Managing tolerance creep in micro-moulding

Paul Runyan looks at what factors can cause tolerance creep when moulding precise miniaturised parts and components

The production of tiny plastic components is a multifaceted task that relies on the professional collaboration of specialists in areas such as design, material science, tool creation, micro-moulding processes, quality checks, and the final assembly to achieve high-volume production for clients.

Precision is paramount in a micro manufacturing context, where the margin for error is limited to mere microns. This necessitates a laser-sharp dedication from everyone participating in the development of the product to proactively prevent any deviation from these stringent tolerances. This article explores the rigorous standards and practices that must be upheld to guarantee the desired outcomes.

Parts created through micro moulding often weigh under a gram, sometimes tipping the scales at a mere 0.001grams, with tolerances that can be as narrow as 2microns. For producers accustomed to larger-scale injection moulding, the notion of consistently fabricating such minuscule components with extreme precision might appear daunting. However, for experts in micro moulding, producing these components — often for parts that are crucial for safety and demand zero defects — with virtually no margin for error is part of their everyday operations.

Additionally, it is essential to execute these tasks not only accurately but also economically and efficiently. The key principle is to pre-empt the necessity for any redesigns, process overhauls, or the remaking of the already costly and intricate micro tools. The overarching mantra for every step is to ensure accuracy from the outset, and to get it right first time.

The cornerstone of precision in micro moulding lies in the vigilant management of stages where discrepancies in exactness are most likely to arise. While opinions may differ on which parts of the product development sequence hold paramount importance in this regard, there is a strongly shared understanding that the aspect of tooling — encompassing both its design and manufacture — is essential to achieving success in micro-moulding projects.

Micro tooling

In the area of micro tooling, experience is the foundation for achieving the minute tolerances required for success. Having a tightly integrated development cycle, where every critical stage, including design, tooling, moulding, metrology, and assembly, is managed under one roof, is indispensable. The mastery over the micro moulding process necessitates this vertical integration as outsourcing any segment of the design-manufacture cycle could lead to inefficiencies in time, cost, and quality control.

The key component when producing millions of products with micro-scale features is a high-quality micro tool. A subpar tool can consume your margin for error in a single stroke, jeopardising the entire project. Therefore, it’s crucial that the tooling department possesses advanced fabrication technologies that ensure the project stays comfortably within the overall tolerance threshold, leaving room for adjustments as the product development advances.

Access to a suite of precision-enhancing technologies is essential for micro-moulders. The use of wire EDM (Electrical Discharge Machining) and highly precise milling equipment is often necessary. Yet, the pursuit of ultra-precision begins well before the physical creation of the tool — it starts with the synergy between tool design and fabrication experts. By collaborating closely from the outset, micro moulders can secure successful results, adhering to the ethos of ultra-precision and the ethos of getting it right the very first time.

Micro-moulding

Upon completion of a precision-crafted and resilient micro tool, attention naturally shifts to the moulding process itself. A proficient micro moulder would have already played an instrumental role in shaping the design of the micro part or component to ensure it can be manufactured effectively and would have selected the most appropriate material for the job. However, when it comes to realising the required tolerances, the moulding phase is yet another stage that can trip up those without the requisite experience.

It is often said that while macro-level moulding is scientific, micro-level moulding combines both the science and the ‘art’ of the craft. This notion underscores the reality that true success in micro moulding hinges on the moulder’s ability to innovate and apply creative problem-solving to each challenge.

But what does this mean in practice? The complexities of micro moulding transcend the simplicity of executing pre-set instructions. It’s not a matter of scaling down macro processes; it’s about adapting and improvising with the equipment and technology available, sometimes even customising solutions. It involves an element of nuanced, insightful experimentation to fine-tune the many variables in play.

These variables, including factors like the time the plastic spends inside the machine (residence time), the force used to inject the plastic into the mould (injection pressure), and the temperature of the mould itself, are all meticulously adjusted by the experienced micro moulder to tailor the process to the material’s characteristics and the intricacies of the component’s design, thus maintaining control over the precision of the final product.

This intricate balancing act again highlights the significance of micro tooling in maintaining tolerances. No matter how many adjustments and calibrations a micro moulder makes during the moulding process, the outcome relies on the injection of high-pressure molten plastic into an exceedingly small and sometimes delicate cavity, which is dependent on the quality of the micro tool. If the tool cannot withstand the process — if it deflects or fractures — then all the moulder’s skill and knowledge go to waste.

Metrology and validation

Next, we address what some may term a ‘necessary evil’, which, in the context of micro-moulding, is actually a critical component, the metrology phase. In such precise manufacturing processes, metrology becomes not just significant, but a critical enabling technology.

The role of metrology in the development of micro moulded products cannot be overstated. Utilising advanced measurement technologies capable of validating the often tiny features on end products is essential. Without such capabilities, it’s impossible to guarantee that a product matches the original design intent and is suitable for its intended use.

The saying ‘what can’t be measured, can’t be made’ holds a dual meaning. Possessing advanced metrology tools that can measure features previously unquantifiable not only enables validation but also expands the possibilities of what can be manufactured. As such, it’s imperative for micro moulders to have access to the latest metrology equipment and to follow meticulous measurement practices.

Even with state-of-the-art tools, inherent variations within any measurement system must be recognised and managed. In micro-moulding, establishing trust in the precision and dependability of the measurement system is crucial. Equipment variation (repeatability) and observer variation (reproducibility) are the primary concerns. Minimising these variations provides greater confidence in the ability to detect deviations in the product’s fabrication.

Regular assessment of the measurement system, particularly Gage R&R (Repeatability and Reproducibility), should be a core aspect of ongoing refinement processes, more so in micro-moulding where the smallest variation can significantly impact the tight tolerance thresholds. Such evaluations fortify confidence in the instruments, operators, and methodologies. Understanding the nuances of measurement variation is essential to grasp the full scope of the manufacturing process, to better predict its capabilities, and to consistently deliver a product that meets customer expectations.

In summary, successful micro moulding entities invest in superior metrology equipment and are relentlessly dedicated to enhancing their R&R metrics.

Conclusion

A successful micro moulding project requires meticulous oversight across all phases of product development. While the focal points of this discussion have been on the crucial areas where tolerances are most stringent — such as tooling precision, moulding finesse, and rigorous metrology — there are numerous other elements that demand vigilant management. Variations in the materials chosen for the micro parts can significantly influence the outcome, with even different batches potentially leading to inconsistencies in the moulding process. Additionally, environmental factors, including humidity and temperature, play a subtle yet impactful role in the dimensional stability of the final products. These factors may not always take centre stage, but they are influential when micro moulding, each capable of reshaping the outcome if not considered.

Everything circles back to a singular, inescapable truth — the experience and integrated operations of a micro moulder are not merely beneficial but essential for success. An experienced micro moulder possesses the nuanced understanding necessary to navigate multifaceted challenges, leveraging a comprehensive, coordinated ‘under-one-roof’ in- approach to ensure every step is optimised to achieve the end goal. This vertical integration isn’t just about having all the tools and processes under one roof, it’s about creating a cohesive and synergistic workflow where expertise, precision, and adaptability coalesce. When micro moulding, where every micron counts, this harmonised pulling together of skills and resources is the key to unlocking consistent, high-quality production that stands the test of market demands and customer expectations.

Paul Runyan is VP Sales & Marketing, Accumold.