378 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS EXTRUSION Extrusion by screw extruders is one of the long-established techniques of the plastics industry. In this process material is heated and softened during its passage along a screw contained in a heated barrel. It is then forced through appropriately heated dies attached to the barrel-end. By this process collapsible tubes from which lengths may be cut and afterwards sealed are easily produced, as also are semi-rigid tubes to which may be fitted end pieces either moulded or pressed from sheet. A development of the extrusion technique is that of extrusion blowing to produce thermoplastic tubes, bottles and containers. The method employed is, in many respects, similar to the long-established practice used to blow and mould hollow glassware and, in principle, comprises the extrusion of a hollow tube, the end of which is sealed. A mould is closed round the extru- sion while it is still soft and air is blown into it, expanding it to the walls of the mould. Both semi-rigid bottles and flexible bottles may be produced. Within limits imposed by orientation of the materials during extrusion, almost any material that will extrude may be used for this process. THERMOPLASTIC SHEET FORMING Thermo-plastic sheet materials are made into their sheet forms by a variety of methods, including slicing from cast or pressed blocks, extrusion, casting and calendering. Materials suitable for forming are celluloid, the acrylates (Perspex), P.V.C. co-polymers and now toughened polystyrene, which in the United States has started an almost entirely new branch of the moulding industry known as vacuum forming, which has passed from infancy to manhood without passing through the stage of juvenile delinquency. Special machines are available which will automatically heat the sheet to its working temperature, clamp it over the mouth of a simple cavity mould of shallow depth and apply a vacuum to the mould causing the soft sheet to snap into the mould and follow accurately its contour and reproduce whatever detail has been provided on the mould surface. The moulds are inexpensive and they may be made from plaster or wood, but aluminium or low-temperature melting alloys are preferred for long runs. Cavities may be single or multiple, and by planned spacing both containers and caps may be formed from the one sheet. A development of this process is to produce polystyrene sheet from an extruder situated immediately behind the vacuum-forming table. By this means only granulated or ground materials are purchased and sheet can be produced in any desired colour or width within the consumer's factory. There is also a saving in cost as it is not necessary to re-heat the sheet before

PLASTICS FOR PACKAGING 379 forming, and off-cuts and scrap may be ground and fed directly back to the extruder. DiP MOULDING Dip moulding consists of dipping a former into a fluid plastic material and removing the former when the plastics shell so formed has set. Threads may be formed in the moulding and caps can be cemented in place. Wall thicknesses from 0.003 in. upwards are obtainable. Colour possibilities very good. PLASTIC FILM There are many processes used for the production of packs from plastics matehal and those making use of plastic film cannot be omitted. Basically the production of bags from plastics film involves cutting of patterns of the required size, folding to shape and sealing the edges together. In the cutting of patterns use is made of the fact that plastics films are sold in reel form. The film may, therefore, be built up in lays and guillotined or stamped out. The most suitable methods of sealing to form the pack may vary accord* ing to the type of plastics film being used and the form of pack required. Table I gives the more common methods used for sealing the various films. HIGH FREQUENCY SEALING This method requires that the surfaces of the two or more plastics films to be joined should be brought into intimate contact and at the same time be subjected to the influence of a high frequency alternating electric field. By this means heat is generated within the films, which become soft and are welded together. TABLE I Cellulose Acetate Cellulose Acetate Butyrate Ethyl Cellulose Polyethylene Terephthalate Polystyrene and Toughened Polystyrene Polythene Nylon Polyvinyl Chloride Copolymers of Polyvinyl Chloride Polyvinyl Alcohol Copolymers of Vinylidene Chloride High Frequency Se• Seali t Solvent xg Welding x x x x x x Adhesive Sealing