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Plastics Processing

The properties of plastics and the many different requirements depending on finished products made from them have led to the development of a number of methods for shaping and molding these materials. From the manufacturers who synthetically produce plastics for industrial use the fabricating industry obtains the specified initial materials, i.e., the appropriate polymers with or without the requisite additives.


In the latter case the user will have to add auxiliary materials such as plasticizers, stabiliziers, pigments and fillers. Batch mixing of powdered ingredients is performed in agitators or mixing drums. Alternatively, kneaders or mixing rolls (Fig.1) are used for plastifiable materials. The last mentioned device comprises a pair of rollers which revolve in opposite directions and which can be heated or cooled as required.

The material entering the gap between the rollers is squeezed and mixed. On completion of this treatment the so-called rough sheet is stripped from the rollers (Fig.1) and passed to a further stage of processing. Continuous mixing is performed in extruders, which offer the additional advantage of filtering the plastics before they undergo further processing (Fig.4).


The shaping of plastic articles and components without the application of pressure is effected by casting. The simplest method of shaping in conjunction with pressure is by molding (Fig.2), which is suitable for both thermosetting and thermoplastic compositions. Thermoplastics can be softened by the application of heat; thermo setting plastics undergo chemical change under the action of heat and are thereby converted to infusible masses which cannot be softened by subsequent heating.


The Plastic Processing programme has been drawn up by the Finnish plastics industry, the universities and Tekes with the aim of improving the competitiveness of companies developing injection moulding, and manufacturing and using injection-moulded components. The programme is finished.

The aim of the entire programme was to improve the competitiveness of the companies in the business chain. This was achieved by speeding up the product development process, the mould manufacturing process and making them more efficient and by controlling the production process considerably better than at present. An additional goal was to improve mould performance, speed up start-up times for production processes and minimize environmental impacts.


Another method of producing molded articles is by injection molding (Fig.5), which has the advantage over ordinary molding that preheating, plasticizing, and shaping are done by the same machine. The only materials suitable for injection molding are thermoplatics of high fluidity. The granules are introduced through a hopper into the cylinder, in which they are heated-by means of a heating jacket- to above their softening point. A moving piston plasticizes the material and forces it through a nozzle into the mold. The plasticizing action can be enhanced by the use of a screw instead of a piston (Fig.6).

Articles or components can also be shaped by the machining of semifinished products films, sheets, rods or tubes. Machining is more particularly employed in cases where the articles are of complex shape or where only a small number are required. Whereas thermosetting plastics can be shaped only by machining (milling, turning, cutting, drilling) once they have hardened, semifinshed thermoplastic materials can be shaped by heating and joined by welding. Hot shaping of thick sheets can be effected by bending or drawing (Fig.3). In the drawing process the material to be shaped is gripped, heated and deformed to the desired shape. If the wall thickness must remain constant, the sheet must be resiliently gripped; with so-called stretch forming a reduction in wall thickness occurs.

In recent years shaping by the vacuum process has gained importance. In the female-mold method or negative mold method, the heated plastic sheet is laid on a concave mold and subjected to further heating. Air is extracted through holes in the mold, so that the sheet is drawn by suction into the mold. For the molding of complex components the plate is prestretched before the actual negative molding operation begins.

Alternatively, a convex master model may be used, in which case the process is known as the male-mold method or positive-mold method. The preheated sheet is placed over the master model and preformed. When the air is evacuated, the desired shape is obtained. The molding techniques are schematically illustrated in Figs.8 and 9.

Endless products such as sections, sheet, strip and thin are produced by extruders (Figs. 4 and 6). Extrusion consists in forcing a plastic material through a suitably shaped die to produce the desired cross-section shape. The extruding force may be exerted by a piston or ram (ram extrusion) or by a rotating screw (screw extrusion) which operates within a cylinder in which the material is heated and plasticized and from which it is then extruded through the die in a continuous flow.

Different kinds of die are used to produce different products e.g., blown film formed by blow head for blown extrusions, sheet and strip slot dies and hollow and solid sections i.e. circular dies. Wires and cables can be sheathed with plastics extruded form oblique heads. The extruded material is cooled and is taken off by means of suitable devices which are so designed as to prevent any subsequent deformation.

For the manufacture of large quantities of film or thin sheet, the sheeting calender is employed (Fig.7). The rough sheet from the two-roll mill is fed into the gap of the calender, a machine comprising a number of heatable parallel cylindrical rollers which rotate in opposite directions and spread out the plastics and stretch the material to the required thickness.

The last roller smoothes the sheet or film thus produced. If the sheet is required to have a textured surface e.g., to resemble wood graining, the final roller is provided with an appropriate embossing pattern; alternatively, the sheet may be reheated and then passed through an embossing calender. The calender is followed by one or more cooling drums. Finally the finished sheet or film is reeled up.

Another field of application consists in coating a supporting material e.g., textile fabrics, paper, cardboard, metals, various building materials with plastics for the purpose of electrical insulation, protection against corrosion, protection against the action of moisture or chemicals, providing impermeability to gases and liquids, or increasing the mechanical strength. Coatings are applied to textiles, foil and other sheet materials by continuously operating spread-coating machines (Fig.10).

A coating knife, also known as doctor knife ensures uniform spreading of the coating materials in the form of solutions, emulsions or dispersions in water or an organic medium on the supporting material, which is moved along by rollers. The coating is then dried. Alternatively, the coating applied to the supporting material may take the form of film of plastic, in which case the process is called laminating.

Metal articles of complex shape can be coated with plastics by means of whirl sintering process. The articles, heated to above the melting point of the plastics, are introduced into a fluidized bed of powdered plastics a rising stream of air in which the powder particles are held in suspension, whereby a firmly adhering coating is deposited on the metal by sintering.