Plastic injection molding offers numerous advantages and challenges.

Many people cringe at the mere thought of getting a shot or an injection while visiting a doctor's office or a hospital. Ironically, a less painful type of injection is giving medical device engineers something to shout about.

Manufacturers have been adopting new plastic injection-molding technology to boost throughput, improve quality and reduce production costs. It's having a big impact on the way cannulas, catheters, connectors, filters, inhalers, orthopedic devices, pumps, syringes, tubes, valves and other medical products are assembled.

Of course, injection molding is not new. It's a proven technology that has been successfully used to manufacture everything from automobile taillights to zoo garbage cans. However, because a wide variety of complex geometries and intricate designs can be easily molded, it is also an extremely attractive process for producing high-volume, one-time-use medical devices.

Plastic injection molding offers numerous advantages, such as:

  • The cost of plastic resin is low relative to other materials.
  • Through the use of multicavity molds, the unit cost for parts can be very low.
  • Injection molding is a very repeatable process, lending itself to tight tolerance specifications.
  • Plastic resins are available with many different properties to cover almost all applications used in the healthcare industry.
  • The injection-molding process yields parts that are very clean relative to other processes; often, no post-cleaning is necessary.

"Injection molding is becoming more popular for medical devices due to its cost effectiveness," says Rocky Morrison, operations director at the Upland, CA, assembly plant operated by Accellent Inc. (Collegeville, PA), a leading medical device contract manufacturer. "Injection molding excels in single-use disposable devices. This helps keep patient cross contamination lower than happens with reusable devices."

"In large enough volumes, molding is the only option," adds Mark White, sales manager for medical products at ATS Automation Tooling Systems Inc. (Cambridge, ON). "Injection molding gives you many more options for surface finish, color and contour, which in turn gives medical device manufacturers better differentiation."

Molding is often a more cost-effective manufacturing process than other alternatives, but it depends on the specific application. "There isn't an easy way to give ballpark estimates in 10 words or less in terms of mold cost," warns White. "It is best to take each component individually to determine if it is cost effective to use injection molding vs. other more expensive processes."

How It Works

Injection molding is a process in which a polymer is heated to a highly plastic state and forced to flow under high pressure into a mold cavity, where it solidifies. High pressure is used to obtain fast filling speeds and to prevent the mold from being overfilled. Typically, injection molding is used to form thermoplastics. But, some elastomers and thermosets can also be molded.

"The mold capabilities, broad availability of materials and extreme controllability are the keys to the success of this process," says Jordan Rothheiser, president of Rotheiser Design Inc. (Highland Park, IL). "Moldmakers have succeeded in constructing incredibly complex molds with side cores coming in from virtually any angle conceivable.

"Collapsing cores are no longer a rarity and with special equipment, even thermosets can be injection molded," adds Rotheiser, author of Joining of Plastics (Hanser Gardner Publications, Cincinnati). "Injection molding is capable of undercuts, side-cored holes, controlled wall thicknesses, extraordinary detail and the tightest molded-in tolerances. Thus press fits and snap fits are readily accomplished."

However, self-tapping and thread-forming screws can also be used with molded parts. Male and female threads can be molded in. In addition, adhesives and plastic welding processes, such as ultrasonics, can be used to join injection-molded parts.

An injection-molding machine consists of two basic components: a plastic injection unit and a mold-clamping unit. The injection unit acts like an extruder. It consists of a barrel that is fed from one end by a hopper containing a supply of plastic pellets.

The clamping unit holds the two halves of the mold in correct alignment and keeps the mold closed during injection by applying a clamping force sufficient to resist the injection force. It typically consists of a fixed platen and a movable platen, which are attached to tie bars.

The injection molding process begins when resin is fed through the hopper into a heated chamber. A rotating screw moves the resin toward a nozzle. The entire screw moves forward, pushing a shot of plastic into a mold under pressure. A backflow check valve prevents the material from pulling back into the chamber when the screw is retracted.

Charles Land, president or Morgan Industries Inc. (Long Beach, CA), says injection molding can be reduced to four simple individual steps:

  • Plasticizing-the conversion of the polymer material from its normal hard, granular form at room temperature to the liquid consistency necessary for injection at its correct melt temperature.
  • Injection-the stage during which the melt is introduced into a mold to completely fill a cavity or cavities.
  • Chilling-the action of removing heat from the melt to convert it from a liquid consistency back to its original rigid state. As the material cools, it also shrinks. This stage can consume 70 percent to 80 percent of the total cycle time; it is largely dependent upon the wall thickness of the part.
  • Ejection-the removal of the cooled, molded part from the mold cavity and from any cores or inserts. As the mold opens, ejector bars make contact with the ejection mechanism and push the part out of the mold.

"Each of those steps is distinct form the others," says Land. "Correct control of each is essential to the success of the total process."

Traditionally, injection-molding machines operate with hydraulic pistons. But, that's starting to change.

"The current evolution is to all-electric or hybrid injection-molding machines," says Accellent's Morrison. "The all-electric machines use less energy and generally are more repeatable than hydraulic machines."

Electric injection molding equipment offers significant cost savings. For instance, they can reduce energy costs by up to 50 percent. A reduced number of parts and a modular design contribute to the savings. The injection units are typically driven by a belt drive and a servomotor via a ballscrew. This allows high shot-weight consistency and precise positioning. In addition, electric machines operate without oil, making them ideal for medical device applications that typically require clean-room environments.

"All-electric injection molding machines open the door to a broad range of applications," says Bob Strickley, marketing director at Milacron Inc. (Batavia, OH). Electric machines can improve productivity by as much as 30 percent, and make it easier to mold small parts, such as medical devices.

Gambro AB (Stockholm, Sweden) is using electric injection molding equipment to mass-produce a small kidney dialysis filter with a shot weight of 1.2 grams. "Up to now, shot weight below 2 grams with piston machines was considered difficult, if not impossible," Strickley points out. "The mechanical stability necessary for the screw prevented any reduction in geometry, and the axial movement during injection necessitated higher precision with small shot weights than with larger ones."

In-Mold Assembly

One of the biggest trends in plastic injection molding is a new process called "in-mold assembly." The process is also referred to as overmolding, multishot, multimolding and two-shot molding. With in-mold assembly, parts are molded and assembled in the mold. Assembly is done after the two parts are molded, which improves repeatability.

"The value-added application is done in the mold," says Strickley, "not after the molding and ejecting of the parts. Sexy applications have two materials with different melt variances and performance variances. In these cases, two parts can be molded into one another for a uniform snap fit-or even a cavity and bearing surface generation."

Ultrasonic welding of individual parts formed beforehand by injection molding is a widely used process for the production of hollow parts with functional interiors. "However, the disadvantage of employing two processes-each requiring their own monitoring-is often compounded by quality issues, especially when multi-cavity molds are used," Strickley points out. "The assembly elements usually come from different cavities and may present slight form variations, leading to high scrap rates during the welding process."

With in-mold assembly, the parts, as well as the joining, are completed within one cycle. The parts are joined together while they are still held by the platen. The combination of these two traditionally separate processes can reduce production cost, while improving quality and productivity.

"In-mold assembly is any of a number of processes that are done using injection-molding technology to eliminate post-molding assembly steps," says Len Czuba, president of Czuba Enterprises Inc. (Lombard, IL), a product design firm that specializes in plastic medical devices.

Examples include:

  • In-mold labeling. This process is widely used for bottles in consumer and household care products.
  • Two-shot overmolding. This process is used to form shock-absorbing corners and elastomer rims on housings for a variety of products, such as hospital pumps, portable computers, remote control devices, PDAs, travel mugs and pens.

"Another in-mold assembly example also uses two-shot molding as its process," says Czuba. "This may include producing a movable set of parts attached by overmolding or two-shot molding." This molding process is commonly used in the toy industry to assemble dolls or action figures that have moving arms or legs.

"Using in-mold assembly, the two-shot molding process can make an action figure with movable arms and legs, but not require separate assembly operations," adds Czuba. "The overmolded part is selected in a way that prevents sticking or fusion of the surfaces. This gives a movable arm or leg."

According to Czuba, a similar two-shot overmolding technique can be used to produce a part that otherwise would require assembling two separate pieces. An example is a syringe plunger with plunger tip. "Here, in-mold assembly can produce an even better part, perhaps having the plunger tip actually adhered to the plunger," explains Czuba. "This would not only eliminate a secondary assembly step, but also produce a better part where the tip is not removable once overmolded."

Foboha GmbH (Haslach, Germany) recently unveiled a revolutionary double turning-cube stack-mold system. It features three "parting lines," allowing a single injection machine to mold two precision parts, from two different materials, label and assemble the finished product. Assembly operations occur during the injection process when the mold is closed, resulting in considerable cycle time savings.

A demonstration system, built for a leading producer of smokeless tobacco, features 16 cavities on each face of two cubes, set between the platens of a Ferromatik Milacron K-TEC 250 two-component injection machine. The finished part serves as a tiny, lidded spittoon that's built into the tobacco can lid. The can lid is injected with red polypropylene homopolymer on one face of cube A at the stationary end of the injection machine, while a smaller black polypropylene lid is injected and in-mold labeled on the B-cube face that's opposite the moving platen.

As the cubes index 90 degrees with each cycle of the machine, the finished parts on each cube face are snap-fit together at the center parting line, with the finished assembly removed by a robot on the next turn of the cube. Cycle time in the demonstration is approximately 6.5 seconds.

The basic machine can be combined with up to six injection units of different size for large-shot preforms and correspondingly smaller shots for secondary colors or materials. At the free station of the first cube, labels or metal parts for in-mold assembly can be inserted into the mold by means of a feeding mechanism. A special drive and coordination mechanism developed by Foboha engineers controls the opening and closing of both cubes.

"This is a potent technology for adding value to a part while it's still between the tie bars," says Bob Hare, U.S. general manager of Ferromatik Milacron. "The double turning-stack system can eliminate a second injection machine; all of its auxiliary equipment; a labeling system; an assembly machine; the energy cost to run all this; the extra labor; the maintenance cost; the controls and interfaces; the work-in-process inventory; and the inconsistency and scrap that result from secondary processes.

"Output per square meter is higher; output per kilowatt is higher; and output per unit of capital cost is greater than any comparable technology for producing this part," claims Hare.

Design Challenges

There are many challenges to molding plastic parts. For instance, manufacturing engineers must ensure that tolerance stack-ups for mating parts do not pose fitment interferences. "Another big challenge is maintaining the required degree of cleanliness found in molding all the way through assembly," says Rob Ecob, tooling manager at ATS Precision Plastic Components (Bowmanville, ON).

Numerous design considerations must be considered when using plastic injection molding technology. "Wall thicknesses, draft, venting, knit-flow lines, and the potential for voids, sinks, gassing or surface defects, [in addition to] shrink and warp," must all be taken into consideration, Ecob points out.

Wall thickness is extremely important, because of its effect on cycle time. Too thick a wall, even in just a small portion of the part, can lengthen cooling time and increase cost. Reduced wall thickness requires lower shot weight and shorter cooling times.

"The three biggest design errors we see are lack of uniform wall thickness, not enough consideration to gating, and not enough consideration to ejection from the mold," notes Accellent's Morrison. "There are many good design guidelines [that engineers] can use. We recommend bringing in plastic experts early in the design process."

When choosing a plastic for injection-molding applications, Morrison says it's important to understand the part function, operating environment, resin properties and resin stability. "Pick the one with the most positives and least negatives," he suggests. "Resins can have large differences in properties. We often see resins selected on the basis of one ‘shining' property without due consideration to the negative aspects which sometimes outweigh that. Look for a balance in requirements and work with the experts early in the design process."

Mold design should also account for how a part will be assembled. For instance, many injection-molded parts have orientation problems in bowl feeders. "The single biggest challenge is figuring out how to feed molded parts into the assembly line," says Nils Hammerich, managing director of Mikron Corp. Rochester (Rochester, NY). "Gripping, holding and placing parts can also cause many headaches.

"Production engineers must understand the unique capabilities of molded parts," adds Hammerich. "Dimensional tolerances, tensile strength and temperature can be extremely complex. They determine how much shrinkage will occur. Of course, you must also avoid overengineering. This can add extra time and expense."

Unfortunately, many manufacturing engineers fail to understand the rheology and constraints of the plastic injection-molding processes. "They tend to view injection-molded parts as the same as machined or die-cast parts," notes Ecob. He says it's important to consider factors such as rigidity, accuracy, repeatability and speed of operation in each stage of a cycle.

Often, manufacturing engineers have an incomplete understanding of the entire injection-molding process and how all the design choices interrelate. "We often see efforts to save money applied in the wrong place, such as the mold," says Morrison. "Attempts to reduce mold costs usually end in failure. Molds are expensive and complex. A thorough understanding of the trade-offs in mold design and construction are needed in any attempt to save cost."

Beyond Plastic Injection Molding

Injection molding is a mature manufacturing technology that is the most widely used plastics processing method. But, manufacturing engineers continue to push the envelope by developing new processes, such as micromolding and metal injection molding.

Micromolding allows manufacturers to produce tiny plastic components for use in medical devices. Micro parts typically have tolerances of 0.0001 to 0.0002 inch. Their geometry can only be seen by microscope. Micro parts feature wall thicknesses as low as 0.0015 inch and overall part weight as low as 0.00012 gram.

Micromolded parts are used in catheters, resorbable implants, stents and other medical devices. Emerging technologies, such as microelectromechanical systems, microfluidics and nanotechnology are expected to spur a huge demand for micromolding equipment.

Metal injection molding is a process similar to plastic injection molding and high-pressure diecasting. It can produce many of the same shapes and configurations. Metal powder is mixed with a thermoplastic binder and molded into a cavity. The parts are initially about 15 percent larger than the finished object.

With metal injection molding, a wide variety of flexible designs, component integration and high-performance characteristics are possible. The technology is typically used to produce implants and surgical instruments, which traditionally must be made from difficult-to-machine materials, such as cobalt-chromium alloy, stainless steel and titanium.