Microforming: Experimental Investigation of the Extrusion Process for Micropins and its Numerical Simulation Using RKEM

Abstract

Microforming using a small machine (or so-called desktop machine) is an alternative new approach to those using full-size heavy equipment for manufacturing microparts. Microparts are commonly defined as parts or structures with at least two dimensions in the submillimeter range, which are used extensively in electronics and micromechanical products. However, when scaling down a conventional forming process to microscale, the influence of the so-called size effect needs to be considered. The individual microstructure (size, shape, and orientation of grains) and the interfacial conditions show a significant effect on the process characteristics. In this paper, the process of extrusion is investigated to establish it as a viable process for microforming. A forming assembly is fabricated and used in conjunction with a loading substage to extrude micropins with a final diameter of 1 mm. The effect of grain size is investigated by using workpieces heat treated to produce grain sizes varying from 32 μm up to 211 μm. Two extrusion dies with different roughness are used to study the effect of surface finish. While experiments lead to interesting questions and new discoveries, theoretical or numerical solutions are necessary tools for process optimization. Here, knowing the limits of the current widely used numerical simulation tools [i.e., the Finite Element Method (FEM)], a new method, the Reproducing Kernel Element Method (RKEM), has recently been developed to address the limitations of the FEM (for example, remeshing issue), while maintaining FEM’s advantages, e.g., the polynomial reproducing property and function interpolation property. The new RKEM method is used to simulate the microextrusion problem. Its results are compared with that obtained from the FEM and the experiment result. Satisfactory results were obtained. Future directions on the experimental and simulation work are addressed.

Fundamentals of Micro‐Manufacturing

“…Mechanical micro-machining (or tool-based micro-machining) as scaled down versions of turning, milling, and drilling is rapidly gaining momentum in industrial applications because of its viability to directly produce miniature 3D functional parts from a wide range of materials with high precision [48,[52][53][54].…”

“…However, there exist some technological barriers. These can be on the size of the cutting tool; for instance, the diameter of small production micro-drills is about 25 μm, and the diameter of micro-milling tools is about 20 μm [17,48,52,53,[55][56][57]. It can have limitations on feature tolerance with error compensation between 250 and 500 nm, or on the flat and round surface produces; for example, roughness of micro-drilled hole walls is about 10-50 nm and roughness of diamond machined surfaces is about 5 nm [17,48,52,53,[55][56][57][58][59][60].…”

“…These can be on the size of the cutting tool; for instance, the diameter of small production micro-drills is about 25 μm, and the diameter of micro-milling tools is about 20 μm [17,48,52,53,[55][56][57]. It can have limitations on feature tolerance with error compensation between 250 and 500 nm, or on the flat and round surface produces; for example, roughness of micro-drilled hole walls is about 10-50 nm and roughness of diamond machined surfaces is about 5 nm [17,48,52,53,[55][56][57][58][59][60]. These limitations are a result of several factors such as lack of technologies to fabricate viable and economical smaller cutting tools, accuracy and repeatability of machine tool drives, tool deflections and vibrations (especially in micro-milling process), size effect, and minimum chip thickness requirements in mechanical micro-machining processes, which is always a problem in ductile-and coarse-grained polycrystalline micro-structure metals [48,53,59,61].…”

“…Many factors that can be neglected in macro-scale machining suddenly become significant in microscale machining; for example, material structure, vibration, and thermal expansion [52][53][54]60]. As a result, the application of micro-mechanical machining process is still limited.…”

“…No chip is produced with a chip thickness less than a critical value, and the entire material is forced under the cutting tool and deformed. Especially, in micro-milling, the elastic portion of the deformation recovers after the tool passes [52,72,73]. The minimum chip thickness requirement significantly affects machining process performance in terms of cutting forces, tool wear, surface finish, process stability, etc.…”