Patent GB-691125-A (GB)
Title: Improvements relating to the production of tubular articles
Inventor: MONCRIEFF LESLIE JOHN
IPC: B29C48/90
Filed: 1950-08-28
Abstract: <PICT:0691125/IV (a)/1> <PICT:0691125/IV (a)/2> Relates to the production of tubes from thermoplastic material such as cellulose acetate or synthetic resins and of predetermined crosssection and consists in extruding the material hot through an annular die, causing the tube so formed to pass while still plastic over a mandrel comprising a shaping member for shaping the inside of the tube to the desired cross-section and carried by a stem projecting from the central core of the die, the plastic tube being kept out of contact with the stem by a cushion of gaseous medium and being cooled while in contact with the shaping member so that it retains its shape on passing the shaping member, and carrying forward the cooled tube by tractive means contacting it beyond the point where it has ceased to be plastic.  The tube 24 passes through three regions.  In the first region it issues from the annular die 11, just beyond which is a ring of air jets (not shown) for initially cooling the tube immediately it emerges from the die, and passes over the stem 13.  Into the central core of the die is screwed the hollow stem 13 which expands at its far end into a hollow, open-ended shaping member 14.  Air is admitted to the inside of the stem 13 behind the extruder.  At a short distance in front of the die the central passage in the stem is blocked, e.g. by a plug 15a, and the air is thus caused to emerge through holes 15 in the wall of the stem and to hold the tube 24 away from it.  The air re-enters the inside of the stem, at the point where the stem merges into the shaping member, through holes 17, after which it passes from the open end of the shaping member into the formed tube.  In this first region the material of the tube is cooled from the fluid to the plastic state, for which purpose the air may be pre-cooled.  In the second region the tube is progressively cooled yet more, as it is drawn over the shaping member 14, by water sprays of increasing velocity (from nozzles 19-23) as the material becomes less plastic.  In this second region the material of the tube passes from the plastic to the rigid state and the tube receives its shape, its bore increasing and its wall thickness diminishing.  The tube also shrinks as it cools and so tends to stick to the shaping member, which should therefore taper slightly towards its far end from a point about half-way along.  The nature of the shaping surface influences the passage of the tube over it, a matt surface being preferable, obtained, e.g., by acid etching or sand blasting.  In addition, lubrication may be provided by charging the air, which passes through the annular space between the stem 13 and the tube 24, with lubricant, either by bubbling it through a body of lubricant or atomizing lubricant into the air feed.  Alternative lubricants to vegetable or mineral oils are a nonthermosetting silicone polymer or non-solvent plasticisers such as tricresyl phosphate. The shaping member may be made separately from its stem (being attached thereto in a manner which allows for the necessary air passages) and may be composed of porous metal charged with lubricant or of material which has lubricating properties, e.g. polytetrafluorethylene.  The stem may be of material with poor thermal conductivity, e.g. ebonite.  In the third region, beyond the shaping member, tractive means are applied which transmit the tension without substantial change of shape of the tube, which has passed from the plastic to the rigid state.  According to a modification, however, the tube is not cooled so much while being shaped and therefore stretches and acquires a smaller cross-section as it is drawn off the shaping member.  The tractive means comprise successive pairs of take-off rolls 25, 26 (one pair being shown), or a belt and idler roll may be used.  The tube will stretch somewhat in the first region due to the traction, which should be such that the stretching is about 10-30 per cent. All the lower rolls 25 are driven at the same speed through an infinitely variable gear, and the upper rolls 26 are spring-loaded and driven by the tube.  The rolls may be shaped to grip the tube round an arc or may be cylindrical and covered with rubber to increase the grip, the latter needing less careful alignment. Arrangements may be made to control the flow of air, or other gaseous medium, which keeps the tube out of contact with the stem in the first region of the process. Fig. 3 shows a mandrel in which the distance between the air outlet and inlet holes 15, 17 is adjustable, the stem 27, carrying the shaping member 14, being a sliding fit in a plug 30 fixed at the forward end of a tube 31 and being screw-threaded to engage a plug 33 at the rear end.  The tube 31 screws into the central core of the extrusion die by a thread 29 and so, when the stem 27 is turned (a square 35 being provided for the purpose) it advances or retracts and so gives the adjustment desired.  Also a sleeve may be fitted in the hollow stem with perforations corresponding to the air outlet holes 15, the sleeve being rotatable from a position in which the holes are fully in register to one where they are completely out of register. The shaping member may be of other cross-sections than circular, e.g. elliptical or polygonal.  The apparatus may operate vertically and not necessarily horizontally as shown.  Other cellulose derivatives may be used for the tube material, or other thermoplastics such as ethylene polymers, polyvinyl compounds, polymers of acrylic acid derivatives, and thermoplastic linear condensation polymers such as the nylons.  Cellulose esters derived from carboxylic acids containing 2 to 4 carbon atoms, and especially cellulose acetate, are particularly suitable. Specification 666,727 is referred to.