THERMOFORMING COEXTRUDED PLASTIC SHEETS

THERMOFORMAGE DE FEUILLES DE PLASTIQUE CO-EXTRUDEES

English Abstract

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THERMOFORMING COEXTRUDED PLASTIC SHEETS
Abstract
A process of making thermoformed plastic ob-
jects comprising coextruding a multilayered sheet
having adjacent layers of plastics that do not
bond together during coextrusion, thermoforming
the multilayered sheet, and separating individual
thermoformed objects of different layers.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS

1. A process of making thermoformed plastic objects comprising coextruding
a multilayered sheet having adjacent layers of plastics that do not bond to-
gether during coextrusion, thermoforming said multilayered sheet into at least
one object, and separating the thermoformed object into objects each comprising
a different layer.


2. The process of claim 1 wherein said multilayered sheet is made of
layers of polystyrene and polypropylene.


3. The process of claim 1 wherein one said plastic is for a separator
layer between layers for said objects.

Description

THERMOFORMING COEXI'RUDED PLASTIC S~IEETS
~ield of the Invention

The invention relates to preparing objects thermoformed from plastic
sheets.
Background of the Invent:ion
Plastic sheets are often produced by apparatus in which plastic
material is heated and supplied under pressure to an extrusion die. The
sheets can then be thermoformed to a desired shape, and individual objects
can be trimmed from the thermoformed sheet.

Summary of the Invention
In general the invention features coextruding a mu]tilayered sheet
having adjacent layers of plastics that do not bond to each other during
coextrusion, thermoforming said multilayered sheet into at least one object,
and separating the thermoformed object into objects each comprising a different
layer. With such a method, the productivity of the extrusion, thermoforming,
and trimming apparatus is multiplied by the number of layers in the sheet.
In preferred embodiments one of the plastics in the multilayered
sheet is polystyrene and another plastic is polypropylene; in some preferred
embodiments one plastic is for a separator layer between plastic layers for

2n the thermoformed objects, and in other preferred embodiments the thermoformed
objects are made from more than one of the plastics of the layers of the
multilayered sheet. The thermoforming of the multilayered sheet with poly-
propylene and polystyrene layers is faster than the slow~ and therefore
expensive, thermoforming of polypropylene alone.




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Description of the Preferred Embodiment
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The preferred embodiment of the invention will
now be described after first describing the draw-
ing, which is a diagrammatic view of a process of
manufacturing plastic objects according to the in-
vention.
Referring to the drawing, there is shown ap-
paratus lO for manufacturing polypropylene bowls 12
and polystyrene bowls 14.
Two heated containers 16, 18 serve as sources
of the polystyrene (70% crystal styrene and 30% rub-
ber modifying plastomer sold by Phillips Chemical
Company under the trade designation Solprene 416P,
a block polymer having a butadiene/styrene ratio
of 70/30, a molecular weight of 140 M, a density of
0.94, and a melt flow index of 2 decigrams/min~,
and polypropylene (available from Rexene Company,
Paramus, New Jersey, and having a melt flow index
o~ 4 decigrams/min), respectively.
Conduits 20, 22l 24, 26, 28 supply the heated
materials to coextrusion block`30. There the ma-
terials merge together to form under pressure a
unitary, five-layer thick stream 32 of generally
circular cross section. Stream`32 passes into
extrusion die 34 ~e.g., Welex standard 54" flex-
lip die~ for extrusion into continuous sheet 36.
Sheet 36 then passes through a series of chill
rolls 38.
The sheet, after coextrusion, may then be
thermoformed into bowls or wound into spools (not
shown) for storage and later themoforming.
To process sheet 36 into bowls, the shee-t is
passed through conventional thermoforming apparatus




40 (the vacuum forming type well known in the art~;
which impresses the bowl shape.
After thermoforming, the shaped sheet 42
passes through trim press 44, in which the indivi-
dual bowls 12 and 14 are trimmed. Because the
polypropylene and polystyrene of bowls 12, 14,
respectively, do no~ bond to each other during
coextrusion or thermoformingl bowls 12, 14 can be
easily separa~ed from each other. The simultaneous
coextruding of five sheets and thermoforming and
trimming of five bowls act to increase the produc-
tivity of the coextrusion, thermoforming, and
trimming apparatus by a factor of five. Also, the
thermoforming of the five-layer sheet is faster
than the siow, and therefore expensive, thermoform-
ing of polypropylene alone.
Other Embodiments
Other embodiments of the invention axe wlthin
the scope of the appended claims. For example,
other plastics that do not bond to each other can
be used in place of the polystyrene and polypropy-
lene. Also t one of the plastic layers could be a
thin separator layer which would be used for scrap
after separating the thermoformed objects from
each other.