GB2109740A - Extruder cross-head for extruding material around an elongated article - Google Patents

Abstract

The nipple holder (15) of the cross-head has an axial hole (25) formed therethrough for passing the elongated article through the extrusion die 51, to receive a layer of a molten plastic material supplied from an inlet (17) from an extruder (4) of the cross head to be by a passage which includes a set of grooves formed in the nipple holder, including: a first groove (27) communicating from the inlet (17) toward the proximal end of the nipple holder; two second grooves (33) communicated to the first groove by branch grooves (29) and extending toward the distal end of the nipple holder; two sets of third grooves (35) extending toward the distal end of the nipple holder, each set from a corresponding second groove; and an annular nipple groove (39) communicating the sets of the third grooves to nipple. A further layer, of a second material, from a second extruder, may be extruded around the material delivered around the nipple holder (15), via a similar set of grooves formed in a guide provided around the nipple holder. <IMAGE>

Description

SPECIFICATION Extrusion apparatus BACKGROUND OF THE INVENTION The present invention relates to an extrusion apparatus for extruding a thermoplastic material such as synthetic resin, rubber or the like about an elongated article such as electric conductor.

In such extrusion apparatus, there has been known an apparatus for extruding a molten material such as thermoplastic resin in a tubular shape about an elongated conductor to thereby apply an insulated coating thereon. Due to the structure of this extrusion apparatus, it is difficult to set the tubular thermoplastic material to be concentric with the conductor on which the thermoplastic material is applied, and thus the so-called eccentric layer phenomenon is likely to occur. The reason why this adverse phenomenon is caused will be explained with reference to Fig. 1 illustrating a typical wellknown extrusion apparatus for producing insulated electric wires.In this extrusion apparatus, a molten plastic material 2 is fed by a screw 4 of an extruder into a tubular molten material passage 6 defined between a cross head 1 and a nipple holder 9, and then passed through the clearance between a die 8 and a nipple 11 held by the nipple holder 9, so that a coating of the plastic material is applied about a conductor 5 which passes through the nipple holder 9 and the die 8. The above-described eccentric layer phenomenon can be caused not only by the misalignment between the die 8 and the nipple 1 1, but also by an uneven circumferential pressure distribution around the tip of the nipple 11 due to the difference in length between the upper and lower passages 6A and 6B, and the difference in resistance and pressure to which the molten material is subjected.

To avoid this eccentric phenomenon, the cross section of an extruded tubular thermoplastic material on the conductor 5 is conventionally examined at times, and then when needed, the die 8 is fine adjusted to be less eccentric with the nipple 1 1 by adjusting eccentricity adjustment bolts 10 of a die holder 12. However, this procedure requires an operator to watch an insulated electric wire coming out of the die 8 to find an unacceptably eccentric layer of the wire when the layer is of a transparent plastic material. When the molten material 2 is not transparent it is necessary to repeat the abovedescribed cross-section examination of the insulated electric wire and the readjustment of the die 8 many times, which results in increases in labor and loss of the raw materials.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an extrusion apparatus for extruding a plastic material about an elongated article in which the circumferential pressure distribution of a nipple is even, so that the occurrence of the eccentric layer phenomenon is minimized, whereby the eccentricity adjustment labor and loss of the raw materials in producing the coated article are largely reduced.

It is another object of the present invention to provide an extrusion apparatus for extruding a plastic material about an elongated article, in which when conditions such as the viscosity and flow rate of the molten plastic material, the diameter of the elongated article, and the thickness of the coating of the plastic material are varied, the molten plastic material is little affected by molten material passages formed within a cross head, and thus the concentricity of a plastic material layer around the elongated article is maintained.

It is a further object of the present invention to provide an extrusion apparatus for extruding a plastic material about an elongated article in which the nipple holder is miniaturized, whereby the cross head is made compact.

With these and other objects in view, the present invention provides an extrusion apparatus in which a cross head has a bore formed therethrough, and a nipple holder having a nipple provided at its distal end is inserted into the bore so that the nipple faces a die, the nipple holder having an axial hole formed therethrough for passing an elongated article through the axial hole whereby a molten plastic material supplied from an extruder is passed through a molten material passage formed within the bore of the cross head into the die to thereby extrude a layer of the plastic material about the elongated article, wherein the passage comprises a set of grooves formed in the nipple holder, the set of grooves including:: (a) a first groove formed therein to extend toward the proximal end of the nipple holder and communicated to the extruder for receiving the molten plastic material; (b) two second grooves communicated to the first groove and formed to extend toward the distal end of the nipple holder, (c) two sets of third grooves formed to extend toward the distal end of the nipple holder, each set of the third grooves being communicated to the corresponding second groove, and (d) an annular groove formed circumferentially therein and communicating the sets of the third grooves to the nipple.

BRIEF DESCRIPTION OF THE DRAWINGS While the specification concludes with claims which particularly point out and distinctly define the subject matter which is regarded as the invention, it is believed the invention will be more clearly understood when considering the following detailed description and the accompanying drawings in which like or corresponding members are designated by like reference numerals.In the drawings: FIG. 1 is a lengthwise vertical section of the cross-head of the conventional molding apparatus; FIG. 2 is a lengthwise vertical section of one embodiment of the present invention; FIG. 3 is a plan view of the nipple holder shown in Fig. 2; FIG. 4 is a bottom view of the nipple holder shown in Fig. 2; FIG. 5 is a cross section taken along the line V-V in Fig. 4; FIG. 6 is a cross section taken along the line VI-VI in Fig. 4; FIG. 7 is a view showing a developed body portion of the nipple holder in Fig. 2; FIG. 8 is a side view, partly in section, of another embodiment of the present invention; FIG. 9 is a front view of the embodiment in Fig. 8;; FIG. 10 is a side view of the guider for an outer layer in Fig. 8; FIG. 11 is a bottom view of the guider for an outer layer in Fig. 10; FIG. 12 is a cross section taken along the line Xll-Xll in Fig.11; FIG. 13 is a cross section taken along the line Xlll-XIll in Fig.11; FIG. 14 is a side view of the nipple holder for an inner layer; and FIG. 1 5 is a plan view of the nipple holder in Fig. 14.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figs. 2 to 7, particularly Fig. 2, there is illustrated an extrusion apparatus for producing an insulated electric wire, in which an attaching bore 3 is formed through a cross head 1 along an advancing direction of an elongated conductor 5, the bore 3 being communicated with a discharge port 7 of an extruder (not shown). Fitted into this bore 3 are a nipple holder 9 which is integrally provided at its distal end with a nipple 11 , and a sleeve 13 tightly fitted around a body portion 1 5 of the nipple holder 9. The sleeve 13 is brought into tight contact with the wall of the bore 3. However, the sleeve 13 may be omitted, in which case the body portion 15 of the nipple holder is designed to tightly fit into the bore 3.Through the wall of the sleeve 13 there is formed a communicating hole 1 7 which communicates the discharge port 7 to an outer face of the body portion 15 of the holder 9. The nipple holder 9 is further provided with a flange 19 from which a sealing ring 21 extends axially. The body portion 1 5 axially projects from the sealing ring 21. The ring 21 is tightly fitted into the bore 3, and then the flange 19 is detachably attached to the crosshead 1 by bolts 23. A center hole 25 is axially formed through the nipple holder 9 to allow the conductor 5 to pass through the center hole.

As clearly shown in Figs. 3 to 7, in the surface of the body portion 15 of the nipple holder 9 there are formed a set of grooves of special shapes which form passages for passing a molten material 2 between the body portion 15 and the sleeve 13. The grooves include a first groove 27 formed radially in the body portion 15 to extend toward the proximal end thereof and communicated through the communicating hole 17 of the sleeve 13 to the discharge port 7 of the extrusion machine. The first groove 27 leads the molten material 2 backwards, i.e., in a direction reverse to the advancing direction (shown by the arrow) of the conductor 5.The first groove 27 is bifurcated into two branch grooves 29 and 29, which circumferentially extend to positions 900 away from the bifurcated point 31 with respect to the axis of the nipple holder 9, that is, 1 800 away from each other. Each branch groove 29 terminates in a second groove 33, which extends toward the distal end of the nipple holder and is bifurcated into two third grooves 35 and 35 each communicating to a sector groove 37. Each sector groove 37 is wider toward the distal end of the nipple holder 9 and its depth is made gradually smaller toward that distal end. All the sector grooves 37, four grooves in this embodiment, communicates with a V-shaped annular groove 39 formed radially and extends circumferentially in the distal end of the body portion 1 5 between the sector grooves 37 and the nipple 11.The annular groove 39 is larger in depth than the sector grooves, and communicates the latter to a clearance 41 defined by nipple 11 and a die holder 43.

The holder 43 is detachably attached to the front side of the cross head 1 by bolts 47 so that a die supporting hole 49 is positioned coaxially with the nipple 11. A die 51 which is fixedly attached to a front nut 53 is tightly fitted into the die supporting hole 49, and is fastened there by the front nut 53 which threadedly engages with the die holder 43.

In operation, molten material 2 within a cylinder of the molding machine is fed by a screw 4 from the discharge port 7 into the first groove 27 of the nipple holder 9. The sleeve 13 is sealedly fitted around the body portion 1 5 of the nipple holder 9, and thus the molten material 2 introduced into the first groove 27 is fed backwards, and divided into two branch flows at the bifurcating point 31, which are then led by the branch grooves 29 to positions away from each other by 1 800 with respect to the axis of the nipple holder 9. Subsequently, the two branch flows of the molten material 2 are fed parallel and forwards by the respective second grooves 33, and then divided by the third grooves 35 into four sub-branch flows 900 apart with respect to the axis of the nipple holder 9. The four sub-branch flows are introduced into respective sector grooves 37. The distance of the flow of the molten material 2 from the first groove 27 to each sector groove 37 is substantially equal, and with respect to each set of grooves 29, 33, 35, or 37 each groove has a substantially equal cross-sectional area. Thus, each subbranch flow is equal in flow-rate and pressure. Then, the molten material 2 is diverged in each sector groove 37, the depth and the cross-sectional area of which are the smallest at its forward edge or edge nearest to the annular groove 39. With such structure, the pressure of the molten material 2 flowing through the sector grooves 37 is raised, and the four sub-branch flows are combined at the forward edges thereof into a cylindrical flow of uniform pressure and introduced into the annular groove 39.The depth of the annular groove 39 is made larger than that of the sector grooves 37, and thus the annular groove 39 serves as an accumulator 39. In the annular groove 39, the pressure of introduced molten material 2 is dropped at its backward side and is raised at its forward side, so that the circumferential pressure distribution of the molten material 2 becomes more uniform at the boundary of the annular groove 39 and the nipple 11. Thus, a tubular molten material 2 of a uniform thickness is fed out of the nipple and then of the die 51, whereby a coating of a uniform thickness is applied on the conductor 5.

The alignment of the main components of the above-described cross head 1 will be described below. In minimizing the eccentric layer phenomenon of the coating layer of the conductor 5, it is required to exactly position the nipple 11 and the die 51 so that they are coaxial. In this embodiment, such requirement is met by forming the attaching bore 3 of the cross head 1 and the die supporting hole 49 of the die holder 43 to be in alignment with each other. Such machining is not difficult, and the misalignment may be held within 1/100 of a millimeter. The nipple 11 and the die 51 are exactly aligned with each other by the following simple procedures. Firstly, the die holder 43 is, as shown in Fig.

2, fastened to the crosshead 1 by bolts 47. Next, the nipple holder 15 around which the sleeve 13 is fitted is inserted into the attaching bore 3 of the crosshead 1, and is projected into the die supporting hole 49 of the die holder 43. Then, the flange 19 of the nipple holder 1 5 is fastened to the cross head 1 by bolts 23. Finally, the die 51 is inserted into the die supporting hold 49, and secured to the die holder 43 by threading the front nut 53 with the latter.

In the above embodiment, the diameter of the attaching bore 3 is larger than that of the die supporting hole 49, but they may be equal. In the latter event, the bore 3 and the hole 49 may be formed by a single boring operation if the die holder 43 is previously fastened to the cross head as shown in Fig. 2.

In place of the conductor 5, the present invention may be applied to the other elongated article such as wire for forming an extruded covering layer thereon.

With respect to the number of the third grooves 35, it may be more than four, depending on the outer diameter of the nipple holder 1 5.

Furthermore, the cross sectional area of the first groove 27, the sum of the cross sectional area of the second grooves 33, and the sum of the cross sectional area of the third grooves 35 may be made equal to each other or gradually decreased in the described order. By doing so circumferential pressure distribution of the molten material in the die 51 becomes more even and thereby the eccentricity of the layer around the conductor 5 is minimized. Further the adhesion of remaining plastic material to the outlet of the die 51 is also minimized.For example, the following dimensions may be adopted: Dimensions Number of Cross sectional (width x depth) grooves area first groove 16mmX 7mm 1 112mum2 second grooves 1 ohm x 55mm 2 110 mm2 third grooves 6mm x 4.5mm 4 108 mm2 Referring to Figs. 8 to 15, there is illustrated a crosshead 1 for a double layer extrusion molding machine as another embodiment of the present invention, in which like or corresponding members are indicated by the same reference numerals as in the first embodiment, and explanation thereof are omitted.This cross head 1 includes an outer guider 61 for forming an outer layer of the insulation covering of conductor 5, the outer guider having a bore 63 formed through a body portion 65 thereof, and an inner or first nipple holder 67 for forming an inner layer of the insulation covering of conductor 5, the body portion 69 of the inner nipple holder 67 being tightly fitted into the bore 63 of the outer guider 61. The nipple holder 67 has a nipple 71 integrally formed at the distal end of its body portion 69. The nipple holder 67 and the outer guider 61 are provided at proximal ends thereof with flanges 75 and 77, respectively.The assembly of the inner nipple holder 67 and the outer guider 61 fitted around the latter is inserted into the bore 3 of the cross head 1 with the outer surface of the body portion 65 of the outer guider 61 being brought into tight contact with the wall of the bore 3. The flanges 75 and 77 of the respective holders 67 and 61 are as shown in Fig. 8 fastened to the cross head 1 by means of bolts 23.

Two outlet pieces 79 and 81 of two cylinders of the extrusion molding machines (not shown) are threadedly attached to the crosshead 1 at symmetrical positions away from each other by 1 800 with respect to the axis of the bore 63 thereof. The outlet piece 79 is in communication with a first groove 27 formed in the outer surface of the inner nipple holder 67 through a communicating hole 83 formed through the crosshead 1 and a supply hole 85 formed through the body portion 65 of the outer guider 61 and being positioned to be in registration with the communicating hole 83. The other outlet piece 81 is in communication with another first groove 87 formed in the outer guider 61 through another communicating hole (not shown) formed through the cross head 1.In each outer surface of the body portions 69 and 65 of the respective nipple holder 67 and the guider 61, there are formed grooves of substantially the same structures as those in the first embodiment, but the corresponding grooves of the nipple holder 67 and the guider 61 are formed 1 800 away from each other with respect to the axis of the nipple holder 67 and guider 61. The grooves formed in the body portion 65 of the outer guider 61 are sealedly covered with the wall of the attaching bore 3 to thereby form molten material passages for the outer layer, and the grooves formed in the body portion 69 of the inner nipple holder 67 are sealedly covered with the inner wall of the outer guider 61 to thereby form molten material passages for the inner layer.

With such construction, the molten material 2 is fed from the outlet piece 79 through holes 83 and 85 into the first groove 27 of the inner nipple holder 67, and then it is forced through the abovedescribed molten material passages for the inner layer into the annular clearance 95 defined between the nipple 71 and the body portion 65 of guider 61 from which clearance the molten material 2 is fed to the annular clearance 93 defined between the nipple 71 and the die holder 43.On the other hand, another molten material 89 is fed from the outlet piece 81 through the hole 84 into the first groove 87 formed in the outer guider 61 , and after the molten material 89 passes through the molten material passages for the outer layer, it is introduced through the annular clearance 91 defined between the body portion 65 of outer guider 61 and the die holder 43 into the annular clearance 93 where it joins the molten material 2 for the inner layer to form tubular double layers, which are then passed through the annular clearance 97 defined between the die 51 and the inner nipple 71 and applied on the conductor 5 to form double tubular insulation layers thereon. In this embodiment, double insulation layers each of which has a uniform thickness are formed on the conductor 5.

In the second embodiment, the supply hole 85 formed through the outer guider 61 is as shown in Fig. 10 located 1 800 away from the first groove 87. The position of the supply hole 85 is not necessarily restricted to this position although it is essential that the supply hole 85 does not intersect the molten material passages for the outer layer.

Furthermore, in the second embodiment, with respect to the nipple holder and the guider, the cross sectional area of the first groove 27, the sum of the cross sectional area of the second grooves 33, and the sum of the cross sectional area of the third grooves 35 may be made equal to each other or gradually decreased in the described order. For example, the following dimensions may be adopted.

Dimensions Number of Cross sectional (width x depth) grooves area first groove #12mm x 6mm 1 72 mm2 second grooves 8mm x 44mum 2 70 mm2 third grooves 4mm x 4,3mm 4 69 mm2 By doing so circumferential pressure of the molten material in the die 51 becomes more uniform and thereby the eccentricity of the double layers around the conductor 5 is minimized. Further the adhesion of remaining plastic material to the outlet of the die is also minimized.

It is to be noted that the present invention may be adapted to extrude more than two layers around an elongated article.

Claims (9)

1. An extrusion apparatus in which a cross head has a bore formed therethrough, and a nipple holder having a nipple provided at its distal end is inserted into the bore so that the nipple faces a die, the nipple holder having an axial hole formed therethrough for passing an elongated article through the axial hole, whereby a molten plastic material supplied from an extruder is passed through a molten material passage formed within the bore of the cross head into the die to thereby extrude a layer of the plastic material about the elongated article, wherein the passage comprises a set of grooves formed in the nipple holder, the grooves including:: (a) first groove formed therein to extend toward the proximal end of the nipple holder and communicated to the extruder for receiving the molten plastic material, (b) two second grooves communicated to the first groove and formed to extend toward the distal end of the nipple holder, (c) two sets of third grooves formed to extend toward the distal end of the nipple holder, each set of the third grooves being communicated to the corresponding second groove, and (d) an annular groove formed circumferentially therein and communicating the sets of the third grooves to the nipple.

2. An extrusion apparatus as recited in Claim 1, said passage further comprising sector grooves formed so as to widen toward the distal end of the nipple holder, each sector groove being communicated at one end nearer to the proximal end of the nipple holder with the corresponding third groove and at the other end with the annular groove.

3. An extrusion apparatus as recited in Claim 1 , wherein said each third groove is wider toward the distal end of the nipple holder and communicated at the distal end with the annular groove.

4. An extrusion apparatus as recited in Claim 2, wherein the cross sectional area of said sector groove decreases toward the distal end of the nipple holder.

5. An extrusion apparatus as recited in any one of Claims 1 to 4, wherein said annular groove is of a V-shaped cross section.

6. An extrusion apparatus as recited in any one of Claims 1 to 5, wherein the sets of said third grooves are parallel and equispaced.

7. An extrusion apparatus as recited in any one of Claims 1 to 6, further comprising a guider being adapted to be tightly fitted around said nipple holder and to be positioned between said bore of the cross head and the nipple holder and including a second set of grooves formed therein, the second set of the grooves forming a second molten material passage for passing a second molten plastic material supplied from a second extruder to said die to thereby form an outer layer of the second plastic material around said first layer.

8. An extrusion apparatus as recited in Claim 7, wherein said second set of the grooves are of structures similar to the first set of the grooves.

9. An extrusion apparatus as recited in Claim 7 or 8, wherein said guider has a communicating hole formed therethrough for communicating said first extruder to said first groove of the nipple holder to thereby pass the molten material to the first molten material passage.