Melt extrusion includes extruders, melt metering pumps, melt filters, and static mixers.
1 melt extruder
The crystallized and dried PET chips were fed into a single screw extruder for heat and melt plasticization. In order to ensure good plasticization of the PET chips and stable extrusion melt pressure, the structure of the screw is very important. In addition to the length to diameter ratio, compression ratio, and various functional sections, it is also required to be a barrier type screw, because the screw of this structure has the following characteristics: 1) It is advantageous for good plasticization of the extruded material. 2) It is beneficial to the uniform temperature of the outlet material of the extruder. 3) The extruder discharge is stable. 4) Good exhaust performance. 5) It is beneficial to improve the extrusion ability.
If the amount of extrusion is not too large, it is recommended to use a vented twin-screw extruder. The exhaust extruder has two exhaust ports connected to two sets of vacuum systems. It has a good function of exhausting and dehumidifying, which can remove the moisture and oligomers contained in the materials, which can save complexity. The pre-crystallization/drying system saves investment and reduces operating costs.
2 melt metering pump
Melt metering is achieved by high precision gear pumps. The function of the metering pump is to ensure that the melt supplied to the die has sufficient and stable pressure to overcome the resistance of the melt as it passes through the filter, achieving uniformity of film thickness. The metering pump usually uses a slanted two-gear pump. In order to further improve the metering accuracy, some three-gear pumps are also used. Since the three-gear pump has a smaller pulse than the two-gear pump, the fluctuation of the pumping amount is also small. The heating temperature of the metering pump is between 270 °C and 280 °C.
3 melt filter
In order to remove foreign matter such as impurities, gel particles, fish eyes, etc. which may exist in the melt, a filter is often installed before and after the metering pump on the melt line. PET film production lines usually use a dish filter made of stainless steel mesh and stainless steel sintered felt. The size of the stainless steel disc is Φ12 inches, and the diameter of the filter mesh is generally 10-30μ. The filter heating temperature is controlled at 275-285 °C.
4 melt tube
The function of the melt tube is to connect the extruder, metering pump, filter, etc. to the die to allow the melt to pass therethrough. It is required that the inner wall of the melt pipe is smooth and has no dead angle, and the length of the melt pipe connected in series should be as short as possible to avoid the stagnation of the melt therein and the residence time is too long to cause degradation.
The melt from the extruder enters the melt tube and flows through the coarse filter, the metering pump, and the fine filter, respectively, and enters the die. For a three-layer coextrusion line, a melt distributor is also placed above the die. Filters, metering pumps and melt pipes can be heated electrically or by heat transfer jackets. The melt tube heating temperature is controlled at 275-285 °C.
5 static mixer
When the melt flows through the melt pipe, the temperature of the melt along the pipe wall has a large temperature difference with the temperature of the center of the melt. In order to make the temperature of the melt entering the die uniform, to ensure uniform discharge of the die, A plurality of static mixers are installed inside one end of the melt pipe connecting die, and when the melt flows through the static mixer, a mixing-separating-separating-mixing action is automatically generated to achieve the purpose of homogenizing the melt temperature.
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