**2. Microinjection molding**

In this section the topic microinjection molding will be developed. The injection molding machine, molding tools and inserts, insert fabrication techniques, rapid heating/cooling process, mould evacuation, demolding, factors affecting the replication quality of micro parts and process monitoring and control will, therefore, be discussed.

#### **2.1 Adaptation of the molding tools and equipment for the micromolding of thermoplastics**

#### **2.1.1 Injection molding machines**

There are a variety of motives why the conventional injection molding machines fail to satisfy the requirements of the microinjection molding. So far, as typical macroinjection molding machines are concerned, the most important issue becomes its minimum metering size, which apparently is too big in comparison to the tiny quantity of material required to produce a microcomponent. The latter turns precise metering very complicated along with a considerable increase of the time of melt residence in the barrel and eventually may lead to material degradation. In addition, clamping force also has to be reduced in order to guaranty free damage release of the micropart (Giboz et al., 2007; Attia et al., 2009). To accomplish precise, free defect molding of the micro components, every functional system of the macro molding machine has to undergo a series of profound modifications (Giboz et al., 2007). So far, several commercially available and home-made machines are utilized for production of the micromolded components and may be divided in two main groups. In the first one, modifications are accomplished by simple rescale/miniaturization of the metering and injection units addressed to precise dosage of polymer melt in every shot (Giboz et al., 2007). Another approach consists of separation of the plasticization from injection unit, where the plasticization is being performed in the extrusion screw or hot cylinder, mounted at angle to the inject axis. Next, polymer melt enters into the injection unit, where the mini plunger pushes the prepared shot to fill the mould cavity. The above mentioned classification includes both hydraulically and electrically driven microinjection molding machines. However, for the sake of accuracy and repeatability of the process, higher precision of plastic metering and clamping may be achieved by the servo mechanisms of the electrically driven machine allowing for more accurate process control along with low noise level and energy efficiency. (Whiteside et al., 2003; Chang et al., 2007)

#### **2.1.2 Mould insert fabrication**

214 Thermoplastic Elastomers

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

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

In this section the topic microinjection molding will be developed. The injection molding machine, molding tools and inserts, insert fabrication techniques, rapid heating/cooling process, mould evacuation, demolding, factors affecting the replication quality of micro

There are a variety of motives why the conventional injection molding machines fail to satisfy the requirements of the microinjection molding. So far, as typical macroinjection molding machines are concerned, the most important issue becomes its minimum metering size, which apparently is too big in comparison to the tiny quantity of material required to produce a microcomponent. The latter turns precise metering very complicated along with a considerable increase of the time of melt residence in the barrel and eventually may lead to material degradation. In addition, clamping force also has to be reduced in order to guaranty free damage release of the micropart (Giboz et al., 2007; Attia et al., 2009). To accomplish precise, free defect molding of the micro components, every functional system of the macro molding machine has to undergo a series of profound modifications (Giboz et al., 2007). So far, several commercially available and home-made machines are utilized for production of the micromolded components and may be divided in two main groups. In the first one, modifications are accomplished by simple rescale/miniaturization of the metering and injection units addressed to precise dosage of polymer melt in every shot (Giboz et al., 2007). Another approach consists of separation of the plasticization from injection unit, where the plasticization is being performed in the extrusion screw or hot cylinder, mounted at angle to the inject axis. Next, polymer melt enters into the injection unit, where the mini plunger pushes the prepared shot to fill the mould cavity. The above mentioned classification includes both hydraulically and electrically driven microinjection molding machines. However, for the sake of accuracy and repeatability of the process, higher precision of plastic metering and clamping may be achieved by the servo mechanisms of the electrically driven machine allowing for more accurate process control along with low noise

parts and process monitoring and control will, therefore, be discussed.

level and energy efficiency. (Whiteside et al., 2003; Chang et al., 2007)

**2.1 Adaptation of the molding tools and equipment for the micromolding of** 

purpose of obtaining parts with high quality requisites.

enhanced thermoplastics.

**thermoplastics** 

**2. Microinjection molding** 

**2.1.1 Injection molding machines** 

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 milling) (Rötting et al., 2002).

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

Microinjection Molding of Enhanced Thermoplastics 217

workpeace movement (Franssila, 2004; Uhlmann et al., 2005). Size and precision of the pattern replication, to a great extent, depend on the size and shapes of the electrodes and eventually with optimum conditions aspect ratios of 100 may be realized (Franssila, 2004). For machining of the patterns of more complex shapes, electro discharge grinding (WEDG) with the electrodes of micrometeric range (down to 20 µm) is reported to realize the micro structures of 5 µm and roughness of 0.1 μm. However, a maximum aspect ratio of micro structure is less than that of μEDM and only reaches 30 times of the wire diameter (Uhlmann et al., 2005). Surface resolution obtained by laser ablation is similar to that of micro cutting techniques making this method very popular for machining of the wide range of engineering materials (Gower, 2000). Ultra short pulsed lasers with optimized pulse energy and focus size are able to produce microstructures with size from 10 µm, aspect ratio of 10 and roughness of 0.16 µm (Heyl et al., 2001). It is worth mentioning, however, that in terms of surface quality and minimum achievable dimensions, the output from the micro

As a rule of thumb, in conventional injection molding process mould temperature is far below the injection temperature. At such conditions, the frozen layer forms near the cavity wall while the core is significantly hotter and continues cooling down to ejection temperature at the end of the cycle. When the polymer melt is injected to the micrometric cavity, high surface-tovolume ratio and reduced dimensions of microparts promote the instantaneous melt temperature drop and as a result incomplete filling even when high pressures are applied (Su et al., 2004). To guaranty complete filling of the microcavity, the mould should be heated up to the glass transition temperature (Tg) for amorphous thermoplastics and melt temperature (Tmelt) for the crystalline ones. This definitely requires implementation of a special rapid heating/cooling (variotherm) system. The latter (Fig. 2) allows for a rise of the mould temperature above Tg /Tmelt during injection with subsequent cool down to the ejection

temperature in order to assist successful part release (Piotter et al., 2008).

Fig. 2. Comparison of the mould temperature profile in the classical and variotherm

laser ablation is inferior to the LIGA and μEDM.

**2.1.3 Rapid heating/cooling process** 

processes (Gornik, 2004).

makes this technique quite satisfactory for molding of the microparts in limited quantities (Heckele & Schomburg, 2004).

Fig. 1. Principle process steps for the fabrication of microstructures by the LiGA technique (Hormes et al., 2003).

The direct structuring techniques are far more attractive economically and can be easily applied for the micro inserts fabrication from hard metals and alloys. When the complex geometries of the micro features are concerned, a variety of micro cutting techniques are currently available and may be enumerated as follows: micro milling (for microgrooves and micro 3D shapes); micro turning (for micro pins); and micro drilling (for micro holes) (Franssila, 2004). To achieve the required cavity surface finishing, diamond and cemented carbide cutting miniaturized tools are currently in use whether their applications will eventually depend on the properties of processed material (Franssila, 2004). Although with diamond cutting tool such fine resolution as of 48 nm of roughness could be achieved, its applicability is limited to the soft non ferrous metals as brass, aluminium, copper and nickel (Davim & Jackson, 2009). More resilient materials like steel are processed with the cemented carbide tool of diameters down to 50 μm yet with less surface resolution (0.1–0.3 μm) (Fleischer et al., 2007). Despite the fact that maximum aspect ratios achieved by micro cutting are considerably smaller in comparison with the other micro fabrication techniques, high level of automation and continuous improvement in the tools precision as well their ability to work with hard metals make it an appropriate technique for the microstructured insert fabrication.

Another popular direct structuring technique for hard metals and alloys is micro electrical discharge machining (μEDM). The heat in the form of pulsed discharge is applied through the thin metal wire (usually brass), allowing to cut through metal by melting and evaporation (Franssila, 2004). Based on this general description, there are a wide range of the μEDM techniques which may be classified by the type of electrode or/and electrode-

makes this technique quite satisfactory for molding of the microparts in limited quantities

Fig. 1. Principle process steps for the fabrication of microstructures by the LiGA technique

The direct structuring techniques are far more attractive economically and can be easily applied for the micro inserts fabrication from hard metals and alloys. When the complex geometries of the micro features are concerned, a variety of micro cutting techniques are currently available and may be enumerated as follows: micro milling (for microgrooves and micro 3D shapes); micro turning (for micro pins); and micro drilling (for micro holes) (Franssila, 2004). To achieve the required cavity surface finishing, diamond and cemented carbide cutting miniaturized tools are currently in use whether their applications will eventually depend on the properties of processed material (Franssila, 2004). Although with diamond cutting tool such fine resolution as of 48 nm of roughness could be achieved, its applicability is limited to the soft non ferrous metals as brass, aluminium, copper and nickel (Davim & Jackson, 2009). More resilient materials like steel are processed with the cemented carbide tool of diameters down to 50 μm yet with less surface resolution (0.1–0.3 μm) (Fleischer et al., 2007). Despite the fact that maximum aspect ratios achieved by micro cutting are considerably smaller in comparison with the other micro fabrication techniques, high level of automation and continuous improvement in the tools precision as well their ability to work with hard metals make it an appropriate technique for the microstructured

Another popular direct structuring technique for hard metals and alloys is micro electrical discharge machining (μEDM). The heat in the form of pulsed discharge is applied through the thin metal wire (usually brass), allowing to cut through metal by melting and evaporation (Franssila, 2004). Based on this general description, there are a wide range of the μEDM techniques which may be classified by the type of electrode or/and electrode-

(Heckele & Schomburg, 2004).

(Hormes et al., 2003).

insert fabrication.

workpeace movement (Franssila, 2004; Uhlmann et al., 2005). Size and precision of the pattern replication, to a great extent, depend on the size and shapes of the electrodes and eventually with optimum conditions aspect ratios of 100 may be realized (Franssila, 2004). For machining of the patterns of more complex shapes, electro discharge grinding (WEDG) with the electrodes of micrometeric range (down to 20 µm) is reported to realize the micro structures of 5 µm and roughness of 0.1 μm. However, a maximum aspect ratio of micro structure is less than that of μEDM and only reaches 30 times of the wire diameter (Uhlmann et al., 2005). Surface resolution obtained by laser ablation is similar to that of micro cutting techniques making this method very popular for machining of the wide range of engineering materials (Gower, 2000). Ultra short pulsed lasers with optimized pulse energy and focus size are able to produce microstructures with size from 10 µm, aspect ratio of 10 and roughness of 0.16 µm (Heyl et al., 2001). It is worth mentioning, however, that in terms of surface quality and minimum achievable dimensions, the output from the micro laser ablation is inferior to the LIGA and μEDM.

#### **2.1.3 Rapid heating/cooling process**

As a rule of thumb, in conventional injection molding process mould temperature is far below the injection temperature. At such conditions, the frozen layer forms near the cavity wall while the core is significantly hotter and continues cooling down to ejection temperature at the end of the cycle. When the polymer melt is injected to the micrometric cavity, high surface-tovolume ratio and reduced dimensions of microparts promote the instantaneous melt temperature drop and as a result incomplete filling even when high pressures are applied (Su et al., 2004). To guaranty complete filling of the microcavity, the mould should be heated up to the glass transition temperature (Tg) for amorphous thermoplastics and melt temperature (Tmelt) for the crystalline ones. This definitely requires implementation of a special rapid heating/cooling (variotherm) system. The latter (Fig. 2) allows for a rise of the mould temperature above Tg /Tmelt during injection with subsequent cool down to the ejection temperature in order to assist successful part release (Piotter et al., 2008).

Fig. 2. Comparison of the mould temperature profile in the classical and variotherm processes (Gornik, 2004).

Microinjection Molding of Enhanced Thermoplastics 219

With this technique, the molecular orientation in the surface region of the micropart has been decreased along with significant improvement of the surface replication. Very quick rise of the cavity temperature may be achieved with induction heating. It was reported that heating could be accomplished in several seconds and consequent cooling is also very fast because of minimum amount of heat generated in the mould (Michaeli & Klaiber, 2007a). However, high-temperature cycling may lead to thermal fatigue and eventual shortage of

Negative effect of the air presence in the mould cavity is a well known phenomenon in the conventional injection molding. The latter is responsible for burn marks on moldings and long-term formation of corrosive residue in the mould which may lead to its permanent damage. While the moulds of conventional size are supplied with special venting channels, this solution cannot be adapted for the micromolding where the part dimensions are frequently comparable in size to the venting grooves. Moreover, high injection speed and complex geometry of micro features may contribute for air entrapment and could eventually lead to incomplete filling, especially in case of the blind hole features with high aspect ratios (Heckele & Schomburg, 2004). In a number of studies, evacuation of air from the cavity prior to injection is referred as an efficient method for improvement of the microparts replication (Despa et al., 1999; Heckele & Schomburg, 2004; Sha et al., 2005; Liou & Chen, 2006; Chang et al., 2007; Sha et al., 2007b). A typical layout of the mould evacuation system is shown on Fig. 3. However, sometimes it is impossible to clearly distinguish the effect of cavity evacuation on the part filling. For example, it could be attributed to the fact that micro cavities are aligned with the parting line of the mould and therefore air can escape easily through the partition (Sha et al., 2005). To assist more efficient air evacuation, the mould platens must be properly adjusted for hermetic sealing of the cavity (Despa et al., 1999).

the mould service time (Tseng et al., 2005).

Fig. 3. Layout of the vacuum mould unit (Chang et al., 2007)

It is also worth mentioning that air suction prior to injection may decrease the cavity temperature. The latter may have a negative effect on the mould filling, being especially

**2.1.4 Mould evacuation** 

Nowadays, the variotherm process is widely applied for molding of the microparts. When mould temperature is close to processing temperature, even microstructures of the high aspect ratios may be eventually filled. Moreover, isothermal filling induces less residual stresses and surface defects in the micro parts (Chang & Hwang, 2006). How it can be easily noticed from Fig. 2, the temperature gradient for variotherm process is significantly higher than that for the conventional process and may lead to longer cycle times compromising the economic feasibility of the micromolding process (Gornik, 2004).

Considering the variotherm process, two main issues have to be addressed. The first is concerned with the choice of the heating technique capable of rising promptly mold temperature above Tg /Tmelt. While the second issue is related to the efficient heat removal for the fastest possible cycle times. So far, there are two principal approaches to transport the required power to the mould: external type (infrared radiation and induction heating) and internal/built-in ( joule/resistive, high-frequency proximity, water and oil heating). The main limitation to use water as a heating media is that the maximum heating temperature does not exceed 95°C, which is less than the glass transition/melt temperature of the majority of polymers. As a result, water is generally used for cooling in the variotherm process (Su et al., 2004; Chang & Hwang, 2006; Xie & Ziegmann, 2008). Heat retention properties of oil and its ability to be heated up to 140°C makes it a more adequate candidate as heating medium. However, the lower heat transfer coefficient, comparing to water, leads to longer cycle times (Gornik, 2004; Whiteside et al., 2004; Tseng et al., 2005).

Application of the joule/resistive heating in variotherm process has been extensively investigated by many researchers (Su et al., 2004; Xie & Ziegmann, 2008). Eventually, nano features with aspect ratios up to 300 could be realized if molded above the glass transition/melt temperature of polymer. Unfortunately, the cycle times may increase up to several minutes due to the thermal inertia of the mould material (Xie & Ziegmann, 2008). Adequate isolation of the heating elements from the mould base will favor the cycle time reduction and increase economic efficiency of the microinjection molding process (Gornik, 2004). Yao et al. (2006) have implemented a local heating of the mould insert by highfrequency current. Unlike the conventional resistive techniques, mould surface heating rate can reach 40°C/s with an apparent heating power of 93 W/cm2, while only the local electrical insulation of the mould insert is necessary.

The main advantage of the external heating over the internally built systems is an ability to heat up locally the insert surface allowing for the faster cycle times. In a number of studies infrared heating has been applied for precise localized heating of the microcavity surface Although the local temperature rise with an aid of halogen lamps requires considerably less time in comparison to the resistive heaters, it may lead to uneven distribution of the mould surface temperature and therefore, special attention should be given to more uniform distribution of the heating sources (Gornik, 2004; Chang & Hwang, 2006). During processing, residual oil or resin particles may burn and contaminate the microcavity surface causing surface defects in the micromolded parts. This problem could be eventually solved with periodical cleaning of the mould cavity (Chang & Hwang, 2006).

Direct infrared radiation heating of polymer inside the cavity has been proposed by Saito et al. (2002). As long as CO2 laser is directed towards the polymer melt through the transparent window in the mould wall, temperature gradient during injection is significantly reduced. With this technique, the molecular orientation in the surface region of the micropart has been decreased along with significant improvement of the surface replication. Very quick rise of the cavity temperature may be achieved with induction heating. It was reported that heating could be accomplished in several seconds and consequent cooling is also very fast because of minimum amount of heat generated in the mould (Michaeli & Klaiber, 2007a). However, high-temperature cycling may lead to thermal fatigue and eventual shortage of the mould service time (Tseng et al., 2005).
