Technology

Pelletizing process

The pelletizing process can take place in two different ways which are differentiated by the pelletizing method that is chosen. It is either pelletizing from extruded strings (from a belt) or head pelletizing.

Head pelletizing is based on the fact that the extrusion machine is equipped with a pelletizing head, typically attached with two screws. It is in fact a perforated plate through which the material is pressed and on which a rotating knife head is held through pressure.

The head is equipped with knives along the perimeter with blades which cut the extruded strings. The pellets are then cooled down by air or water to prevent them getting stuck together. Typically, these cooling media are also used for the transport of the manufactured pellets and their further processing. The pellets made this way have the characteristic shape of lentils or kernels.

Production of PVC plastisols

Production of PVC Plastisols is based on the principle of mixing the powder and liquid components using a mixing machine. Slow-running planetary arm mixing machines or high-revolution mixing machines, called dissolvers, are the most used machines for the production of plastisols.

A powder paste-forming polymer along with powder components is gradually added to the mixture of liquid components (plasticizers). During mixing, the powder components are dipped in the plasticizer, creating the plastisol. The excess air is sucked from the plastisol in the final mixing phase. Plastisols are typically stored in large containers (homogenizers), where the PVC plastisol is slowly stirred and acquires its final characteristics.

Extrusion

Extrusion is a technological operation during which the polymer (polymer mixture with additives) is melted in an extrusion machine and the molten plastic is continually pressed through a profiling device (extrusion head) in a free space. The extrusion technology serves for the manufacture of either final shapes or for the preparation of semi-finished products.

According to the shape of the final product or the shape of the semi-finished product, we can divide the extrusion technology into three basic groups:

• Production of tubes and profiles
• Production of foils and boards
• Production of fibers and sheeting

These technological methods are used mainly by screw extrusion machines (extruders) which do not work alone, but are a part of a manufacturing line where other machines and devices ensure calibration, suction, division, and final adjustment of the shape or surface.

The line for extruding profiles and tubes is composed of a screw extruding machine, an extruding head, calibration, cooling and afterheat removal zone, a towing device, division, winding, and eventually a tilting spout.  One-screw extrusion machines are commonly used, and the non-plasticized PVC is extruded using a two-screw extrusion machine. PVC, HDPE, LDPE, PP, PA, POM, or PBT is typically used to manufacture tubes. Profiles are made of PVC, PS, and their copolymers PMMA, PC, POM, etc.

Fluid application (whirling)

Fluid application uses a powder polymer which is kept in a fluidized bed so it behaves as a liquid. By submerging a heated metal object in the powder, a similar layer of plastic is created on the object’s surface.

“Dipping” technology

Liquid PVC plastisol is used for dipping. Dipping a heated metal object in plastisol creates the required layer of plastic coating on the object’s surface which subsequently undergoes gelatinizing. The dipping method can be used for applying protective or decorative coatings on metal objects. Adhesive varnishing treatment is used to ensure the adhesion of the coating to the metal surface.

Dipping metal molds with a subsequent removal of the products can be used for manufacturing plastic covers and sleeves.