Micro metal injection molding (MicroMIM)(15.1-15.4.2)

Table of Contents

• 15.1 Introduction

• 15.2 Potential of PIM for microtechnology

• 15.3 Micromanufacturing methods for tool making

• 15.3.1 Manufacture of microtools-General issues

• 15.3.2 Options for manufacturing microstructured mold inserts

• 15.3.3 The LIGA method

• 15.4 PIM of microcomponents

• 15.4.1 Feedstocks for microcomponent PIM

• 15.4.2 MicroPIM-Molding procedure

15.1 Introduction

This chapter discusses the state of research and development of microcomponent powder injection molding (MicroPIM) of metals and, to a lesser degree, ceramics. Ceramics are referred to where innovative developments have taken place which have not yet been tried with metal materials, but will most probably be attempted in the near future.

The particular microspecific features of PIM are discussed in the following sections. Section 15.2 considers the contribution of PIM to microsystems technology. Since it is necessary to review microspecific tool making, Section 15.3 describes the various methods needed to produce microstructured mold inserts. Section 15.4 covers the special features of MicroPIM, the powders used, molding and thermal process steps, and the current capabilities of the process. Particular variants of the process are described in Section 15.5. Section 15.6 deals with the simulation of MicroPIM,focusing on the challenges and possibilities for process improvement using modified material models. Section 15.7 summarizes important research and development requirements and discusses the most promising developments in the field of MicroPIM.

15.2 Potential of PIM for microtechnology

Microsystems technology is considered one of today's most promising future technologies. Its innovations have been used in various markets, including information technology, life sciences, automotive engineering, and power engineering, in the white and brown goods industries, in machine construction, and in physical and chemical process engineering, to name but a few examples. The most successful microsystems products are largely manufactured from silicon or plastics, rather than metals or ceramics.

Microsystems technology is considered one of today's most promising future technologies. Its innovations have been used in various markets, including information technology, life sciences, automotive engineering, and power engineering, in the white and brown goods industries, in machine construction, and in physical and chemical process engineering, to name but a few examples. The most successful microsystems products are largely manufactured from silicon or plastics, rather than metals or ceramics.

In addition to the previously mentioned sectors, there are some fields, for example, chemistry, telecommunications and biology, and some products, for example, midget gears or counter mechanisms, that require highly resistant components made of metals or ceramics (see also Table 15.2 later in Section 15.4). The potential of metal- or ceramic-based materials is well known within precision mechanics applications that are subjected to high forces, corrosion, wear, high temperatures, or that demand low thermal expansion, biocompatibility, or sterilizability. It is necessary therefore to enhance manufacturing methods for products using metal-based components in microdimensions. Emphasis must be placed on the development of mass production methods to obtain profitable medium- and large-scale batches of complex microcomponents PIM is expected to become increasingly popular for the precision manufacture of complex microparts. Being a suitable method for use with nearly any powder material, it offers a wide range of processible materials (soft and hard magnetic materials, refractory metals, and functional ceramics). Often, there are hardly any other ways to process and apply these materials economically, other than by MicroPIM. 

Table 15.1 Optional methods for manufacturing microstructured mold inserts

 ^aOptional process step.

^bFeasibility boundary.

^cTolerances vary according to etching depth; tolerances will be 10-50 times larger if etching depth exceeds considerably 10 μm. 

^dGeometry-dependent.

^eConsidering line widths from 1 to 10 μm.

^fIntermediate mask.

^gShim.

^hDepending on geometry.

ⁱUnder restricted conditions only.

^j3D plus hollow and cut-back features.

Metal injection molding (MIM) should be compared to other manufacturing technologies in terms of the economic viability and production capability of the process. As a method that can be replicated and scaled up, MIM has advantages with regard to medium- or large-scale production when compared to typical machining processes such as milling, drilling, electrical discharge machining (EDM), and grinding. The same holds true for MIM when compared to laser structuring, particularly as less scrap is produced. When compared with well-established, large-scale processes like stamping, embossing, or powder pressing, MIM shows a much better capability for near-net-shape fabrication of complex geometries, including significantly reducing the need for finishing processes. These advantages, however, have to be judged against the higher processing costs caused by the addition of steps such as feedstock preparation and debinding.

15.3 Micromanufacturing methods for tool making

15.3.1 Manufacture of microtools-General issues

The molding of components with structural details in the micron or even submicrometer range requires appropriate tools and mold inserts. However, as the overall dimensions of the respective parts may range from hundreds of micrometers to several centimeters, special methods are required to manufacture these tool components. These specially manufactured microstructured mold inserts are usually incorporated into injection molding tools as interchangeable parts. Unlike mold inserts, injection molding tools can be manufactured using conventional precision tool construction methods.

If microparts with high aspect ratios (≥5) have to be manufactured, certain processes like core evacuation or variothermal temperization have to be used. The large variety of microparts means that nearly an equally large variety of single- or multicomponent micro powder injection molding (2C-MicroPIM) tools has to be produced. These tools may have, for example, two- or three-plate molds or may be designed with or without hot runners.

15.3.2 Options for manufacturing microstructured mold inserts

As described earlier, the tools typically used for microinjection molding are manufactured using conventional methods. The tools are then fitted with microstructured mold inserts that can be produced in various ways, including lithographic methods, laser ablation, erosion, optimized precision mechanics processes, and various other techniques. For a more detailed discussion of production methods. Micromachining and microdischarge machining are currently the most popular and most established methods. Finally, two or more structuring methods can be combined to manufacture microstructured mold inserts. Table 15.1 surveys the main manufacturing methods and major parameters.

Issues of wear resistance, which have not yet been thoroughly investigated for macroscale injection molding, become very important when looking at microinjection molding. An overview of interesting studies describes experiments performed in special test stands, which compared mold insert materials according to their respective wear resistances. It turned out, rather unexpectedly, that rates of wear are lowest for materials of a lesser hardness, for example nickel, while being highest for dispersion-hardening steels whose precipitated grain particles are virtually washed away by the action of the feedstock as it flows past. However, more research is required focused on mold wear as a major issue for future PIM technologies.

It is beyond the scope of this chapter to describe all the methods listed in Table 15.1 in detail. We will confine ourselves instead to discussing the LIGA method, as described in the following section.

15.3.3 The LIGA method

For certain applications, lithographic and electroforming methods are combined to manufacture mold inserts with intricate structural details and high demands on the side wall roughnesses and the aspect ratios. This method is referred to as LIGA. This German acronym stands for LIthographie, Galvanoformung, Abformung (lithography, electroforming, and molding). Over the last few decades research institutes around the world have used this process.

At the Karlsruhe Institute of Technology, the LIGA mold inserts are manufactured by gluing an aligned small plastic plate (mostly polymethyl methacrylate (PMMA)) onto a high-precision copper substrate with a properly pretreated, highly polished surface. The surface of the resist is also highly polished. The resist is structured by emitting hard X-radiation through a gold absorber mask with the desired structure. The irradiated resist areas are then dissolved through wet-chemical development (X-ray lithography). After optional partial or full-surface metallization through vaporization (as electroforming starter layers), the pattern of the plastic structure is invertedly transferred into metal (mostly nickel or nickel alloy) through electroforming. Wire-cut EDM is applied to give the tool its outer shape prior to or after substrate separation and final finishing of the mold insert and its complex surface by wet-chemical etching and wet-chemical ultrasonic cleaning. Examples for mold inserts produced by ultraviolet (UV)-based lithography are shown in Fig. 15.1A and B.

Fig. 15.1 (A) Typical LIGA mold inserts made by UV irradiation of SU-8 resist and subsequent electroplating. Both inserts carry inverse structures for replication of microfluidic functional units, the upper insert is made of nickel while the surface of the lower one consists of gold (the yellow micromold is an intermediate step of the final part; the next step consists in dissolving this thin gold layer (30 nm, yellow) and then the final nickel cavity is obtained). (B) SEM detail view of a further UV-LIGA master in photoresist SU-8; note the smooth surfaces and the different levels of microstructures generated by a three-step irradiation process. Electroforming onto this master will generate the micromold. 

The LIGA mold inserts are used, for example, as injection molding tools for series manufacturing of hollow waveguide microspectrometers or the manufacturing of optical or mechanical components, such as gearwheels or lenses. Such inserts are subject to very high demands relating to the material and surface properties, as well as the lateral structural and outer dimensions. To meet these demanding requirements, various improvements in handling, production sequencing and tooling have been developed in recent years and have considerably enhanced the safety of the LIGA process. LIGA technology, of course, has its limitations. In the case of narrow standing structures, detrimental overlapping effects might occur during electroplating, while filigree features, which have a large side-wall surface area, may get damaged during resist removal.

15.4 PIM of microcomponents

PIM of metal- or ceramic-filled feedstocks is well established in industry for the production of macroscopic parts which are used in products such as industrial machines or automobiles, white goods and brown goods, microelectronic and medical devices. High-precision, narrow-tolerance nozzles for bonding wires and ferrules serving as ceramic guide elements in optical fiber plugs are examples of microelectronic applications that come very close to MicroPIM. It seems only natural, therefore, that PIM technology, throughout the world, is being developed for the manufacture of microcomponents.

15.4.1 Feedstocks for microcomponent PIM

Binder systems

Microcomponent PIM requires the careful selection of feedstock components and compounding techniques. Classical PIM of hard metal or ceramic components uses medium-size powder particles of around 0.5-10μm. Macroscopic MIM is commonly performed with particles sized up to 20μm or even larger. MicroPIM, however, requires particle sizes in the micro- or even submicrometer range in order to meet the specific requirements regarding surface roughness, true-to-detail design, and the mechanical properties of the sintered part. In this context, it should be mentioned that surface roughness plays a much more important role than in macroscopic applications. One significant reason for this is the higher ratio between ground elevations and the structure size itself. A considerable argument for using powders that are as small as possible in MicroMIM is so that grain sizes are kept to a minimum after sintering in order to maintain a polycrystalline morphology. One option to try and ensure this outcome is to start the process sequence with the smallest possible powder particles.

MicroPIM needs feedstocks of a very low viscosity for rapid filling in the presence of high flow-length-to-wall-thickness ratios and to avoid the premature setting of the melt on account of the high thermal conductivity of the feedstocks. The binder systems, being responsible for the viscosity of the feedstocks, play an important role in compounding. A further issue covers the interaction of chain length and constitution with the capability of the binder to disperse and keep the powder particles separated effectively. Viscosity, however, is not the only criterion for determining the suitability of feedstocks for micromolding purposes. Since microcomponents are subject to high demolding forces due to relatively large surface-to-volume ratios and small load-bearing cross sections, equal importance is attached to the strength and stability of the green parts. For example, polyoxymethylene-based binders usually have high viscosities excluding them, at first glance, from the list of binders suitable for MicroPIM. The above-average green strength, however, displaces this disadvantage if the high viscosity is compensated by, for example, a variothermal process conduct. In addition, it must be mentioned that the microcavities cannot be cleaned easily once they have become blocked with feedstock.

Besides requiring low feedstock viscosities and high strengths of the green parts, MicroPIM, in the same way as classical PIM technology, depends on factors such as feedstock homogeneity, shelf life and recyclability, easy and environmentally friendly debindering, and controllable shrinkage behavior. The most popular binder systems for commercial and scientific uses are categorized as follows:

• polymer compounds consisting of waxes and/or thermoplastics;

• thermoplastic-based binders;

• thermoplastic-reinforced polyethylene glycol-based binders;

• water-based or coagulating binders.

While thermoplastic-based feedstocks and several variants of wax-based binders have become widely accepted, there are few examples of the application of thermoplasticreinforced and water-based types. However, as these two latter binder systems have advantages, such as the environmental friendliness of debindering in uncritical aqueous media, they are expected to be used increasingly in future.

The workability of feedstocks is improved by the inclusion of additives to the binders. These additives may consist of, for example, low-molecular-flow agents and dispersants with cross-linking properties that are intended to ensure an optimum distribution of the particles in the binder and to prevent re-agglomeration. The dispersants have to be selected taking into account both the surface characteristics of the powder and the chemical structure of the binder substances (polarity). In addition, care must be taken to provide for an optimum powder-binder coupling to ensure higher filling degrees which, in turn, reduce the sintering shrinkage and minimize dimensional inaccuracies. High-grade MicroPIM feedstocks are also treated with such additives. The chemistry of these is not discussed later because it is similar to that of the additives used in the feedstocks of most macroscopic applications.

Metal powders

MicroPIM is performed using mainly metal powders from the standard PIM steels 17-4PH (1.4542, X5 CrNiCuNb 17 4) and 316L (1.4404, X2 CrNiMo 17 13 2), nonferrous metals such as copper, and the recently developed titanium, tungsten and tungsten alloy powders. Zirconium ceramic and aluminum oxide ceramic powders are mostly used for molding optical fiber ferrules and wire bond nozzles. Nitride ceramics, for example silicon nitride ceramics, are still at an advanced stage of development. An overview covering the range of the most important types of metal powders used for MicroPIM to date is provided by Table 15.2.

Table 15.2 Metal materials currently applied for MicroMIM, typical powder sizes

^aFiner fractions tested on laboratory level.

^bAR <1.

Micromolding is mainly performed using metal powders that consist of particles with less than medium-sized diameters. As a general rule, the powder particle sizes should amount to no more than one-tenth or, preferably, one-twentieth of the size of the smallest detail of the cavity. The d50 values of the steels are typically in the range of 1.5-4.5μm, but fractions up to 10μm or above are also used. The major MicroMIM steels 316L and 17-4PH are available as finest <5μm fractions. Powders of still finer structures or other types of steel can be obtained by, for example, air separation. Pure iron powders, just as the steels mentioned earlier, are available as fractions with particle sizes of 1.5μm and below.

Looking at the nonferrous metals, while fine fractions are available for copper and nickel, the minimum d50 values of titanium amount to approximately 20μm, which is rather large for the purpose of microcomponent molding. On the other hand, it must be mentioned that due to the expressive pyrogenic nature of titanium, the powder particle size cannot be reduced infinitely. 

Prealloyed powders are recommended to avoid the high shear rates that often occur during MicroPIM molding and cause rapid separation of the individual alloying substances. Furthermore, prealloyed powders show advantages with respect to dimensional details and surface quality as a result of a better homogenized mixture. Micromolding works best with gas-atomized powders consisting of globular or spherical particles that ensure high powder filling rates and acceptable feedstock viscosities.

15.4.2 MicroPIM-Molding procedure

MicroPIM techniques have been developed using special machines and tools designed for polymer-based microinjection molding. This special basic equipment will be introduced later. It should be noted that the expression "micropart" covers a lot of quite different devices. These range from tiny singular parts with a weight of only a few milligrams to so-called microstructured parts which are large, mostly flat base plates with microstructural features on the surface. As a consequence, different machine types have to be applied. Custom microinjection molding machines have been developed by companies like Sodick Co. Ltd., Japan, or Arburg GmbH +Co KG, Germany In the case of microstructured parts, precise but still commonly configured injection molding machines with clamping forces in the range of 15-50t are sufficient. To achieve the best accuracies, electrical driven units are preferable.

For the filigree singular parts, however, special machines or at least sophisticated injection units had to be developed. To obtain the smallest injection volumes, they are usually not equipped with standard screw units but with one or two plunger injection facilities. As the plunger diameters can be much smaller than those of the screws, even the lowest shot volumes can be measured out and injected precisely. As an appropriate example a microinjection molding machine developed and distributed by Wittmann Battenfeld GmbH, Austria, is shown in Fig. 15.2.

Fig. 15.2 Injection molding machine MicroPower 15 equipped with a special screw/plunger system for replication of microcomponents. Although originally designed for utilizing polymers it has been proven for the processing powder filled feedstocks as well.  

With respect to the relatively high injection pressures, and to avoid flashing effects, the clamping force in general shall be chosen higher than calculated by the projected area of the microparts.

Several microinjection molding machines or injection units, respectively, have been developed and are now available on the market. They require only a few additions in order to adapt them for MicroMIM. For example, they should have the advantage of better wear resistance, i.e., coated steel units or even hard metal should be used. As the typical micromolding machines have been designed for polymer materials, the plasticizing units should be modified for a good homogenization of PIM feedstocks, for example by an optimized geometry of the extruder screw.

Mold inserts for MicroPIM are characterized by their filigree detail dimensions. It is not surprising, therefore, that additional features had to be developed to adapt injection molding processes to the requirements of micromold insert manufacturing. Two relevant examples, evacuation and variothermal tempering of the injection molding tools, will be discussed later.

Cavities in a typical micromolding tool are "blind holes" i.e., they are impermeable to gas at the bottom. The compressed, heated air that would develop if the heated feedstock was pressed into such cavities would cause burning of the organic share of the binder. While this so-called "diesel effect" can be remedied when occurring in macroscopic injection molding processes, it cannot be avoided in microstructures whose cavities cannot be provided with common ventilation slots as used in laminate tools or bores. This problem can be adequately solved by evacuating the tool directly prior to injection using a tool-connected vacuum pump. As a rule, pressures of 1 bar or less are achieved in the cavities.

Variothermal temperature control is performed in case of tools with high flowlength-to-wall-thickness ratios. During this special process, temperatures close to the melting point of the feedstock have to be reached prior to injection, thus ensuring that the viscosity of the latter remains high enough for molding the<