The injection molding industry grew very quickly during World War II in the 1940s due to the incredible demand for cheap, mass-produced products. In 1946, another American inventor, James Watson Hendry, built a screw injection machine. While the Hyatts’ machine worked like a large hypodermic needle, using a plunger to inject plastic through a heated cylinder and into a mold, Hendry’s new and improved version used a screwing motion which allowed for more precise control over the speed of injection and the quality of articles produced. This machine also allowed material to be mixed before injection, so that colored or recycled plastic could be added to virgin material and mixed thoroughly before being injected. In the 1970s, Hendry went on to develop the first gas-assisted injection molding process, which permitted the production of complex, hollow articles that cooled quickly. This greatly improved design flexibility as well as the strength and finish of manufactured parts while reducing production time, cost, weight and waste.
Today, Hendry’s screw mechanism lives on. Screw injection machines account for the vast majority of all injection machines.
“Injection molding processes begin by loading raw plastic resin, usually in pellet form, into a loading bin or ‘hopper,’” explained Jared Sunday, director of engineering for Latrobe, Pa.-based Classic Industries Inc., a plastic injection molding company for medical devices. “This is then gravity-fed into a long heated barrel containing a reciprocating screw. The heated barrel, combined with the friction forces of the rotating screw, transform the raw plastic resin into an homogeneous melt. As the screw transitions back, it builds a reservoir of the melt in the front zone of the barrel. Under controlled high pressure and speed, the screw is propelled forward pushing the melt into the mold. Production-grade injection molds typically are two or more halves and made out of high strength steel. However, they can be intended for shorter runs or prototyping by being constructed out of softer steels, aluminum, or even types of plastics. The inside of the mold contains the reverse geometry of the intended shape of the component being manufactured. As the melt is pushed into this mold, it flows into the internal open areas of the closed mold. The melt is cooled through a controlled process until it has solidified. The mold then opens and the cooled component is ejected or robotically removed.”
Today’s machines are capable of very tight tolerances and repeatability. Because of its ability to produce large volumes, injection molding also remains one of the most cost effective methods of manufacturing. Tight tolerances, repeatability and cost efficiency form the holy trinity of advantages injection molding offers medical device OEM customers.
“‘Tight tolerance’ is a term that is often tossed around loosely in the [manufacturing] industry,” wrote Ken Glassen, vice president of engineering for plastic injection molding company Kaysun Corporation, in a Manufacturing.net article last year. “However, if tight tolerance is not done correctly, parts and products will underperform or possibly fail, resulting in customer dissatisfaction and a tooling and/or process overhaul. Other tight-tolerance benefits may include the elimination of secondary operations like machining, making it easier to procure mating parts, and allowing the possible conversion of metal parts to plastic—all of which help reduce costs.”
“Tight tolerance” simply means the ability to produce components with very minimal variation. According to Glassen, injection molding typically operates with a tolerance of +/- 0.002 inches. A very tight tolerance is +/- 0.001 inches. Part design and complexity, material, tooling, and process design and control play a part in the level of tolerance. Compare this with additive manufacturing (Also known as 3-D printing or rapid prototyping), in which parts are created by the laying down of thin layers of material bit by bit. According to Quickparts Inc., a manufacturing services company that offers both additive manufacturing and injection molding, the standard tolerances for most additive manufacturing processes start at +/- 0.005 inches and compound from there as the design increases in size. This is an important consideration for OEM customers to remember when the success of a project depends on how well different components assemble with one another.
Additive manufacturing as a manufacturing process is expanding very rapidly due to patents expiring on various 3-D printing processes and techniques. The process offers great advantages in terms of cost when only a small number of devices or components are required. It is also a standard process used to make prototype components in the design and development stage of a medical device, as it is fast and cheap.
“Additive manufacturing is brilliant for product development in the early stage—coming up with a design, having it printed to see if it works from a form and function standpoint,” Ryan Case, director of sales for Orchard Park, N.Y.-based medical plastic injection molding company Polymer Conversions Inc., told Medical Product Outsourcing. “It really brings down the cycle times of development so you can get closer to your device faster.”
However, many industry experts agree that once a device enters into large volume production, injection molding might still be the best choice. Additive manufacturing is a much younger process (about 30 years old) and is evolving rapidly, but has not yet reached the point where it can produce as durable components in as large volumes as injection molding can.
Corroborating Glassen’s position, Classic’s Sunday told MPO that tight tolerances to +/-0.001 inches are “not uncommon” in injection molding. “This can be accomplished using a wide variety of off-the-shelf and custom plastic materials and colors,” he said. “Injection molded parts can be very soft and pliable to the touch, or they can be almost as strong and robust as steel.”
The ability of injection molding to produce extremely strong components with tight tolerances has facilitated the move some OEMs have been making from metals to plastics. According to Mark Fuhrman, director of sales and marketing for C&J Industries, a plastic injection molding company based in Meadville, Pa., the company is seeing many customers come to it seeking to move away from metals to plastics for the same devices. C&J uses PEEK (polyetheretherketone), which besides being very strong, is also reusable, can be autoclaved, is stable, and has limited shrink characteristics. Even though PEEK is an expensive polymer, injection molding a plastic still has proven more cost effective than machining metal for certain devices.
“The secondary operations often required for metal parts come along for free in plastic injection molding,” Fuhrman explained. “A metal part may require that it have multiple operations for machining, adding treads, adding undercut, etc. There are usually multiple setups for a machined part, and then the part has to get a secondary treatment whether it’s anodizing or painting or whatever. There are some secondary processes that need to take place to make that metal part look attractive. These processes are inherent to plastic injection molding and come along at no cost.”
Indeed, one of the main limitations of 3-D printing is the process’ inability to provide a polished finish on printed metal devices. Direct metal laser sintering, a 3-D printing process for metals, produces a grainy finish, necessitating the device to be printed a hair larger than required so it can be polished down to a mirror-like finish. If metal is forgone completely, this ceases to be an issue, as molded plastic parts come out of the mold ready without the need for further surface treatment. But is it possible to simply replace metal with plastic for most devices? Fuhrman told MPO about a client that came to C&J with a very robust, stainless steel part, wanting to convert to polymer to save money:
“We have a medical device that is a very high impact part that goes onto a medical/surgical tool made for machining the femur bone. It has to be very stout and strong,” Fuhrman said. “The customer converted from stainless steel to plastic injection molding, and it gave them some additional features that cannot practically be created in steel without welding and machining—the process just didn’t lend itself at all to a device that would be attractive and cost effective. So converting to plastic injection mold was quite a cost savings to them due to the elimination of those secondary processes.”
“Material choices are much more widely available currently for injection molding, but this is quickly changing as technology for additive manufacturing is growing by leaps and bounds,” said Classic’s Sunday. “Additive manufacturing is also able to produce components using plastic, ceramics, or metals. Although many choices of general materials are available, specifically tuned custom grade choices are limited. If the intended product is a tube connector, filter, syringe, or standard device, injection molding would be an intelligent choice. But if you are creating a brace, cast, or implantable custom fit for a single person, additive manufacturing will meet your need.”
Inject, Mold, Repeat
Repeatability is the key attractive feature of injection molding. The nature of the process inherently lends itself to the ability to produce the same exact device over and over and over again with very tight tolerances and exactness. This process is only cost effective, however, if the device is being made in the thousands. For smaller runs, 3-D printing makes more sense, because unlike for injection molding, a new tool (mold) does not have to be created every time a new device is made. Usually, the same printer can be used.
“The primary advantage of any molding process is replication, which is the ability to make literally millions of identical products conforming to the shape and size of the mold,” said Thomas Hicks, director of product development for Charlton, Mass.-based MTD Micro Molding. “Replication is the foundation of manufacturing whereby interchangeability is the key to mass production. However, not all applications require mass production, and some implants are examples. They may need only a very limited number of units. The time-consuming efforts and costs associated with creating a permanent mold capable of manufacturing do not lend themselves to these limited needs. Similarly, some steps in the development of high production components also do not lend themselves to hard tooling, with the main example being quick prototyping which often includes a succession of iterative design changes. On the other hand, the main advantages of additive processes are the speed to create physical parts and the ability to do so at low costs. The disadvantages compared to molding are often the necessary compromise required in terms of part design (resolution of feature geometry, clarity of clear materials, smooth surface finishes, micro-close dimensional tolerances, and often any semblance of the desired feature at all) as well as material selection.”
And repeatability translates to volume, which translates to cost savings.
“The cost effectiveness of molding can depend on the volume,” Polymer Conversions’ Case explained. “Where additive manufacturing is great is when you’re doing very small runs in the tens or possibly hundreds. When you start scaling up into the multiple thousands, you can make multi-cavitation tooling where you can make more than one part at a time. Every time you double the amount of parts you make at a time, you take approximately 25 percent of costs out of the part.”
Repeatability is an essential consideration for medical devices, because sterility and precision is of utmost importance. For this reason, process validation and controls are important parts of the repeatability of injection molding. The Injection Molding Handbook (Rosato et al, 2000) notes several factors that affect repeatability, including fill time of the flowable material into the mold and screw design. Consistent melt quality is an important factor in ensuring shot-to-shot repeatability, and for this reason, the very geometry of the screw that injects the flowable material into the mold must be specifically matched to the polymer being used. This is but a tiny insight into the many details that need to be considered in order to achieve exactness and good repeatability.
“Its all about control,” said Case. “Do you have the controls in place to ensure you are delivering a product that not only from a manufacturing standpoint is repeatable, but also controls within the broader manufacturing environment to make sure you have the right resin going into the component every time; and that the operators handle the parts the exact same way every time; and that you have the rigor and culture involved that can handle the repeatability of the process, not only itself in terms of the molding process, but also in terms of getting material through the press and handling the products, etc.”
These mechanical details are what cause injection molding to be relatively expensive for prototyping or small-run productions. Creating an appropriate mold and calibrating the machine for a component is not worth it if only a few hundred devices are needed. Such a run would be much more appropriate on a 3-D printed machine that can be calibrated via software rather than hardware.
“The need for tooling for the molding process is perhaps the largest drawback in comparison to additive technologies,” added Mike Borst, global program manager for SMC Ltd., a contract manufacturer for medical devices headquartered in Somerset, Wis.
“Increased time and money, comparatively, is required to produce parts with molding. However, when these parts are produced, oftentimes thousands or tens of thousands of parts are possible. Knowing where you are in the development process and the need for ‘test’ parts will dictate which technology to use.”
Exactly how small can a molded part get? Well, not as small as a micron, Fuhrman said. But at C&J, components can be molded small enough to fit into Roosevelt’s ear on a dime piece. But a micro-molded part does not necessarily have to mean a small part. Parts that are inches large can benefit from micro-molding on a thin wall section that is 0.004 inches thick, for instance, or a 0.008 inch hole. C&J manufactures a suture ring for eye surgery, which, as can be imagined, is tiny.
“Micro-molding is a specialized subset of injection molding, and while in general theory it is described the same way, the finished parts are now so much smaller (some the size of a grain of rice, or smaller) and the dimensions are naturally also correspondingly much smaller,” said MTD’s Hicks. “This size shift demands a special understanding of the behavior of plastic materials as they behave in these smaller dimensions, and it requires specialized techniques for constructing the molds, as well as handling, measuring, and packaging the finished products.
Hicks speaks to a common misunderstanding of what micro-molding actually means. The process is not about crafting a particularly tiny mold. According to another expert in micro-molding, Ankeny, Iowa-based Accu-Mold LLC, a micro-molder is not classified by hardware alone. The combination of innovation, processing, and expert tool-building together make up the elements of true micro-molding. It takes years of experience to master the process, and a good press is only as good as the technology that comes before it.
And what about additive manufacturing? This process has become very advanced in its short lifetime. Last year, a research team from Harvard University and the University of Illinois at Urbana-Champaign 3-D printed a tiny lithium battery approximately the size of a grain of sand (less than 2000 microns). To make the microbatteries, the team printed precisely interlaced stacks of tiny battery electrodes, each less than the width of a human hair. Such a battery, of course, makes it possible for use in equally as tiny medical devices. Researchers are even printing live tissue, with a team at the University of Connecticut printing artificial kidneys this past May and at the University of Oxford creating a printer capable of printing synthetic skin tissue.
As exciting as these innovations are, 3-D printing cannot match molding in strength and durability. While molding may not be able to create delicate tissues, it certainly can produce robust parts.
“With respect to micro-sized parts, usually the only process capable of producing the part is through micro-molding,” Hicks said. “As compromises are introduced and the design changes, the material changes. As the part becomes less and less a true ‘micropart,’ the created object might bear a similarity to the original design, but it will not be a true prototype, and the likelihood that it will test and perform like the desired part is very slight. The part becomes more of ‘model’ than a ‘prototype’ and even less like the original desired part.”
Case likens the whole process to boiling spaghetti—and the metaphor makes everything a little clearer.
“One of the main advantages of molding over additive—beyond the tolerance—is also the strength of the component,” Case said. “In additive manufacturing you’re building it layer by layer whereas in molding you’re making it all at one time. The polymer itself is made up of a chain of molecules. Think of it like spaghetti: If you boil a whole bunch of spaghetti, dump it on a plate and let it cool, all the spaghetti strands are entangled together and they form one part, and this is like molding; whereas if you lay the spaghetti on top of each other, it easier to break apart—this is like additive manufacturing.”
So while additive manufacturing has the capability to add thin layers and make a delicate component, its “strength,” so to speak, lies elsewhere.
From Prototype to Final Product
While additive manufacturing has proven ideal for prototyping—hence the moniker “rapid prototyping”—C&J’s engineering manager Mike Yurkewicz warned that device designers should remain very aware of the implications of moving the production of a device from additive manufacturing to injection molding. What works in one process may not work in the other. And, as Hicks noted to MPO, “The closer a component design is to final, the more attractive molding becomes as a manufacturing option.”
“Design for manufacturability,” Yurkewicz said. “A product designer can create something with a 3-D additive process that may not be moldable. So once you get beyond the initial quantity needed just for your 510(k) application or studies and so on, and then transfer that process to injection molding, design characteristics such as a certain wall thickness can create tolerance issues. If your volumes ever justify making the jump into injection molding, you could be painting yourself into a corner if you design in such a way in your rapid prototyping stage.”
Molding is manufacturing’s workhorse. The process can create volume like none other, and it’s a reliable, repeatable process. While molders laud the cost efficiency of their process, and 3-D printers do the same for their own process, cost effectiveness really comes down to volume.