2023年7月25日发(作者：)

EXTRUSION HEAD FOR PLASTIC

TUBING

Charles E. Carter, Easrliampton, and Stanley C. Wells, Jr., Southampton, Mass.,

assignors to United Plastic Corporation, Easthanipton, Mass., a corporation of

Massachusetts

This invention relates to an improved extrusion apparatus and method for

extruding tubing of plastic and more particularly of plastic having a short extrusion

range.

In the past the extrusion of tubing of plastics of long extrusion range, such as

polyethylene has been effected in an apparatus in which the polyethylene, melted to a

suitable viscosity, is introduced in the form of a round rod into the side of an extrusion

head which is provided with a hollow mandrel and an extrusion die forming with the

mandrel an accurately dimensioned and positioned annulus. The plastic is forced in

under pressure and is extruded in the form of a tube. Since the plastic is not sufficiently

hardened to be completely self-supporting the end of the tube is sealed tight at the start

of any extrusion and a small pressure of air is maintained through the hollow bore of the

mandrel. This distends the tube slightly, supporting it and it is then set by cooling with

air or by other well known means. Take-up rolls pull the tube out and may, if desired,

stretch it, determining the tube wall thickness. Different extrusion dies may be bolted

on to the head for producing tubing of different sizes. No problem is encountered with a

plastic such as polyethylene which is sufficiently flowable so that it forms a uniform

tube which is perfectly clear. The method and the apparatus have been used on a very

large scale and constitute one of the standard methods of producing tubing from

suitable plastics, such as polyethylene.

When it is attempted to extrude tubes of polyurethane by the method and the

apparatus described above for polyethylene, problems are encountered. Polyurethane

does not have the long extrusion range of polyethylene and has to be introduced at more

perfectly controlled viscosities. As a result when the solid rod of polyurethane is forced

to turn and fill the annular space between the mandTel and the extrusion head, and

further down the extrusion die the results are unsatisfactory. At the point of maximum

change of direction optical and dimensional imperfections in the tubing result. These

imperfections do not necessarily affect the strength of the tube but they do produce a

seam because the zone of imperfections is not perfectly homogenous and hence not transparent. The tubes, although they may be mechanically of suitable strength, do not

look well and in order to avoid this drawback it is customary to pigment the

polyurethane or otherwise color it so that the seam is masked. Where an opaque tube is

acceptable polyurethane tubing produced with pigmentation or coloring has been sold

and is a practical, though not so perfect product. However, for many purposes it is

desirable to have a perfectly uniform, clear, transparent tubing of maximum strength

and this has hitherto not been practical at least on a reliably reproducible schedule. It is

with the problem of producing uniform, transparent tubes of polyurethane or similar

type of plastic that the present invention deals.

Essentially the present invention utilizes an extrusion die mandrel and air pressure

for supporting the tube until it is hardened in exactly the same manner as in the

extrusion method and apparatus used for polyethylene tubing. However, instead of

introducing a solid rod of polyurethane into the extrusion head the polyurethane

introduced is formed into a somewhat flattened ribbon.

Now when the ribbon has to change direction in order to flow down through the

annular space between the mandrel and the extrusion head and further down the

extrusion die, the curvature, which is coaxial with the thin 5 dimension of the ribbon,

does not produce imperfections which show up as a seam. On the contrary, perfectly

clear polyurethane tubing can be extruded continuously and reliably without any

off-grade material.

In order to produce the tubing by the method described above, a different form of

extrusion head and channel for the introduction of the polyurethane is required and this

combination constitutes the apparatus phase of the present invention. The use of a

removable die, while desirable, and provided in the present invention, does not

constitute the distinction from the prior art. These dies may be of substantially the same

design as used with polyethylene type materials for the production of tubing. Preferably

they are removable in order to accommodate tubing of different sizes. As far as the

main part of the head is concerned, the present invention eliminates production of an

undesirable seam in the finished tubing but the easier curving of the ribbon shaped

polyurethane coming in also minimizes pressure drop and turbulence in the head. This

is however, of less major importance than the production of uniform, transparent

tubing which is the main advantage of the present invention. The fact that no

disadvantages result, and no compromises are needed is an added advantage of the

present invention. The exact means by which the extrusion head is made forms no part

of the present invention in its broader aspect. There will be described however, a method of forming the extrusion head and design therefor which is advantageous and so

in a more specific aspect of the invention this is an additional feature. The preferred

method involves a split block head which is then bolted or otherwise fastened together.

However, any other method of producing the cavities necessary to introduce a ribbon

may be used, for example by an insert plug, and so in the broader aspect the present

invention includes an extrusion head having the desired configuration regardless of the

method by which the head is made.

Institutions diagram

The invention will be described in greater detail in conjunction with the drawings

in which: FIG. 1 is a vertical section through extrusion head die and material supply;

FIG. 2 is an elevation of the material supply element along the line 2—2 of FIG. 1

looking toward the left; FIG. 3 is an elevation of a portion of the extrusion head at the

point where the material supply is attached and is situated along the line 3—3 of FIG. 1

looking toward the right; and

FIG. 4 is an isometric view of the two blocks of which the extrusion head is formed

the said blocks being shown separated.

The extrusion head in FIG. 1 has a main casting which is shown as 1A because in

the view it is this block. The other block, IB, is shown in FIG. 4 but does not appear in

FIG. 1.

A mandrel tip 2 is screwed into the hollow mandrel 8 and a die 26 is held in two

rings 3 and 5. The former is bolted to the extrusion head by bolts 4 and the latter is

bolted to the ring 3 by bolt 6. An adjusting set screw 7 serves to position the die

accurately with its central part, which is the mandrel tip 2, screwed to the mandrel 8 as

described above.

The mandrel 8 is hollow, the hollow center communicating with a source of air

under slight pressure which is not shown and is indicated only by arrow and legend on the drawing. In the two blocks 1A and IB, which form the extrusion head, there is a

large bore at the top takes a hollow transition plug 9 having a beveled sealing

seat, fastened to the extrusion head by bolts 10. The bottom portion of this element

extends farther down on the right hand side of FIG. 1 as shown at 24 and not so far at 25.

The shapes of the bottom edges being segments of smooth spirals. Inside of the plug 9

is a hollow packing gland 11 into which the mandrel 8 is screwed by threads 12 and

held in position by a lock nut 13.

On the side of the extrusion head there is mounted an element 15 into which is

screwed a conduit 17 from a source of polyurethane 19 under pressure. This source is

not shown and is indicated only by an arrow on the drawing as the feed of the

polyurethane is by any conventional means. The element 15 is bolted onto the extrusion

head 1A and IB by bolts 16 and is provided with a tapering hollow center 18 which

gradually transforms the circular rod of plastic coming through the tube 17 into a ribbon

form. The final ribbon shape is shown at14 in FIGS. 2, 3 and 4. It enters into the blocks

1A and IB of extrusion head and is curved down as is shown in FIGS. 1 and 4, the

ribbon being turned to fill the annular space by the spiral shape of the lower portions 24

and 25 of the transition plug 9. After the ribbon has turned and is now in the form of an

annulus of polyurethane between the mandrel 8 and the extrusion head it passes down

through a relief or surge chamber 21 in the form of a groove in the extrusion head

perpendicular to main axis of head. This further assures that there are no strains or

imperfections in the annulus of plastic moving on down through the tapered annular

channel of the extrusion die 2. A tube is extruded and at the start the end is shown sealed

at 20 in FIG. 1. This sealing and the air pressure through the hollow m the mandrel 8 is

used in the same manner as m the standard polyethylene tube manufacture and, while

necessary to the present invention, is not the portion distinguishing from the

polyethylene extrusion head.

Turning to FIG. 4 it will be seen that the two blocks permit the machining of the

ribbon shaping channels 14 and then when clamped together the bores 23 and 25 are

made in the normal manner. The two blocks are bolted together by bolts 22.

In operation polyurethane, or a material haying similar characteristics, is

continuously introduced by a conventional extruding mechanism and is transformed

into a ribbon m the portion 18 of the element 15 This ribbon is gradually curved by the

spiral surfaces or the portions 24 and 25 of the transition plug 9 and the curving is so

gradual that no imperfections result which would otherwise produce a seam. Any

strams introduced are equalized m the chamber 21 and uniform, clear polyurethane ,.tubing is extruded. The tubing is normally extruded into a water bath as

this is the standard way of hardening polyurethane and is carried on by rollers which

may stretch the tubing. As these elements are standard and do not differ from the

method used before, they are not shown .

In the specific description of the drawings an extrusion head has been described

for extruding hollow tubing. This is the most important single field for the present

invention but it is not the only field in which the invention has advantages. It is possible

to use a mandrel tip which is solid and which is normally referred to as a blind tip. This

will result in the annular plastic material coalescing before it passes through the end of

the extrusion die and so will result in extruding a solid shape which may be circular,

that is to say a rod, or it may have other shapes such as rectangular, fluted and the like.

The particular shape, of course, is determined by the form of the opening in the

extrusion die. When a solid form is to be extruded the mandrel need not be hollow.

However, in many cases it is desirable to be able to change over from the extrusion of

hollow tubing to the extrusion of a solid shape at will. In such a case the hollow

mandrel can remain, as a blind tip, when screwed in, will block off the hollow center

of the mandrel. Such a construction which Uses a single head with interchangeable

dies and mandrel tips has the advantage of greater flexibility while retaining all of the

advantages of the invention in producing uniform homogenous extrusions,

I claim:

1. In an extrusion head for the extrusion of plastic tubing which is provided with

a mandrel, a die, means for introducing plastic into the annular space between the

mandrel and the extrusion head and die, the improvement which comprises;

(a) means for introducing the material to be extruded in the form of tubing in

the shape of a ribbon, means in the extrusion head to divide and turn the ribbon into

the annular space between mandrel and head,said means being positioned so that the

turn of the ribbon is about the short axis of the ribbon which is along the long

dimension of a cross section thereof,whereby the plastic is introduced into the annular

space without forming nonuniformities on one side of the tube.

2. An extrusion head according to claim 1 for the extrusion of plastic tubing in

which the mandrel is hollow throughout its length and means are provided for

introducing gas under pressure into the hollow mandrel.

3. An extrusion head and die according to claim 1 in which the annular space

between mandrel and extrusion head is expanded into a strain relief chamber between

the introduction point of the plastic ribbon and the extrusion die. 4. An extrusion head and die according to claim 1 in which a transition plug is

provided around the mandrel above the point of introduction of plastic, filling the

annular space between mandrel and extrusion head and provided with spirally

inclined faces against which the material is forced.

extrusion head according to claim 1 in which the head is formed of tW0

blocks with mating surfaces bisecting the longitudinal dimension of the mandrel and

means are provided t0 fasten the tw0 blocks together to form an integral extrusion

head.

6.A ss of extruding material of short extruding temperature range through an

extruding head and die with a mandrel,comprising in combination, introducing

the material to be extruded into the side of the extrusion head in the form of a

ribbon,turning it into the head about an axis parallel to the long dimension of the

ribbon cross section into the annular space between the mandrel and the

extrusion head and forming it into a tubular shape around the mandrel

temporarily lowering pressure on the tubing and extruding it tubular form.

7. A process according to claim 6 in which the mandrel is hollow and gas under

pressure is introuduced into the hollow mandrel.

8.A process according to claim 7 in which the extruduate is an elastomer and the

extruded tubing is introduced into a cooling bath.

9.A process according to claim 8 in which the elastomer is a polyurethane

elastomer.

10. A process according to claim 6 in which the material is a polyurethane

elastomer.