The mixing of PVC low-foaming dry blend is carried out on a standard heating and cooling mixing unit (this unit is a combination of a high-speed mixer and a cooling mixing unit). The mixing principle of the high-speed mixer is that the material is rapidly scattered along the container wall under the impetus of the high-speed rotating stirring blade and falls from the center of the pot to form a vortex motion to achieve the effect of rapid mixing and uniformity. During high-speed mixing, when the materials are mixed, friction between the materials will be generated, and frictional heat will be generated at the same time. There is also friction between the material, the stirring blade and the pot wall, and frictional heat is also generated. Under the combined action of the external heating source of the container, various additives are quickly infiltrated into the PVC resin to form a semi-gelled dry blend. The principle of the cooling mixer is to pass cooling water through the inner jacket of the pot, and under the low-speed stirring of the blade, the dry blend at about 110-120℃ is rapidly cooled to below 45℃ to prevent the hot material from agglomeration, decomposition and foam. The mixed dry blend discharged from the mixer should be sieved to remove agglomerates and molten particles for the next step of extruding and granulating, or directly extruding low-foaming products.
There are many components in PVC low-foaming formulations. When adding materials, various additives should be added in order, such as stabilizers, impact modifiers, lubricants, foaming agents, foaming regulators, pigments, fillers, etc. This is very critical. The wrong order of feeding will seriously affect normal production and product quality.
The normal feeding sequence is as follows:
(1) Add PVC resin to the high-speed mixing chamber at low speed.
(2) Add stabilizers and metal soaps to the PVC resin at a high speed of 60°C.
(3) Add processing aids, internal lubricants, pigments, impact modifiers, and foaming agents to the material at a high speed of 75°C.
(4) Add foaming regulator and external lubricant under high-speed stirring at about 90℃.
(5) Filler is added at a high speed of about 100°C.
(6) At about 110℃～120℃, remove the material at low speed and discharge it into the running cooling mixer.
(7) In the cold mixer, the mixture is cooled to below 45°C and discharged.
(8) Sieved and packed (or conveying to storage tank).
PVC dry blend should be placed for more than 48 hours after mixing, and then conveying to the pelletizing process or directly to the extrusion molding process.