2.3. Demoulding force in micro injection moulding

Replicated part will forcibly demoulded or ejected from the replication tool when it reaches stable condition and solidified using a series of injector pins. A sufficient amount of force is needed to overcome the retarding forces that are present at the component and tool interface due to friction and adhesion. Shear stress present from friction and thermally-induced stress due to cooling process can result in a demoulding failure. When a part size reduces or shrinks after cooling process, it reduces the potential sites where the ejector pins can act and this could cause damage to the part when mechanically forced out from tool cores. The shrinkage causes the stress to build up in the cross section of the part [12]. Stresses which are developed from shrinking are really associated to normal force. Therefore, a tangential force is needed to overcome the normal force and create relative motion between the surface part and tool during demoulding process. The component and tool must avoid being overly stressed or damaged during the process to prevent failure on the parts.

2.4. Taguchi method in replication process

Design of experiment is a mathematical and structured method to investigate the influence of many factors simultaneously when there is variation due to controllable and uncontrollable factors. It gives the opportunity to make the best decision scientifically when encountered with the situation that depends on many influencing variables, inputs, factors, and parameters. Taguchi method introduces an experimental technique in the form of design of experiment that is suitable for a wide range of applications. In engineering, the Taguchi method is used for investigating the optimal performance characteristics from a set of factors through design of experiment. It is composed of five basic phases which are mainly planning experiments, designing experiments, conducting experiments, analysing results, and confirming the predicted results [13].

3. Experimental  procedure

This section explains the structure of the experimental design and how the experiments were carried out. In micro injection moulding, the selection of test materials is generally determined from a range of polymers according to the application. Every polymer has different properties and shows different results. The test part design is described as it is important for observing the effects of the process parameters considered. Then, the design of experiment with the process parameters considered for this study and the measured response is outlined. The machine used to manufacture the micro injection moulded test parts is presented.

3.1. Test materials

Polymer may be moulded into any shape or structure in conventional injection moulding. However, the restriction of surface  area  to  volume  in  micro  injection  moulding  only

allows those polymers with special properties such as low viscosity and good mechanical strength. The common polymers that are suitable for micro injection moulding are: PMMA, PP, PSU, POM, PE, PA, PEEK, LCP, COC, PBT,

and PPE [14]. In this experimentation, TOPAS COC 5013 has been selected to carry out the planned experiment. Topas® is the trade name for Topas Advanced Polymers cyclic olefin copolymers (COC).

3.2. Test part and tool design

The design of a 15 mm x 20 mm x 1 mm microfluidic platform shown in Fig. 1 (a) was used to analyse the effect of process parameters. The features commonly found in microfluidic components in system design include aspects such as reservoirs, channels, and waste compartments. The pin dimensions are 500 μm in diameter and 600 μm in height, and the cross section of the main channels differ between 50 and 200 μm. In addition, the overall design includes a draft angle of 2 to 3 degrees on the side walls.