Medical Device Link.

But experts say that a lot less can be left to chance in the blister-forming process if the manufacturer has a sufficient grasp of material properties and works closely with experienced blister film suppliers to come up with an adequate design for the packaging. Also important are using proper validation techniques and consistent control systems.

When it comes to thermoforming blister films, variables such as temperature, pressure, and dwell time can throw a process into inconsistency if they are not controlled properly. Therefore, it is crucial to understand the properties of each material being run.

Fortunately, the most commonly used blister film is polyvinyl chloride (PVC), which is considered to be a very forgiving material. "Most materials to date have been PVC based. There are differences among PVCs, but in relation to the bigger world of processing, the differences are quite small," says R. Michael Cain, sales and marketing manager for Tekni-Films, a division of Tekni-Plex Inc. (Somerville, NJ). Cain adds, "Since everything is mostly PVC based, you do not experience radical process changes. You can put the machine on-line and start running the film. With a few subtle changes and not much material used, you can have the process adjusted."

Other higher-barrier films tend to present more challenges. For example, Cain says, "if you are running a barrier coating such as PVdC [polyvinylidene chloride] or Aclar, you have to look at the temperature of your setup. Most people do that by running the Aclar- or PVdC-facing side cooler than the PVC-facing side."

Converters say that PVdC is known to release a gas that will oxidize tooling, which means that tooling needs to be coated with a material, such as Teflon, to prevent PVdC from sticking to the sealant. Meanwhile, Aclar is always run with PVC, but it shrinks more than PVC, which produces curl if not properly accounted for.

Polypropylene (PP), while rarely used in the United States, "is most difficult in terms of temperature ranges, shrinkage, and being unforgiving to new equipment," says Kevin Carter, market development specialist—pharmaceutical films, Klöckner Pentaplast of America Inc. (Gordonsville, VA).

However, such problems do not appear to be the case with the newest blister film, cyclic olefin copolymer (COC), which is run with PP. "In the structure, the COC is the core, and the PP is used like a film and has no structural function, and it has been perfectly processable," says Mattias Otto, director and chief operating officer of Uhlmann Packaging, Machinery Division (Towaco, NJ). He notes, "we learned that it does not crystallize. It's more or less an amorphous material. So, you may need to run your forming station at a different temperature than you do for PVC."

The choice of lidding material and sealing parameters should also be considered in conjunction with film material selection. "The same materials that seal to PVC may not seal to PVdC due to slight differences in the chemistry of heat-seal coatings," explains Joseph Kaplan, director of marketing for Alcoa Flexible Packaging (Richmond, VA). "For example, if the product requires high-barrier substrates, you may choose an Aclar laminated–thermoformed film to seal to the foil lidding." He adds, "The Aclar may be PVC- or PVdC-coated, and the appropriate vinyl coating selection must be made to obtain seal integrity in the finished package."

"It is unfortunate that the lidding material is often an afterthought in the period before a product launch," says Kaplan. He points out that good design choices and early supplier involvement ensure smooth product rollouts.

In addition, "the seal and the type of heat-seal coating are important," adds Markus Haid, Uhlmann's aftermarket director. Haid says that old machines had maximums of 50 cycles and long dwell times. Although this was sufficient for getting the coating materials flowing, new machines are capable of more than 50 cycles and have different dwell times, which calls for new coatings that are capable of flowing faster.

Foil lidding has its own parameters under which it will hold up during the sealing process. "We can create heat-seal curves for lidding materials and heat-seal coatings in relation to specific coatings to help customers determine what conditions they should be operating under," says David R. Sciubba, healthcare sales manager, Hueck Foils LLC (Wall, NJ).

If using cold-form foil, the process is a bit easier to predict because heat is not involved, and the materials are responding only to mechanical strain. Usually, the aluminum is laminated to two other materials, most often PVC and oriented polyamide (OPA), to resist cracking. Sciubba notes that "there's a lot of work by the supplier that goes into specifying the exact web materials and how they work together to affect the formability of the material. You get the best formability from the right combination of materials and the knowledge of how to laminate them." Another consideration for the cold-form process, says Bob Parker, manager of tool design for Uhlmann, is that "since the cavity is so large, it always has to be fed with a dedicated feeder."

The design of the individual blister cavities plays a large role in determining what is feasible in a forming process. The size and depth of the cavities may determine what materials and techniques can and cannot be used. And the product itself is the main factor in determining the size and depth of the cavities.

"For example, it is important to consider what the best configuration is, based on the customer's requirements and the product's characteristics," says Howard Thau, president of Sonic Packaging Industries Inc. (Westwood, NJ). "You may need to modify the cavity size and shape to ensure proper fit for filling and to avoid any damage to the product. And based on the blister configuration and the depth of the draw, the gauge and forming material must be taken into consideration to guarantee optimum package integrity."

Preventing any area of the cavity from getting too thin is perhaps the biggest challenge to the forming process. When the cavity is formed, whether by some form of pressure or by plug assist, the film is stretched to make the new space, but no extra material is being added. So, making the cavity thicker in one spot means making it thinner in another.

Plug assist is designed to optimize the distribution of material within the cavity, and some manufacturers have come to rely on it. The deeper the cavity, the more likely it is that plug assist will be needed. Then again, it can easily become a detriment to the forming process if it is not used properly.

"When you first start the plug-assist process, you make adjustments by the feel of the cavity," Carter explains. "You touch the bottom, but it's still thin, so you thicken it until it feels rigid. But the thermoforming equipment is not making additional material for the cavity. What has occurred is that 80–90% of the material is now in the bottom of the cavity, and the side walls are extremely thin. Even in tablet forms, you can thin out the side wall and decrease the barrier. And the side wall is often the biggest part of the configuration, depending on the package design."

Therefore, Carter recommends measuring two points on the cavity when using plug assist. He explains, "On a capsule it should be the midpoint of the long axis of the side wall and the midpoint of the cavity bottom. Optimally, we consider the best distribution to be equal between those two points. We recommend getting 40–60% of the original material thickness at both midpoints. You target 50% at both locations, but in reality you have process variation. However, you can maintain a range of 40–60%."

Another thing to keep in mind is that a decision to change the thickness of the material can have consequences for the forming process. "We had a customer who decided to move from 10 to 7 µm PVC," recalls Walter Berghahn, director of machine sales and marketing for Uhlmann. "The cavity walls got thinner and they lost rigidity in the whole card. They tried to save money, but they ended up getting negative effects."

The overall layout of the blister card also plays a role in what can and cannot be accomplished in the forming process. "Customers call us with the layout of what marketing wants, so if you know it's a tricky design, you have to tell the customer that and avoid making problems for yourself later," says Michael Maslo, service manager, Pharma Tool Corp. (Warminster, PA). "Most of the time, they'll give us a well-laid-out design, and we'll work the tooling around that, but you have to realize that there are certain limitations to what marketing can do."

These considerations are especially important with physician samples, Berghahn says, because "there is a lot of area on the card with no cavities but marketing information instead. This will curl unless you use stiffening ribs or don't seal in certain areas."

Another potential impact is the "downstream design," or secondary packaging, says Tom Fillis, director of engineering for PCI Services (Philadelphia). "The final pack-out determines how you lay everything out on the web," he says. "It can make a big difference if it goes into a carton or if it is used as a card."

If the card is designed to be child-resistant (CR), that can present another set of variables to consider. "If you are running a CR design, make sure you have sufficient flange area," Thau says. "Make sure the lidding material is securely bonded to the blister. When you use perforation, make sure there is ample room between the pockets for a sufficient seal area to apply heat and pressure to that portion of the blister."

While the factors that go into a successful blister-forming process may not have changed much, there have been some new technological developments that have had an impact. "The control systems around the forming systems have improved because of servo technology," Carter notes. "Some companies are also using servo on plug assist and can achieve an incremental depth control of 0.001 in. Because of these new control systems, you can get more precise parameters when you're setting up."

Servo technology can also bring more consistency to the materials, Berghahn says. "How material comes out of the extruder affects how it stretches, shrinks, and forms. New machines all have devices to put things back in order. We can use a servo index to adjust for shrinkage and stretch."

There are now software programs that can predict the outcome of a forming process before any material is ever put on the web. Most of them deal with the cold-form process, which is more consistent than thermoforming because of the absence of heat. However, at last year's Blister-Pack 2000+ conference in Jamesburg, NJ, Rexam Healthcare Packaging (Bristol, United Kingdom) unveiled a thermoforming simulation program. It requires specific material data that must be obtained under the conditions found on the thermoforming machine.

But in the end, says Berghahn, "experience goes further than anything, even spec sheets. Specs are static, but machines are dynamic."