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Novel Micromolding Process Reduces Cost, Boosts Production Volumes

The micromolding technique was developed for thin-walled cannulas, but it may have applications in other sectors besides medical.

Joe Darrah

September 21, 2023

5 Min Read
micromolded plastic parts
Image courtesy of Accumold

A novel approach for micromolding thin-wall cannulas has been developed by precision micromolding company Accumold. The new process is expected to reduce costs and failure rates while increasing production volumes. The thin-walled cannula design is also said to be customizable to meet specific applications and project lengths, material requirements, and head design.

Small-scale parts with tight tolerances

Conventional production of cannulas typically involves extrusion, tipping, and gluing to a metal hub, all of which introduce limitations in efficiency and quality, said Accumold. Extrusion, which involves forcing material through a die to create the cannula shape, becomes especially challenging for small dimensions because of 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, which can impact the precision required for medical procedures. There have also been general concerns about the gluing process, as adhesives can degrade over time and lead to safety issues if the cannula detaches from the hub.

With today’s demand for smaller, thinner components continuing to rise, micro injection molding can play a significant role within the medical industry, particularly as minimally invasive procedures and the need for smaller devices become more prevalent.

Micro injection molding also enables the production of intricately designed medical components that often require tight tolerances.

Officials at Accumold anticipate that this molding approach will be applicable to applications in other industries, including electronics and automotive, because of its ability to produce small-scale parts in tight tolerances and consistent quality.

Miniaturization and manoeuvrability

Demand for small, thin-walled cannulas is projected to grow to facilitate invasive patient-care procedures, such as laparoscopic surgery, endoscopy, and catheter insertions. The diminutive size and thin walls of the cannulas also allow for minimal tissue disruption during insertion, as well as reduced pain, faster recovery times, and decreased risk of complications. The specialized cannulas play a particularly important role in fields where manoeuvrability is essential, such as neurosurgery and cardiovascular interventions. Their slender design enables access to complex anatomical structures that could be challenging to reach with larger instruments. Additionally, as medical technologies continue to evolve toward miniaturization and minimally invasive techniques, small and thin-walled cannulas become essential components for innovative medical devices.

Process particulars

Perfected over five years of research at Accumold, the micro injection molding process requires several design for manufacturability (DFM) considerations to be addressed for effectiveness. Ensuring uniform wall thickness is paramount, as variations can lead to warping, cooling inconsistencies, and inadequate filling. Proper gate placement is essential for influencing material flow and minimizing 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 mold and prevents potential damage.

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 demolding.

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.

An appropriate aspect ratio directly impacts the manufacturability and quality of the molded 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, making it vital to strike the right balance that promotes both accurate molding and structural stability.

Achieving the ideal aspect ratio is crucial not only for the successful filling of the mold but also for ensuring consistent quality throughout the production process. A well-balanced aspect ratio minimizes the risk of defects such as sink marks, flow lines, and uneven surfaces that can compromise the cannula's functionality and overall performance. Additionally, the aspect ratio affects the ease of demolding and assembly, contributing to efficient production and reliable end products.

Material selection

During the five-year development process, particular attention was paid to material selection, which is paramount in optimizing outcomes. The unique challenges posed by micro-scale manufacturing, such as precise cavity filling and intricate geometry replication, require materials to possess specific properties such as low viscosity, excellent flowability, and minimal shrinkage. Material selection also affects the durability and biocompatibility of medical devices, ensuring they can withstand the rigors of use while being safe for patients. 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 micromolding endeavors, according to Accumold.

Current medical applications

The micromolding process reportedly has been successfully performed in a range of end-use applications in a variety of materials. One example cited by Accumold is an ophthalmological cannula application in which polycarbonate was used to produce short cannulas for use in eye surgery. The cannula had an outer diameter of 0.035 in. (0.889 mm), inner diameter of 0.027 in. (0.6858 mm), and a wall thickness of 0.004 in. (0.1016 mm).

The company also prototyped a molded cannula for a drug manufacturer’s cancer-drug-delivery device using polypropylene. The device has an inner diameter of 0.027 in. (0.6858 mm), outer diameter of 0.015 in. (0.381 mm), and wall thickness of 0.006 in. (0.1524 mm). For this application, the wall thickness tapered down to the needle, and was thinner at the tip.

A prototype of two molded cannulas for a large diabetes company’s drug-delivery devices also utilized polypropylene for a cannula with an outer diameter of 0.022 in. (0.5588 mm), inner diameter of 0.011 in. (0.2794 mm), and wall thickness of 0.0055 in. (0.01397 mm).

The cannulas were molded on conventional presses and on Accumold’s proprietary micromolding presses. Accumold’s researchers discovered that conventional micromolding presses had difficulty with non-fill and flash. Through the use of the company’s fully automated in-house presses and 16-cavity micromold tooling, reliable, repeatable, and high-volume production of 40 million parts per year from a single production cell was reportedly achieved.

About the Author(s)

Joe Darrah

Joe Darrah is an award-winning freelance journalist based in the Philadelphia region who covers a variety of topics, including healthcare and medical technology. His articles have been published in more than 40 publications.

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