Extrusion die for rigid foam sheet

An extrusion die for foam sheet including a die body defining a manifold for conducting a split stream of heated and pressurized foamable thermoplastic melt through separate distribution channels to a die terminus. A pair of mating die lips are arranged on opposed sides of the melt and are constructed to adjustably slide toward and away from each other to vary the flow of a melt. The lips are each mounted to slide along an axis which is tilted rearwardly at an acute angle relative to an orthogonal to the sheet being formed. The mandrel is configured and adapted for quick withdrawal and replacement from an end of the die through a passageway extending through the die transverse to the direction of extrusion of a sheet.

TECHNICAL FIELD 
The present invention relates generally to extrusion apparatus, and more 
particularly to an extrusion die for rigid foam sheet products. 
BACKGROUND ART 
In recent years the marketplace has exhibited an increasing demand for 
rigid expanded thermoplastic products in the higher density ranges, 
particularly in geographical areas experiencing a shortage of raw wood. 
Such products include decorative moldings and the like, but more 
particularly larger panels for use as cabinet doors, room doors, 
furniture, packaging, construction board, and the like. 
It is important in such applications for rigid foam sheet products that the 
product have a smooth and hard outer skin, that the sheet have favorable 
mechanical properties, and low manufacturing cost. The cell size and the 
cross-sectional uniformity of the cells from the inside to the surface of 
the sheet will determine the density of the material as well as its load 
bearing characteristics and its weight, all of which are important 
properties for its intended use as a wood substitute. As used herein, the 
term "rigid foam sheet" is intended to refer to any foam sheet which has a 
hard or firm external skin, whether or not the sheet has a hardened center 
sheet-like core as is typical of many extruded foam sheet products 
manufactured by inward-foaming extrusion processes. 
The present invention concerns an extrusion die for use in the continuous 
manufacture of thermoplastic rigid foam sheet products by a process known 
as "foaming inwards" or "inward-foaming" foam extrusion. The method of 
"foaming outwards" is an older, simpler method in which a thermoplastic 
polymer with a foaming agent is extruded through a wide slit. The hot, 
soft, still-foaming extrusion material is led directly to an in-line 
calibrating facility where the process of expansion of the foam is stopped 
by contact of the foaming extrudate with a cooled surface. 
The inward-foaming process, sometimes termed the "Celuka" process, is 
described in a 1973 U.S. Pat. No. 3,764,642to Boutillier. It has the 
advantage over other extrusion processes used to form rigid foam sheet of 
not requiring any calendaring or other post processing to form a hard skin 
on the sheet. 
In the '642 process, a mandrel or "torpedo" is employed in the extrusion 
die to split the path of the melt and create a hollow space inside the 
melt into which the melt can foam (hence the term "inward-foaming"). The 
outer surfaces of the melt are cooled by a closely following cooled 
calibrator, forming a skin in which no foaming takes place. The melt foams 
into the hollow form towards the center of the melt until the hollow is 
filled and an equilibrium counter pressure is created inside the melt. 
The Celuka inward-foaming process described in the '642 patent is discussed 
in U.S. Pat. No. 4,399, 086 to Walter. The '086 Walter patent states that 
by use of the inward-foaming process, the density of the foamed material 
can be varied by appropriate choice of parameters such as torpedo shape 
and size, cooling rate and withdrawal rate of the melt, composition of the 
melt, etc. The resulting outer skin is relatively thick, however, and the 
foamed core layer contains cells widely varying in size, and cell 
diameter. The structure is said by Walter to often be non-homogeneous, in 
particular in the cross section at the center line where the two inwardly 
foaming layers meet. "Not infrequently a laminar, linear texture, or even 
internal separation of the material can be observed at the center line. 
The resultant weakening of the core can be of disadvantage in applications 
where the sheet is subject to high mechanical stressing and rigidity is 
especially important." 
Importantly, the '086 patent criticizes the inward-foaming process for its 
inability to produce thin sheets more than 1000 mm wide and less than 10 
mm thick, except at great expense. Two reasons are given: 1) it is not 
possible to create the space in the die with a torpedo which has a very 
small height to width ratio, and 2) the outer skin cannot be produced 
thinner than a certain value due to the intensive cooling needed to create 
the hollow section during calibration. 
The Walter '086 process purports to overcome the problems of the 
inward-foaming process by a process in which the polymer melt is permitted 
to foam unrestrained, and is then post-processed by sequential application 
of cooling and heat to create the desired properties of the resulting foam 
product. 
The January 1994 issue of Modern Plastics, pages 21-22, describes an 
extrusion die of the inward-foaming type manufactured by Cincinnati 
Milacron. The Milacron system utilizes an adapter after the extruder which 
horizontally splits the melt stream. The resulting dual melt streams are 
guided to a ringed lip and mandrel system which is said to eliminate the 
need for a strut or spider to support the mandrel. 
The melt passes over the die lips which are oil-cooled to 140 degrees. The 
material then enters a 20 mm precalibrator where the outer edge is cooled 
so quickly that the melt (PVC) does not have a chance to bubble. 
The lips are of the flexible hinge adjustable type. The Milacron die is 
believed to be capable of producing rigid foam sheet no thinner than 10 
mm. 
Despite the enumerated favorable attributes of prior art rigid foam 
extrusion dies of the inward-foaming type, shortcomings exist. No known 
prior rigid foam extrusion die is capable of producing acceptable rigid 
foam sheet in thickness less than 10 mm. 
The Milacron die can handle a very limited range of extrusion slit gauge 
adjustments due to the use of flexible-hinge-type adjustment lips. To make 
a significant change in sheet gauge with the Milacron die requires 
effectively disassembling a major part of the die and replacing major 
components, including the adjustable lips, with consequent down time and 
component cost burden. Flow and pressure control is inadequate to produce 
acceptable rigid foam sheet product across in a wide range of foam sheet 
gauges. 
U.S. Pat. No. 4,797,083 to Reifenhauser depicts an extrusion die of the 
inward-foaming type which utilizes an elongated melt-splitting mandrel 
supported without ribs. The elimination of such ribs is said to avoid 
undesirable texturing of the sheet caused by such ribs and non-uniform 
foaming in the edge regions of the extruded product. Flow control is 
provided by adjustable flow control members positioned in throttling 
passages in each of the melt streams. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, an extrusion die for rigid foam 
sheet includes a die body defining a manifold comprising separate 
distribution channels for conducting a split stream of heated and 
pressurized foamable thermoplastic melt to a die terminus where the melt 
is caused to merge and foam. A pair of mating die lips are located at the 
die terminus and are arranged on opposed sides of the melt. The lips are 
constructed to adjustably slide toward and away from each other to vary 
the flow of the melt. In one aspect of the invention, the separate 
distribution channels defined by the die body means are configured to 
merge within the die to create a chamber between the lips in which the 
melt foams. In another, the separate distribution channels defined by the 
die body means are configured to cause the melt to merge and foam outside 
the die. 
Preferably, the lips are each mounted to slide along an axis which is 
tilted rearwardly at an acute angle relative to an orthogonal to the sheet 
being formed. According to another aspect of the present invention, the 
nearest surfaces of the separate distribution channels are defined at 
least in part by an intervening mandrel. The mandrel is preferably 
configured and adapted for quick withdrawal and replacement from an end of 
the die through a passageway extending through the die transverse to the 
direction of extrusion of the sheet. To change the thickness of a sheet 
product to be extruded, the operator need merely quick-change the mandrel 
and adjust the separation of the lips. 
The die passageway in cross-section desirably has a portion which is 
convergent toward the rear of the die and the mandrel has a mating 
cross-sectional configuration. The mandrel is adapted to be quickly and 
securely locked into the convergent portion of the passageway by a screw 
or other attachment means applied from the rear of the die. 
Desirably, the extrusion die of the preferred form of the invention 
includes a pair of cooled precalibration blocks for assisting in forming a 
skin on the outer surfaces of the melt, the precalibration blocks being 
affixed to the lips to move therewith. Preferably, an air gap is formed at 
an interface between each of the precalibration blocks and the lip to 
which it is affixed to enhance the thermal isolation of the precalibration 
block from the lip. 
In a preferred implementation of the principles of the invention, a pair of 
adjustable restrictor bars are located in the die upstream of the lips for 
regulating the pressure and melt flow in the separate channels in the 
region of the lips. 
The extrusion die of the present invention is capable of producing quality 
rigid foam sheet product in a wide range of thickness from at least 30 mm 
to 3 mm or less, thus overcoming the serious small gauge sheet 
capabilities of prior inward-foaming extrusion dies. Through the 
convenience offered by the quick-change, side-exit mandrel and the dual 
slide-action adjustable lips, set-up time and cost to change the gauge of 
foam sheet to be produced is substantially reduced. 
Further, other limitations of prior inward-foaming extrusion processes 
identified in the Walter '068 patent--too-thick skins, center separation, 
and poor mechanical properties--are also overcome by the die of the 
present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1-3 illustrate an extrusion die 8 for making rigid foam sheet in 
accordance with the teachings of the present invention, comprising an 
upper die body 10, a center die body 11 and a lower die body 12. The upper 
die body 10 and center die body 11 define an upper manifold 14. The center 
die body 11 and lower die body 12 define a lower manifold 16. The upper 
and lower manifolds 14, 16 conduct a split stream of heated thermoplastic 
melt from an extruder (not shown) through separated upper distribution 
channel 18 and lower distribution channel 20 to a die terminus. 
At the die terminus, the nearest surfaces 22, 24 of the upper and lower 
distribution channels 18, 20, respectively, are defined by an intervening 
mandrel 26. The remote surfaces of the upper and lower distribution 
channels 18, 20 in the terminal region of the die are constituted by the 
faces 28, 30 of upper lip 32 and lower lip 34, respectively. (See FIG. 3.) 
Referring particularly to FIG. 2, in accordance with an aspect of the 
present invention, the mandrel 26 is configured and adapted for quick 
withdrawal and replacement from an end of the die through a passageway 36 
extending through the die 8 transverse to the direction of extrusion of 
the foam sheet. (The foam sheet is not shown.) 
Specifically, the passageway 36 formed in the center die body 11 in cross 
section has a portion 38 (see FIG. 3) which is convergent toward the rear 
of the die body. The mandrel 26 has a configuration in cross-section with 
a male convergent portion 40 which mates with the female convergent 
portion 38 of the die body 11, forming a keyway configuration which 
assures secure and geometrically accurate positioning of the mandrel 26 in 
the die body when properly located in the passageway 36. 
In order that the mandrel may be quickly and securely locked into the 
convergent portion 38 of the passageway 36, attachment means such as a 
series of bolts, one of which is shown at 41, engage the convergent 
portion 40 of the mandrel 26 through a rear opening 44 in the die body 11. 
(See FIG. 3.) 
A pair of end seal plates 46, 48 are held in place against a gasket 49 by 
end seal retainer plates 50, 52 which are in turn secured by bolts 42. The 
end seal plates 46, 48 are separated by adjustment screws 47 which are 
turned to adjust the pressure of end seal plate 48 and gasket 49 against 
the die body. The mandrel 26 can be quickly removed and replaced with a 
mandrel adapted for a different gauge sheet or different properties, or 
removed for maintenance and/or repair, by simply removing the end seal 
plates 46, 48 and sliding the mandrel 26 out of the passageway 36 in the 
center die body 11. 
The mandrel 26 has a conduit 54 throughout its length for conducting a 
coolant. A coolant supply is coupled to the mandrel 26 by means of a 
fitting (not shown) connected to the conduit 54 through openings in the 
end seal plates 48, 50. The opening in end seal plate 46 is shown at 56. 
In accordance with an aspect of the present invention, the upper and lower 
lips 32, 34 are constructed and arranged to adjustably slide toward and 
away from each other to vary the melt flow in the upper and lower 
distribution channels 18, 20. The upper lip 32 is mounted to slide along 
an adjustment axis 58 (FIG. 3) by rotational adjustment of a series of 
like lip adjusting spools 62. The spools 62 each comprise a unitary 
collar-sleeve-nut member 63 which turns on a threaded shaft 64 screwed 
into a threaded bore in the upper lip 32. 
A series of like lock bolts 67 secure the upper lip 32 firmly against the 
upper die body 10 when the upper lip 32 has been set to the desired 
position by adjustment of spools 62. The lock bolts 67 can be set to 
immobilize the lip, or can be set just firmly enough to permit adjustment 
of the lip without fluid leakage. 
The lower lip 34 is similarly adjustable along an adjustment axis 68 and, 
like the upper lip 32 described immediately above, is adjustable in 
position and longitudinal configuration by means of a series of lip 
adjustment spools, one of which is shown at 70. The lower lip 34 is 
secured in place by a series of like lock bolts 72. 
The upper and lower lips 32, 34 are cooled by introduction of a coolant 
from a supply (not shown) into conduits 66, 69 in the lips 32, 34 through 
fittings (not shown) passing through openings 71a, 71b, 73a, 73b in end 
seal plates 46, 48. The openings 71a, 71b, 73a, 73b are angled at the tilt 
angle 65 of the upper and lower lips 32, 34 to permit the lips to be 
adjusted without interfering with the coolant fittings. 
It is known to use an adjustable lip having a construction somewhat similar 
to that described above in the manufacture of large gauge non-foamed 
thermoplastic sheet products, however, in accordance with a novel aspect 
of the present invention, the adjustment axes 58, 68 of the adjustable 
lips 32, 34 are tilted rearwardly at an acute angle relative to an 
orthogonal to the sheet being formed. In the illustrated preferred 
extrusion die embodiment, the adjustment axes 58, 68 of the upper and 
lower lips 32, 34 are both tilted rearwardly by the same tilt angle 65. 
The tilt angle 65 is preferably between about 10 degrees and 20 degrees to 
a line orthogonal to the foam sheet being extruded. In a die successfully 
tested, the tilt angle 65 is 15 degrees. 
In accordance with another aspect of the invention, referring particularly 
to FIG. 3, the face 28 of the upper lip 32 has a forward face portion 74 
which is parallel to the sheet being formed, and a rear face portion 76 
which forms an acute angle with the plane of the sheet. The lower lip 34 
has a similar forward face portion 77 and rear face portion 79. This 
truncation of the rear corners of the adjustable lips 32,34 improves the 
streamlining of the upper and lower distribution channels 18, 20, 
particularly when the die is set up to produce small gauge foam sheets. 
Upper and lower precalibration blocks 78, 80 are affixed to the upper lip 
32 and lower lip 34, respectively, by means of a series of like bolts, 
shown at 82 for the block 78, and at 84 for the block 80. The 
precalibration blocks 78, 80 have conduits 86, 88, respectively, formed 
therein for conducting coolant through the blocks 78, 80. 
It is often desirable that, in order to enhance the formation of a hard 
skin on the foam product being formed, the temperature of the 
precalibration blocks 78, 80 is maintained at a significantly lower 
temperature than the temperature of the adjustable lips 32, 34. 
In order to improve the thermal isolation between the blocks 78, 80 and the 
lips 32, 34, recesses are provided in the blocks 78, 80, respectively, to 
create air gaps between the blocks 78, 80 and lips 32, 34. The recess in 
the lower block 80 is shown at 92. 
In accordance with another aspect of the present invention, the extrusion 
die 8 includes a pair of adjustable upper and lower restrictor bar 
assemblies 96, 98 which are adjusted vertically and laterally in concert 
with vertical and lateral adjustments of the lips 32, 34 in order to 
control and properly profile the flow of thermoplastic melt in the 
terminal region of the die 8. 
The restrictor bar assemblies 96, 98 may have a structure which is similar 
to that found in restrictor bar assemblies for extrusion dies for 
non-foamed extruded thermoplastic sheet products. The upper restrictor bar 
assembly 96 comprises an upper restrictor bar 100 which is adjustable 
along a restrictor bar adjustment axis 102 by means of a series of like 
adjustment spools 104. The adjustment spools 104 may be constructed 
similar to the lip adjustment spools 62, 70 described above for adjusting 
the lips 32, 34. 
The upper restrictor bar 100 is located upstream of the upper lip 32 and 
regulates the melt flow in the upper distribution channel 18 in the region 
adjacent the upper lip 32. It is preferred that the adjustment axis 102 of 
the upper restrictor bar 100 be tilted rearwardly by the same angle as the 
tilt angle 65 of the adjustment axis 58 of the upper lip 32 such that the 
upper lip 32 and the upper restrictor bar 100 move in parallel. 
The lower restrictor bar assembly 96 is preferably constructed similar to 
the upper restrictor bar assembly 98. 
FIG. 4 illustrates another embodiment of the illustration wherein a mandrel 
110 adapted for the manufacture of very thin rigid foam sheet is 
substituted for the mandrel 26 of the FIGS. 1-3 embodiment, as described 
above. Upon quick-exchange of the mandrel 110 for an existing mandrel, the 
lips 115, 117 are simply readjusted to establish proper process conditions 
for the new run. 
As noted, employing the teachings of the present invention, to change the 
thickness of a sheet product to be extruded, the operator need merely 
quick change the mandrel and adjust the separation of the lips 32, 34. In 
contrast, using the extrusion die of Cincinnati Milacron discussed above 
in the BACKGROUND ART section of this specification, to make a significant 
change in the thickness of a foam sheet product to be produced, a major 
disassembly of the die is required. A complete replacement of the 
adjustable lip assemblies may be required, due to the limited range of 
motion of the flexible hinge type adjustable lip utilized in the Milacron 
extrusion die. 
FIG. 5 depicts yet another embodiment of the illustration wherein a mandrel 
114 is employed which is structured to terminate intermediate the forward 
and rearward ends of the lips. The distribution channels 116, 118 defined 
in part by the mandrel 114 thus merge within the die to create a chamber 
120 between the lips 122, 124 within which the melt foams. The FIG. 5 
embodiment, providing mandrel cooling of the nearest surfaces 126, 128 of 
the distribution channels 116, 118 only part way through the region of the 
lips 122, 124, is adapted to produce very thin rigid foam sheets with 
favorable mechanical properties. 
Numerous modifications and alternative embodiments of the invention will be 
apparent to those skilled in the art in view of the foregoing description. 
Accordingly, this description is for the purpose of teaching those skilled 
in the art the best mode of carrying out the invention and is to be 
construed as illustrative only. The details of the structure may be varied 
substantially without departing from the spirit of the invention, and the 
exclusive use of all modifications which come within the scope of the 
appended claims is reserved.