The paper is organized as follows. The micro injection moulding process is described in section 2. The experimental procedure is explained in section 3. In section 4 the results and analysis of the results are presented. The paper is concluded in section 5.

2. Micro injection moulding

Researchers have given the definition of micro injection moulding as process for producing polymetric parts with structure dimensions in the micron or sub-micron range [7]. Component parts that have been manufactured by micro injection moulding fall into one of the following categories which are type A having sizes of overall dimension less than 1 mm and type B having larger sizes on overall dimension but incorporate micro features with sizes typically smaller than 200 μm [5]. Micro injection moulding has similar basic steps with conventional injection moulding. Typically conventional injection moulding stages consist of filling, packing, holding and cooling phases. The micro injection moulding starts from closing the mould, followed by polymer being injected into the mould, after which holding and cooling takes place until the polymer is solidified, lastly the mould opens and ejects the part from the mould. The injection stage requires plasticization of polymer material as the polymer melts and fills the mould cavity. At holding stage, the holding pressure secures the melt from escaping from the mould cavity and allows the melt for additional shrinkage. The cooling stage starts from the end of holding pressure until the mould is open [8]. The cooling process is essential as to solidify the polymer before it gets ejected from the mould in one piece. The parameters of micro injection moulding and conventional injection moulding are the same which are mould temperature, holding pressure, injection speed, holding and cooling time.

2.1. Replication capability of process parameters

The micro injection moulding process has many factors which affect its replication capability. Therefore, many researchers have studied the significance of this process parameters such as melt and mould temperature, injection speed into mould, and holding pressure. The recent developments in micro injection moulding machine have enabled the introduction of specially designed systems for the fabrication of miniature parts incorporating micro features. At the same time in an attempt to improve the process performance, new process parameters such as the metering size and a small forward movement of the injection plunger for controlling the holding pressure have been considered [9]. Previous researchers who looked into the conventional injection moulding process have identified the melt and mould temperature, injection speed and pressure as the the main factors due to their effect on melt flow property.

In some cases there were differences within the findings. This could be explained by different experimental conditions such as different sizes of test parts and polymers which were used. Although, high settings of process parameters like high melt and mould temperature can be used to improve the melt flow in micro cavities, it can also generate negative effects on the injected parts [4, 6]. A high melt temperature could increase the rate of degradation of polymer, high mould temperature may increase the cycle time and cost for cooling equipment and high injection speeds can result in high pressure at the screw tip causing material degradation [10].

In recent research, process parameters such as melt temperature, mould temperature, injection speed, and holding pressure which influence the pressure in cavity of micro injection mould were investigated [11]. Hence, a design of experiment is needed to determine an optimum level of process parameters necessary for tuning the MIM process.

2.2. Polymers in micro injection moulding

There are many types of polymer materials that have been used in micro injection moulding for a variety of micro component parts in industry. Researchers have been using various types of polymers in their studies for better understanding of the behaviour of those polymers in micro injection moulding process. Polymers are known for their low cost and good electrical, mechanical and optical properties. The flow and thermal properties of polymers are the main reason for their usage in injection moulding although it can affect the replication capabilities and quality of moulded parts. Further, micro cavities can lead to strict dimensional limits and cooling circumstances that makes it difficult for melt fills. For incomplete melt fills can cause defects in moulded parts in micro injection moulding which contributes to waste and production costs. Hence, polymers with good flow properties with low viscosity are preferred for micro injection moulding purposes.