Originally Published MDDI November 2001
New materials and equipment improve the molding process and the parts it produces.
William Leventon
A healthy dose of innovation has been injected into plastic molding. The changes have been a boon for medical device OEMs, according to injection molders, bringing more design options, better parts, and more-consistent product batches.
In the injection molding industry these days, the operative word is new—new materials are flowing into new molding machines, which are producing new kinds of parts. New processes are being used, and these processes are monitored by new types of equipment.
Going back some years, medical device manufacturers were often limited to materials developed for the automotive and consumer product markets, according to Mike DeAngelo, manager of injection molding for the Burron OEM Division of B. Braun Medical Inc. (Bethlehem, PA). But more recently, resin manufacturers have been formulating materials specifically for the medical market.
"I think medical [technology] is driving a lot of the new material innovations," says Kelly Stichter, opportunity development manager for Phillips Plastics Corp. (Hudson, WI).
USEFUL PROPERTIES
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| Products molded for consumers must reflect aesthetic and ergonomic considerations. |
The new materials offer injection molders a variety of useful properties. For implantable devices, Inland Technologies Inc. (Fontana, CA) uses Bionate, a biodegradable material from The Polymer Technology Group Inc. (Berkeley, CA). And at Unimark Plastics Co. (Greenville, SC), molders are using a clear plastic to manufacture lens carriers. This new plastic allows customers to check the quality of lenses after they've been placed in the carriers.
For improved chemical resistance, Burron and Bayer Corp. (Pittsburgh) have developed a lipid-resistant polycarbonate material that helps injection-molded medical devices stand up to aggressive chemicals such as those in new chemotherapy drugs. Other materials are designed to handle the high temperatures that medical devices encounter during autoclave sterilization, which is regaining popularity. High-temperature options include PEEK, Radel, Udel, and Santoprene 8000.
To add strength to medical parts, molders are turning to liquid-crystal polymers such as Questra, a strong structural material manufactured by Dow Plastics (Midland, MI). "If you drop a part molded out of Questra on your desk, it sounds like a metal part," says Stichter. The rigid, stable material is particularly good for mechanisms, she adds.
Manufacturers are also boosting the strength of molded parts by using materials that contain special fillers. Glass-filled resins, for example, "make a pretty rigid structure," according to Tilak Shah, president of Polyzen Inc. (Cary, NC). Glass-filled plastics are used to make laparoscopes and other scope-related products.
Fill materials offer more than strength. For example, devices made using radiopaque fill material aren't transparent to radiation. During radiation therapy, Shah notes, this fill material shields the body from radioactive sources.
Few things worry medical device manufacturers more than the possibility of contamination. So Battelle Memorial Institute (Columbus, OH) is working on inert substances to take the place of potentially harmful chemicals that can leach out of polyvinyl chloride (PVC). By replacing these chemicals with soy-based additives, Battelle hopes to minimize the chances that PVC will adversely affect drug purity or potency, explains Kelly Jenkins, program manager of Battelle's polymer center.
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| The development of molding materials still lags behind that of molding machines, which are more efficient and more precise than ever before. |
Unfortunately, the special properties of these materials usually don't come cheap. To keep costs down, many of Unimark's customers are actually shying away from engineering plastics. A few years ago, these manufacturers were making medical devices out of what Joe Pack, Unimark's vice president of sales and marketing, calls "strange materials." This was done mainly for marketing reasons and to differentiate a product from its competitors. But those reasons are no longer sufficient to justify the extra cost of special materials. Customers "have to have a fit-and-function reason for using those materials, or they won't do it," Pack says.
For many OEMs, he adds, the choice of materials is now driven more by price than by function. To get a price break, large medical device companies are making their own deals with material manufacturers. Engineers at these companies try to use only the discounted materials when designing new products.
Manufacturers are also being more careful about using plastic additives. "In the past, you might use a plastic with five different additives in it," Pack says. "Now you don't use the additives unless you have to have them."
For cost-conscious manufacturers, there's at least one additive on the market that's actually designed to reduce costs. This additive, known as MuCell, produces air bubbles in plastic resins, Stichler notes, thereby reducing the weight of finished parts.
PARTS ON A DIET
Material costs are being reduced in other ways as well. "We're asking plastic parts to do more than they did 10 years ago," says Jenkins. "Everything's being reduced by weight and by wall thickness."
Today, Jenkins notes, molders are getting down to wall thicknesses of about 0.02 in.—half as thick as the walls of conventional injection-molded parts. Molding these thin-wall parts requires both new materials and new processing methods.
As their name suggests, high-flow materials flow more easily than the conventional materials from which they're derived. According to Jenkins, high-flow materials include a flow agent that boosts their melt-flow rate to 20–30 g/10 min.
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| Molded devices have played a large role in the advancement of minimally invasive surgery. |
High-flow materials hold up well to the extraordinary pressures generated by thin-wall injection molding. During thin-wall processes, Jenkins says, pressures can reach 40,000 psi, compared with pressures of 10,000–20,000 psi typical of conventional injection molding. Such high pressures can produce tremendous shear forces that damage plastics. But because they flow more easily than ordinary plastics, high-flow materials are subjected to less shear force, which reduces processing damage and helps the materials maintain their ultimate strength.
As material flows, it's cooled by contact with the chilled walls of the mold. Eventually, the cooling process brings the material down to its transition temperature, at which point it stops flowing and solidifies. This happens more quickly in thin-wall molding because the thin mass of flowing material doesn't maintain its heat as well as a thicker mass.
As a result, Battelle and others are working on methods to increase the "flow length" of thin-wall molding materials. In one scheme, directed energy is used to heat the flowing plastic, which extends the time it remains in a liquid state. Another technique features a special high-flow nozzle that increases flow length by ensuring random orientation of injected material.
Molding materials may have come a long way in recent years, but their development still lags behind that of molding machines, contends Gary Hengeveld, vice president of Inland Technologies. In the next three to five years, however, Hengeveld expects plastics manufacturers to gain greater control over the properties of their materials. To-day, for example, a molding material might have a melt-flow index of 10 to 15. "That means that one lot might push real hard and another lot might push real easy," Hengeveld explains. "So your molding machine has to adjust to that."
Hengeveld believes manufacturers will dramatically reduce the spreads of melt-flow indexes in the coming years. Instead of 10 to 15, the spread of a future material may be only 14 to 15. The result: fewer machine adjustments and molded parts with more-consistent dimensions from one lot to the next.
EQUIPMENT AND PROCESSES
Of course, material developments are only half the injection molding story. The other half deals with injection molding equipment and processes, and the drive to make them more efficient, more precise, and more capable than ever before.
One process that's becoming increasingly popular is two-shot overmolding. This process consists of two molding operations that combine to make a single part. First, the basic plastic part is formed. Then, before the part comes out of the press, a second shot overmolds another material onto the part.
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| Molding accuracy enables manufacturers to design and develop products of great complexity, despite small size or intricate components. |
Manufacturers often use two-shot overmolding to add a gripping surface to medical devices. "You get rigidity from the first material, and on top of that you overlay another material that gives the product a softer feel," Shah says. Other second-shot additions include gaskets, bumpers, and materials of different colors.
"Customers who used to get a four-piece part now want a one-piece part in two colors," Pack says. In addition, Unimark uses two-shot overmolding to make syringes with a functional layer and a sterilization layer.
As medical products get smaller, so do the injection molding machines that make them. "We're seeing a downsizing of injection molding equipment," Jenkins says. "Part sizes are driving down machine sizes."
At Battelle, Jenkins and his colleagues have a custom-built machine they refer to as a "desktop" injection molder. Measuring about 2½ ft long and 1 ft wide, this fully functional injection molding machine includes scaled-down motors, drives, and all the other components found in larger machines of its kind.
But in the realm of the small, there's much more to successful injection molding than making tiny machine components. "When shot size is in the milligram range, it doesn't take much screw travel to inject your shot," Jenkins notes. "How do you do that with a great deal of accuracy?" Process accuracy becomes ever more challenging as the size of the molded part shrinks. "If you're dealing with micron-sized features, you don't have to be off much to be out of spec."
Not surprisingly, machines that can do the job aren't easy to build. "In the past, we used off-the-shelf components to make these machines," Jenkins says. "Now they're designed and engineered from the ground up."
Whether they're large or small, old hydraulic molding machines are being replaced by all-electric machines. Electric machines eliminate the danger of oil contamination posed by their hydraulic counterparts, DeAngelo says, which makes them ideal for cleanroom applications. Moreover, he adds, electric machines are as much as 85% more energy efficient than hydraulic machines. And they allow true computer control, which greatly improves shot-to-shot repeatability and accuracy. "We want our products to be the same from the first unit to the 10 millionth unit we make," DeAngelo says. "The electric machines help us reach that goal."
Now, all-electric technology can also control mold tooling. Mounted to the tooling, ac servodrive motors are taking over functions that are usually handled by hydraulic equipment. Besides eliminating the danger of oil contamination, electric tooling equipment is more accurate than a hydraulic cylinder, DeAngelo notes. "With an ac servodrive, you can set [the tool] to move 2.874 inches, and it will move exactly 2.874 inches," he says. "You can't get that type of control from a hydraulic cylinder and fluid. With hydraulics, there's always some slack and some play."
AUTOMATING FOR SUCCESS
DeAngelo says manufacturers need precision accuracy if they want to automate their molding operation. No longer a luxury, automation has become crucial to the success of American injection molders, according to Allan Johnson, manager of sales and marketing for Scientific Molding Corp. (Somerset, WI).
"Because of foreign competition, U.S. molders have become much more automated during the last several years," Johnson says. "And the rate of automation continues to increase. At this point, I doubt if there are any molders that are serious about being competitive that don't have robots and pickers on every press."
These machines can slash the amount molders spend on labor, which accounts for about 30% of the cost of a typical part, according to Bill Pittman, vice president of OEM services for DeRoyal Plastics Group (Powell, TN). In addition, automation dramatically improves the repeatability of the molding process. For instance, Johnson says, "if you're molding metal inserts into parts, those metal inserts are put into the tool exactly the same way each time, which means they'll be in exactly the same location in the plastic part each time."
What's more, the timing of each operation is almost exactly the same every time. "You have repeatability from one part to the next, down to minute fractions of a second," Johnson says. If you depend on a human operator, a task might take 7 seconds one time and 17 seconds the next. In that extra 10 seconds, mold and material temperatures can change. "So you have a potential for changes in part dimensions, part quality, and even the surfaces or aesthetics of the part. But with a robot, those parts are identical time after time."
As manufacturing becomes more automated, inspection and process control are following suit. "A lot of customers are telling us: 'You're required to have your process in control,'" Pack says. "So tools that help us do that are becoming a big deal."
Automated process-monitoring equipment includes computers, sensors, machine vision systems, and coordinate-measuring machines. According to Pack, most molding presses come with a local process-monitoring system. Molders can also purchase off-the-shelf systems that monitor process variables such as time, temperature, pressure, and position.
Process monitoring "is a big selling point for us," says Pittman, whose company uses an off-the-shelf system that keeps tabs on mold temperatures, water pressure, injection pressure, and cycle times. The data are collected and stored so the company has a detailed history of every part it makes. "If we make a bad part, we can go back and look at our process to see why the part was bad," Pittman says.
Monitoring systems also enable molders to analyze process and part-dimension data in real time. By contrast, Johnson says, it might take a human inspector half an hour to go through all the manufacturing data. And even then, the inspector might not be able to interpret the data without calling in someone else.
If a process variable moves out of its preset range, an automated monitoring system can set off an alarm or shut the press down until a technician looks at it. The molding operation can also be set up so that when a process variable moves out of range, the parts made during that time are separated from the rest of the batch.
The process deviation "might not have been enough to push the parts out of spec, so I might have thrown away good parts," Pack says. "But I'd rather get them out of the stream so I'm sure I'm not shipping any defective parts. If a customer gets a bad part from us and that part gets to one of his customers, we've lost him for life." Not to mention, Pack adds, "we could be involved in a lawsuit."
Some of Unimark's customers want written certification that the molding process was in control the entire time their parts were being made. In addition, customers are now asking for process data. No problem, Hengeveld says; thanks to PC-based controllers, it's easy to send process information to customers who want it.
In the past, many machine controllers used a language specific to one type of machine. But with the spread of PC-based controllers, Windows software has become common in molding equipment. "This lets you download process data into a normal computer and send it to a customer," Hengeveld explains.
CONCLUSION
Injection molding has been changed by a host of new technologies aimed at improving the process and its products. New materials offer a variety of properties and increase the options of designers. New molding techniques and machinery turn out better products and hold costs down. And sophisticated monitoring equipment keeps a close watch on process variables to boost accuracy and repeatability. Taken together, these innovations add up to a boon for medical device OEMs shopping for injection molding services.
William Leventon is a New Jersey–based freelance writer who frequently covers the medical device and diagnostic industry.
Copyright ©2001 Medical Device & Diagnostic Industry