Attention Problems Of PVC Rolling Biaxial Tensile Film
- Feb 21, 2018 -

In the PVC rolling biaxial tensile film production, often encountered such as shrinkage is too large, thin film thick, perforation and many other defects, serious film will cause a lot of waste, customers can not use or even normal production. How to overcome the above defects is the key to ensure product quality, reduce production cost and improve enterprise competitiveness. This paper introduces several problems and corresponding countermeasures in the production of biaxial stretching film.

PVC Calendering Technology is still the best and most economical process in thin film production, the newest development of this field has greatly expanded its application field. All kinds of PVC film is still the most popular products, such as geomembrane, plastic film, light box film, water bed film, food fumigation film. While the two-way tensile film is mainly imported production line for large-scale manufacturing, but its production technology, namely biaxial tensile film technology, China has independent intellectual property rights. In other words, the Chinese are using their own technology, the use of imported production lines to produce various kinds of biaxial tensile film products. Because the manufacturers introduced different production lines, the products are not the same, so in the solution to the problem of product quality are not the same. Therefore, the method of this article can only play a role in the discussion. The difference between the production of two-way tensile film and the ordinary rolling film lies in the subsequent biaxial stretching process, therefore, the requirements of the environment, the formulation of materials, the control of the process are very different.

Too much shrinkage.

Contraction too large contains double meaning: the film in the tensile process of shrinking back too large to reach the width of the product requirements, the film in the two-time composite process of heating back shrinkage too large, exceeding the standard requirements, the width of the product requirements.

Problem 1 can be explained by the ductility of the plastic. PVC is amorphous polymer, in a certain temperature range is greater than the yield strength of the tensile effect, the formation of plastic extension deformation, in the deformation process of polymer structural units (chain segment, macromolecules and Microcrystalline) due to tensile and start orientation. With the increase of orientation degree, the force between macromolecules increases, which leads to the tendency of hardening of polymer viscosity, and the deformation tends to stabilize and no longer develops, this phenomenon is called "stress hardening". By increasing the temperature appropriately, the ductility of the plastics can be further improved, and the tensile ratio may be increased, and even some less malleable polymers can be stretched. Usually the deformation near room temperature to TG (minimum processing temperature) is referred to as "cold stretching", which will limit the flow of polymer molecules in the process of tensile stress hardening, thus preventing the further improvement of draw ratio. The "cold stretching" of the film occurs after the tensile force is lifted, resulting in a larger retraction.

In fact, in the design of tensile film workshop should consider the workshop warm, air convection problem, because the film in the tensile process of heat dissipation is very fast, thin film, stretching wider, the faster the heat dissipation. The author measured that the film from the tensile inlet to the stretch exit of the 3m surface temperature from 175 ℃ to 135 ℃, so the temperature of the production workshop is generally not less than 20 ℃. The workshop is designed to leave only the exhaust windows above, and the bottom is not to leave the window. In addition, in order not to have the cold stretching phenomenon, the greenhouse can be made in the transverse stretching position to ensure the ambient temperature of the tensile film. Reduce the speed ratio of each machine properly, also can reduce the tensile of the film. For problem 2, the lower the general tensile temperature (above TG), the greater the tensile ratio, the faster the tensile speed and cooling speed, the higher the orientation, the tension-cooled polymer will be heated to more than TG temperature, will obviously occur back shrinkage, shrinkage film is made according to this principle. In order not to produce heat shrinkage, the product can be kept stretching in the case of the highest temperature treatment for a certain period of time, and then cooled to room temperature, so that the product is good stability. This can be added to the bottom half of the drawing machine to maintain the tensile temperature; In addition, it can also give the No. 2nd temperature refining Wheel pass warm water, so that the tensile film cooling slowly, so that can effectively reduce the thermal shrinkage of the tensile film.

Thin film Thick

The Thick of tensile film is mainly caused by two kinds of conditions: from the calender out of the film itself thick uneven, resulting in tensile products thick different; In the process of stretching, because of the air convection, resulting in different film transverse temperature, resulting in the tensile film thick different, resulting in the final product volume along the transverse thickness of different. You can adjust the four-roll calender to solve, the concrete can adjust the shaft crossover device, the next round of the reverse bending device, but also to control the rollers between the deposit, so as to avoid the cold material into the impact of the film tensile uniformity. The front wheel, the upper whorl deposit diameter is not more than 250mm, the upper whorl, the middle wheel saves the material 15~30mm, the middle wheel, next round saves the material 10~15mm. The º is due to different air convection along the transverse, resulting in different temperature of the film, so the transverse thickness of the film is inconsistent.

According to practical experience, the general thin film ends easily thickened, which is because the two sides of the air convection than in the middle of the faster, so both sides of the temperature is lower. Therefore, it can also be used to mask the two sides of the widening machine. If the above method can not solve the problem, you can install the nozzle to make up. In the distance from the film 150mm, the horizontal placement, the nozzle for the duck-mouth type, there is a micro-valve to adjust the exhaust, so that the air flow is relatively soft. Thin film where the large exhaust valve can be properly opened, the rest of the exhaust valve will be closed.

Perforation problem

Perforation of the main production of the following 3 kinds of situation: the material is too dirty, the filtration network fineness is not enough; four-roll calender has cold material mixed with the effect of filling materials and plasticizer.

The condition 1 can be solved by increasing the fineness of the filter mesh and changing the frequency of the filter screen; The situation 2 can be solved by controlling the deposit between the rollers, and the material should be plasticized as much as possible so as not to mix with the raw meal, and to control the speed ratio of each machine, and minimize the hole problem.

The perforation of the tensile film is much more obvious than the ordinary calendering film, because the tensile of the ordinary calendering film is one-way, smaller than the unidirectional tensile film, and can be compensated to some extent by the calendering device. The biaxial tensile film is tensile, which is much larger than biaxial stretching, 2m wide film if Lacoine to 5m, then the stretch ratio of 215.1mm hole after stretching becomes 215mm. In fact, the hole is larger than 215mm, this is because the stress concentration around the hole, vulnerable to damage. Therefore, it is very important to solve the problem of perforation of tensile film.

The situation 3 filling material mainly uses the light calcium carbonate, the amount of calcium carbonate can increase the strength of the film, but when the amount of calcium carbonate reached 50~60 (relative to 100 PVC), the film tearing strength was significantly reduced, when the perforation of the tensile film increased significantly. Therefore, controlling the dosage of calcium carbonate is an important factor to reduce the pore of tensile film. When the dosage of plasticizer increases, the hole decreases because the film becomes softer and the elongation at break increases, whereas the hole increases.

Precipitation problem

In the process of producing tensile films, sometimes the precipitation phenomenon, that is, in the film surface or calendering roller (or traction roll) on the formation of a layer of white frost, serious impact on the use of the film, or even film scrap (such as the Light box film will be due to the surface of the light box cloth ink absorption). From the production situation, the material precipitates mainly lubricants, powder stabilizer and calcium carbonate. In the production of PVC calendering film process, in order to make the product surface smooth, beautiful, but also to improve the fluidity of plastic melt, reduce and avoid the adhesion of equipment and friction, and in the plastic to add a certain amount of lubricant. However, improper use of lubricants, more than the compatibility limit, easy to precipitate on the surface of the product, that is, usually called the cream, affecting the appearance of the product, but also affect the ink-jet printing advertising cloth of the inking; the amount of lubricant is too small to be lubricated, the amount of lubricant is generally less Commonly used lubricants are stearic acid, polyethylene wax and so on.

The processing temperature of PVC and its decomposition temperature is very similar, when processing under 160~200℃, will occur intense thermal degradation, product discoloration, performance deterioration. Therefore, in the process of PVC processing, heat stabilizer must be added. Many heat stabilizers not only have stable effect, but also have certain lubrication effect, such as barium stearate, cadmium stearate. If the compatibility of these materials is poor, there will be frost, migration and other phenomena. In order to improve the mechanical properties of the products, reduce the cost and the single consumption of resin and the right amount of filling materials. Adding the right amount of additives will reduce the linear expansion coefficient of plastics and product shrinkage, and can improve the heat resistance, flame retardancy and strength of plastics. The dosage of the additive should be appropriate, too much will precipitate from the film surface. Increase the fineness of the additive, can increase the dosage of additive, such as superfine calcium carbonate than ordinary calcium carbonate dosage. can also use coupling agent to treat calcium carbonate and increase the combination of calcium carbonate and PVC material tightness.