GB883585A - Improvements in or relating to the thermo-forming of thermoplastic materials - Google Patents

Abstract

Thermoplastic covers are formed on rigid articles by a thermoforming process comprising disposing the article in an evacuable enclosure partly defined by a sheet of thermoplastic material, heat-softening the thermoplastic, moving the article relative to the enclosure to cause the article to become surrounded by the heat-softened thermoplastic, evacuating the enclosure to draw the thermoplastic into conformity with the article, coding the thus covered article to harden the thermoplastic and removing any excess from the article. Suitable thermoplastic materials include non-inflammable cellulose esters such as cellulose acetate and cellulose butyrate, polyethylene and polypropylene, polyvinyl chloride, polymethyl methacrylate and polystyrene. As shown, Fig. 4, there is provided an evacuable chamber 24 formed with an integral flange 27 provided with grooves 28 in its upper surface for the reception of sealing rings 29. The <PICT:0883585/III/1> <PICT:0883585/III/2> <PICT:0883585/III/3> chamber is provided with a compressed air inlet 31 and a vacuum take-off pipe 32. A bushing 34 is provided centrally of the base and receives a ram 35 formed at its lower end with a piston 36 p moving in a cylindrical pot 37 secured to the base of the chamber. The pot is provided with compressed air inlets 41 above and below the piston. The ram 35 carries on its upper end a rigid horizontal platform 42 which is in its turn provided with an accurately centred well for receiving the base 46 of a mandrel 45, which supports the bobbin or other article to be coated. Both the cylindrical portion 48 and the base 46 of the mandrel are grooved to allow evacuation of air from inside the bobbin. The chamber 24 is closed by a plate 30 having a hole 51 (of the same cross-section as that of the article to be coated) accurately disposed above the mandrel. A sealing gasket 52 is placed on the upper surface of plate 30 and the sheet of thermoplastic material 56 placed thereon, and clamped in position by clamping frame 54. Above the clamping frame there is mounted a movable heater 57 running on guide rails 58. A sealing member 67 for sealing and shaping the thermoplastic is also provided above the heater. In operation the piston 36 is raised and a waisted bobbin 49, which has been heated approximately to the softening point of the thermoplastic to be used, placed on platform 42. The platform is lowered, a sheet of polystyrene clamped in position and the sheet heated. When the sheet is soft enough, the heater is removed and the piston raised until the whole of the article to be covered is above the clamping frame 54 (see Fig. 6). The sealing member 67 is lowered until it fits into the top of the bobbin and air is exhausted from chamber 24 to cause the stretched sheet to conform to the bobbin (Fig. 7). The sealing member is raised, the ram lowered and cooling air directed on to the bobbin, which is removed when cool and suitably trimmed.