Normally, building an injection molding machine would be well beyond the capabilities of a home shop. In his book, Gingery simplifies the technology enough to enable building a machine capable of inexpensively injection molding small parts from common recycled plastic. He offers step-by-step instructions of building a small, relatively inexpensive tabletop injection molding machine able to mold up to a half ounce of plastic. That doesn’t seem like much, and Gregory advises that the machine “will not be the right equipment if you intend to produce parts in great quantity.”
It’s an excellent project for the budding inventor interested in injection molding technology. Hannon built a modified version of the machine, because, as he explained, “being an engineer, [he] couldn’t resist making improvements.” Some of the deviations from Gingery’s instructions included using hot rolled steel instead of cold for the machine’s heater block (where the plastic is heated before injection into the mold), which allowed him to add a second cartridge heater for quicker warmup and a hotter achievable temperature. He also welded the machine together instead of using screws (as indicated by Gingery’s design). Obviously, this method calls for some welding equipment in the shop, so beginners may be more comfortable sticking to drilling bolt holes.
“Using the machine is easy,” Hannon wrote in MAKE:. However, it is recommended to make a test mold of two half-inch diameter plastic pellets to feed the machine for real parts.
“To try it out, I cut some 3/8 inch wide strips of polyethylene from an old plastic tote lid, set the controller to 380°F, and fed the plastic strips into the cylinder,” Hannon explained. “After the cylinder is filled with sufficient molten plastic, the mold is placed under the nozzle and raised into place. (If the mold is put in place while the plastic is being loaded, some of the plastic will dribble out of the nozzle and clog the sprue of the mold.) Then, pull the lever and inject!”
With his home-built machine, Hannon manufactured a number of test-tube caps 16 millimeters in diameter for his science experiments. He made a sketch of the caps, went to the machine shop to cut some metal for the molds, and used the same plastic he made the pellets from, which were recycled from a tote lid. Unfortunately, the first set of caps he produced were too stiff and difficult to put on his test tubes.
Hannon reasoned that a more flexible plastic was necessary for his molding project to achieve his desired specifications (simple as they were). He then tried LDPE (low-density polyethylene)—a biocompatible synthetic polymer becoming popular for use in manufacturing medical devices—that was harvested from the lids of oatmeal boxes. “The caps made with the LDPE work just fine,” he concluded.
Though the DIY injection molding machine is limited to usage of simple molds made with a lathe or mill, Hannon’s materials challenge demonstrates it is still subject to demands similar to those a large-volume injection molder might encounter. Choosing the correct fabrication material is only the beginning of the necessary considerations for a successful molding project.
Mixing Molding Materials
Of all manufacturing processes, molding is perhaps the most versatile in terms of the types of fabrication materials available for use. A number of liquid or pliable materials can be molded, including glass, metal, ceramics, and plastics. Device manufacturers have undoubtedly used most or all of these materials for their projects, but the great strides polymer science has made over the past few years is prompting OEMs to convert many devices and components into plastic.
Polymers (also known as resins) are now able to last longer, endure high temperatures, and exhibit a wear resistance on par with metals. Injection molding, the typical process used for molding plastics, can produce very strong components with tight tolerances. It is also one of the most cost-effective and repeatable methods of manufacturing, as injection molding is able to generate large volumes of parts.
However, even next-generation polymers aren’t quite doing enough to satisfy the demands of device manufacturers. There has recently been heightened interest in customized materials with properties tuned to the end-use application. In these increasingly prevalent cases, molders adopt either the strategy of working with new materials, or combining existing materials to meet the device manufacturer’s specifications.
“As R&D pushes the boundaries, molding materials need to have a wider range of properties that may not be typically found in one resin,” noted Jeremy Williams, TZERO consultant and trainer for RJG Inc., a Traverse City, Mich.-based provider of injection molding process optimization technology and services. “This leads to compounding more additives in with the base polymer chain or blending of several materials to achieve a specific set of mechanical or thermal properties.”
“The trend continues of customizing material properties to meet functional needs of medical device components,” commented John Clark, a project manager for Charlton, Mass.-based MTD Micro Molding, a micro-injection molder for the medical device industry. “This includes experimenting with new materials and combinations of materials. For example, Chronoflex is a polycarbonate-based thermoplastic polyurethane (TPU+PC) elastomer that offers strength, flexibility, and dimensional stability to the latest device component designs.”
However, blending polymers to achieve the device maker’s desired material capabilities can fundamentally alter the molding process. There’s no real way to know how the mix of materials will react during processing.
“Variability inevitably increases the more materials that are combined to meet the application’s requirements,” advised Williams. “This can make predicting its behaviors in processing much more difficult.”
Another consideration molders are wary of for OEM projects is a particular polymer’s intrinsic viscosity (IV). This is a measure of the polymer’s molecular weight, and reflects its melting point, crystallinity, and tensile strength. Resin with the appropriate IV should be used when molding to ensure processability and final product specification are kept within the desired range. A particular batch of resin’s average chain length can be controlled by measuring viscosity during polycondensation (a condensation reaction in which a monomer with two functional groups leads to the formation of a polymer), but the molding process can have an impact on the IV of the final product. This is another challenge molders must overcome when combining materials.
“In some cases, the challenge of implant materials is that the IV window can impact ability to manufacture a production component,” explained Scott Herbert, founder and president of Rapidwerks Inc., a Pleasanton, Calif.-based plastics manufacturer specializing in precision micro molding. “Sometimes, the processes involved degrade the end product to the point where it is no longer useful. So, the ability to custom blend to raise IV becomes an advantage when you start to think about drying raw material, injection molding, sterilization, and any other in-process acts [that] apply heat to the material—ultimately affecting the IV or its drop.”
Device manufacturers may also select molding materials based on aspects beyond chemistry and processability. While no manufacturer is looking to shortcut the regulatory process and face costly delays and resubmittals, some are choosing materials that facilitate swift travel down the regulatory path for their projects. Examples of these materials might be those with plenty of supporting documentation regarding biocompatibility, extractables and leachables, guidelines for reprocessing, and others.
“We are seeing more clients select materials based on ease of regulatory path more so than before,” observed Brian Matachun, director of technical sales for MTD Micro Molding. “This may be driven by more material manufacturers supplying medical OEMs with documentation that allows for human implantation, which can take the place of lengthy and costly testing.”
However, while use of such materials may potentially offer a speedier track to commercialization, they may not be suitable for the manufacturer’s intended product design. Injection molding is a process equipped to handle a large variety of plastics, but not necessarily the “easy” ones.
“From an injection molder’s perspective, this can be a double-edged sword,” Matachun continued. “While these materials can allow for a faster path to commercialization, they may not be suitable for every component design and can present challenges that may be difficult, costly, and time consuming to solve. If the design cannot be molded from these materials and they are committed to using these materials, then an alternate means of manufacturing may need to be utilized.”
Sometimes Smaller Is Better
It’s a battle the medtech industry is constantly fighting—patients clamor for their devices to shrink enough not to be cumbersome, and device manufacturers strive to meet this need by using the smallest components feasible in their designs. But how small of a plastic component can injection molding actually produce? As more and more polymers are finding use as implantable device components, molders may find themselves with requests for nearly microscopic parts.
“On a regular basis, we are seeing smaller parts designs for implantable devices or to replace high-cost machined parts,” said Williams. “With these requirements, the molds must be manufactured to a tighter tolerance and the molding process must be optimized to ensure consistency.”
Despite being right there in the name, however, micro-molded parts are not necessarily microscopic. In fact, some micro part features visible to the naked eye necessitate more specialized tools and techniques than a microscopic part with simple geometry. Some of the more challenging parts to micro mold are actually on the larger side but contain micro features, like an inch-long part with an .1-mm thin wall section or a .2-mm diameter hole.
“As medical components continue to get smaller and smaller, precision tooling and molding enable a part to have multiple functioning features,” explained Matachun. “That is the biggest thing. A high degree of tooling resolution allows OEMs to design for smaller parts and even take two- or three-part designs and make it into one injection-molded component.”
The demand for more complex micro-assemblies is also driving the micro-molding business. Device makers want their assemblies to fit into tiny and delicate pumps, catheters, or endoscopes that also contain working micro components. These might have a particularly challenging geometry because they were originally designed as two or more parts. In order to avoid costly and time-consuming assembly under a microscope, device makers seek a molder who can both fabricate and assemble complex parts in micro conditions. That way, the time and cost for manufacturing their device will be reduced, and complex functions can exist in a very small device.
“Micro molding has been the gateway to numerous advancements in medical device design and innovation,” noted Aaron Johnson, vice president of marketing and customer strategy for Ankeny, Iowa-based Accumold, a manufacturer of precision micro-molded plastic parts and components. “Micro molding not only helps make things smaller, it also helps devices do more. When an OEM can design a housing, a connector, or complex component that can do more in the same space or less, the whole device is transformed.”
One such example of a complex component would be one including microelectronics. This can be challenging for molders because it involves transferring other traditional injection molding processes at the micro scale. In a difficult collusion of miniaturization trends and digital health endeavors, device manufacturers are demanding micro-sized lead frame and insert molding to accommodate microelectronics in their devices. The ability to overmold a micro part on a very small insert while maintaining consistency and accuracy is integral to manufacturing devices with embedded microelectronics.
“A hearing aid not only provides sound, it delivers high quality audio, connections to phones and TVs, and in some cases, provides bio or environmental feedback,” continued Johnson. “This is, in part, advanced because the micro-molded plastics provide it the correct complex structures. Micro molding allows device designers the opportunity to push the limits with their designs, thus bringing higher quality or more advanced devices to market.”
The manufacturing technique has proven to be both cost-effective and avoids wasting materials.
“Micro molding is worth its weight in gold when discussing small shot size, lower part cost, and minimal [effects] to material degrading, due to long cycle times and/or a long time in the barrel,” said Herbert.
When working with a molding provider, there are occasionally factors concerning specification for a component that device makers overlook. Some of these can have a significant impact on the cost of development or production lead times. OEMs should do their best to adopt a forward-thinking attitude in terms of their project, so there are no costly surprises along the way. Molders, therefore, should consider the long-term view early on in development—sometimes more so than the device maker.
“In the initial phases of device development, the typical process focuses on answering the question, ‘Can I even make this?’” stated Johnson. “While this is an important part of the process, it cannot be overlooked that long-term view should factor in early, especially when pushing the limits of your designs. Suppliers must have the initial capability but also the scalability and sustainability to provide the same high quality output throughout the whole life of the product. Don’t minimize the importance of this in the early stages. The cost of quality will always reveal itself along the way—be ready for it.”
A thorough understanding of the properties of materials being used to mold a component is very important as well. There is a vast array of materials with unique physical characteristics available for molding a part, but not all will be suitable for a medical device—for example, some molding materials may not work with a particular sterilization method. Additionally, during initial molding, material attributes and their interaction with a specific mold design should be addressed.
“Limitations of materials properties can lead to the incorrect selection based on application,” commented Williams. “Designers often select tolerances not applicable to injection molded parts; we often see prints that designate stamped or machined metal tolerances. These types of practices lead to increased validation cost and/or time, while often increasing piece price higher than expected.”
Design for manufacturability (DFM) urges a part’s design to facilitate the manufacturing process in order to reduce manufacturing costs. Using DFM practices allows potential problems to be tackled during the design phase, which is the least costly area of production to address the issues. Further, in adopting a DFM strategy, there are considerations unique to molding that other manufacturing methods don’t encounter. Some device makers may forget or lack experience in designing complex parts under a DFM strategy. Robust tooling is also important in these early phases, and it’s not always on an OEM’s mind.
“One factor I often see overlooked is designs to manufacture are often neglected, in the way of adding draft to allow a part to come out of the tool undamaged,” observed Herbert. Additionally, each injection mold design must have a gate—an opening that allows molten plastic to be injected into the cavity of the mold. Gate type, design, and location can impact the part in terms of part packing, gate removal or vestige, aesthetic appearance, and part dimensions. Gates vary in dimension based on the type of plastic being molded and the part’s size. Larger parts necessitate larger gates for a heavier flow of resin to shorten mold time. Small gates create a better appearance, but take longer to mold or might need higher pressure to fill correctly.
“Gate location is also often overlooked and not considered when the part is created in CAD,” Herbert continued. “Then, when it comes to tool design, there are few surfaces remaining in which a gate location is OK. Creative measures need to be taken, and in some situations, creates a tool design challenge.”
“During our design reviews, we make recommendations to our customers to create successful designs for micro molding from the start,” explained Clark. “Recommendations for micro part specifications to improve/enable manufacturability in micro include avoiding lopsided tolerances, remembering flash exists in injection molding (avoid “no flash” specifications), and allowing your molder to participate in the decision for gate location when possible.”
Finally, it is important to consider a component supplier as more than “just a molder” and integrate them entirely into the product development process. Experienced molders will do more than make the part; they will also provide valuable input into the design, ultimately making manufacturing more cost efficient. They will also guide OEM customers through the design phase with molded prototypes (often with rapid tooling) to bring about the shortest possible lead times and provide a speedy first impression of the part’s properties.
“It is surprising how many OEMs overlook the importance of using the same supplier during the new product development lifecycle,” said Randy Ahlm, CEO of NPI/Medical, a provider of complex, injection molded components and assemblies for medical device, life sciences, and healthcare customers. “The few suppliers that can do prototype to production in-house truly accelerate the new production introduction process and lower their overall costs due to a more seamless transition with fewer iterations. At the same time, they are also consolidating their customer’s supply base.”
A good molding vendor will also pay close attention to all regulatory requirements concerning process validation and quality assurance. Value-added services make the journey to a cost-efficient component much more feasible for a medical device OEM.
“We have had several customers come to us after missing launch milestones because their prototype supplier wasn’t able to meet their validation requirements and/or their parts needed significant redesign for production molding,” Ahlm explained.