As medical devices continue to become more complex, OEMs must continue to seek strategic partnerships with manufacturers that enable them to meet consumer demands. Due to the tight tolerances of a plastic medical device component— especially one that handles fluid or gas—effective collaboration is necessary between the medical device OEM, the mold builder and the injection molder. All parties have to work effectively as one team in order to deliver the specified product features, while also working within the inherent physical limitations of mold building and injection molding. In order to successfully produce a complex medical device, there are numerous considerations to navigate among all three parties. Common design considerations (of many) include:

Fig. 1 – First shift supervisor Ed C. operates an injection molding machine at Plastikos.

Gate location and type: Oftentimes, the best place to gate (i.e., thickest wall section) will compromise the aesthetics of the finished device. Therefore, a give and take exists between the mold designer and builder, the molder, and the OEM product designers to find an acceptable alternative gate location. That alternative location may satisfy the surface finish criteria; however, the risk of other molding imperfections could increase, such as sink marks, internal voids, hesitation regions, warp/bow, and/or weld lines.

Fig. 2 – Finding the right partner for quality moldmaking is critical when working with medical device components. This worker is machining a mold.
Draft and surface finish: Another common (yet very challenging) design request is for a near-perfect surface finish and little to no draft on the side walls and/or internal diameter of the part. This request poses a significant challenge from the moldmaking and molding perspective. The part orientation within the mold needs to be taken into consideration in order to properly eject the part and minimize drag marks on surfaces without draft. In some cases, polishing and mold plating techniques, along with minor design refinements, can be utilized to yield an acceptable part that can consistently be molded in the production environment. Again, all three organizations must collaborate to determine a realistic acceptance criterion.

Parting lines and witness lines:

Especially when working with fluid-handling components, it is critical to have a smooth finish so the liquid can flow easily through the part. A give and take exists to find an acceptable tolerance for parting lines and witness lines. Also, a mold is often designed with the flexibility to change out a core pin to run different gauge sizes on a particular port or fluid path.

Material Consideration: Materials can seem extremely attractive on the material data sheets with regards to dimensional stability, chemical resistance, and surface finish. However, it’s important to read the fine print. Some materials today have extremely high minimum order quantities and/or lengthy lead times. It’s important to communicate your material interests to the molder to determine if there are other alternatives available during the R&D phase of the project.

Careful Design Specification Critical to Produce Quality Parts

High-cavitation injection molds are common in medical molding since most devices are one-time use, which requires a highvolume demand. The drawback, however, is that any subtle inherent variability within the entire production system, which consists of (1) raw material, (2) hot runner system (if applicable), (3) the steel that comprises each individual cavity of the mold, (4) injection molding machine, (5) any auxiliary equipment, and (6) injection molding process is amplified as the number of cavities increase. An amplification of that system and process variability increases the difficulty level on the production side. Fortunately, there are tools such as cavity pressure transducers, material testing equipment, in-line inspection equipment, and scientific process training available to overcome and monitor such inherent variability. The real challenge is identifying early on what can realistically be accomplished and repeatable when developing a fluid-handling device. (See Figure 1)

With this in mind, it’s imperative that the manufacturer understands what is critical on the part and how the individual components will interact with one another, once assembled. The upfront communication will help provide the information that is needed to effectively determine what tolerances can be attained. Furthermore, the OEM and manufacturer should come to an agreement on measurement method, as this can impact the capability results of a critical dimension. The First Article Testing results may look promising if the initial samples are within tolerance. However, it becomes increasingly more challenging to prove a certain feature will not fluctuate throughout a Production Qualification run. (See Figure 2)

Manufacturer Applies Principles to Mold Unique Fluid- Handling Part

An example of this close collaboration between the medical device OEM, mold designer and builder, and injection molder occurred when Micro Mold Co. Inc., Erie, PA, an advanced mold designer and fabricator, and Plastikos Inc., also Erie, PA, a custom injection molder specializing in medical molding, set out to mold a disposable insulin cartridge for a portable insulin pump. The t:slim® Insulin Pump, designed by Tandem Diabetes Care, San Diego, CA, is worn by consumers around the clock, and contains a 300-unit disposable insulin cartridge.

The project presented an interesting fluid-handling challenge. The pump had to deliver a slow, continuous supply of basal insulin to control blood glucose levels at a rate of as low as 1 microliter/hour (0.1 units/hr). Additionally, the pump had to deliver a quick bolus dose of insulin during meals or when blood glucose levels were elevated. Further, the t:slim pump had to deliver bolus increments as low as 0.1 microliters (0.01 units). To put this in perspective, there are about 30 microliters in a single drop of water. Components the size of a single plastic pellet with a critical tolerance of +/- 0.0005 would play an integral role in the micro-dosing performance.

A close partnership between Plastikos, Micro Mold, and Tandem Diabetes ensured this type of micro-dosing performance could be achieved on the disposable cartridge. The challenge for Micro Mold and Plastikos was to design and manufacture multiple high-cavitation molds—ranging up to 32 cavities with fully interchangeable inserts—that were capable of producing the cartridge components, which fit into the durable body of the pump.

Overall, this unique project presented numerous challenges for the engineers, molders, and moldmakers involved, but taught all involved a valuable lesson. OEM device makers and manufacturers throughout the supply chain must truly collaborate in order to effectively meet consumer expectations as end user demand for higher quality medical devices continues to grow, driven by a home healthcare surge.

This article was written by Dan Snyder, Technical Sales Engineer, Plastikos, Inc., Erie, PA. For more information, Click Here " target="_blank" rel="noopener noreferrer"> MD&M East, Booth 439