Method and apparatus for extruding thermoplastic shape

A method and apparatus for continuously cooling a thermoplastic shape having two open sides in which the thermoplastic shape, in its at least partially molten state, is extruded and/or drawn along an elongated form having a contour complimentary to the contour of one side of the two open sides and, simultaneously with moving the thermoplastic shape, at least partially cooling the thermoplastic shape by cooling the form with a liquid cooling medium and drawing a vacuum, on the previously mentioned one side of the two open sides of the thermoplastic shape, through the form.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and apparatus for cooling a 
thermoplastic shape. More specifically, the present invention relates to a 
method of continuously cooling a thermoplastic shape having two open 
sides. 
It is well known in the art to form thermoplastic pipe by drawing or 
extruding molten thermoplastic through an annular space formed between an 
inner male element or mandrel and an outer female element, bushing or 
sizing ring, while simultaneously cooling the thermoplastic to solidify 
the same. While dies of this type accurately form the inner and outer 
surfaces of the pipe and form a controlled uniform thickness, even though 
the thermoplastic may have a tendency to warp and shrink, there are 
numerous disadvantages to their use. A major disadvantage is that the dies 
are extremely complex and expensive. Since a given die generally forms 
only one size and thickness of pipe, in many cases the dies are shipped 
from one plant to another in order to avoid the expense of buying 
duplicate dies for each manufacturing facility. An attendant problem, 
again leading to complexity and expense, is the necessity of providing 
uniform cooling of the thermoplastic. Finally, since pipe is usually 
utilized for transporting fluids under pressure, it must have specified 
and uniform pressure rating and, hence, the wall thickness is maintained 
within a close tolerance, consequently, the dies utilized are incapable of 
producing pipe which varies in thickness along its length. 
Since the drawing or extrusion of thermoplastic shapes other than pipe and 
having two open sides, such as channel shapes, is less widely practiced 
than the formation of pipe, little attention has been directed to 
improvements in the forming devices, even though the formation of such 
differing shapes creates numerous additional problems not encountered in 
the formation of pipe. Consequently, in many instances dies of the same 
character as those utilized in the formation of pipe are also utilized in 
the formation of thermoplastic shaped items having two open sides. 
Obviously, the wide variety of shapes with two open sides which can be and 
are formed greatly exaggerates the problem of utilizing complex and 
expensive forming equipment. In addition, there are times when it is 
desired to form a single elongated thermoplastic shape whose thickness 
varies along its length. This, of course, is impossile when utilizine 
pipe-type dies which surround the entire cross section of the shaped item. 
The desire to form an item which varies in thickness along its length also 
exaggerates the problem of cooling, since changing to a different 
thickness obviously requires changing the degree of cooling. Accordingly, 
there have been some attempts to simplify dies or means for forming 
thermoplastic shapes having two open sides. However, while these attempts 
have either completely or partially solved some of the above-mentioned 
problems, they have by no means solved all problems mentioned. Further, 
these more simplified methods are generally incapable of forming 
thermoplastic shapes having two open sides from thermoplastics which have 
a tendency to warp or shrink. 
It is, therefore, an object of the present invention to overcome the 
above-mentioned and other problems of the prior art. Another object of the 
present invention is to provide an improved method and apparatus for 
continuously forming thermoplastic shapes having two open sides. Another 
further object of the present invention is to provide an improved method 
and apparatus for continuously forming thermoplastic shapes having two 
open sides which are inexpensive both as to method and equipment. A 
further object of the present invention is to provide an improved method 
and apparatus for forming thermoplastic shapes having two open sides which 
involves the utilization of improved cooling means. A still further object 
of the invention is to provide an improved method and apparatus for 
forming thermoplastic shapes having two open sides which can be varied in 
thickness along their length. Another and further object of the present 
invention is to provide a method and apparatus for forming thermoplastic 
shapes, having two open sides and which vary in thickness along their 
length, in which the cooling may be adjusted to compensate for the changes 
in cooling required by changes in thickness. Yet another object of the 
present invention is to provide an improved method and apparatus capable 
of accurately forming thermoplastic shapes having two open sides from 
thermoplastics which have a tendency to warp and shirnk. These and other 
objects of the present invention will be apparent from the following 
description. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a method and apparatus is 
provided for continuously forming a thermoplastic shape having two open 
sides in which the thermoplastic shape, in at least a partial molten 
condition, is moved across a form having a contour complimentary to the 
contour of one side of the two open sides and, simultaneously with the 
movement of the thermoplastic shape, the thermoplastic shape is cooled by 
cooling the form with a liquid cooling medium and drawing a vacuum on the 
one side of the two open sides of the thermoplastic shape through the form 
.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
While the description herein illustrates the invention by reference to a 
particular thermoplastic shape which can be formed in accordance with the 
present invention, it is to be understood that the term, "thermoplastic 
shape having two open sides," is meant to include any elongated shape 
having an open cross sectional form with all sides open, as opposed to a 
closed cross sectional form, such as pipe, having inside and outside 
surfaces. 
FIG. 1 of the drawings shows one such thermoplastic shape 2 having two open 
sides, in this particular case being a channel-type shape. Obviously, in 
accordance with the definition herein, other thermoplastic shapes having 
two open sides could include flat bar, v-shaped configurations, t-shaped 
configurations and similar other configurations which would be apparent to 
one skilled in the art. 
FIG. 2. of the drawings is an isometric view partially in section and shows 
one embodiment of a means for forming a thermoplastic shape having two 
open sides in accordance with the present invention. Specifically, in FIG. 
2, the numeral 4 designates a form across which a thermoplastic shape, in 
its at least partially molten condition, is continuously moved by drawing 
and/or extrusion from an appropriate profile die and sizing sleeve. The 
top surface of form 4 has a contour complimentary to the contour of one 
side, in this particular instance the underside, of the thermoplastic 
shape 2 of FIG. 1. Passing through the upper surface of form 4 into the 
interior of form 4 are apertures 6 designed for the passage of air 
therethrough. Apertures 6 are in turn internally connected to channel 8, 
which is an open channel extending substantially the entire length of form 
4. Not shown is a fixture connecting to the front end of channel 8 through 
which a vacuum may be drawn by an appropriate vacuum means (not shown). 
Otherwise, form 4 is solid. Form 4 can be made of an appropriate material, 
such as aluminum. Form 4 is appropriately mounted within a container 10 
adapted to maintain a body of a cooling medium, such as water, on all 
sides of form 4. Preferably, the water is continuously circulated through 
water bath 10, through water inlet 12 and through water outlet 14. The 
plastic shape to be formed, in at least a partial molten state is 
continuously drawn over the top surface of form 4 through appropriate 
inlets and outlets 16 and 18 in the ends of water bath 10. Since water 
bath 10 is filled with water, openings 16 and 18 will, of course, 
generally conform to the upper surface or contour of thermoplastic shape 2 
of FIG. 1. Form 4 may also be provided with appropriate upstanding sides 
20 to form the edges of thermoplastic shape 2 of FIG. 1. While as 
indicated above, the openings 16 and 18 are of the general configuration 
of the upper surface of thermoplastic shape 2 of FIG. 1, these openings do 
not in and of themselves, have a direct bearing on the shape of the upper 
contour of thermoplastic shape 2 of FIG. 1. Instead, the vacuum drawn 
through form 4 draws the thermoplastic shape against the top surface of 
form 4 and contributes to the formation of the upper contour of 
thermoplastic shape 2 of FIG. 1. Drawing a vacuum on form 4 also has the 
very distinct advantage of preventing warping and excessive shrinkage of 
the thermoplastic shape as it is being formed and cooled. This is 
particularly important where the thermoplastic shape is to be formed from 
a thermoplastic which is very sensitive to warping and shrinkage, such as 
high density polyethylene. The vacuum may also be utilized to aid in the 
cooling of the thermoplastic shape by withdrawing heat through the air 
apertures and channel. It is also possible, in accordance with the 
embodiment shown, to vary or change cooling along the length of the form 
by appropriately tapping into air channel 8 along the length of the form 
and/or tapping into the water bath at appropriate points along the length 
of the water bath and circulating water, or other cooling medium, at 
different temperatures. The thickness of the thermoplastic shape can also 
be varied utilizing the forming means of FIG. 2 by providing slide means 
(not shown) at each end of water bath 10 to permit adjustment of the 
inlets and outlet openings 16 and 18, respectively. 
FIG. 3 of the drawings is an isometric view of another embodiment in 
accordance with the present invention. In accordance with FIG. 3, an 
elongated form 22, having an upper contour complimentary to the contour of 
the lower surface of thermoplastic shape 2 of FIG. 1, along which the 
thermoplastic shape is drawn. Formed through the upper surface of form 22 
are air apertures 24, of the same character as those shown in FIG. 1, 
which are connected to an elongated internal air channel 26, again similar 
to that of FIG. 2. Mounted on the end of air channel 26 is fixture 28, 
which in turn is connected to an appropriate source of vacuum (not shown). 
In this particular embodiment, a portion of the cooling of the 
thermoplastic shape is provided by interconnected cooling medium channels 
in the interior of form 22. Water is supplied to the innerconnected 
channels through water inlet 30 with appropriate fixture 32 connected 
thereto and leading to a source of water supply. Obviously, a water outlet 
would be formed in the opposite end of form 22. In this particular 
instance, as in the use of the forming means of FIG. 2, the thermoplastic 
shape is fed to form 22 from an appropriate profile die and sizing sleeve 
(not shown). In this particular embodiment, it is to be noted that the top 
surface of the plastic shape to be formed is at least partially cooled by 
atmospheric air, rather than water or other cooling medium as in the 
embodiment of FIG. 2. However, it should be recognized that the embodiment 
of FIG. 3 could also be mounted in an appropriate cooling medium bath in 
the same manner as the form of FIG. 2. Also, as in the embodiment of FIG. 
2, the embodiment of FIG. 3 could have appropriate taps into the main air 
and water channels to vary the cooling along the length of the form. Also, 
as in the embodiment of FIG. 2, the vacuum drawn on form 22 serves the 
same multiple purposes, and in particular, cools a plastic shape having 
two open sides in a manner to prevent warping and shrinking. 
While the present invention has been described in connection with specific 
structures and modes of operation, it is to be understood that these are 
by way of illustration only and variations, modifications and equivalents 
thereof will be readily apparent to one skilled in the art.