Aaron Johnson, VP marketing & customer strategy, Accumold, analyses the use of micro moulding in microfluidic devices.
There are few applications that are as exacting as the manufacture of microfluidic devices. Not only is there a requirement for ultra-precise and ultra-small features that need to be manufactured repeatably, but it requires a laser-like focus on the area of design for micro moulding (DfMM).
When looking at the micron-level of precision inherent in a microfluidic device, it is necessary to understand that the rules of injection moulding and the behaviour of plastics change. One area that also requires detailed attention is flatness. A microfluidic device usually needs to be completely flat, as any curvature will compromise the required sealing of channels through which gases or liquids need to pass. Traditionally, injection moulding is characterised by such issues as deflection, warpage, and shrinkage which can all compromise the integrity of the finished device, so your chosen micro moulder must have the knowledge and experience to work round such potential problems, and this may require the moving or modification of features.
When looking at microfluidics, we are not dealing with devices that afford a lot of room for manoeuvre, and there are often a lot of interconnected features and geometries, movement, or redesign of one potentially altering another. With all this in mind, a micro moulder will necessarily place a disproportionate focus on up-front design reviews involving representatives from all phases of the product development process.
Key among these are representatives from the micro-tooling team. Tooling for micro moulding projects does not require an extrapolation of the rules governing tooling in traditional injection moulding. Features in micro moulded parts often exceed the allowable tolerances in traditional injection moulding, and similar issues are confronted in the areas of venting and tooling mismatch.
Other tooling issues specific to micro moulding are requirements for an understanding of polishing for micro-mould cavities, and the heating and cooling implications when dealing with extremely thin steel inserts that can be negatively affected by temperatures involved in many moulding applications. In addition, as many micro moulding applications use high temperature or high performance materials, such as liquid crystal polymers and PEEK, it is necessary to use and understand the nature of tooling materials such as stainless steel rather than traditional tool steels which may not be able to withstand the high temperatures necessary, and can corrode.
It is vital to consider the likely stresses and strains that a specific microfluidic device design may impart on the necessary micro tool to make it. Very often microfluidic devices are characterised by extremely small micron-level distances between features, and this implies that the tool will have very thin regions that are prone to wear and tear. Such a tool if not designed correctly may well be appropriate for only low quantity production but would be troublesome when looking at high volume mass production, and then the cost and time implications of mould maintenance become an issue.
Catching such problems before a design is locked down is extremely important, as it means that the time and cost of design re-iterations and revisions can be avoided.
When working with your chosen micro moulder, it should all be about reviewing and a total focus on DfMM and design for micro assembly (DfMA) when producing microfluidic devices. Standard mould flow analysis does not always work when looking at micro tools and working at micron tolerances, so it is about the knowledge and experience of the micro moulder.
Microfluidic devices require tooling that will allow the production of parts that see the correct depth and length of features is replicated sometimes millions of times perfectly and repeatably. When looking at the micro features in such tools, your chosen micro moulder needs to have a detailed knowledge of the different material flow, viscosity, and solidification characteristics of the materials being used to ensure that feature integrity is maintained. Quite often such devices are used in safety critical applications, so failure is not an option.
As so often is the case, the micro moulder that will be able to navigate the intricacies of the design and manufacture of highly complex microfluidic devices is one that will combine access to in-house technologies that push the envelope in terms of what it is possible to manufacture (the kit), with the knowledge and understanding of all the different aspects of a product development process from design to mass volume production and assembly (the experience).
The emphasis must also, as always, be on the nature of the relationship that is forged between customer and micro moulder. The discussion of the huge importance of DfMM and DfMA above in respect of the manufacture of microfluidics involves a true partnership ideally from the point of product conception to achieve optimised goals as quickly and cost-effectively as possible.
