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Tube membrane method in extrusion method for manufacturing PVC film

by:LINYANG     2020-02-04

The Tube membrane method

in the extrusion method for manufacturing PVC film is to pass the melt plasticized by the extruder through an annular die to form a thin-walled round tube, and pulled up by the traction device (There are also horizontal or downward directions) The air injected from the mouth mold inflates the round tube and causes the tube to swell to form a membrane bubble, and at the same time cools it rapidly with air, the hot film Bubble is solidified at a certain distance above the die and the shape of the film bubble no longer changes (This is called condensation line) After that, the cured film is clamped between the two rollers, and the film bubbles are stretched and then rolled to obtain the product.

1. Using dry mixture to blow twist conjunctiva

The production of food grade hard PVC twist conjunctiva in China was originally imported from Italy, Japan, Austria and other countries, later, this kind of equipment was also developed in China and sold as hard pvc food packaging film units. This unit uses dry mixing as raw material, which is composed of feeding device, exhaust extruder, rotating die, cooling device, traction and coiling device, etc.

According to the literature, the production equipment of PVC twist conjunctiva is composed of two parts: high-speed heating/cooling unit and food packaging film blowing unit. When mixing materials, firstly measure according to the formula. Under the low-speed operation of the high-speed heating mixer, PVC resin, solid lubricant, liquid additive, MBS and acrylic acid processing aid are added in batches, and then turn to high-speed operation, when the temperature rises to 110 ~ Unload the materials into a cold mixer at 120 ℃ for mixing, and discharge them when the material temperature is as low as 40 ℃.

In Φ 65 when producing conjunctival torsion on a single screw extrusion and blowing unit, the temperature setting is as follows: barrel temperature (℃) 165/185/175/175 (Connection)/170 (Rotary)/175/180/190 (Mouth mold)

The temperature of the relevant parts decreased by 3 ~ 5 ° C, production 1 ~ 1. After 5 h, it will drop by 1 ~ 2℃ to prevent PVC decomposition caused by too high temperature. In the production process, attention should be paid to the control of temperature, screw speed, inflation ratio, bubble type, condensation line height, stretch ratio, herringbone plate opening angle, bubble stabilizer height, etc, the longitudinal and transverse mechanical strength of the film is close. Such as tensile strength (Vertical/horizontal)55: 52MPa, elongation (Vertical/horizontal) 150%: 130%, right angle tear strength (Vertical/horizontal)85: 77 (N/mm).

semi-hard and soft PVC films can also be directly blown with dry mixture.

2. Film blown by pellets

The pellets made by plasticizing the dry mixture have the characteristics of convenient feeding, low content of air and volatiles, uniform dispersion of additives and partial gelation, therefore, ordinary single screw extrusion can be used instead of exhaust extruder. In addition, some processing materials, such as vinyl chloride-Vinylidene Chloride Copolymer (PVDC) Stabilizers, plasticizers and other additives are often added to synthesis plants and made into pellets for sale.

The film blowing unit is composed of a single screw extruder, an annulus die, a cooling device, a traction device and a winding device. The die is generally a central feed type. This type of die is supported by a connecting sleeve from the bottom, and a part of the die can be rotated or swung mechanically to change the film thickness distribution.

The Technological points of blowing the film with soft PVC pellets are as follows: barrel temperature/℃ 150 ~ 160/160 ~ 170/170 ~ 180/160 ~ 170 (Head)/180 ~ 190 (Mouth mold) The inflation ratio is 1. 5 ~ 3 draw ratio 4- 6 position of cooling air ring/mm 30 ~ 200 condensation line height/mm 150 ~ 400

for PVDC materials, special film blowing units are used to produce food packaging films. The technological conditions are as follows:

barrel temperature/℃ 130 ~ 150/155 (Mouth mold) Quenching temperature/℃ 5 ~ 8 preheating temperature/℃ 28 ~ 40 blowing ratio, stretching ratio 3 ~ 5
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