In the production process of TPU film, TPU particles need to be extruded and blown film, extruded cast or extruded calendering, and the TPU base film of the invisible car coating generally adopts the extrusion casting molding method, of which extrusion molding is a very critical step, this article will take you to understand TPU extrusion processing technology and common problem solutions.
Thermoplastic polyurethane (TPU) can be processed on most general-purpose extruders. However, the TPU melt viscosity is high, and in order to ensure that the melt can be fully plasticized, the TPU extruder should be equipped with a high-power motor. Generally speaking, the power of the extruder motor of 45mm and 65mm screw should not be less than 18KW and 30KW.
1. Equipment
1. a screw
Since thermoplastic polyurethane (TPU) is very sensitive to shear, we recommend the use of gradient screws or abrupt screws with long compression segments.
Screws with mixing devices and nylon screws with short compression sections are not suitable for TPU machining. The former has a strong shear effect, which is easy to degrade TPU, resulting in a sharp decrease in melt viscosity and inability to form. The latter can not make the TPU melt fully plasticized, and the surface of the product is easy to produce crystal points.
The following figure is a schematic diagram of the TPU extrusion screw:
The length ratio of the feeding section, the compression section and the metering section is in the range of 4:6:5.
The length-diameter ratio of the screw should be between 25:1 and 30:1, and the compression ratio should be around 3:1.
2. Multi-well plates and strainers
In order to prevent non-meltable impurities in the TPU melt from entering the die, a perforated plate and a strainer should be installed between the barrel and the connector. The mesh count is generally between 80-120 mesh.
Another important role of the filter is to make the melt generate enough pressure in the machine head, so as to increase the strength of the melt to facilitate molding.
3. a die head
Since TPU is a heat-sensitive material that is extremely sensitive to shear, the design of the extruder die and connector needs to be smooth and geometrically smooth and there are no dead spaces. This prevents TPU from being difficult to remove due to accumulation in dead spots.
In addition, due to the high viscosity and elasticity of the TPU melt, the die setting section should be long enough to eliminate the memory deformation of the polymer.
II. Conditions for Submission
1. Drying
The moisture in the TPU chips can degrade the melt, affecting the processing performance as well as the mechanical properties and appearance of the product. Therefore, TPU must be dried before it can be extruded. The drying temperature varies depending on the hardness, generally between 95-110°C, and the time is about 3 hours.
2. Processing temperature
TPU melt viscosity and viscosity are very large. In order to ensure that the TPU melt can be fully plasticized and at the same time minimize the viscosity of the surface of the product, we recommend a low to high temperature distribution from the feed section to the metering section, and a descending temperature distribution from the metering section to the die. The lower die temperature also helps to improve the melt strength. The temperature distribution is as follows:
In some cases, a high-to-low temperature distribution can also be used. This means that the temperature of the feed section is the highest, and the temperature of the die head is the lowest. This temperature distribution is more conducive to the full plasticization of the TPU melt. However, when processing TPU with high crystallinity, the material may adhere to the screw in the feeding section and not move forward, resulting in feeding blockage.
TPU processing temperature varies depending on the size of the screw. Due to the large shearing effect, the processing temperature of large-size screws should be lower than that of small-size screws.
3. Screw speed
As mentioned earlier, TPU is very sensitive to shear, so the screw speed should not be too high. However, if the screw speed is too low, the melt will stay in the barrel for too long and degrade. Generally, the speed of the screw should be between 20-50RPM. The selection of the appropriate screw speed should be based on the complete plasticization of the melt and the smooth surface of the product without bubbles.
3. Analysis of common problems
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frequently asked questions |
Possible causes |
Suggest a workaround |
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The size of the extruded parts is uneven |
The pressure is unstable, the feeding is unstable |
Increase the pressure of the machine head, replace the finer filter screen, reduce the temperature of the feeding section, and reduce the speed of the screw |
|
The surface of the extrusion has an unfused crystal point |
The melt crystallizes in the nose |
Increase the temperature of the machine head, reduce the speed of the screw, check whether the design of the machine head is reasonable, and check whether the design of the screw is reasonable |
|
There is foam or bubbles on the surface of the extruded part |
The water content of the slice is too high, and the processing temperature is too high |
Dry thoroughly and reduce the processing temperature |
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There are black spots on the surface of the extruded part |
Material dissolution |
Clean the equipment, clean the filter, check whether the design of the machine head is reasonable, and reduce the processing temperature |
|
The surface of the extrusion is rough |
The temperature of the machine head is too low, the moisture content of the slice is too high, the melt problem is too low, and the setting section of the machine head is too long |
Increase the temperature of the head, dry it sufficiently, increase the temperature of the barrel, shorten the length of the qualitative section of the head, and reduce the speed of the screw |
|
There are line marks on the surface of the extruded part |
The melt crystallizes in the machine head, the machine head is damaged, the water content of the slice is too high, and the head temperature is unreasonable |
Clean the head, replace the head, dry it thoroughly, and adjust the temperature of the head |
Source: Dongguan Bangtai Plastic Technology Co., Ltd
