JPS6078718A - Extrusion of foaming body - Google Patents

Abstract

PURPOSE:To stably manufacture high quality foaming body in constant extrusion and expansion by reducing the groove depth of the front end, providing cut-off parts in screw flight and using the screw having many flights in the lengthwise direction of the body. CONSTITUTION:In a method for obtaining a foaming body by mixing foaming agent with extruding material and extruding the mixture from the front die of the extruder to the low pressure zone, the screw to be used has the groove depth of the screw 6 smaller than that of other part in the front end part of the screw, a part 7 short of screw flights and many flights 8 installed in the direction of the screw length near the above part. In this manner, it is made possible to stably manufacture of high quality cable free from fluctuation of extrusion and expansion per revolution of the screw, that is, excellent in characteristics of standing wave ratio.

Description

[Detailed description of the invention] The present invention relates to improvements in extrusion methods and devices for foams and the like.

It is known to use extruders with screws to produce rubber-plastic foam products and pellet-like moldings.

For example, when extruding plastic foam products, the foam material is fed into an extruder with a screw, heated and melted, and then
There is a method of obtaining a foam by extruding it into a low pressure zone from a die at the tip of an extruder.

FIG. 1 is an example for extruding foamed plastic,
The material is supplied from the material supply port (2) of the extruder (1).

Then, as a blowing agent, a gas such as N, Co8, or a fluorocarbon liquid is injected into the extruder through an injection port (not shown), or a chemical blowing agent is kneaded into the material in advance. A foaming agent is mixed into the material. After kneading and stirring the blowing agent and materials in the scree (3) in the extruder (1), the conductor (
5) is run to cover the foam material, and then extruded into a low pressure zone to obtain a foam.

In this method, the conventional screw uses a screw having flights installed over its entire length, but it has the following drawbacks.

(1) When material is fed into the crosshead through the scree, pressure fluctuations occur as the screw rotates.

(2) The above pressure fluctuations cause fluctuations in the extrusion amount, outer diameter, foaming degree, etc. during extrusion, resulting in deterioration of standing wave ratio (VSWR) characteristics in the case of coaxial cables, for example.

The present invention has been made in order to improve such drawbacks, and the shape of the tip of the scree is modified such that one or more pitches of the flight portion of the scree are removed, or the depth of the groove of the scree is shallower than that of other portions.

Next, one screw example of extrusion using a screen having a zone with a large number of flights in the length direction of the extruder will be shown.

Conventionally, screw flights are installed over the entire length, so the flow of material changes in a spiral shape each time the screw rotates, and this flow change is not completely corrected before being fed into the crosshead. For this reason, the material pressure within the crosshead fluctuates periodically as the screw rotates, and the amount of material supplied also fluctuates. On the other hand, in the screen of the present invention, the part (7) shown by the broken line in FIG.
), or the depth of the screw groove is made shallower than in other parts, and subsequently, a large number of flights 8 are formed in the length direction of the screw.

Therefore, the fluctuations in the flow of material caused by screw rotation, which conventionally occur, can be removed in zone (7), and the flow can be rectified in the length direction of the extruder in zone (8), making it possible to The problem can be completely solved.

In other words, by using this screw, there is no variation in the extrusion amount or foaming degree with each screw rotation, for example, VSW
High quality cables with good R characteristics can be stably manufactured.

The part where the depth of the groove of the screw is made shallow or the flight is removed requires at least one pinch, and preferably the flight is removed. It is preferable that four or more flights in the length direction of the extruder are provided in the circumferential direction.

〔Example〕

Using tetrafluorodichloroecune as a foaming agent in polyethylene, conductor diameter 4-3 mm, insulation outer diameter 17.5111+
++.

Table 1 shows the results of manufacturing a CATV cable core with −ε=1.26 and evaluating its characteristics.

Table 1 (G) 2 pitches have no flights and the groove depth is set to 172 mm from the groove depth of the other flight parts.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a conventional method for extruding a foam, and FIG. 2 is a diagram showing a method for extruding a foam according to the present invention. In the figure, 6 is a flight IJ used in the method of the present invention, -, 7 is a missing portion of the flight, and 8 is a part provided in the length direction.

Claims (2)

[Claims] (1) In the method of mixing a foaming agent into the extrusion material and extruding it from a die at the tip of the extruder to a low pressure zone, the shape of the screw tip should be such that the depth of the groove in the screw hole is shallower than in other parts. A method for extruding a foam, characterized by using a screw having a structure in which a portion lacking flights is provided, and a large number of flights are subsequently provided in the longitudinal direction of the screw. (2) In the screw, a screw is used in which one or more pitches of screw flights are removed, and four or more flights are subsequently installed in the longitudinal direction of the screw. ) The method for extruding the foam described in section 2.