In the process of calendering, with the help of the shear force produced between the rollers, the material is extruded and sheared many times to increase plasticity, and it is extended into a thin product based on further plasticization.
Calendering is the use of strong shearing force between rollers and the corresponding processing temperature, so that the viscous-flowing material is extruded and extended many times, and finally becomes a sheet product with width and thickness. Both plastics and rubbers have a calendering process, and polyvinyl chloride resin is the raw material in plastics.
Calendering Process and Calendering Principle?
In the manufacture of rubber products, the pre-mixed rubber compound is made into a film of a certain thickness, width, or shape, or the process of hanging a thin rubber layer on textile is calendaring. The use of calendering can complete operations such as pressing, pressing, gluing, wiping, laminating, thinning, and filtering.
When the sizing material is added between the two working rollers of the calender, the sizing material is brought into the nip due to the rotation of the rollers. The sizing material is rolled into a film having a thickness and width. During the calendering process, the rubber material undergoes viscous flow on the one hand and elastic deformation on the other hand. Therefore, various process phenomena in calendering are related to the fluidity of the rubber compound and the viscoelasticity of the rubber compound.
The extrusion and shearing action of the roller on the plastic changes the macrostructure and molecular shape of the material, and plasticizes and extends the plastic under the coordination of temperature. As a result of the rollers, the layer becomes thinner, while stretching increases both the width and the length of the layer. During the calendering process, while the roller is extruding and shearing the material, the roller is subjected to the reaction force from the material. The force that tries to separate the two rollers is called the separation force. Usually, the rollers can be designed and processed into a slightly waisted drum shape, or the shafts of the two rollers can be adjusted to cross a certain angle or prestressed. Which can overcome or reduce the harmful effect of the separation force to a certain extent and improve the calendered products' uniformity of thickness. During the calendering process, the thermoplastic is subjected to large shear stress, so the macromolecules will be oriented along the advancing direction of the film. The resulting film has anisotropy in physical and mechanical properties. This phenomenon is called the calendaring effect. The size of the calendering effect is affected by the calendering temperature, rotation speed, feed thickness, physical properties, etc., heating up or increasing the calendering time can reduce the calendering effect.
What Kinds of Calendars are There?
- According to the number of working rolls: two rolls, three rolls, four rolls.
- According to use:
- Sheet calendar: Used for sheet pressing or textile gluing, usually with three or four rolls, and each roll rotates at the same speed.
- Glue rubbing calendar: Used for rubbing glue on textiles, usually three rolls, with a certain speed ratio between each roll.
- Universal calendar: This calendar has both the functions of the above two calenders, usually three or four rolls, and the speed ratio of each roll can be adjusted by auxiliary gears.
- Calendering machine: It is used to manufacture films with patterns or certain shapes on the surface, and one of the rollers is engraved with patterns.
- Steel wire calendar: Used for gluing steel cord fabric, consisting of four rollers.
- According to the arrangement of the rollers, there are I-type, T-type, L-type, Z-type, S-type, etc.
Why Should Compensation Measures be Adopted When Designing a Calendar?
During the processing of the calendering machine, when the rubber material passes through the roller distance, the lateral pressure on the roller is very high, and the weight of the roller itself will cause the roller to produce a certain elastic bending. As a result, the calendered film appears thick in the middle and thin on both sides. The phenomenon. To overcome these disadvantages, some compensation measures are usually taken when designing the calendar to ensure the precision of the calendering. The commonly used compensation methods include the concave-convex coefficient method, the axis crossing method, and the reverse bending method.
What is the Calendering Effect and What Effect does it Have on the Performance of the Product?
The phenomenon that the properties of the calendered products in the longitudinal and transverse directions are different is called the calendering effect, the breaking strength along the longitudinal direction of the film is large, the elongation rate is small, and the shrinkage rate is large. The tear strength along the transverse direction of the film is small, the elongation rate is large, and the shrinkage rate is small. The reason for the calendering effect is that the rubber molecules and needle-like or flake-like particles are aligned after calendering. The calendering effect, from the processing point of view, should be eliminated as much as possible. The deformation of the semi-finished product will be caused, which will bring difficulties to the operation. However, from the product point of view, some products require high longitudinal strength and should be used. In some products, the strength distribution needs to be uniform, and it should be eliminated. When carrying out the physical and mechanical properties test, it should be noted that the direction of the cutting piece should be consistent with the calendering method.
Elimination method: try not to use or not to use needles and sheet fillers. Raising the calendering temperature increases the activation energy of the molecules.
What are the Calendering Preparation Processes?
- Hot smelting: Before adding the rubber compound to the calendar, it must be rolled in the open mill, to improve the dispersion uniformity and thermoplasticity of the filler in the compound, and make the compound soft and easy to calendar. This process is called thermal refining or preheating. The thermal refining of the rubber compound is generally carried out on an open mill with a diameter of 390-550 mm, including two steps. The first step is roughing, which uses a low-temperature thin pass to make the rubber compound soft and uniform. The second step is the fine chain, the roller distance is larger and the roller temperature is higher to increase the temperature of the rubber compound and obtain a uniform and larger thermoplasticity. The range of 1:1.7~1.28 is desirable. During operation, the amount of accumulated glue on the heating mixer should be kept at a certain level.
Conditional control of thick and thin chains:Roughing: roller 2-5 mm, roller temperature 40-45 ℃, thin pass 7-8 times.
Thin chain: the roller distance is 7-10 mm, the roller temperature is 60-70 ℃, and the pass is 6-7 times.
- Feeding and conveying: The feeding method has an important relationship to preventing air from mixing into the rubber compound. Generally, continuous feeding is used, and the feeding amount is equal to the consumption. The number of additions should be more, and the amount of each addition should be small, to prevent air from being trapped.
- Preparations for the operation of the calendar: Check the equipment before starting the machine. The key points are: whether the surface of the roller is normal, whether the position is normal, whether the temperature is uniform, and whether there are foreign objects or marks. Check whether the rotation of the bearing is normal, and observe the oiling condition, whether the surface thermometer, thickness measuring instrument, splint, and winding shaft are ready, whether the adjustment of the motor is normal, and whether the rotation sound is abnormal.
- Temperature control: Controlling the roller temperature of the calender is an important part of ensuring the quality of the calendering. Generally, the calendering roller temperature of synthetic rubber is lower than that of natural rubber, and the temperature of the upper roller is higher than that of the middle and lower rollers. The roll temperature also depends on the formula, plasticity, process method, rolling method such as rotation speed, etc.
- Textile drying: The textile must be dried before calendering to reduce the moisture content of the textile and increase the temperature of the textile. The dried textiles should not be parked to avoid moisture absorption. The moisture content of textiles should generally be controlled at 1-2%. Excessive moisture content will reduce the adhesion between rubber and textiles, but drying will damage the textiles or make the textiles hard and reduce their strength.
What are the Operation Methods of the Tablet Pressing Process?
Pressing is to press the preheated rubber material into a film with a certain thickness and width with a calendar at the same roller speed. There are two kinds of operation methods: no glue accumulation and glue accumulation in the middle and lower rollers. The temperature of the middle and lower rollers is like or slightly lower than that of the middle roller. A proper amount of glue can make the film smooth and reduce air bubbles, and the film has good compactness, but it will increase the calendering effect. This method applies to styrene-butadiene rubber. But it should not be too much during operation, otherwise, it will bring in air. The opposite of the no-gum rule applies to natural rubber.
What Factors Should be Paid Attention to When Pressing?
- Roller temperature: The roller temperature should be determined according to the properties of the rubber. Usually, the rubber with high rubber content or high elasticity should have a higher mixing temperature. To make the film move smoothly on the roller, the temperature of each roller should have a certain temperature difference. For example, natural rubber tends to stick to hot rollers, and when the film is moved from one roller to another, the latter's roller temperature should be higher. On the contrary, in styrene-butadiene rubber, its adhesion to the roller increases with the decrease of the roller temperature.
- Plasticity of rubber compound: High plasticity, good fluidity, easy to obtain a smooth film, but too large plasticity, and easy to produce the sticking phenomenon, small plasticity, the surface of the tablet is not smooth, the shrinkage rate is large.
- Roller speed: The roller speed is fast and the production capacity is large, but the roller speed should be determined according to the plasticity of the rubber compound.
What are the Requirements for Tablet Quality?
Tableting quality requirements are smooth surface, uniform Baidu, no wrinkles, no self-sulfur rubber particles, dense internal rubber, and no bubbles.
Quality faults and causes that often occur in the process:
- Internal bubbles: The reasons are the high moisture content of the compounding agent in the rubber compound, the high volatility of the softener, the high calendering temperature, the excessive glue accumulation, the improper loading of the film, the high-water content of the returned glue, and the too thick calendered film.
- Surface shrinkage: The reason is that the plasticity of the rubber compound is low, resulting in a large shrinkage rate. The ratio of rubber material and return rubber is uneven, and the heat refining is uneven so that the rubber material itself is not soft and hard, and the shrinkage is inconsistent.
- Rough surface: The reasons are the small plasticity of the rubber compound, the low temperature of the roller, the high calendering speed, and the self-sulfurizing rubber particles of the rubber compound.
- Uneven thickness of the film: The reason is the uneven plasticity of the rubber. The temperature of the glue fluctuates greatly, resulting in uneven shrinkage. The roll distance on both sides is inconsistent or the winding tightness is not equal during operation, which will also affect the thickness of the film.
What is Fitting, and What are the Process Methods and Requirements for Fitting?
Lamination is the operation of combining two layers of thin films into one film by a calendar. It is usually used to manufacture thicker films with higher quality requirements, as well as films composed of two different rubber materials, interlayer films, etc.
The bonding process methods are:
- Two-roll calender lamination: It is carried out with an ordinary constant-speed two-roll rubber mixer. The advantages are that the lamination thickness is large, the operation is simple, but the precision is poor.
- Three-roll calender lamination: Use a three-roll calender to attach the pre-rolled film or tape to the newly calendered film.
- Four-roll calender lamination: Two films can be calendered and laminated at the same time, with high efficiency, good quality, and accurate specifications, but the calendering effect is large. When laminating films, each film is required to have a consistent degree of plasticity, delamination and bulging will occur. When laminating two layers of film with different formulas and thicknesses, it is best to use the method of simultaneous lamination, that is, to attach two fresh films from the calendar, to achieve complete adhesion and no bubbles. And the films are not it will wrinkle, but the calendering effect is large.
What Process Points Should be Paid Attention to When Pressing?
Pressing is to press the rubber material into a semi-finished product with a certain cross-sectional shape or a film with a pattern on the surface.
Forming requires accurate specifications, clear patterns, and dense rubber. Therefore, there are strict requirements for the compound formulation and process conditions.
In the compound formulation, the glue content should be mainly controlled. When the glue content is high, the pattern after pressing is easy to disappear. Therefore, in the formula of the pressing compound, an appropriate amount of filler and softener should be added. Adding reclaimed rubber and vulcanized ointment to the rubber compound is an effective measure to prevent the pattern from collapsing.
Pressing is mainly to use the fluidity of rubber to shape. It is required that the rubber compound has a certain plasticity, and the plasticity of the compounded rubber, the degree of thermal refining, and the proportion of return rubber should be strictly controlled. After the film comes out of the calendar, the method of rapid cooling should be adopted to shape the pattern and prevent it from collapsing and deforming.