There are usually two methods for producing plastic films, namely cast film and blown film. The production process of cast film is to melt the raw material resin with an extruder first, and the molten resin is cast through the die head onto a cooling roller with a smooth surface and quickly cooled into a film. After thickness measurement, traction, corona treatment, and flattening, the edge material is cut off and rolled into a film roll.
With the tightening of environmental protection supervision, brand owners not only hope that packaging can extend the shelf life of food, but also hope that the production process of packaging can reduce costs. As a result, multi-layer co-extruded cast composite film came into being and made great progress.
Processing technology of multi-layer co-extrusion composite cast film
Compared with other composite methods, cast film is less likely to delaminate, and different layers can have different colors; the materials used to process co-extruded composite cast film need to consider the adhesion between resins; cast film production equipment is developing towards large-scale, rapid, efficient and automated directions. The increasing number of film layers is one of the development trends of extruded cast film.
Multi-layer co-extruded composite cast film is a cast film with a multi-layer structure produced by co-extrusion of multiple extruders. It uses more than two extruders to combine different, molten resins inside the die or at the outlet of the die, and through one-step processing, it is manufactured into a multi-layer composite film. Its equipment is mainly composed of multiple extruders, multi-channel distributors, T-die heads, cast systems, traction systems, winding systems and other parts.
Because multi-layer co-extruded cast film can achieve different materials and different performances in each layer. Therefore, it has a variety of characteristics in combination. Compared with other composite methods, cast film is less likely to delaminate, and different layers can have different colors
Solutions to common problems in cast film production
Cast film process is a common film production process. The film produced by this process has the characteristics of non-stretching and non-directional, and is often used in composite substrate film and wrapping film applications. In general, cast film refers to polyolefin cast film, which is specifically divided into cast polypropylene film (CPP) and cast polyethylene film (CPE). CPP is characterized by high transparency, good flatness, good rigidity, moisture resistance, oil resistance, high temperature resistance, and good smoothness. These characteristics determine that it is widely used in the inner substrate of cooking-resistant composite bags and vacuum aluminum-plated film substrates. CPE is characterized by softness, high flatness, good moisture resistance, good transparency, and excellent heat sealing performance. It is widely used in the production of inner substrates of composite films.
The fundamental reason for the uneven transverse thickness of the film is the uneven discharge amount of the die. The specific influencing factors and some solutions are as follows: ① The die lip gap is improperly adjusted, which requires partial adjustment or complete readjustment; ② The lateral temperature distribution of the die head is uneven, and the heater needs to be checked for faults; ③ There are debris in the mold cavity, which causes logistics disorder and uneven discharge, and it needs to be cleaned regularly several times a year; ④ The melt pressure is too high or fluctuates too much, which will cause unstable extrusion and affect the uniformity of thickness. It can be improved by adjusting the temperature, extrusion speed and adjusting the raw materials; ⑤ The position and direction of the air knife are inappropriate or the air outlet is uneven, resulting in uneven cooling and uneven thickness; ⑥ Too much or uneven addition of recycled material. Generally, the amount of recycled material added should be less than 15-20%, and it should be added evenly.
Crystal points are caused by the failure of plasticization of a small amount of resin with too high molecular weight in the raw materials. Static mixers and other methods to increase shear and melt pressure can be added to the equipment. The following situations are prone to crystal points: ① The barrel temperature is not enough or too high. If the temperature is low, the raw materials cannot be completely plasticized; if the temperature is too high, the melt pressure is small, and the shear force is small, which is not conducive to the plasticization of polyolefin plastics; ② The melt pressure is small, so the shear degree is small, and the plasticization is poor. The number of filter layers and mesh can be appropriately increased; ③ The extrusion speed is slow and the shear is insufficient; ④ The raw materials used are poor. It is necessary to select raw materials of appropriate grades and pay attention to whether there are materials with large differences.
The root cause of the film roll having ribs is uneven thickness. Even if it is very small, it may cause obvious ribs after accumulation, affecting the appearance and use. It can be analyzed and solved from the following aspects: ① The film thickness is uneven, find out the influencing factors and adjust them carefully; ② The winding tension is too large and the film roll is too tight; ③ The unstable melt pressure causes uneven thickness; ④ The air knife position and direction are improper or the air outlet is uneven, causing uneven thickness;
⑤ The shrinkage rate of the film is large after placement, and it needs to be adjusted from the raw material production process and film roll storage.
Polyolefin is a crystalline polymer. The key to improving transparency is to reduce the crystallinity and make the spherulites small. The main influencing factor is the resin raw material. Adjustment of the cast film production process can only improve the transparency of the film to a certain extent. The measures are as follows: ① Reduce the temperature of the quenching roller to achieve a better quenching effect of the melt film; ② Shorten the distance between the die and the chilling roller to reduce crystallization; ③ Slow down the extrusion speed to allow the melt film to be fully cooled on the chilling roller.
Multi-layer co-extrusion casting machine recommendation:
https://www.potop-polymer.com/lab-cast-film-machine/coextrusion-flow-film-stretching-machine-cpp.html
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