Extruder for highly foamed plastic material

Abstract: The present invention provides an extruder for highly foamed plastic material for coating a central conductor of a coaxial insulated cable with an insulator made of a mixture of a plastic material, preferably polyethylene, and an inert gas, such as nitrogen gas, to serve as a foaming agent. An inert gas is introduced approximately halfway down the extruder through a lead-in portion. At the lead-in portion of the extruder is a vent portion wherein the screw is provided with a plurality of stirring blades extending radially from the screw toward the inner surface of the extruder. These stirring blades provide a means by which the inert gas is mixed thoroughly into the plastic material. The above invention provides a means of producing an insulating material in which an inert gas (such as nitrogen), rather than hydrocarbon chloride fluoride, is used as a foaming agent, thus avoiding potential harm to the ozone layer.

Description: BACKGROUND OF THE INVENTION The present invention relates to an extruder for highly foamed plastic material for coating a central conductor of a coaxial insulated cable or the like with an insulator of for example a high foam plastic material, preferably polyethylene, having at least a 70% or more foaming rate. Conventionally, as the method for producing a coaxial insulated cable of the kind described above, there is a known technique in which hydrocarbon chloride fluoride (flon) or the like is used as a foaming agent in two extruders connected to each other. The foaming agent is led into the first-stage extruder at its middle portion so as to be kneaded with a plastic material such as polyethylene or the like, and then the kneaded plastic material is sent to the second-stage extruder so as to be further kneaded and the highly foamed plastic material is applied onto a central conductor as a coating. Fig. 4 (Prior Art) is an external view showing an example of a screw used in the conventional first-stage extruder. Here, the screw having a total length L is constituted by a feed portion 2, a kneading portion 3, a metering portion 4, a vent portion 5, a kneading portion 3', and a metering portion 4' having lengths l 2 , l 3 , l 4 , l 5 , l' 3 , and l' 4 respectively. A foaming-agent lead-in portion 6 is provided in a cylinder at a portion where the vent portion 5 of the screw 1 is positioned. The foaming-agent lead-in portion 6 has a structure such as a poppet valve, which is not illustrated in detail in the drawing, for preventing a plastic material in the cylinder from flowing back. The pellet-like plastic material supplied from a portion a of the cylinder is kneaded through the lengths l 2 , l 3 , and l 4 , further kneaded through the lengths l' 3 , and l' 4 together with a foaming agent such as hydrocarbon chloride fluoride (flon) poured under volume-regulation at the vent portion 5, and then led into a second-stage extruder 10 (Fig. 3). As portrayed in a schematic view in Fig. 3, the foaming agent and the mixed plastic material which are led into the second-stage extruder 10 are further kneaded so that the foaming agent is equally dispersed in the plastic material, and the thus kneaded foam plastic is applied onto a central conductor 16 as a coating therefor by a crosshead 15 constituted by a nipple and a die positioned at a top end of the second-stage extruder 10, thereby producing a high foam insulated wire-like body 17. Conventionally, in extruding a plastic material such as high foam polyethylene or the like in which hydrocarbon chloride fluoride (flon) or the like is used as a foaming agent, the foaming agent performs an expanding action onto the plastic material, so that the conventional screw performs sufficient kneading. Recently, however, the theory has been advocated that hydrocarbon chloride fluoride, or flon, destroys the ozone layer, causing destruction of the ozone layer on a world-wide scale. Thus, in achieving a method for extruding a high foam plastic material using an inert gas (for example, a nitrogen gas) as a foaming agent, a conventional high foam extruding method cannot be used because a stable consistent foaming state is not achieved. When such a method is used for extruding a high foam plastic material having an inert gas, both the electrostatic capacitance required as a characteristic of cables and the outer diameter of the cables vary extremely. Accordingly, an extrusion method for extruding high foam plastic material using an inert gas as a foaming agent and in which the foaming state is consistent and stable has been required. DE-A-1 554 773 discloses an extruder having a cylinder with an inlet portion, the cylinder defining a cavity, into which cavity feeds a extruder screw having a vent portion and a plurality of blades extending radially into said vent portion. US-A-3 806 099 discloses a first and second extruder for extruding a foamed plastic material using a inert foaming agent to cover a central conductor according to the preamble of the claim. SUMMARY OF THE INVENTION In order to solve the above problems, as a result of various investigations, the present inventor has completed the present invention in which a plurality of stirring blades are provided in an extruder for highly foamed plastic material at its vent portion so that a high foam plastic material such as polyethylene or the like and an inert gas as a foaming agent can be evenly mixed with each other to thereby make it possible to stably extrude an insulator in an even, consistently foaming state. That is, according to the present invention, the high foam insulation extruder using an inert gas, such as a nitrogen gas, as a foaming agent for producing an electric wire insulated with high foam polyethylene having a foaming rate not lower than 70 %, comprises the features as defined in the claim. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a side section showing an extruding screw of a first-stage extruder unit of a high foam insulation extruder; Fig. 2 is a sectional view taken on a line A-A of Fig. 1; Fig. 3 is a schematic view showing the present invention of the high foam insulation extruder constituted by the pair of extruder units; and Fig. 4 (Prior Art) is a side section showing an example of a conventional extruding screw. DETAILED DESCRIPTION: OF THE PREFERRED EMBODIMENTS Fig. 1 shows an extruding screw of a first-stage extruder unit of a high foam plastic extruder in which an inert gas is used as a foaming agent. Fig. 2 is a sectional view taken on a line A-A of Fig. 1 and showing a structure of a vent portion of the screw 1. As shown in Figs. 1 and 2; the present invention is proposed so as to solve the disadvantage in the conventional art and an object thereof is to provide an extruding screw 1 with a vent portion 5, which may be tapered and which is circumferentially provided with a plurality of stirring blades having an effective height extending to the vicinity of an inner wall of a cylinder 8. The stirring blades are provided in place of a flight of the screw threads in the vent portion 5. Fig. 3 is a schematic view of the present invention of the extruder for highly foamed plastic material constituted by the pair of extruder units. Next, the operation will be described. An inert gas having predetermined pressure is led from an inert gas lead-in portion which is formed at the vent portion, the inert gas being fed into a completely melted plastic material through the lengths l 2 , l 3 , and l 4 of the screw 1. The fed inert gas is physically mixed with the plastic material by a plurality of stirring blades 7 provided in the vent portion 5 with the stirring blades having an effective height extending to the vicinity of the inner wall of the cylinder 8. The mixture is further kneaded through the lengths l' 3 , and l' 4 , so that the inert gas is equally dispersed in the plastic material in the form of fine inert-gas bubbles to thereby obtain a stable extruding state in which an even, consistently foaming state is achieved. Here, variations in the characteristics of cables coated with a high foam insulator depending on the difference in screw structure are as shown in the following Table 1. Forming agent Screw Outer diameter (mm) Outer diameter variation (mm) Electorostatic capacity variation (PF/m) Hydrocarbon chloride fluoride Conventional screw 8.5 ±0.03 ±0.05 Inert gas (nitorogen gas) Conventional screw 8.5 ±0.2 ±0.3 Screw of the invention 8.5 ±0.05 ±0.08 It is apparent from the result of Table 1 that the extruding screw according to the present invention has an extrusion performance equivalent to that of the extruding method in which hydrocarbon chloride fluoride (flon) is used as a foaming agent. Low-density polyethylene, middle-density polyethylene, high-density polyethylene, polypropylene, butyl rubber, or the like, or a mixture of two or more kinds of the polyolefin-group resin can be used as the high-foam plastic material, and particularly polyethylene resin and polypropylene resin are preferrably used. Further, as the high foam plastic material according to the present invention, the polyolefin resin may be mixed with a small quantity of other thermoplastic resin or rubber. The inert gas used as a foaming agent in the present invention may be, typically, nitrogen, argon, carbon dioxide, or the like. There is no specific requirement of which one so long as the inert gas is in a gaseous state under the conditions of temperature not lower than 100°C and pressure not higher than 30 kg/cm 2 . In foaming, a small amount of bubble nucleus forming agent such as azodicarbonamide or the like may be mixed with the foaming plastic material in advance. As described above, according to the present invention, in the extruder for extruding the high foam plastic material using an inert gas as a foaming agent, the flight of the screw thread at the vent portion is removed and a plurality of stirring blades with an effective height extending to the vicinity of the inner wall of the cylinder are provided around the circumference of the screw in a radial direction in place of the flight of screw thread, so that it becomes possible to physically mix and mull an inert gas and a high foam plastic material evenly with each other. Accordingly, a stable extruding state in which the foaming state is consistent can be obtained. According to the present invention, it is possible to obtain extremely good effects in that the foaming agent such as the conventional hydrocarbon chloride fluoride or the like which may destroy the ozone layer in the air may be replaced with an inert gas which does not cause any environmental problem at all by replacing the conventional screw with a relatively inexpensive screw according to the present invention. Besides avoiding environmental problems, the cost of raw material can also be reduced. Accordingly, the present invention is effectively used in the cable producing field in which a high foam plastic material is applied as an insulator coating onto, for example, a central conductor of an insulated coaxial cable.

Claims: An extruder for applying onto a central conductor a highly foamed plastic material using an inert gas as a foaming agent comprising: a first-stage extruder unit (9) comprising a first cylinder (8) defining a first cavity and a first screw (1) extending through said first cavity for propelling plastic material through said first cavity as said screw (1) rotates; a second-stage extruder unit (10) for extruding said foamed plastic material and comprising a second cylinder defining a second cavity and a second screw extending trough said second cavity; wherein said second-stage extruder unit (10) is connected to said first-stage extruder unit (9) for receiving, propelling and mixing a mixture of said plastic material and said foaming agent; a combination of a nipple and a die, located at the end of said second-stage extruder unit (10) and applying said mixture onto the central conductor; characterised in that the first cylinder (8) has a foaming-agent lead-in portion (6) through which the foaming agent enters into said first cylinder (8); said first screw (1) has a vent portion (5) and a plurality of stirring blades (7) extending radially from said vent portion (5) adjacent to said lead-in portion (6) for stirring said foaming agent into said plastic material as said plastic material is moved through said first cavity; said second-stage extruder unit (10) receives the mixture of said plastic material and said foaming agent for further propelling said mixture through said second cavity and further mixing said mixture as said second screw rotates.

Inventors: KAWABATA SABURO, KAWABATA, SABURO, KAWABATA, SABURO C/O YOKOHAMA WORKS OF
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1991
Main classification: B29C47
IPC codes: B29C44, B29C47, H01B13