Microinjection Molding of Microsystem Components: New Aspects in Improving Performance
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IOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING J. Micromech. Microeng. 23 (2013) 093001 (21pp) doi:10.1088/0960-1317/23/9/093001 TOPICAL REVIEW Microinjection molding of microsystem components: new aspects in improving performance Can Yang1, Xiao-Hong Yin and Guang-Ming Cheng Laboratory for Advanced Material Processing, Zhejiang Normal University, Jinhua, Zhejiang, 321004, People’s Republic of China E-mail: [email protected] Received 17 March 2013, in final form 17 May 2013 Published 1 August 2013 Online at stacks.iop.org/JMM/23/093001 Abstract Microinjection molding (μIM) is considered to be one of the most flexible, reliable and cost effective manufacturing routes to form plastic micro-components for microsystems. The molding machine, mold tool fabrication, material selection and process controlling in this specific field have been greatly developed over the past decades. This review aims to present the new trends towards improving micro-component performance by reviewing the latest developments in this area and by considering potential directions. The key concerns in product and mold designing, essential factors in simulation, and micro-morphology and resultant properties are evaluated and discussed. In addition, the applications, variant processes and outlook for μIM are presented. Throughout this review, decisive considerations in seeking improved performance for microsystem components are highlighted. (Some figures may appear in colour only in the online journal) 1. Introduction required in certain applications. Since the 1990s, polymer materials have been increasingly incorporated into MEMS Micro-electromechanical-systems (MEMS) or microsystem industries. An important reason is that they provide not only technology (MST) has already become a familiar part of controllable thermal, mechanical and electrical properties, but our daily life in that various micro-components can be also unique chemical, structural and biological functionalities found in a number of devices such as laptops, microphones, not available in any other material. On the other hand, polymer- accelerometers, gyroscopes, electronic compasses, cell phones based MEMS components possess significant cost advantages and many more [1]. Moreover, MEMS exhibits a vast potential thanks to the cheap materials and corresponding processing for future development with increasing demand for maximal procedures without cleanroom confinement [4]. functionalities in minimal space. According to a recent report To date, micromolding techniques, such as injection [2], the MEMS market is forecast to grow to approximately molding, reaction injection molding, hot embossing, US$21 billion in 2017 from US$10.2 billion in 2011 with a injection-compression molding and thermoforming, have compound annual growth rate of 13%. been established for manufacturing micro products from In MEMS industries, there are several materials, such thermoplastic polymers (see a topical review [5]). Among as metal, glass, quartz, silicon and polymer, available for these processes, microinjection molding (μIM) has the manufacturing micro-components [3]. Each material has advantages of mass-production capability, short cycle time, its own characteristics and offers suitable functionalities easiness for full-automation, accurate dimensional control and so forth [6]. Obvious growth of activities in this field can be 1 Author to whom any correspondence should be addressed. demonstrated by a simple search in the Engineering Village 0960-1317/13/093001+21$33.00 1 © 2013 IOP Publishing Ltd Printed in the UK & the USA J. Micromech. Microeng. 23 (2013) 093001 Topical Review • Special machinery is needed for the purpose of accurate melt metering. Specifically, most μIM machines adopt separate plasticization and injection units (see figure 2 for demonstration). Based on previous works [10–13], the updates of some currently available commercial μIM machines are provided in table 1, from which one can find that a screw diameter down to 8 mm and a piston diameter as small as 3 mm have been utilized. It should also be noted that in most cases screws are employed in plasticization units thanks to their better mixing performance, while pistons are used in some cases where the material degradation becomes an issue. • Due to the extremely high cooling rate caused by a tiny cavity, the mold temperature (Tm) needs to be increased over the glass transition temperature (Tg)ofthematerial of interest to avoid premature solidification of the melt (usually T > T + 30–40 ◦C[14]). μ m g Figure 1. Numbers of IM-relevant publications as a function of • Thorough venting (mostly in the form of vacuuming) is the year. indispensable during μIM in order to prevent the melt platform for μIM-relevant publications (figure 1). Even though from being stopped or even burnt by high-temperature some publications might adopt ceramic or metal powders gas trapped inside the micro-cavities. instead of polymers as materials, the trend of continuous • Some normally neglected factors such as microscale growth is clear. Similarly, from a practical viewpoint, the viscosity, wall slip, melt viscoelasticity and surface growth can also be seen in the industrial activities reflected tension effect, should be considered owing to their by the increased number of issued μIM-relevant US patents increased influence. Therefore, special care has to be taken (762 in 2001 versus 3962 in 2011) [7]. in μIM-relevant simulation and industrial practice. When it comes to micro-components, an often-asked • Novel μIM flow field associated with large deformation question may be how small is really small, or how do we and cooling rates provides preconditions for forming define small? Since the proposal by Kukla et al [8], the micro-components with distinct micro-morphology and criterion as to what can be considered as micro products has resultant performances, leading to an extension of μIM been continuously renewed, thanks to the rapid development applications. of micro fabrication technologies. As a sub-subject of After three-decades of development, great micromolding, μIM shares three kinds of definitions for accomplishments have been fulfilled in the μIM area. micro products [6]. (i) Micro-parts possessing a weight in For instance, the advancements in machine development, the order of milligrams or being a fraction of a polymer mold fabrication, temperature control, and process optimiza- pellet approximately spherical in shape and 3 mm in diameter. tion have greatly increased the opportunities of producing Actually, an amazing micropart weighing only 0.12 mg has smaller and more complex micro products (refer to a recent been successfully microinjection molded [9], which means review [13]). It is noted that the existing research has focused that a sesame-seed-sized plastic pellet is enough to produce 520 pieces of such tiny components. (ii) Micro-featured on the manufacturability of micro products, mainly from parts having microstructures with a characteristic dimension the viewpoints of material selection and process control. In typically less than 100 μm. (iii) Micro-precision parts which recent years, new trends have emerged toward obtaining more have tolerances in the micron range (typically between 2 and accurate and performance-diverse microparts, stepping more 5 μm) without an overall dimension limit. As we have limited closely to the practical applications. The success in charac- the scope of this review to microsystem components, only terizing and controlling micro-morphologies and resultant μ aspects regarding micro and micro-featured parts, i.e., the first properties has led to increased possibilities of tailoring IM two definitions, will be reviewed and discussed in the rest of products for wider applications. The present work intends this work. to complete Giboz et al’s work [13] on recent developments μ As a miniaturized version of conventional injection in IM, highlighting both new trends and possible future μ molding (CIM), μIM shares the same main procedures directions, which could benefit the IM as well as the relevant including plasticizing granular resin, metering molten micro manufacturing communities. material, injecting melt into the mold cavity, packing the injected melt for shrink compensation, cooling the molded 2. Considerations at design stage products, and finally demolding (see figure 2). However, this does not mean μIM is a simple scaling down of the Benefiting from the available CAD/CAE/CAM techniques, CIM process since some negligible factors tend to play an entire manufacturing chain consisting of product design, significant roles, and accordingly, need to be taken into numerical prediction, mold fabrication and then injection account. Comparing with CIM, the main differences for the molding has been well established in the CIM process. Some μIM process can be summarized as follows: manufacturability issues (e.g., minimal product thickness, flow 2 J. Micromech. Microeng. 23 (2013) 093001 Topical Review (a)(b) (c)(d) Figure 2. μIM process demonstration with separate plasticization and injection units: (a) plasticization, (b) mold closing, (c) injection, packing and cooling, and (d) demolding and re-plasticization for the next cycle. Table 1. Commercially available μIM machines with their key characteristics. Max. Theoretical Max. Screw (S) or Max. clamping injection injection piston (P) injection Model force volume pressure diametera velocity Manufacturer name (kN) (cm3) (MPa) (mm) (mm s–1) Lawton Sesame nanomolder 13.6 0.082 350 P10 1200 Desma FormicaPlast