The filtration system is an indispensable part of the plastic film extrusion process. Conventional melt filtration devices may cause pressure, flow velocity fluctuations, production shutdowns, and the like. For high-speed extruded film production processes, product defects or production outages can have a huge impact on production efficiency as well as on company benefits. The constant pressure and fully automatic filtration system successfully developed by Gonos Plastics Technology GmbH in Germany solves this problem.
Single, biaxially stretched film production overview
The production of a stretched film is that the plastic particles are melted in an extruder and then extruded through a long hole or a circular die, and then stretched by a stretching apparatus into a film product.
The plastic particles enter the extruder through a corresponding compounding and mixing system. In order to form a uniform and melted raw material, the plastic polymer usually needs to be filtered and processed into a film. Before the stretching process, the film roll must be cooled below the melting point. In order to ensure the correct operation of the stretching, the temperature needs to be further reduced to a temperature slightly lower than the melting point of the crystal by means of rolls, heaters or heaters. In the next molding process, the film can be reheated and relaxed before being rolled in a stress-free condition cooled to room temperature.
The heating-mechanical treatment of the stretching of the film leads to the reorientation of the plastic molecules. This process can increase the strength, give the material a higher elastic modulus and lower tension and better stability.
In general, there are some differences between one-way and two-way stretching. In the uniaxial stretching process, the injected film material enters the drawing unit composed of a plurality of rolls after forming and the roll rotation speed is different. As a result, the thickness of the film can be correspondingly changed as the length changes. After uniaxial stretching, the tear strength of the film can be increased, so the braiding of cords, packaging tapes or bags is produced using this process. Thickness of film produced between 25 and 800 μm
In a uniaxial stretching device, the change in the shape of a certain volume of material is achieved by uniaxial stretching. In the two-step biaxial stretching, the warp prestretched film is stretched in the orthogonal direction with the same length as the extrusion direction. With the development of linear motors, the above-mentioned stretching process can be flexibly connected together and high-efficient molding of longitudinal and transverse biaxial stretching becomes possible. This is biaxial stretching called “synchronous” stretching without complicated transfer and Chain structure. Manufacturers of such simultaneous stretching apparatuses believe that the higher the uniformity of polymer stress, the higher the elastic modulus, tear strength, and tensile strength that can be imparted to the product.
Biaxial stretching also gives the final product better uniformity and excellent transparency. Because of its characteristics, it is often used in the production of packaging films, films, recordings, and video tapes. The thickness of the product is between 1 and 400 μm.
During the biaxial stretching process, the produced hose passes the subsequent reheating process and is stretched by blowing. During this process a new bubble is created. This production process is also called the "double bubble" production process.
Depending on the process and the materials used, the draw ratio can reach 50:1. Under such stretching conditions, very tiny particles may become breaking points. Some stretched films are only a few microns thick, and the importance of filtration in the production of such films is self-evident. When the melt leaves the nozzle, there cannot be the presence of particles 2-3 times larger than the film thickness. In addition, larger particles can have important effects on factors such as surface quality and optical properties.
Different Filtration Systems Discontinuity and so-called continuous screen changers The production of uniaxially stretched films requires relatively little polymer purity and the processing accuracy is between 40 and 100 μm. A plunger or double plunger screen changer is commonly used.
In a double-plunger filter, the melt stream splits into two strands, each passing through a plunger attached to a stop plate. After filtration, the two streams recombine into the mold. Each plunger has a hole plate. In order to facilitate the network change, the plunger is first removed by the hydraulic cylinder and the melt flow channel is blocked by the plunger. The production continues during the network change, but the filter area is only The original half, so that the melt pressure is much higher than the original.
In the production of thin films using these quasi-continuous double-plunger or slide filter, the following problems are mainly caused: When the screen is changed, a large pressure rise will occur because a filter hole will be completely closed briefly. As a result, shear enhancements may affect the flow characteristics of the melt. These changes can often affect the thickness and purity of the film and can even cause the film to break.
In addition, the plunger structure used in the system is not easy to clean, including the decomposition particles produced by the scorching in the plunger when the screen is changed. In the production process, in order to ensure high processing accuracy, a long sealing structure is often used, and dead spots are formed in the melt flow channel. During cleaning, high cleaning material losses will occur due to the effective cleaning of all filter holes, which will lead to a decrease in output.
Plungers or skateboard filters, even with very good filtration capabilities, are not feasible in the manufacture of biaxially oriented films. If the tension device is interrupted due to the frequent replacement of the filter screen, it will have a great impact on the efficiency.
