Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ackground of the Invention
Thls invention relates to a process and apparatus for
preparing a hollow, rib-reinforced, lamlnated artlcle and to
the article prepared thereby.
A general process for making lamlnated artlcles by
flrst maklng a lamlnate preform from two thermoplastic sheets,
then heating the preform, placlng the heated preform between
two mold halves, and blow-forming the article by lntroducing a
~luid between the sheets to expand the sheets lnto thelr
respectlve mold cavlties, ls well known in the art.
When the sheets are lamlnated to form a preform,
however, they stlck together and are very dlfflcult to ~eparate
without tearlng. m ls tendency of the sheets to stlck together
creates problems ln lntroduclng the blowlng fluld between the
sheets and ln sheet separation as the blowing fluid expands the
sheets lnto the mold cavlty.
U.S. Patent 3,281,301 dlscloses a method typlcal of -
the prlor art wherein certaln preselected areas of the lnter-
faclal sur~ace between two lamlnated thermoplastlc Qheets are
20 treated prlor to lamlnatlon ln such a manner as to make them - -
nonadherent to each other. Thls can be accomplished by using - --
a partlng membrane such as paper or a layer of talc.
:In addition, a bleeder member, such as a tube, i8 --~
lnserted between the sheets and is used to introduce air - --
between the qheets. Air forced ln between the sheets through
the bleeder member ~ollows the path progressively through the
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1041891
nonadherent areas distending the sheets in such areas.
In similar processes, see U.S. Patents 3,271,846;
3,289,281; and 3,394,446 as the process appies particularly to
thermoforming metal sheets.
These prior art processes have the disadvantage of
requiring the use and placement of a nonadherent masking
material on the sheets, which necessitates a separate operation
and precludes subsequent sealing of the sheets where the masking
material has been placed. Moreover, the laminated sheets must
be precisely registered in the mold in order that the nonadherent
areas conform with the recessed portions of the mold, and a
bleeder member must be properly aligned with and inserted between
` the laminated sheets for the introduction of air.
Other related prior art includes processes wherein two
thermoplastic webs are heated and fed either continuously or
incrementally between two advancing opposed mold halves which
have an opening for the insertion of a blow tube. jThe thermo- -
plastic webs, however, are not laminated prior to or during -
forming but instead are spaced apart to aid in introducing air
between the sheets.
In U.S. Patent 3,537,138, for example, the advancing
webs are spaced apart and brought together only at points for~ing
the edge of the final article and at all times leaving an opening,
such as the mouth of a bottle being formed, where air can be in-
troduced between the sheets to blow the~sheets apart and into
their respective mold cavities.
; In still another related patent, namely, U.S. Patent
3,099~043~ the advancing webs are spaced apart and brought into
,:,
contact with two opposlng and partially opened mold halves. The
....
-~ 30 webs~ respectively, are vacuum drawn into their corresponding
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lo~i8~1
mold half and thereafter the mold halves each containing half
of a formed article are brought together ~using the edge~ of ~he
article together forming the whole article while constantly main-
taining communication between the atmosphere and the interior of
the article being formed.
While many prior art processes are known for thermo-
forming plastic materials, there is a continuing need for a
process having a convenient method for introducing air between
the thermoplastic sheets. The need is especially apparent where
the thermoformed article has an intricate design, such as a hollow
rib-relnforced lamlnated article, --
Summary of the Invention
Accordingly, this invention provldes an improved
process for preparing a hollow, rib-reinrorced, laminated ~: :
article by: :
. . .(a) placing two sheets between opposlng mold platens, -
the sheets belng allgned such that the sheet surfaces ~. ;
oppose each other, at least one of the sheets .
belng a thermoplastic material heated to its :
thermoformlng temperature and at least one of the
sheets belng provided with grooves or integral
pro~ectlons which form fluid passageways) and at
least one of the mold platen-s being provided with a
mold cavity to form a shaped artlcle having ribs;
! (b) closing the mold platens to contact the sheets; and
(c) lntroducing a fluld into the fluid passageways to
di~tend the thermoplastic sheet lnto lts mold
cavity forming a shaped article having ribs~ whlle
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the sheet~ malntain contact in the nondistended areas.
Thi~ lnvention also provides an apparatus for pre-
paring a generally hollow, rib-reinforced, lamlnated thermo-
plastic article compri~ing:
(a) means for heating a composite thermoplastic preform
to the thermoforming temperature of the thermoplastic
whereln said preform comprises at least two thermo-
plastic sheets and the interfacial ~urface of at
least one ~heet is provided with fluid passages;
(b) means for placing the preform between two opposing
mold platens wherein the mold is shaped with a rib-
forming mold cavlty facing the preform to produce the
thermoplastlc artlcle; and
(c) mean~ for lntroduclng a fluid into the fluid passages
of the preform forclng the preformed sheets apart to
distend the sheet~ lnto their respective mold cavities
while the ~heets maintain contact in the nondistended
:, areas.
This invention further provides a composite thermo-
plastic laminated preform whereln sald preform is a lamlnate of
i at least two thermoplastlc sheets and the lnterlor sur~ace
o~ at least one sheet ls provided wlth fluid passageways.
This invention stlll further provldes a thermoplastic
artlcle comprised of: - -
(a) at least two laminated thermoplastic sheets wherein the
interfaclal surface of at least one sheet is provided
with fluld passageways and
(b) dlstended and nondistended areas ln the thermoplastlc
sheets formed by a fluid lntroduced into the fluid ~.
passageways between the two sheets to ~eparate the ~-~
~heets and force the sheets into a desired configura-
tlon wherein the oppo~ing nondistended areas provide
contact polnt~ to hold the article together, the
dlstended areas provlde the article wlth the deslred
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shape and the comb~nation of distended and nondistended
areas provide integral ribs that reinforce and con-
tribute to the structural integrity of the artlcle
of product claims.
In alternate embodiments, the thermoplastic material can contain
a filler; the fluid passageways can be formed by a pattern of
interconnected or disconnected pro~ections, grooves, embossed
pits, valleys~ or corrugations in one or both of the sheets;
the mold can be of various configurations, such as one mold
half having a smooth surface and the other mold half having a
rlbbed surface, or both halves can be shaped to a desired con-
figuration; and fluid can be forced into the passageways by
positlve pres~ure, or drawn into the passageways as the sheet
is distended into a ~acuum mold, or by a combination of these
methods. mi9 process is particularly useful in the preparatlon
of laminated structures having an intricate design of sealed
hollow portions.
This invention also provides apparatus ~or carrying
out the proce~s.
Brief Description of the Drawlngs
Flg. 1 is a schematic drawing showing the preparation
of a preform useful ln the present invention.
Flg. 2 shows an embodiment of a preform wherein the
interfacial surface of one sheet is emboseed with a pattern of -:
parallel, extendlng, rectilinear-~haped lands and grooves. The
fluld passageways are not interconnected.
Flg. 3 shows an alternate embodlment of a preform where- - -
in the lnterfacial surface of one sheet is embossed wlth a
pattern o~ lsland-llke, rectangular-shaped protuberances formlng ~ --
Y 30 lnterconnected fluid passageways. ~
Fig. 4 1~ ~till another alternate embodiment of a pre- -
form whereln the interfa¢ial surface of one sheet ls embos3ed
wlth a pattern of lsland-like, generally circular-~haped
protuberan¢es ~orming interconnected fluid pas~age~.
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104~891
Figs. 5 through 8 show various stages in the thermo-
forming process of the present invention. Fig. 5 shows the pre-
form being heated in an oven. Fi~. 6 shows the heated preform
placed between two opposing mold cavities. Fig. 7 shows the
-- mold halves closed on the prefonm and air under positive pressure
forced between the laminated layers forming the preform distend-
ing the preform into the mold cavity. Fig. 8 shows the comple-
tion of thermoforming process wherein the mold halves are opened
and the thermoformed article is released from the mold cavity.
Fig. 9 shows an alternate embodiment of introducing air
between the laminated preform by forcing air through the marginal
edges of the preform. In addition, the mold incorporated therein
is a two-position mold wherein the first position is held to form
the product and thereafter the mold is closed an incremental
amount to pressure fuse the nondistended areas of the preform.
Flg. 10 shows an alternate embodiment offintroducing
` alr between the laminated preform by drawing a vacuum in the
mold cavity which in turn draws air in through the marginal
edges of the preform, into the lnterfacial surface between the
laminated layers through the fluid passageways provided in the
interfacial surfaces, forcing the laminated sheets to separate ~--
and conform to the mold cavity. Optionally, the mold can be
further closed an incremental amount to pressure use the non- -~ -
tistended areas of the prefo~m.
Fig. 11 is a fragmented cross section of the preferred
preform taken along the longitudinal axis of the preform. - -
:'
Pig. 12 is a ragmented plan view of a bobbin tray. -
Pig. 13 is a fragmented cross section of the bobbin -
tray shown in Pig. 12.
Plg. 14 i8 a cross section of a heater useful in
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~04~891
preheating the preform, taken in the vertical plane.
Fig. 15 is a cross section of an embodiment wherein
the mold and preform are vertically aligned.
Fig. 16 is a cross section in the horizontal plane
of the mold of Fig. 15 in the closed position.
Description of the Preferred Embodiments
The invention will now be further described in tenms -
of a preferred embodiment wherein two thermoplastic sheets are
formed into a composite preform prior to being heated and
lQ positioned for molding. Use of preform is preferred for
convenience in handling, process control, and to maintain the
sheets in proper alignment one with the other.
The thermoplastic composite preforms can be prepared
from any sheet-forming thermoplastic that is thermoformable.
While at least two sheets are contemplated to form the composite,
three or more sheets can actually be used and each sheet can be
mate from~the same or a different thermoplastic. Useful thermo-
plastics include polyolefins 2 to 10 carbons such as poly-
ethylene and polypropylene, polyesters such as polyethylene
terephthalate, polyamides such as nylon, polysulfones, poly-
carbonates, vinyls such as polyvinylchloride and polyvi*yl
acetate, polystyrene, and cellulosics such as cel~ulose acetate
butyrate ant cellulose propionate. Also contemplated are thermo- ~ -
.~ . .
` plastic materials which, after thermoforming, can be thermoset
to form articles whlch maintain their shape over wlde temperature
ranges.
These thermoplastics csn optionally be filled with
~arlous types of fillers. The particular type and amount of
flller chosen for use ls dependent on the end result desired.
Yor example~ asbestos or aluminum oxide can be used to provlde
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fire resistance. Fiberglass fillers can be used to provide re-
inforced films. Various colored fillers can be used as pigments,
such as titanium dioxide. Clays can be used as bulk fillers and
many other fillers can be used to reinforce the film to add
strength or to modify other properties of the film such as stiff-
ness, optical properties, etc.
Useful types of fillers include the following: asbestos,
fiberglass, carbon, calcium phosphate, calcium carbonate, kaolin-
ite clay, silica, titanium dioxide, bentonite, talc and mica.
The amount of filler-used can vary from about 1 to 70
weight percent based on the weight of the polymer and filler
present. It is preferred that the filler be present in the amount
of about 1 to 60 weight percent based on the total weight of
polymer and filler present. The preferred type filler for use is
:,
clay and it is preferred that the clay be present in the amount
of about 30 to 60 weight percent based on the total weight of the
` poly~er and filler present. -
The thermoplastic polymers can be prepared by known
methods. When fillers are used, they can be added to the thermo- ~ -
plastic polymers during the preparation of the polymers according
to the conventional methods.
After the polymer is prepared, it is extruded through
an extrusion die into a film that is cast onto a quenching drum.
While being quenched, the surface of the film that will eventually
- be an interior interface in the composite preform structure is
embossed, grooved or otherwise marked to provide passageways
along the surface of the film. Alternatively, the passageways
can be provided by extruding the film through a serrated die.
Whlle only one of the contacting surfaces at the interface in
the co-posite structor- oecd be provided with paseageway6, It
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~041891
is s~metimes desirable that both surfaces at the interface
that oppose each other in the interior portion of the composite
be embossed or grooved to insure and provide an increased number
of interior passageways~ After marking, the film can be mechani-
cally stretched to orient the film in order to improve strength
properties.
A variety of methods can be employed to form the
composite, depending on the thermoplastic material and on the
desired degree of bonding. The bonding, however, must be weak
enough that the sheets readily separate as they are distended
into the mold cavity during thermoforming. Moreover, the method
of forming the composite must preserve the fluid passageways.
Ultrasonic spot welding is a convenient method of
forming composites of clay-loaded polyethylene sheets since the
spot welds readily rupture during distension. Adhesive bonding
.:
can be employed for excellent contact bonding or electrostatic ~ '
bonding can be used for loose bonding by providing a high
voltage electrical discharge of opposite polarity on the
exterior of the sheets. Thermal bonding can also be used by
passing the sheets between heated rollers. If the sheets are
susceptible to strong thermal bonding, or if strong adhesive
bonding is used, the passageways should be formed by pro~ections
having a width less than the thickness of the sheet so that the
pro~ections, and not the sheets, rupture as the sheets are ~
distended during thermoforming. -
A convenient method of making the film composite pre-
forms will be discussed in relation to Fig. 1. Referring to
Flg.~l~ fil~-forming extrusion devices 1 and 2 each extrudes
thermoplastic fllms 3 and 4 that are cast onto quenching drums
30 5 snd 6, respectively. The quenching drums 5 and 6 meet in
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10418~1
nip-formin~ relationship wherein the nip 7 is a narrow gap with
a space about equal to the combined thickness of the as-cast
films 3 and 4. Roll S rotates clockwise and roll 6 rotates
counterclockwise so that both films are conveyed into the nip.
An embossing roll 8 is positioned in nip-forming re-
lationship with quenching drum 6 at a convenient location between
the extrusion die and nip 7. The embossing roll marks that por-
tion of film 4 with an appropriate embossing pattern that will
form the interior surface of the resulting laminate.
After the films have been cast, and the interior sur-
face of at least one film embossed, they pass through the nip
formed by the quenching drums where they are lightly pressed
together forming a continuous film composite that can be wound
onto a mill roll or forwarded to a cutter and chopped into
predetermined sheet lengths.
The fluid passageways can be formed by a number of
different patterns on the film surface. Figs. 2, 3 and 4 are
examples of pro~ection patterns found useful in forming the
fluid passages. The term ~pro~ection~ as used herein refers
to raised portions of the thermoplastic sheet.
Fig. 2 shows a laminated preform. The preform is com-
posed of two thermoplastic sheets 9 and 10 laminated together.
The interior surface 11 of sheet 9 is smooth and the opposing
Interior surface 12 of sheet 10 is embossed with a pattern
which in cross section consists of rectangular-shaped pro-
tuberances 13 that extend in a continuous parallel pattern
across the width of the sheet. The fluid passages 14 are
formed by the open valleys between the protuberances bounded
on top by fllms 11. The parallel fluid passageways!are not
lnterconnected. Thls pattern is particularly useful when
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relatively thin sheets are involved and intricate molding
patterns are required. Where the hollow portions of the mold-
ing pattern are not interconnected, however, passageways
communicating with each hollow portion must also communicate
with the fluid source.
Figs. 3 and 4 show other surface patterns with inter-
connected fluid passages that are useful in providing fluid
passages between the composite sheets. The patterns shown have
interconnected fluid passages. Fig. 3 shows two sheets 15 and
16 as a composite with the interior surface of sheet 16 having
a pattern of protruding truncated rectangular projections 17
embossed thereon with fluid passages 18 provided between the
projections.
Fig. 4 shows two sheets 19 and 20 laminated together
with the interior surface of sheet 20 having a pattern of pro-
truding truncated cylindrical-like projections 21 embossed
thereon with fluid passages 22 provided between the projections.
Many other useful in patterns, whether regularly or
irregularly occurring on the interior surface of at least one
sheet, are sufficient if they provide fluid passages between ; ~-
the sheets .
Once the film composite is prepared, depending on --
its physical properties such as thickness, flexibility and
winding characteristics along with its ultimate intended use,
it can be wound onto a mandrel or cut into sheets that are
stacked for further use. For clarity in definitions, the terD
"preforml', herein, is used to denote a composite of at least
two film sheets wherein the interfacial surface of at least
one ~heet is provided with fluid passages. -
Use of the composite preform to make a hollow, ribbed,
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1891
shaped article will now be described with particular reference
to Figs. 5, 6, 7 and 8 of the drawin~s.
The preform is heated to the thermoforming temperature
of the thermoplastic. Thermoforming is described as a process
of heating a thermoformable material, e.g., thermoplastic, to
soften the material, forming the softened material into a
desirable article by forcing the material into contact with a
shape-forming mold, and quenching the formed material whereupon
the material permanently takes on the shape of ~he mold.
The temperature to which the material is heated is
called the ~thermoformingl~ temperature and is described as that
-~ temperature of the material (usually a temperature range) where-
in the material can be shaped to a selected contour and can --
permanently retain that shape when cooled and released from the
shaping force.
The thermoforming temperature range can be empirically
determined for a given material. The following test has been ; -
found useful:
The lowest forming temperature is that temperature at
which a square box with fairly sharp and distinguishable corners
can be drawn fron a sheet of the material without any visual
damage to the material. The highest forming temperature is that
temperature at which a sheet of the material either gets so soft
and fluid that it sags under its own weight (melt sag) or changes
ln appearance, scorches or degrades.
Some typical values of thermoforming temperature are
as followc: branched polyethylene, 250-375F.; linear polyethyl-
ene, 300-395 P.; polypropylene, 300-395F.; unoriented poly-
ethylene terephthalate, 350-400F.; oriented polyethylene
terephthalate, 380-490P.; and polycarbonate (polybisphenol-A - ~-
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1041891
carbonate), 440-475 F.
Many different techniques of heating can be used in-
cluding radiant heating, dielectric heating, convection heating
or contact heating as between two hot platens. For relatively
thin preforms, it has been found desirable to heat the preform
between two hot platens. The weight of the platens aids in
keeping the thin sheets from curling or warping. Very thin pre-
forms may exhibit a tendency to sag during heating. In such
cases it may be desirable to hold the preform in the vertical
~` 10 plane to prevent disruption of the passageways during heating
and while the preform is in the heated condition.
; Referring to Fig. 5, a preform 23, made up~of two
sheets 23 and 24 laminated together, is heated in an oven 26 to
the thermoforming temperature of the film. After the preform 23
` is heated, it is placed between two opposing mold platens 27 and
28-shown-in Fig. 6. The mold platens are provided with cavities
shaped to some desired configuration. It is important to note
that the portion of the preform to be distended into a cavity of
the mold does not touch any portion of the mold prior to blowing.
"'s
Therefore, the sheet is not quenched by the mold in such areas
and they remain relatively close to the original heating tem-
perature resulting in excellent blowing characteristics.
This invention is particularly useful when at least one
mold half 28 is shaped to reproduce distended portions 29 in the -
.:, .
~l sheet to be blown. In this manner, articles such as automobilehoods and automotile paneling can be made having one side smooth
for visual aesthetic effects and one side ribbed to provide im-
provet strength properties. ~
The present invention is, however, also useful in
making articles havlng shapes on both sides such as a stacking
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1041891
tray where the t~ay is shaped to fit over one article and under
another.
Once the preform is between the mold platens, the mold
platens are mated as shown in Fig. 7 and a fluid is introduced
into the fluid passageways 30 provided at the interior contact
surfaces between two films. The fluid can be a gas such as air,
which is the most practical fluid for use, but it can also be
other gases such as nitrogen. In addition, the fluid can be a
liquid such as water, a molten polymer, a liquid prepolymer, or
foam in liquid form. Selection of heated air is preferred to
avoid premature chilling of the preform. When the shaped article
has thin walls, the heated air should be introduced under pressure
to avoid formation of a partial vacuum, and possible wall collapse,
as the article cools after molding.
The flùid can be forced into the internal passageway
under posltive pressure through one or more ports provided along
the perimeter of the laminated sheet structure or the fluid can
be drawn into the internal passageways by vacuum. In addition,
the combination of vacuum and positive pressure can also be used
to introduce the fluid into the fluid passageway. This is par-
- ticularly useful in reducing mold cycle time.
When the vacuum technique is used, a practical way to
apply the vacuum is to provlde tiny pinholes through the mold
plstens, drawing a vacuum from the interior to the exterior of
the mold through the~e pinholes. When a positive pressure is ~
used, care must be taken to evacuate or otherwise eliminate the - - -
problem of trapplng alr in a mold cavity. This can be done by -
known technlques lncludlng the use of a vent ln the bottom of the
cavity~ or llning the cavity with a screen, or roughenlng the
surface of the molt cavity.
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Figs. 6, 7 and 8 show the use of positive pressure to
force air into the fluid passageways between the two films
forming the preform. Air from a compressor, not shown, is forced
through conduit 31, through passageway 32 in the mold platen,
` through holes 33 in film sheet 24 into the fluid passageways 30
between the two sheets 23 and 24 forcing the films to blow apart
Il~U conform to the mold cavity and particularly distending sheet
24 into the hollow sections 29 of the mold. Hollow sections 29
of the mold have a roughened surface. Fig. 8 shows mold halves
27 and 28 parted and the article shaped from sheets 24 and 25
partially removed from the mold.
As fluid enters the internal passageways, it forces the
laminated sheets apart at the places where a mold cavity exists
forcing the sheet to conform to the mold cavity. Where the mold
cavity exists, the sheets distend into the cavity forming a hol-
low shaped rib. Nhere a cavi~y does not exist, the sheets remain
in contact. These contact points add strength and rigidity to
the hollow articles being formed. An important aspect of this
invention is the fact that the fluid passages provide im~ediate
distrib~tion of fluid between the sheets. This permits rapid
.-.t
distention of the sheets before any significant quenching affects
the sheets, thereby providing excellent mold replication.
If it is desired that structural rigidity of the molded ~- -
srticle be improved, adhesion can be established at the inter~
~i faclal contact points of the sheets. Adhesion is conviently -
i establl~hed, when the sheets can be thermally bonded, by heat
i~ or pre~sure or both. In this case, the mold can be designed with
selectlve heating elements at the contact points on the mold and
the molt further provitet with two positions, one position to
form the artlcle ant one posltlon to seal the article, the article
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10411~91
being formed with the mold platens slightly apart and subse-
quently the sealed areas formed with the mold platens tightly
pressed to each other. Alternatively, a temperature activated
adhesive can be employed when the two sheets will not bond to-
gether by high temperature and pressure alone. When such an
adhesive is employed, the article is heated to the activation
temperature after being molded.
By the application of sufficient pressure in sealing
the article, one can also achieve a hermetic seal if desired.
For example, the ridges shown in Figure 2, or the pro~ection
pattern shown in Figure 3, will readily flatten during the sealing
operation to brin8 the two sheets into intimate bonded contact.
` Fig. 9 shows in cross section the use of a two-position ~ -
mold and a method of introducing air into oblique directed pas-
~-` sageways of the preform at the sheet interface along two edges of
;~ the preform. In addition, the preform is made of three
f;' thermoplastic sheets laminated together with fluid passages pro-
vided only at the interface between sheets 35, 35a and 36. ~ -
Optlonally, sheet 36 could also be laminated to still another
thermoplastic sheet or sheets 35 and 36 could be coated either ~ -
prior to or after thermoforming.
Blow forming takes place in a two-step operation --
whereby in the first step theemold halves are first brought to-
i gether a predetermined amount indicated as "h" in the drawings - - - -: -.
` ~ust prior to the introduction of the pressurized fluid. As
; shown in Fig, 9, the actual gap "h" separating mold halves 34
- snd 34a is coincident with the combined thickness of the upper 35
plu8 35a (a barrier or gloss layer) and lower 36 preheated film
sheets ln the unblown state. This gap distance "h" will vary de-
peDdlng on the combined thlckness of the sheets. ~ -
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1041891
As soon as the mold halves are brought together, the
air is introduced through air inlets 37 and 38. The air travels
along air passages 39 and 40 into air gap 41 and 42 and into the
interface between sheets 35 adn 36 through air passageways 43.
Air in passageways 43 between the film sheets (that were pre-
heated to their thermoforming temperature) blows the sheets
apart to separate and distend them into the predetermined rib- -
forming cavity regions 44 and 45. Mold cavity regions 44 and 45
have a roughened surface. This is followed immediately by an
incremental closing of the mold halves 34 and 34a reducing gap
height "h" by an amount sufficient to pressure seal or fuse
together the existing contact points between sheets 35 and 36
remaining after the blowing step. To aid in sealing the mold
chamber and act as a limit stop to keep the mold halves apart, a
resilient gasket 46 is disposed along the top of a raised rim
portion 47 of mold half 34. The raised rim portion also func-
tions as a guide to the mating of the mold halves.
The additional movement compresses the layers tightly -
together causing minor deformation of the open grooves and slight ~ -
expansion or flattening of the bonded portions of the protuber-
ances. The net effect of the final motion is an increase in the
bond area and a stronger final product. Thereafter, the mold
halves are separated and the finished product removed. ;-
~ Fig. 10 shows an alternate method of thermoform~ng
- using a vacuum technique to draw air into the interfacial bound-
ary between the two sheets constituting the preform.
Réferrin8 to Fig. 10, mold halves 48 and 49 are shown
' in mating relationship havlng a preform 50 sandwiched between the
i molds, PrefGn~ 50 i8 comprised of two sheets 51 and 52 and the
lnterfaclal surfaces of both sheets are provided with fluid
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1~41~
passages 53 and 54, respectively.
` In operation, a vacuum is drawn on vacuum chambers 55
and 56 which evacuates the mold cavity by drawing air through
multiple passages leading from the mold cavity to the vacuu~
chambers wherein the passages are designated by numerals 57 and
58. This in turn draws air from the atmosphere into the mold
through ports 59 and 60 and into the interfacial boundary between
sheets 51 and 52 through fluid passages 53 and 54 forcing the
sheets to distend and conform to their respective mold cavity.
To aid in sealing the-~mold chamber and act as a limit
stop~Jto keep the~mold halves slightly apart, resilient gaskets 61
and 62 are disposed along the top of a ralsed rim portion of mold
half 49.
Example 1
A preferred embodiment of the present invention will
now be described with particular reference to Figs. 11, 12 and 13.
High-density polyethylene, namely, Alathon@~7030
~ (trademark of E. I. du Pont de Nemours & Co. for polyethylene
7 resin), having a density of 0.96 and a melt index of 2.8 in the
amount of 60 parts by weight is melt blended with 40 parts by
weight of kaolinite clay (Harwick GK Clay) and cast into two --
~` sheets each 30 mils thick, having dimensions of about 28.5 inches ---
in length and 21.5 inches in width.
One side of one sheet is embossed with a regular pat-
tern of raised ridges and valleys. The ridges and valleys run
parallel to each other traversing the width of the sheet in a
line perpendicular to the long axis, i.e., the longitudinal axis,
of the sheet.
The ridges and valleys have a general pattern similar
to that shown ln Fig. 2 with the exception that the pro~ections
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do-not have squared off corners but instead the pro~ections have
a cross section in the shape of a trapezoid.
After the sheets have been cast and the surface of one
sheet embossed, the sheets are placed together with the embossed
surface in the interface between the two sheets to form a preform.
The sheets are laminated under light pressure, during
- the heating step, by placing the sheets between two flat-surfaced
platens heated to a temperature of about 270F. The sheets are
left between the platens for a time that can vary from abo~lt 3.5
`~ 10 to 4 minutes. Depending on the residence time, the sheets them-
selves will be hested to a temperature of about 250 to 270F.
Fig~ 11 is a fragmented cross section of the preform
taken perpendlcular to the ridges and valleys. Referring to -
^ Fig. 11, the preform is made up of two laminated sheets 63 and
64. Sheet 63 is 30 mils thick and has two smooth surfaces and
sheet~ 64 has one embossed and one smooth surface. The embossed
surface is at the interface between the two sheets.
~l Sheet 64 has a thickness~ of 29 mils in the areas
~ where there are no pro~ections and a thic~ness of 39 mils in the ;
-~ 20 area of the pro~ections 65. The pro~ections themselves are 10
?. mi}s high and are spaced apart a distance`of-90 mils center~to- -
center. The trapezoidal section itself is 10 mils wide at the
~.
top, 21.6 mils wide at the bottom and has a height of 10 mils.
` The~heated preform is placed in a mold that is designed - --
to reprotuce a bobbin tray. The mold is at room temperature~ ----
, The mold is closet and a vacuum of about 29 inches of mercury is -
~ appliéd to the mold cavity. As the,~mold is evacuated, it draws
`yl air into the fluid passages in the preform through the marginal
j' edges o the preform. The air in the fluid passages separates
the ~heet~ distending them into the recesses of the mold cavity.
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~041891
Vacuum is applied for about 5 to ô seconds, the mold halves are
separated, and the finished article removed.
The thermoplastic articles of the present invention are
comprised of (a) at least two laminated thermoplastic sheets
wherein the interfacial surface of at least one sheet is provided
with fluid passageways and (b) distended and nondistended areas
in the thermoplastic sheets formed by a fluid introduced into the
fluid passageways between the two sheets to separate the sheets
and force the sheets into a desired configuration wherein the
opposing nondistended areas provide contact points to hold the
article together, ehe distended areas provide the article with
the desired shape and the combination of distended and nondis-
tended areas provide integral ribs that reinforce and contribute
to the structural integrity of the article. ~-
` Fig. 12 is a fragmented view of a hollow, rib-
reinforced, laminated structure in the form of a bobbin tray,
representing the preferred embodiment. Fig. 13 is a cross
section along line 13-13 of Fig. 12. The circular portions 66 of --
~ the tray provide recesses on both sides of the tray to contain
; 20 one end of a bobbin. With a recessed pattern on both sides of
, the tray, trays containing bobbins can be stacked on top of each
; other. The pattern of island-like recesses 67 and protuberances -
68 form ribs that add to the strength of the tray which is im- -
portant during the handling of the trays in use. In general, - ~
; the ribs can be short or elongated~ curved or straight, and - --form a pattern in one or multiple directions. These ribs and ~ ;
significantly to the structural integrity of the article.
Exam~e 2
A second embodiment will now be described with
particular reference to Figures 14 through 16.
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1041891
Two sheets of clay-loaded polyethylene, having the
composition described in Example 1, are laminated to form a
composite by heating the surface of the sheets and bringing
them together by means of a nip roll, or by broad area ultra-
sonic bonding. Each sheet is 45 mils thick and 31 inches long
by 24 inches wide. One sheet has an embossed pattern of ridges
having a height of 10 mils, and the composite has the configura-
tion shown in Figure 11 except that the ridges are spaced 100
mils center to center.
Referring to Fig. 14, the composite 69 is hung from
a clamp-air manifold mechanism 70 with the ridges in the
vertical direction, which mechanism in turn is suspended from
a conveyor chain (not shown). The clamp-air manifold both holds
the composite as it is suspended between two banks of radiant
heaters 71 and serves as an air manifold during the subsequent
thermoforming operation.
The heaters are inclined at a slight angle such that
the top and bottom of the heaters are spaced 12 and ô inches,
~ respectively, from the composite. mis spacing helps compensate; 20 for a chimney effect in which the bottom of the~-composite tends
to cool as the heat rises. Heat distribution is also regulated --
by a modulating screen 72 of wire screening or perforated sheet
metal placed between the heat source and the composite to shield
central portions of the lamlnate from excessive heating. A
perforated sheet metal screen having about 50% open area, tri-
angular openings which increase heat intensity of the vertical
and lower horizontal edges of the composite, and circular ~ ---
openings wh~ch increase heat intensity in a selected narrow
bsnd at~acent the upper horizontal edge of the composite, has
proven effective.
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~0418~1
The composite remains between the radiant heaters
for approximately 2 minutes and is heated to about 250 to 280 F.
Referring to Figure 15, the heated composite 69
still gripped by the clamp-air manifold mechanism 70 is placed
between two mold halves, 73 and 73a. Mold half 73 is mounted
on a stationary mold platen 74 which is firmly anchored to a
frame (not shown). Mold half 73a is mounted on a movable ~old
platen 74a which is actuated by a hydraulic ram 75. The mold
halves are designed to produce the bobbin tray of Figure 12 and
are at room temperature.
After the composite has been placed between the two
mold halves, a closing pressure of 6,000 to 7,000 pounds is
applied to mold half 73a by the hydraulic ram. This closing
pressure causes the mold to close to a first position at which
there is a gap between the mold halves approximately equai to
the thickness of the composite. The gap width is regulated
, by control springs 76 disposed along the rim of the stationary
- mold platen which engage the moving platen as the mold closes.
Pinch bars 77 are provided on the bottom edge and two --~
vertical edges (not shown) of the mold halves. The pinch bars ~ - -
are ad~acent to and extend about 7 mils beyond the face of the
mold halves so that they seal the bottom and two vertical edges
of the composite as the mold closes to its first position. A -
' sealed vertical edge is shown in Figure 16 which is a partial
horizontal cross-section taken through the composite and mold
j in the first position.
As the mold is closing, a vacuum of about 20 inches
mercury i8 applied to the mold cavity through holes provided ~-~
in the mold. me vacuum draws the sheets toward the mold cavity,
separating and distendlng the sheets into mold cavity recesses 78,
s ,
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104~891
and drawing ai~ into the recesses through the composite fl~id
passageways.
After the pinch bars have sealed all edges except the
upper horizontal composite edge held by the clamp-air manifold
mechanism, warm air under 15 to 20 psi pressure is fed from the
air manifold, through the composite fluid passageways, and into
the mold ca*~ty recesses. The warm air pressurizes the tray,
preventing collapse of hollow portions of the final product
when cooled. The pressurized air also assists in achieving
mold replication.
A closing pressure in excess of 7,000 pounds is then
applied to advance mold half 73a lO mils forward toward mold
half 73. During this high pressure stage the two tray halves
are permanently welded together, the fluid passageways are
elimlnated by flattening of the pro~ections, and the fourth
edge of the composite is sealed.
me total time required for both the low and high
pressure mold stages i8 2 1/2 to 3 seconds, after which the
mold is held closed to allow the formed tray to cool. The mold
, 20 halves are then separated and the finished article is removed.
The process of the present invention is particularly
adapted for the manufacture of thermoformed articles having a
complex pattern of hollow spaces which rib-reinforce the article.
Rapid cycles can be achieved since the fluid passageways pro-
vided in the composite preform permit use of a pressure assist
to vacuum drawing~ The use of pressurized fluid gives better
mold replication than that achieved through vacuum drawing alone,
and permits thermoforming of sheets not formable by vacuum alone.
The process obviates the need for using a patterned
nonatherent material (stop-weld) on the interfacial surfaces,
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eliminating the requirement that the preform be precisely
- registered with the mold. Moreover, the process avoids the
need to insert a fluld supply means at locations of the hollow
portions of the thermo~ormed articles, likewise, eliminating
registry problems.
-~ While the process has bccn described in detail with
respect to use of a sheet which has been extruded through a
serrated die, embossed, or otherwise marked to provlde the fluid
passageways, the passageways can also be formed by other methods
which form integral pro~ections on the sheet, e.g., by melt
extruding a contlnuous or discontinuous pattern of polymer onto
the sheet, such as patterns shown ln Figs. 2 through 4. By ~-
"integral" is meant that the proJections flrmly adhere to the
sheet-sothat they are not dislodged before the sheet is thermo- -
formed. When melt extruding polymer onto the sheet to form the
proJections, a polymer is generally selected whlch will thermally
bond to the sheet prior to or during the thermoformlng proces~.
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