WO2002026470A1

WO2002026470A1 - Crosshead with accurate adjustment for uniform dispersion of extrudate

Info

Publication number: WO2002026470A1
Application number: PCT/US2001/030451
Authority: WO
Inventors: David A. Kaye
Original assignee: Genca Corporation
Priority date: 2000-09-28
Filing date: 2001-09-28
Publication date: 2002-04-04
Also published as: AU2001296387A1

Abstract

A crosshead (1) has a tip holder (5) for pivotally holding the tip (82). Set screws (14a, 14b) are used to apply adjustable forces to the tip holder (5) against a constant return force. By adjusting the set screws, the tip (82) can be pivoted relative to the rest of the crosshead (1) to ensure that the tip (82) and the die (81) are concentric. The tip has a pivot point within the tip holder (5), and the distances from the pivot point to the set screws (14a, 14b) and from the pivot point to the outlet of the die are selected to provide accurate adjustment of the tip (82).

Description

CROSSHEAD WITH ACCURATE ADJUSTMENT FOR UNIFORM DISPERSION OF EXTRUDATE Field of the Invention

The present invention is directed to a crosshead. for extruding a plastic sheath or jacket around a wire core to form a cable and more particularly to such a crosshead having a tip and die configured to be adjusted for uniform dispersion of the plastic sheath or jacket. Description of Related Art

It is a common industrial practice to encapsulate a wire core such as an electrical wire, cable, optical fiber, filament, etc. with a jacket or sheathing to insulate or isolate the core from the ultimate environment in which it is to be placed. The insulation is usually of a polymer plastic composition supplied from an extruder in a molten state and dispensed onto the wire core via a crosshead through which the core is continuously fed. The molten plastic enters a flow path which forms a concentric and uniform distribution about the core. Desired qualities in the jacket or sheathing include uniformity of thickness and the absence of voids.

An example of a crosshead for applying such a jacket or sheathing is taught in U.S. Patent No. 5,358,570 to Drawbaugh, the disclosure of which is hereby incorporated by reference herein. In that crosshead, the molten plastic enters through an inlet and flows through two oppositely directed primary flow paths. Each primary flow path splits into two secondary flow paths and a ramped diversion dam. The secondary flow paths and ramped diversion dams are configured to form an annular flow path for the molten plastic. From the annular flow path, the molten plastic enters a space between a tip and a die. The tip is hollow, and the core is fed through a bore in the tip. Thus, the molten plastic is extruded to form the sheath or jacket around the core.

For the wall of the jacket or sheath to be even, the tip and the die must be concentric. Ensuring such concentricity has traditionally required painstaking adjustment. Summary of the Invention

It will be readily apparent from the above that a need exists in the art for a tip and die which overcome the above-noted problems of the prior art. It is therefore an advantage of the present invention to provide accurate adjustment for the wall concentricity of the extrudate in a simple manner.

To achieve the above and other advantages, the present invention is directed to a crosshead die apparatus in which the relative positions of the tip and the die, and thus the wall concentricity of the extrudate, can be adjusted by using a variable force reacting against a constant response force. For example, the tip can be mounted in a pivotable tip holding device having a ball socket or spherical seat to seal the tip holding device against extrudate leakage. The tip holding device is retained by

mechanical tension. Such a tip holding device can be pivoted radially over 360° by

using only two variable sources of force to control the radial pivoting. The forces can be generated through any controllable method.

The two variable forces move in finite slip-stick free movement. Also, the pivot center and the force center of the tip holding device are positioned relative to each other to provide a length ratio which, in conjunction with the finite slip-stick free movement, allows an adjustment of concentricity more precise than any known in the art. Brief Description of the Drawings

A preferred embodiment of the present invention will be set forth in detail with reference to the drawings, in which: Fig. 1 shows a cross-sectional view of a crosshead according to the preferred embodiment;

Fig. 2 shows a front view of the same crosshead; and

Fig. 3 shows a rear view of the same crosshead. Detailed Description of the Preferred Embodiments A preferred embodiment of the present invention will be set forth in detail with reference to the drawings, in which like reference numerals refer to like components throughout.

Fig. 1 shows a cross-sectional view of a preferred embodiment of a crosshead according to the present invention. Figs. 2 and 3 show front and rear views, respectively, of the same crosshead. Except as described below, the crosshead functions essentially like the crosshead of the above-referenced Drawbaugh patent. The crosshead 1 includes a head cartridge 3 into which a helicoid or other manifold 4 is inserted. The helicoid 4 has an inlet 2 for receiving molten plastic and is held in place with screws 9a (shown in Fig. 3). A tip holder 5 is inserted into the helicoid 4 and held in place with a retainer 7, and a die holder 6 is bolted onto the front of the head cartridge 3 with screws 9b. A die 81 is disposed in the die holder 6. A tip 82 is disposed in the tip holder 5, and an O ring 10 is disposed between an outer surface of the tip 82 and an inner surface of the helicoid 4 to form a seal therebetween to prevent the passage of molten plastic. The crosshead 1 is heated with a heater 15 held in place with clips 15a and a screw 15b (Figs. 2 and 3). To measure the temperature of the crosshead 1, a thermocouple 41 is held to the crosshead 1 with a thermocouple adapter 42 and plugged into a thermocouple j ack 43. Set screws 14a, 14b apply variable forces in two orthogonal directions to the tip holder 5. The response force is provided by a spring 13 applying a force, typically on the order of 100 lbs., on a push rod 11 within a spring housing 8. An extractor 12 is provided to facilitate disassembly.

The crosshead has three set screws 14a, 14b. Two set screws 14a are provided above the push rod 11 and are separated from a vertical line N passing through the

push rod by 45° on either side. The third set screw 14b is below the tip holder 5 and

is parallel to the vertical line N.

The two set screws 14a located at 45° angles from the vertical line N apply

forces in orthogonal directions on the tip holder 5. The two of them can be adjusted

to apply a combined force at any angle over a 360° range from the vertical line N.

The three set screws 14a, 14b are adjusted to ensure that the tip 82 and the die 81 are concentric. A slight gap 5a is left around the tip holder 5 to allow such adjustment. The tip holder 5 is engaged with the tip 82 at a pivot point 5b, at which the tip holder 5 and the tip 82 have spherical surfaces to form a bearing. A distance A between the pivot point 5b and an outlet plane 81a of the die 81 is typically 1.084 inches. A distance B between the pivot point 5b and the vertical line N along which the push rod 11 applies the return force is typically 3.000 inches. The distance B is selected to be sufficiently larger than the distance A such that the tip 82 can be positioned within the die 81 with great accuracy. Various modifications can be made to the invention as disclosed above. For example, the adjusting forces could be applied by force applying elements other than the set screws 14. Also, while the preferred embodiment has been disclosed with reference to a particular type of crosshead, it can be adapted to other types of extruding devices. Moreover, instead of two variable forces, a single variable force having a variable angular orientation could be used.

Although only preferred embodiments are specifically illustrated and described herein, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.

Claims

What is claimed is:

1. A crosshead die apparatus comprising: a die; a tip extending into the die to form a flow path between the tip and the die; a tip holding device for holding the tip at an adjustable position within the die; and a force applying system for adjusting the position at which the tip holding device holds the tip within the die by applying a controUably variable force and a response force to the tip holding device.

2. The crosshead die apparatus of claim 1, wherein the force applying system applies two controUably variable forces.

3. The crosshead die apparatus of claim 2, wherein the two controUably variable forces are orthogonal to each other.

4. The crosshead die apparatus of claim 3, wherein the force applying system comprises two set screws, each positioned against the tip holding device, for applying the two controUably variable forces.

5. The crosshead die apparatus of claim 4, wherein the force applying system further comprises a push rod and a spring for applying the response force, the push rod being positioned against the tip holding device.

6. The crosshead die apparatus of claim 1, wherein the tip holding device holds the tip to allow the tip to pivot around a pivot point at which the tip and the tip holding device are in contact.

7. The crosshead die apparatus of claim 6, wherein both the tip and the tip holding device have spherical surfaces at the pivot point.

8. The crosshead die apparatus of claim 7, wherein: the die has an outlet plane; the pivot point is separated from the outlet plane of the die by a first distance; the pivot point is separated from a location on the tip holding device to which the controUably variable force and the return force are applied by a second distance; and the second distance is greater than the first distance.

9. A method of adjusting a crosshead die apparatus comprising a die, a tip extending into the die to form a flow path between the tip and the die, and a tip holding device for holding the tip at an adjustable position within the die so that the die and the tip are concentric, the method comprising:

(a) applying a controUably variable force and a response force to the tip holding device; and

(b) adjusting the controUably variable force to cause the tip to be concentric with the die.

10. The method of claim 9, wherein step (a) comprises applying two controUably variable forces.

11. The method of claim 10, wherein the two controUably variable forces are orthogonal to each other.