Cooling device for extrusion heads for tubular films of plastic material

Abstract: A cooling device (1), for extrusion heads (4) for tubular film (8) of plastic material, can be constructed in two versions, each having the dual purposes of cooling a tubular film and at the same time giving it as uniform a thickness as possible. The first embodiment comprises a set of keys (25) which when operated by means of actuators (26) cause a ring of elastic material to bend, which locally throttles an annular conduit (10) directly carrying a current of cooling air to the tubular film (8). In the second embodiment the temperature of the current of cooling air passing through the said conduit is varied by thermo-electric means.

Description: The present invention relates to a cooling device for extrusion heads for tubular films of plastic material. Tubular films of plastic material, which constitute a semi-finished article forming a basis for a variety of products are at present obtained by extrusion using machines which comprise at least one die operatively associated with at least one cooling device as indicated in the preamble to Claim 1 (see e.g. US-A-4443 400). The cooling device acts on tubular material continuously emerging from the die, by directing currents of ambient air against the external annular surface of the tubular film to remove its heat by convection, thus converting the material from a pasty to a solid state. From US-A-4443400 and WO-A-90/15707 cooling devices for extrusion heads for tubular films of plastic material, of the type comprising at least one conduit delivering the cooling fluids are known. Said conduit is provided with means which serve to regulate the cooling action of the cooling fluid and which can be operated in accordance with data provided by means for detecting variations in the circumferential direction of the thickness of the extruded film, in such a manner than if locally the thickness of the film exceeds a reference value the cooling action exerted thereon is reduced. The said regulating means comprise a plurality of elements which modulate the cooling action and which are situated in positions corresponding to the entire circumference of the film; each of the said modulating elements acts on a corresponding and limited portion of the circumference of the film. Other similar devices are described in USA-A-4.606.879, DE-A-3.707.826 and 2.237.236. In all these devices the elements modulating the cooling action act on the flow rate of the cooling fluid. Such devices, in which the fluid flow is locally modified, present many problems about the local stability of the tubular film, that is in a semifluid condition and is subjected to remarkable stresses by the cooling fluid. The document DE-A-2.658.518 discloses the use of either cold air or heat; document US-A-4.426.239 shows means of heating and/or cooling and heating elements switched on for a shorter duration thereby giving substantially constant air temperature. Using all the devices of the prior art it is quite impossible to obtain a sufficiently constant thickness over the entire length of the tubular film in view of the unstable equilibrium to which the fused matter is subjected. The purpose of the present invention is to obviate the foregoing drawbacks, i.e. to provide for extrusion heads for tubular films of plastic material, a cooling device capable of producing a film having not only the known required characteristics but also a predetermined uniform thickness throughout, the thickness of the film thus being far more uniform than that normally obtainable with extrusion heads using known cooling means. This object is achieved in accordance with claim 1. Particular embodiments of the invention are subject of the dependent claims. The device according to the invention, is capable of modifying the cooling locally, by raising the temperature of the cooling fluid in relation to the average temperature of said fluid along the entire circumference of the film. The said device acts on the film in its consolidation phase in a manner which differs from one zone to the next, only taking effect where it becomes necessary to adpat the void thickness to a certain reference value. Those portions of film of which the thickness exceeds that preselected are subjected to a slower cooling process than the remaining portions, of which the thickness is nevertheless adequate or close to the thickness mentioned, this in order to allow more lengthening during the stretching action . Obviously, the greater the number of modulating elements the better the uniformity of thickness obtainable, the maximum number possible being governed by considerations of a technical-economic nature. The invention is illustrated by the diagrams on the accompanying sheets of drawings, purely by way of an example and without any limitative effect. Figure. 1 schematically illustrates an extrusion machine to which a device according to the invention has been applied. Figure. 2 is a sectional view of an extrusion machine provided with a device according to the invention. Figure. 3 is a sectional view of a device according to the invention, applied to an extrusion head. Referring to the drawings, the cooling device to which the invention relates, marked 1, as a whole, is used on the extrusion machine illustrated in Figure.1 and marked 2 as a whole. Machines 2 of these types produce via an extrusion head 4 a cylindrical tubular film 8 of plastic material, which, after being consolidated by the cooling process, is flattened and wound onto rollers 3. The extrusion head comprises a die consisting of a member 5 and a cavity 6 which are coaxial and define an annular conduit 7 of adjustable thickness from which emerges the film 8, already formed in a pasty state. The function of the cooling device 1, is to cool the film in such a way as to consolidate its structure. Referring particularly to Figure 3, the cooling device 1 comprises an annular body 22 which is operatively associated with means 23 (Figure 1) which detects variations in the thickness of the film. The annular body 22 comprises a first second, third, fourth, fifth, sixth and seventh element (referenced 14 -20 respectively) of substantially annular construction positioned coaxially and symmetrically in respect of the extrusion axis 21, which corresponds to the axis of the tubular film 8. The annular elements 14-20 define first and second annular conduits, 9, 10 respectively, for delivering cooling fluid (preferably ambient air) to the entire external surface of the tubular film 8 via first and second ports, 11, 12 respectively, as shown by the arrows F and G. The second annular conduit 10 is fed from the first conduit 9 via one or more ports 13 whose axis is substantially parallel to the axis 21. Means for regulating the cooling action, marked 24 as a whole, are mounted between the annular elements 14-20. As illustrated in Figure 3 the device 1 is designed to cause the regulating means for the cooling action to modulate the action by acting on the temperature of the cooling fluid (preferably consisting of air) by varying it in relation to the average temperature of the said fluid along the entire circumference of the film preferably by increasing it. The regulating means 24 preferably comprise electrical resistors, which term does not preclude the possibility of employing bivalent heating/cooling elements, such as thermoelectric elements known on the market under the name "PELTIER". These elements too can be controlled by the means 23 for detecting the thickness variation occurring in the film. The temperature of the fluid (usually air) passing through the second conduit 12 is thus varied in accordance with the thickness required. There is therefore a localised acceleration and/or deceleration in the cooling action, with consequent localised control, for this purpose, of the molecular creep. Where the thickness of the film emerging from the conduit 7 exceeds that required and thus that of the other zones the cooling air undergoes a localised heating action in such a way as to slow down the cooling process. This ensures that the molecular creep phenomena are maintained until the required thickness is obtained. The heat exchange between the air flow to be modulated, passing into the conduit 10, is assisted by the interactions occurring between the air flow and a series of projections or teeth 40 aligned in such a way as to form a comb-type structure.

Claims: Cooling device (1,) for extrusion heads for tubular films of plastic material (8), of the type comprising at least one conduit (10,) delivering the cooling fluid, the said conduit (10,) being provided with means (24,) which serve to regulate the cooling action of the cooling fluid and which can be operated in accordance with data provided by means (23) for detecting variations in the circumferential direction of the thickness of the extruded film (8), in such a manner that if the thickness of the film exceeds a reference value the cooling action exerted thereon is reduced, said regulating means (24,) comprising a plurality of elements which modulate the cooling action and which are situated in positions corresponding to the entire circumference of the film (8), each of the said modulating elements acting on a corresponding and limited portion of the circumference of the film (8), characterised in that the elements modulating the cooling action act on the temperature of the cooling fluid. Device in accordance with Claim 1, characterised in that the elements modulating the cooling action operate by raising the temperature of the cooling fluid in relation to the average temperature of the said cooling fluid along the entire circumference of the film. Device in accordance with Claim 1 or 2, characterised in that the regulating means (24) consist of bivalent thermoelectric elements of heating or cooling preferably of the PELTIER-type operatively positioned in at least one annular conduit (10). Device in accordance with Claim 2, characterised in that the elements modulating the cooling action are electrical resistors operatively positioned in at least one annular conduit (10). Device in accordance with Claim 3, characterised in that the bivalent thermoelectric elements comprise a series of projections or teeth (40) arranged to form a comb-type structure interacting with the flow of the cooling fluid.

Inventors: DEPONTE EDOARDO, DEPONTE, EDOARDO
Applicants: DEPONTE EDOARDO, DEPONTE, EDOARDO
Application year: 1992
Main classification: B29C47
IPC codes: B29C47