Radio-Frequency Sealing for Disposable Medical Products

Medical Device & Diagnostic Industry Magazine
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An MD&DI December 1999 Column

An extensive variety of polymers can be processed with RF to form seals that are as strong or stronger than the material itself.

For years, the medical industry has used radio-frequency (RF) sealing to manufacture bags for IV fluids and for blood and urine collection. As the industry has grown apace with an aging population, the demand for more bags and disposable devices has created increased interest in this processing technology. This article explains what RF sealing is and how it operates. Also reviewed are machine sizing and tooling requirements, product-handling systems, and various ways to maximize sealing efficiency.

It is not clear exactly when RF technology was first employed in plastic sealing, but estimates point to somewhere around 1945. Plastic raincoats, three-ring binders, and novelty items such as wallets were some of the first products sealed with radio waves. Later applications included medical bags, swimming-pool liners, various automotive products, tents and awnings, and, in the last 10 years, packaging.

In the medical industry, RF energy is used to seal together two or more layers of film—typically PVC, EVA, Saran, or polyurethane—to form a container or pressure device. Bags for IV fluids, chemotherapy, blood, enteral feeding, ostomy and urology, laparoscopy, enema, ileostomy, and fluid filtering are all made with RF sealing. In addition, the process is used to produce blood-pressure cuffs, hot and cold packs, leg-compression sleeves, aircasts, body bags, wheelchair pads, immobilizing pillows, breather bags, implants, IV arm boards, stretchers, centrifuge devices, sterilization indicators, tourniquets, catheters, and fluid-pump cassettes, among other disposable devices. Although these products are extensive and varied, the technique for sealing them is essentially the same.

Medical bags are typically processed via radio-frequency sealing.

The RF sealing process is similar to microwave heating of food, and indeed may have stimulated the development of microwave ovens. For example, just as not every material will heat in a microwave, not all plastics can be heated with radio frequency. Polystyrene, polypropylene, and polyethylene are common materials that do not respond to RF energy.

Table I: Radio-frequency sealability of plastic sheeting.¹

Because of the significant investment many medical manufacturers have made in RF systems, there is a strong incentive to retain RF compatibility when new materials—such as those formulated to replace PVC—are created. There are many new polyolefins currently being developed for medical products that can be sealed with RF. In fact, one way to determine if a plastic is a good candidate for RF sealing is to see if it heats up in a microwave (Table I).

WHAT IS RF SEALING?

RF sealing is the use of radio waves—typically at 27.12 MHz—directed through two or more layers of a dielectric plastic in conjunction with pressure so that molecules of all layers of the plastic combine when the material becomes molten. The changing polarity of the radio waves being passed through the plastic causes polarized molecules in the polymer to vibrate back and forth, inducing friction at the molecular level and producing heat. This heat, when created with sufficient energy, causes the plastic to become molten and, under pressure, the layers seal together by free exchange of molecules. The RF energy is then shut off while the tooling holds the plastic sheets together for a very short time to cool under pressure.

What results is a seal with equal or greater strength than the material itself. The seal is consistent, nearly clear, and uniform in appearance and measure. Adding a thin, raised exterior edge to the die will result in a tear seal, which is a thinning of the plastic layers to approximately 0.001 in., which allows an operator to strip the outside waste from the very edge of the seal. This configuration is very common in the production of medical bags and is also used in clamshell sealing.

HOW RF WORKS

A basic RF sealer comprises a radio transmitter and an air-operated press that opens and closes the power applicator. The transmitter is called an RF generator because it generates radio-frequency-wave power rather than transmitting radio signals. The generator includes three components: the power supply, the oscillator, and the controls. The power supply converts power from an ac line source into high-voltage direct current. An oscillator tube/resonator combination then converts the direct current into 1000 to 1500 V of alternating current operating at a frequency of 27.12 MHz ±0.6%. The controls regulate and monitor the operation of the generator as its output heats the plastic in the seal area.

The sealing appliance is almost always an air-operated press with interchangeable electrodes (tooling or dies) that are typically made of brass. Bag tooling comprises a top die and a bottom nest with raw-material locators, and mandrel assemblies for tube seals.

The press consists of a steel frame with an air-operated, moveable platen assembly that squeezes the sealing die against the plastic film resting in the bottom die nest. A closed path is created where the top die presses through the material and against the bottom nest. The polymer heats as the RF current flows from the generator along the path through the seal area—where the brass tool contacts the plastic—and back to the generator. The actual operating sequence is as follows:

1. Press lowers and closes current path.
2. RF energy flows through the path and heats the seal area.
3. Seal is accomplished and energy flow is stopped.
4. Seal cools under pressure.
5. Press opens and releases the finished product.

SIZING RF SEALERS

When processors calculate the power level (in kilowatts) required for a given sealing application, they first convert the seal area into square inches. If a tear seal is involved, the linear inches of the tear seal must be assumed to each be the equivalent of a 0.125-in. bar seal. Thus, a bag with 24 in. of perimeter tear seal will have 3 sq in. in tear seal alone. To this must be added the area of the corresponding bar seal as well as any internal seal areas to determine the total seal area. This total is then divided by three to determine the power required to seal the application in minimal time, with power to spare. Some manufacturers will simply use 5 sq in. per kilowatt, but this method does not allow for the fastest possible cycle times nor for the predictable decrease in power of the oscillator tube over time (Figure 1).

Figure 1: Typical RF power output for minimum cycle time.

TOOLING

Tooling for the RF sealing of medical bags consists of an upper die, typically made from brass, mounted to an aluminum tool and jig plate, and a bottom die or nest that is typically made from aluminum. Dies have also been constructed solely from aluminum, steel, or copper beryllium, and any metal that conducts electricity will work. The nest usually has a buffer board or sheet attached in the seal area to minimize the heat-sinking effect of the metal nest and provide an insulated landing for any tear-seal tool edge.

The reason for most tools being brass is threefold: the metal conducts electricity very well, it transfers heat very evenly, and it is easy to machine, and therefore, repair. The key considerations in RF tooling are to maintain parallelism (uniform thickness throughout the tool, within 10% of the preseal thickness of the plastic) and to remove any sharp edges and corners. Because RF voltage increases per area when applied to sharp edges and corners, the higher the voltage, the greater the chance that the material will break down and arc across, ruining the plastic and possibly harming the die. A good arc suppressor in the generator will stop the RF output before die damage can occur, but arcs—even if stopped within milliseconds—can damage the film and the buffer.

Bags with tubes sealed into them will require double-cycle tooling, which includes mandrels that are inserted into tubes and placed between layers of film. Often, these mandrels will incorporate a stripper to help get the bag and tube off of the mandrel after sealing.

Tooling can be made to seal more than one piece per cycle, depending on the power level of the sealer and the die platen dimensions. Typically, tooling changes require from 10 minutes on a two-station shuttle to 30 minutes on multiple-station turntables. Optional quick-change features can substantially reduce changeover time.

EFFICIENT RF SEALING TECHNIQUES

To determine the maximum amount of RF energy that can be fed into the seal area, generator output is increased until the arc suppressor fires. The RF output is then reduced until firing only occurs due to an irregularity, such as the presence of foreign matter, material imperfections, or improperly located material. At this point, the system is operating at optimum energy levels and below the arc-through threshold. When maximum power output is insufficient to trip the arc suppressor, the output level needed to minimize sealing time is probably unavailable. If the available RF output falls well below the power requirement for setup, the temperature requirement for a seal will never be attained. When this occurs, higher-output equipment or tooling that requires less power is indicated.

Sealing can be made more efficient by controlling heat loss. The heat is generated in the plastic; therefore, the tooling actually sucks heat out of the material since the tooling temperature is below the melt temperature of the plastic (240°–320°F). Heat losses can be minimized by warming the tooling to approximately 120°–150°F and/or by insulating the tooling surfaces from the product with a buffer. Both methods reduce the heat-sink effect of the tooling, while allowing for efficient cooling after the RF is turned off.

Tooling that is run at temperatures higher than ambient needs to be leveled on the press at the temperature at which it will be run. The natural tendency of metal plates to expand when heated tends to warp platens at higher temperatures. Leveling adjustments are built into most RF platens to address the expansion and avoid warping. RF output can also be enhanced by increasing press pressure so that the material will bond before its interface reaches the temperature needed for a full melt, but this parameter has the smallest effect when compared with energy and time.

MAXIMUM THROUGHPUT WITH AUTOMATION

An automated RF bag sealer is capable of completing 8 to 10 cycles per minute (cpm). On soft PVC film applications without tube seals, throughput can occasionally increase to as much as 20 cpm. However, 8–10 cpm remains the average for fully automated product feeding, with 5–7 cpm for manually loaded turntables and shuttles. Actual production output will depend upon how many items are being sealed at one time and on the speed of the operators. Most items occupying 1 sq ft or more are sealed 2-up employing a power output from 12.5 to 20 kW. Smaller items are typically sealed 4-up or 6-up using a 4–10-kW power output.

DOUBLE-CYCLE SEALING

Bags with tubes sealed into them are best made on what is called double-cycle equipment. For each press closure, there are actually two separate RF seal cycles. The first cycle seals the perimeter of the tube or tubes. The second cycle seals the bag perimeter and any internal bar seals. The best equipment to perform this task would include completely independent controls for tuning, preheating, welding, cooling, power, and arc-suppression sensitivity, which would ensure full control in tuning each seal exactly as desired. In addition, a system to balance the top and bottom of the tube seal is often necessary for even flow of RF current around the tube.

On the first cycle, RF energy is applied to the mandrel inserted inside the tube and between the sheets of film, while the upper and lower half-circle dies (or cradles) are both grounded. This way, the seal will be even all the way around the tube. On the second cycle (the press has not yet opened), the mandrel is disconnected from the RF and from the ground (i.e., is electrically neutral). Through the double-cycle RF switch, the upper die is then connected to the RF and the bottom die remains grounded, allowing the perimeter and any bar seals to be accomplished. The double-cycle process eliminates the need for a second unit to seal tubes and avoids the double handling that would ensue. It also produces a more attractive product, with no obvious seal overlaps that could weaken the integrity of the bag.

Product feeding time is shortened through the use of an automated product in-feed system. The two most popular configurations are a shuttle system and an indexing turntable. A shuttle system has two complete in-feed stations. In operation, one station is loaded while product in the other is being sealed; moving the newly loaded station into the sealing position moves the sealed station out. Shuttle systems are usually placed at the end of two conveyor belts to handle the flow of presealed film and finished product.

Turntable or rotary systems are typically built to accommodate from four to eight stations, allowing more operators access to the tool nests. Typically fed precut film and tubes so that machine speed can be maximized, these systems can be set to run in an automatic mode in which the operators must keep up with the set pace. Tooling is more expensive for rotary systems than for shuttle units, as more nests are required (Figure 2).

Figure 2. A common configuration for semiautomated RF sealers is this 12-kW, four-position, double-cycle medical turntable.

With total automation, the operator is eliminated as a variable. Fully automated systems use mechanical means such as web indexers, feeder bowls, and pick-and-place robots to load the die nests. These systems are best suited to long, dedicated runs of products with seals that are the same size and shape, as they do not lend themselves to frequent changeover.

COMPARING RF WITH OTHER SEALING TECHNOLOGIES

With RF sealing, everything between the upper and lower dies heats evenly, at least in theory. In actual use, however, the dies heat-sink the plastic on contact, such that a temperature profile would indicate the hottest spot is at the interface of the two materials. This works to great advantage in bonding, since the interface is where the most heat is required.

Other methods, such as thermal, impulse (a switched thermal), or ultrasonic sealing, do not share this advantage. For example, temperature profiles taken during thermal and impulse processing indicate that the hottest spot is where the dies touch the outside of each layer of plastic—a condition that often causes degradation of the outside of each layer before the interface reaches melt temperature. (Thermal and impulse sealing functions best with certain very thin [<0.006 in.] films and with polyethylene, polypropylene, or polystyrene.) Ultrasonics, on the other hand, works like a jackhammer, pounding the plastic from 20,000 to 40,000 times per second, with the resulting friction creating heat and thus melting the plastic. Again, the temperature profile is less desirable than that with RF. These alternative processes are limited in area of seal, lack repeatability of acceptable seal quality, and do not have the ability to produce tear seals. They are often employed in small-area spot sealing or in applications for which product appearance is not important, such as polybagging or tack sealing to locate parts.

CONCLUSION

Few products are as well suited for a particular manufacturing process as are medical bags for RF sealing. No other bag-sealing method yields the consistency, capacity, and quality afforded by RF technology. Depending on the type of bag produced, the required quantity, the manufacturing environment (e.g., labor costs), and the unit price, there are several processing options available to the RF sealer, ranging from completely manual to fully automated techniques.

In addition to choosing the most efficient RF sealing technique for a particular job, bag makers should work with reputable RF companies to ensure the proper design of equipment and tooling. It is also important to remember that RF interference can affect many sensitive electronic devices within a manufacturing facility, and appropriate shielding meeting or exceeding OSHA, IEEE, and FDA regulations should be verified during equipment selection. The fact that, in many cases, data feedback from sealing equipment can help validate the manufacturing process and eliminate costly product testing is another important consideration.

Although RF sealing is a long-established technology, new product developments and option packages have solidified its performance, reliability, and safety. At first glance, RF sealing may seem complicated—the term "black magic" has even been used to describe it. However, RF sealing merely obeys the laws of physics, and is in fact a predictable, dependable, and robust manufacturing process.

REFERENCES

1.For more information on the mechanical and chemical properties of the materials listed in Table I, see the Plastics Encyclopedia, Thermoplastics and Thermosets, 8th ed. (San Diego: Condura Publications, 1986).

BIBLIOGRAPHY

Dittrich, HF. Tubes for RF Heating. Eindhoven, Netherlands: Philips Technical Library, 1971.

Farkas, RD. Heat Sealing. New York: Reinhold, 1964.

FCC Rules and Regulations, CFR 47, Part 18. Washington DC: U.S. Federal Communications Commission, 1985.

Metaxas, AC, and Meredith, RJ. Industrial Microwave Heating. London: Peter Peregrinas Ltd., 1983.

Ott, HW. Grounding and Shielding Techniques in Instrumentation, 3rd ed. New York: Wiley, 1986.

Ott, HW. Noise Reduction Techniques in Electronic Systems. New York: Wiley, 1976.

Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Field—300 KHz to 100 GHz, ANSI Standard C95.1. New York: ANSI, 1982.

Sobotka, H. Industrial RF Heating Generators. Eindhoven, Netherlands: Philips Technical Library, 1963.

Terman, FE. Electronic and Radio Engineering. New York: McGraw-Hill, 1943.

Von Hippel, AR (ed.). Dielectric Materials and Applications. Cambridge, MA: MIT Press, 1966.

Westman, HP. Reference Data for Radio Engineers, 5th ed. New York: Howard Sams & Co., 1974.

Zade, HP. Heat Sealing and High Frequency Welding of Plastics. London: Temple Press, 1959.

Steve Myers is general manager at Callanan Co. (Elk Grove, IL), a wholly owned subsidiary of Alloyd Co. and a supplier of RF welders and sealers to the medical device industry.

Photos courtesy of Callanan Co.

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