How to eliminate the wrinkle of blown film

2021-03-27  723

In the blowing process, the instability of the film bubble from entering the air ring to the maximum diameter is the main reason for the wrinkle of the film. If any slight center deviation occurs during the process of blowing and pulling, the contact part of the bubble will have a great difference in length when it touches the folding frame and the traction roller. Then, the tension band and relaxation band will appear alternately on the film bubble during winding, and it is easy to wrinkle at the slack zone.

The harder the resin and the higher the crystallinity, the more likely to form crease, and the more difficult to eliminate the crease caused by the tension band is not easy to stretch. Therefore, the film made of HDPE, nylon, polyester, polystyrene and polycarbonate is more prone to wrinkle than that of soft resin, such as LDPE, LLDPE and EVA.

Control melt temperature

The movement of the bubble is caused by the random change of melt temperature, and the higher the crystallinity of the polymer, the more sensitive the melt modulus is to the change of melt temperature. In addition, the control failure of wind ring and internal cooling system (IBC system) may cause the offset of membrane bubble, but these faults are easy to diagnose and correct.

The stable equilibrium of extrusion melt temperature can effectively prevent the migration of the film bubble. At this time, the temperature distribution of the screw barrel shall be checked and adjusted, and the heat generated by the screw barrel shall be transmitted to the material as soon as possible while the material delivery is not affected.

Generally, the heat transfer process occurs from the feeding section of the screw to the homogenization section. The flow of solid materials can be improved and the material softened by heat transfer, thus reducing the generation of friction heat.

The friction shear heat produced by screw usually causes the local non-uniform overheating, which makes the melt temperature in this area higher than other areas, forming a discrete region. When the melt in the discrete region is extruded from the die, it will react with the air ring cooling device, which will affect the cooling of the air ring, and thus lead to the film bubble migration. This problem can be solved by generating more conduction heat at the beginning of processing, because it can reduce the melt viscosity of material in the screw compression section, and thus reduce the shear heat acting on the melt.

The temperature distribution of "temperature, heat, cold temperature" is conducive to stabilize the extrusion temperature of melt. In the feeding section, the melt temperature shall be set according to the recommendation of resin supplier. The temperature setting of compression section shall be 10-37.8 ℃ higher than the initial setting temperature, while in the end section and the front section of metering section, the temperature setting shall be 10-37.8 ℃ lower than the initial setting temperature. Finally, the temperature of the outlet is adjusted to the initial setting temperature to facilitate extrusion. If the temperature is still unable to solve the problem, sometimes changing the screw design or surface repair of screw thread may reduce the shear heat and improve the stability of extrusion temperature.

The random change of temperature makes more hot film bubbles exposed to cold air, which may increase the blow up ratio (bur). Although the sensitivity of the bubble to the extrusion temperature can be reduced by reducing the area of the unsupported membrane bubble exposed to the cold air, and thus the blow up ratio can be reduced, but the layflat width will also be reduced.

If the die head and required bending diameter are established, and the hard resins such as nylon, propiononitrile copolymer or polycarbonate (PC) are processed, and these resins can only be continuously produced under a very small blowing ratio. At this time, the cooling area can be reduced by increasing the diameter of the inner bubble of the wind ring, and the reduction diameter will not be reduced.

The diameter of the inner bubble of the wind ring can also be increased by using a wind ring with a tapered extension of the upper lip, which is because the membrane bubble is not supported by the main cooling air flow. The longer the middle cone-shaped air ring lip, the larger the caliber that can be set to adjust the upper lip, and the more obvious the effect. In addition, a pair of stacked air rings can be used, and the wind rings located on the top are slightly larger than the bottom, or the size of one of the air rings is larger than the size of the die head to increase the bubble diameter.

Keep the bubble stable

If the measures such as maintaining the stable equilibrium of extrusion melt temperature and reducing the cooling area of the foam can not eliminate the crease caused by the offset of the bubble, the mechanical method can only be used to keep the stability of the bubble. The most common stabilizing device is the bubble guides made of rollers. For soft film materials, such as LDPE, they are not easy to wrinkle, so they can be stabilized by wood strip fixation.

Cage type foam stabilizer provided by IBC system is also a very effective method to keep the foam stable. If hard resin is used, a more solid round stable frame is needed to prevent the bubble from shaking left and right, and the foam stabilizer is placed on the condensation line.

It can also be used to install some advanced foam stabilization devices with two or four-sided herringbone splints. One is internal bubble spring, which starts from die head, and can pre shape the membrane bubble through its special arm device before the bubble is completely rolled, thus eliminating creases. Another stable device is the internal bubble mandrel, which is cooled before use, so as to ensure that the melt will not adhere to it during the process of processing. It is worth noting that the inner membrane cylinder needs to control the inner membrane pressure of the upper and lower parts of the rotating cylinder respectively.

If the above measures can not eliminate wrinkle, the final method is to change the design of herringbone splint. When two-sided herringbone splint is used for winding, the winding speed of the film bubble part is faster than that of other parts because the two sides of the herringbone splint are asymmetric. The solution is to increase the length of the plywood, make the bubble part of the contact herringbone splint more uniform on its circumference, but at the same time, it is necessary to overcome more friction resistance. The guide roller with small friction or lubricated surface can reduce friction resistance, but it needs maintenance and regular cleaning, otherwise wrinkle will also occur.

The cross-section of the membrane bubble can be square by hinged or pivoted four side winding system. Even if the film materials are processed with PP, nylon or PC resin, the foam can be more stable and even, thus avoiding wrinkle. However, this process also requires that the friction resistance of the surface must be very small.