Polyvinyl chloride shrink film production process conditions

1) Rapid cooling after biaxial stretching, "freezing" or "memorizing" the molecular orientation of the biaxially stretched PVC film, and shrinking when heated during use. This process is the key to the production of shrink film.

2) Inflation ratio and traction ratio, the inflation ratio is actually the main factor that determines the degree of transverse orientation and expresses the degree of transverse stretch. The draw ratio is actually the main factor in determining the degree of machine direction stretch orientation. Therefore, its size directly affects the shrinkage rate of the shrink film. When the die gap is 0.4-0.7mm and the inflation ratio is 3.5-4.5, a shrinkable film with better shrinkage can be obtained.

3) The height of the frost line. For the upward blowing method, the transverse shrinkage rate increases rapidly with the rise of the frost line. When the longitudinal shrinkage rate gradually increases to 15mm, it tends to be flat. An excessively high frost line reduces the stability of the tube bubble , generally controlled at around 150mm.

4) Die gap, when the film thickness and bubble diameter remain unchanged, increasing the die gap will inevitably lead to an increase in the traction ratio and increase the longitudinal shrinkage rate. When the die gap is small, the melt will expand rapidly when it comes out of the die and make the film thinner, the draw ratio becomes smaller, and the shrinkage rate decreases. Therefore, it is also very important to determine the size of the die gap. When producing 0.02-0.04m film, the die gap is usually 0.6-0.8

5) The relationship between film thickness and shrinkage rate, the general rule is that when the film thickness increases, the longitudinal and transverse shrinkage rates will gradually decrease, and when the shrinkage rate is less than 0.02 ~ 0.06mm, the shrinkage rate does not change much, so the shrinkage rate is determined as a value , try to keep the film thickness tolerance within the specified range to maintain a balanced shrinkage rate.

6) The influence of cooling speed, the faster the cooling speed, the faster the bubble will be shaped, and the greater the frozen stress will be, which will increase the orientation degree of the film and increase the shrinkage rate. For this reason, the cooling air flow inside and outside the bubble can be adjusted. Change the cooling rate.

7) Example of process condition control.

30mm diameter shrink film unit, the temperature control of each section of the extruder is: feeding section 140-150°C; compression section 160-180°C; homogenizing section 170-190°C; head 170-190°C.

Host screw speed: 40～50r/min

Heat stretching temperature: 110-150°C for the first stage; 100-140°C for the second stage.