Магистерская диссертация тема работы Разработка цифрового двойника технологического процесса с использованием производственных данных

1.4.1 Temperature program for coextrusion

There are limits for setting the stretching temperatures of the MDO since the film will stick to the chromium-plated rolls. The sticking point for PP homopolymer is at approx. 140 – 145°C. For PP copolymer, it is approximately 120 - 125°C. Consequently, a plain film can be stretched at temperatures of 140 – 145°C, while a co-extruded film with PP copolymer as outside layers must be oriented at temperatures of 120 – 125°C. The required lower stretching temperatures for a co-extruded film would result in an increased thermal shrinkage and impaired sealing properties.

To be able to stretch the PP homopolymer in the inner layer at higher temperatures than 120 – 125°C, the Teflon-coated pre-heating rolls are used. Considerably higher temperatures can be adjusted for the Teflon-coated rolls without making the film stick. Experience has shown that for coextruded film temperatures up to 134°C can be set for Teflon-coated rolls.

Thus, also the film core can be heated up to 132°C. For the ensuing stretching process, however, chromium-plated rolls are necessary. For the process, the outside layers of the film must be cooled down again to 120 – 125°C.

Since the radii of the stretch rolls are considerably smaller than the radii of the pre-heating rolls and therefore the residence time of the film on these rolls is shorter, only the outside layers of the film are cooled.

By overheating the sealing layer on the Teflon-coated pre-heating rolls and by subsequent cooling down of the outside layers on the cooling rolls, it is possible to

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orient the coextruded film at optimum temperatures for each layer, i.e. approximately 120°C for the sealing layer and approximately 132°C for the core layer.

1.4.2 Pre-heating rolls

The preheating section heats the base web to just below the sticking point of the copolymer. The latter rollers are Teflon-coated to reduce sticking. Crystallinity is not alerted at this temperature but stretching is made easier.