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

1.3.5 Air knife position adjustment

The ideal casting angle is the one where the web makes a natural tangent to the chill roll surface. Pinning should take place at the point of contact with the chill roll

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and the air knife jet should be normal to the web surface at this point. The jet stream of air used to pin the surface should ideally be extracted away from this area and not be allowed to form vortices between the air knife and die. Vortices cause web flutter which can affect the gauge.

The air knife angle should be adjusted to remove the “crater-like” appearance of the base web on the chill roll surface.

Air knife deckles are used to direct the flow of air to the thick edges.

1.4 MDO machine (general description)

On the first stage, the machine direction oriented (abbreviated MDO) orients the unstretched cast film in the longitudinal direction.

This requires a specific stretching temperature. To warm up the cast film to stretching temperature several pre-heating rolls are needed.

The preheated cast film can be oriented in longitudinal direction in the stretching zone. This zone consists of stretching rolls running at different speeds.

After stretching, the longitudinally oriented film is thermo-fixed by the so-called “heat-setting rolls”.

The pre-heating rolls of the MDO are heated. The temperature is always set for one roll. The first rolls have a chromium-plated surface, the last rolls are Teflon-coated. The Teflon-coating is necessary for a specific temperature program to produce the co-extruded film. The stretching unit consists of 4 rolls. The usual stretch ratio is between 4,8 and 5,3.

Limits are set by the minimum and the maximum stretch ratio. A too low longitudinal stretching ratio will cause excessive thickness tolerances in a longitudinal direction. If a too high stretch ratio is chosen, the film is oriented too much in a longitudinal direction and it cannot be stretched in transverse direction without breaking in the TDO.

Increasing the stretch ratio increases the tensile strength of the film and reduces the elongation at break.

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During longitudinal orientation, the film becomes thinner concerning the stretch ratio. In the short-gap stretching system the neck-in of the film during the stretching process is so small that it can be disregarded.

After longitudinal stretching the film passes over the heat-setting rolls for thermo-fixing. By increasing the temperature and decreasing the speed between the heat-setting rolls it is possible to reduce the thermal shrinkage of the film during heat treatment. At an excessive fixing temperature, the film becomes hazy. Too low speed can cause wrinkles.