What is the reason why the stretch film shrinks too much
2017-07-18 11:05:52

What is the reason why the stretch film shrinks too much

Stretch film shrink too large contains two reasons: stretch film in the stretching process is too large rebound, and it in the secondary compound process heat shrinkage is too large, more than the standard requirements ,both Can not reach the width of the product requirements.

Question 1:

Stretch film raw material is an amorphous polymer, in a certain temperature range if the stretch film suffer a power is greater than the yield strength, it will become extension and deformation, in the deformation process the polymer structural units (segments, macromolecules and microcrystalline) start to orientation when it be stretched. With the increase of the degree of orientation, the interaction force between macromolecules will be enhance, and the viscosity of the polymer increases, and the deformation tends to be stable without further development. This phenomenon is called "stress hardening". Properly increase the temperature, the ductility of the stretched film is further improved, The elongation will increase, and even some of the less ductile polymers can be stretched. The deformation whith adjusting the room temperture to nearly tg (minimum processing temperature) is usually referred to as "cold stretch", and the polymer is subjected to stress hardening during the stretching process, which restricts the flow of the polymer molecules, thereby preventing further improvement in the draw ratio. The "cold stretch" stretch film is restored to its original state after the tensile force release, resulting in a larger retraction.

In fact, in the design of stretch film workshop will consider the workshops temperture, air convection problem, because the stretch film in the process of stretching will loss the heat is very fast, the thinner stretch film, the wider the stretch, the heat faster. I measured that the stretch film from the inlet to the outlet only 3metre surface temperature from 175 ℃ down to 135 ℃, so the production workshop temperature is generally not less than 20 ℃. In the design of the workshop only install window in the top of the exhaust. In addition, in order not to cause cold stretch, a thermal insulation room may be formed at the lateral stretching portion to ensure the ambient temperature of the stretched film. It is appropriate to reduce the ratio of the respective machines and to reduce the stretching of the stretched film.

Question 2

Generally, the higher the stretching temperature (above Tg), the greater the elongation, the faster the stretching speed the cooling rate, the higher the degree of orientation, reheating the polymer whith been cooled and stretching orientation to a temperature higher than Tg , Will be obvious retraction, stretch film is made according to this principle. In order not to make the products cause heat shrinkability, the products can be treated at the highest temperature for a certain period of time while maintaining the stretching, and then cooled to room temperature, so that the obtained product has good stability. This can be added in the lower part of the extension of the lower part of the heating device to maintain the tensile temperature; also can be given to the No. 2 temperature through the warm water, so that the stretch film slow cooling, so you can effectively reduce the stretch film Heat shrinkability.