Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1 This invention relates to the manufacture of thin
walled plas-tics material articles which are formed from
web or sheet plastics materials (which may he laminates~O
Any suitable -technique such as vacuum forming,
pre~sure ~orming, plug assist or mechanical formin~ or any
combination of such techni~ues may be used in the forming
of the articles, but in all cases the sheet material i~ . :
heated to effect a degree of softening of the material so that
it can be formed and therefore -~he forming of the articles
will be referred~to simply and generally as "thermoforming".
Thermoforming is of cour~e well known and is wldely
practised, and there are many thermoformed article~ ln
every day use in many countries o~ the world, ~uch articles :.
including do~estic holloware and drinking cups. . :~
Certain materials, and in particular polypropylene,
polyethylene and expanded plastics sheet re~uire to be :~
thermoformed at very accurate temperatures in order to achieve . .
high ~peed production and high quali-ty produ~ts. Conve~tional
methods of heating such as infra-red heaters do not provide
suf~icient accuracy of heating to achieve these desirable
features even when the sheets being thermoformed are of uniform
thickness which in commercial practice i~ not usually the case. ~ .
Furthermore, when the heating is carried out by a
radiation process, many types of plastics sheet wrinkle or ~ :
sag under the heating ac-tion which causes even further
temperature variations in the heated shee-t.
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1 The present invention seeks to provide a method of
heating which is useful for a very wide range of plastics
sheets including polystyrene, ~.B.S. acrylonitrile, nylon,
polyester, polyvinyl chloride, cellulose acetate, the so-called
"filled" plastics material comprising plastics and inorganic ~-
particular material and indeed any sheet which is capable of
being thermoformed, and in which the aforesaid disadvantages
are obviated or mitigated.
According to the present invention there is provided a
~ method for the manufacture of thin walled plastics material
articles, wherein the articles are thermoformed in a heated web
af the plastics material and the web is heated by being passed
round heated roller means.
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~ Ieating the web by contact with heated roller means
and hot gas and/or vapour ensures even heating of the web across
its width, and the web can at no point reach a temperature which
is greater than that of the roller means and gas and~or vapour.
The web may be fed into and out of a chamber containing
the roller means and gaseous and/or liquid environment.
The web preferably is continuous and travels directly
to a thermoforming machine in which the articles are formed
and the method is repeated. The construction of the articles
is of course immaterial to this invention.
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1 The web preferably travels into and out of the cha~ber
in a horizontal direction and in the same plane, and the roller
means may comprise three rollers lying with their axes
horizontal, two of the rollers lying above the third roller
and disposed symmetrically relative to the vertical plane
containing the third roller. Each roller i8 preferably
heated by circulating hot oil therethrough.
The web travels over and round one of the upper rollers,
round the lower roller and round and over the other upper
roller, so as to deEine a pa~h having said vertica} plane
as an axis of sy~unetry. The rollers preferably are of such
size and are so dl~posed that the web, in travelling through
the chamber, has e~ual contack with the rollers on each side
of the sheet.
There may be inle-t means and ou-tlet means by which
gaseous and/or vapour medium can be injected into and removed
from the chambers. The temperature of the rollers and the
gaseou~ and/or vapour medium praferably is accuxately
controlled, so that the web will be heated to an even,
accurate temperature, which preferably is such that the web
i8 ready for thermoforming.
The temperature of -the rollers and gaseous and/or vapour
medium will preferably be thermosta-tically controlled in order
to ensure that the web is maintained at the critical temperature.
The minimum length of time which the sheet must be in the chamber !:
to reach the desired temperature will depend upon the dimensions
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1 of the sheet but in any case -this time can be adjusted to
suit the material being heated. There is no maximum heating
time as the temperature of the web cannot get higher than the
chamber temperature.
Articles, such as domestic holloware, formed from
polypropylene heated in this manner can be reliab}y
produced in large quantities and taking large numbers on
average they exhibit an excellent surface finish, toughness
and clarity which are bet-ter than those characteristics ln
conven-tionall~ formed polypropylene articles.
As the thermoorming machine will pxobably operate on
a cyclic basis requiring a dwell or a batch of the articles
can be formed, the web will also require to dwell cyclically
in contac-t with the rollers.
It is not necessary that the heating effected in the
heating chamber should comprise the whole of the heatin~
of the sheet. Pre-heating and/or post heating ~uch a~
steam heating, radiant heating or the like may be employad.
Thi~may be suitable where the plastics material web being
heated is polystyrene, as the heating chamker could be used
to perform the major portion of the heating of the sheet, and
for example, conventional radiant heaters, which are
inefficient, could be used for only a small post heating
step of bringing the web up to forming temperature from the
temperature at which it leaves the li~uid bath. In any event,
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1 the roller contact and gaseouq and/or vapour heating in the
heating chamber is always beneficial because of th~ even
temperature di~tribution which is achieved.
The invention thus provides a simple method whereby ;~
plastics material in web form can be heated evenly, enab7ing
the effective use of plastics materials such as the polyolefins
e ~. polypropylene, for thermoformed articles produced in large
numbers; the cycle time can be made very short and in many cases
a cheaper yet superior article can be produced.
The invention also provides articles produced according to
the method as aforésaid.
Al~o according to the invention there i~ provided apparatus
for use in the manufacture of thin wal}ed plastic3 materlal
articles comprising means defining a heating chamber, exit and
entry means whereby a web of plastic~ material can be fed into
and out of the chamber, roller means around which the web can
be passed, means whereby the roller means can be heated, and inlet
and outlet mean~ whereby hot gaseous and/or vapour can be
injected into and removed from said chamber.
An embodiment of the invention will now be described, by
way of example, with reference to the accompanying diagrammatic
drawing, of which the single figure is a sectional side elevation
o~ heating chamber.
Referring to the drawing, the heating chamber is defined
by a cabinet having front and rear walls lO and 12, top and bottom
walls 14 and 16, and two side walls, of ~ich onLy slde wall
18 i~ shown, as the figure i8 a sectional elevationO The front
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1 wall at its top portion is provided with an entry slot 20, whilst
the rear wall 12 is provided with an exit slot 22. These slots
20 and 22 are in horizontal alignment and enable the passage of
a thermoformable web 24, such as a web of polypropylene to enter
the chamber through slot 20 and exit from the chamber through
slot 22. -
The front wall 10 and the rear wall 12 are also pro~ided
with inlets 26 for gaseous and/or vapour heating medium. In
this example, the heating medium is air. In use, heated air is
10 continuously inserted into the chamber through these inlets 26 ~
and is extracted by extraction pipes 28 which are located, as - -
shown, above and below the entry slot 20 and above and below the
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exit slot 22.
Inside the chamber, there are three heated rollers 30,
32 and 34. These rollers are disposed with their a~is parallel
and horizontal, and the rollers 30 and 32 are disposed at a
higher level than the roller 34. The rollers 30 and 32 are in
fact symmetrically disposed about a vertical plane passing
through the axis of roller 34. These rollers are heated by
circulating heated oil through the hollow interiors thereof, but
their peripheral surfaces are metallic or good conduction ~ -
of heat from the roller surfaces to the web 2~, in contact
therewith. The web 24 after passing infra-red pre-heaters 36
travels, as shown, over roller 30, round roller 34 and then
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I over roller 32. The web eventually leaves via exit slot 22
and it will be seen that the entering stretch of web and the
leaving stretch in relation to the chamber, are disposed in the
same horizontal plane.
The temperatures of the rollers and the environment in
the chamber will be maintained within accurate limits to ensure
that the web 24 is heated to an even and regular temperature,
so that it will be suitable for passing directly from exit
22, to a thermoforming machine in which the articles are
produced in the web 24. These tempera-tures will be controlled
by the circulation both of the environment o the chaTnber,
and the oil of -the heating rollers, and suitable thermostats
will be employed to maintain these temperatures within accurate
lim~s. For example the air which is injected through inlets 26
may be at 160 centigrade for polypropylene sheet and the
rollers 30, 32 and 34 would be maintained at a similar
temperature.
The roller sizes and disposition are preferably selected
so that as -the web 24 passes through the apparatus there will
be the same amount of contact between roller and web, at each
side of the web at all times, and the rollers 30, 34 and 32
may be driven at slightly higher speeds, in that order, in
order to compensate for any lengthwise expansion of the sheet
24 which may take place during the heating process and to ensure
that the web 24 remains in intimate contact with the rollexs.
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1 Furthermore, it is possible to have the rollers 30, 34 and
32 so positioned as to define nips between roller 30 and 34
on the one hand, and roller 34 and 32 on the other hand in
order to effect or assist the feeding of the web through the
chamber.
Whilst the preferred construction o~ machine has been
described, it is to be appreciated that modifications may
be made without parting from the scope of the invention.
For example, it would be possible to provide an apparatu~
ln which only two rollers are provided, with the web travelling
in "S" formation round these rows and having e~ual web to
roller con-tact on each side o~ the web. Furthermore, the
chamber may contain suitable baffling in order to ensure
that the hot air which is being injected into the chamber will
tend to be directed towards the rollers.
