By Doug Dodrill | VP of Technology and Henk Blom, PhD | Director of Technical Services
The flow wrap or fin-seal package offers a highly efficient method for packaging a wide variety of products. As economic demands change, the necessity to increase the flow wrap output rate grows. There are many variables that go into optimizing the material and machinery used in this type of application, each with their advantages and disadvantages.
This paper will highlight five of the major considerations for designing and selecting the ideal flow wrap material for your application. It will then address the options for evaluating and testing the quality and integrity of the finished flow wrap package.
Coefficient of Friction
As with most product design decisions, competing goals must be balanced. When it comes to the sealant surface, the coefficient of friction is a critical design point.
A high COF (sticky surface) is highly desirable in terms of product loading. As the product is transferred from the feeding mechanism to the surface of the unwinding film, it is free to move and slide. This movement can happen immediately when placed on the flow wrap film surface but more commonly is seen as the leading end sealer engages, compressing the fin-seal tube and squeezing the product back upstream. When the product changes location as it slides, it can end up in the location of the end seal jaws. The result can be anything from a defective end seal to a crushed product with a damaged sealing tool.
Conversely, a low COF (slippery surface) is desirable for other reasons. A tacky surface tends to bunch up and not readily slide over itself as the flow wrap package is formed. In critical areas like the end seal where the film must transition from a 3-dimensional tube to a 2-dimensional flat seal, it becomes difficult to avoid wrinkles and seal channels when the inner surface sticks and grabs the opposing surface rather than sliding and conforming to the height change.
The end-user experience also must be considered. In general, it is much easier to remove the product from the flow wrap package when the contact surface is slippery. The potentially competing interest of processing line speed versus the end-user preference has to be carefully balanced.
The choice of sealant type is undoubtedly one of the most critical variables that affects run speed. A wide range of polymers are commonly used that each offer unique advantages as well as limitations. The appropriate balance of cost and performance should be evaluated relative to the application.
One of the most difficult locations in achieving a hermetically sealed package is the point where the fin seal meets the end seal. At this location, the polymer needs to flow and fill in the void created by the webs coming together. The melt flow properties of the sealant polymer dictate the ability of it to “caulk”. Polymers that readily move and flow are ideal for this purpose.
Unfortunately, there is a downside to polymers that offer good caulkability. These high-flow materials inherently tend to exhibit high elongation and a low modulus. When it is time for the package to be torn open, this high elongation results in stretching and webbing rather than a clean tear. The attribute that makes the film seal efficiently can become frustrating and challenging for the end-users’ access to the product.
Through the use of coextrusion technology and new polymer advancements, it is possible to create hybrid materials that offer enhanced caulkability without sacrificing the end-user experience.
Seal Initiation Temperature
As the speed of the flow wrap process increases, the time during which heat energy must transfer through the film to the sealing interface decreases. In the newest high-speed, high-volume applications, this time is mere fractions of a second. During this brief moment, it is not possible for the critical sealing surface to ever achieve equilibrium with the heated tool. Thus, higher line rates are best accomplished with polymers that melt and activate at lower temperatures. While traditional LDPE performs acceptably at traditional speeds, highly engineered polymers are necessary to push through those limits
As a rule of thumb, a decrease in the density of polyolefins will lower the seal initiation temperature and improve the heat sealing window. However, the lower density options, once again, create end-user opening challenges as the structure becomes more elastic and resistant to tearing.
The overall mass that separates the sealing tool from the seal interface is a critical factor that can limit process speeds. Each layer in the flow wrap film or laminate conducts heat at a fixed rate for that material. The bulk of the film is effectively acting as an insulator, limiting the speed at which seals can be made. Not surprisingly, the more mass or thickness in the film, the longer it takes to drive energy through the structure. From the perspective of heat transfer, thin structures will perform better than thick structures. However, adequate sealant bulk is still required to flow, caulk, and deliver robust hermetic seals. The durability of the film and package walls themselves also must be considered in parallel with the seal optimization. Hermetic seals serve little purpose if the film itself is compromised by tears, abrasion, or punctures.
The integrity of a flow wrap package is critical to its function, whether it is related to keeping moisture or aroma and flavors in the package, oxygen out of the package, or maintaining package sterility in the case of medical and diagnostic devices. The absence of through-pinholes (hereafter pinholes) in the film or channels in the seals ensures that the product inside the package will perform as designed over its expected shelf-life. So it is important, from a process perspective, to be able to detect channels and pinholes in packages during production.
There are a number of test methods available today that will detect pinholes of various sizes. A full list of those methods that are currently maintained by ASTM International can be found here. It is important to note that all of the methods in that list have precision and bias statements, and as such one can reasonably expect that they can be validated. Some of the methods in that list (such as the Helium tracer gas method) are more suited for research and development efforts, while others can be readily implemented in a quality control lab or on a production line.
The most common package integrity test methods used in production environments today are visual inspection (ASTM F1886), the dye leak test (ASTM F3039 or ASTM F1929) and the bubble leak test (ASTM F2096 or D3078). The visual inspection test method provides a qualitative (accept/reject) visual inspection method to evaluate the appearance characteristics of unopened, intact seals in order to determine the presence of defects that may affect the integrity of the package. The sensitivity of this method, which requires that at least one of the webs of the package is clear, is about 0.003 in (75 µm), depending on the lighting, contrast, and experience of the inspector.
Of the two dye leak tests, typically only F3039 would be applicable for flow wrap packages, since F1929 is for packages containing a porous web such as paper or spun-bonded polyolefin. F3039 uses a blue dye to detect 0.002 in (50 µm) channels in seals or 0.00039 in (10 µm) pinholes in the film. The test is very simple to perform, can be completed in less than a minute, and provides a very clear detection method for channels and pinholes.
The bubble leak tests previously mentioned are similar in that both are performed under water, and both require the presence of stream of bubbles to detect a leak in a package. They differ in that in the case of D3078 a vacuum is pulled on the package while it is submerged. This test depends on the presence of a head space within the package. In the case of F2096, on the other hand, the package is pressurized through a hole in the package. F2096 is suited for detecting gross defects (0.01 in or 250 µm) in packages, while D3078 is estimated to be able to detect holes as small as ~0.00002 in (~0.5 µm).
Ultimately, all manufacturing processes have a rate limiting constraint. This limitation may be related to production of the product itself, feeding of the product into the packaging line, or forming and sealing the flow wrap package. If the flow wrap process is the rate limiting factor, the finished package requirements should be carefully evaluated to see if any of the previously discussed variables can be modified to reduce or remove that limitation. With careful packaging material and machine design, the flow wrap output rate can be drastically increased to offer significant efficiency and value to the manufacturer.
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