October 24, 2022 By MDO Contributors Network Leave a Comment
TransMed7’s SpeedBird biopsy devices are made with injection molding, among other manufacturing processes. [Photo courtesy of Fictiv] Plastic Molding Development
Injection molding is known for producing high volumes of tight-tolerance parts. What medical designers may not realize, however, is that some contract manufacturers can also cost-effectively prototype functional samples for testing and evaluation. Whether it’s for single-use devices, repeated-use devices or durable medical equipment, plastic injection molding is a versatile process that can help you bring products to market faster.
Like any manufacturing process, there are best practices for injection molding. They fall into four major areas: part design, material selection, tooling and quality assurance.
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By considering what works well and working closely with an experienced manufacturer, you can avoid common mistakes that result in added costs and delays. The following sections explain what medical designers need to consider when outsourcing an injection molding project.
Design for manufacturability (DFM) is the process of designing parts so they are easy to manufacture. Parts with looser tolerances have larger part-to-part dimensional variations and are usually easier and less expensive to make. However, most medical applications require tighter tolerances that those used with commercial products. Therefore, during the part design process, it’s important to work with your manufacturing partner and add the right type of commercial or precision tolerances to your drawings.
There isn’t just one type of injection molding tolerance, and omitting drawing details can result in parts that don’t fit correctly or cost too much to produce. In addition to dimensional tolerances, consider whether you need to specify tolerances for straightness/flatness, hole diameter, blind hole depth and concentricity/ovality. With medical assemblies, work with your manufacturing partner to determine how all the parts fit together in what’s known as a tolerance stack-up.
Tolerances vary by material, so don’t just evaluate plastics based on properties and pricing. Choices range broadly from commodity plastics to engineering resins, but these materials all have something important in common. Unlike 3D printing, injection molding can produce parts with exact end-use properties. If you’re designing pilot prototypes, recognize that you have the flexibility to use the same material as in production. If you need a plastic that conforms to a specific standard, consider asking for a certificate of assurance (COA) to ensure that the injection molding material — not just its individual ingredients — complies.
Manufacturers mostly create injection molds out of aluminum or steel. Aluminum tooling costs less but can’t match steel tooling’s support for high volumes and precision. Although the cost of a steel mold may take longer to amortize, steel is cost-efficient across a high volume of parts. For example, if a $10,000 steel mold for a single-use medical product is amortized across 100,000 parts, the tooling cost is just 10 cents per part.
Steel tooling can also be the right choice for prototypes and lower volumes, depending on your injection molder’s capabilities. With a master die unit and frame that includes sprues and runners, leader pins, water lines and ejector pins, you only pay for the mold cavity and the core details. Family molds that contain more than one cavity can also reduce tooling costs by having multiple different designs inside the same mold.
With medical injection molding, it’s not enough to produce good parts most of the time and then have the QA department catch any defects. In addition to tight tolerances, medical parts need a high degree of accuracy. DFM, T1 samples and post-production testing and inspection are important, but process control is essential for variables such as temperatures, flow rates and pressures. So along with the right equipment, your medical injection molder needs to be able to identify critical-to-quality (CTQ) attributes.
For disposables, repeated-use medical devices and durable medical equipment, injection molding can help you bring products to market faster after alpha and beta prototyping are complete. Injection molding is known for supporting high-volume production, but cost-effective pilot prototyping is also possible. Injection molders have different capabilities, so consider making careful vendor selection an additional best practice for your next project.
Robbie Long is a senior technical applications engineer with Fictiv. [Photo courtesy of Fictiv]
The opinions expressed in this post are the author’s only and do not necessarily reflect those of MedicalDesignandOutsourcing.com or its employees.
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