Aaron Johnson, VP of Marketing and Customer Strategy at Accumold, explains how to solve complexity problems in micro injection molding and microfluidics
When it comes to the art and science of microforming, there are few applications as sophisticated as making microfluidic devices. Not only is there a need for ultra-precise and ultra-small features that must be fabricated repeatably, but the fabrication of microfluidic devices also requires a laser-like focus on the Design for Micro Molding (DfMM) area.
In fact, in microfluidics, what a designer sees as equitable efficient design on screen very rarely works when the required tooling, injection molding, and assembly phases of product development are taken into account. Therefore, when considering manufacturing microfluidic devices, it is critical to collaborate and work with a microformer with the experience and knowledge necessary to optimize designs for manufacturability.
Microfluidics is an extremely dynamic niche today and for many years to come the importance of micromoulding as a technology for promoting innovation in the medical sector in applications such as drug delivery devices, implants and diagnostics (so-called laboratory-on-a-chip systems). However, microfluidic technology is used to control the behavior of gases and liquids on a microscale and is therefore widely used in many chemical and engineering applications in a number of industrial sectors.
There are a number of technologies that can be used to manufacture a microfluidic device, such as lamination, 3D printing, and high resolution nanofabrication techniques. However, when it comes to the repeatable mass production of high-precision microfluidics, the focus is still on the use of micro injection molding.
It is important to emphasize that micro-injection molding is not just about making tiny plastic parts. It also means making larger parts with micro-features and micro-tolerances where microfluidic devices are typically located. When looking at the micrometer precision inherent in a microfluidic device, it is necessary to understand that the rules of injection molding and the behavior of plastics are changing.
One area that also requires detailed attention is flatness. A microfluidic device must normally be completely flat because any bend will compromise the required sealing of channels through which gases or liquids must flow. Traditionally, injection molding has been characterized by problems such as deflection, warpage, and shrinkage, all of which can affect the integrity of the finished device, moving or changing functions.
In microfluidics, however, we are not dealing with devices that offer a lot of leeway, and there are often many interconnected features and geometries, movements or redesigns that may change others. With this in mind, a microformer will inevitably place a disproportionate focus on pre-design reviews that involve representatives from all phases of the product development process.
The critical component – micro tools
These include in particular representatives of the microtooling team. In the area of tools for micro-molded parts in particular, time and money can be wasted if the microformer does not have the necessary know-how. Tools for micro injection molding projects do not require extrapolation of the rules for tools in traditional injection molding. The characteristics of micro-molded parts often exceed allowable tolerances in traditional injection molding, and similar problems arise in the areas of venting and tooling mismatch.
Other tool-specific problems for microforming are requirements for polishing microforming cavities and the effects on heating and cooling when handling extremely thin steel inserts, which can be adversely affected by the temperatures encountered in many injection molding applications. Also, because many microforming 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 tool materials such as stainless steel in place of traditional tool steels, which may not be able to withstand the high temperatures required and can corrode.
It goes without saying that it is always better to work with a vertically integrated microformer where the microtooling is done in-house, and this is especially important in microfluidics as tooling design is often the key to success or failure Failure of mass production.
It is critically important to consider the likely stresses and stresses that a specific microfluidic device design can place on the micro-tool required to manufacture it. Very often, microfluidic devices are characterized by extremely small, micrometer-sized spaces between features, and this implies that the tool has very thin areas that are prone to wear. Such a tool, if not properly designed, may well only be suitable for low volume production, but would be problematic when considering high volume mass production and then the cost and time implications of mold maintenance become an issue.
It is extremely important to identify such issues before locking a design as it means avoiding the time and expense of design repetitions and revisions, as well as the worst of all worlds, weeks of waiting and thousands of dollars on a tool, that is not suitable for the purpose.
When working with your chosen microformer, microfluidic device manufacturing should be all about verification and a complete focus on DfMM and Design for Microassembly (DfMA). When looking at micro-tools and working with micrometer tolerances, standard mold flow analysis doesn’t always work, so it’s about the knowledge and experience of the microformer.
The purpose of this article is not to get too technical, but instead to highlight the critical role of microtooling in considering microfluidics. We just touched on the subtleties of some areas of a typical microfluidic device tool, but sprue location also needs to be considered, which can help overcome common problems related to plastic stagnation after injection when attempting to infiltrate thin section areas.
Gates for micro tools are also often measured in micrometers, which has a significant influence on the behavior of plastic, the most obvious being the influence of such small gates on the shear heat, which can lead to high reject rates if handled improperly, weight and dimensional deviations between cavities or between Shots, warpage and longer cycle times than expected.
Microfluidic devices require tools that enable the manufacture of parts that have the correct depth and length of features, sometimes perfectly and repeatably replicated millions of times. When considering the micro-features in such tools, the microformer you choose must have detailed knowledge of the various material flow, viscosity, and solidification properties of the materials used to ensure that functional integrity is maintained. Quite often such devices are used in safety critical applications, so failure is not an option.
Equipment and experience are key
As is so often the case, the microformer, who will be able to master the intricacies of designing and manufacturing highly complex microfluidic devices, will combine access to in-house technologies that push the limits of what is possible to manufacture (the kit) with the knowledge and understanding all the different aspects of a product development process from design to mass production and assembly (the experience).
We have seen the focus on micro tools in microfluidics manufacturing, and that means your chosen micro injection molding partner should be able to access the full range of tooling technologies that enable them to be manufactured in a timely and cost-effective manner. complicated shapes. As a minimum, this should include in-house CNC machining, micro-milling and micro-wire EDM.
But the type of relationship that is established between the customer and the microformer must, as always, be in the foreground. The above discussion of the enormous importance of DfMM and DfMA with regard to the production of microfluidics includes a real partnership ideally from the product conception in order to achieve optimized goals as quickly and cost-effectively as possible.