Paul Runyan, VP of sales, Accumold
Micromoulding specialist Accumold has developed a way to injection micromould thin-walled cannulas in very high volumes. Cannulas are typically produced via an extrusion process, which is expensive, prone to high fall out rates and does not lend itself to the high volumes required by medical device OEMs for some applications.It took Accumold five years to perfect the injection micromoulding of thin-walled cannulas at volume, and this article describes how it was done, the issues at play when injection micromoulding such intricate devices, the advantages of the injection micromoulding process, and what this means for medical device applications moving forward.
Cannulas produced by Accumold.
The case for micromoulding
Injection micromoulding is a precision manufacturing process that involves the production of small and intricate plastic parts with high accuracy and repeatability. It utilises specialised machines and moulds to inject molten plastic into tiny cavities, typically on the scale of micrometres to millimetres. The process allows for the creation of complex geometries and extremely thin-walled components that are difficult to achieve through traditional moulding techniques. Injection micromoulding finds applications in various industries, including electronics, automotive and medical devices, due to its ability to produce small-scale parts with tight tolerances and consistent quality.
For medical device OEMs, injection micromoulding is of paramount importance as the demand for smaller and thinner components continues to rise. In the medical field, where minimally invasive procedures are becoming increasingly prevalent, the need for smaller medical devices and tools is crucial. Injection micromoulding enables the production of intricate medical components such as catheters, implants, connectors, cannulas and drug delivery devices with precise dimensions and intricate features. These devices often require intricate designs, tight tolerances and biocompatible materials, which injection micromoulding can accommodate. This technology empowers medical OEMs to create more advanced and efficient devices that enhance patient care, minimise invasiveness and improve overall medical procedures.
Thin-walled cannulas
Small and thin-walled cannulas play a pivotal role in various medical applications due to their significance in minimising patient discomfort and optimising medical procedures. These slender and delicate tubes are instrumental in facilitating less invasive catheter insertion in procedures such as laparoscopy and endoscopy. Their diminutive size and thin walls allow for minimal tissue disruption during insertion, resulting in reduced pain, faster recovery times and decreased risk of complications. Small and thin-walled cannulas also contribute to enhanced precision during delicate procedures, enabling healthcare professionals to access targeted areas within the body with greater accuracy.
Moreover, these specialised cannulas are particularly crucial in fields where intricate manoeuvrability is essential, such as neurosurgery and cardiovascular interventions. Their slender design enables access to intricate anatomical structures that might be challenging to reach with larger instruments. Additionally, as medical technologies continue to evolve towards miniaturisation and minimally invasive techniques, small and thin-walled cannulas become essential components for innovative medical devices. Overall, these cannulas empower medical professionals to provide more effective treatments, reduce patient trauma and advance the landscape of modern medical procedures.
Traditional cannula production
High-volume production of small and thin-walled cannulas using traditional methods such as extrusion, tipping and gluing to a metal hub present several limitations that hinder efficiency and quality. Extrusion, which involves forcing material through a die to create the cannula shape, becomes challenging for extremely small dimensions due to the risk of material inconsistency, wall thickness irregularities and potential defects. Tipping, the process of adding a plastic or metal tip to the cannula, introduces variability in terms of bonding strength and tip alignment, impacting the precision required for medical procedures. Moreover, the gluing process poses reliability concerns as adhesives might degrade over time, leading to potential detachment of the cannula from the hub and compromising patient safety.
Furthermore, these traditional methods are labour-intensive and time-consuming, making it difficult to meet the demands of high-volume production efficiently. In contrast, modern manufacturing technologies such as injection micromoulding offer a more streamlined approach. Injection micromoulding enables the creation of intricate and consistent cannula designs with precise wall thickness control. It eliminates the need for separate extrusion, tipping and gluing steps by producing the entire cannula in a single mould, enhancing product reliability and reducing the risk of defects. This approach not only improves the quality and reliability of the cannulas but also offers a more scalable and cost-effective solution for meeting the demands of high-volume production in the medical industry.
Injection micromoulding for cannula production
While the use of injection micromoulding for high-volume cannula manufacture is obviously advantageous, there were issues that Accumold needed to address and overcome to make it a viable manufacturing process.
For example, material choice is paramount when optimising outcomes in injection micromoulding due to its profound influence on product performance, reliability and manufacturability. The unique challenges posed by micro-scale manufacturing, such as precise cavity filling and intricate geometry replication, demand materials with specific properties such as low viscosity, excellent flowability and minimal shrinkage. Material selection also impacts the durability and biocompatibility of medical devices, ensuring they can withstand the rigors of use while being safe for patient interaction. By choosing materials that align with the intended application and manufacturing process, manufacturers can achieve consistent quality, dimensional accuracy and functional reliability, ultimately driving the success of micromoulding endeavors.
Accumold has successfully micromoulded cannulas in a variety of materials for a variety of end-use applications. For example, polycarbonate (PC) was used to produce short cannulas for use in eye surgery. This cannula had an outside diameter of 0.889 mm (0.035 in.), an inside diameter of 0.6858 mm (0.027 in.) and a wall thickness of 0.1016 mm (0.004 in.). The rigidity of the cannula works well in the eye but is not ideally suited for the skin.
The company also prototyped a micromoulded cannula using polypropylene (PP) for a cancer drug delivery device. The cannula had an outside diameter of 0.6858 mm (0.027 in.), an inside diameter of 0.381 mm (0.015 in.) and a wall thickness of 0.1524 mm (0.006). For this application, the wall thickness tapered down to the needle and was thinner at the tip.
Finally, Accumold successfully prototyped two micromoulded cannulas using PP for the drug delivery devices of a large diabetes company. The cannulas had outside diameters of 0.5588 mm (0.022 in.), inside diameters of 0.2794 mm (0.011 in.) and wall thicknesses of 0.01397 mm (0.0055 in.).
Other critical considerations
During its five years of learning, Accumold found that to successfully micromould thin-walled cannulas for applications such as the ones mentioned, while material choice was critical, so too was design and aspect ratios.
Several critical design for manufacturability (DfM) considerations must be addressed. Ensuring uniform wall thickness is paramount, as variations can lead to warping, cooling inconsistencies and inadequate filling. Proper gate placement is essential, influencing material flow and minimising stress points, while suitable venting channels are crucial to prevent air traps that can result in surface defects. Incorporating appropriate draft angles facilitates seamless ejection from the mould and prevents potential damage.
Furthermore, maintaining accurate parting line alignment prevents flash and surface mismatches. Strategic placement of features such as ribs and supports enhances structural integrity without compromising the overall design, while carefully considering the positioning of ejector pins prevents interference with critical features during demoulding.
Additionally, addressing assembly considerations in some instances can be vital, particularly if the cannula is part of a larger device. Ensuring mating surfaces, alignment features and interlocking mechanisms are well-designed enables smooth integration. By thoroughly addressing these DfM issues, Accumold is able to successfully micromould small, thin-walled cannulas with precision and reliability at volume.
An appropriate aspect ratio directly impacts the manufacturability and quality of the moulded part. Maintaining a balanced aspect ratio is essential to avoid challenges associated with flow dynamics, cooling and structural integrity. An excessively high aspect ratio can lead to difficulties in material flow and cavity filling, potentially resulting in uneven thickness and defects. Conversely, an aspect ratio that is too low might hinder proper cooling and cause warping. It is therefore vital to strike the right balance, promoting both accurate molding and structural stability.
Achieving the ideal aspect ratio is crucial not only for the successful filling of the mould but also for ensuring consistent quality throughout the production process. A well-balanced aspect ratio minimises the risk of defects such as sink marks, flow lines and uneven surfaces, which can compromise the cannula’s functionality and overall performance. Additionally, the aspect ratio impacts the ease of demoulding and assembly, contributing to efficient production and reliable end products.
The micromoulding process
Beyond the choice of materials, design considerations and aspect ratios, there was one other variable that has been critical to Accumold’s success in the high-volume manufacture of thin-walled cannulas, and that was the use of its proprietary micromoulding presses, developed over generations. Cannulas were moulded on conventional presses and on Accumold’s presses, and it was discovered that conventional micromoulding presses had problems with non-fill and flash. Through use of Accumold’s fully automated, in-house developed micromoulding presses and high-cavitation micromould tooling, reliable, repeatable, and high-volume production of 40 million parts a year from a single production cell was achieved.
Summary
Through a combination of expertise and persistence, Accumold can now offer high-volume production of cannulas with simplified, all-plastic bodies and heads designed to maximise manufacturing efficiencies and product integration. The cannula design is customisable to meet specific project length, material and head design, and available materials include PC, PP, fluorinated ethylene propylene (FEP), polyether block amide (PEBA), thermoplastic polyurethane (TPU) and other customer-specified options.