Process of and apparatus for making sheets or ribbons of organic plastic materials

摘要

An organic thermoplastic substance free from volatile solvent is softened by heating and is then forced through a nozzle to form a flat ribbon at such a high temperature and low viscosity that it p is not self-supporting; the edges only of the ribbon are chill-pressed close to the nozzle and the remaining hot plastic portion of the ribbon is supported out of contact with solid surfaces until it has hardened by gripping the chilled edges of the ribbon and pulling it transversely, and by engaging and longitudinally pulling the fully hardened portion. Apparatus for carrying out the process comprises a screw type extrusion device provided with means for heating it, a nozzle through which the plastic material is forced to form a flat ribbon, chilling and gripping rollers for chill-pressing the edges of the ribbon, additional rollers for gripping and conveying the chilled edges of the ribbon in diverging paths to produce transverse stretch, and a drawing roller engaging the ribbon at the place where it has set to its final shape and dimensions, for pulling the ribbon longitudinally. As shown in Fig. 1, the moulding compound which may be initially in dry granular or powdered condition is placed in feeder F and forced by a heated screw stuffer S containing screw 22, into a homogenizer H. The now fluid plastic is forced through a fine porous medium such as a wire cloth in homogenizer H which is subjected to controlled heating. From the homogenizer the plastic compound flows into extrusion nozzle N. To obtain a sheet or ribbon of uniform thickness, the transverse distribution of the plastic delivered to the slot is controlled so as to compensate for the greater frictional resistance to flow at the ends than in the middle. The control device may take the form of an adjustable plug or baffle D. As the plastic extrudes in sheet form it is engaged at the edges by pairs of chill-pressing rolls R which may be water-cooled. Further rolls R1 are provided to assist rolls R in drawing the plastic from the nozzle and stretching the sheet transversely in the region of the nozzle. Further rolls R2, R3 support the sheet and apply a longitudinal tension. A pair of rolls P extend across the entire width of the sheet and apply longitudinal tension. They are placed at or beyond the point where the sheet is sufficiently hardened to resist marring of the surface. The rolls R-R3 are adjustably mounted on a grill G so that their positions may be changed to vary the amount and rate of stretch applied to the sheet. Beneath the sheet and in the stretching zone a heater K is provided to apply heat to the sheet and control its viscosity. The sheet is wound into a roll W. The moulding compound may be a colloided cellulose ester or ether such as cellulose acetate or ethyl cellulose. Any residual moisture or volatile solvent is removed, e.g. by passing the compound through a rotary drier against a counter current of hot dry air. Feeder F, as shown in Fig. 2, contains a screw 23 operating in the inlet to the stuffer chamber and mounted on shaft 24. The shaft 24 is driven through gear 26 and clutch 28. If an excess of material is fed to the stuffer, a back-pressure exerts an axial thrust on screw 23 forcing it upwardly and disengaging the clutch 28. The stuffer, homogenizer and nozzle may be heated by means of hot oil circulating through their casings. The homogenizer is shown in Fig. 3. It comprises an inlet chamber 59 and outlet chamber 61, with plate 53, screening 54 and backing plates 55 extending vertically. These plates are grooved so that when stacked together channels are formed permitting the plastic to flow to the outlet chamber. As shown in Fig. 9, the plastic material enters the nozzle through pipe 58. The inlet chamber 73, rotatable plug 74 and slot 75 extend from end to end of the nozzle. As shown in Fig. 7a, the plug 74 is shaped so as to restrict the flow of plastic material in the central portion of the stream through slot 75. As shown in Fig. 10, the transverse stretching rolls R are driven by a chain 87. The rolls R are mounted in brackets which may be bolted at any desired angle and position in the grill G. Cooling liquid such as water is circulated through the brackets from pipe 105. Heating of the extruded sheet in the stretching zone may be effected by means of gas burners, and means may be provided for diffusing the heat, e.g. a lattice of metal strips or a screen containing a layer of pulverulent ceramic or other material.