**1. Introduction**

212 Thermoplastic Elastomers

D, Biro,. G, Pleizier,. & Y, Deslandes. (1992), Application of the microbond technique Part III

Rohchoon, Park,. & Jyongsik, Jang. (2003), Effect of laminate thickness on impact behavior of aramid fiber/vinylester composites, Polymer Testing, Vol. 22, pp. 939-946 Ming, Cheng,. Clint, Hedge,. Jean-Pholippe, Dionne,. & Aris, Makris. (2010), Ballistic

Philip, Cunniff. (1992), An analysis of the effects in woven fabrics under ballistic impact,

Effects of plasma treatment on the ultra-high modulus polyethylene fibre-expoxy

resistance enhancement by adjusting stress wave propagation paths, Proceedings

Michael, Piggott. (2002), Load Bearing Fibre Composite, Kluwer Academic

of 25th international symposium on ballistic, Beijing China

Textile Research Journal. Vol. 62, pp. 1-15

interface, Journal of Materials Science Letters Vol. 11, pp. 698-701

Injection molding is, nowadays, a well known and wildly used manufacturing process to produce both thermoplastic and thermosetting polymeric material components, in a large scale, with accuracy and at low prices. Even though the electronics industry provides an economy of scale for the silicon industry, polymer devices can be produced in huge volumes maintaining the requested features and quality, with a great variety of material characteristics, a fact that has considerably open the market to injection molding of microcomponents.

A simple miniaturization/rescale of the conventional injection molding process is not valid due to problems related with the rheology of the polymer flow in the micro cavity/channel and requires that all the process layout, as well as adjacent technologies, should be reconsidered, enhanced and properly adapted. The dimensional reduction of the components requires a higher control of the overall accuracy of the devices. The molding blocks of this type of objects are bounded to an amplified wear due to the fact that the surface roughness is dimensionally very close to the dimensions being controlled. The wear of the molding surfaces and the adhesion forces involved in the part extraction stage are greatly influenced by the nature of the material being injected and are enlarged when the molded object shrinks in the core or in the pins of the molding blocks. This is even more critical due to the flow behavior of enhanced polymer materials, developed for highly demanding applications in what concerns material mechanical or chemical properties.

Tailored special polymers are of fundamental importance in the supplement of microcomponents. Complex polymeric materials engineered to detain both the mechanical properties and the memory shape suitable for applications such as active control, impose enormous challenges in what concerns both the process and the molding tools.

In conventional injection molding tools, surface engineering is used to improve the molding block performance to obtain parts with superior mechanical quality. In micromolding, surface engineering has a more important role due to the above-sited requisites, in order to reduce the deterioration of the molding impressions, increase its durability and reduce the need for corrective intervention on the tool. Different thin film coatings may be used, but

Microinjection Molding of Enhanced Thermoplastics 215

In the conventional injection molding process, cavity is usually machined directly in the mould plate. The procedure, however, has undergone substantial alterations for the tools tailored for the microinjection molding applications. It turns to be more practical, in terms of energy saving and versatility of the mould tool, to machine the impression in the interchangeable mould insert. This way all necessary mould tool transformations can be applied locally and the microparts of different configuration could be produced using the same injection mould. The choice of the technique for the insert fabrication generally depends on three main factors: insert material used, surface finishing (roughness) and the aspect ratio demanded by the application. A number of techniques currently in use for microfabrication include: LIGA-process; silicon etching; laser ablation, micro electrical discharge machining (μEDM) and mechanical micro machining (diamond turning, micro

The LIGA process is a technology developed in the Forschungszentrum Karlsruhe in Germany in 1980s. Since the start of the micro technology development, is has been referred as a suitable technique for fabrication of the high aspect ratio microstructures with surface roughness down to 30 nm and such a low lateral resolution as 200 nm (Despa et al., 1999; Hormes et al., 2003; Munnik et al. 2003). Being a multi stage process, LIGA may be divided in three main steps: lithography, electroforming and plastic molding (Fig. 1). At first step (Lithography), the CAD information for the micro features is stored on the mask membrane (a very thin metal foil) covered with a layer of absorber generally Cu or Au. Synchrotron radiation passes through the transparent part of a lithographic mask and penetrates several hundred microns into a layer of sensitive X ray resist polymer (PMMA). The plastic modified by radiation is removed by solvent, leaving the template of resist structure. On the second step (Electroforming), the space generated by removed plastic is filled by electro deposition with metal normally Ni or Ni based alloys and negative replication of the resist structure is obtained. On the final step of LIGA process, the obtained metal structure is used as a mould insert for micro plastic parts production (Hormes et al., 2003). Among the variety of factors affecting the quality of X-ray LIGA generated patterns, the latter to a great extent

is correlated with radiation intensity, mask composition and substrate type.

Novel approaches for improvement of X-ray LIGA process for micro insert fabrication are in permanent research in academy (Kim et al., 2006; Meyer et al., 2008). Although with deep ray X LIGA it is possible to obtain very accurate patterns of micro features with high aspect ratios up to 100 and micro structures with size less than 250 nm, it is still not a widespread commercial technique for micro replication, being time consuming and costly (Despa et al., 1999; Meyer et al., 2008). UV–LIGA and IB (Ion Beam) LIGA technologies are less complex and costly comparing to the X-ray LIGA process. In the former application, the ultraviolet source instead of the X-ray is used to expose the resists, while in the latter impressions is obtained by irradiating of photoresist materials with light ions (Munnik et al., 2003; Yang et al., 2006). It, however, should be pointed out that despite the fact that size of the micro structures obtained by this technique may reach 100 nm, the electrons are very light and may cause the loss of resolution and poor surface finishing at depth (Munnik et al. 2003). In case of low volume production, silicon microstructured inserts may be a suitable alternative to the expensive LIGA process. The impressions in silicon are usually obtained by wet etching and in spite of the inherent fragility of the material low roughness surface finishing

**2.1.2 Mould insert fabrication** 

milling) (Rötting et al., 2002).

diamond films seem to assume an extreme importance in these types of applications due to its superior properties. The latter are not problem free coatings and not typically used on conventional molding tools. Nevertheless, its use may constitute the premises to solve the problem posed by highly abrasive and shear intensive polymer flows on cavities with geometrical detail and poor access, due to the micro-scale developed with the solely purpose of obtaining parts with high quality requisites.

In this chapter, the challenges that involve the micro-injection of enhanced thermoplastics will be discussed. Special attention will be given to the microinjection technology and tooling, but also to the injection materials, which impose further challenges, the so called enhanced thermoplastics.
