This article will focus on five main considerations for designing and selecting the ideal fluid wrap material for your application. It will then handle the option of evaluating and testing the quality and completeness of the finished flow package.

1. Coefficient of friction

As with most product design decisions, competing goals must be balanced. For the surface of the sealant, the coefficient of friction is a key design point.

High COF (sticky surface) is ideal for product loading. When the product is transferred from the feeding mechanism to the surface of the unwinding film, it can move and slide freely. When placed on the surface of a fluid-wrapped membrane, this movement can occur immediately, but more commonly, the front end sealer engages, compresses the fin seal tube, and squeezes the product back upstream. When the product changes position during the sliding process, it can reach the position of the end sealing pliers. The result may be a defective end seal to a product crushed by a damaged sealing tool.

On the contrary, for other reasons, a lower COF (slip surface) is desirable. The sticky surface tends to bundle up and is not easy to slide on itself because of the formation of flow wraps. In critical areas like end seals, the film must transition from a three-dimensional pipe to a two-dimensional flat seal. When the inner surface sticks and grabs the opposite surface instead of sliding and adapting to height changes, it is difficult to avoid wrinkles and seal the channel.

The end-user experience must also be considered. Generally speaking, when the contact surface is smooth, it is much easier to remove the product from the flow package. The potential competing interests between processing line speed and end-user preferences must be carefully balanced.

2. Caulkability

The choice of sealant type is undoubtedly one of the most critical variables that affect the operating speed. A wide range of polymers is commonly used, each with unique advantages and limitations. The appropriate balance of cost and performance should be evaluated relative to the application.

One of the most difficult positions in achieving a sealed package is at the point where the fin seal meets the end seal. In this position, the polymer needs to flow and fill the voids formed by the network structure. The melt flow characteristics of this sealant polymer determine its “caulking” ability. Polymers that are easy to move and flow are ideal materials for this purpose.

Unfortunately, polymers that provide good caulking properties have a disadvantage. These high fluidity materials themselves tend to exhibit high elongation and low modulus. When the package is torn, this high elongation causes stretch and webbing rather than a clean tear. The attributes that make film sealing effective can become frustrating and a challenge for end-users to access the product.

Through the use of co-extrusion technology and the advancement of new polymers, it is possible to create hybrid materials with improved caulking properties without sacrificing the end-user experience.

3. Sealing start temperature

As the velocity of the fluid encapsulation process increases, the time during which heat must be transferred through the membrane to the sealing interface decreases. In the latest high-speed, high-volume applications, this time is only a fraction of a second. In this brief moment, the critical sealing surface cannot reach equilibrium with the heated tool. Therefore, polymers that melt and activate at lower temperatures are best able to achieve higher linear speeds. Although the performance of traditional LDPE is acceptable at traditional speeds, to break through these limitations, highly engineered polymers must be used

According to experience, a decrease in polyolefin density will lower the initial temperature of the seal and increase the heat seal window. However, the low-density option once again brings open challenges to end-users, as the structure becomes more elastic and tear-resistant.

4. Thickness

The overall quality between the sealing tool and the sealing interface is the key factor limiting the processing speed. Heat is conducted at a fixed rate in each layer of the flow wrap film or laminate. Most of the film effectively acts as an insulator, limiting the speed of sealing. Unsurprisingly, the greater the mass and thickness of the film, the longer it will take to drive energy through the structure. From the perspective of heat transfer, thin structures have better heat transfer performance than thick structures. However, sufficient sealant volume still needs to flow, caulk, and provide a strong seal. The durability of the film and the packaging wall itself must also be considered while optimizing the seal. If the film itself is damaged due to tearing, abrasion, or puncture, the hermetic seal is useless.

5. Integrity test

The integrity of flow packaging is critical to its function, whether it is maintaining moisture or aroma and flavour in the packaging, oxygen in the packaging, or maintaining sterility of the packaging in the case of medical and diagnostic equipment. There is no penetrating pinhole (hereinafter referred to as pinhole) in the sealing film or channel, which can ensure that the product in the package runs in the designed way within the expected shelf life. Therefore, from a process point of view, it is important to be able to detect channels and pinholes in the packaging during the production process.

There are many test methods available to detect pinholes of various sizes. A complete list of these methods, currently maintained by ASTM International, can be found here. It is worth noting that all methods in this list have precision and bias statements, so we can reasonably expect them to be verified. Some methods in the list (such as the helium tracer gas method) are more suitable for R&D work, while other methods are easy to implement in quality control laboratories or production lines.

In production environments today, the most common packaging integrity tests 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 (accept/reject) visual inspection method to evaluate the appearance characteristics of an unopened, intact seal to determine whether there are defects that may affect the integrity of the package. The sensitivity of this method is about 0.003 in (75µm), and it requires at least one spider web on the package to be transparent, depending on the light, contrast, and the experience of the inspector.

Of the two dye leakage tests, usually, only F3039 is suitable for flow packaging materials, because F1929 is suitable for packaging materials containing porous fibre mesh (such as paper or spunbond polyolefin). F3039 uses blue dye to detect 0.002µm channels or 0.00039µm pinholes in the sealing sheet. The test operation is very simple and can be completed in less than one minute, providing a very clear detection method for channels and pinholes.

The aforementioned bubble leak test is similar because both are performed underwater and both require the presence of a bubbling stream to detect leaks in the bag. The difference between them is that in the case of D3078 when the package is immersed in water, the vacuum is pulled. This test depends on whether there is a headspace in the bag. On the other hand, in the case of F2096, the packaging is pressurized through the holes in the packaging. F2096 is suitable for detecting major defects (0.01µm or 250µm) in packaging, while D3078 is estimated to be able to detect holes as small as 0.00002µm (0.5µm).

6. Conclusion

There is always a limit to the rate at which a manufacturing process can operate. This restriction may be related to the production of the product itself, sending the product to the packaging line, or forming and sealing a flow packaging package. If the process packaging process is the rate-limiting factor, then the completed packaging requirements should be carefully evaluated to see if any of the variables discussed earlier can be modified to reduce or eliminate the limitation. Careful packaging materials and flow wrapping machine design can greatly increase the output speed of flow packaging and provide manufacturers with significant efficiency and value.