GB2188273A

GB2188273A - A plastic net extension die, with heating means

Info

Publication number: GB2188273A
Application number: GB08710144A
Authority: GB
Inventors: Mario Beretta
Original assignee: RDB Plastotecnica SpA
Current assignee: RDB Plastotecnica SpA
Priority date: 1983-12-23
Filing date: 1987-04-29
Publication date: 1987-09-30
Also published as: GB8414844D0; IT8324353A0; GB2151972B; FR2557015A1; GB8710144D0; FR2557015B1; JPS60159030A; GB2151972A; CA1239267A; IT1203718B

Abstract

The strands making up the net are extruded as a tubular net from between a pair of counter-rotating annular dies, and the net tube is taken from the dies around a tubular mandrel immersed in cooling fluid, on which it is cut along a helical path to be continuously removed as a net strip. According to the invention, temperature irregularities within the die are avoided, by heating it by an annular heater (8) e.g. a resistance heater, disposed around the outer die at a clearance e.g. of a few tenths of a millimetre. <IMAGE>

Description

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GB 2 188 273 A

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SPECIFICATION

Process for producing nets of plastic material and apparatus for carrying out said process

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The present invention discloses a process for producing plastic nets obtained by extrusion, with meshes of any dimensions. A particular advantage thatthe process offers is to allowthe production of 10 net having high characteristics of uniformity and dimensional constancy in the diameter of the strands.

The invention relates also the extrusion apparatus for carrying out said process.

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Prior art

Belgian Patent No. 641951 of Dec. 30,1963, in the name of DU PONT,teachesa process forthe extrusion of an oriented plasticfilm, wherein an 20 anular extrusion head is used to form a tubular element of plastic material which is made advance around a substantially cilindrical spindle, rotating around its own axis so as to wind the tube spirally while it is advancing.

25 During this phase the plastic material is stretched so as to orient it.

The tube thus obtained is subsequently cut, thus obtaining a pairof stripsthataresubsequently coupled so as to obtain an only strip of film, with the 30 molecules oriented in two directions.

German Patent No. 1907699 with British priority No. 8977 of Febr. 23,1968, teaches a process for producing net of plastic material, whereinthe wires making up the net are extruded from nozzles located 35 along a pairof circular crowns one of which, the outer one, is still, while the inner one rotates around its axis.

The strands coming out of the stationary crown, hereinafter called longitudinal, are wound spirally 40 around a fixed cylindrical spindle, dipped in cooling water, and they run along the surface of this spindle, therefore covering the same route. The strands coming out of the rotating crown, instead, join up with the formers thus forming a net the meshes of 45 which have an inclination that isfunction of the ratio between the output speed of the longitudinal strands and the rotation speed of the mobile crown.

This tube, formed by the meshes running along the spindle, is then cut by means of a fixed device the 50 cutting plane of which is set along the spiral route, so as to cut the netin parallel with thelongitudinal strands and thus obtain a continuous strip.

This process has, however, several inconveniences. In fact, the diameter of the strands 55 depends, among otherthings, on the viscosity of the melted polymer that is extruded, the viscosity being strictly bound to temperature.

It is obvious therefore that any fault relevant to the temperature control in the extrusion unit affects the 60 strands dimendions.

Since the outer extrusion bell is fixed, it is particularly sensitive to localized temperature differences, due for instance to draughts orthe like and it is very difficult, consequently, to obtain 65 strands all of the same thickness.

Moreover, it is necessary to consider also that an electric resistance, anular as well, is generally built-in the extrusion bell; said resistance is interrupted in correspondence of the area of 70 connection with the outerfeeding cables.

Also in this area, therefore, a difference of temperatu re costantly occu rs with respect to the surrounding areas. The case may be thatthe strands diameter is not the same in all of them, but rather 75 varies slightly from one another and this causes considerable problems.

In fact, since the net as produced is wound up in reels, if one of the strands is, e.g., thicker than the others, what happens is that these thickness 80 differences come together always in the same spot, thus making the produced reel unstable. These reels have, generally, a diameter of 3-4 meters and therefore when the plastic material forming the net shrinks slightly furtherto the cooling, considerable 85 tensions occur, that deform the reel which, instead of having substantially flat sides as in Figure 1, gets deformed taking the shape as illustrated in Figure 2.

This causes difficulties of packaging or at the time of handling the reel.

90 Asecond disadvantage of the known processes is thefactthat, since the longitudinal strands run on the spindle keeping constantly the same position, they wear off the spindle always on the same spot each thus cutting its own running route. In the area limited 95 between two of these routes, moreover, the calcar contained in the cooling water deposits.

It is obvious, then, that if the extrusion unit is substituted e.g. to produce a net with meshes of different dimensions, practically insurmountable 100 difficulties are met.

Therefore, to overcome the above mentioned disadvantages, the present invention teaches a process and the relevant apparatuses allowing on one hand to improve radially the characteristics of 105 dimensional uniformity of the strands, and on the other hand to distribute possible lacks of uniformity of the strands dimensions overthe entire width of the reel.

This result is obtained substantially setting also 110 the outer extrusion member in rotation motion, so as to prevent the strands and the cutting planefrom being parallel, varying, as a consequence, the position of the longitudinal strands in each stretch of net.

115 The present invention will be illustrated now in detail, byway of non-limiting example with particular reference to Figures 3 to 5 as annexed, wherein:

Figure 3 illustrates schematically an apparatusfor 120 carrying outthe process according to the invention, partially in section;

F/fift/re^illustratestheschematical view, partially in section, of an extrusion head according to the invention;

125 /r/srure5illustrates an example of net produced by the process according to the invention.

The process consists of extruding the strands intended to make up the net by means of an extrusion head indicated as a whole by 1 andformed 130 of two concentrical crowns or dies 2 and 3, rotating

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GB 2 188273 A

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and provided with a series each of nozzles set out withanularoutline.

The extruded strands are wound up around afixed cylindrical spindle4, dipped in a coolant which can 5 bejustwater.Thestrandsextrudedfromoneofthe 70

two crowns, e.g. from the outer crown 3, are wound around spindle 4 with an helicoidal outline.

The strands extruded from the inner crown 2 join up with the formers forming with them and angle 10 that is function of the ratio between the rotation 75

speed of the two crowns around their own axes.

In the proximity of the surface of spindle 4there is a cutting unit 5 that cuts the cylinder of net just solidified thus forming a continuous strip 6. 15 Therouteofthestrandscomingoutofcrown3is 80 not fixed. Asa result of the rotation ofthecrown,

these strands move parallelly to themselves as the strip is unwound. In this mannerthe cutting plane of unit 5 is not parallel to the longitudinal strands of 20 strip 6, but rather slightly inclined with respectto 85

them.

Therefore, the strip of netthus produced looks as it is illustrated in Figure 5, with the longitudinal strands slightly inclined with respectto the net edges. 25 Inthismannerifsomestrandshaveathickness 90

different from the others, as e.g. the ones indicated with 7 in Figure 5, these strands, as the net is being wound up on the reel, vary their position with respectto the ones of the previous coil, so thatthe 30 irregularity is distributed along the entire width of 95

the reel.

Toheatupand keep at a control led tem peratu re crowns 2 and 3, a heating device is used, consisting of an anular body 8 which is heated by means of an 35 electricresistanceorthelike,setaroundoutercrown 100 3, extremely close to it, so as to transmit it the heat.

The distance between crown 3 and ring 8 is of few tenths of millimeter.

This system offers several advantages: 40 firstofall,asimplerconstructionisrealizedthan building-in the heating means directly in the extrusion crown, as previously.

Moreover, the temperature inside the crown is considerably more constant, since it rotates and 45 therefore is not affected byslightlocaltemperature changes that may be detected in the ring.

The process according to the invention offers,

moreover, another advantage.

In fact the mass of melted polymer insidethe 50 extrusion member moves with different speeds according to the distance between the extrusion member walls. Preferential flux lines are therefore created while, inthe proximity ofthe walls, a certain stagnation of polymer often forms, which may 55 carbonize gradually and cause sediments such as to stop up the nozzles. Moving, instead, also the outer extrusion crown, turbulences are created in the mass of polymer, which prevent or anyway limit drastically this disadvantage.

60 Substantially, therefore, the process according to the invention makes possibleto obtain thefollowing advantages:

A) the cutting plane ofthe net results to be inclined with respectto the helix thatthe strands follow, so 65 thatthe position ofthe strands varies with continuity.

thus distributing possible irregularities on the entire length ofthe net;

B) it allows an easier control ofthe temperature of the extrusion means, eliminating the influence due to possible discontinuities ofthe heating means orto external agents;

C) it allows a better mixing ofthe melted polymer ^ insidethe extrusion member, preventing deposits from forming on the inner walls.

Obviously, a person skilled in the art may provide *

for several modifications and variations, which however must be all considered as included in the ambit ofthe present invention.

Claims

1. Apparatus for producing a net of plastics material comprising extrusion means in correspondence with the extremity of a fixed cylindrical spindle immersed in a coolant, wherein said extrusion means comprises a pair of annular concentric dies, each die being rotatable around its axis but at a different speed and/or in a different direction to the other, and means for controlling the temperature of said dies.

2. Apparatus according to claim 1, wherein said meansfor controlling the temperature of the dies comprises an annular member set out around the outerdie inthe immediate proximity of the latter,

said annular member being provided with means to produce heat and maintain a constanttemperature therein.

3. Apparatus for producing a net of plastic material substantially as herein described with reference to Figures 3 to 5 of the accompanying drawings.

Printed for Her Majesty's Stationery Office by Croydon Printing Company (UK) Ltd, 8/87, D8991685.

Published by The Patent Office, 25 Southampton Buildings, London, WC2A1 AY, from which copies may be obtained.