Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF 'I`~IE INVENTION
Eield of the in ention
The present invention relates to a molding method and
apparatus for molding articles of expanded plastic material.
Molding methods for articles of expanded thermoplastic
material have been known heretofore and especially methods for
manufacturing articles obtained by agglomerating pre-expanded
polystyrene beads.
- Presently these methods are commonly used for manu-
facturing blocks, supports or shells for packaging food products,
machinery or household electrical appliances etc... . They
have excellent impact resistance, they are weatherresistant and
their cost is relatively low.
Description of the prior art
Usually, the known molding techniques include the
following steps;
- filling a molding cavity of a mold with pre-expanded
polystyrene beads
- injecting steam for heating said beads to the softening
temperature of the polymer and above the boiling point of the
porogeneous fluid which is contained in said beads. Said steam
causes, the beads to expand and bring them to a partially molded
state causing them to agglomerate into a homogeneous cellular
mass.
- cooling the product obtained in a cellular state
contained in the mold in order to prevent a second expansion or
a distortion of the molded article; and, once the mold is cooled
- withdrawing the molded article from the mold, after
which the mold can be heated again for the next molding cycle.
This method involves the use of considerable heating
system and means for rapidly cooling the mold in order that the
two alternate steps, i.e. heating the mold and cooling thereof,
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do not increase excessively the manufacturing of the article.
l3Owever, since the article is made from an excellent
heat insulating material, this method results in a very limited
production rate, and reduces drastically the efficiency and the
profitability of the molding equipment.
Another disadvantage which effects this method is the
consumption of energy. For example, for a medium size article
when heating 0.150 kg of thermoplastic material contained in the
mold it is also necessary to heat, at each molding cycle, the
150 kg of the mold material itself. It can be realized that this
mass ratio of about 1 to 1000 brings in a considerab]e loss of
energy which heavily affects the cost of the finished product.
In order to reduce the manufacturing cost it has been
proposed to provide molds with multiple molding cavities. This
approach is valid for small size articles only and on the other `!
hand the design and the manufacturing of the molds which have to
be machined accurately, from a good heat conducting material, such
as aluminum, are very expensive.
Also the expenses for unavoidable servicing and
maintenance of so complex an equipment have to be taken into
- consideration. In order to carry out successive cooling operations,
the mold is provided with internal passages which allow the cooling
water to circulate. If this water is not treated, calcar deposits
thereof will block the injection nozzles and attack the walls of
the mold or of its auxiliary equipment. Further, for economical
reasons, the factories that are using the known method, use
cooling systems which operate in close circuit and consequently
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include storage containers, tanks for water treatment, water
cooling towers and pumping stations which considerably affect the
production cost of the molded articles of expanded polystyrerle
produced in mass production.
The present invention provides a method and apparatus
overcoming the above mentioned disadvantages.
According to the present invention there is provided
a method for forming shaped articles from pre-expanded thermo-
plastic beads, in two cycles, comprising a first cycle which
includes the stages of introducing said beads into a mold cavity
of a first mold to fill said mold cavity, wherein said first
mold comprises two members, a male member and a female member,
and wherein said first mold is maintained at a substantially
constant elevated temperature throughout said two cycles, inject-
ing steam into said first mold to cause the beads to expand,
partially melt, agglomerate and form a shaped cellular preform,
disengaging said two members of the first mold whereby said pre-
form is retained by one mold member of said first mold, and a
second cycle which includes the stages of transferring said
shaped cellular preform at substantially ambient pressure, from
the first mold to a second mold, by conveying said preform on
said first mold member which retains said preform to said second
mold, whereby the shaped preform is free to expand under the
conditions of the stage of transferring, wherein said second
mold is at a second substantially constant temperature throughout
said two cycles, wherein said second substantially constant
temperature is lower than said substantially constant elevated
temperature and wherein the cavity of said second mold has the
shape of the ultimate shaped article, cooling and compressing
said shaped cellular preform in said second mold, and removing
the finished article from said second mold.
The apparatus of the invention includes a first mold
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comprising male and female members defining a molding cavity
having at least approximately the shape and size of the article
to be produced; means for maintaining this mold at a constant
temperature between 90C and 130C, and preferably about 120C;
an injection device for introducing beads of thermoplastic
material into the molding cavity; one or more injection nozzles
for injecting steam into the molding cavity; a mechanical or
pneumatic device for ejecting the preformed article on one said
member from the heated mold; means for transferring said article
,
on the one member and introducing the article into a second
mold comprising a molding cavity having the exact shape and
size of the article to be produced, this mold being maintained
at a temperature lower than that of the first heated mold; and
mechanical or pneumatic means to withdraw the article from said
mold.
The method of the invention has many advantages over
the prior art since it enables the production time to be reduced,
the production cost to be reduced and, considerably simplifies the installation
lay-out to increase its efficiency and profitability; in addition,
with the method of the present invention, lower density materials
can be obtained hence permitting material savings.
According to a preferred embodiment of the present
invention, the article formed or premolded inside the heated mold
is withdrawn as soon as the first molding step is completed for
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being transferred in the cold mold. From this very moment,
a new article can be injected in the heated mold thus permitting
to reduce considerably the total molding cycle time, this time
being reduced to about 10 seconds/article, for medium sized
items.
The mold being no longer subject to the stresses due
to the repeated and substantial temperature variations, can be
constructed from any material including metals, synthetic
materials or even wood. The construction of the mold is only
limited by well known design and techniques of the injection or
compression molds. The mold design is simplified in that the
passages for circulating cooling water into the mold which are
necessary with the conventional methods and apparatuses are
eliminated in the apparatus of the invention or are at least of
a much simplified structure.
Brief Description of the Drawings
The objects and advantages of the present invention will
become apparent upon reading the following detailed description
of a preferred embodiment and alternatives and upon reference
to the drawings in which:
Figure 1 is a schematic view of-an embodiment of the
apparatus of the invention,
Figures 2 and 3 show details of the transfer mechanism of
the apparatus of figure 1,
Figure 4 is a sectional view of an embodiment of the
heated mold shown in its closed position,
Figure 5 is a sectional view of an embodiment of the
cold mold shown in its open position,
Figure 6 is a sectional view of a second embodiment of
the heated mold of the invention, shown in closed position, during
molding operation,
Figure 7 is a sectional view of the molded article
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during its transfer ~rom the heated mold to the cold mold, and
Figure 8 is a sectional view of the second embodiment
of the cold mold of the invention shown in closed position during
the final molding operation.
Detaile_ description of the drawings
Referring now to figure l, the apparatus of the
invention for manufacturing expanded thermoplastic material
acticles includes a frame 1, made from tubes or steel sections
supporting a container 2 to store the raw material in the form
of expanded polystyrene beads 3, a container 4 for collecting
the articles produced, and various units for the transformation
of the raw material and for the control of the apparatus.
These various units comprise essentially the male
portion 5 and the female portion (not shown) of a heated mold,
the male portion 6 and female portion 7 of the cold mold
(described in detail with reference to figures 4 and 5) and one
member 8 to transfer the article from the heated mold into the
cold mold. The male portion 5 of the heated mold is mounted at
the lower end of the member 8 which is positioned so that it
can slide on a shaft 9 under the action of a piston lO received
in a housing ll of prismatic configuration. In addition, the
member 8 can pivot in its plan like a pendulum about said shaft 9
between a first position into which the male portion 5 of the
heated mold is placed in front of its corresponding female
portion and a second position wherein the portion 5 is in front
of the portion 7 of the cold mold to effect the transfer of the
article and introduce it in mold portion 7. This swinging motion
is obtained through a mechanism 12 which is shown in more detail
in figures 2 and 3.
The male portion 6 of the cold mold is mounted on the
free end of a piston rod 13 which is slidably received into a
cylinder 14 to reciprocate horizontally. Also supported by the
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frame 1 is a control panel 15 for the control of various elements
of the apparatus.
The transfer mechanism will be now described with
reference to figures 2 and 3. The mechanism 12 substantially
comprises two slides 16, 17 and a plate 18 which can slide
vertically in slides 16 and 17 under the action of a piston 19
slidably received into a cylinder 20 and connected to the plate
18 by a rod 21. A curvilinear opening 22 is formed in the plate
18 into which is engaged the end of a pin 23 connected to the
housing 11. Once the piston 19 is moved downwards (as shown
on figure 3) by a fluid under pressure fed to the cylinder 20,
the end of pin 23 is guided in the curvilinear opening acting as a
cam and driving the housing 11 together with the pin to a
position 11' shown in dotted lines. The housing 11 rigidly
fixed to the member 8, the pendulum or swinging motion of the
latter is derived from the mechanism 12. The position 11' (as
shown in figure 3) of the housing 11 corresponds to the new
position 18' of the plate 18 and 23' of the pin end 23.
Figure 4 shows schematically an embodiment of the heated
mold which comprises a male portion 41 and a female portion 42
into which is formed a molding cavity 43 to be filled with
thermoplastic material. An inlet passage 44 for steam in the male
portion 41 is connected to internal passages 45 which themselves
terminate into nozzles 46. The purpose of these nozzles is to
inject steam into the molding cavity 43 to expand the thermo-
plastic beads therein. The female portion 42 is aiso provided
with a tapered injection inlet passage 47 for the thermoplastic
material with steam exhaust nozzles 48 for the excess steam which
are in communication with the outlet steam ports 49, and with a
condensed water exhaust port 50.
Figure 5 shows schematically an embodiment of a cold
mold including a male portion 51 and a female portion 52 of this
mold. Both members are so constructed as the molding cavity
has exactly the configuration and size of the article 53 to be
produced. The mold portion 52 is equiped with pushing means
adapted to eject the finished article out of the cold mold. This
means comprises a plate 54 which can be mechanically moved by
a rod 55 or pneumatically pushed by a device connected to the
port 56.
In operation of the device described above, the male
portion 5, 41 of the heated mold is urged against the female
portion 42 of said heated mold by means of a piston 10, thus
providing a molding cavity having approximately the configuration
; of the article to be produced. This mold is heated by well-
known means not represented. Once the heated mold is ready for
the injection process the pre-expanded polystyrene beads 3
contained in the storing container are introduced into themolding
cavity through an injector. The expansion and agglomeration of
the thermoplastic material beads are obtained by means of steam
injected into the heated mold through the injection nozzles 46.
As soon as the expansion of the material is completed, the male
portion 5,41 of the heated mold integral with the member 8 is
moved to the left (as shown on figure 1) by means of the
piston 10; the article injected into the molding cavity remains
onto this male portion of the heated mold and can freely further
expand thus providing stress relieving.
Thereafter, the piston 19 is moved downwards (as seen on
figure 3) inside the cylinder 20 moving together with the
plate 18 thus moving the end of pin 23 from its initial position
shown in figure 2 to its final position 23 shown in dotted lines
in figure 3. The motion of pin 23 pivotably moves simultaneously
the housing 11 and themember 8 about shaft 9.
In the final position of the member 8, the male portion 5,41 of
the heated mold is aligned with the female portion 7,52 of the
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cold mold into which the now completely relaxed molded article will
be partially engaged. At that timet:hepiston 19 is moved again
upwards causing simultaneously the housing 11 and the element 8
to swing back to their initial positions. For completely inserting
the preformed article into the cold mold, a fluid under pressure
is injected into the cylinder 14 in order to move the piston 13
to the right (as seen on figure 1). The male portion 6,51
which is rigidly fixed to the free end of the piston rod 13
will forcibly engage the preformed article 53 into the molding
cavity of the cold mold which has exactly the configuration and
size of the article which is sought to be obtained. Thereafter
the piston 13 will move to the left (as seen on figure 1) and the
article 53 formed exactly to the size and configuration of the
cold mold will be ejected by means of a mechanical action on said
push rod 55 or by the pneumatic means to finally be collected
in the collecting container 4.
Another embodiment of the apparatus of the present
invention will now be described with reference to figures 6,7 and8.
Specifically, in figure 6 is shown the first step of the process
for the premolding injection step in the heated mold. The
reference numeral 61 shows the female part of the heated mold for
preforming the article, reference 62 shows the male portion of the
mold. Reference 63 shows the molding cavity, reference 64
shows the injection device through which the pre-expanded
material beads are forced into the molding cavity reference 65
shows the injector body, reference 66 shows heating bands and
references 67 and 68 are inlet and exhaust passages for steam.
Figure 7 is a section view of the molded article at the
time of its transfer from the first heated mold to the second and
final cold mold.
Figure 8 shows the second and final molding step into
the cold mold. In this figure reference 69 shows the female
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portion of the cold mold, reference 70 shows the male portion of
same, references 71 and 72 show passages for circulation of cooling
water, reference 73 shows the molding cavity and reference 74
shows the ejector for the finished article.
While the present invention has been described with
reference to two preferred embodiments, many alternatives changes
and modifications will appear to those skilled in the art such
as particularly:
- as the transfer time of the pre-formed article from
the heated mold to the cold mold is not critical per se, it is
possible to design a machine having two stations, one station
for hot molding positioned at some distance of a second station
for cold molding, both stations being connected by the transfer
means,
- the preformed articles can be molded under vacuum
into the cold mold. This technique enables to considerably increase
the relative pressure inside the expanded thermoplastic material
beads which permits easier production of low density articles.
- both molds can be on the same molding press or on
separate presses,
-one first preforming heated mold can be provided for
several second cooled molds or vice-versa,
- the transfer device between molds can be mechanical,
pneumatic or of another type,
- heat transfer from the heating mold to the thermo-
plastic material can be made by any known means such as electrical
resistors, steam and heated oil.
Further to the multiple advantages already mentioned,
the method of the invention enables to obtain articles having a
higher density which are more resilient than similar articles
molded with the standard method. These articles are more flexible
and less brittle, which gives them an improved ability for acting
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as shock or impact absorbing pads, especially when they are used
for packaging.
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