[Science Stickers] Partial Analysis of PVC Calendering Production Process (2)

The impact of production rate on production

The vehicle speed is too high:

1. The internal frictional shear force and thus a large amount of "heat accumulation" may occur between the nip, resulting in the entire molecular chain orientation (the molecular orientation and the degree of relaxation are different), resulting in poor flattening (the molecular chain orientation is not the same) , Resulting in different retraction, even if the circumference is good) and rewinding air (the degree of orientation is different, the circumference is controlled very well in real time, but due to the tightness of some points, the air cannot be discharged)

2. The elastic potential energy increases (the elastic deformation becomes larger), the internal stress release of the material decreases when in the equipment, and the melt strength decreases (the melt strength decreases with the increase of the melt flow rate. The higher the speed, the material is easy to sag , The melt strength is also low) The melt landscape pattern is aggravated, and the surface is more prone to unevenness, which affects the subsequent printing.

(Increasing the stress or strain rate within a certain range will increase the reversible elastic deformation in the runner and increase the release expansion; reducing the polymer melt temperature will increase the elastic strain entering the die area, and the extension of the relaxation time will also cause the release Increased expansion) (elastic deformation, relaxation degree, melt strength)

3. The separation force becomes larger, resulting in a change in the roller's latitude, resulting in a medium-high.

4. Affect the metal peelability, the speed is too fast, the horizontal material temperature due to the "heat accumulation" point and the heat dissipation effect, the material is unstable in the lower wheel, the lifting material fluctuates, affecting the circumference, the bait is easy to run, and the circumference problem is caused. (It is related to the size of PVC's metal peeling property) (The first buildup becomes unstable even at high production rate, and the edges are easy to grow)

5. High speed, although the shearing force is increased, the heating time of the material is shortened, which is more likely to cause the accumulation of material and the uneven temperature of the material attached to the wheel surface, which makes the mixed system contain raw material and produces deep air spots; At the same time, there are inconsistencies in the degree of molecular orientation.

The bait runs, the circle changes

Reasons for bait running:

1. Material: (formulation) produced different adhesion properties (calcium carbonate content, hardness, lubrication and precipitation, the effect of R4 temperature on material temperature, etc.)

2. Melt strength: low melt strength, causing the product to be intolerant of pulling; where there is a defect, a slight pull will result in greater defects (calcium carbonate, lubrication, hardness)

The higher the normal calcium carbonate content, the more serious the bait will run (1. The melt strength becomes worse and cannot withstand pulling; 2. The adhesion between the material and the metal becomes worse and unstable.)

3. Whether the adhered surface is clean and the surface temperature: (Let the material pass through normally) (the surface of the lower wheel and the surface of the take-up wheel)

Uncleanness leads to unstable and large fluctuations in the hoisting material at R4; the dirty first take-in wheel can be equivalent to fluctuations in the speed of the take-in wheel, resulting in the phenomenon of one meal and one meal, which affects the stability of the hoist. Appropriately increase or decrease the temperature of R4 to balance the adhesion between the material and the next wheel.

Whether the surface of the No. 1 take-up wheel is clean or not, the film will not pass through the wheel smoothly, which indirectly affects the normal lifting of the material on the lower wheel.

4. Material accumulation and shearing problems:

Adjustment: make the edge of the accumulation material rounded, so that it can be stretched 1. The first accumulation material is a little bit smaller and rotten, and there is no raw material on the side; 2. The second and third accumulation materials come out a little, and make them larger properly (first The raw meal coming down from the side of the accumulation material is replenished and plasticized (fluidity) uniform in the second accumulation material; 3. The speed ratio of the main machine is smaller, the gap is reduced, and the shearing is enhanced;

The accumulation of material is too small, resulting in more severe rotation, larger shear heat, and it is easy to become unbalanced, which affects the stability of the R4 hoisting material, and the size of the third material accumulation can be appropriately increased;

When the material accumulation is large, the area of ​​the roller contacting the accumulation material becomes larger, the temperature of the roller is forced to rise, and the lower wheel hoisting material rises, and the hoisting material may be unstable and easy to shake.

5. Improper hoisting position: easy to shake up and down (especially at the edge position): increase or decrease the ratio appropriately, and less fluctuation

6. Speed: During high-speed production, the problem of bait running is prone to occur, and the hoisting material is unstable.

Additional solution: use the guide wheel to hold down the take-up wheel to reduce running

For thin products: over-mixing, the material is more likely to adhere to the metal, easy to shake in the next wheel, causing the bait to run; insufficient mixing, the melt strength of the material is insufficient, and it is not resistant to pulling, causing the bait to run away.

Cold spots, half pinholes

Influencing factors:

1. R1, R2, R3, R4 speed, speed difference (control the first material accumulation, moderate rotation, strengthen the first exhaust, reduce the impact on the second and third material accumulation) and the rabbi of the main engine

When the speed ratio is large, the material will cover the roller well. If the speed ratio is too large, the roller will be wrapped. The thickness of the product will be uneven, and excessive internal stress will be generated, which will increase the size shrinkage rate of the product. On the contrary, if the speed ratio is too small, the material will suck. The roll properties are poor, and bubbles are easy to be mixed to cause bubbles to appear in the product.

If the main speed is too fast, the heating time of the material becomes shorter, and the temperature of the material is easy to be uneven, which causes the raw material to be contained in the material and causes air-conditioning spots. 12# In 2018, the motor of A rolling mill failed, the production speed of this line was reduced, and the air-conditioning spots were much reduced compared with before.

2. The size and degree of rawness and maturity of the first accumulation

3. The size and shape of the second and third deposits (mainly affected by the difference in the height and the degree of intersection of the rollers)

Each machine has a suitable third accumulation size, and sometimes the third accumulation cannot be too small (1. Too small rotation speeds up, causing air to be encased in, resulting in a large number of air spots; at the same time due to control factors, leading to the first Changes in the size and raw material accumulation will affect the size of the third accumulation, resulting in half pinholes; the rising and falling speed causes the size of the third accumulation to change), the third accumulation cannot be too small, and the production of each section must be stable to ensure the first accumulation The material size and raw and cooked are basically unchanged.

The uniformity of the third stock size will seriously affect whether the production is smooth. 9# 2017 will produce thick products and high degree of polymerization orders. By adjusting the oil pressure of the middle wheel gap, the third stock size becomes very uniform. Become normal.

4. Roller temperature, temperature difference

The difference in roller temperature setting is too large, and the material accumulation temperature is lowered by the low temperature roller, which causes the accumulation of material flow to become worse, rotation problems occur, and raw materials are easily mixed in the middle, resulting in half pinholes.

The greater the degree of polymerization, the harsher the processing conditions, the higher the roller temperature, the smaller the temperature difference.

In order to balance the height of the lower wheel hoisting material, the temperature of the middle wheel can be increased and the temperature of the lower wheel can be lowered to ensure the fluidity of the material

5. The degree of plasticization of the material (the processing aid PA-40, if added, the force between the PVC chains will increase, and the air bag will not get out)

6. Slippery in the formula

There are precipitates attached to the metal roller, and the material does not cover the roller well, and it is easy to trap air. At this time, the roller should be cleaned immediately to remove the precipitate.

When each filler or additive in the formula is mixed and the compatibility is not good, there will be a tendency to precipitate

Too much external lubrication: too much lubrication or poor material compatibility, the shearing force of the roller to the material is reduced, the shearing heat is reduced, the fluidity is deteriorated, and the exhaust is not smooth

(Pearl powder products, directly added in 10,000 horsepower, the surface of the host is severely precipitated, and there are more air-conditioning spots; when added in a high mixer, the precipitation is reduced, and the air-conditioning spots become less;)

 Insufficient external lubrication: Too much adhesion to the wheel, poor peelability to metal, and air bag cannot be discharged inside

The formula, fillers/pigments affect the viscosity of the material (fluidity), compatibility and force (melt strength: shear heat affects fluidity), as well as the effect on the peelability of the metal (rolling issues)

Blue material is prone to air spots: the surface of the phthalocyanine pigment is non-polar and has poor compatibility with PVC. At the same time, the pigment precipitated on the surface of the roller is carbonized after a long time to produce sticky wheels. Colors are precipitated on the white silicone wheels, and the product is prone to cold air spots. . ) Fillers generally have a better inhibitory effect on precipitation. This is because they have a grinding effect on the roller, thereby adsorbing the precipitate.

During production, it was found that when the stabilizer's external lubrication is poor, the material is easy to adhere to the host, and there are more problems with cold air spots and semi-pinholes.

7. Roller surface material

   Different materials have different adhesion to PVC

8. The surface roughness of the roller affects the smoothness of exhaust (reciprocating traction polishing, vapor honing)

 When producing thick products, some manufacturers will sandblast and grind the rollers to make the surface of the rollers rough.

In injection molding, too high a degree of polishing will cause a vacuum in the mold, which will cause the product to be adsorbed on the inner wall of the mold. On the other hand, a too low degree of polishing will also cause difficulty in demolding (for calendering, the rollers are too shiny and sticky Enhanced performance, poor exhaust; the roller is too rough, easy to stick to the material)

The main reasons for the formation of surface roughness are: 1. Tool marks during processing; 2. Plastic deformation during cutting separation; 3. Friction between the tool and the processed surface; 4. High-frequency vibration of the process system. No matter which processing method is used, the surface of the part is not absolutely flat and smooth. If you observe it under a microscope (or a magnifying glass), you can see the microscopic uneven peaks and valleys. Therefore, when the new equipment is produced, there will be problems with poor exhaust and molecular flow orientation, which will cause problems during winding or storage.

9. The condition of the materials entering the host

Whether the material temperature (fluidity) meets the requirements, whether the temperature is uniform (whether there is cold material in the middle), and whether the cold material can meet the production requirements of the host temperature and shear heat.

10. Equipment problems

Does the temperature of the equipment cause some oil pressure to appear to be no problem on the surface, but due to the dispersion of the force, the oil pressure changes indirectly, resulting in changes in wheel deformation, resulting in changes in the shape of the third accumulation, resulting in half pinholes

To sum up: when the PVC formula precipitation and adhesion are normal, as long as the fluidity of the material (temperature or processing aids) is normal, the problems of cold air spots and semi-pinholes can basically be solved, and the production conditions and various stages of control are not required So strict.

Unwinding

Influencing factors:

1. Rewinding tension : causes changes in the adhesion of the material to the wheels, rewinding tightness, and exhaust performance

2. Winding temperature : When the material temperature is too low, the soft PVC will be harder, and the adhesion to the wheel will become poor. It is easy to appear because the air bubbles become larger, resulting in discounts, and the temperature will be better if the temperature is higher; but the temperature should not be too high. High, too high is easy to warp edges.

3. The flow direction of the accumulation material changes strangely because of the roller

Causes: 1. The size and position of the blowing air are not correct; 2. The flow direction changes caused by the topography of the wheel surface; 3. The arrangement of the equipment causes the relative position between the wheel and the embossing wheel to be incorrect, resulting in embossing There are bubbles in the material, which affects the uniformity of the temperature, resulting in different winding stresses, and air-packed winding discounts.

4. Is the temperature uniform:

Including: 1. The uniformity of the roller temperature; 2. Whether the material temperature is uniform (whether there is a heat accumulation point); 3. The influence of air blowing and other factors that cause the degree of molecular orientation to be different. The internal stress is different, and some points are tightened tightly.

5. Product specifications (thickness, width, softness and hardness). Thinner and harder products are more likely to be discounted.

Reasons: 1. The production speed is fast and the static electricity is large. The static electricity of the material and the winding roller form repulsion, which is easy to bubbling and cause discounts.

Although the production rate of thick products is about the same as that of thin products, because of the weight of thick products, the electrostatic charge repulsion is less than the weight of products, so the degree of influence is much smaller.

2. The material is thin, the production speed is fast, and the temperature of the cooling wheel cannot be increased, resulting in the material being colder.

3. The material is hard, a little bit of external force will be discounted, and there is no toughness.

To sum up: the three effects of adhesion, molecular orientation, and electrostatic effect are jointly affected. For products of different specifications, the three factors have different influencing factors.

Middle wheel

cross:

The greater the crossover, the thicker the two sides will increase and the lower the middle height; the smaller the separation force (the principle of steel rolling) and the lower the middle height; and the smaller the 2nd and 4th points.

Clearance:

The size of the oil pressure setting for clearance elimination will affect the deformation of the middle wheel (affecting the middle-high and middle-low), thereby affecting the shape of the second and third deposits, especially the third deposit is the most obvious.

In normal production, the greater the amount of crossover, it means that the gap between the rollers has increased and the compensation has increased. However, the gap change between the rollers and the compensation change do not coincide, leading to more and more serious problems of three highs and two lows. 3. The shape (not the size, but the uniformity of the size) of the second accumulation has changed, resulting in abnormal production.

The separation force (change), the middle height of the middle wheel (fixed value), and the deformation of the wheel's own weight (fixed value) produced by the production of each specification at a certain speed and speed ratio should be adjusted and compounded. The gap eliminates the oil pressure (variable value), and the shape of the material accumulation can be better controlled through the deformation of the middle wheel.

Three high and two low

the reason:

Separating force: Forcing the rollers to bend and elastically deform along the axial length, so that the distance between the rollers is the largest in the center, and gradually decreases to the two sides, forming a waist drum shape.

Heat dissipation: The difference in the axial heat dissipation speed of the calender roller affects the processing temperature on both sides of the blank.

Shrinkage: During the cooling process of traction, the two ends (especially the 3-5cm range of the edge is more obvious) are more likely to cold shrink earlier, which is often called the "shrinkage" phenomenon.

This explanation can explain reasonably: why there is no cross-axis, no roll bending, or even a calender that does not even have the height of the roll, and the film produced also has "three highs and two lows". ("Mid height" is produced by the extrusion deformation of the roller, and "side height" is caused by the shrinkage of the side and the heat dissipation from the edge of the roller)

On the equipment, the roller grinding method of the calender can be used to make the roller slightly concave in the near-center area on both sides.

Strengthening factors:

The greater the crossing, the more serious the three highs and the two lows (the difference between the roller axis cross curve and the roller deflection difference curve, so the compensation of the roller axis cross to the roller deflection is not completely compensated, which makes the roller cross calendering In the cross-section of the product, the thickness of the product often produces "two highs and three lows", showing an uneven saddle shape. This phenomenon is usually called the U-shaped effect. At the same time, after the roller axes cross each other, it produces a torsion effect on the calendered product.  U-shaped effect and calendering torsion will increase with the increase of the cross angle of the roller axis, so the cross angle of the roller axis is usually limited to within 2 degrees)

Weakening factors:

1. The product is taller

2. The accumulation of material (especially the second accumulation) is a bit larger, so that the accumulation of material rotates differently and causes the inflection point at this point to move outward. (The temperature accumulates at points 2,4. The temperature is high, the roller expands, and the thickness becomes thinner here) (Spindle shape: the smaller the accumulation, the more severe the rotation; the accumulation at both ends rotates the most, but due to heat dissipation, it causes The thickness is rather large)

3. Increase the temperature of the middle wheel , and make the second accumulation temperature higher, so that the temperature of the accumulation decreases with the size and rotation (spindle shape) of the accumulation, reducing temperature inconsistency; at the same time, the temperature of the middle wheel rises and contacts the edge of the PVC material. The temperature of the roller rises, the roller expands, and the thickness becomes thinner, which reduces the amount of cross-use. It can also move the inflection point caused by the edge effect, so that the 2, 4 points are not so low (the temperature of the middle wheel increases, and the second The middle temperature of the secondary material has little effect, and has a greater impact on the edge) (2017.3 6# production, the temperature of the middle wheel increased by 15℃ than normal, the circumference of the two ends became smaller, collapsed, basically unable to adjust, and the lower wheel hoisted the material at the same time Very high)

4. The speed ratio is close (especially the speed ratio of the upper wheel and the middle wheel) to reduce the speed of the accumulation of material and reduce the excessive temperature deviation of the accumulation caused by the rotation

5. Swing position to make the first accumulation of material evenly cooked and cooked.

Why the wider the door, the more serious the three highs and two lows

The wider the door width, the more serious the spindle of the second accumulation , the more likely it is to have an inflection point (definition of the inflection point: the spindle of the accumulation is serious, and the heat generated due to the difference in the rotation speed of the accumulation between the near middle area and the near side area is different. One, the edge is due to the heat dissipation of the equipment and the shrinkage of the stretched knife during production, which causes a smaller area).