Ready Meal End-o-Line for Flexible Packaging TraysCredit:ELITER Packaging Machinery
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Sealing technologies is one of the most problematic concerns when it comes to food packaging with which the storage, preservation, freshness are of utmost importance to preventing food waste caused by improper packaging. Popular recent food packaging involving sealing technologies including form-fill-seal mechanisms and machines which deal with flexible packaging, and thermoforming and tray sealing process that is frequently used in prepared meal and takeaway food industries.
The seal integrity plays a critical role in the reliability of head sealing that has a detrimental effect on how well the packaging can maintain and extend the food containment&#;s shelf life, and along with the seal mechanism, the process parameters and properties of materials are also key factors for the same.
Despite the fact that ÉLITER Packaging Machinery does not engage in the packaging automation of form-fill-seal of flexible packaging, our passion in packaging still drives us to set some in-depth research into this topic now that they are usually packaging from our client&#;s upstream machinery and that our own equipment would frequently be supposed to deal with.
On the contrary to the convention form of rigid packaging like cartons and boxes, flexible packaging refers to those packaging that can change and vary in its shape when being used and filled with different containments like liquids, solids, or powders. Typical packaging materials used for forming flexible packaging can be plastics, paper, aluminium foil, or a combined packaging with several layers made of different materials.
Typical Examples of Flexible Packaging
Credit: ELITER Packaging Machinery
Understanding the various options of sealing technologies and mechanisms for flexible packaging closure is a first step to a quality preservation and storing method to make sure food product&#;s life span, as the comments goes:
&#;Fail to choose the right seal&#;you risk losing business through product spoilage and recall due to quality issues.&#; [1]
Pouch.Me, Heat Sealing vs. Cold Sealing: What&#;s The Difference?
The heat sealing processes engages in the contact of two surfaces of the packaging materials, typically plastic film, which consists of print layer, barrier, and sealant layer.
With this concept being mentioned, it is necessary as well to introduce the film material&#;s different layers engaged during the heat sealing process, the diagram of which, and explanations are as follows:
Plastic Packaging Film Layers
Credit: ELITER Packaging Machinery
The print layer is the outermost layer of the flexible packaging film and is designed for printing graphics, branding elements, product information, and other marketing messages.
The barrier layer is responsible for providing protection against external factors such as moisture, oxygen, light, and odours, which can affect the quality and shelf life of the packaged product.
Barrier layers are typically composed of materials like metallized films, aluminium foil, or specialized polymer coatings with high barrier properties which create a barrier that prevents the ingress or egress of gases, moisture, and light, thereby extending the shelf life of the product and preserving its freshness and flavour.
The sealant layer is located between the barrier layer and the product being packaged. Its primary function is to provide a hermetic seal when heat is applied during the sealing process.
Sealant layers are usually made from heat-sealable polymers such as polyethylene (PE), polypropylene (PP), ethylene vinyl acetate (EVA), or ionomer resins. Sealand layer consists of materials that melt and flow when exposed to heat, allowing the packaging to be securely sealed, ensuring product integrity, and preventing leakage or contamination. The sealant layer should exhibit excellent seal strength, seal integrity, and compatibility with the packaging equipment used for sealing.
The sealant layer plays a key role in the process of wetting, melting, and adhesion and diffusion, which are the processes we will be discussing in the following chapters that deals with the microscale mechanisms of heat sealing.
Heat-Sealing in Flow Wrapping Processes, HFFS Examples
Credit: ULMA Packaging [2]
The packaging materials used for cold sealing shares the same features as that for heat sealing in terms of the multi-layer structure.
Cold sealing is a method used in flexible packaging to create a secure seal between layers of packaging material without the need for heat. Instead of relying on heat to melt a sealant layer, cold sealing utilizes pressure-sensitive adhesives (PSAs) that remain tacky at room temperature.
Cold Sealing Example of Flow-Wrapped Chocolate Wafer
Credit: ELITER Packaging Machinery
When pressure is applied to the adhesive-coated layers, it activates the adhesive, causing it to flow and form a strong bond between the layers. Cold sealing is particularly useful for sealing temperature-sensitive products or packaging materials that cannot withstand high temperatures, an example of which is the flow wrapping packaging used for chocolate bars and wafers.
Common heat sealing techniques on packaging equipment including sealing jaw, hot wiring sealing with knife, hot air sealing and so forth. The most basic process is by applying a constant source of heat with a combination of temperature, and pressure to melt the sealant layer and result into the fusion of the molten surfaces.
Heat Sealing Mechanisms of Flexible Packaging
Credit: Ilknur Ilhan, University of Twent, Reproduced by ELITER Packaging Machinery
The processes involve some critical parameters and technical features of the materials and microscale processes, including:
The steps engaged in the heat sealing mechanisms starts from the initial status when the films get in contact through the overlapping of two surfaces, named the sealing area, where they are hot pressed and melted. The steps that follow are adhesion and diffusion across the contact surfaces that result in the exchange of molecular and lead to the entanglement at microscale. A final step after the sealing is the cooling and recrystallization, by which the sealing area will form a new composition in bulk. To introduce the steps in a clearer presentation, they are:
From a microscopical view and at the level of molecular, the heat integrity, if not done properly, will result in an interface of millions of tiny gaps and holes despite that from the microscale view it can prevent the permeation of external substances like gas or liquids.
Wetting is the process here that plays a key factor in the interface application now that it is during the first transient moment of seconds that the tiny gaps between the interface of film is supposed to be filled. The contact surface&#;s well match is critical for the good quality sealing and to avoid the risk of pinholes in sealing area that will cause leak formation. However, it depends on a range of parameters, internal and external factors such as the material&#;s surface roughness, apart from the topography, hydrophilic and hydrophobicity properties [3].
With these concerns being suggested, selecting packaging materials with adequate properties pave down the primary foundation to a good heat sealing. The selection involves the calculation and figure-based simulation to verify of the wettabilities of the film and sealant.
A key metrics that stands for the wetting effectiveness is the surface free energy of the material. The higher the surface free energy is, the better the interaction potential between the two surfaces that come into contact during wetting processes.
For flexible packaging pouches made of PE (Polyethylene) film, the surface energy of PE plays a significant role in determining how well the liquid (e.g., contents of the pouch) wets or spreads over the surface. Lower surface energy of PE typically results in poor wetting, leading to issues like poor adhesion of inks or coatings, or even poor barrier properties.
Melting and Wetting of Heat Sealing
Credit:University of Twent, reproduced by ÉLITER Packaging Machinery
The calculation can be carried out with 2 options of equations. One of which is Young&#;s equation to calculate the surface free energy:
$$ \sigma_s = \sigma_\text{sl} + \sigma_l \cdot cos \theta \qquad(1) $$
or in another form:
$$ \sigma_\text{sl} = \sigma_s &#; \sigma_l \cdot cos \theta \qquad(2) $$
whereby the elements are:
Supposing a calculation case study involving the VFFS sealing processes for (PE) film, whose typical surface energy values might be in the range of 30-40 mN/m. And then a value as \( \sigma_l \) for common liquids used in packaging which might have surface energy values ranging from 30 to 60 mN/m, depending on factors such as viscosity, polarity, and additives.
Notes: To accurately apply Young&#;s equation and calculate the interfacial energy \( \sigma_\text{sl} \), experimental measurements or reference to known values for the specific materials involved would be necessary. These values can be obtained through techniques such as contact angle measurements using a goniometer or surface energy analysis methods like theTo accurately apply Young&#;s equation and calculate the interfacial energy \( \sigma_\text{sl} \), experimental measurements or reference to known values for the specific materials involved would be necessary. These values can be obtained through techniques such as contact angle measurements using a goniometer or surface energy analysis methods like the Owens-Wendt-Rabel-Kaelble (OWRK) method
Supposing the given parameters:
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Carry out the calculation as follows:
\begin{align}
\sigma_\text{sl} &= \sigma_s &#; \sigma_l \cdot cos\theta \\
& = 30 &#; 45\times \frac{\sqrt{3}}{2} \\
&= 35-23.2 = 11.8 mN/m
\end{align}
An alternative option of Dupré-Poisson Equation to calculate the energy bound by wetting, defined as follows:
$$ W_a = \sigma_s + \sigma_l -\sigma_\text{sl} \quad(3) $$
where, apart from the 3 elements mentioned, the rest \(W_a \) stands for the workd of adhesion, refers to the work or energy required to separate two surfaces that are adhered together.
Diffusion plays a crucial role in the heat sealing process, wherein the movement of molecules along the sealant is influenced by various internal and external factors. Factors such as molecular weight and chain branching significantly affect the diffusivity of molecule chains, among which, for example, longer chains exhibit greater effectiveness than shorter chains in facilitating strength build-up at the interface.
Adhesion and Diffusion
Credit:University of Twent, reproduced by ÉLITER Packaging Machinery
In a general sense, the lower the molecular weight \(M_W\), the faster will be the heat sealing process, nevertheless, molecular chain with lower \( M_W \) will result in weaker adhesion and entanglement of the formed interface. \(M_W\) is also a critical parameter for the molecule chain&#;s diffusion rates of the used packaging material, most frequently different kinds of polymers.
The use of polymer in forming sealing interface and sealing area of flexible packaging can be by same polymers or two different kinds of them. In the former case, the interdiffusion rate will be the self-diffusion rate [4].
For the mention case of heat sealing area between same polymer materials, one can turn to use the Rouse model for calculating the self-diffusion rate, related to the material&#;s melt visiocity:
$$ D_\text{int} = D_\text{self} \quad(4)$$
$$ D_\text{self} = \frac {RT}{N_a \zeta_o N} \quad(5) $$
$$ \eta_o = \frac{\zeta_o n \rho R^2_g N_a}{6M_W} \quad(6) $$
$$ n_o D_\text{self} = \frac{\rho RT}{6} \times (\frac{R^2_g}{M_W}) \quad(7)$$
where,
The following is a assumptive calculation case to understand the diffusion behavior of polymer chains during heat sealing in a Horizontal Form-Fill-Seal (HFFS) process, based on Rouse Model.
Supposing the following given parameters:
Rouse Model is a Efficient Tool for Polymer Motion Study [5]
Credit: Mikhail V. Tamm, Kirill Polovnikov
The calculation starts by acquiring the zero-shear viscosity of the polymer melt \( \eta_o\) with equation \( 6 \),
\begin{align}
\eta_o &= \frac{\zeta_o n \rho R^2_g N_a}{6M_W} \quad(6) \\
\eta_o &= \frac{ (5\times 13^\text{-13} \times n \times 900 \times (5\times 10^\text{-9})^2 \times (6.022 \times 10^\text{23}) }{6 \times 0.1}
\end{align}
then to calculate \( D_\text{self} \) defined by equation \( \quad(5) \), substuting the values within the equation
\begin{align}
D_\text{self} & = \frac {RT}{N_a \zeta_o N} \quad(5) \\
D_\text{self} & = \frac {8.314 \times 423} {6.022 \times 10^\text{23} \times (5\times 10^\text{-13}) \times }
\end{align}
After which we may calculate the adhesion work \( W_a \) as
$$ W_a = D_\text{self} $$
Entanglement within the seal area is intricately linked to the characteristics of the molecules involved, including their type, length, molecular weight (Mw), and branching content, which influence their diffusion through the seal thickness. As molecules move due to the application of heat during sealing, new interaction points emerge, leading to the formation of attractive forces between them.
The entanglement process initiates during the early stages of sealing and persists throughout the cooling phase while the molecules remain in a molten state, gradually diminishing as molecular movement slows down. The density of chain entanglement significantly impacts the ultimate strength of the seal, underscoring its importance in the sealing process.
Entanglement and Recrystalization
Credit:University of Twent, reproduced by ÉLITER Packaging Machinery
In the case of semicrystalline polymers, the sealing process entails an additional step known as recrystallization, which complements other primary sealing mechanisms such as melting, wetting, interdiffusion, adhesion, and entanglement. During recrystallization, the rate of cooling assumes a critical role in determining the growth of crystals.
Slower cooling rates result in the formation of larger crystals albeit in lesser quantities, while faster cooling rates lead to the proliferation of smaller crystals across the sealed area. This variation in crystal size and distribution directly impacts the mechanical properties of the seal, thus necessitating careful consideration of cooling rates in the heat sealing process.
Flexible packaging inevitably involves the use of plastic packaging materials now that we have been discussing all about polymers in above chapters.
A stereotype would be that sustainability can be reached by decreasing the packaging metarials used or cutting down the energy consumption during the automation processes to form the same. However, in the case of form-fill-seal, the reliability sealing goes to the contradictory side of forming sealing area and packaging that is free from leakage formation.
In the field of packaging design, we have three approahces commonly used to achieve the target of savings, stated as follows with their respective downside brought to the formation of sealing area and disadvantage to the whole packaging
Saving with packaging material consumption to achieve sustainability with heat sealing and plastic packaging FFS, can be achieved by 2 methods wihch are:
Both, regardless of their contribution to saving material use and minizie consumption, rasie as backfire the risk of pinhole formation and thus the leakage formation in sealing area.
In a given interval of time, the less the sealing time for each cycle will conduct to more units of packages produced, correspondingly decrease the energy consumption for each of them.
Neverthelsss, to view from the perspective of preservation, less sealing time inevitably results in the insufficient time for the process of difussion, which is critical to the cross-segments molecular exchange for forming a reliable sealing area.
Packaging systems used for forming flexible packaging where heat sealing is engaged has its basic operation sequence of forming-filling-sealing that starts from the producing and forming containers (like thermoformed plastic trays) or flexible packages (flow-wrap packs or pillow bags), which will subsequently containe or wrap over the products, either solid or liquid to be contained. And following the above processes is the final step of heat sealing.
The 3 typical packaging systems where heating sealign is engaged for forming packaging are FFS machines, either HFFS or VFFS, as well as the recently popular options thermoform-fill-seal (TFFS) as automtaed by packaging system called thermoformers, featuring the machines manfuactured by the well-know German company Multivac.
VFFS systems operate by continuously pulling a flat sheet of film from a roll, which is then formed into a tube around a filling pipe. The product is dispensed through the pipe, filling the formed tube before the machine seals it vertically along the back and horizontally at intervals to create individual packages. This process is particularly efficient for high-speed packaging operations.
VFFS Packaging System Operation Sequence Diagram
Credit: ILHAN, University of Twent
HFFS flows a operational pattern where the film is fed from a roll and formed into a pouch at a horizontal orientation. The product is filled into these pouches, which are then sealed. The horizontal setup is particularly suitable for products that are difficult to handle or that require a more gentle filling process, such as prepared foods, fresh produce, or delicate items.
VFFS Packaging System Operation Sequence Diagram
Credit: ILHAN, University of Twent
HFFS, on the contrary, tend to form lager flexible packaging containers rather than those most flexibke packaging like pillow bag, pouch or sachet. A typical example of packaging container formed by TFFS are the modified atmosphere packaging trays.
TFFS Packaging System Operation Sequence Diagram
Credit: ILHAN, University of Twent
The approach of thermoform Fill Seal (TFFS) packaging involves the use of two distinct materials. One material is thermoformed to create a base such as a cup or tray, while a flexible film serves as the material for the top lid. During the TFFS process, once the base has been filled with the product, a barrier lid made of flexible film is then applied and securely sealed using heat. This method allows for a durable and protective packaging solution, accommodating a variety of contents by effectively combining the rigid structure of the base with the versatility of a flexible top lid.
Similar to our induction seal and cap liner product line, our lidding is used to protect the product from degradation by moisture or oxygen, but the lidding is designed to seal with varying methods, such as conduction, ultra-sonic or induction. Many of these lidding structures are designed to be punctured for the introduction of pressurized super heated water for coffee or other beverage creation, so the seal integrity has to be very robust, but at the same time the lidding is designed to be easy to puncture. Our custom-developed structures protect products against oxidation, external odors and moisture while protecting the unique aromas and flavor profiles intended for the consumer. Our lidding solutions are perfect for branding your application through surface printing.
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