In the world of flexible packaging there is a close relationship between the packaging machine, the packaging material and the product to be packaged; this interrelation is called PACKAGING APPLICATION and implies that these three variables should not be moved or changed without testing, since the change in one of them affects the behavior of the interrelation, which can cause very critical problems in the production processes.
The sequences of operations in packaging machines can be described as a series of basic operations. Said basic operations are for example, the transport of the packaging material through the machine, the formation of the packaging material into the final form of the package, such as by folding it, the dosing and filling of the product to be packaged and the closing or sealing of the filled package, such as through the thermosealing. There are some basic operations for all kinds of packing machine.
The objective is to find out the relevant properties of the packaging materials in each of the basic operations and determine a range of numerical values for them, so that there is a guaranteed productive process that flows smoothly.
In this first part we will analyze the process in a vertical neck forming machine, vertical forming, filling and sealing machine (VFFS), which is one of the most commonly found systems. In this process we will emphasize the tightness of the packaging seals, which are very typical of plastic materials and laminated materials, which is very important to maintain the quality of the product.
The operations carried out on a packaging machine should be done without interruptions, since each stop of the process affects the total cost of packaging. To achieve this, one must know the reasons for the possible problems, and these must be caused either by the packaging machine, by the packaging material or by the product being packaged.
The packaging process in packaging machines always consists of a sequence of operations, and from now on we will refer to basic functions, ranging from the dragging of the roll of flexible packaging material, cutting of the material, forming of the packaging, filling and sealing of the packaging. Therefore, each packaging machine must be seen as the sum of the specific basic functions that it performs.
The basic functions have to be related to the packaging material and the product to be packaged involved and its characteristics, taking into account each functional step. The research in the field of packaging technology has to be raised by itself in the following objectives:
Through the correlation of the basic functions with the properties of the packaging material, the term “RUNS WELL ON MACHINE” of flexible packaging materials has been used, and can be defined as which basic functions and under which operating conditions (such as machine speed), a packaging material can run smoothly. From the above it can be deduced for which packing machine a packing material is appropriate, if the machine has been correctly adjusted.
On the other hand, from such treatment of packaging operations, conclusions can be drawn for the manufacture of the packaging machines. This can be designed from the beginning in such a way that they are constructed to match the properties of the packaging material and products to be packaged.
One of the types of packaging machine, more extensively studied in its relationship with the packaging material is the VFFS, very widely used as we had already mentioned. Single-layer or multi-layer plastic films and coated papers run well on this type of machine.
A forming, filling and sealing machine can be observed dragging or advancing the material by the intermittent movement of the horizontal jaws (MIM).
In the next chapter we will continue talking about the packaging application, we will see the strip-dragging system and its difference from the jaw-dragging one. We will emphasize the two most important factors that affect the processing on VFFS machines: the tension of the material during unwinding and the drag forces.
Continuing with the different dragging systems in vertical neck forming machines, we will see the belt dragging system and its difference from the jaw dragging system. We will emphasize the two most important factors that affect the processing on VFFS machines: the tension of the material during unwinding and the drag forces.
In a second type of vertical machines the packaging material is pulled or dragged with the help of belts or bands instead of the jaws. Thus, the two machines differ in how the basic functions Us4 and Us9 are performed, in the current blog, compares both types of machine. Both are intermittent in operation and in each cycle the material advances the length of a bag.
All previous studies of the performance of packaging materials on vertical neck-forming machines assume that the most important factors affecting their processing are: tension during unwinding and drag forces. These can induce the formation of longitudinal folds, instability of the material and excessive tension. The results can be transverse and/or longitudinal wrinkles and distortion in the printed design; with high drag forces the material can break.
The measure of this dragging force is based on the fact that the fixing of the forming neck can be related as a beam supported on both sides and loaded in the middle and whose bending is directly proportional to the supplied dragging force that produces only small deformations. This condition is certainly satisfied in this case. The bending of this part therefore provides a measure of the drag force. This is commonly measured with strain gauges that serve this purpose very well.
The drag force is very dependent on the rigidity of the material to allow it to deform (bending stiffness (BS)) and increases markedly with it.
Extensive studies by Hohmann and Woyke confirmed this. The monolayer plastic films gave the lowest values of drag force (âia ™ ), the highest values were given by coated papers and papers laminated with aluminum and polyethylene and in the middle laminated plastics. For a problem-free operation the study showed that values of 80 x 10 -3 mNm2/m was the limit for bending stiffness. The forming neck on the machine was made of aluminum-bronze with an angle â° of 17 degrees.
A clear connection between the drag force and the friction force on the packing material in the machine, especially on the forming neck, could not be established. The influence of friction seemed to be masked by the greater influence exerted by bending stiffness (BS). However, the influence of friction was obvious with the processing of packaging materials with values close to the same (BS), showing drag force values measured for low density polyethylene films with different friction coefficient values, with a neck with an angle âatº of 15 degrees; to achieve good machine operation, the maximum value for the friction coefficient was 0.25; otherwise, there were gaps in the ventral seal, over stretching of the packaging material and abrasion of the inks in case the material was printed on the surface.
Bending stiffness (BS) and friction are therefore two properties of the packaging material that play a decisive role in the operation of VFFS machines, with bending stiffness being the most influential. Stress to tension, however, also plays an important role, since as mentioned in the study, there were breaks of the material repeatedly. Trying to relate the stress to the tension to the drag force, it could be assumed that the material will not break when the ratio is greater than 1, the studies showed that minimum values should be between 2.5 – 4.0 so that there is no rupture of the material.
The question to all the above is the following: if the tensile stress of a material cannot be increased, nor the bending stiffness decreased, what changes can be made to the baling machine to reduce the drag force?
Firstly, flatter necks, i.e. larger neck angles could be a solution. More inclined necks have the advantage of guiding the material better; experience has shown, however, that stiffer materials, in particular coated and paper-laminated papers, cannot at all or only with difficulty be processed into very inclined forming necks.
A second possibility to reduce the drag force is in the selection of the material for the forming neck. For example, in the case of an LDPE film, the drag force for an approximate angle of 46 degrees, between a bronze-aluminum forming neck and a molded resin one, would be
145.0 ± 2.2(N/m) vs 79 ± 1.8(N/m) respectively.
Problems unrelated to the drag force are caused by friction between the packaging material against the filling tube and can occur on machines of any type of drag. The critical point is at the bottom of the filling tube, where the tube of already formed material is pushed rather than pulled down.
In the next chapter we will continue talking about the packaging application, but focusing directly on the sealing area and a perfect understanding of the variables involved, and thus finish this correlation between packaging machine – packaging material – product to be packaged.
Usually the sealing of the seams in the packages made of flexible materials is done through thermoplastic materials by means of heat sealing.
For this purpose the temperature, the pressure and the residence time must be adjusted depending on the packaging material. In most contact heat systems such as constant heat jaws or the impulse sealing system, the heat from the sealing tool or jaw is applied from the outside through the packaging material to the sealing área or seam.
By other methods the heat is transferred directly to the inner part of the packaging material or is generated inside the material by high frequency or ultrasonic welding.
When a hot tool or clamp is used for sealing, the required residence time, (t), essentially depends on the temperature of the clamp, (T), and the thickness of the packaging material (d). The following two relationships apply:
T = (const./ t) + Ts (1)
Where Ts is the temperature required to form the seam or seal at the contact surface (>=at the melting temperature), and
t = constant. X (d2c∂/∑) (2)
where c = specific heat, ∂ = density and ∑ = thermal conductivity of the packaging material
The faster a packing machine runs, the shorter the time available for the thermosealing and therefore the jaw must be heated more in line according to equation (1). This heating, however, is limited by the thermal damage that may be caused to the packaging material such as, for example, burning of the outer film, delamination of some of the layers of the lamination, or deformation of the plastic films in the sealing área.
It can be deduced from equation (2), that by doubling the thickness of the packaging material, the residence time for a proper seal is quadrupled. If the heat can be transferred to the sealing area from both sides, only one third of the sealing time is required, compared to if the heat is transferred from only one side.
The findings of comparative studies between heat sealing and ultrasonic sealing showed interesting data, since in all cases ultrasonic sealing showed higher hot tack values than contact heat sealing while the resistance of heat sealing was higher.
The effect of contamination with product in the area with respect to the sealing force was also analyzed in the study. The packaging material was contaminated with shampoo, silicone oil, milk, coffee powder and flour, and subsequently sealed.
The above turned out that in all cases the sealing force was lower at a higher or lower level compared to the forces of the seals that were not contaminated.
The nature of the contamination and the material determined the extent of the reduction in the sealing force.
In packages whose product is heavy (>= 500 g), the residual heat in the seals can cause failures in the package (seal opening) unless the machine is slowed down to allow the seals to cool down. Ultrasonic sealing does not have hot tack problems, because the tools or jaws remain cold and less heat has been applied to the packaging material. Ultrasonic seals exhibit full strength almost immediately after sealed.
We need to differentiate between two circumstances in the case of gas or steam transportation due to the loss of tightness in the packaging:
If there is a difference in the total pressure between the inner part of the package and the outer atmosphere, both gases and steam will be transmitted as one flow.
If the total pressure inside and outside the package is the same, there may still be a difference in partial pressure for a gas or vapor; this will result in diffusion.
The first usually occurs with vacuum packaging and occasionally with modified atmosphere packaging, the second, as a rule with modified atmosphere packaging and where the packaging must protect its contents from gaining or losing moisture.
There is a complex interrelation between the packaging machine and the material to be worked, and we have dealt with only part of it in these three sections. The example we have taken of a vertical machine, shows us how through a systematic analysis, it is possible to determine the properties that affect the operation of packaging materials and gain indications for the design of packaging machines.
The sealing of thermoplastic materials not only provides strong seals, but also leak-proof seals and seams, which is decisive for maintaining the quality of the packaged product. Depending on the purpose of the packages, leaks can be increased or completely eliminated during their manufacture.
It is for this reason that in order to fully comply with the packaging application, a close cooperation of the manufacturers of the packaging materials, packaging machines and product to be packaged is highly recommended.
In the next blog we will talk about ultrasound sealing, an aspect that has recently been taking relevance in the sealing of materials for flexible packaging, we will make an analysis of the variables involved, and thus finish this correlation between packing machine – packing material – product to be packed.