Rotoforming as a method of polymer processing
Rotational molding is a technology for processing thermoplastic polymers for the production of hollow products of various configurations and sizes with a wall thickness of up to 20 mm and a volume of up to 25 m3: containers, containers, tanks, large containers, etc.
The method is based on uniform coverage of the inner walls of a hermetic form, rotating in two mutually perpendicular planes, with a dosed by mass of molten material. Split rotor molds are made of sheet steel, copper-nickel and aluminum alloys, melting occurs as a result of external thermal action, distribution – the action of centrifugal forces and good adhesion to the metal surface. The choice of shape is determined by the geometry of the product, its dimensions and requirements for surface quality.
Polymers are used as raw materials in liquid, granular or powder form – linear polyethylenes of different densities, including metallocene, certain types of polypropylene, polycarbonate, polyamide and thermosetting compounds. Thanks to modifying additives, they are distinguished by good fluidity, ease of molding, high strength properties, resistance to cracking, UF-, thermal and light stability.
Stages of the production process
Rotational molding is a high-performance process that consists of several main stages:
•Stage 1. Dosing and loading of polymer into cold mold.
The mass of the material depends on the wall thickness of the final product, the particle size ranges from 50 μm to 200 μm.
• Stage 2. Heating of the mold.
The rotating rotoform is placed in an insulated oven-chamber, where it is heated: with hot air, optimal for products with a wall thickness of up to 15 mm, infrared radiation when using molds of simple geometry, or a gas flame, suitable only for steel equipment. Less commonly used methods include the use of molten salts of KNO3 or NaNO3, as well as heated oils. The working temperature can be from 2200 to 4000, the rotational speed varies from 4 to 20 rpm. Due to the high viscosity and biaxial rotation, the molten homogenized polymer does not run off but is distributed along the inner walls.
• Stage 3. Form cooling.
The cooling process is carried out by blowing air or supplying cold water with the continuous rotation of the mold, which ensures uniform hardening. In simpler plants without cooling chambers, rotary molds are equipped with jackets, where mineral oil or water is supplied. The final mold temperature depends on the type of polymer.
• Stage 4. Removal of the product.
Removing the finished product from the depressurized mold is not difficult, due to its shrinkage during cooling.
Technological parameters of the rotational molding process: rotation frequency, heating temperature, heating time, the volume of feedstock are determined experimentally.
Advantages of rotomolding
The main disadvantage of rotomolding – the duration of the process – is easily offset by a number of its advantages and excellent characteristics of finished products.
The method is highly profitable due to the absence of production waste and makes it possible to manufacture products with different parameters, varying the wall thickness, dye and material, to obtain multilayer structures of a complex configuration with provided technological holes. The finished products are distinguished by high strength due to the absence of internal stresses and violating the integrity of welds.