**4. High performance molding tools**

In this section, it will be presented the results of surface modification of the molding blocks. The performance of diamond coated molding tools will be compared with the performance of other typically used coatings and non-coated tools. Different morphologies of the diamond coating will also be presented in order to evaluate their influence on the molding process.

#### **4.1 Surface engineering in molding tools**

Although the advances acquired in the past couple of years in the microinjection molding technology process, there are still some problems on the downstream that must be overcome. The micro parts may become statically charged and tend to adhere to surfaces around the molding area, making free fall extraction difficult or even impossible. In the conventional thermoplastic injection molding, the wear of molding tools is known to be one of the main sources of breakdown failures, resulting in production losses. In microinjection of thermoplastic parts, the moulds for components that are miniature complex and require

The enhancement of UHMWPE properties is of utmost importance, essentially for medical applications, as UHMWPE is largely used for that purpose. However, the incorporation of CNTs into UHMWPE, for medical applications, requires special attention in what concerns to toxicity and biocompatibility of these composites. Reis et al. (2010) describe the response of human osteoblasts-like MG63 cells in contact with particles generated from UHMWPE/CNT composites. The results show the absence of significant elevation of the osteolysis inductor IL-6 values, pointed out for possible use of this superior wear-resistant

As it was discussed earlier, with the addition of particles to thermoplastics it is possible to modify the physical, thermal and mainly the mechanical properties of the raw thermoplastic. With these enhanced thermoplastics it is of utmost importance to evaluate the relationship of the new materials with the processing parameters, and optimize them, especially when new processing techniques are arising, like microinjection molding. In the bibliography it is possible to read some studies on microinjection molding of neat polymers (Chien, 2006; Sha et al., 2007). Few studies did consider polymer compounds containing fillers, such as glass fibers, glass particles, nanoceramic materials and carbon nanotubes (Huang et al., 2005; Huang, 2006; Hanemann et al., 2009; Abbasi et al., 2011). Huang et al. (2005) study the moldability and wear particles of composites of polymeric matrices reinforced with nanoceramic particles. Their rheology analysis reveals that with increasing nanoparticle loading the shear viscosity increases, meaning that, in microinjection moulding, high pressure is needed to produce high quality parts with micro-features.

Abbasi et al., (2011) prepared PP/CNT and PC/CNT composites and studied the effects of processing conditions on its structure, mechanical properties and electrical conductivity. They concluded that the high deformation values of the microinjection molding only slightly changed the overall crystallinity due to the short cycle time of process. They also observed that the crystals were all oriented in the flow direction. Other interesting result is that the type of processing strongly affects the electrical conductivity of the composites.

In this section, it will be presented the results of surface modification of the molding blocks. The performance of diamond coated molding tools will be compared with the performance of other typically used coatings and non-coated tools. Different morphologies of the diamond coating will also be presented in order to evaluate their influence on the molding process.

Although the advances acquired in the past couple of years in the microinjection molding technology process, there are still some problems on the downstream that must be overcome. The micro parts may become statically charged and tend to adhere to surfaces around the molding area, making free fall extraction difficult or even impossible. In the conventional thermoplastic injection molding, the wear of molding tools is known to be one of the main sources of breakdown failures, resulting in production losses. In microinjection of thermoplastic parts, the moulds for components that are miniature complex and require

composite for future orthopaedic applications.

**4. High performance molding tools** 

**4.1 Surface engineering in molding tools** 

**3.3 The rheology properties and molding tools** 

high precision tolerances are not wear free. On the contrary, the molding surface wear can be even much more critical than in conventional molding. The wear out of the molding tools creates demolding problems, compromising the molding finish quality, speeding up the corrosion of the tools and resulting in costly maintenance stops. Polymer abrasion, adhesion and corrosion are the catalysers of these mechanisms. Furthermore, the increasing usage of polymers reinforced with glass fibers, minerals, or even carbon nanotubes, enhance the abrasive power of polymers.

Coatings technologies have been strongly developed in the last decades, as did their application on mould and die tools. Ion implantation and unbalanced magnetron sputtering PVD (Bienk & Mikkelsen, 1997), High Velocity Oxy Fuel (HVOF) and Atmospheric Plasma Spraying (APS) (Gibbons & Hansell, 2008), diamond-like carbon and silicon carbide (Griffiths et al., 2010), nanostructured TiB2 (Martinho et al., 2011), Oxide coating (Alumina) and Nitride coatings (AlN, CrN, NiCr(N), TiN) (Navabpour et al., 2006), among other surface engineering coating have been tested in order to evaluate their performance to avoid molding surface wear and assist the demolding process.

Chemical vapor deposition (CVD) of polycrystalline diamond, in microcrystalline and nanocrystalline morphology, detain a number of extreme properties that point it as a technology suitable for exploitation in numerous industrial applications. It possesses a high mechanical hardness and wear resistance, high thermal conductivity and is very resistant to chemical corrosion. Its properties and their optimization by means of the deposition process, in order to fulfill the application requisite have been investigated by several research teams (Ahmed et al., 2006; Das & Singh, 2007; Gracio et al., 2010). Some successful work has already been performed on the evaluation of diamond coating on molding tools in special for micro-featured tools (Neto, 2008b, 2008c, 2009). As refereed above, in this sub-chapter, the application and evaluation of CVD diamond thin films as a surface engineering technique to improve operation and durability of microinjection mould tools will be highlighted.

### **4.2 Diamond coatings**

Polycrystalline diamond, in microcrystalline or nanocrystalline morphology, detains a number of extreme properties that point it as a technology suitable for exploitation in numerous industrial applications. It detains an extreme mechanical hardness (ca. 90 GPa) and wear resistance, one of the highest bulk modulus (1.2 × 1012 N.m−2), the lowest compressibility (8.3 × 10−13 m2 N−1), the highest room temperature thermal conductivity (2 × 103 W m−1 K−1), a very low thermal expansion coefficient at room temperature (1 × 10−6 K) and is very resistant to chemical corrosion (Das & Singh, 2007; Grácio et al., 2010).

Most of these properties are attractive for the application on cavities and mould tools; nevertheless coating an entire cavity with polycrystalline diamond is presently an utopia. CVD systems are considerably size limited due to the means of activating (thermal, electric discharge, or combustion flame) the gas phase carbon-containing precursor molecules.

A second problem related to the usage of CVD on mould tools is concerned with the fact that diamond cannot be directly coated onto ferrous substrates, the widest used row material to produce mould tools. Carbon, the precursor element of diamond, easily diffuses into the ferrous matrix, leaving behind no matter to start the diamond nucleation process.

Microinjection Molding of Enhanced Thermoplastics 231

A set of AISI P20 modified steel molding inserts was prepared and pre-coated with a 2 μm thick PVD chromium nitride (CrN) film, in order to block the mutual diffusion between the ferrous substrate and the diamond growth atmosphere. All the steel plates had the molding surface polished with silicon carbide paper till grit #2000. The samples to be diamond coated were ultrasonic abraded with a diamond solution and then cleaned with isopropyl alcohol. Diamond growth was performed in a hot-filament CVD reactor, using time-

In a first experiment, four different samples were prepared to evaluate their performance has molding surfaces. Sample AC1 was submitted to 4h30 of diamond deposition and sample AC2 to 9h00. The full deposition conditions used in steel substrates can be seen in Neto et al. (2008b). Sample AC3 had only the CrN film an AC4 was a bare steel substrate. The two last samples were used in order to compare their performance with the diamond

Figure 7 shows the SEM images of inserts AC1, AC2, AC3 and AC4 before injection molding. Both diamond coated samples exhibited diamond crystallites mainly displaying (111) crystal orientation, although (100) oriented crystals were also observed. These growth directions are typical for the processing temperatures employed in this investigation. The average crystal size of these films is 1.74 and 1.25 μm, and the measured average roughness is 0.18 and 0.16 μm, for samples AC1 and AC2, respectively. Inserts AC3 and AC4 displayed an average roughness of 0.10 and 0.11 μm, respectively. Raman spectroscopy was used to assess the diamond Raman quality, on the diamond coated samples, as proposed by Kulisch et al. (1996) and to estimate the residual stresses of the diamond film according to Ralchenko et al. (1995). Calculated quality factor values for the diamond coatings were 56.0 and 58.3%, for samples AC1 and AC2, respectively. Samples AC1 and AC2 presented diamond peak shifts (∆φ) of 11 and 13 cm−1, respectively. Calculated residual stress (σ) values for the

diamond coatings were 6.2 and 7.4 GPa, for samples AC1 and AC2, respectively.

tarnished surface than the samples molded with bare steel or with the CrN coating.

of the plastic sample became heterogeneous.

This may be due to the slight increased roughness that the diamond coated inserts presented, compared to the non-diamond coated samples, and also due to the crystalline nature of the diamond coatings. It is also worth mentioning that the injected samples 100 by the bare steel insert was very clean, but as the number of injections are increased, the surface

After this preliminary analysis using the molding inserts, they were placed in a mould tool specially designed to accommodate the 10 x 10 x 3 mm inserts and mounted in an injection molding machine to perform a cycle of 500 high-density polyethylene (HDPE) sample plates. The molded samples were analyzed using optical microscopy. Figure 8 displays 150 times magnified images of HDPE objects from the run number 1, 100, 300 and 500. Whatever the type of insert surface, the first injected object presents more heterogeneous surface than the objects injected in cycles 100, 300 or 500. After the first set of injections, the polymeric objects molded with both diamond coating are very alike. The molded pieces by AC3 and AC4, present some surface scratches, but maintain the optical brightness (observed by the naked eye). The samples molded with the diamond coated inserts presented a slightly more

**4.3 Performance of diamond coatings on steel substrates** 

modulated CVD process.

coated inserts.

To bypass the latter, appropriate interlayers can be used. A suitable interlayer is the one that promotes a diffusion block from and to the substrate material, enhances the adhesion between the diamond coating and the mould, and does not affect the properties of the diamond film or those of the mould tool, as pointed by Neto et al. (2008a).

A third constraint is the typical diamond deposition temperatures. The process temperature could be a limitation, since high temperatures may change the heat treatment induced in the steel material and alter its properties. Nevertheless, in the past years, deposition has been achieved at lower temperatures then the typical 800 to 900 °C (Dong et al., 2002; Petherbridge et al., 2001).

Not all molding tools are made out of steel material. Hybrid moulds or multi-material moulds are currently used for injection molding prototyping or to enhance mould heat extraction effectiveness. Aluminium, copper, silicon, silicon carbide, among others, are also used. The uses of different tool material place new constrains but also open new possibilities. The use of a silicon insert as the molding block, per example, may benefit from the microfabrication technology attained in the electronic industry. As the demand for smaller devices continues to increase, current manufacturing processes will find it more challenging to meet cost, quantity, and dimensional requirements. By merging silicon microfabrication techniques with appropriated surface engineering techniques, nano-scale features may be produced to replicate nano-features devices (Gourgon et al., 2005; Guo, 2007). Additionally, the deposition of diamond coating on silicon material is vastly reported by the scientific community, making it an excellent candidate material to be surface engineered with tuned diamond films.

CVD diamond deposition is possible by means of activating hydrogen and hydrocarbon. The growth of a diamond film starts from distinct nucleation sites. As individual randomly oriented nuclei grow larger, its diameters equal the average distance between the nucleation sites and start to form a continuous film. The subsequent film growth is dominated by competitive growth between differently oriented grains. With increasing film thickness, more and more grains are overgrown and buried by adjacent grains. Only those crystals with the direction of fastest growth perpendicular to the surface will survive. Thermal or plasma energy is the key factor to promote the fluctuations of the density to achieve small aggregates and to promote the thermodynamic environment to lead to the crystal growth. Nevertheless, it has been gradually recognized that the superequilibrium concentration of atomic hydrogen has also an important role on diamond growth. Various activating methods are used, such as DC-plasma, RF-plasma, microwave plasma, electron cyclotron resonance-microwave plasma CVD, and their modifications. More recently, nanocrystalline (NCD) and ultrananocrystalline diamond (UNCD) deposition have been researched. Many synthesis processes are described in the literature. Most of these processes require an extra gas source such as Ar, N2 or He. The basic idea behind the deposition process is to enhance the diamond secondary nucleation rate during the deposition, thus leading to the formation of the films. (Sharda & Bhattacharyya, 2004; Spears & Dismukes, 1994)

In the following subsections, the conditions for the different diamond films deposited on steel and silicon will be presented, characterized and evaluated as polymer moulding surfaces.

To bypass the latter, appropriate interlayers can be used. A suitable interlayer is the one that promotes a diffusion block from and to the substrate material, enhances the adhesion between the diamond coating and the mould, and does not affect the properties of the

A third constraint is the typical diamond deposition temperatures. The process temperature could be a limitation, since high temperatures may change the heat treatment induced in the steel material and alter its properties. Nevertheless, in the past years, deposition has been achieved at lower temperatures then the typical 800 to 900 °C (Dong et al., 2002;

Not all molding tools are made out of steel material. Hybrid moulds or multi-material moulds are currently used for injection molding prototyping or to enhance mould heat extraction effectiveness. Aluminium, copper, silicon, silicon carbide, among others, are also used. The uses of different tool material place new constrains but also open new possibilities. The use of a silicon insert as the molding block, per example, may benefit from the microfabrication technology attained in the electronic industry. As the demand for smaller devices continues to increase, current manufacturing processes will find it more challenging to meet cost, quantity, and dimensional requirements. By merging silicon microfabrication techniques with appropriated surface engineering techniques, nano-scale features may be produced to replicate nano-features devices (Gourgon et al., 2005; Guo, 2007). Additionally, the deposition of diamond coating on silicon material is vastly reported by the scientific community, making it an excellent candidate material to be surface

CVD diamond deposition is possible by means of activating hydrogen and hydrocarbon. The growth of a diamond film starts from distinct nucleation sites. As individual randomly oriented nuclei grow larger, its diameters equal the average distance between the nucleation sites and start to form a continuous film. The subsequent film growth is dominated by competitive growth between differently oriented grains. With increasing film thickness, more and more grains are overgrown and buried by adjacent grains. Only those crystals with the direction of fastest growth perpendicular to the surface will survive. Thermal or plasma energy is the key factor to promote the fluctuations of the density to achieve small aggregates and to promote the thermodynamic environment to lead to the crystal growth. Nevertheless, it has been gradually recognized that the superequilibrium concentration of atomic hydrogen has also an important role on diamond growth. Various activating methods are used, such as DC-plasma, RF-plasma, microwave plasma, electron cyclotron resonance-microwave plasma CVD, and their modifications. More recently, nanocrystalline (NCD) and ultrananocrystalline diamond (UNCD) deposition have been researched. Many synthesis processes are described in the literature. Most of these processes require an extra gas source such as Ar, N2 or He. The basic idea behind the deposition process is to enhance the diamond secondary nucleation rate during the deposition, thus leading to the formation

In the following subsections, the conditions for the different diamond films deposited on steel and silicon will be presented, characterized and evaluated as polymer moulding

of the films. (Sharda & Bhattacharyya, 2004; Spears & Dismukes, 1994)

diamond film or those of the mould tool, as pointed by Neto et al. (2008a).

Petherbridge et al., 2001).

engineered with tuned diamond films.

surfaces.
