Domestic film biaxial stretching equipment short board and improvement

Two-way film stretching equipment can be divided into off-line two-way stretching equipment and on-line two-way stretching equipment according to production continuity. Off-line biaxial stretching equipment refers to a biaxial stretching device in which different sets of equipment are used to complete different processes in a film production process. Generally used in the production of special films such as PTFE , PVC, PI, etc. The connection biaxial stretching equipment refers to the biaxial stretching equipment that the film production process is completed in the same equipment. It is generally used in the production of packaging films and some differentiated films such as BOPP, BOPET, BOPA, OPETG, BOPET thick films, and the like. The bidirectional film stretching apparatus can be divided into an asynchronous biaxial stretching apparatus and a synchronous biaxial stretching apparatus according to whether the longitudinal stretching and the transverse stretching are simultaneously performed. The following focuses on the connection of asynchronous biaxial stretching equipment.

Development status of packaging film stretching equipment

1. Development of Packaging Film Stretching Equipment in China

Domestic two-way stretching equipment began in the 1980s and introduced production lines with an annual output of 1,000 tons to 1,500 tons from abroad. With the development of the packaging industry, the number of plastic flexible packaging requirements has been increasing, and at the same time, the film manufacturers have to reduce Cost, the production capacity of each line is also increasing, from 1,500 tons to 3,000 tons per year, to 6,000 tons, 12,000 tons, 16,000 tons, and now has reached an annual output of 25,000 tons. The increase in production speed increases the yield of the film, but the film performance is limited to increase. In addition to the film thickness uniformity, the film's limit deviation is increased from the original ±5% to ±3%. The strength of the film is increased from the original 170-190Mpa to the current 200-230Mpa. The equipment structure is developed from single-point stretching to two points. Stretching, the control system's transmission speed and precision are constantly updated with the development of the electronics industry.

2. The actual situation of the current double pull device

At present, 80% of the biaxially stretched film production line is a production line for producing packaging films, and 20% is a special production line for special films. These special production lines, except those specially designed to meet the process requirements, are basically common to packaging film stretching equipment. Equipment manufacturers are more willing to pursue the production of a single line to meet the customer's heart of scale production. Even the purpose of two-point stretching in the longitudinal drawing design is not to improve the film performance but to increase the speed of the production line. These factors have led to the double pull devices we are using now, and there are some problems in the production of films with special requirements.

Existing stretch equipment problems

1. The uniformity of stretching is difficult to increase

1.1 The reasons for extrusion plasticization, after the increase in production speed, should pay attention to the design of the extruder, can meet the plasticization requirements of the material, otherwise there will be uneven plasticization, affecting the film thickness uniformity and physical properties of each The opposite sex, such as uneven strength, uneven heat shrink, etc. Although experts also studied a lot of structure of the screw, but the most important thing is the speed of the material, how to make the material according to the material properties does not degrade the material and improve the plasticization Under the premise of sex, increasing screw speed is the development and research direction of the extruder in the future.

1.2 The correspondence between the first thickness gage and the second thickness gage is an important indicator of the uniformity of tensile and the temperature difference across the double pull device. Some companies currently provide equipment that does not advocate the use of two thickness gauges before and after, mainly to reduce the price, and in the actual production does not affect the production of thin films, so it continues. In fact, two thickness measurements have different uses. The thickness gauge in front is mainly to measure the thickness uniformity of the slabs. What shape of the slab can produce a qualified film, there is no uniform standard, some bow arch, bow height is not the same, some straight, which reflects the uniformity of the tensile strength of different equipment and the lateral temperature deviation of the equipment . Generally speaking, the smaller the bow height of the slab, the more accurate the position of the bolts before and after the thickness gauge, and the thinner the film thickness is, the smaller the bow height of the slab due to the increase of the molecular orientation ratio. Therefore, when we produce a thick film, the curve bow height of a slab is large; when a thin film is produced, the curve bow height of a slab is small. This does not prove that the stretching is uniform when the film is produced, and the lateral temperature difference is small. But because of the small amount of heat needed to reach the glass transition temperature in the horizontal drawing machine, the film reaches the stretching temperature almost at the same time, although there is a temperature in the transverse direction. In addition, the stretch rate per unit volume of the slab increases, and a portion of the longitudinal high point in the unit area is easily eliminated.

1.3 The problem of longitudinal stretching requires efforts to solve the problem of scratching and evenness of MDO in two-step stretching. At present, thicker films are more difficult to solve in MDO scratching in two-step stretch production. It is a comprehensive reflection of the problems of control, machinery and technology. Scratch on the surface of the film affects the reflection and refraction of light. It is scratched in the MDO process. It is difficult to find in the inspection of the film. Only the polarimeter and the difference in the depth of the color after the coloring process of the reflective film can be easily seen because of the reflective film. The requirements for surface roughness and gloss are very high, and the existing additive masterbatch and equipment structure cannot be satisfied. The non-uniformity of the longitudinal stretching has a great influence on the deviation of the thickness of the film. Although the deviation in the thickness can be reduced when the stretching ratio is large in the transverse stretching, it is difficult for the thick film, and all the equipment manufacturers need to Research and solution in this area. .

1.4 The reasons for the horizontal pull: To solve the problem of low temperature on both sides of the horizontal drawing machine, the stretching ratio is not enough, the film thickness deviation is large, and it also affects the uniformity of the strength and heat shrinkage of the films on both sides.

2. Improve the applicability of equipment to temperature-resistant films

Efforts to improve the temperature resistance of the film, the general film can only be used at 130 - 150 °C for a long time, there are many customers now require temperature 180 ~ 200 °C , which in the existing process conditions to achieve more difficult. According to experiments, it is known that the temperature resistance of the film is not simply related to the crystallinity of the film, and the crystalline crystal form also has a great influence. High crystallinity, not necessarily high temperature resistance, but also according to the proportion of crystal form can be determined. The proportion of typical fiber crystals is high, and the temperature resistance of the film is good. Therefore, efforts to increase the content of fibrous crystals is a direction to improve the temperature resistance of the film. As a thin film equipment manufacturer, it is necessary to strive to increase various conditions that contribute to the formation of thin film fiber crystals, and to meet the requirements for the production of heat-resistant films.

3. Solve the film surface oligomer contamination

The problem of oligomer contamination is a strong demand for thin film production plants for products such as optical films, sunshade films, and UV-resistant glass films. Due to the local adhesion of the oligomers, differences in local gloss and transparency of the film are caused, affecting the quality of downstream products. Highly transparent films not only reduce dust particles in the ambient air, but also reduce oligomer contamination. This problem is also a problem that is difficult to solve for a long time and involves a wide range of issues:

3.1 Raw material oligomer content problem. It is generally believed that the stretch film PET chip has a higher viscosity, which is a misunderstanding. In order to increase the molecular weight, the reaction time of the high viscosity material in the synthesis polycondensation reaction is prolonged. Since the time is long, the reverse reaction can also increase. Therefore, the molecular weight distribution varies greatly. Even though the average molecular weight is the same, it is obvious that there are many molecules with low molecular weight chains with wide molecular weight distribution. Therefore, raw materials of BOPET films are better in equipment and the viscosity of raw materials is lower, generally 0.62 to 0.65. Suitable.

3.2 Reduce the amount of dust generated during the transport and drying of PET chips, increase the separation effect of the cyclone separator, and reduce the oxygen drop generated during drying. .

3.3 Select the appropriate melt extrusion temperature and melt line temperature, and reasonably set the adjustment of the heating power of the PID to prevent thermal degradation of the PET in the pipe due to thermal inertia. .

3.4 Seriously solve the problem of dust reduction and separation from the debris in the film crushing. Recycling granulation should strive to reduce the viscosity drop problem and reduce the ratio of recycled materials to the raw material ratio as much as possible.

3.5 TDO cross-pulling machine uses a hot air purification and reuse system, especially the TDO heat-setting section increases the amount of exhaust air, and reduces the concentration of oligomers in the hot air of the hot air circulation system.

3.6 increase TDO cooling section number and length, so that the temperature drop of the film has a certain degree of cooling gradient, (the first cooling section 160 °C ; the second cooling section 100 °C ; the third cooling section 50 °C ) and increase the circulation and exhaust volume So that the oligomer can not be attached to the film surface is rarely attached to the cooling bellows.

4. Ensure the transverse strength of the film

The uniformity of heat shrinkage is the manufacturing difficulty of the equipment: From the design of the equipment, it is considered to minimize the bow height in the process of transverse stretching, and it is very good for improving the uniformity of the film. Theoretically, the tensile arch is completely eliminated. It is impossible, but from the equipment design can reduce the bow height. The first is to improve TDO, the uniformity of the temperature in the oven, so the air volume control of the air duct, tuyere, and the inlet of the clip makes the design more difficult. Equipment manufacturers' technical difficulties. .

5. Ensure that the horizontal cooling section is evenly cooled

It is generally ignored that the uniformity of the temperature and air volume in the cooling section of the TDO has been taken into account in the production of optical films, and a new topic has been proposed for the design of the cross-pulling machine. The film has no tension without thickness deviation. When the visible light has a different refraction, the reaction in the film is uneven. The main reason is that when the high-temperature film passes through the cooling section, there is a phenomenon of non-uniform cooling, resulting in a difference in local contraction, so there is a visible light irradiation Different light refraction now.

6. Corona treatment of film contamination

Corona treatment is an important process for increasing the surface tension in film production. However, due to the impact of a high-voltage electric field, macromolecules on the surface are broken, the number of carboxyl groups increases, and the number of low molecular weight polymers increases. Some low molecular weight polymers Being sucked away by the hood, there is also part of the oligomer attached to the surface of the film. When running to the cooling roller, the oligomer is attached to the roller surface. If the time is longer, the accumulated film thickness will affect the cooling of the film. If in summer, the temperature in the room is high, the surface temperature of corona-based film is very high, and the cooling roller is too thick to affect the heat transfer cooling, causing the hot film surface to partially omit the oligomer. The adhesion layer, when the adhesion layer is aluminized, or printing and drying, it will cause the aluminum plating layer and the ink layer to drop. Therefore, the corona device's oligomers and degradation products are excluded from the future double-pull device. Improve the direction of improvement.