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Effect of film blowing process on film properties

2021-03-27 5

Blown film technology

The process flow of blown film is plasticizing and extruding material to form tube blank, which is inflated, cooled, pulled and coiled. In the process of blown film forming, according to the different extrusion and traction directions, it can be divided into three kinds: flat extrusion up blowing method, flat extrusion down blowing method and flat extrusion flat blowing method.

1. Flat extrusion and top blowing

In this method, a right angle die is used, that is, the discharge direction of the die is perpendicular to the extruder, the extruded tube billet is pulled up to a certain distance, and then it is clamped by a herringbone plate. The extruded tube is inflated into a foam tube by the compressed air introduced from the bottom, and its transverse dimension is controlled by the amount of compressed air, and the longitudinal dimension is controlled by the traction speed. The blown thin foam tube can be obtained by cooling and shaping Membrane.

Due to the tenacity and stable traction of the foam hanging on the top of the membrane tube, it can produce a wide range of thickness and wide film, so it is most widely used. The main plastic varieties suitable for up blowing method are PVC, PE, PS and HDPE, especially the wide film of PVC and PE.

2. Flat extrusion blow down method

Down blowing method has a good cooling effect because the tube billet is pulled downward, and the film is drawn into the traction roll by self weight, so the operation of film drawing is more convenient. However, the installation of the machine is too high, and the film bubble is hung on the plastic section which has not been cooled and shaped. When the pulling speed is fast or the production of thick film and the material density is large, it is easy to break the tube film PP, PA, PVDC (vinylidene chloride) and other high crystallinity plastics, to reduce the crystallinity by quenching.

3. Flat extrusion and flat blowing

This method uses a straight die concentric with the screw of the extruder, and the foam pipe and the center line of the die are on the same horizontal plane, which is called flat extrusion and flat blowing method. This method is only suitable for blowing small diameter film products, such as LDPE, PVC, PS film. The flat blowing method is also suitable for the production of blown heat shrinkable films.

Main process factors affecting blown film

There are many factors that affect the stability of blown film processing. Understanding these factors is conducive to the control of the production process, and also to improve the physical and mechanical properties and appearance quality of the film.

1. Temperature control

Temperature control is a key link in film production, which directly affects the product quality. There are two kinds of temperature control conditions: one is that the temperature gradually rises from the charging section of the fuselage to the exit of the nose; the other is that the temperature of the middle and front sections of the fuselage is the highest, and the temperature of the nose is lower. For the equipment with small aspect ratio, especially the transparency, the second method is used. The selection of temperature in each section of the main engine is related to the materials used. If the extruder with diameter of 65 mm, length diameter ratio of 20:1 and compression ratio of 3:1 is used to produce PE film, the temperature control is as follows: feeding section 140-150 ℃, middle section 170-180 ℃, front section 180-190 ℃ and die head 180 ℃.

2. Expansion and traction of membrane tube

The molten material is extruded from the die head to form a cylindrical tube blank. At the same time, air is introduced into the die to inflate the film blank into a film bubble. The formation of the film bubble makes the melt stretch and thin, and combines with the stretch caused by the traction effect to form a biaxial tensile film. In order to prepare films with equal longitudinal and transverse strength, it is required that the transverse blowing ratio (the ratio of the bubble diameter to the die diameter) and the longitudinal drafting ratio (the ratio of the linear velocity of the drawn film tube to that of the non drawn film tube) should be the same. In the actual production, the die with the same specification is often used to make films with different thickness and width by adjusting the blowing ratio and traction ratio.

The blow up ratio is usually controlled at 1.5-3. If the blow up ratio is too small, the transverse strength is low; if the blow up ratio is too large, the process stability, film forming and production continuity are low. The draft ratio is generally controlled between 3 and 7. In production practice, the draft ratio is determined when the film thickness, blowing ratio and film gap are determined. Therefore, adjusting the draft ratio is a way to adjust the film thickness. In order to reduce the film thickness, the traction speed can be increased, that is, the draft ratio should be increased, but the film tube should be prevented from being broken when the draft ratio is too large.

3. Cooling and setting

The cooling of film tube is very important. It should be air-cooled immediately after blowing. If the air cooling is not good, the film will stick and cannot be lifted, or they will stick to each other under the pressure of traction roller. Therefore, the cooling effect and cooling rate will directly affect the quality and output of products. The common cooling method is to use compressed air to blow directly around the membrane bubble through the wind ring. In order to improve the cooling effect, cooling air, secondary air ring, mandrel inner cooling and other technologies are also used.