Flow Wrap Materials: 5 Considerations for Optimizing Speed
By Doug Dodrill | VP of Technology and Henk Blom, PhD | Director of Technical Services
The flow wrap or fin-seal package offers a very efficient method for packaging a wide variety of products. As economic demands change, the need to increase the flow envelope output rate increases. There are many variables that go into optimizing the hardware and machinery used in this type of application, each with their pros and cons.
This article will highlight five of the main considerations for designing and selecting the ideal Flow Wrap material for your application. It will then discuss options for evaluating and testing the quality and integrity of the finished flow wrap packaging.
Coefficient of friction
As with most product design decisions, competing goals must be balanced. With respect 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. When 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 occur immediately when placed on the surface of the packaging film, but is most often observed when the front end sealer engages, compressing the finned tube and tightening the product upstream. When the product changes location by sliding, it may end up in the location of the end joint jaws. The result can be anything from a failed end seal to a crushed product with a damaged sealing tool.
Conversely, a low COF (slippery surface) is desirable for other reasons. A sticky surface tends to bunch together and not slide easily on itself as the wrapper forms. In critical areas like the end seal where the film must transition from a three-dimensional tube to a two-dimensional flat seal, it becomes difficult to avoid wrinkles and sealing channels as the inner surface sticks and grips the opposite surface rather than slipping and conforming to the pitch change.
The end-user experience should also be taken into account. In general, it is much easier to remove the product from the flow wrap packaging when the contact surface is slippery. The potentially competing interest of processing line speed versus end-user preference must be carefully balanced.
The choice of sealant type is undoubtedly one of the most critical variables affecting execution speed. A wide range of polymers are commonly used, each offering unique benefits as well as limitations. The appropriate balance between cost and performance should be assessed based on the application.
One of the most difficult places to get a hermetically sealed package is where the fin seal meets the end seal. At this point, the polymer must flow and fill the void created by the bringing together of the canvases. The melt flow properties of the sealant polymer dictate its ability to “caulk”. Polymers that move and flow easily are ideal for this purpose.
Unfortunately, there is a downside to polymers that provide good caulking ability. These high flow materials inherently tend to exhibit high elongation and low modulus. When it’s time to tear the wrap, this high elongation results in stretching and webbing rather than a clean tear. The attribute that allows the film to seal effectively can become frustrating and difficult for end users to access the product.
Through the use of co-extrusion technology and new advances in polymers, it is possible to create hybrid materials that provide improved caulking ability without sacrificing the end-user experience.
Gasket initiation temperature
As the speed of the dynamic wrapping process increases, the time that thermal energy must be transferred through the film to the sealing interface decreases. In the latest high-speed, high-volume applications, this time is only fractions of a second. During this brief moment, it is not possible for the critical sealing surface to reach equilibrium with the heated tool. Thus, higher line rates are best achieved with polymers that melt and activate at lower temperatures. While traditional LDPE performs acceptably at traditional speeds, highly engineered polymers are required to push those limits.
Generally, decreasing the density of polyolefins will lower the seal initiation temperature and improve the heat seal window. However, lower density options, once again, create opening challenges for the end user as the structure becomes more elastic and tear resistant.
The overall mass that separates the sealing tool from the seal interface is a critical factor that can limit processing speeds. Each layer of flow wrap film or laminate conducts heat at a fixed rate for that material. Most of the film acts effectively as an insulator, limiting the speed at which joints can be made. Not surprisingly, the greater the mass or thickness of the film, the longer it takes for energy to pass through the structure. From a heat transfer point of view, thin structures will perform better than thick structures. However, an adequate volume of sealant is still required to pour, caulk and provide strong airtight seals. The durability of the film and the packaging walls themselves must also be taken into account alongside the optimization of the seal. Airtight seals are of no use if the film itself is compromised by tears, abrasions or punctures.
The integrity of a flow wrap package is critical to its function, whether related to maintaining moisture or aromas and flavors within the package, oxygen out of the package, or maintaining the sterility of the packaging 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 intended during its intended shelf life. It is therefore important from a process point of view to be able to detect channels and pinholes in packages during production.
There are a number of test methods available today that can detect pinholes of different sizes. A complete list of these methods that are currently maintained by ASTM International can be found here. It is important to note that all the methods in this list have precision and bias statements, and as such it can reasonably be expected that they can be validated. Some of the methods in this list (such as the helium tracer gas method) are more suitable for research and development efforts, while others can be easily implemented in a quality control lab or on a production line.
The most common packaging integrity test methods used in production environments are visual inspection (ASTM F1886), dye leak test (ASTM F3039 or ASTM F1929), and bubble leak test (ASTM F2096 or D3078). The visual inspection test method provides a qualitative visual inspection (pass/fail) method to evaluate the appearance characteristics of intact, unopened seals to determine the presence of defects that could affect the integrity of the seal. ‘packaging. The sensitivity of this method, which requires that at least one of the strips of the package be transparent, is about 0.003 in (75 µm), depending on lighting, contrast, and the experiment of the user. inspector.
Of the two dye leakage tests, only the F3039 would be generally applicable to wrap-around packages, since the F1929 is intended for packages containing a porous web such as paper or spunbonded polyolefin. The F3039 uses a blue dye to detect 0.002 in (50 µm) channels in joints or 0.00039 in (10 µm) pinholes in film. The test is very simple to perform, can be performed in less than a minute and provides a very clear detection method for channels and pinholes.
The previously mentioned bubble leak tests are similar in that they are both performed underwater and both require the presence of a stream of bubbles to detect a leak in a package. They differ in that, in the case of the D3078, a vacuum is applied to the package while it is submerged. This test depends on the presence of a headspace inside the package. In the case of the F2096, on the other hand, the packaging is pressurized through a hole in the packaging. The F2096 is suitable for detecting coarse flaws (0.01 inch or 250 µm) in packaging, while the D3078 is said to be able to detect holes as small as ~0.00002 inch (~0.5 µm).
Ultimately, all manufacturing processes have a rate limiting constraint. This limitation may be related to the production of the product itself, the feeding of the product into the packaging line, or the forming and sealing of the wrap-around packaging. If the continuous packaging process is the throughput limiting factor, the finished packaging requirements should be carefully evaluated to see if any of the previously discussed variables can be changed to reduce or remove this limitation. With careful packaging materials and machine design, packaging throughput can be significantly increased to deliver significant efficiency and value to the manufacturer.
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