Plastics,Rubber and Composites: Macromolecular Engineering

Today, microinjection moulding represents an established special variant among the different replication techniques strongly promoted by the growing market demand for microcomponents. Spearheading progress towards more sophisticated technologies, micro powder injection moulding offers a large scale fabrication process for metal/ceramic microparts. Minimal details in the range of 10 μm and replication accuracy down to ±0·3% (in special cases, ±0·1%) can be reached. With a twofold aim of reducing mounting costs and enabling the production of highly integrated devices two-component microinjection moulding is under development. Interesting examples are immovable or movable ceramic shaft wheel components.

Achieving perfect replication of micro- and nanostructured surfaces without creating sink marks in the parts is challenging. Therefore, gas assisted injection moulding (GAIM) was investigated as a method to enhance moulding quality in polypropylene, polymethylmethacrylate (PMMA) and thermoplastic polyurethane (TPU) parts. For all three polymers, the GAIM did not improve replication (depth ratios) of low aspect ratio microfeatures moulded using tooling with positive features, but did significantly enhance replication of higher aspect ratio trenches and tapered holes. The enhanced replication was attributed to better filling, and with the semicrystalline polymer, significantly less shrinkage. GAIM sometimes enhanced the edge definition of the features, and as expected, reduced sink marks in the polypropylene surfaces.

Moulded interconnect devices (MIDs) are plastic substrates with electrical infrastructure. The fabrication of MIDs is usually based on injection moulding, and different process chains may be identified from this starting point. The use of MIDs has been driven primarily by the automotive sector, but recently, the medical sector seems more and more interested. In particular, the possibility of miniaturisation of three-dimensional components with electrical infrastructure is attractive. The present paper describes possible manufacturing routes and challenges of miniaturised MIDs based on two-component injection moulding and subsequent metallisation. This technology promises cost effective and convergent manufacturing approaches for both macro- and microapplications. This paper presents the results of industrial MID production based on two-component injection moulding and discusses the important issues for MID production that can modulate the qualities of final MID. The results and discussion presented here can be a valuable user guide for mass production of moulded interconnect devices.

The microinjection moulding process is subject to microspecific phenomena, such as rapid cooling or high shear rates, which greatly affect part properties. While the correlations between morphology, crystallinity and the mechanical properties are well known for parts of usual macroscopic dimensions, there is less information available for microparts. In this paper, these correlations are discussed, related to the dimensions of semicrystalline thermoplastic parts. Results indicate that, if submitted to rapid cooling, microparts exhibit a fine structure, with low crystallinity, low yield strength and low elastic modulus. Experimental investigations have shown the influence of process parameters to be negligible. More important are the material's rate and ability to crystallise, which allow for properties to be significantly enhanced. Another possibility to considerably improve the performance of microparts independent of the used polymer is processing with slow cooling in thermally conductive moulds.

An ultrasonic injection moulding (UIM) system, which applies ultrasonic waves to injection moulding, as a precision injection moulding technology was developed. Replication properties of the microstructure of the moulded surface were evaluated by the UIM system. In particular, the effects of oscillation conditions on the replication ratio of the moulded surface were investigated. As a result, the replication ratio of the moulded surface was significantly improved in UIM compared with that in conventional moulding. The replication ratio increased when the ultrasonic wave was applied immediately after the resin was filled in a cavity. Results showed that, by applying ultrasonic waves, oscillatory flow was generated inside the cavity and consequently the surface replication was increased. The surface replication during packing and holding stages was improved by the oscillatory flow provided by the ultrasonic vibration.

Three-dimensional finite element method simulation was performed to clarify the mechanism on surface replication in microinjection moulding and thermal nanoimprinting. In particular, the filling flow behaviour into micro- and nanosurface features was discussed in comparison with the experimental results. The simulation results and the experimental results of injection moulding show possibility of the generation of air traps in the filling stage and it is considered that those air traps have a strong relation with replication shape and replication rate. The simulation results of thermal imprinting revealed the penetration behaviour of polymer melt into nanosurface features and showed that the aspect ratio of the surface features and imprinting pressure and temperature influenced flow behaviour in thermal imprinting.

Three-dimensional nanoimprint lithography was carried out using a photocurable resin. First, a fabricated nanoimprint lithography mould was coated with an antisticking layer. Then, an ultraviolet photocurable resin was dispensed onto cleaned glass slides or poly(ethylene terephthalate) films. The mould was pressed against the resin on the substrate, and the impressed photocurable resin was exposed to ultraviolet radiation. The mould was then removed, leaving a replica of its pattern. Using a three-dimensional mould with a very rough surface, it was possible to evaluate the photocurable resins, and it was found that the use of monomers with weaker intermolecular forces improved the transfer and release properties.

A microinjection moulding machine was used to obtain micromouldings of polyoxymethylene, in order to study morphology development during the process. The method of design of experiments was used to investigate statistically the effects of processing variables on the microstructural features of the mouldings. The morphological features were identified by microtoming the samples in both transverse and longitudinal (flow) directions and observing the microtomed sections under a polarised light microscope. Morphology evolution along the flow direction was followed by microtoming the specimens along the centre plane longitudinally and sequentially. A five-layer skin core structure was identified for micromoulded polyoxymethylene. The development of the structure was explained, based on mechanisms which were similar to those proposed for conventional injection moulding. Injection speed was found to be the most significant factor affecting morphological features of the final moulding. Moreover, the average plunger velocity, which is directly related to the cavity filling flow rate, was found to have good correlation with skin layer thickness. The distributions of crystalline polymorphs were observed and explained, in light of the distributions of the flow and thermal patterns in the mould. Morphology evolution along the flow direction reflected the distribution of pressure, temperature and velocity of the polymer melt during the microinjection moulding process. The results provided some indications regarding micromoulding mould design.