The tool is manufactured in brass using conventional milling to perform the experiments. The overall size of the mould insert is 25 x 28 x 5 mm with four 3 mm holes for ejectors as shown in Fig. 1 (b). A single open gate design was used to reduce pressure and temperature that could affect the gating. Then, the tool halves were assembled and inspected using a primary mould tool for parallelism before the mating faces were shut off. The part design is made up of many features that could determine the mould accuracy. The influence of the process parameters in replication capabilities could be determined by comparing the dimensions of the tool and mouldings and the part weights.

Fig. 1. (a) Microfludic test part design (b) and it’s mould insert

3.3. Design of experiment

In this study, four process parameters at two levels given below were employed:

A - Barrel temperature (Tb); Level 1=240, Level 2 = 300 B - Mould temperature (Tm); Level 1 = 70, Level 2 = 130 C - Holding pressure (Ph); Level 1 = Off, Level 2 = On D - Injection speed (Vi); Level 1 = 200, Level 2 = 800

In order to investigate the level of process parameters that affect the replication process in micro injection moulding process, the demoulding force during the ejection process was focused as the measured response in this experiment. Because this experiment has multi factor effects it will not be easy   to

study the effects of so many factors since every factor can behave differently at certain point [15]. At the same time, it is essential to know which factors and their interactions are significant for a proper design of experiment. Therefore, it is necessary to use a factorial design to investigate several factors. In this study, the Taguchi method was used to reduce the number of experiments that needs to be conducted without compromising the outcome. Thus, for the four process parameters considered at two levels each a Taguchi L16 Orthogonal Array was selected (see Table 1). Then, the demoulding force was measured as the response for each set of control parameters. The trial was repeated ten times at the same parameter settings in order to measure process variation.

Table 1. The Taguchi L16 Orthogonal Array

Fig. 2. Battenfeld Microsystem 50 injection moulding machine

4. Experimental results and analysis

In studies of influencing factors that affect the replication capabilities in micro injection moulding, the demoulding force was used as a response variable to indicate the output of the process. As discussed before, the demoulding force of micro injection moulding is important as it affects the quality of the moulded part. The experiment was conducted by repeating each run 10 times and the average demoulding force was noted as given in Table 2. The test results were analysed using Signal to Noise (S/N) ratio and ANOVA. S/N ratio is very useful measure for improvement of quality through reduction of noise and improvement of measurement [Ranjit, 2001]. Table 2 presents two different set of results based on the measured responses where S/N ratio is considered to be nominal-the-best  and larger-the-better.

Table 2. The Taguchi L16 experimental results

3.4. Experimental machine used

The machine used in this experiment was Battenfeld Microsystem 50 injection moulding machine shown in Fig. 2. This machine is designed as a handling system for the manufacture of micro components. The machine has capabilities of high repeatable process control, accurate dosing of material, and working in clean room environment. An important aspect of this machine is the homogeneous plasticisation at extruder screw that is mounted 45 degrees to the injection axis. In addition, this machine has a precise injection of material into the mould and can achieve injection speeds of 750 mm/s. The experimental results are given in the following section.