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In the production of PVC calendered biaxial stretching film, defects such as excessive shrinkage, uneven thickness of film, perforation and so on are often encountered, which will cause a lot of waste of film, customers can not use or even normal production. How to overcome the above shortcomings is the key to ensure product quality, reduce production costs and improve the competitiveness of enterprises. Today, Xiao Plastic will introduce to you some problems that should be paid attention to in the production of calendered biaxial stretching film and the corresponding countermeasures.

PVC calendering process is still the most economical process in film production. The development of this field has greatly expanded its application field. Various PVC films are still the most popular products, such as geomembrane, greenhouse film, lamp box film, water bed film, grain fumigation film. Although biaxial stretching film is mainly produced by imported production lines, its production technology, namely biaxial stretching film technology, China has its own intellectual property rights. That is to say, the Chinese are using their own technology, using imported production lines to produce all kinds of biaxial stretching film products. Due to the different production lines and products introduced by different manufacturers, the solutions to product quality problems are also different. Therefore, the methods discussed in this paper can only play a role in attracting valuable ideas. The difference between the production of calendered biaxial stretching film and that of common calendered film lies in the follow-up biaxial stretching process. Therefore, there are great differences in environmental requirements, material formulation and process control.

Excessive contraction

Excessive shrinkage includes two meanings: the film shrinks too much in the stretching process to reach the required width of the product; the film shrinks too much in the secondary composite process to meet the required width of the product.

Question 1:

The ductility of plastics can be explained. PVC is an amorphous polymer. When it is subjected to tensile force greater than the yield strength in a certain temperature range, plastic elongation will occur. During the deformation process, the structural units (segments, macromolecules and microcrystals) of the polymer begin to orient due to stretching. With the increase of orientation degree, the interaction force between macromolecules increases, which leads to the increase of polymer viscosity and the tendency of hardening, and the deformation tends to be stable but no longer develops. This phenomenon is called "stress hardening". With the increase of temperature, the ductility of plastics can be further improved, the tensile ratio can be increased, and even some polymers with poor ductility can be stretched. Deformation from room temperature to Tg (**** processing temperature) is usually referred to as "cold stretching". Stress hardening of polymers during stretching will restrict the flow of polymer molecules, thus preventing the further increase of stretching ratio. When the tensile force is relieved, the film with "cold stretching" will be restored to its original state, resulting in larger shrinkage.

In fact, when designing the stretching film workshop, we should consider the problems of heat preservation and air convection in the workshop, because heat dissipation is very fast during the stretching process. The thinner the film is, the wider the stretching is, the faster the heat dissipation is. The author measured that the surface temperature of the film from the drawing inlet to the drawing outlet decreased from 175 to 135 degrees C in a short span of 3 meters, so the temperature of the production workshop should not be lower than 20 degrees C. Workshops are designed with exhaust windows only above and no windows below. In addition, in order to avoid cold stretching, thermal insulation room can be made at the transverse stretching part to ensure the ambient temperature of the stretched film. Reducing the speed ratio of each machine appropriately can also reduce the stretching of the film.

Question 2

Generally, the lower the drawing temperature (above Tg), the larger the drawing ratio, the faster the drawing speed and cooling speed, the higher the orientation degree. When the polymer cooled by the drawing orientation is heated to the temperature above Tg, the shrinkage will occur obviously. The shrinkage film is made according to this principle. In order not to produce heat shrinkage, the product can be treated at ** temperature for a certain period of time, and then cooled to room temperature, so that the product has good stability. This can add a heating device under the back half of the stretcher to maintain the stretching temperature. In addition, warm water can be supplied to the No. 2 tempering wheel to slow down the cooling of the stretching film, which can effectively reduce the thermal shrinkage of the stretching film.

Uneven thickness of thin films

The uneven thickness of the stretched film is mainly caused by two situations: the uneven thickness of the film itself from the calender results in different thickness of the stretched product; in the stretching process, due to air convection, the transverse temperature of the film is different, resulting in different thickness of the film after stretching, resulting in different thickness of the finished roll along the transverse direction.

It can be solved by adjusting the four-roll calender, which can adjust the cross-axle device of the middle roll and the reverse bending device of the lower wheel. In addition, it is necessary to control the storage between the rolls so as to prevent the cold material from entering into the film and affecting the uniformity of the film drawing. The storage diameter of front and upper wheels is not more than 250 mm, the storage diameter of upper and middle wheels is 15-30 mm, and that of middle and lower wheels is 10-15 mm. The reason is that different air convection along the transverse direction causes different temperatures at different points of the film, so that the transverse thickness of the film after stretching is not the same.

According to practical experience, it is easy to get thicker at both ends of the film because the air convection on both sides is faster than that in the middle, so the temperature on both sides is lower. Therefore, film can also be used to shield both sides of the stretcher. If the above method can not solve the problem, the spray pipe can be installed to make up for it. It is required that the nozzle should be duck-billed and the exhaust volume should be regulated by a miniature valve, so that the airflow emitted is more gentle. Large exhaust valves can be properly opened in thin film areas and closed in other places.

Perforation problem

There are three main reasons for perforation: dirty material, inadequate fineness of filter screen, mixing of cold material in four-roll calender, and influence of filling material and plasticizer.

Situation 1

It can be solved by increasing the fineness of the filter and changing the frequency of the filter. If there are still perforations, material replacement is considered.

Situation 2

It can be solved by controlling the storage between the rollers. In addition, the material should be plasticized as far as possible so as not to mix raw materials without plasticization, and the speed ratio of each machine should be controlled to minimize the resulting hole problems.

The perforation of the stretched film is much more obvious than that of the ordinary calendered film, because the tensile force of the ordinary calendered film is unidirectional, smaller than that of the unidirectional tensile film, and can be compensated to a certain extent by calendering device. The tensile force of biaxially oriented films is biaxial, much larger than that of biaxially oriented films. If the film with 2 m width is widened to 5 m, the hole with 215.1 mm tensile ratio will become 215 mm after stretching. In fact, the hole is larger than 215 mm, because the stress around the hole is concentrated and easy to be destroyed. Therefore, it is very important to solve the problem of perforation of stretched films.

Situation 3

Light calcium carbonate is the main filling material. Appropriate amount of calcium carbonate can increase the strength of the film. But when the amount of calcium carbonate reaches 50-60 phr (relative to 100 phr of PVC), the tear strength of the film decreases obviously, and the perforation of the tensile film increases obviously. Therefore, controlling the amount of calcium carbonate is also an important factor to reduce the voids in tensile films. When the amount of plasticizer increases, the voids decrease because of the softening of the film and the increase of the elongation at break. On the contrary, the voids increase.

Precipitation problem

In the process of producing stretching film, precipitation phenomenon sometimes occurs, that is, a layer of white frost on the surface of film or calender roll (or traction roll), which will seriously affect the use of film, and even make the film scrap (such as lamp box film will affect the ink absorption of lamp box cloth due to surface precipitation). From the production situation, the main materials precipitated are lubricants, powder stabilizers and calcium carbonate. In the process of producing PVC calendering film, in order to make the surface of products bright and beautiful, and to improve the fluidity of plastic melt, reduce and avoid the adhesion and friction of equipment, a certain amount of lubricant was added to the plastic. But when the lubricant is used improperly, it is easy to precipitate on the surface of the product when it exceeds its compatibility limit, that is to say, frosting will affect the appearance of the product as well as the ink absorption of the spray-painted advertising cloth; the amount of lubricant is too small to play a lubricating role, and the amount of lubricant is generally less than 1%. The commonly used lubricants are stearic acid, polyethylene wax, etc.

The processing temperature of PVC is very close to its decomposition temperature. When processed at 160 - 200 C, severe thermal degradation will occur, and the products will change color and deteriorate their properties. Therefore, heat stabilizer must be added in the processing of PVC. Many heat stabilizers not only have stabilizing effect, but also have lubricating effect, such as barium stearate and cadmium stearate. If the compatibility of these materials is poor, frost and migration will occur. In order to improve the mechanical properties of the products, reduce the cost and the unit consumption of the resin, the filling materials should be added appropriately. Addition of appropriate additives can reduce the linear expansion coefficient and shrinkage rate of plastics, and improve the heat resistance, flame retardancy and strength of plastics. The amount of additives should be appropriate, too much will precipitate from the film surface. Increasing the fineness of additives can increase the amount of additives, for example, the amount of superfine calcium carbonate is more than that of ordinary calcium carbonate. Coupling agent can also be used to treat calcium carbonate to increase the bonding tightness between calcium carbonate and PVC materials.