This invention relates to an apparatus and method for densifying plastics material, whether in expanded (foamed) or other form.

Plastics products in some forms (e.g. bottles and other containers) and expanded plastics material have a low weight-to-volume ratio. The waste material is difficult to dispose of, particularly from the environmental point of view and particularly in view of the large volume which it occupies for transporting and dumping. The material can also be destroyed by incinerating but this requires high temperatures to ensure complete combustion.

It has been proposed in the past for scraps of plastics materials to be re-used in extruding apparatus. In GB 1 572 623, such scraps of plastics material are fed by a rotary arrangement of mechanical blades into an elongate V-section furnace having an extruder screw disposed along its bottom. The plastics scraps are melted within the furnace and an extruded product issues from an orifice at the end of the extruder screw. The elongate V-section furnace is formed on its inside surface with a series of vertical fins.

We have now devised an improved apparatus and method for densifying plastics products and material, to reduce very considerably their volume and thus enable the material to be tansported and stored more easily and at the same time enabling the material to be recycled or reused for new products.

In accordance with this invention there is provided a densifying apparatus for plastics material, comprising a chamber for receiving the material to be densified, the chamber having a wall which is cast from metal and incorporates a heating element cast therein, and an outlet in the bottom of the chamber through which molten plastics material flows under gravity.

Preferably the inner surface of the heated wall of the chamber is coated with a polymeric material e.g. PTFE such

as Teflon. This polymeric coating seals the grain structure of the metal casting, and gives the surface a low friction finish to assist the gravity-flow of the waste materials through the chamber.

Preferably the inside surface of the heated wall of the chamber tapers towards the bottom outlet. Preferably this tapering surface is circular in horizontal cross-section, and may be conical.

The molten waste plastics material flows under gravity out of the apparatus through the bottom outlet, or it may flow under gravity through the bottom outlet and into a cooling chamber where it solidifies. In either case the passage of the melted plastics is unaided by any mechanical means. The cooling chamber may have valves at its inlet and outlet. In one alternative, the cooling chamber may comprise a rotor having a number of cavities for receiving a quantity of the molten plastics, cooling to solidify that quantity of plastics, and ejecting the solidified body, at successive rotary positions of the rotor. In another form, the cooling chamber may comprise a horizontal slide plate with at least one .cavity for receiving the molten plastics, for cooling and solidifying the plastics, and slidable to another position at which the solidified body is ejected.

The apparatus may be used for densifying expanded polystyrene, or products formed of polypropylene, polycarbonate, PET or other plastics.

The apparatus preferably includes a thermostatic control system arranged to maintain the chamber close to a predetermined temperature. This temperature needs to be high enough to melt the plastics without being so high that it will carbonise or ignite the material, thus giving off noxious fumes in large volumes. Small volumes of such fumes are however given off and an exhaust port for this is provided. Preferably also the chamber is provided with a meter for measuring the

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concentration of pentane or other noxious fumes within the interior of the chamber, and giving a warning and/or shutting down the apparatus should the concentration of these noxious fumes exceed a predetermined level.

Also in accordance with this invention there is provided a method of densifying plastics material, comprising placing the plastics material in a chamber which has a wall cast from metal and incorporates a heating element cast therein, energising the heating element to render the plastics material molten, and allowing the molten plastics material to flow under gravity through an outlet in the bottom of the chamber.

The solid plastics bodies formed by the apparatus or method of this invention may be used as fuel elements, e.g. to assist incineration of other waste or to fuel industrial boilers.

Embodiments of this invention will now be described by way of examples only and with reference to the accompanying drawings, in which:

FIGURE 1 is a front elevation of an embodiment of apparatus for densifying plastics material;

FIGURE 2 is a vertical section through another embodiment of apparatus for densifying plastics material;

FIGURE 3 is a vertical section, on enlarged scale, through an outlet chamber of the apparatus of Figure 2;

FIGURE 4 is a vertical section through one alternative form of outlet chamber for the apparatus of Figure 2; and

FIGURE 5 is another form of outlet chamber for the apparatus of Figure 2.

Referring to Figure 1 of the drawing, there is shown an apparatus for densifying plastics, the apparatus comprising a closed chamber 10 supported on a frame 12 having legs 14. The chamber comprises a vertical cylindrical portion with a conical portion 16 at its lower end and a conical portion 18 at its upper end. The upper conical portion 18 and the cylindrical portion are preferably formed of steel and the lower conical portion 16 is preferably cast from aluminium or aluminium alloy. Electrical

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heating elements 20 are cast into the lower conical portion 16. An inlet 22 is provided into the top of the upper conical portion and an outlet 24 is provided from the bottom of the lower conical portion: a ilter 26 is provided in the outlet and the filter and outlet are heated in order to maintain the material in molten form as it flows from the outlet. The interior surface of the chamber is provided with a low friction coating e.g. Teflon.

A thermostatic control system 28 responds to a temperature sensor (e.g. a thermocouple) embedded in the wall of the lower conical portion 16, to control the current supplied to the electrical heating elements and so maintain the conical portion 16 close to a predetermined temperature. A temperature sensor 30 (e.g. a bimetallic switch) is mounted on the outside of the lower conical portion 16 to switch off the electrical heating elements 20 if the sensed temperature rises above a threshold.

The chamber 10 is provided with an exhaust 32 for noxious fumes given off from the plastics material in use of the apparatus. The chamber is also provided with a meter 34 for monitoring the concentration of noxious fumes within the chamber 10 and switching off the electrical heating elements 20 if the concentration of such fumes exceeds a threshold.

In use of the apparatus shown in Figure 1, plastics material or products are first granulated and then entrained in a stream of air which is directed into the chamber through the inlet 22. The granulated material circulates over the interior surface of the chamber as it falls under gravity, and in so doing it is heated and finally, when in the lower portion of the chamber, reaches a temperature at which it melts, but without carbonising or igniting. The plastics in molten form then flows under gravity through the filter 26 and out of the chamber through the outlet 24. The molten material is collected in suitable vessels and, now occupying very little space, can be transported easily and moreover can be recycled, i.e. reformed into expanded plastics products.

Referring to Figures 2 and 3, there is shown another

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embodiment of densifying apparatus, which is particularly useful for plastics material or products which are contaminated. The chamber 10 is housed within a cabinet 40. The top of the chamber has an opening 41 in the front communicating with a cylindrical container 42 through an opening 43 in the latter. Once the waste material is put into the container 42, it is turned by hand around its axis to close the opening 41 and tip the waste material into the chamber 10. The arrangement prevents direct access into the chamber 10. The container 42 is mounted for rotation on weight-sensitive bearings 44, so that if anything too heavy is put into the container these bearings lock up to prevent the container 42 being turned. The outlet of the chamber has a sliding valve 44 and below this is an outlet chamber 45 having a sliding valve 46 at its lower end. A heating element 47 is cast into the wall of the outlet chamber 45 at its upper end and a cooling duct 48 is cast into the wall of the outlet chamber 45 at its centre and lower end.

When the valve 44 is open and valve 46 closed, the outlet chamber 45 fills with molten plastics material from the main chamber 10. The heating element 47 helps to maintain the material in molten condition so that it will flow into ,he outlet chamber. Then the valve 44 is closed, and cooling fluid is passed through the duct 48 to solidify the material, whereafter the valve 46 is opened to allow the solidified body to fall from the outlet chamber and onto a tray 50. An opening 51 is formed in the front wall of the cabinet and has a door 52. The solidified bodies from the cooling chamber can be removed through the opening 51 on the tray 50: however, a timer is provided to keep the door 52 locked until a predetermined time interval after the elments of the apparatus have been switched off.

Alternatively with valve 44 in the open position and valve 46 replaced by a slot die, a continous casting process can be achieved for feeding direct into a hot melt granulator or onto a cooling conveyor feeding direct into a pelletising machine.

In a modification, the lower portion of the chamber 10

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may form part of a sphere (e.g. it may be hemi-spherical) instead of being conical, so as to increase its surface area.

Figure 4 shows an alternative form for the outlet chamber of the apparatus of Figures 2 and 3. This outlet chamber includes a cylindrical rotor 60 mounted for rotation about a transverse, horizontal axis A. The rotor 60 has a number of inwardly-tapering cavities 62 in its outer surface, the example shown having four such cavities 62. An electric motor and a drive transmission are provided to index the rotor around at intervals of time. One cavity 62 is aligned with the outlet from the melt chamber and fills with molten plastics, then the rotor turns to bring this cavity to a cooling position whilst the following cavity is brought to the top or filling position, and then the rotor turns again to bring the cooled and solidified body of plastics to the bottom or ejection position where it falls from the rotor under gravity into the tray 50. The rotor is mounted within a casing 64 of the outlet chamber. This casing preferably incorporates heating elements adjacent the outlet from the melt chamber and preferably includes cooling tubes cast integrally adjacent the cooling cavity. Preferably the casing 64 comprises a casting of aluminium or aluminium alloy with any heating and/or cooling elements cast therein. Preferably the rotor also comprises a casting of aluminium or aluminium alloy. Preferably the surfaces of the casing 64 and rotor 60 which come into contact with the plastics material are coated with low-friction material e.g. PTFE such as Teflon.

Figure 5 shows another form for the outlet chamber. A horizontal slide plate 70 is disposed below the outlet of the main chamber. In the example shown this outlet is divided into two by a central web 71. The slide plate 70 is supported on a horizontal table 72, and is formed with two cavities 73, 74. Alternately, cavity 73 will be positioned below part 75 of the outlet from the main chamber, then the slide 70 will be moved to the left so that the body of plastics (now cooled and solidified) within cavity 73 can be ejected under gravity whilst cavity 74 is

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positioned below part 76 of the outlet from the main chamber, whereafter the slide 70 will be moved to the right for cavity 73 to receive fresh molten plastics and for the body in cavity 74 to be ejected.

The plastics bodies which are ejected from the apparatus described above may be used as fuel elements to assist incineration of other waste or to fuel industrial boilers.

It will be appreciated that the apparatus described rely upon the thermal conductivity of the metal of the chamber wall, together with an improved slip factor of this wall due to a PTFE or other coating, to alter the physical or molecular condition of the plastics material by densifying it or solidifying it. The inner surface of at least the lower portion 16 of the chamber is circular in every horixzontal cross-section and the interior at least of this portion 16 of the chamber is an open volume, free of any vanes, baffles, fins or other obstructions. The waste material introduced into the top of the chamber falls under gravity, without the assistance or drive of any mechanical means, eventually reaching the outlet in molten form. Furthermore, the molten material at this point flows through the outlet under gravity and without the assistance of any mechanical means.

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