Although the membrane form is not a blown film component, it is directly affected, so it is also in this chapter. Typically, after the diameter is small in the mouth of the film bubble bottom portion, it is gradually solidified along the axis to form a cylinder having a large diameter thickness. After a certain height, the membrane is completely cured in diameter and no longer change.
Membrane form can be described by the following parameters
- Mouth diameter
- Dictation gap
- Cream height
- Membrane diameter
- Membrane thickness
The mouth diameter determines the diameter of the film bubble start, and the mouth-mode gap determines the film thickness at the beginning of the membrane bubble.
The raw material is melt molten and is cooled. After a certain temperature, the raw material condensation is solidified. The distance required for this segment is referred to as the cream height. “Frost” is an operator’s image metaphor, where the polymer melt cooling solidification forms a crystal, and the transparency of the film decreases, it seems like frost. The convention is confirmed, where the cream is located in place in the largest diameter of the membrane, which is no longer a blow to stretching the membrane. The cream is called the cavity, especially this paragraph looks more like a neck. The frostline is solidified by membrane, generally to be described in the thickness of the bubble and film. When the membrane enters a traction, the cylindrical membrane bubble is sandwiched as a two-layer film, which is widely used to describe the membrane. Two times the width is the membrane perimeter. Since the width is better than the measurement, the film bubble diameter is defined using a width.
Several process parameters can determine the shape of the membrane:
- Melt longitudinal flow velocity
- Traction speed
- Membrane internal volume
- Cooling efficiency
The longitudinal flow rate of the melt refers to the flow rate of the machine when the material leaves the mouth. It is determined by the screw speed but is different from it. The traction speed refers to the line speed of the membrane bubble in the traction clamp. In fact, the movement speed of the film above the cream is equal to the traction speed. All blown film processes are so that the film is moved at the melt extrusion mode in the melt out of the melt, and moves above the frostline. Intermediate Acceleration Movement The thin film makes it possible to obtain a suitable thickness of film products. The internal volume of the membrane is also characterized by the volume of the air in which the film can also be used. The cooling efficiency is currently flowing through the surface of the membrane surface and its temperature determination. Other factors also affect the morphology of the membrane, such as process temperature, mold design, formulation, and raw material flow, but these are basically quantified for a certain processing process.
There are two membered shapes (Figure 3.9) mainly have two applications for different types of raw materials formulations. The bowl of meticulubbubs have almost no bubble, blown directly on the surface of the die.
This bubble is suitable for processing of LDPE, LLDPE, and PP. Bowl film bubbles were supernatated as early as possible, and the bubble shape was relatively stable.
Another kind of foam is a long-necked film. Mainly suitable for processing of HDPE. Since the melt strength of the HDPE is relatively low, the long-necked method causes the melt lateral tensile time, and there is sufficient space to cool, increase the intensity, thereby obtaining a relatively stable film bubble.
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