In the production of PVC calendering biaxial stretch film, defects such as excessive shrinkage, uneven film thickness, and perforation are often encountered, which will cause a large amount of waste of film, and customers can not use or even normal production. How to overcome the above defects is the key to ensure product quality, reduce production costs and improve the competitiveness of enterprises. Today, small plastic will introduce to you several problems that should be paid attention to in the production of biaxial stretched film and corresponding countermeasures.

PVC calendering process is still the best and most economical process in film production, and the latest development in this field has greatly expanded its application field. All kinds of PVC film is still the most popular products, such as geomembrane, greenhouse film, light box film, water bed film, grain fumigation film. The bidirectional stretch film is mainly imported production lines for large-scale production, but its production technology, that is, bidirectional stretch 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 all kinds of bidirectional stretched film products. Because the production lines introduced by each manufacturer are not the same, the products are different, so the methods to solve the product quality problems are also different. Therefore, the method discussed in this article can only play the role of throwing bricks to attract jade. The difference between the production of calendered biaxial drawing film and the production of ordinary calendered film lies in the subsequent biaxial drawing process. Therefore, there are great differences in the environmental requirements, the formulation of materials and the control of the process.

The shrinkage is too large

The shrinkage is too large: the film in the stretching process is too large, can not reach the width required by the product; Film in the secondary composite process of heat retraction is too large, more than the standard requirements, can not reach the width of the product requirements.

Problem 1:

can be explained by the ductility of plastic. PVC is an amorphous polymer, in a certain temperature range under the tensile force greater than the yield strength, it will produce plastic extension deformation, in the deformation process of polymer structural units (segments, macromolecules and microcrystals) due to stretching and begin to Orient. With the increase of the degree of orientation, the force between the macromolecules increases, causing the viscosity of the polymer to increase and the tendency to harden, the deformation also tends to stabilize and no longer develop, this phenomenon is called "stress hardening". Appropriately increase the temperature, the ductility of the plastic is further improved, the tensile ratio can be increased, and even some polymers with poor ductility can be stretched. The deformation from room temperature to Tg(the lowest processing temperature) is usually called "cold stretching". After the stress hardening of the polymer during the stretching process, the flow of polymer molecules will be restricted, thus preventing further improvement of the tensile ratio. Films that undergo "cold stretching" revert to their original state when the tensile force is relieved, resulting in a large retraction.

In fact, when designing the stretched film workshop, it is necessary to consider the warm and air convection problems of the workshop, because the heat loss of the film during the stretching process is very fast, the thinner the film and the wider the stretch, the faster the heat dissipation. The author measured that the surface temperature of the film from the stretching inlet to the stretching outlet was reduced from 175 бу C to 135 бу C within a short period of 3m, so the temperature of the production workshop should generally not be less than 20 бу C. In the design of the workshop, only exhaust Windows are left above, and no Windows are left below. In addition, in order to avoid cold stretching, a greenhouse can be made in the transverse stretching part to ensure the ambient temperature of the stretched film. Appropriate reduction of the speed ratio of each machine can also reduce the stretching of the film.

Question 2

Generally, the lower the tensile temperature (above Tg), the larger the tensile multiple, the faster the tensile speed and cooling speed, the higher the degree of orientation. When the polymer cooled by the tensile orientation is reheated to a temperature above Tg, it will obviously shrink back, and the shrink film is made according to this principle. In order not to make the product produce thermal shrinkage, the product can be treated at the highest temperature for a certain period of time while maintaining the tensile temperature, and then cooled to room temperature, so that the product has good stability. This can be added under the back half of the stretching machine heating device, in order to maintain the stretching temperature; In addition, warm water can be given to the No. 2 temperature wheel to cool the stretched film slowly, so that the thermal shrinkage of the stretched film can be effectively reduced.

Uneven thin film thickness

Uneven thin film thickness is mainly caused by two situations: from the calender out of the film itself uneven thickness, ultimately resulting in the thickness of the stretched product is different; In the process of stretching, due to air convection, the transverse temperature of the film is different, resulting in different thin film thickness after stretching, and ultimately lead to different transverse thickness of the finished material roll along the finished product.

can be solved by adjusting the four-roll calender, specifically can adjust the roller axis crossing device, the lower wheel anti-bending device; In addition, it is necessary to control the storage between the rollers, so as not to have cold material into the film to affect the uniformity of tensile. The diameter of the front wheel and the upper wheel is not more than 250mm, the upper wheel and the middle wheel are 15~30mm, and the middle wheel and the lower wheel are 10~15mm. The situation is due to the different air convection along the transverse everywhere, resulting in different temperatures at each point of the film, so that the transverse thickness of the film after stretching is inconsistent.

According to practical experience, the general film at both ends is easy to become thick, this is because the air convection on both sides is faster than the middle, so the temperature on both sides is low. Therefore, it can also be used to block the film on both sides of the widening machine. If the above method can not solve the problem, you can install a jet tube to make up. In the distance of 150mm from the film along the lateral placement, the nozzle is required to be duck-billed, there is a micro-valve to adjust the exhaust volume, so that the air flow out is softer. The thin film can be properly opened in the large exhaust valve, the rest of the exhaust valve is closed.

perforation problem

perforation is mainly produced in the following three cases: the material is too dirty, the fineness of the filter is not enough; Four roller calender with cold material mixed; The effect of filling materials and plasticizers.

Case 1

can be solved by increasing the fineness of the filter and replacing the filter frequency; If there are still perforations, consider replacing the material.

Case 2

can be solved by controlling the stock between the rollers; In addition, the material should be plasticized as far as possible, so as not to have unplasticized raw materials mixed, but also to control the speed ratio of each machine, as far as possible to reduce the resulting hole problem.

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, which is smaller than the unidirectional stretched film, and can be made up to a certain extent by the calendering device. The bidirectional stretch film by the tensile force is bidirectional, much larger than the bidirectional stretch, 2m wide film if the width is pulled to 5m, then the tensile ratio of 215.1mm holes after stretching into 215mm. In fact, the hole is larger than 215mm, because the stress around the hole is concentrated and easy to be damaged. Therefore, it is very important to solve the perforation problem of the stretched film.

Case 3

Filling material mainly uses light calcium carbonate, the right amount of calcium carbonate can increase the strength of the film, but when the amount of calcium carbonate reaches 50 to 60 (relative to 100 PVC), the film tearing strength is significantly reduced, then the perforation of the stretched film is significantly increased. Therefore, controlling the amount of calcium carbonate is also an important factor to reduce the holes of the stretched film. When the amount of plasticizer increases, the pores are reduced because the film is soft and the elongation at break is increased. On the contrary, the pores are increased.

precipitation problem

In the production of stretched film process, sometimes precipitation phenomenon, that is, on the surface of the film or calender roller (or traction roller) produced a layer of white frost, serious will affect the use of the film, and even make the film scrapped (such as light box film will affect the ink absorption of the light box cloth due to surface precipitation). From the production point of view, the precipitated materials are mainly lubricants, powder stabilizers and calcium carbonate. In the process of producing PVC rolled film, in order to make the surface of the product smooth and beautiful, but also to improve the flow performance of the plastic melt, reduce and avoid the adhesion and friction of the equipment, a certain amount of lubricant is added to the plastic. However, the improper use of lubricant, exceeding its compatible limits, easy to precipitate on the surface of the product, that is, commonly known as frosting, affecting the appearance of the product, but also affecting the ink absorption of the printing advertising cloth; The amount of lubricant is too small and can not play the lubricating effect, the amount of lubricant is generally less than 1%. Commonly used lubricants are stearic acid, polyethylene wax and so on.

PVC processing temperature and its decomposition temperature is very similar, when processed at 160~200бц, there will be severe thermal degradation, product discoloration, performance deteriorates. Therefore, in the processing of PVC, heat stabilizer must be added. Many heat stabilizers not only have a stabilizing effect, but also have a certain lubricating effect, such as (powder) barium stearate, cadmium stearate. If the compatibility of these materials is poor, frosting, migration and other phenomena will occur. In order to improve the mechanical properties of the product, reduce the cost and the single consumption of resin and appropriate amount of filling materials. Adding the appropriate amount of additives will reduce the linear expansion coefficient of plastics and product molding shrinkage, and can improve the heat resistance, flame retardant and strength of plastics. The amount of additives should be appropriate, too much will precipitate from the surface of the film. Increasing the fineness of the additive can increase the amount of the additive, such as the amount of ultra-fine calcium carbonate is more than ordinary calcium carbonate. Coupling agent can also be used to treat calcium carbonate and increase the tightness of calcium carbonate and PVC material.

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