasma revealed that alumina hydrate was formed by plasma irradiation at low temperature of less than 40°C. It is assumed that formation of alumina hydrate promoted the removal rate of sapphire.

Abstract: Due to the small feed rate used in micro-machining, ploughing force needs to be considered in addition to the chip formation force. A new analytical model has been proposed to calculate cutting forces of micro-grinding process based on the process configuration, work piece material properties, and micro-grinding tool topography. The proposed approach allows the calculation of cutting force comprising both the chip formation force and ploughing forcec considering single grain interaction.

Abstract: Atmospheric Pressure Plasma Processing (APPP) of silicon-based optics and wafers is a form of chemical etching technology developed in recent years. The material removal rate is comparable to those of conventional mechanical processing methods in precision fabrication. Moreover, there is no mechanical contact or physical loading on the substrate surface, hence no surface or sub-surface damages are induced. Inductively coupled plasma is one realization of APPP. In this work, inductively coupled plasma torch is used to generate plasma and excite etchant particles at atmospheric pressure. These active particles then diffused to the workpiece surface, react with its atoms to form volatile products. The activity and number of particles in plasma are influenced by processing parameters such as input power, distance between nozzle and substrate surface, flow rate of plasma gas argon and precursor gas CF₄. These factors have various impacts on material removal rate. Processing experiments are conducted on fused silica to investigate the parameters’ influences on material removal rate. The basic interaction between substrate surface and plasma is illustrated, then the relationships between processing parameters and material removal rate are analyzed. From the experiments some trends are derived. Material removal rate rises with the increase of power and flow rate of CF₄, whereas decreases with the increase of processing distance, etc. The etching footprint is proved to be near Gaussian-shaped and believed to have high potential for deterministic surface processing.

Abstract: Microstructured optical elements made of glass are generally replicated by hot pressing with super-hard materials, such as binderless tungsten carbide (WC) and precision ceramic. However, in grinding of microstructures, problems frequently occur in terms of rough ground surface, chipping and rounding of micro-structures edges when compared to conventional grinding. In order to overcome these technological constraints, a promising precision grinding method for microstructured surfaces that applies ultrasonic vibration to improve the surface quality, and protect the edges and tips of microstructured surfaces is presented. The experimental investigation of ultrasonic vibration assisted grinding of microstructures on binderless WC is researched. The effects of ultrasonic vibration on surface roughness, form accuracy and edge radius were analyzed. The morphology of surface and array edges was examined with a scanning electron microscope (SEM), while the surface roughness was measured by a laser interferometer. And a contact probe profilometer was used to assess the form of array and radius of microstructured edges. Experimental results showed that the application of ultrasonic vibration leads to significant improvements of the surface roughness and edges of microstructures compared with traditional precision grinding processes. A micro cylinder lens array of binderless WC with surface roughness of 78nm and edge radius of less than 1μm was obtained. The novel grinding method is feasible and applicable in machining higher form accuracy microstructures.

Abstract: Biodegradable stents have been used for the treatment of cardiovascular diseases and are often placed inside coronary arteries. The manufacture of Biodegradable stents is an challenging issue because of the features, like geometry design, machine processing way and the material select of stents. An improved manufacturing process of biodegradable stents is presented. The proposed processes consist of the following steps: tube drawing (is use tube sinking manufacture), laser cutting and surface modification (annealing and ultrasonic cleaning). The precision injection and drawing operation is used to obtain the required wall thickness for seamless tube. The laser cutting is performed to change the shape of the stent, and the finishing operations are selected to modify the surface features, like smoothness and texture of stents. The smoothness of surface finish is one of the properties that determine the performance of a stent. Therefore, the surface modification process (vacuum annealing and ultrasonic cleaning) is important in the production of stents and in clinical treatments involving stents. Annealing is the key technology that affects the material’s final crystal grain size. Ultrasonic clean was of the stents to remove the slag (oxides) and burrs formed in laser production. Poly Lactic Acid material sample with an outside-diameter of 1.5 mm, a wall thickness of 0.08 mm, and a length of 10 mm, had been manufactured and demonstrated the proposed manufacture technology. The surface roughness of a stent manufactured is the value of Ra 13.7 nm which basically meets the design requirement for further performance evaluation. It is hoped that the above finding can be used for future study of stents.

Abstract: This paper presents an integrated roller embossing process named Continuous Injection Direct Rolling (CIDR) process. The CIDR process is similar to Continue Casting Direct Rolling (CCDR) process for metal material. The process consists of Plastic Injection Module, Injection Nozzle, Precision Rolling Module, and Automatic Coiling Module. In the proposed CIDR process, the plastic plate with microstructured pattern is produced by the integrated injection and rolling process, which is different from the current rolling or plastic injection process. Some key technologies including injection nozzle design, roller design, roller coupling, and ultra-precision machining of rollers, are presented. An on-machine measurement method is also proposed and presented to avoid the incontinence and errors caused by disassembling the roller workpiece for off-line measurement. The results of the pilot study are also discussed. The research work provides an enabling solution for precision manufacturing plastic plate and film with microstructured patterns.