Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF T~IE INVENTION
1. Field of the Invention
The present invention relates to a method and
apparatus for molding expandable polystyrene and, more
particularly, to such a method and apparatus in which
shaped articles are molded in a fixed mold cavity from
expandable polystyrene beads.
The invention is especially adapted for molding
high density expanded polystyrene into shapes such as
tubes, cones, cups and the like. Such articles are
generally molded in a production type process, and it is
the purpose of the invention to provide both qualitative
and quantitative improvements in such production.
2. Description of the Prior Art
It is well known to mold expandable polystyrene
beads into article configurations such as tubes and cups.
In general, the mold is charged with the requisite
- amounts of pre-expanded but further expandable polystyrenebeads, and heated at a time sufficient to soften the
polystyrene whereupon the beads expand a second time and
fuse together. The mold is then generally cooled with
cooling water, and the article or articles removed
therefrom. These processes are generally satisfactory,
but efforts have been made to effect improvements in such
molding processes in order to increase the speed of
production from a given unit, and to improve the
properties of the molded composition.
The filling of cavities in molds is normally
accomplished by transporting the pre-expanded beads into
~o these cavities with the help of an air stream. Venting
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slots at different locations within these cavities are
designed to permit the escape of the air and produce
uniform filling. Such vents consist either of holes
drilled into the walls of said cavities or inserts
containing numerous slots. These surface interruptions
are then found on the molded parts again since the
expanding polystyrene during the molding step produces
an exact replica of the surface it comes in contact.
In addition, efforts have been made to increase
the application of steam by providing perforated walls,
and U. S. Patent 3,081,488 shows the use of such
perforated walls. This patentee also utilizes a vacuum
interior to hold the beads on perforated walls to improve
filling prior to molding. However, the perforated walls
do not provide the desired smoothness for certain appli-
cations such as high density tubular parts. In U. S.
Patent 3,259,677, there is a teaching of the use of
porous permeable walls in a mold form, however, the
patentee states that the mold is not suitable for molding
expandable materials, because the material itself forms
a closed barrier. Instead, the patentee limits his
application to the formation of regular heat-softenable,
non-expandable plastic material.
In addition, certain problems have been
encountered with respect to obtaining a uniformly expanded
article, because both a uniform distribution and an even
softening of the bead particles is required to prevent
certain areas from being heavy and other areas from being
comparatively light.
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Thus, it is a primary object of the invention
to provide a method and apparatus for molding expandable
polystyrene in which the molding operation is carried
out in a minimum of time with a minimal amount of equip-
ment.
Another object of the invention is to provide
a method and apparatus for molding expandable polystyrene
in which tubular parts and the like may be formed which
are substantially uniform in density throughout.
It has now been found that these and other
objects are achieved by providing a mold cavity having
smooth, permeable walls together with means for providing
a vacuum, steam and compressed air in order to operate
the mold under improved process conditions. The smooth,
permeable walls are preferably made with sintercore metal,
which has the advantages of allowing passage of air or
steam through the walls, which provides a smooth molding
surface, and which provides good heat transfer character-
istics.
Thus, the invention provides a mold assembly
for forming a high density expandable polystyrene article
of an open-ended, closed-curved, sheet-like configuration
having an interior surface and an exterior surface. The
assembly comprises an interior mold section formed with
smooth permeable wall means having an interior surface
defining an interior chamber and an exterior surface shaped
to match the interior surface of the article to be molded
and provide a part of a mold cavity, an outer mold section
formed with smooth permeable wall means having an inner sur-
face shaped to define the exterlor-s~rface o~ the article to
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be molded and formed for fitting against said inner
section to form said mold cavity, a jacket means disposed
to provide an exterior chamber adjacent to the outer
surface of the outer section of the mold, said jacket
means and outer mold sections being removable to expose
the mold cavity, means for evacuating the interior or
exterior chamber to provide a vacuum within the mold,
means for supplying steam to the jacket or inner chamber,
and means for cooling the mold.
Preferably, the mold cavity is tubular in
configuration, and the assembly is adapted to form
tubular parts, spools or cups. The permeable wall means
are preferably made of sintercore metal, and the cooling
means comprises means for supplying compressed air or
vacuum or water to the steam jacket. In the preferred
form, compressed air can also be supplied to the interior
- chamber. In its process form, the invention provides a
method of molding expandable polystyrene comprising the
steps of providing a mold assembly having a mold cavity
substantially enclosed by a permeable sintercore material,
a first chamber in the center of the mold cavity and a
second chamber on the other side of the mold cavity;
evacuating said first chamber to provide a vacuum therein;
placing a measured amount of pre-expanded but further
expandable polystyrene beads in said mold cavity; charging
said second chamber with steam whereby steam will move
through the permeable mold walls and mold cavity into
the first chamber; said steam being charged for a time
sufficient to expand and fuse the expandable polystyrene
within the entire mold cavity, shutting off said steam,
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cooling the molded polystyrene, and removing the molded
polystyrene article from the mold.
It is preferred to use metallic materials in
the mold assembly to provide good heat transfer character-
istics. The vacuum should best be used when loading the
mold to retain the beads in an even distribution and
when charging with steam so that steam is initially
directed through the entire mold. Compressed air, water
or a water mist are used for cooling assist in forming
a cool outer surface on the outer surface of the poly-
styrene article. Similarly, compressed air is directed
to the inner chamber to assist interior cooling and help
release the article from the inner mold walls.
The invention described in this patent appli-
cation uses sintermetal to produce maximum venting by
utilizing the entire surface of the mold.
The steaming process is normally carried out by
injecting steam through the aforementioned vent holes
which then penetrates into the beads and causes their
expansion and subsequent fusion. The even distribution
of the steam over the entire surface is particularly
critical for high density applications. It was also
found that the steaming rate is a decisive factor for
uniform results. If the steam is applied too fast, then
the expandable polystyrene tends to seal off at the
surface and prevent the further penetration of the steam
into the core of the part to be molded. It may also
cause local melting of the expandable polystyrene, an
effect known as "burning".
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The use of sintermetal permits the application
of steam in two ways:
First, by applying low pressure the expanded
beads in the cavity are preheated and start to expand
from the heat conducted from the surface. The appli-
cation of a vacuum through the sintercore permits then
the even distribution of the steam over the entire outer
surface of the part and penetration through to the core
which permits even fusion throughout the cross section
of the part.
If desired, the vacuum in the core can be
substituted with steam and a vacuum applied to the outer
shell so that the direction of steaming is reversed.
Cooling the molded article is normally required
to form a sufficiently hard layer on the surface which is
able to withstand the residual inner pressure from the
expanded beads in the article. This is accomplished by
the application of air, water or vacuum or combinations
thereof to the walls of the mold for a certain period of
time. It is generally advantageous to keep the quantity
of water to a minimum and thus the application of a fog
or water mist is all that is required to lower the wall
temperature of a mold sufficiently.
The molded article is then removed from the
mold. This removal presents usual difficulties with
cylindrical cores, but is helped greatly in this
invention by providing air pressure to the core and thus
loosen up that article.
Thus, it is seen that the invention is best
adapted to form tube-like configurations such as tubes,
spools, or cups. The invention is also particularly
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valuable in forming the so-called high density foam
articles.
BRIEF DESCRIPTION OF THE DRAWING
In the accompanying drawing, in which like
numerals are used to designate like parts throughout:
Figure 1 is a cross-sectional view of a mold
constructed according to the invention together with a
diagrammatic view of the associated parts;
Figure 2 is a cross-sectional view of the mold
of Figure 1 taken in the plane of line 2-2 thereof; and
Figure 3 is a cross-sectional, elevational view
of a means for charging the mold with feed material.
While only the preferred embodiment is shown
in the drawings, it should be understood that the
invention is to be limited only by the spirit and scope
of the appended claims.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now more particularly to the drawings,
there is shown a mold assembly 11, comprising a mold 12
composed of a sintercore metal and formed with an inner
mold section 1~ and an outer mold section 14. Between
the inner and outer mold section, a mold cavity 16 is
provided, in which the molded article 17 is formed. The
inner mold section 13 is carried on a frame plate 18,
which is mounted in conventional fashion on suitable
supports (not shown).
In the embodiment of the drawing, the inner
mold section 13 is generally cylindrical, and is
permanently secured to frame plate 18, with the plate
~0 closing one end of the inner part of the cylindrical
section. The other end is closed by a plate 19, which
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is permanently secured thereto. In this way, a central
chamber 21 is formed within the sintercore inner mold
section 13. The outer mold section 14 is also generally
cylindrical, and forms the inner part of jacket assembly
22. The jacket assembly 22 also comprises an outer shell
23, which is cylindrical and concentrically disposed to
the outer mold section 14 to form a jacket chamber 24
therebetween. The ends of the jacket chamber 24 are
closed with end plate 26, and spacer plate 27. As here
shown, a circular closure plate 28 is provided to close
an end of the mold cavity 16, but it will be appreciated
that plates 26 and 28 may be combined into a single plate
unit, if desired.
In order to remove the molded article 17 from
the mold, the jacket assembly 22 is removably attached
to frame plate 18 through a circular flange 29 formed
on outer shell 23. If desired, the flange 29 can be
removably attached to frame plate 18 by suitable bolts
or the like; however, it is preferred to use suitable,
fast acting, removable clamping means (not shown) to
speed up the production of the mold assembly.
As here shown, the jacket assembly 22 is
constructed in two sections, with each section having a
pair of longitudinal flanges 31. These flanges are also
removably attached by suitable means (not shown), and
removal of the jacket is facilitated by use of the two
sections. However, it will be appreciated that the
jacket assembly could be made in one piece, and removed
axially, if desired.
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The mold assembly 11 also comprises feeding
means 30 for supplying expandable beads to the mold at
point F, means 32 for supplying steam to jacket 24,
means 33 for supplying compressed air to the jacket 24
and central chamber 21, and means 34 for providing a
vacuum in central chamber 21.
Feeding means 30 comprises a venturi tube 35
having a feed inlet 40 for receiving pre-expanded
polystyrene beads from a suitable source such as a hopper
(not shown), means 30 also comprises a movable compressed
air supply tube 45 for directing a stream of alr and
polystyrene beads into the mold.
Steam supply means 32 comprises a source of
steam 36, line 37, which is controlled by multiposition
valve 38, entry port 39 to jacket 24, and exhaust port 41
which can be closed by valve 44. As here shown, a branch
line 42 is shown communicating with a second entry port 43.
However, it will be appreciated that more or even less
entries and/or exhausts than those shown may be utilized,
if desired. It will also be appreciated that the vacuum
chamber and steam chamber could be reversed, along with
their associated parts.
The vacuum means 34 comprises a source of
vacuum such as vacuum pump 46, operative through line 47,
valve 48 and line 49 to evacuate the central chamber 21.
Valve 48 is a multiposition valve capable of connecting
line 47 to line 49 for connecting the vacuum pump 46 to
the central chamber 21, and capable of connecting line 49
with a line 51. Valve 48 may also have a shut-off
position, if desired.
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The compressed air means 33 comprises a
compressed air supply 52, a line 53 connecting supply 52
to a multiposition valve 54, and lines 51 and 56. Line 56
is in communication with the jacket 24, and line 51 is
attached to valve 48 for communicating with central
chamber 21 when valve 48 is positioned to connect lines
49 and 51. Water or water mist can be used for cooling.
For example, water may be aspirated into the compressed
air line 53 by means of a venturi 57 in water line 58
connected to water supply 59 through valve 61.
Thus, it is seen that with the sources of
vacuum, compressed air and steam available, the central
chamber 21 can be subjected to vacuum or compressed air
and the jacket 24 can be charged with steam or compressed
air or a reverse operation achieved simply by appropriate
operation of valves 38, 48, 54 and 61. These valves are
preferably operated automatically by appropriate control
and timing devices (not shown).
In operation, the processlform of the invention
is carried out by performance of the steps hereinafter
given. First of all, the jacket and any article that may
be in the mold are removed, the mold closed, and the
valve 48 operated to evacuate chamber 21. Feeding means
30 is then positioned at F and air is charged to air
supply tube 45 to supply expanded polystyrene beads evenly
along the exterior surface of the outer surface of mold
section 13. As herein used, the term "beads" is intended
to cover any particulate form of expandable polystyrene
regardless of shape, and preferably pre-expanded beads
are used to minimize the amount of final expansion. These
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particles or beads will be distributed evenly on the
inner mold wall by virtue of the vacuum within providing
a suction through the pores of the permeable sintercore
material, permitting an efficient means of venting.
With the beads in place, the jacket is charged
with steam of suitable pressure by operation of valve 38.
Steam will pass through the porous wall 14, mold cavity
16, and through the porous wall 13 into the vacuum chamber
21, assisted by the suction maintained therein. Exhaust
port 41 is provided to assure steam flow to the entire
jacket and preheat the expandable beads in cavity 17
indirectly. This port is equipped with a suitable control
valve 44 to provide a desired positive steam pressure in
the jacket.
After a time sufficient to expand and fuse the
polystyrene beads has elapsed (which time can be
minimized by experimentation), finally the steam is shut
off and the mold is cooled. The cooling is achieved by
charging the jacket 24 with compressed air which draws in
water through the venturi 57 and forms a mist. This
purges the steam and rapidly cools the mold. In addition,
the compressed air may be the sole cooling agent after a
certain period of time and it also maintains a positive
pressure on the exterior surface of the molded article to
assist in the separation of the article from the mold wall
as the valve 57 and exhaust 44 are shut.
The vacuum is shut off in the interior chamber
and the jacket is removed. Compressed air alone or a
water mist is then directed into the interior chamber to
provide final interior cooling and to separate the molded
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article from the interior wall. The article is then
removed, and the compressed air is shut off leaving the
apparatus in position to start another molding cycle.
It will be appreciatecl that mold release
compounds and other uses of the usual applicable
technique will be employed as is usual in the art. In
addition, a preferred process utilizes the step of
applying a fine spray of mineral oil to the beads prior
to the expansion thereof. This step may be conveniently
carried out when the beads are in position on the mold
wall prior to the assembly of the jacket. It has been
found that such spray assists in the formation of a good
unitary article of uniform strength.
In accordance with the invention, improved
articles are made by assuring an excellent uniformity of
density. Surface characteristics of the article are also
maintained as well as those made with solid mold walls.
In other words, the process advantages of perforated
mold walls are obtained without the usual disadvantage
of surface imperfection, and the invention provides
excellent results with the so-called high density foam
articles, i.e., articles having a density greater than
2 pounds per cubic foot.
In its broad aspect, the invention may be
utilized for articles of any sheet-like configuration.
However, the invention is particularly advantageous when
used to mold an article having a cylindric body, slightly
conical body or conical body having at least one open end.
Thus, tubes, spools and cups may be made with particular
advantage when fabricated according to the invention.
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From the foregoing description, it is seen
that the present invention provicles an improved method
and apparatus for molding expandable polystyrene.
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