Micro injection-moulding projects are by their very nature complicated, and the fact that many require the repeatable attainment of extremely high tolerances demands that there is a collective focus on accuracy when looking at micro tool fabrication, moulding, validation, and automation processes.
However, material selection and design for micro moulding are also hugely important, and must be a primary consideration as time, cost, and the frustration of creating a design that needs to be radically altered before it can be manufactured need to be eliminated as much as possible.
It is because of this that it is especially important that manufacturers work with micro moulders as close to the beginning of the product development process as possible, as accessing expert micro moulding design insight early is the key to cost-effective, timely, and right-first-time manufacturing.
When looking at the rule book for design in micro moulding, it is worth taking on board a few things that will help even at the product inception stage. However, it is also worth speaking to a micro-moulding expert as soon as you can, as then you will access the precise application-specific advice you will need.
The design conundrum
Many OEMs view micro-moulding practitioners as manufacturers. While this is in itself self-evident, we very often view ourselves as consultants. Micro moulding experts that are truly vertically integrated, with all stages of the product development process under one roof, have expertise in design, tooling, moulding, validation and automation.
What this gives them the ability to do is pool this expertise, and the best time to do this is at the conceptual design stage of any product development programme. There is no better time for tooling managers to put their hands up and seek advice as to the best design for seamless moulding than early in the design stage. Likewise, a judicious word from the assembly manager will potentially save customers weeks of delay and spiralling costs, if problems are detected early.
As manufacturers begin to conceive a micro moulding project, it is important to bear in mind some key issues from the get go. Every project – as with so much in life – should start with the end in mind. You need to understand the basic design approach, really bottom out what you can and cannot do, as with micro moulding, even the smallest design change can be catastrophic and completely derail a project.
For example, if a manufacturer is considering an application with a 0.005in (0.127mm) wall thickness, a 5% change in dimensions can be the difference between whether a part will fill or not. A material change can also have a profound effect on a part with this level of detail. Any design engineer will be used to considering stack tolerances and stack dimensions, but when parts get smaller and smaller, the amount of change becomes more of a limiting factor.
Design for Manufacture (DfM) is a fundamental consideration when micro moulding. In fact, there should be a subset of DfM, basically DfMM (design for micro-manufacturing), as the rules of the game are so different when designing miniature parts, or parts with very precise elements and tight tolerances.
Design engineers have all the computer power and expertise they need to create the most astoundingly innovative products on screen, but they can be utterly impossible to make. Sometimes the reason for this is simple to deduce, other times less so. Size is often part of the problem. It is possible to blow up a particular part of a design to fill the screen and nuance aspects of it through tinkering and adding functionality at the scale of fractions of an inch or a few microns. Theoretically, this can be the difference between perfectly optimised design or a slightly better than average design, but the design can be impossible to mould. Any design at the micro scale requires careful attention to what might otherwise not be a big deal for larger parts.
Each micro moulding project is unique in terms of geometric complexity and shape, and can be made in a wide range of materials, all of which will ultimately affect the ability to manufacture them.
Design for micro moulding
So what constitutes a micro-moulded part? Typically if you are looking to manufacture a part or component that weighs less than a gram, you are micro moulding. However, it’s one thing to mould something small, and quite another to mould it efficiently and precisely time and time again. Micro moulding must be capable of producing thousands, millions, or even billions of parts with the same precision.
As a general rule of thumb when looking at the DfMM, dimensionally at Accumold we can accommodate parts up to 0.5in (13mm) in the largest dimension, and to date the smallest we have moulded is 0.031in x 0.012in x 0.015in (800 x 300 x 380 microns).
General guidelines for design engineers to consider are as follows. Thin wall sections need to be 0.004in (0.1mm) or thicker and there needs to be a focus on thick- to thin-wall transitions, as well as on wall thickness uniformity. Feature aspect ratios are around 6:1 (although this is highly material dependent), gates can get as small as 0.1mm, and ejector pins can be a small as 0.25mm. It is also vital to have an understanding of how shrink rates will affect the part, and to be cognisant of parting line mismatch.
Taking all this together, you can see that there are numerous variables, and numerous ‘rules’, but it is important to discuss ideas for micro plastic parts and components with an expert micro moulder before you abandon an idea as being too complicated. Micro moulding pushes the boundaries of what is possible in manufacturing every day, and challenges from manufacturers help push the envelope. A good micro moulder will relish the opportunity to work to innovate, and only if the objective is truly impossible will they advise that manufacturers make design changes.
Materials and design
Material selection is an area that can have a dramatic influence of successful design for micro moulding. Micro moulding materials come in almost as many forms as there are ideas in the mind of a design engineer. In the world of micro moulding, mission-critical components often require exotic or highly engineered compounds. Materials like PEEK, PEI (Ultem), carbon-filled LCP, or glass-filled nylons are commonplace. Soft durometer or elastomeric resins are also prominent. Direct experience with these materials in the context of micro moulding is another part of valuable know-how needed to maximise the performance of the resin/part design combination.
Even in macro product design scenarios, material selection can be complicated as it is a trade-off between what is best for the functionality of the part or component being designed, and if it is possible to mould it into the shape required. When looking at micro-moulding applications, however, where limits are often extreme, the issue becomes more complicated.
The typical route to material selection is to match the functional part requirements with material datasheets. However, most datasheets are based on the manufacture of much larger parts, and this will reflect on recommended gate size and flow properties etc.
So manufacturers looking to micro mould will follow datasheets, and then see sub-optimal results when the parts are moulded. This can lead to premature design iterations, or in extreme cases the abandonment of projects altogether.
The central issue is that datasheets, advice from resin suppliers, and part design criteria are all at odds at the micro scale. It is because of this that it is vital to engage with expert micro moulders early in the conceptual design phase of product development, as they are best equipped to advise based on years of experience navigating material selection issues, and are best placed to indicate whether a chosen material could be an issue in part production.
Summary
Design for micro moulding is complicated as the ‘norms’ in terms of geometric complexity and material performance are different from moulding larger-scale products. Working with micro moulding experts early in the product development cycle and at design reviews is important, as they can offer vital insight into issues such as design for tolerance achievement, optimal gate locations, accounting for flash and mismatch, advice on prioritising and limiting critical dimensions, and material selection for optimal outcomes.
Aaron Johnson is VP of marketing and customer strategy at Accumold
