**2.2 Process monitoring and control**

Microinjection molding is inherently a black box process, where the extreme processing conditions and rapid process variations do not lead to the linear correlation of the inputs and outputs. Monitoring of the micro molding process parameters is critical to assess the rheological state of polymer throughout the molding cycle. Hitherto, the majority of sensors currently available at the market are too big to be installed into the micro cavity. To overcome those limitations, an indirect monitoring is frequently applied to gather the information about the mould temperature, heat flux, injection pressure, injection speed and

critical in the case of the thermally sensitive polymers as polyoxymethylene (POM) (Sha et

Implementation of the vacuum assisted cavity evacuation might be carefully considered in every case of the microinjection molding taking into account polymer type, geometry and

Demolding of microparts becomes a critical issue as failure frequently occurs at the onset of ejection. The interactions of polymer shrinkage and the coefficient of friction between the polymer and the molding tool may have detrimental effects on the component failure during ejection (Pouzada et al., 2006). Moreover, the micro features with high aspect ratios have a larger contact surface between the mould and the polymer which results in higher frictional resistance during part release. Smooth surface finishing is highly desirable for decreasing the friction on the polymer/insert interface (Attia et al., 2009). Coatings are frequently used to improve the roughness properties of the mould cavity and achieve superior surface quality of microparts. For example, if a cavity is coated with diamond-like carbon (DLC), fewer forces are required for the PC and ABS microparts ejection (Griffiths et al., 2008). In order to avoid breaking during ejection, the adhesive bond between polymer and stainless steel mould tool should not exceed the tensile strength of the polymer (Navabpour et.al., 2006). In addition to the experimental techniques, the magnitude of the stress on the polymer/metal interface can be accessed via finite elements stress analysis

Considering fragility of the microstructured parts, conventional ejection with pins becomes unlikely and may eventually lead to their irreversible damage. In order to overcome those weaknesses, alternative solutions have to be considered. For example, if geometry of micro parts allows for a positive draft angle, the latter could assist the proper demolding of the microparts (Grave et al., 2007; Wu & Liang, 2005). A concept of more even distribution of ejection forces has been applied when ejection with pins was substituted with the micro ejection block used as striper plate to thrust the microparts (Wu & Liang, 2005). Ejection of the high aspect ratio micro structures, molded within the insert of rough surface finishing, may be successfully accomplished with vacuum assisted demolding (Michaeli et al., 2000). Choice of demolding system at micro scale will eventually depend on the cavity geometry, surface finishing and the material to be molded. In addition, a special attention should be given to correct calculation of the ejection forces and proper design of the ejection system, so

Microinjection molding is inherently a black box process, where the extreme processing conditions and rapid process variations do not lead to the linear correlation of the inputs and outputs. Monitoring of the micro molding process parameters is critical to assess the rheological state of polymer throughout the molding cycle. Hitherto, the majority of sensors currently available at the market are too big to be installed into the micro cavity. To overcome those limitations, an indirect monitoring is frequently applied to gather the information about the mould temperature, heat flux, injection pressure, injection speed and

far as both of those factors could be detrimental for the micropart quality.

al., 2005; Sha et al., 2007b).

surface finishing of the cavity.

**2.1.5 Demolding** 

(Grave et al., 2007).

**2.2 Process monitoring and control** 

displacements (Yan et al., 2003; Zhao et al., 2006). Frequently pressure sensors are embedded into the plunger of the microinjection molding machine for monitoring of the metering size and pressure evolution. It, however, does not represent the polymer behavior inside the microcavity (Zhao et al., 2006). Cavity pressure monitoring is considered to be a better approach to verify the process variation and control the quality of the molded parts. For example, more relevant information about polymer melt behavior can be acquired by single axis force sensor, embedded into the mould-core extrusion mechanism (Yan et al., 2003). In a number of studies, an integral approach has been adapted towards the monitoring of the pressure and temperature inside the cavity. Pressure monitoring has been performed with piezoelectric force transducer embedded into the ejector pin. Whereas temperature, which has early been mentioned to affect the microcavity filling, may be measured with the aid of the J type thermocouples embedded at depths of several millimeters in the cavity wall. This way, the polymer solidification inside the cavity may be recorded (Whiteside et al., 2004).

When the direct monitoring of the micromolding is concerned, only a few sensors have been referred to be appropriate for the micro applications. The majority of the above-mentioned sensors hardly satisfy the micro criteria with the contact area diameter varying from 1 to 4 millimeters (Luo & Pan, 2007; Ono et al., 2007). However, those sensors were reported to provide reliable data about the temperature and pressure evolution in the cavity and if properly located, could be used to control the micromolding process. Further miniaturization of the sensors is one of the key factors to provide an insight to rheology of the polymer flow in the microcavity. The obtained data could be applied for the better process control as well for the improvement of the rheology models used for numerical simulation of the micro molding process. So far, by the author's knowledge, the market offer of the micro pressure and temperature sensors is very limited. For example, a combined sensor for measuring of the mould cavity pressure und contact temperature in the cavity with the front diameter of 1 mm, was recently launched by KISTLER (Kistler, 2011). Further miniaturization has been accomplished by PRIAMOS, for measurement of the melt temperature at contact with the cavity wall, with the sensor of 0.6 mm of diameter (PRIAMUS®, 2011).

The real time monitoring and control of the micromolding process are highly dependent on the sensors capable of reliably monitor the instant process variations. With further sensors miniaturization and an increase in precision, it will eventually be possible to achieve the higher level of automation and superior economic viability of the micromolding process.

#### **2.3 Effect of the process/tool/polymer interaction on the quality of micromolded components**

Quality of the micromolded parts is, to a great extent, determined by complex interaction of the process parameters, polymer and mould tool properties. Considering variability of the microparts design and purpose, their quality criteria are much diversified as well, including filling length, dimensional stability, surface finishing and a range of the mechanical properties of interest (Attia et al., 2009). Currently, two main approaches exist for correlation of the process/polymer/tool factors with the quality output: one factor at time experiment (OFAT) and design of experiment approach (DOE). While the former method highlights the leading trends, the latter addresses the optimization of the process by identifying the

Microinjection Molding of Enhanced Thermoplastics 223

A brief introduction to the current meaning of enhanced thermoplastics will be provided. It will be given some emphasis to enhanced thermoplastics through nanoparticles loading. Then it will be presented research work done on this issue, giving special evidence to the effect of mixing carbon nanotubes (CNT) to a thermoplastic material. The consequences on the rheology and interaction with the molding tools of these materials will also be discussed.

Thermoplastics are long chain polymers than can be structurally amorphous or semicrystalline. These polymers possess long chains, where macromolecules are bonded through the weak van der Waals forces. Their general properties are toughness, resistance to chemical attack and recyclability, i.e. they can be re-processed as many times as needed, till their degradation, due to processing. Despite that the continuum search for better and cheaper materials has been guiding the scientists to the development of new thermoplastics with enhanced properties, such as better resistance to water and UV; better mechanical properties (toughness, stiffness); and enhanced electrical conductivity. These new class of

There are different techniques to manipulate polymeric materials in order to obtain enhanced properties, such as: (a) by modifying their molecular structure, the hard and soft segments. Through this technique, the molecular structure of the polymer is changed by different combinations of: chain flexibility and hard segments, chain entanglement, the orientation of different segments, the hydrogen bonds and their intermolecular interactions. Through the modifications of the hard/soft segments ratios of the polymer, it is possible to obtain different physical, thermal and mechanical properties (Chattopadhyay & Raju 2007); (b) the incorporation of plasticizers, which are substances, usually plastics or elastomers, that are incorporated in a material in order to increase its flexibility, workability or extensibility, and modify the thermal and mechanical properties (Wang et al. 1997; Rahman & Brazel 2006); (c) throughout the incorporation of particles into a polymeric matrix (Tjong 2006). Some of the most commonly used reinforced particles are: CaCO3, glass, carbon fibres, and in the last years, carbon nanotubes have been frequently used due to their extraordinary thermal,

**3.2 The effect of incorporating carbon nanotubes (CNTs) into thermoplastic materials**  The incorporation of carbon nanotubes (CNTs) in thermoplastic materials has been one of the hottest topics in materials science in the last years, since their discovery by Iijima (1991),

CNTs can be classified has single-walled carbon nanotubes (SWCNT), which consist on a single grapheme sheet wrapped into a cylindrical shape and are characterized by a small diameter (0.4 - 3 nm) and lengths up to centimeters; and multi-walled carbon nanotubes (MWCNT) that detain a number of grapheme layers coaxially rolled together to form a cylindrical tube, and their outer diameter ranges from 1.3 - 100 nm and their length can be as long as tens of micrometers (Baughman et al., 2002). It has been reported that CNTs possess an elastic modulus in a range of TPa (Yu et al., 2000) and detain high aspect ratios (>100),

electrical and mechanical properties (Wang et al., 1998; Coleman et al., 2004).

**3. Enhanced thermoplastics** 

thermoplastics are known as "enhanced thermoplastics".

essentially due to the extraordinary properties of CNTs

**3.1 Definition** 

interactions among factors. Both methods are extensively explored in recent research work (Zhao et al., 2003; Liou & Chen, 2006; Michaeli et al., 2007b; Sha et al., 2007b; Theilade & Hansen, 2007; Tofteberg & Andreassen, 2008).

Variotherm mould cycling along with the high injection temperature could significantly improve the weld line strength of the micro tensile test samples (Theilade & Hansen, 2007). Moreover, at higher mould temperature viscosity of the polymer melt significantly decreases and therefore requires less injection pressure and speed (Despa et al., 1999). Even in the absence of the variotherm cycling, higher mould temperature was reported to promote better cavity filling for a wide range of the tested polymers (Zhao et al., 2003; Sha et al., 2005; Michaeli et al., 2007b; Sha et al., 2007a; Sha et al., 2007b; Tofteberg & Andreassen, 2008). In the case of over-molded micro needles, an increase of bonding strength has been accomplished by the combination of the high holding pressure, high mould and low melt temperature (Michaeli et al., 2007b). An attempt of the empirical correlation of the part quality with process parameters has been proposed by Tofteberg & Andreassen (2008). In their study, replication of the micro features of Cyclic Olefin Copolymer (COC) and Poly(methyl methacrylate) (PMMA) has been significantly improved with an increase of the mould and melt temperature. It is also worth mentioning that with an increase of the micropart complexity the mould temperature factor was found out to prevail over the other process parameters (Attia et al., 2009).

Premature solidification of polymer may be reduced, to some extent, by injection at high injection speed. The latter is reported to assist in filling of the micro pins, having on the other hand, an adverse effect on the surface finishing (Sha et al., 2007a). The positive effect of the injection speed has also been confirmed by the other researchers. High injection speed was decisive for high-quality replication of the micro walls as well as its interaction with the injection temperature (Theilade & Hansen, 2007). When moulded with cold runners, it is not rare to encounter that volume of the feeding system may contain several times the volume of the micropart. Such discrepancies make precise metering unlikely, increasing the probability of incomplete filling and invalidating the holding pressure effect (Zhao et al., 2003). Nonetheless, some difficulties related to the metering precision and process fluctuations can be attenuated by applying higher holding pressure (Liou & Chen, 2006). Although with the holding pressure increases improvement of filling the micro and submicron structures has been reported, it seems uneasy to differentiate single holding pressure influence from its interaction with the mould temperature effect (Liou & Chen, 2006).

In the recent research, discrepancies in rheological behavior of the different grades of plastics have been widely reported. In order to guarantee the proper filling and acceptable surface quality, the easy flow grades should be preferred (Despa et al., 1999; Zhao et al., 2003; Michaeli et al., 2007b; Tofteberg & Andreassen, 2008). Adhesion forces between polymer and mould tool may vary significantly for different polymer grades and should be accounted for successful demolding of the microparts. The latter may be achieved by fine surface finishing of the mould cavity, positive draft angles and by using release agents (Wu & Liang, 2005; Grave et al., 2007). The variety of factors involved into the transformation of the polymer melt within the microcavity makes the interpretation of the cause-effect relationship uneasy task even for neat (unfilled) thermoplastics. Furthermore this relationship may become substantially more complicated for enhanced thermoplastics as their new properties will eventually influence rheological behavior at micro scale.
