Rob Connelly, principal engineer, Trelleborg Healthcare & Medical explains how precision tooling, validation, and skilled engineers make miniaturisation possible.
The demand for miniaturisation of silicone and thermoplastic moulded products for medical devices has heightened the importance of process validation, precision tool design, and skilled tool operators and engineers. According to Plastics News Research, advancements and miniaturisation in the medical device industry have improved healthcare around the globe, making treatments and monitoring of conditions simpler, less invasive, and more portable.
Tap into tooling and precision
To miniaturise devices, components must become smaller. This significantly increases the complexity of the manufacturing process and requires equipment, tooling and processes that promote a high level of precision. Additionally, experts should Design for Manufacturability (DfM) to shorten time to market, improve future production yield and ensure delivery of product in high volume.
Typical moulding tools and processes are not usually capable of providing the quality and precision needed for miniature devices. Therefore, it’s important to partner with a supplier that understands the complexities of micromoulding.
A robust micromoulding process starts with high-quality injection moulding tooling which will determine the quality of the final product. All micromoulding tools, whether producing a plastic or silicone component, require a high degree of precision in tolerances and shut-off surfaces to precisely control the flow of material into the mould cavity. To achieve this level of quality, high-precision machining equipment is needed for mould fabrication.
In micromoulding processes, it is also vital to use equipment capable of injecting small doses of material into the mould cavity. Material doses or shot sizes of less than 1.0 grams are typical for these micro-sized components. Shot size control is critical for low viscosity Liquid Silicone Rubber (LSR) and is, therefore, a major focus with micromoulding equipment.
Without tight control over shot size, 5 to 10% of micro part cavity volume can easily “leak” from the mould cavity. This produces excess material around the perimeter of the part, commonly referred to as flash. In a larger, more typical sized moulded component, the size of a hand for instance, a small amount of flash is acceptable and can be removed in a secondary process. However, when a part is smaller than a pin head, the flash could be as large as the part itself, and flashless production becomes obligatory. This requires precision tooling, smaller injection units, better control of heat zones, and greater precision of injection units.
When working with a highly capable manufacturer of micromoulded components, more advanced capabilities, such as multicomponent injection moulding, also commonly referred to as 2K, 2shot, or 2C LSR, are ideal. Involving the simultaneous injection of LSR in combination with technical plastics, this highly advanced technique allows the combination of parts within an assembly into a single integrated component, eliminating potential assembly failures and dead space in which bacteria can breed. However, it requires sophisticated tool and process engineering.
Putting value in validation
In addition to high-precision moulds and equipment, robust process validation and measurement methods are also critical to ensure miniaturised components are manufactured within specification. Validation of micromoulded components is challenging due to process control complexity and difficulty in measuring, handling, and inspecting small features. For example, micromoulded parts are so small, static can literally cause the parts to fly away during handling. Therefore, special handling and static-mitigation devices are necessary throughout the manufacturing process.
Automated grippers are generally too large when demoulding extremely small silicone parts, while ejectors are not suitable for silicone parts regardless of size. This is because of silicone’s propensity to flow into the clearance of an ejector system. Another alternative is using brushes to remove parts from the mould, but the tiny micromoulded parts stick to the brush itself, rather than release from the mould and “fall” into a container below. The solution is specialised grippers designed specifically for micro-sized parts.
Packaging also needs to be considered in the production process, since standard polyethylene bags are not practical for extremely small parts. The parts will statically stick to the surface of the bag and be extremely difficult to remove. Therefore, parts may need to be packaged in small hard plastic containers or on double sided tape, for instance.
Gauge repeatability and reproducibility studies play an important role in ensuring the measurement processes for micromoulded components. These tests measure accuracy of the measurement process and help ensure measurements are repeatable by a single inspector and reproducible by multiple inspectors. As parts or part features and their associated tolerances become smaller, measurement equipment and methods increase in complexity. Methods and equipment must be engineered to achieve minimal measurement error and demonstrate statistical process capability and control.
Experienced micromoulding manufacturers balance the trade-offs between increasing the number of cavities in a mould and the challenges of validating those additional cavities. Higher cavitation in a tool enables increased throughput and potentially lower costs. However, the additional cavitation increases the challenge of tooling and process control, which increases the complexity of validation. It’s also important to include in-process controls that enable components to be individually analysed by cavity.
Experience is critical for success
The precise nature of micromoulding demands consistency and a deep working knowledge of tooling, equipment, process and material science, and engineering. Experts are needed to understand the challenges and variations that exist in micromoulding to anticipate and mitigate challenges before they occur. Finding a partner with longevity in the medical device industry and a significant depth of capabilities is invaluable for micromoulding.
Conclusion
Micromoulded parts are highly beneficial in the healthcare and medical industry because they facilitate development of medical devices to improve patients’ quality of life, shorten hospital stays, and reduce healthcare costs. The requirement for quality and invasive surgical methods, and a larger demographic of aging people are driving demand for miniaturisation. Device manufacturers should consider the critical elements of micromoulding - tooling, equipment, process controls, knowledge of materials, measurement systems, process and product validation, and overall engineering expertise - and partner with experienced moulders to ensure devices deliver the desired quality, consistency and patient outcomes.