Diffuser shaped film cooling holes are increasingly being applied in the turbine blades of advanced aeroengines. It is a difficult problem to machine this kind of cooling holes due to their complicated shape and superalloy material. Currently, die-sinking electrical discharge machining (EDM) is regarded as an available process for machining the diffuser holes. However, the change of tool electrode shape caused by the electrode wear and difficult renew of dielectric fluid affect the processing precision. The worn tool electrode has to be replaced with a fresh one after machining one or several holes. The replacing procedure is time consuming to break the machining process. In the previous work, a novel block divided EDM process was proposed to efficiently machine the diffuser holes, but the machining accuracy needs to be further improved. In this research, the block divided EDM process firstly is analyzed and simulated in detail to know the key factor affecting the machined accuracy. Furthermore, a self-repair method and a profile error compensation method of the tool electrode wear are proposed for improving the processing accuracy. The self-repair method is to plan and control the rotation motion of the tool electrode for maintaining its shape. The profile error compensation method is to modify the Z coordinate of the tool electrode for decreasing the machining error. Machining experiments verified that the proposed methods can meet the accuracy requirements of the film cooling holes with diffusion angle error < 0.1° and average profile error < 10 μm.

Tool electrode wear compensation in block divided EDM process for improving accuracy of diffuser shaped film cooling holes

In this article, we present a fabrication method that is capable of making (3D) microfluidic devices with multiple layers of homogeneous polystyrene (PS) film. PS film was chosen as the primary device material because of its advantageous features for microfluidics applications. Thermopress is used as a bonding method because it provides sufficient bonding strength while requiring no heterogeneous bonding materials. By aligning and sequentially stacking multiple layers (3 to 20) of patterned PS film that were achieved by a craft cutter, complicated 3D structured microfluidic devices can be fabricated by multiple steps of thermopress bonding. The smallest feature that can be achieved with this method is approximately 100 μm, which is limited by the resolution of the cutter (25 μm) as well as the thickness of the PS films. Bonding characteristics of PS films are provided in this article, including a PS film bonding strength test, bonding precision assessment, and PS surface wettability manipulation. To demonstrate the capability of this method, the design, fabrication, and testing results of a 3D interacting L-shaped passive mixer are presented.

A 3D microfluidic device fabrication method using thermopress bonding with multiple layers of polystyrene film

A rapid prototyping process was presented to fabricate a nylon honeycomb microstructure coated with parylene C. The surface structure was designed to obtain a hydrophobic surface using the volume of fluid (VOF) model. With the micro-molding technique, the contact angle of the polymer surface could be designed and fabricated by changing the different microstructure surface die-steel mold inserts. For the honeycomb (20 μm width and 60 μm depth) microcavity side wall, an average micro-molding filling percentage of 95% could be achieved by using a three-section constant-pressure molding process. The solid surface wettability is governed by both the geometrical microstructures and the surface energy. A 2 μm parylene C layer was deposited on the nylon honeycomb microsurface to reduce the surface energy. To design honeycomb structures with different microcavity densities, the contact angle of these artificial surfaces could change from 91° to 130°. From a comparison of the contact angle measurements with the different models, the honeycomb-structured microsurface could be described by the Cassie–Baxter model. The errors between the VOF simulation and the measured data were <10%. The drag reduction performance of the honeycomb microplates was investigated in a water tunnel with a high Reynolds number (from 0.5×106 to 4.6×106). As a result, the honeycomb microplates showed a maximum drag reduction rate of 36±0.6% in comparison with the bare plates in such turbulent flow. Benefiting from the replaceable mold insert, more designable microstructure polymer surfaces can be manufactured by this rapid prototyping technique. A fast and efficient method for mass-producing hydrophobic surfaces with various designs could be used to reduce drag in ships. Techniques for rendering surfaces with hydrophobicity, which can reduce frictional drag considerably and improve a ship's fuel efficiency, have attracted considerable attention. However, fabricating hydrophobic surfaces with different architectures has proved challenging. This led Zhenyu Wang and colleagues from Peking University in China to develop a speedy and versatile technique for mass-producing micro-beehive cavities in a polymer substrate, greatly increasing its hydrophobicity. By using a fast exchange mold-insertion process, the researchers were able to efficiently fabricate and assemble micro-beehive surface structures with the desired contact angle, leading to hydrophobic surfaces that can be tailored to specific applications, including antifouling and drag reduction in ships.

/pdf/a-designable-surface-via-the-micro-molding-process-3x40wf1itm.pdf

A designable surface via the micro-molding process

A review on the use of microslit for various applications and its fabrication

It is challenging to fabricate micro features on an injection-molded polymer product. Ultrasonic vibration induced into micro-injection molding is helpful for flow of polymer melt. In this paper, a micro-injection mold integrated with ultrasonic vibration was designed and fabricated, and micro embossment was machined on the surface of the ultrasonic vibrator. Poly(methacrylic acid methyl ester) (PMMA) was used for injection molding experiments, with four ultrasonic power levels (0, 300, 600, and 900 W), three injection speed levels (60, 80, and 100 cm3/s), two injection pressure levels (60 and 90 MPa) and a mold temperature of 60 °C. It was found that ultrasonic vibration perpendicular to the middle surface of the cavity is beneficial in forming transverse microstructure, but is not conducive to generating longitudinal microstructure. Increase in injection pressure can improve molding qualities for both the longitudinal micro groove and the transverse micro groove. Increase in injection speed is not conducive to forming the longitudinal micro groove but benefits formation of the transverse micro groove. When ultrasonic vibration is applied at the injection and packing stages, molding quality of the longitudinal micro groove becomes worse, while that of the transverse micro groove becomes better.

/pdf/experimental-study-of-injection-molding-replicability-for-3qvalepp.pdf

Experimental Study of Injection Molding Replicability for the Micro Embossment of the Ultrasonic Vibrator

A method of fabricating micro-core on hard materials by electro-discharge machining (EDM) with multi-station narrow slit electrode was proposed. Instead of changing electrodes traditionally, the method of transforming the electrode station was applied, and the positioning error could be avoided. With the narrow slit structure of the electrode, the discharge debris could be excreted easily and the high depth-to-width ratio of micro-core could be fabricated. NAK80 was selected as the core material, while copper-tungsten alloy (70 % W) as the electrode material to carry out the fundamental experiments in which the relationship between the number of electrode stations and forming accuracy of micro-core was researched. The results demonstrate that using four electrode stations, the dimensional precision is within 3 μm, the root radius is within 10 μm, and the surface roughness is Ra 0.198 μm, and all this can be satisfied with the demand of molding for microfluidic chip. On the basis of the fundamental experiment results, a variable section micro-core was manufactured. The machining results show that the forming accuracy of the convex becomes worse with the decrease of the forming convex height. This phenomenon was analyzed in EDM principle and the error reflection mechanism.

Fabricating micro-core by EDM with multi-station narrow slit electrode

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Fabricating micro-core by EDM with multi-station narrow slit electrode