The number of high-performance films has steadily increased since the introduction of shrink-wrap packaging, but how many layers of film are required to form a stable carrier? Polifilm Extrusion posed this question to itself and compared 11, 13, 55 film performance.
Polyolefin shrink films are of crucial importance for the tertiary packaging of goods in global trade, and only when the packaged goods are placed together on a unit, usually in the form of a European pallet, can the requirements for a stable and secure carrying unit be met One of the main tasks of the shrink film is to firmly bind the carrier units together in order to withstand the test of transport and storage without damage.The mechanical properties of the film are crucial for ensuring the integrity of the carrier unit but , Shrink film in addition to the need for high retentivity, but also have the ability to bind the goods together, the combination of solid enough between the layers.
The task of the shrink film is to combine the goods as a carrier unit to provide a stable connection between the packaged goods and the pallet and to prevent the goods from slipping, displacing, scattering from the pallet and protecting them from dampness influences.
Load cell type and shrink film packaging
When using a shrink film, pay attention to the type of tray and the items to be shrink-wrapped, which requires knowing the shape of the load cell in advance (Figure 1): flat or flush with the tray (Class A) Prominence (category B), or items that need to be packed appear irregularly shaped on the pallet (category C). The weight and shape stability is a major influence on the load cell, so the decisive question is whether items on the pallet Keep the shape stable in size, relatively strong, or very flexible, and deform under strong compressive forces.
Figure 1 Loaded units of different shapes: Straight structure, flush with the pallet (left A), over or under (middle B), extremely irregular product (right C) (Source: Polifilm)
In addition to a wide range of load cells, there are many different shrink packaging products and methods on the market, depending on the packaging process, automatic, semi-automatic shrinkwrappers and manual shrinkwrappers, Pre-stretch and film loading are not the same, so the free arm wrapping machines in fully automatic packaging lines for beverage bottle shrink wrapping are much faster than semi-automatic shrink wrappers. Each system uses a special pre-stretch unit to Determine the pre-stretch, and prepare for a specific job (Figure 2) .When the film is stretched beyond the peak point, to reach the re-stretch will result in an increase in grip area.At this point, the polymer chain toward The direction of stretch (machine direction, MD) thus exhibits maximum grip, which is why custom films are required to achieve the specific properties required for the specific combination of articles and trays in a particular shrink packaging technology.
Fig. 2 Elongation of shrink film, main elongation (A), peak (C) and working area (D) (Source: Polifilm)
Shrink film type
In general, shrink film can be distinguished according to the above requirements in the following types (Figure 3):
◆ low elongation, strong grip;
◆ medium elongation, medium grip;
◆ high elongation.
Figure 3 Schematic diagram of tensile strength-elongation at break for low elongation at break and high hold; medium elongation at break, medium hold, and very high elongation at break (Source: Polifilm)
But is there a lot of layers required for most films? There have been many discussions of how many layers should be available in order to obtain stable load cells in the market. In the past, single- and triple-layer films were used, and now, The 9th floor is the technical standard, but the trend is now 11th or 13th floor and there are already systems capable of processing 27, 33 or up to 55 floors.
Some terms that are still circulating in the market, such as multi-layer or single-layer films, are not well-defined because there is no uniform definition of what is called multi-layer and what is a monolayer film. A key issue is For example, if the film thickness is the same, does the 55-layer film perform better than the 13-layer film? For further discussion, suppose the 13th and 55th layers are the same The latest cast film production line.
Investigate diversity and complexity
Polifilm produces films with thicknesses of 12, 15 and 23μm in cooperation with Slenz Maschinengesellschaft mbH from Lenzing GmbH, Austria and Dow Packaging and Specialty Plastics from Midland, Mich. The SML PowerCast XL production line connects seven Extruder (2x90 / 33; 5x75 / 33), a cooling roll with a diameter of 1600 mm and a width of 5000 mm, and automatic wide slot die (4800) with a maximum speed of 850 m / min. The structure of the feed section is not the same (see Figure 4 layer separation).
Fig. 4 Layer structure of co-extruded 13 and 55-layer films: Extruders A, D to G, each thickness 10% of the total thickness, Extruders B and C, thickness 25% of each layer (source: Polifilm)
Thickness As shown in the figure, a shrink film of 500 mm in width is processed using the same process parameters.The tensile and holding forces in the formulation are different to reflect the needs of commercial use (Table 1). Dow Polyethylene Film such as The Elite AT 6111, an ethylene-octene copolymer engineered with Elite AT technology, meets the requirements of high performance films.
Table 1 Summary of Formulas Tested on Production Lines 12 and 55 (Source: Polifilm)
test
The FPT-750 thin film test system from ESTL, Deerlijk, Belgium, was used to test the film (Figure 5). The maximum possible pre-tensile elongation at break, puncture and seismic performance were tested at various pre-stretch and test speeds. Conventional 4000 mm / s coating speed.
Figure 5 Test Equipment and Procedures: FPT-750 from ESTL (left), and actual puncture and tray tests (right) on the test tray (center) (source: Polifilm))
Test trays were tested on a rotary table topper from Highlight Industries, Wyoming, MI, USA The films were coated using a standard coating procedure and based on the grip on the corners and the surface of the tray, The puncture resistance of the shrink film selected for the program was evaluated for film properties.
In addition, a drop weight test (ASTM test, 1709-01) was also performed on the film in the laboratory and the Elmendorf tearing test was conducted in the machine direction.
Fig. 6 Tensile strength-elongation at break for 13-layer and 55-layer films a) flowable films 12, 15, and 23 μm in thickness b) high strength films 12 and 15 μm c) high elongation and elongation films, Thickness 15, 23 μm (Source: Polifilm)
Evaluation
Using the production and test methods described above, 3800 data points were measured and the formulations were evaluated according to Table 1. The characteristics of the 13th and 55th layers of different types and thicknesses (puncture resistance, shock resistance, tear strength, elongation at break Length, drop weight performance, Elmendorf tearing properties, application tests and grip, etc.) were compared and found no significant difference in statistical bias, regardless of level 13 or level 55.
Similar results were obtained by comparing the tensile strength and the elongation at break documented in the Limit Test Procedure In the 'Limit Curves' of the 13,55-layer film line, a comparison of the thickness, a formulation of a flowable film 6a) and strong film.
No significant differences were found in the elongation at break or the holding force of the films of different layer structures.According to the holding force corresponding to the film thickness, a standardization was made to obtain a curve according to the thickness variation so that films having different thicknesses There was also no significant difference in films with high elongation at break for different layer structures (6c).
Figure 7. Tensile Strength-Elongation Graphs of Highly Elongated Films of 15, Thick, 11, 13, and 55 Layers (Source: Polifilm)
In a further test, the performance of a similar recipe on a SML 11-layer production line was investigated for similar formulations, where again the recorded property values were almost identical to those of the 13th and 55th lines (Figure 7).
in conclusion
Extensive tests have shown that shrink films for fixed load cells can be produced on the latest generation of cast film lines 11, 13 and 55, with the same thickness at the same thickness, and in this work also found The success depends on the raw materials, film formulations and process parameters used, and the ability to control the line and its cleanliness are also important to ensure consistent quality.
Determining performance is the recipe for a particular application, depending on the product. In order to serve the needs of a robust, sustainable load cell for the growing transportation market, it is essential to understand the packaging process and the consistent quality. Less.