**2.1.4 Mould evacuation**

218 Thermoplastic Elastomers

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

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

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

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

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.

economic feasibility of the micromolding process (Gornik, 2004).

to longer cycle times (Gornik, 2004; Whiteside et al., 2004; Tseng et al., 2005).

electrical insulation of the mould insert is necessary.

with periodical cleaning of the mould cavity (Chang & Hwang, 2006).

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).

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

Microinjection Molding of Enhanced Thermoplastics 221

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

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

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.

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

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

recorded (Whiteside et al., 2004).

(PRIAMUS®, 2011).

**components** 

critical in the case of the thermally sensitive polymers as polyoxymethylene (POM) (Sha et al., 2005; Sha et al., 2007b).

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 surface finishing of the cavity.
