[Science Stickers] Partial Analysis of PVC Calendering Production Process (1)

Uniformity of the calendering production process

1. Various fillers and additives cannot be uniformly dispersed in each equipment section;

2. The temperature of the material is unbalanced in each equipment section; the throwing of the material is more likely to cause uneven dispersion and uneven temperature, which will bring a series of problems.

3. The degree of molecular orientation (that is, the same point, both front and back sides are uneven) (when placed in hot water, the material will naturally curl toward the front): the shape of the accumulated material is different (many spindle-shaped) and uneven heat dissipation ( Rack cooling).

The direction of temperature transfer during the calendering process

In practice, people have found that when running at low speed, heat is usually transferred from the pressure roller to the product, and when the speed increases, the heat is transferred in the reverse direction.

The temperature in the middle of the roller is often higher than that at the ends. During the operation of the roller, due to the bending deformation caused by the lateral pressure of the material, the middle of the calendered product should be thicker in the transverse direction, but the phenomenon of the middle of the product being thinner occurs more frequently.

In order to understand that the "heat" flows from the roller to the material or vice versa: the term "critical speed" is used. The critical speed of the roller refers to the speed when the linear velocity of the roller surface reaches the heat generated by the extrusion and shear friction of the roller to the melt equal to the heat required for plastic molding processing.

When the roller surface linear speed is less than this speed, the roller needs to be heated; on the contrary, when the roller surface linear speed is greater than this speed, the roller not only does not need to be heated, but needs to be cooled. Therefore, the critical speed of the roller is the turning point of the roller from requiring external heating to requiring external cooling. It is mainly related to the properties of the processed material, the thickness of the product, and the roller speed ratio. Under different conditions, the critical speed of the roller is different. Therefore, it is generally represented by a speed range. For example, when calendering hard PVC plastic, the critical speed range of the roller is 25~30m/min. In the production of soft PVC, the normal production accumulation temperature is about 190℃, and after the speed is reduced for a period of time, the accumulation temperature is sometimes only 160-170℃.

PVC resin powder properties

No phase change, amorphous, highly polar plastic

1. Strong electronegativity makes it easy to adhere to metal (adhesion to metal and high temperature)

2. Strong polarity and large intermolecular forces cause PVC softening problems and high melting temperature. Generally, it needs 160-200℃ to process.

3. Poor stability, easy to decompose

4. High melt viscosity (shearing during processing will cause frictional heat to increase rapidly)

5. The melt strength is small (poor ductility), which causes the melt to be easily broken (PVC is a straight-chain molecule with short molecular chains and low melt strength

6. Melt relaxation is slow, which easily leads to rough, dull and shark skin on the surface of the product.

7. Thermal expansion and contraction (object characteristics)

8. Molecular chain length, orientation effect

9. Poor fluidity, shear thinning (non-Newtonian fluid, pseudo-plastic)

10. PVC resin does not transmit heat and shear force strongly, and the melt formed is uneven

11. There are chiral carbon atoms in the main chain and weak crystallization ability-chlorine atoms are more electronegative, and adjacent chlorine atoms on the molecular chain repel each other and are staggered and arranged, which is conducive to crystallization (this explains the anti-plasticization Principle of effect)

Abnormal molecular flow

Molecular orientation is the inevitable trend of materials in the oppositely moving wheels; the uniformity of the degree of orientation and the uniformity of molecular stress relaxation and creep during the process are the basis for affecting whether the orientation is normal, and whether there is a problem with winding and spreading.

1. The internal frictional shear force that restricts the speed of thin products may be too high, and a large amount of "heat accumulation" may occur between the roll gaps, resulting in inconsistent fluidity and peeling properties to metals, and the object expands with heat and shrinks with cold. Variation in thickness and uneven winding stress.

2. The precipitation formula will cause uneven heat transfer in the roller, and will also affect the direction of molecular flow, resulting in uneven winding stress.

3. The grinding direction of the roller surface may affect the molecular flow direction, resulting in uneven winding stress.

4. Improper air blowing control of the main engine will also affect molecular flow (stress relaxation, creep), resulting in uneven winding stress.

5. The non-uniformity of temperature change when the film is stretched.

6. Whether there is sloshing or air bubbles during the pulling process of the film (the fundamental reason is the uneven change of molecular stress relaxation and creep caused by temperature changes)

7. Whether the flow rate of the heat transfer oil in the main engine wheel can smoothly take away the overheating of the material, so that the temperature of the material is basically uniform.

The impact of material accumulation on production

Poor rotation of the accumulated material will cause uneven thickness of the product in the horizontal direction, bubbles in the film, and cold scars in the hard film.

Reasons for poor stock rotation:

1. The material temperature is too low or the material fluidity is poor due to the formula

2. Roll temperature is too low

3. Improper adjustment of roller pitch

The first accumulation: the size, raw and cooked affects the size of the second and third accumulations, resulting in changes in thickness and circumference.

The size of the second accumulation can be adjusted appropriately to reduce the influence of the change of the first accumulation (changing the die head, etc.) on the thickness and circumference.

The second accumulating material: the benefits of making it larger appropriately: 1 Make the accumulating material temperature more uniform and reduce the influence of heat accumulation; 2.2 and 4 point circle is better controlled (the inflection point moves outward); 3. Reduce the change of the first accumulating material to the third The impact of material accumulation (the degree of influence is mitigated by the second material accumulation); 4. When the second material accumulation has a lot of edges (about 20cm or more), the edge gap caused by the raw material of the first material accumulation is caused by the second material accumulation. Buffer, there is not much missing material to the next round, and the deviation of the bait is reduced.

The third material accumulation: the size affects the height of the lower wheel hoisting material and the stability of the hoisting material (1. The temperature change of the material accumulation; 2. The change of the area of ​​the roller contacting the accumulation material causes the temperature of the roller to change)

The role of accumulation:

Proper accumulation of materials can make the film smooth and reduce bubbles, and the film has good compactness, which will increase the calendering effect. This method is applicable to styrene butadiene rubber.

The no accumulation law is the opposite, which is suitable for plastics or rubbers with higher plasticity, such as natural rubber.