The melt mixing extrusion method is an internationally used method for producing thermosetting powder coatings. Its technological process is:
First, premix. The pre-mixing process is to uniformly mix resin powder, pigment and filler, curing agent, leveling agent and various additives according to the formula to create a good physical condition for melting and mixing, which is more conducive to uniform dispersion. During the premixing, the premixing time can be determined according to the experiment. Once the mixing time is stabilized, it is better not to change it arbitrarily, otherwise it will affect the degree of dispersion of the material and the final color change of the coating film.
This is followed by melt mixing extrusion. The melt mixing extrusion process is an important link in the manufacture of powder coatings. Through kneading, the components of the powder coatings are uniformly dispersed in the molten state of the resin. It overcomes the separation of components caused by different specific gravity of materials during dry mixing.
The melt extrusion equipment is an extruder, and there are two types of extruders, one is a single-screw extruder, and the other is a twin-screw extruder. For materials with different properties, it is necessary to determine reasonable process parameters to ensure a good mixing effect. The extrusion process of thermosetting powder coatings is very strict, and it is necessary to control the extrusion temperature and the residence time of the material in the barrel.
The molten material from the extruder should be extruded into thin sheets immediately and cooled in time to prevent the heat accumulation of the molten material and prevent chemical changes in the components from affecting the quality of the product.
The last process is fine crushing. At present, in order to make the production process reasonable and convenient, the fine crushing machine used in CHINA often combines the crushing machine and the particle size classification part to form a crushing equipment unit. Therefore, the pulverized powder particles are sent into the cyclone separation or rotating cylinder by wind force to achieve classification and finally collect the product.
In the crushing operation, attention should be paid to the problem of the feeding speed and the temperature rise inside the machine. The temperature rise of the machine body will affect the output. Generally, the temperature rises faster at the beginning. If the feeding speed is controlled, the growth will be slow in the future, and gradually increase. It tends to be stable, and continuous production can be carried out if the airflow temperature in the machine does not exceed 40°C.
Sampling should be taken intermittently during production to check the particle size and particle size distribution of the product, find out the cause after any problem occurs, and take timely measures to ensure the high quality of the product and prevent the difference in particle size caused by broken screen or other reasons.
