Note: Descriptions are shown in the official language in which they were submitted.
10776f~8
The invention relates to a method for forming
involute or elongate plastic articles from a sheet of
thermoplastic material.
Current methods of forming thermoplastic material
have certain basic deficiencies which restrict their
utility in forming involute or elongate articles. Involute
plastic structures, i.e., structures having a neck smaller
than an interior cross-section, are generally formed by
bonding two or more separately formed pieces of material
together, rather than forming the article from a single
piece, and the seam or joint required degrades the optical
properties of the article. Injection blow molding can be
used to form thermoplastic articles, but this method is
limited to relatively small articles due to the magnitude
of the equipment involved. Extrusion blow molding can also
be used to form thermoplastic articles, but the closure
on the end opposite the opening forms objectionable marks
and loss of optical properties.
The present invention discloses a method for
forming elongated and involute articles from thermoplastic
material having a variety of novel and innovative features
which overcome deficiencies in the methods used to date.
The invention provides a methodology for accurate control
of the heating of a sheet of thermoplastic material, and
the establishment of preselected concentric temperature
zones within the thermoplastic material, to facilitate sub-
sequent formation of the
~L
7~ -
1077668
articles. The thermoplastic material is rapidly heated so
that the material forms easily without unwanted stretching.
After the sheet of thermoplastic material is heated, it is
placed in a device for forming the thermoplastic material to
the desired shape. The heated sheet of thermoplastic
material is planarly supported by the device and held in
accurately monitored compression to control slippage of the
material toward the center thereof. A plug is used to
project the center of the sheet of thermoplastic material
through an aperture in the planar support, and the material
"self-forms" around a sharp corner on the interior of the
aperture into an elongated shape. The planar support
disclosed herein also provides a means for inducing axial
stress in the sheet of material to control the thickness of
the thermoplastic material as it is formed from the flat
sheet into the elongated piece.
In the formation of involute articles, the elongated
piece is formed within an involute female mold. The involute
female mold disclosed herein has a first aperture section
forming the entrance to the mold, and a sharp corner is
provided at the junction of the flat aperture section and
the interior surface of ~he female mold to eliminate poten-
tial formation of stress cracks. The ambient environment of
the thermoplastic material as it is elongated is closely
controlled to prevent cooling of the material below its
forming temperature so that it can be expanded within the
mold by injection or pressurized air into the interior
cavity of the elongate piece. As the material expands, the
concentric temperature zones control relative expansion of
different areas of the material so that the final article
has controlled and approximately uniform wall thickness.
-2-
, . ~ .
1077668
The invention provides an infrared oven with
voltage and ambient temperature controls to rapidly and
accurately heat a sheet of thermoplastic material. During
the heating of the thermoplastic material, metallic screens
are interposed between the oven heating elements and the
thermoplastic material to establish radial temperature
gradients in the material. After the sheet of thermoplastic
material has been heated, it is placed between a pair of
opposed platens which provide planar support. The pair of
opposed platens have apertures through the centers thereof
to allow for transverse extension of the thermoplastic
material and allow for controlled slippage of the thermo-
plastic material toward the center thereof. A heated plug
is used to transversely extend the thermoplastic material
into an elongated conical piece. During the elongation
process, the thermoplastic material "self-forms" as it
translates from between the pair of opposed platens into the
elongated shape, and a small radius corner must be provided
at this junction to properly allow the material to self-
form.
If involute articles are to be formed, the elon-
gated conical shaped piece is formed within an involute
female mold, and compressed air is injected into the in-
terior cavity of the elongated conical shaped piece to
expand and press the thermoplastic material against the
surface of the mold. Proper design of the entrance of the
involute female mold and proper maintenance
-
" . . ' ~ , . ' ' ' - --':
1077668
of the temperature gradients in the thermoplastic material
are essential for the proper formation of an involute
article without optical defects. Throughout the entire
process, maintenance of the proper ambient conditions and
structural details of the forming mechanism are essential
for proper operation of the process.
The novel features which are believed to be character-
istic of the invention, both as to organization and method
of operation, together with further advantages thereof will
be better understood from the following description con-
sidered in connection with the accompanying drawings in
which several preferred embodiments of the invention are
illustrated by way of example. It is to be expressly
understood, however, that the drawings are for the purpose
lS of illustration and description only and are not intended
as a definition of the limits of the invention.
FIGURE 1 is a perspective view of the apparatus used
for heating the sheet of thermoplastic material.
FIGURE 2 is an exploded view of the interior of the
heating apparatus.
FIGURE 3 is a perspective view of the apparatus used
to elongate and form the involute plastic articles.
FIGURE 4 is a cross-sectional elevation view of the
apparatus illustrated in Fig. 3 showing the sheet of thermo-
plastic material in position to be formed.
FIGURE 5 is the cross-sectional elevation view of
Fig. 4 after the sheet of thermoplastic material has been
formed into an elongated conical shaped piece.
-- 4 --
- - . . ~ -: : .:
. : : : .
~ - :
~"" 1077668
Figure 6 is the cross-sectional elevation view
illustrated in Figures 4 and 5 after the elongated conical
shaped piece of thermoplastic material has been partially
formed into an involute plastic article.
S Referring to Figure 1, a circular sheet of thermo-
plastic material 10 is mounted inside an oven enclosure 11.
The circular sheet of thermoplastic material 10 is supported
on a grid 12 of very fine, widely-spaced wires which are
attached to a rotatable ring 13. The rotatable ring 13 is
mounted on brackets 14 and 15 attached to the side of the
oven enclosure 11. In this manner, contact of the mounting
structure with the sheet of thermoplastic material is
minimized, and only the thin wires are interposed between
the thermoplastic material and the heating mechanism.
The sheet of thermoplastic material 10 is heated
by means of linear infrared heating elements 16. An array
of heating elements is disposed adjacent both the top and
the bottom faces of the thermoplastic material 10, but the
upper array is hidden in the perspective view of Figure
1. A scalloped reflecting structure 17 is used to direct
the radiant heat from the infrared heating element 16 in the
direction of the thermoplastic material 10. Potential
heating irregularities due to the linear nature of the array
are compensated for by rotating the sheet of thermoplastic
so that the heating is concentrically uniform.
In the present invention, it is absolutely essen- -
tial that the heating of the sheet of thermoplastic material
10 be
-5-
` 1077668
accurately and precisely controlled, since only slight
deviations from the desired heating will seriously degrade
the integrity of the entire forming process. Hence a
voltage regulator 18 is used to provide a precisely con~
trolled and constant input voltage to the linear infrared
heating elements 16 through leads 19. However, the heat
loss or gain of the sheet of thermoplastic material 10 and
the radiant output of the heating elements 16 is dependent
on the ambient air temperature within the oven enclosure 11.
Therefore, it is not sufficient to merely control the input
voltage to the heating element 16 in order to accurately
control the heating of the sheet of thermoplastic material
10, but the ambient air temperature within the oven en-
closure 11 must also be controlled. Such control is pro-
vided by a venting mechanism which exhausts heated air outvent 20. The venting mechanism 19 controls the venting of
air and maintains the ambient temperature of the air inside
the oven enclosure 11 at a~¢onstant temperature. The door
21 to the oven enclosure ll does not fully seal off the
enclosure, but rather provides for a slight intake of air to
compensate for the hea*ed air vented through the exhaust
; vent 20. By accurately and precisely controlling the
radiant heating of the sheet of thermoplastic material 10,
the process becomes very repeatable and provides constant
quality control of the end product.
To control subsequent formation of the material,
it is not only desirable to control only the general heating
of the sheet of thermoplastic material 10, but it is also
desirable to set up various temperature gradients throughout
the thermoplastic material. Figure 2 is an exploded view
illustrating the sheet of
~' .
.
`` ~077668
thermoplastic material 10 is separated from the support grid
12 of thin, widely spaced wires attached to ring 13. Both
the upper and lower arrays of linear infrared heating
elements 16 with their scalloped reflecting structures 17
are illustrated. The mechanism for mounting and rotating the
- ring 13 is not illustrated in Figure 2. To establish radial
temperature gradients in the sheet of thermoplastic material
10, concentric rings of metallic screen 30 and 31 are
interposed between the sheet of thermoplastic material 10
and the support grid 12. Although shown separately, in the
exploded view, in actual operation they are positioned one
on top of the other. In the Figure, two concentric rings of
metallic screen are illustrated for simplicity, but in
practice a multitude of concentric rings of metallic screen
are normally utilized, with some rings having internal
cutout sections as in concentric ring 31 and some without
such cutouts as in concentric ring 30. The purpose of the
concentric rings of metallic screen is to block a portion of
the radiant energy from the infrared heating elements 16 to
prevent it from reaching the sheet of thermoplastic material
10. Since the metallic screen is formed in concentric rings
a set of radial temperature gradients will be established.
The heating apparatus is designed to heat the
sheet of thermoplastic material 10 such that the edge of the
sheet is heated to a greater extent than the central area of
the sheet. Concentric heat zones are formed by the metallic
screens so that the center is heated to a temperature near
the low end of the forming temperature range, the edge is
heated to a temperature near the high end of the forming
temperature range, with possible intermediate zones between
the center and the edge.
11~77668
After the sheet of thermoplastic material 10 is
heated, it is immediately transferred to the forming apparatus
illustrated in Fig. 3 and placed between a pair of opposed
plates consisting of an upper platen 40 and a lower platen
41. The upper platen 40 is attached to air actuators 42
and 43 which control the compression that the pair of opposed
platens 40 and 41 maintain on the heated sheet of thermo-
plastic material 10. An aperture 44 is formed through the
upper platen 40 and the lower platen 41 whereby the center
of the heated sheet of thermoplastic material 10 is exposed.
A plug 45 mounted on shaft 46 is passable through the
aperture 44 through the pair of opposed platens 40 and 41
to form the heated sheet of thermoplastic material into an
elongated conical shaped piece. The elongated conical
shaped piece is formed within an involute female mold 47
if involute articles are to be produced. If merely elongated
conical articles are to be produced, the involute female
mold 47 is not necessary.
- 8 -
:
1~77668
During the formation of the elongated conical shaped
piece of thermoplastic material, it is desirable to gradually
increase the compression of the pair of opposed platens 40
and 41 on the sheet of thermoplastic material 10 to prevent
flange wrinkling and to control slippage of the thermo-
plastic material relative to the pair of opposed platens.
The preferred method of controlling the increase in com-
pression is by means of a closed loop control system wherein
the pressure exerted by air actuators 42 and 43 is increased
based on the amount of thermoplastic material which has
been used in the forming process. To provide such a closed
loop control system, a row of thermoinsulative pegs 48, 49
and 50 is provided which are placed in a row of radially
disposed holes on the upper platen 40. As the edge of the
sheet of thermoplastic material 10 moves in the direction
of the aperture 44 during the formation process, switches
51, 52 and 53 attached to the pegs 48, 49 and 50 are
successively actuated by the falling of the pegs through
the holes to monitor the movement of the thermoplastic
material toward the aperture. The switches successively
increase the pressure exerted by air actuators 42 and 43
whereby the compression exerted by the pair of opposed
platens 40 and 41 is gradually increased.
A plurality of pegs 54 are located in a ring of holes
in the upper platen 40 disposed about the aperture 44.
Switches 55 are connected to the pegs 54, and the interior
peg 50 and connecting switch 53 of the radically disposed
row of pegs form part of the ring. The ring of pegs is
adapted to cause the air actuators 42 and 43 to exert
sufficient pressure to clamp the heated sheet of thermo-
plastic material 10 between the pair of opposed platens
40 and 41 to prevent any slippage of the
7766~3
thermoplastic material relative to the opposed platens. Any
one of the ring of pegs 54 and 50 will activate the clamping
procedure so that no part of the edge of the sheet of
thermoplastic material 10 can enter the aperture 44.
In order to control the exact temperature of the
sheet of thermoplastic material 10 at all times, parts of
the apparatus which come into direct contact with the
thermoplastic material are heated. A heater 56 is attached
to the plug 45, and a heater 59 is also attached to the
involute female mold 47. Portions of the apparatus which are
actively heated, i.e., the plug 45, and the mold 47, are
constructed of thermoconductive material to evenly dis-
tribute the heat throughout the structure.
The involute female mold 47 can be split along
seam 60 so that it can be divided into sections. Air holes
61 are provided to allow for exhaust of air from the in-
terior of the mold 47 during the forming process.
The actual formation of the involute plastic
articles can be more fully illustrated by viewing Figures 4,
5, and 6 in series. Figure 4 illustrates the heated sheet
of thermoplastic material 10 in position between the upper
platen 40 and the lower platen 41. The plug 45 is in the
raised position above the aperture 44 through the pair of
; opposed platens 40 and 41. Pegs 48, 49, 50 and 54 are in
abutment with the sheet of thermoplastic material 10 and are
thus maintained in position. -
The upper platen 40 is composed of thermocon-
ductive material, but has a coating of thermoinsulative
material 70 on the face adapted to contact the heated sheet
of thermoplastic material 10. The lower platen 41 is
composed of an interior
--10--
.-, - : , .
.:- . - . . . .
1077668
annular section 71 of thermoconductive material which is
coated by a layer of thermoinsulative material 72 on the
faces of the lower platen adapted to contact the heated
sheet of thermoplastic material 10. Teflon (a trademark for
polytetrafluoroethylene) is preferably used as the thermo-
insulative material since is facilitates slippage of the
thermoplastic material relative to the platens. The ex-
terior section of the lower platen 41 is mounted circumferen-
tially about the periphery of the interior annular section
71, and is preferably composed of semi-rigid thermoinsulative
material such as wood. The exterior section 73 is slightly
depressed relative to the interior section 71 such that a
slight lip 74 is formed on the surface of the lower platen
41 at the junction between the interior section 71 and the
exterior section 73. The slight lip 74 allows control of
slippage through pressure without compressing the entire
sheet of thermoplastic material.
If involute plastic articles are to be formed, the
interior section 71 of the lower platen 41 is preferably
attached directly to an involute female mold 47 such that the
aperture 44 through the lower platen 41 forms the entrance
opening to the female mold. The involute female mold illu-
strated in Fig. 4 is basically spherical, but the method-
ology illustrated herein is applicable to any shape wherein
the neck of the article is smaller than an interior cross-
section, i.e., wherein the article is involute, as well as
to tapered, i.e. non-involute articles. The mold 47 has
holes 61 therein adapted to allow for exit of air from the
interior of the mold during formation of the articles. In
addition, a sensor 76 is mounted on the interior surface of
the involute female mold 47 and is adapted
-- 11 --
1077668
to sense contact of the thermoplastic material with the
interior surface of the involute female mold to indicate that
the formation of the article is completed.
Formation of the elongated conical shaped piece is
illustrated by way of reference to Fig. 5. The outer dia-
meter of the plug 45 with the thermoplastic material thereon
is smaller than the interior diameter of the aperture 44
so that the thermoplastic material is not compressed between
the plug and the walls of the aperture. Hence, the thermo-
plastic material "self-forms" as it translates from between
the pair of opposed platens 40 and 41 into the aperture 44.
Due to the self-forming nature of the process, the construc-
tion of the corner 81 over which the elongated conical
shaped piece 80 self-forms is critical. Normal molding
technology would dictate that a large radius corner be
used to alleviate friction. However, one of the principal
features of the invention disclosed herein is that a
relatively sharp, small radius corner as illustrated is
far preferable to a large radius corner.
In the formation of the elongated conical shaped piece
80, the thermoplastic material is drawn radially inwardly
toward the aperture 44 as the plug 45 extends the material.
As the edge of the heated sheet of thermoplastic material
is drawn toward the aperture, the thermoinsulative pegs
48 and 49 successively fall whereby the compression exerted
by the pair of opposed platens 40 and 41 on the thermo- `-
plastic material is increased. When the edge of the thermo-
plastic
- 12 -
1(~77668
material is sufficiently close to the aperture so that one
of the pegs 54 or 50 in the interior ring falls, sufficient
compression is exerted by the pair of opposed platens to
clamp the thermoplastic material in place between the
platens.
The plug 45 extends to such a depth that the tip
of the elongated conical shaped piece 80 is substantially
adjacent the interior surface of the involute female mold 47
opposite the entrance opening 44. A lid 82 is mounted on
the shaft 46 which holds the plug 45 and is adapted to fit
over the aperture through the upper platen 40 whereby the
; interior cavity of the elongated conical shaped piece 80 is
sealed. Hence, pressurized air can be introduced through
nozzle 83 also located on shaft 46 in order to form the
desired involute plastic article. The sequence of opera-
tions illustrated by Figures 4 and 5 are performed in a
relatively short amount of time so as to prevent substantial
cooling of the thermoplastic material. Hence, the elongated
conical shaped piece 80 basically retains the temperature
configuration imparted to it during the heating process.
; However, to facilitate formation of the involute article,
the surface heat of the material is allowed to conduct to
the center plane so that the material will more readily
stretch when pressurized. The temperature zones remain
basically unaffected. -
As air is injected into the interior cavity of the
elongated conical shaped piece, the walls of thermoplastic
material do not expand simultaneously, but the formation of -
the article proceeds in various steps. The initial step in
the formation of the involute plastic article is illustrated
-13-
1077668
in Figure 6. The first formation takes place in the area of
the entrance to the involute female mold 47 as illustrated
by contour 93. Since the original temperature zones es-
tablished in the heating process has been maintained,
thermoplastic material which was near the edge of the
original sheet of thermoplastic material is at a greater
temperature than material intermediate the edge and the
center. The thermoplastic material which was adjacent the
edge of the original sheet of thermoplastic material is now
adjacent the entrance opening 44 of the involute female
mold 47, and due to its greater temperature forms first.
The thermoplastic material in the region of the entrance 4
to the involute female mold 47 comes into contact with the
entrance opening and the interior surface of the female mold
adjacent the entrance opening as air is first injected into
the cavity. Although the involute female mold is heated, it
is heated to a lesser temperature than the thermoplastic
material, so that the thermoplastic material is cooled
rapidly by the relative temperature differential when it
comes in contact with the thermoconductive surface of the
mold. Hence, the shape of the junction between the flat
surface 90 and the interior surface of the female mold 47 is
critical. The invention provides for a small radius corner
90 at this junction, rather than a large radius corner as
would be expected, together with a vertical surface 90 of
thermoconductive material at the entrance to the mold. As
compressed air is introduced through nozzle 83, the hot
plastic first contacts the vertical surface 90 and imme-
diately cools. This prevents further slippage and eli-
minates stress cracking during the balance of the formingprocess.
-14-
1077668
It is apparent from Figure 6 that the initial
expansion, which took place near the entrance opening 44, is
in a region wherein the material has only a short distance
to travel before reaching the interior surface of the mold
47. Hence, the portion of thermoplastic material which is
heated more than other portions has the shortest distance to
travel to reach the interior surface of the mold. The
thermoplastic material which was less heated has a rela-
tively long distance to travel before it reaches the interior
surface of the mold 47 in its final form as illustrated by
the dashed contour 93. In this manner, the material which
has the shortest distance to go stretches most rapidly, and
the end result is the formation of an involute thermoplastic
article with controlled wall thickness.
