Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Layered – stratified traversely of length – or multiphase...
Reexamination Certificate
2002-02-08
2004-06-01
Eashoo, Mark (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming continuous or indefinite length work
Layered, stratified traversely of length, or multiphase...
C264S177170, C264S211000, C264S236000, C264S237000, C425S197000
Reexamination Certificate
active
06743387
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a process for producing cables, in particular cables for the distribution of electrical energy or cables for telecommunications.
More particularly, the present invention concerns a process for the production of cables having at least one covering layer comprising a polymeric base composition having high viscosity.
Still more particularly, the present invention concerns a process for the production of cables having at least one covering layer comprising a polymeric base composition to which is added a mineral filler capable of imparting one or more specific properties to the aforesaid cables.
The present invention further concerns a device for the purpose of performing the production process referred to above.
2. Description of the Related Art
In general, an electric cable comprises at least one conducting element, consisting of a single wire or of a plurality of wires stranded together in a suitable way, and one or more covering layers of the aforesaid conducting element, which provide electrical insulation and/or fulfil a function of mechanically or chemically/physically protecting the cable against external factors.
This covering layer or several covering layers are applied onto the conducting element via a deposition stage, generally performed by means of an extruder.
In general, an extruder comprises: a hollow cylindrical casing; an extrusion screw of preset pitch positioned within said casing and having an axis of rotation parallel to the axis of said cylinder; a charging hopper located at a first end of said casing for the introduction of a predefined composition based on at least one polymeric material, optionally premixed with other components in an upstream device, such as for example a Banbury mixer; a filtration section located close to the head of said screw, positioned perpendicularly to the axis of the latter and thus perpendicularly to the direction of advancement of said composition; a connecting flange positioned downstream from the filtration section; an extrusion head comprising a conveyor element and a die communicating with the exterior, so as to define the second end of said casing, and whose purpose is to impart a predefined shape to the material emerging from the extruder.
More specifically, according to a form of embodiment known in the art, the extrusion head is provided with an inlet port through which the conducting element to be covered with the aforesaid covering layer is introduced inside of the extruder.
According to a technology known in the art, the conducting element is introduced into the extruder head perpendicularly to the direction of advancement of the material fed into the extruder through the hopper referred to above.
In the case where the cable production process includes the use of an extrusion operation, said process then comprises the following stages:
feeding the extruder with the composition forming the covering layer which it is desired to deposit on the conducting element;
unwinding a conducting element from a feeding reel and conveying it inside of the extruder head where the deposition of said covering layer onto said conducting element is effected;
cooling the cable thus obtained and winding it onto a collecting reel.
Said polymeric material and said other components can be premixed together in a device upstream from the extruder, creating a mixture which is fed into the latter via the charging hopper referred to above.
Furthermore, prior to said cooling operation, a crosslinking operation can be performed in the case where polymers of crosslinkable type are used.
This type of process known in the art, includes at least the following devices for the purposes of its implementation:
at least one charging hopper for feeding in the polymeric material, optionally premixed with other components of said composition;
at least one extruder comprising an extrusion screw and an extrusion head inside of which is contained a die for the purpose of fitting said covering layer around at least one conducting element of said cable;
one or more units for cooling the cable thus produced;
devices for unwinding the conducting element, and
devices for winding the cable downstream from the production plant.
As stated above, said device can also include one or more crosslinking units if a polymeric material of crosslinkable type is used.
It is also known that the extrusion operation can take place in several distinct stages, above all in the case where it is desired to cover the conducting element with a plurality of covering layers.
For example, if it is desired to deposit a pair of covering layers onto the conducting element, the extrusion operation can involve, in a first step, the extrusion of an internal covering layer, in direct contact with the conducting element, and then, in a second step, the extrusion of an external covering layer, deposited on said internal covering layer.
This covering process can also take place in a single step, for example by a “tandem” technique, in which at least two individual extruders placed in series are used, or by coextrusion with a single extrusion head.
In the present description and in the claims which follow, the term “covering layer of a cable” means any covering comprising at least one polymeric material deposited on the conducting element of said cable, where this covering may consist of one or more layers, each having, for example, electrical insulating properties or being capable of protecting the cable from the action of external factors.
Furthermore, for simplicity of explanation, in the course of the present description the expression “operation of extrusion of a covering layer onto the conducting element of a cable” should be taken to mean an extrusion operation performed onto the conducting element itself, in the case where it is intended to make a cable provided with a single covering layer, or an extrusion operation performed onto a covering layer previously deposited onto the conducting element, in the case where it is intended to make a cable provided with a plurality of covering layers.
In addition to the stages mentioned above, in general the production processes of a cable, upstream from the deposition stage of the covering layer onto the conducting layer, include the provision of a filtration stage for the purpose of removing the impurities present within the components that form the composition referred to above.
This is because these impurities can be contained inside of said components, for example if the latter are fed into the extruder in the form of granules or pellets, in which case the impurities are embedded in the interior of the granules or pellets; or said impurities are introduced together with said components as a result of the interaction of the latter with the external environment during the usual operations of handling, transport or storage to which said components are subjected.
Furthermore, not only the quantity of impurities present in the covering layer, but also the dimensions of said impurities, are of fundamental importance.
This aspect is particularly critical, for example, in the case where the covering layer of a cable for use at high tension, for example 150 kV, has to be produced, a case where it is necessary to confine the dimensions of the impurities to below a critical value, for example less than 300 &mgr;m, more preferably less than 150 &mgr;m.
In particular types of application, for example when it is desired to produce a cable of the self-extinguishing type, the external covering layer of the conducting element is provided with a high percentage of mineral filler, which imparts the desired flame-resistant properties.
The presence of the mineral filler renders the material very viscous and considerably worsens its processability. In particular, the filtration stage of the process of extrusion of a material containing flame-resistant mineral fillers is particularly critical, even though for this type of cable impurities of relative large dimensions are tolerated and th
Belli Sergio
Locatelli Angelo Giuseppe
Pozzati Giovanni
Veggetti Paolo
Eashoo Mark
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Pirelli Cavi E Sistemi S.p.A.
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