The increasing demand for barrier films in food packaging sector poses a new challenge to the production process of co-extrusion barrier sheets. A production line simply combining three extruders to produce five layers of co-extrusion symmetrical structure film and sheets fails to satisfy the needs of customers for more layers of composite and customization. In this article, we describe how SML successfully copes with this challenge in cooperation with films & sheets manufacturers.
In recent years, with the changing of dietary habits, more and more people buy food of small packages from supermarkets, resulting in growing demands for barrier films for food packaging.
Simply put, when an extruder is used for the extrusion of barrier material EVOH, in addition to barrier material PA , a second extruder for barrier layer is needed. In order to prevent the separation of the barrier layer from the base layer, it is necessary to add a bonding layer on both sides of the barrier layer, so a bonding layer extruder is needed. "Mild processing" and accurate measuring are very important for the processing of barrier material EVOH and bonding material. The remnant materials after trimming and hot forming or the recovered materials from the edges constitute two base layers. If the color of the upper and lower layer is different, two additional extruders are needed. In addition, another two extruders are needed for the extrusion of new materials on the outer layer. Some special films may have "functional layers" (e.g. peeling-off layers) on both sides, which requires an additional extruder. To sum up the above, the production line shall be equipped with seven extruders to produce a "composite film".
In order to reduce the cost of raw materials (EVOH, binder, additives), we may use uncolored homopolymer and packaging material flashes containing low-grade PP , which requires use of the eighth extruder.
In the runners, each material is firstly combined in an order according to the required structure, and then extended to the corresponding die width. It is very important that the transverse thickness distribution of each layer in the film be as uniform as possible. Therefore, a distribution bolt can be used to adjust the distribution of material layer in the distributor, so that the thickness of each layer can be further uniformly optimized.
SML multi-layer co-extrusion calendered sheet production line.
Why we need eight extruders?
In principle, the simplest barrier film production line equipped with three extruders can produce five layers of symmetrical co-extrusion films and sheets. But in todays highly competitive market, film manufacturers will not be satisfied with simple products, they need to find reliable partners to produce products that meet specific requirements in real life.
SML Lanjing is very familiar with the film and sheet production field, and can manufacture "customized" production lines for customers to meet requirements for complex products. Last year, SML Lanjing delivered several 9-layer composite film production lines.
Firstly, in design, in-depth and accurate planning is very important for the layout and connection of production lines.
For example, eight extruders and their respective melt pipes are arranged around the runners. If not carefully designed, the components will be subject to interference with each other.
A basic question is: Since three extruders are enough for the simple production line, why we need eight extruders?
The melt from the die is polished and cooled at the three-roll calendering unit. In this process, in order not to destroy the distribution of materials in each layer of the composite film, the pressure distribution and accurate control on the melt film are very important. Therefore, on-line measurement and adjustment of the actual pressure on the film are of great help to the production of high-quality films. Because of the large amount of raw materials output from eight extruders, it is necessary to design and install post-cooling rolls, which are directly placed in calendering units to cool thick films evenly.
After trimming, the film will be coiled into a large film coil up to 2 meters in diameter by the coiler trolley. If the film coils need to be further processed or hot-formed in the factory, these large film coils have very significant economic benefits. Compared with the small film coils with a diameter of only 1 meter, the number of large film coils can reach four times that of small film coils, which greatly reduces the downtime of rewinding in subsequent processing.
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