Rotational molding common materials

Both neat polyolefins and their blends and composites are commonly processed by blow molding, injection molding, extrusion, compression molding, rotational molding, and thermoforming. The final materials can be produced as bulk materials, fibers, and thin films. 

TABLE 8.14 Recommended Wall Thioknesses for Commonly Rotationally Molded Materials... 

For the economic manufacture of seamless hollow articles made of plastic materials, the well-known procedures for blow molding and rotational molding can be used. In the origins of rotational molding, PVC or plastisols were used for the manufacture. Since "industrialization of rotational molding in the 1950s, PE is the most widely used processed material. PE is available in different densities and qualities, dry blended and compounded, ultraviolet-(UV)stabilized, electrically conductive, and phosphorescent. Other common materials are PVC, as well as PP, PA 6, PA 12, and PC. All of the RAL colors are feasible and even colors that imitate natural colors are possible (e.g., stone, terracotta, etc.). 

For powders, the low-shear melt viscosity must be low enough to allow the material both to wet the internal surface of the mold and to fuse into itself in a homogeneous mass. The common measure used as a guide to the melt flow properties of materials is the Melt Index (MI) measured in g/10 min. Most commercially available rotational molding polyethylenes have an MI between 2 g/10 min and 8 g/10 min (2.16 kg/190°C). Higher flow rates may be required for parts with complex molding details. 

PVC is available in two distinct forms for rotational molding, either liquid or dry. Liquid materials in the form of plastisol and organosol have been the mainstay of PVC molding since their introduction in 1947. Dry powders or vinyl micropellets were introduced in the early 1980s. Vinyl plastisol is the most common form of PVC used for rotational molding. 

The bulk of materials ground for rotational molding are polyethylenes (95%+ of the market). Polyethylene is a relatively tough material that can be difficult to divide. The most common method of pulverizing is performed using high-speed attrition mills that grind pellets approximately 3/16-1/4 in. (5-6 mm) in diameter down to the required particle size-distribution. 

These factors in combination with the overall structural requirements dictate common choices of mold materials for different processes. For example, high-grade tool steel is normally required for injection molding because of high pressures, while rotational molding tools can normally be built by fabricating metal plate. The requirements for low pressure forming molds can even be simpler—wood or nickel-coated epoxy will often suffice. 

Co-injection molding involves injecting two or more different materials into a mold, generally in sequence. To accomplish this, molding machines are fitted with multiple injection units (Figure 14-53). Two-shot molding, the most common version, often utilizes an indexing mold. In this process, the mold is rotated 180° after the first shot fills a portion of the cavity, then the second shot finishes the part... 

A typical arrangement for a direct screw-transfer injection-molding machine for thermosets is shown in Figure 2.16. The machine has two sections mounted on a common base. One section constitutes the plasticizing and injection unit, which includes the feed hopper, the heated barrel that en closes the screw, the hydraulic cylinder which pushes the screw forward to inject the plasticized material into the mold, and a motor to rotate the screw. The other section clamps and holds the mold halves together under pressure during the injection of the hot plastic melt into the mold. 

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