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New plasma-coating process for molded parts

New plasma-coating process for molded parts

Сase studies

By Mikell Knights

Waldorf Technik GmbH & Co. KG, Engen, Germany, has developed a barrier technology for injection molding that places a plasma-enhanced chemical vapor deposition coating (PeCVD) onto the part. Waldorf Technik says its Cavonic 3-D coating acts as a barrier to all kinds of gases, vapor and chemical influences. “The barrier protects food products from flavor scalping or leaching, and extends shelf life substantially as it protects the food against oxidation,” says Wolfgang Czizegg, CEO at Waldorf Technik and its spinoff, Cavonic GmbH, which is focused on the barrier coating.

A product with the coating can be used in high-temperature sterilization and pasteurization, hot-fill and retort processes. The Cavonic 3-D coating has no impact on the weight of the molded container, and it eliminates the need for a multilayer barrier structure, allowing the container to be recycled in common recycling streams.

Coating system

Waldorf Technik’s Cavonic 3-D coating system.

A typical multilayer structure features PP, a bonding tie layer, an EVOH barrier material, another tie layer and PP. The EVOH prevents gas permeation but provides less protection against moisture and temperature fluctuations. There are other challenges, says Czizegg. “For parts with an EVOH barrier layer, it is … helpful if the part is round and symmetrical to ensure that the barrier layer is equally distributed. Square or rectangular shapes with sharp corners can thin the barrier in those areas. And parts with functional elements such as stacking ribs or nonsymmetrical geometries may not be suitable for multilayer structures with an EVOH barrier,” he says.

The principle of the Cavonic system is similar to PeCVD systems for PET blow molded bottles, like the Actis system from Sidel International AG, Hünenberg, Switzerland, and the InnoPET Plasmax system from KHS Plasmax GmbH, Hamburg, Germany. However, the design and application of the barrier is significantly different, says Czizegg.

“The barrier coating approaches for blow molded PET bottles create a vacuum chamber inside each individual bottle, then apply the plasma and the coating inside each bottle. Each bottle has its own coating, plasma device and gas line,” says Czizegg. The Cavonic process can coat hundreds of parts at one time, he says. This reduces the investment by around 70 to 80 percent.

The system places a glass-like layer of silicon dioxide on the part surface, which is not a new process. Amcor Ltd., Hawthorn, Australia, in Europe and Toppan, Tokyo, have developed silicon coatings for extruded films that have been approved for food and beverage contact by the FDA and regulatory agencies in Europe and elsewhere for years. The company says the Cavonic process is the first system designed for injection molding containers that have wide mouths, spouts or tube shoulders, such as capsules, coffee pods, jars, cups, lids, cartridges.

plasma process 2

The plasma-coating process in progress.

Commercial applications
At present, the Cavonic system is in use in three commercial applications in Europe. In one application, it is being used to produce 95mm-
diameter jars for foods such as cream cheese, salad and jam, says Czizegg. Greiner Packaging International GmbH, Kremsmünster, Austria, uses the Cavonic system in making containers.

 “The process is not designed for use with bottles, because the plasma cannot get down past the bottle opening. It is best for open-mouth containers and small parts such as fitments for pouches or single-serve packaging [items],” says Michael Urquhart, North American consultant for Cavonic.

Cavonic is ideal for use with a wide range of standard polymers, including PP, PS, HDPE, PET and polylactic acid (PLA). It is not appropriate for LDPE or LLDPE where softeners in the material can migrate to the surface and become liquid, which interferes with chemical bonding.

The process uses approximately 30 kilowatts per hour of electricity, mainly to drive the vacuum pumps. Overall costs are very competitive with multilayer or co-injected structures, says Urquhart. “EVOH can cost up to eight times the price of the main polymer. With Cavonic, a user can purchase a canister of gas for $20 and it lasts a week,” says Urquhart. The canisters actually contain silicone oil, which becomes a gas in a vacuum.

cavonic production

Containers produced using barrier coating technology.

A thin-walled, 40mm-diameter coffee pod produced from PP, PS or PET at a rate of 45,000 pods per hour would require a total investment for injection molding and coating systems of $5 million. Consumption of the silicon and oxygen gases would amount to less than $250 per week. The barrier cost per 1,000 units would equal 1 cent for energy and 3 cents for the barrier material, totaling 4 cents per 1,000 units.

Spouts produced from HDPE or PP for flexible pouches, manufactured in a 64-cavity tool running in an 8-second cycle, would require a total molding cell and coating system investment of $3 million. Silicon and oxygen consumption would amount to less than $180 per week.

Batch treatment of parts is key
Waldorf Technik’s Cavonic spinoff developed the system as a batch process that treats multiple products simultaneously. Waldorf Technik created its sister company, called Cavonic GmbH, five years ago to handle applications and process development relating to its plasma-coating technology.

The Cavonic system uses a three-chamber oven and trays. The trays have an area of 11.5 square feet. The oven is designed to hold up to seven trays. Each tray can hold hundreds or thousands of parts, depending on part dimensions, says Czizegg. “In this way, a single piece of equipment is used to treat all of the parts at the same time, which minimizes the space requirements.”

When the trays enter the oven, a vacuum is applied in the first chamber to create an atmosphere free of any reactive gases. In the second chamber, an electrode-driven plasma energy charge is applied. “The function of the plasma treatment in the vacuum atmosphere is to absorb oxygen and hydrogen atoms off the surface of the plastic container. Polymers are normally structures based on carbon, hydrogen and oxygen. The plasma absorbs hydrogen and oxygen, leaving electrically loaded carbon molecules  on the part surface that are radical, in other words, free to partner with other molecules,” says Czizegg.

Small valves positioned on one wall of the central chamber open to separately introduce silicon in gas form and oxygen in a precise volume and ratio. The silicon and oxygen gases form a covalent bond with the radical carbon molecules on the part surface. “The vacuum atmosphere immediately distributes the gases uniformly throughout the chamber and across the part, creating a homogeneous layer structure. Everywhere, the same density of gas lays down on the part surface,” says Czizegg. More than 50 percent of the silicon and oxygen mixture that enters the chamber bonds to the part surface, while the rest is deposited on the inner chamber walls or exits as exhaust.

The silicon and oxygen mixture combines with the carbon to create a coating 6 to 7 nanometers thick on the part surface. The Cavonic process produces a coating comprised of multiple layers of silicon and oxygen, alternating the chemistry of the mixture from layer to layer. The first coat has a little more oxygen to create a layer that Czizegg refers to as soft glass. “It is not like a hard glass, but has elastic and sticky characteristics more like silicon. It is a barrier that performs as a fantastic base layer. It forms an unbreakable chemical bond with the polymer and maintains its integrity if the part is subjected to heating and cooling and experiences expansion and shrinkage,” says Czizegg.

The Cavonic process then introduces gas silicon and oxygen to create silicon dioxide, a hard glass-like barrier layer that is also 6 to 7 nanometers thick. The process produces a layered coating, with alternating soft glass and hard glass layers until it reaches a final coating structure of about 100 nanometers, says Czizegg.

The structured layer is like a skin and is flexible and strong. The final top layer can be tailored to deliver a soft-touch surface.  After coating, the part moves to the third chamber, where it returns to atmospheric pressure.

For use in-line with injection molding
The process takes three to four minutes; however, it is designed for use in-line with high-cycling injection molding applications, as parts are treated in batches.

“Initially, one would think a four-minute process that creates a vacuum, applies a plasma process and a coating, then returns to atmospheric pressure does not fit together with fast cycling injection, but collecting and buffering the production of the molding machine makes it work,” says Czizegg. Because parts can be handled in batches, the process is efficient, he says.

The Cavonic system is scalable. Processors can use fewer than seven carrier trays. Cavonic GmbH supplies the transfer system that moves the product to the treatment oven. It designed the oven, the system that controls the vacuum and the valve technology that introduces gas into the chamber, as well as the ultraviolet-resistant tray.

Simpler product testing
In addition, product development and testing is much simpler than developing a new multilayer barrier product. “The processor is not required to create a test mold and/or configure a molding cell with multiple injection units, as is required with a co-injection or multilayer barrier structure. The part is placed on a carrier tray and treated,” says Czizegg.  Cavonic conducts initial product testing with its coating technology for very low costs. The parts can be sent to an independent testing lab to determine the oxygen transmission rate, which can demonstrate how well the barrier is functioning.

Cavonic has heated coated parts to 446 degrees Fahrenheit for an hour, and also cooled parts to minus 108 Fahrenheit and struck them with a hammer for impact testing. In both cases, the barrier performance of the coating was good, says Czizegg. Even after the hammer torture, the coating remained on the surface due to the covalent bonding of the coating to the polymer.

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