1. Introduction to thermoplastic elastomers
Thermoplastic elastomer TPE/TPR, also known as artificial rubber or synthetic rubber. Thermoplastic elastomers have the elasticity of rubber at room temperature, and a type of elastomer that can be plasticized at high temperatures. Its products not only have the excellent properties of high elasticity, aging resistance and oil resistance of traditional cross-linked vulcanized rubber, but also have the characteristics of convenient processing and wide processing methods of ordinary plastics. It can be produced by injection molding, extrusion, blow molding and other processing
methods and 100% of the corners of the nozzle can be directly used for secondary use after being crushed. It not only simplifies the processing process, but also reduces the processing cost. Therefore, thermoplastic elastomer TPE/TPR material has become the latest material to replace traditional rubber. It is environmentally friendly, non-toxic, comfortable to the touch, and beautiful, making the product more creative. Therefore, it is also a more humane and high-grade new synthetic material, and it is also a world-standard environmental protection material.
2. Formula and raw material characteristics
(1) Formula: SBS/SEBS+PP+OIL+filler
(2) Raw material characteristics:
SBS/SEBS, the bulk density of raw materials is very low, 0.2kg/m3, very light, foamy, takes up a lot of space, and is not easy to add into the screw.
OIL: Added to the extruder as a lubricant, slowing down material delivery.
3. Process of Twin Screw Extruder for Thermoplastic Elastomers
Ingredients – premixing – melt mixing – pelletizing
4. Selection of Twin Screw Extruder for Thermoplastic Elastomers
Among many mixing and plasticizing extruders, the parallel co-rotating twin-screw extruder is used in plastic compounding due to its excellent mixing and plasticizing performance, good screw self-cleaning function, and the “building block combination” function of the screw barrel. It is widely used in mixing and fully used in the production of thermoplastic elastomers. In the selection of production equipment for thermoplastic elastomers, the configuration and selection of twin-screw extruders is a very important link, which has an important impact on future production. The selection mainly has the following characteristics:
4.1 Selection of parallel twin-screw extruder
4.2 Screw diameter: According to the production scale, the screw specification is generally selected from 50mm to 90mm.
4.3 Length-to-diameter ratio L/D: ≥44, generally 48
4.4 Screw speed: ≥600rpm
4.5 Screw material: It is recommended to use wear-resistant and corrosion-resistant types, such as W6Mo5Cr4V2 high-speed steel, and the cylinder body should use a high-wear-resistant bushing structure, such as lining a-101 high-wear-resistant alloy.
4.6 Motor power: high power.
4.7 Gearbox: small speed ratio, high torque grade, specific torque is usually M∕A3≥8.
4.8 Host configuration: There must be a sufficient L/D ratio to make the material fully heated sheared and plasticized; the entrance must have a large free volume to enhance the effect of feeding materials, and it must have a good exhaust function to remove low-molecular substances and monoliths.
5. Feeding method
Since the raw material is a powdery or granular mixture, considering the characteristics of the material and the conveying efficiency, it is recommended to use a twin-screw feeder with stirring. Forced tamp-feeding is not recommended.
6. Screw combination and barrel design
6.1 Feeding section: Since the SEBS/SBS material is bulky, it is not easy to eat the material when it is added to the screw, so large lead and deep groove screw elements should be used. The feeding port of the barrel is rectangular.
6.2 Conveyor section: Choose a large compression ratio to compress and convey materials quickly.
6.3 Mixing and plasticizing section: choose more kneading blocks, 30 degrees, 45 degrees, 60 degrees kneading blocks can be used, and cooperate with the banburying rotor to realize distribution mixing. The mixing rotor increases the drag and stretch, and also increase the dispersion of the material.
6.4 Exhaust section: There must be a large exhaust space, usually using large-lead screw elements; setting a reverse element on the screw in front of the exhaust port to realize the pressure building function.
6.5 Homogenizing metering section: It must have strong conveying function and steady flow function to achieve stable extrusion pressure and uniform granulation. Usually, small lead elements are used, and the L/D ratio greater than 6.
7. Vacuum exhaust:
When producing TPE with a parallel twin-screw extruder, use vacuum to remove low-molecular volatiles and odors in the product, which is very beneficial to improve product quality.
The vacuum port is easy to leak, and it is the key to control the vacuum port not leaking.According to the formula and operation process, the factors that cause the vacuum port to leak under normal circumstances are:
7.1 Too much oil is added. The material flow speed is accelerated, and the material delivery is unbalanced near the vent hole.
7.2 The processing temperature is low.
The solution is to improve the formula and increase the process temperature;For the extruder: the vacuum port should be set in a proper position, a suitable stopper should be selected, and a reasonable screw structure should be designed.
If the above method still can not solve the problem of leaking at the vacuum port, the screw must be reassembled.
8. Cutting method
Generally, water ring pelletizing, underwater pelletizing, or air-cooled die surface pelletizing are used. Select according to material performance requirements. For materials with a hardness below 10A, air-cooled die surface pelletizing should be chosen.
Comparison of the characteristics of these three pelletizing methods:
| water ring pelletizing | underwater pelletizing | air-cooled die surface pelletizing | |
|---|---|---|---|
| pelletizing | dried pelletize | Pelletize in water | dried pelletize |
| cooling | water cooling | water cooling | air cooling |
| output | Moderate | large | small |
9. Ways to increase the output of the extruder
9.1 To increase the screw speed, a large L/D must be used to ensure that the material is fully plasticized.
9.2 Side feeding can increase the output by 40%, which is applied in many manufacturers.
Case
Product performance: Shore hardness 55A, density 0.98
SHJ-75H high-performance co-rotating twin-screw extruder
Screw diameter: 72mm
Screw aspect ratio: L/D=48,
Screw speed: 600r/min,
Extruder power: 250kW
Operating temperature: 190°C, 200°C, 210°C, 210°C, 210°C, 210°C, 200°C, 200°C, 190°C, 180°C, 170°C
Underwater pelletizing: Output: 500-600kg/h
Screw combination from blanking to machine head: 9 (conveying parts) + 5 (large and medium kneading blocks, ban burying rotor) + 4 (conveying parts) + 8 (medium and small kneading blocks, ban burying rotor) + 3 (conveying parts)+3(medium and small kneading blocks)+6(conveying parts)+4(medium and small kneading blocks, mixing rotor)+8(conveying parts)
