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1. Longitudinal stretch
The longitudinal stretching process includes heating and stretching the non-oriented cast substrate in the longitudinal direction so that the molecular chains are aligned in the stretching direction. Longitudinal stretching is usually carried out in equipment with several preheating rollers and a pair of stretching rollers, each of which moves at a different rotation speed. The longitudinal stretch ratio is defined as the ratio of these roller speeds.
(A) Long free stroke stretching
(B) Short free stroke stretching
(C) Stretching of small diameter closely arranged rollers
(A) Long free stroke stretching
The substrate is stretched between a pair of nip rollers rotating in the same direction. During the stretching process, the radiant energy provides heat to the substrate. The system usually has a long free stroke, which can cause significant "necking" (the width decreases during stretching). Due to the formation of thick edges, a large amount of "neck" is disadvantageous.
(B) Short free stroke stretching
The substrate is stretched between counter-rotating rollers. Due to the short free stroke, the system is very good in terms of avoiding "necking". However, the weakness of this system is the formation of surface defects, such as sticking and scratches.
(C) Multi-roll stretching
The substrate is gradually stretched between a series of many small rollers. The system showed moderate "necking" phenomenon. However, due to too many rollers, there are still sticking and scratching defects. The longitudinal stretching system actually used is a combination of these three basic systems. But the system used by different companies is different.
The materials used on the roller surface are:
ˇ¤Hard chrome plating;
ˇ¤Sintered ceramics;
ˇ¤Fluoropolymer;
ˇ¤Silicone Rubber.
Each material has a different initial bonding roller temperature, above which a strictly non-oriented substrate adheres to the surface of the roller. Generally, hard chromium is below 80ˇăC, ceramics are smoothed at 100ˇăC, and the conditions are suitable, and fluorine and silicon polymers are below 120ˇăC.
(1) At room temperature, the polymer chains freeze and are in a glassy state. This situation continues until the glass transition temperature (T);
(2) Above the Tg temperature, the polymer is in a "supercooled" state, which is maintained until the crystallization temperature (T.);
(3) When the temperature is close to T., polymer molecules begin to crystallize, which is called thermally induced crystallization.
In order to make the molecular chain of the substrate straighter, stretching should be carried out in the temperature range of T8 and T., that is, in a supercooled state. In fact, the suitable temperature for longitudinal stretching is between 80 and 120ˇăC.
The uniaxially stretched cast PET substrate is stretched below 70ˇăC, and the curve has a clear yield point, which means that the temperature is not suitable for stretching. This kind of stretching is called "neck stretching", it cannot provide good linear molecular orientation for the substrate, and usually has poor thickness uniformity. Above 80ˇăC, the stress-strain curve becomes smooth, indicating that the conditions are suitable for stretching. Above 100ˇăC, the stress does not increase much with the increase of strain. This kind of stretching is called "over-traction", and it is a "flowing" process, not a "stretching" process. In the process of "over-traction", as the thickness and width decrease, the small molecular weight becomes straight.
By combining "over-drawing" with normal stretching, a higher stretch ratio is obtained due to the increase in film forming speed. This technical idea originated from the production of PET fiber and then transferred to the production of film. The higher longitudinal stretch ratio should be 4-9 times the original length. For longitudinal stretching, its influence on continuous transverse stretching must be considered. Due to the occurrence of super strain crystallization, a too high longitudinal stretch ratio will cause poor transverse stretchability. In order to maintain good transverse stretchability, the physical properties of the longitudinally stretched film should be restricted, such as the density below 1.355g/cm3 (the density of amorphous PET is 1.335g/cm3), and the birefringence is less than 0.100. In order to obtain this For longitudinal stretch film, the stretching temperature should be between 90~100ˇć, and the stretching ratio should be 3.5~4.0 times the original length.
2. Horizontal stretch
The transverse stretching is usually done with a tenter, and the thermosetting stage is followed by the tenter. The two sides of the longitudinally stretched film are clamped by the tenter clamp, and the film is introduced into the tenter heating furnace, where the film is preheated, and the transverse stretch is 3.0 to 4.5 times the original width within the range of 90-120ˇć . In the stenter heating furnace, the film is heated by hot air blown from the upper and lower nozzles, and stretched by the spreading clamp chain. The important point of transverse stretching is how to control the heat in the preheating zone to initiate crystallization. High temperature and/or long-term preheating usually accelerate crystallization, and crystallization leads to poor stretchability, such as poor thickness uniformity. A good parameter related to the level of thermally induced crystallization is the "crystallization initiation temperature". The lower the temperature, the earlier thermally induced crystallization occurs. For reference, this temperature is around 140ˇăC for an amorphous cast film, and around 110ˇăC for a longitudinally stretched film.
3. Heat curing
Biaxially stretched PET film contains a lot of internal stress caused by stretching. Therefore, when the film is exposed to high temperature, even at room temperature, the film shrinks, and the film shrinks slowly over time. Therefore, the film is usually thermally cured between 180~235ˇăC. Thermal curing increases the crystal size and degree of crystallinity, and relaxes the strained amorphous region. This relaxation has a great effect on improving thermal dimensional stability. The thermosetting temperature below 180ˇăC does not improve the thermal stability much, while the thermosetting temperature above 235ˇăC will reduce the crystal crystallization caused by stretching, and the mechanical properties will be seriously deteriorated.
Nanjing Sumino Precision Machinery Co., Ltd. is a high-tech enterprise integrating scientific research, production and sales. It has always adhered to the business philosophy of integrity, equality, and customer benefits first. In order to better improve the quality of equipment, it has entered a more professional and standardized Production track. We have established independent research and development technologies for seven types of equipment including plastic sheet material, casting, bi-stretching, non-woven non-woven meltblown cloth, laminating film composite, recycled granulation environmental protection recycling system and coating machine manufacturing system. The center and production department provide a complete set of professional technical solutions to meet the special needs of each customer. Among them, the biaxially oriented film manufacturing system includes: BOPP biaxially oriented film manufacturing system, PVC biaxially oriented shrink film production line, PET biaxially oriented film production line, BOPA biaxially oriented film production line, BOPA biaxially oriented film production line, etc.
Company Address: No. 11, Kaifeng Road, Moling Street, Jiangning District, Nanjing City, Jiangsu Province
The longitudinal stretching process includes heating and stretching the non-oriented cast substrate in the longitudinal direction so that the molecular chains are aligned in the stretching direction. Longitudinal stretching is usually carried out in equipment with several preheating rollers and a pair of stretching rollers, each of which moves at a different rotation speed. The longitudinal stretch ratio is defined as the ratio of these roller speeds.
(A) Long free stroke stretching
(B) Short free stroke stretching
(C) Stretching of small diameter closely arranged rollers
(A) Long free stroke stretching
The substrate is stretched between a pair of nip rollers rotating in the same direction. During the stretching process, the radiant energy provides heat to the substrate. The system usually has a long free stroke, which can cause significant "necking" (the width decreases during stretching). Due to the formation of thick edges, a large amount of "neck" is disadvantageous.
(B) Short free stroke stretching
The substrate is stretched between counter-rotating rollers. Due to the short free stroke, the system is very good in terms of avoiding "necking". However, the weakness of this system is the formation of surface defects, such as sticking and scratches.
(C) Multi-roll stretching
The substrate is gradually stretched between a series of many small rollers. The system showed moderate "necking" phenomenon. However, due to too many rollers, there are still sticking and scratching defects. The longitudinal stretching system actually used is a combination of these three basic systems. But the system used by different companies is different.
The materials used on the roller surface are:
ˇ¤Hard chrome plating;
ˇ¤Sintered ceramics;
ˇ¤Fluoropolymer;
ˇ¤Silicone Rubber.
Each material has a different initial bonding roller temperature, above which a strictly non-oriented substrate adheres to the surface of the roller. Generally, hard chromium is below 80ˇăC, ceramics are smoothed at 100ˇăC, and the conditions are suitable, and fluorine and silicon polymers are below 120ˇăC.
(1) At room temperature, the polymer chains freeze and are in a glassy state. This situation continues until the glass transition temperature (T);
(2) Above the Tg temperature, the polymer is in a "supercooled" state, which is maintained until the crystallization temperature (T.);
(3) When the temperature is close to T., polymer molecules begin to crystallize, which is called thermally induced crystallization.
In order to make the molecular chain of the substrate straighter, stretching should be carried out in the temperature range of T8 and T., that is, in a supercooled state. In fact, the suitable temperature for longitudinal stretching is between 80 and 120ˇăC.
The uniaxially stretched cast PET substrate is stretched below 70ˇăC, and the curve has a clear yield point, which means that the temperature is not suitable for stretching. This kind of stretching is called "neck stretching", it cannot provide good linear molecular orientation for the substrate, and usually has poor thickness uniformity. Above 80ˇăC, the stress-strain curve becomes smooth, indicating that the conditions are suitable for stretching. Above 100ˇăC, the stress does not increase much with the increase of strain. This kind of stretching is called "over-traction", and it is a "flowing" process, not a "stretching" process. In the process of "over-traction", as the thickness and width decrease, the small molecular weight becomes straight.
By combining "over-drawing" with normal stretching, a higher stretch ratio is obtained due to the increase in film forming speed. This technical idea originated from the production of PET fiber and then transferred to the production of film. The higher longitudinal stretch ratio should be 4-9 times the original length. For longitudinal stretching, its influence on continuous transverse stretching must be considered. Due to the occurrence of super strain crystallization, a too high longitudinal stretch ratio will cause poor transverse stretchability. In order to maintain good transverse stretchability, the physical properties of the longitudinally stretched film should be restricted, such as the density below 1.355g/cm3 (the density of amorphous PET is 1.335g/cm3), and the birefringence is less than 0.100. In order to obtain this For longitudinal stretch film, the stretching temperature should be between 90~100ˇć, and the stretching ratio should be 3.5~4.0 times the original length.
2. Horizontal stretch
The transverse stretching is usually done with a tenter, and the thermosetting stage is followed by the tenter. The two sides of the longitudinally stretched film are clamped by the tenter clamp, and the film is introduced into the tenter heating furnace, where the film is preheated, and the transverse stretch is 3.0 to 4.5 times the original width within the range of 90-120ˇć . In the stenter heating furnace, the film is heated by hot air blown from the upper and lower nozzles, and stretched by the spreading clamp chain. The important point of transverse stretching is how to control the heat in the preheating zone to initiate crystallization. High temperature and/or long-term preheating usually accelerate crystallization, and crystallization leads to poor stretchability, such as poor thickness uniformity. A good parameter related to the level of thermally induced crystallization is the "crystallization initiation temperature". The lower the temperature, the earlier thermally induced crystallization occurs. For reference, this temperature is around 140ˇăC for an amorphous cast film, and around 110ˇăC for a longitudinally stretched film.
3. Heat curing
Biaxially stretched PET film contains a lot of internal stress caused by stretching. Therefore, when the film is exposed to high temperature, even at room temperature, the film shrinks, and the film shrinks slowly over time. Therefore, the film is usually thermally cured between 180~235ˇăC. Thermal curing increases the crystal size and degree of crystallinity, and relaxes the strained amorphous region. This relaxation has a great effect on improving thermal dimensional stability. The thermosetting temperature below 180ˇăC does not improve the thermal stability much, while the thermosetting temperature above 235ˇăC will reduce the crystal crystallization caused by stretching, and the mechanical properties will be seriously deteriorated.
Nanjing Sumino Precision Machinery Co., Ltd. is a high-tech enterprise integrating scientific research, production and sales. It has always adhered to the business philosophy of integrity, equality, and customer benefits first. In order to better improve the quality of equipment, it has entered a more professional and standardized Production track. We have established independent research and development technologies for seven types of equipment including plastic sheet material, casting, bi-stretching, non-woven non-woven meltblown cloth, laminating film composite, recycled granulation environmental protection recycling system and coating machine manufacturing system. The center and production department provide a complete set of professional technical solutions to meet the special needs of each customer. Among them, the biaxially oriented film manufacturing system includes: BOPP biaxially oriented film manufacturing system, PVC biaxially oriented shrink film production line, PET biaxially oriented film production line, BOPA biaxially oriented film production line, BOPA biaxially oriented film production line, etc.
Company Address: No. 11, Kaifeng Road, Moling Street, Jiangning District, Nanjing City, Jiangsu Province