Development of Process Chain for Micro-Injection Molding

Abstract

In today’s continuously growing demand for components with increasingly smaller dimensions and features, micro-manufacturing is gaining more significance. For the mass production of plastic components with micro-features, injection molding is particularly suitable and still customary in order to keep target costs. Due to high requirements regarding lifetime and resistance to wear, the molds are made of hardened steel. The shaping of these molds involves electrical discharge machining (EDM). This process allows the generation of micro-structures in micrometer range having small inner radii, high dimensional accuracy and extreme aspect ratio independent of the workpiece hardness. As a work tool for EDM, electrodes made of pure copper (Cu) and tungsten reinforced copper (WCu) are commonly used. The shaping of the electrodes is conducted by micro-milling. For an overall understanding of this process chain, the interaction between micro-milling, micro-EDM, and micro-injection molding must be evaluated. This paper provides knowledge on the limits of each process with regard to burr formation, form accuracy, structure size and aspect ratio. Many factors throughout the process chain affect the size of an attainable micro-feature of a final product. The proper selection of the electrode material is a key factor in the process chain. As the feature size of the electrode defines the dimension of the final product shape, the smallest possible structures have to be found during micro-milling. The dimension of the eroded cavity is defined by the feature size of the electrode in combination with the lateral working gap. Eroded cavities with small inner corner radii and steep flanks can be generated when applying flawless and burr-free electrodes. However, using electrodes in inadequate conditions can lead to worse outcomes. The quality and reliability of the final product are determined to a great extent by the design of the injection molding process. Due to the arising vacuum when evacuating the remaining air in the mold, the inflowing melt is being distributed equally. It must be guaranteed that during injection molding, the mold is thermally-controlled. This prevents premature solidification. Through a combination of these two strategies, a form filling rate in micro-cavities up to 100 % is carried out. By deriving an impeccably adapted process chain, a precise micro-molding can be performed. This leads to the capability of manufacturing bars on a final plastic part with a width and height as low as 55 x 100 µm, and a length of 2 mm.