4 minute read
Tolerance attainment in micro molding
from AMT JUN/JUL 2022
by AMTIL
Tolerance attainment in micro moulding
Much is made of the need for manufacturers embarking on a micro moulding project to partner with experienced and highly capable specialist micro moulders. Aaron Johnson of Accumold gets into the nitty gritty.
The trend towards miniaturisation across industry is accelerating, and in many sectors increasingly small micro parts and components are used in safety critical applications, so the demand is for micronlevel tolerances that can be achieved repeatably and which can be verified to ensure compliance with design intent. It is only by working with a qualified micro moulder that such repeatable tolerance attainment can be assured, and is also the only way that tolerance creep can be arrested at every point in the product development process. The key is to select a partner with experience that has all stages of the product development process under one roof. In the area of true micro moulding there are only really a handful of companies that can credibly claim to achieve the level of tolerances required, and even less that can prove true vertical integration. An eagle eye on tolerances
When manufacturing micro parts and components, it takes a coordinated team of experts in design, materials, tooling, moulding, validation, and automated assembly to successfully manufacture customer parts at high volume. When tolerances are measured in microns, it is critical that all involved in the product development process maintain an obsessive focus on mitigating the risk of non-adherence to these tight tolerances. For the uninitiated, the thought of volume manufacturing parts with tolerances sometimes as low as two microns may seem the stuff of fantasy, but there are qualified micro moulders that do this daily, while at the same time maintaining zero failure rates. Attention to detail and attention to process is everything, as not only do micro moulders need to make tiny parts with tiny features repeatably and attaining micron tolerances, but they need to do this in a cost-effective and timely fashion. The viability of a project in terms of cost and time means the avoidance of multiple design iterations, constant review and change to manufacturing processes, and above all having to recondition extremely expensive and timeconsuming micro tools. The key is to ensure that areas where most “variation” in accuracy can occur are managed with absolute focus. Micro tooling
Micro tooling is perhaps the key to the success of a micro moulding project, be that its optimal design or its ultimate fabrication. 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. Micro moulders must be in a position to employ an array of moulding technologies, including the latest CNC machining technologies, and also — as is often required — EDM 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. I addition, as many micro moulding applications use high temperature, high performance materials, such as bio-resorbables, 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. When looking at tolerance creep, if tool design and fabrication is not optimal, then the project may be lost at this stage. Tooling is a true enabling technology when micro moulding, and the ability to fabricate tools significantly within final tolerance requirements of a part and to accommodate tolerance loss in other areas of product development is vital but beyond all but the most experienced of micro moulders. Metrology and Validation
Metrology has a key role to play in any micro moulding product development process, and it is vital to use measuring technologies that are able to validate end-use products with feature sizes that are always extremely tiny. In a micro moulding scenario, it is hugely important to establish confidence in the accuracy and reliability of the chosen measurement system(s). While other sources of measurement variation exist, the two key sources are equipment variation (repeatability) and appraiser variation (reproducibility). Evaluation of the measurement system in terms of Gage R&R should be an important element of any continuous improvement activity, especially with micro moulding applications where any variation in the repeatability and reproducibility of the measurement could crucially eat into the low margins for error in terms of tolerance attainment. The evaluation builds trust in the instruments, operators, and processes. Material Considerations
Ultimately, material choice has a direct influence on end-use part functionality and it affects tolerance attainment, the achievement of dimensional objectives, and strength parameters. It also has a direct and obvious impact on the cost and timeliness of manufacturing. Summary
Micro 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 (DfMM) are also hugely important. It is because of this 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.
