Method for producing microcellular foamed plastic material with smooth integral skin

The present invention is directed to the continuous production of microcellular plastic web material having integral unmodified laminar skin. In accordance with the invention a web of plastic material impregnated with an inert gas is delivered continuously to a degassing device in which the degree to which gas diffuses out of the surfaces of the web can be selectively and continuously controlled, whereupon the web enters a foaming station where it is reheated to induce foaming. The temperature of the reheated web and the duration of the foaming process prior to quenching are also selectively controllable to produce the desired web characteristics. The gas-impregnated plastic web can be produced initially by any of several novel means including a continuous device for impregnating a previously formed web and several versions of die devices which operate either by rapidly quenching the extruded gas-impregnated plastic material before any significant foaming has occurred or by maintaining the material above its foaming pressure within the die until the material has cooled to below the temperature at which foaming occurs.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to the production of foamed plastic 
material and more particularly to the production of sheets, webs or 
strands with integral unmodified smooth skin from microcellular foamed 
plastic material, by which is meant a plastic material having uniformly 
distributed voids or cells of very small size, i.e. on the order of 2 to 
25 microns. 
2. Description of the Prior Art 
As disclosed in U.S. Pat. No. 4,473,665, issued on Sept. 25, 1984, and 
assigned to the Massachusetts Institute of Technology, foamed plastic 
material with very small and uniformly distributed voids or cells can be 
produced by impregnating a plastic material under pressure with an inert 
gas, which nucleates and expands to provide the desired cellular structure 
when the material is at a temperature within the range of its glass 
transition temperature and the pressure is reduced. The resulting product 
is referred to as microcellular foam, which characterizes a product having 
uniformly distributed voids or closed cells of very small size, e.g. 2 to 
25 microns. In one embodiment, a batch process is disclosed in which a 
previously formed plastic sheet or other article is impregnated with gas 
under pressure, the pressure is reduced to ambient, the material is heated 
to a softening point to effect foaming, and the material is quenched to 
terminate foaming when the desired degree of foaming has been achieved. As 
soon as depressurization occurs, the absorbed gas begins to diffuse out of 
the impregnated plastic. Accordingly, for maximum foaming, the heating of 
the material to its foaming temperature should take place as soon as 
possible after depressurization. However, because the gas diffuses most 
rapidly from the surface regions of the material, an appropriate delay 
between depressurization and reheating of the material will result in a 
cellular core with an integral, unmodified laminar skin. Because the skin 
results from the inherent inability of this surface portion of the 
material to foam, the skin surface is defined by the extruder, and i.e. 
thus as smooth as the surface of a corresponding extruded unfoamed sheet. 
Furthermore, during the subsequent reheating of the sheet to induce 
foaming, the entire sheet can be heated to its softening or glass 
transition temperature, whereby the sheet surfaces are free to expand in 
response to the foaming, i.e., the entire sheet is free to expand in all 
directions without inducing significant stresses in the sheet to the 
above-described process. 
In contrast, there are basically two other previously known techniques for 
forming a skin on foamed plastic material. In one of these techniques, the 
foam surface is reconstituted by using heat and pressure to soften and 
recompress the surface material, which can be done in the foaming die 
itself or by a secondary operation. In the other technique, either before 
foaming, prior to or during the foaming stage, the material is brought 
into contact with a relatively cool surface, typically within the foaming 
die; so that the surface of the material is cooled below its foaming 
temperature before it has foamed and the resulting skin is then stretched 
by the expanding foam to accommodate the final size of the articles. While 
both of these techniques are commonly used to provide foamed articles with 
a very thin cosmetic skin, such a surface is generally not entirely 
smooth. Furthermore, if either of these techniques is employed to produce 
a substantially thicker skin, the cycle or processing time can become 
prohibitively long and the structural integrity of the material is likely 
to be impaired because of the unequal expansion, contraction and pressures 
experienced between the skin regions and the cellular core. Additionally, 
in using either of these two basic techniques, it is very difficult to 
selectively control the thickness of the desired skin without altering 
some other parameter that should preferably be determined by other 
considerations, e.g. extrusion speed or extrusion temperature. Such 
control is particularly important in the case of relatively thin webs e.g. 
10-25 mils which are made of a transparent material in which case it may 
be critical that the foamed core be sufficiently thick to provide uniform 
opacity but also that the integral laminar skin be as thick as possible to 
provide desired physical characteristics. 
To distinguish the material produced in accordance with the present 
invention, which employs the degassing process disclosed in the M.I.T. 
patent from that produced by the two techniques just described, the term 
"integral unmodified laminar skin" is intended to characterize the skin as 
being formed from the same parent material as the cellular core, as 
opposed to being laminated to the core (integral); to characterize the 
skin as not having been foamed and then reconstituted as not having been 
foamed and then stretched significantly at below its softening temperature 
(unmodified); and, to characterize the skin as having sufficient thickness 
to comprise a definite lamina, e.g. at least 1 mil (laminar). 
While the batch technique of the M.I.T. patent will produce the desired 
type of skin in an article with a microcellular foam core, it is obviously 
suitable only for experimental or very limited production purposes and 
cannot possible be economically viable for the production of commercial 
quantities of foamed plastic sheet or the like. 
An alternative process, disclosed in the same M.I.T. patent discussed 
above, involves extruding a web of pre-impregnated molten plastic material 
into a pressurized chamber, in which it passes first through a heated bath 
that prevents so-called "freezing-off", i.e. adhesion to the extruder die 
and, then, through a cooling bath, that cools it below its foaming or 
glass transition temperature. Thereupon, it passes through a pressure seal 
and into a reheating bath at ambient pressure, which reheats it to an 
appropriate temperature to induce foaming. The plastic can be impregnated 
either by previously exposing the plastic pellets to pressurized gas 
before they are introduced into the extruder or by injecting gas into the 
molten plastic within the extruder. It should be noted that the term 
"impregnated" as used herein does not necessarily mean that the cooled 
material has been completely saturated to the solubility limit when it is 
initially depressurized, but rather, that a substantial portion of that 
amount of gas is absorbed or dissolved generally homogeneously throughout 
the material. Although very desirable from a continuous production 
standpoint, this technique also suffers from several disadvantages, e.g. 
the difficulty in threading the initial extruded web along a convoluted 
pathway through inaccessible pressurized portions of the apparatus, 
difficulties in controlling tension within the apparatus, and problems 
associated with sealing the web passageway to prevent leakage of 
pressurization. More significant, however, is the fact that because the 
reheating of the web to produce foaming occurs immediately adjacent the 
exit seal, it is not possible to vary the diffusion of gas out of the 
material prior to foaming. Accordingly, the finished material has little 
or no unmodified laminar skin and there is no way in which skin thickness 
can be controlled except to a limited extent, by varying the extrusion 
speed and for the temperature of the foaming medium, either or both of 
which may be undesirable for other reasons. 
A somewhat analogous process for producing foamed extruded plastic material 
is disclosed in Japanese Patent Kokais Nos. SHO 59(1984)-169824, published 
on Sept. 25, 1984 and SHO 60(1985)-99629, published on June 3, 1985, both 
of which are assigned to Mitsubishi Yuka Co., Ltd., Tokyo, Japan. In 
accordance with these disclosures, a molten resin (e.g. polystyrene) is 
blended or impregnated with a volatile foaming or blowing agent and is 
extruded at elevated pressure into a long die. In the portion of the die 
nearest the extruder, the material is maintained under sufficiently high 
pressure to prevent foaming as the material is cooled to its optimum 
foaming temperature while passing along the die. To counteract the 
frictional resistance caused by the corresponding increase in viscosity of 
the material, a lubricant is injected between the die surfaces and the 
adjacent faces of the plastic material. A restriction is preferably 
employed at the end of the first portion of the die to aid in maintaining 
the pressure in that die region and, beyond the restriction, the die 
throat is expanded to define the size of the desired finished product. As 
the plastic enters the larger portion of the die, it foams and is further 
cooled to a dimensionally stable temperature before exiting the die, with 
its passage through the die still being assisted by the previously 
mentioned lubricant. 
Another closely related process, for providing foamed plastic insulation on 
electrical wire, is disclosed in U.S. Pat. No. 3,988,404, issued Oct. 26, 
1976 and assigned to the Furukawa Electric Co., Ltd., Tokyo, Japan. 
According to this disclosure, plastic pellets are pressurized with a gas 
in a two stage pressurization process and, while maintained under 
pressure, the pellets are fed to an extruder through which the electrical 
wire is drawn. As the material, cooled to its desired foaming temperature, 
is extruded out of the die around the wire, it foams to provide an 
insulating layer that is described as having minute and homogenous cells. 
In investigating the techniques disclosed in the foregoing patents assigned 
to Mitsubishi Yuka Co. and the Furukawa Electric Co., Ltd., we have found 
that smaller, more symmetrical and more uniformly distributed cells appear 
to be produced by reheating a rod or web of plastic material that was 
impregnated with gas and cooled to below its foaming temperature before 
being allowed to foam than by allowing the material, at a foaming 
temperature, to expand into an enlarged die region or into the atmosphere. 
More particularly, our observation has been that these latter prior art 
techniques tend to produce relatively elongate cells that are larger and 
less uniformly distributed; which adversely affects the physical 
properties of the finished product. Although other factors may be very 
significant, it seems likely that this difference in cell size, shape and 
distribution may be attributable principally to two distinctions between 
the prior art technique, and the technique to which the present invention 
is directed, namely that in the latter case the cells are nucleated and 
grown at a relatively low temperature, which produces smaller cells, and, 
that the reheating technique allows the material greater freedom to expand 
uniformly in all directions and thereby avoids internal stresses. It 
should also be noted that, any skin produced by these processes results 
primarily from either reconstituting the foam surface or from preventing 
foaming by chilling the surface of the material before the surface regions 
have foamed. In other words, such processes are not capable of producing 
integral unmodified laminar skin. 
SUMMARY OF THE INVENTION 
The present invention is directed to the continuous production of 
microcellular plastic web material having integral unmodified laminar 
skin. In accordance with the invention a web of gas-impregnated plastic 
material is delivered continuously to a degassing device in which the 
degree to which gas diffuses out of the surfaces of the web can be 
selectively and continuously controlled, whereupon the web enters a 
foaming station where it is reheated to induce foaming. The temperature of 
the reheated web and the duration of the foaming process prior to 
quenching are also selectively controllable to produce the desired web 
characteristics. The gas-impregnated plastic web can be produced initially 
by any of several novel means including a continuous device for 
impregnating a previously formed web with gas and several versions of die 
devices which operate either by rapidly quenching an extruded 
gas-impregnated plastic material before any significant foaming has 
occurred or by maintaining the material above its foaming pressure within 
the die until the material has cooled to below that temperature.

DESCRIPTION OF THE ILLUSTRATIVE PREFERRED EMBODIMENTS 
As shown in FIG. 1, the depicted illustrative system comprises a 
conventional plastic extruder 10 provided with a feed hopper 12 and 
connected by a nozzle member 14 to an extrusion die 16. As previously 
mentioned, the plastic material extruded into and through the die is 
impregnated with an inert gas (e.g. nitrogen, argon, carbon dioxide, etc.) 
either by previously impregnating plastic particles or pellets with gas 
under pressure and maintaining pressurization as they are fed into and 
stored in the hopper, or by introducing gas into the molten plastic within 
the extruder itself. Various plastic materials and gases appropriate for 
use in practicing the invention are disclosed and discussed in the 
previously mentioned U.S. Pat. Nos. 4,473,665 and in 3,833,625, issued on 
May 13, 1975 and assigned to the Dow Chemical Company. 
The illustrated die 16, is of the type employing a coating fluid, and will 
later be described in detail with reference to FIG. 2 Briefly, however, 
the die directs the incoming gas-impregnated molten plastic into the form 
of a web and cools it to below its foaming temperature before it emerges 
from the die. Control of temperatures within the die is provided by a 
heating control unit 18 and a cooling control unit 20. As previously 
mentioned, a coating fluid is introduced into the die by a pump unit 22 to 
provide a lubricant, a coolant and/or heat transfer medium and means for 
sealing and controlling extrusion forces exerted on the web. The coating 
fluid is preferably glycerine, but alternative coating fluid materials are 
disclosed in the above-mentioned Japanese Kokai No. SHO 59 (1984)-169824. 
Although the term "die" may conventionally imply only that part of a device 
that initially shapes a material to its desired form, the term "die", as 
used herein, includes whatever structure is employed beyond the point 
where the plastic material assumes its desired cross-sectional form to 
prevent it from foaming by maintaining it under pressure until it has 
cooled to below its foaming temperature. 
Upon emerging from the die, the web, designated by numeral 24, enters a 
washing unit, depicted at numeral 26, which removes the coating material 
from the web and which may also serve to further cool the web. This unit 
can include nozzles for spraying water on both surfaces of the web to 
dilute and wash away the coating material and to cool the web and 
appropriate squeegee blades, rollers, suction nozzles, etc., to remove the 
water and diluted coating material from the web. Because similar units are 
well known in the web cleaning art, further details are unnecessary for 
understanding the present invention; typical analogous units of this type 
being shown, for example, in U.S. Pat. Nos. 3,158,886 and 4,244,078. if no 
coating material is used, as in the connection with FIG. 4, the washing 
station is unnecessary and may simply be eliminated. 
Beyond the washing unit, the web is diverted by guide roller 28 to a 
degassing control device comprising fixed rollers 30, tension control 
roller 31 and movable rollers 32, the latter being supported on frame 33 
which is slideably carried by guide rods 34. Lead screw 35, selectively 
driven by motor 36, can move frame 33 and thereby rollers 32 between the 
extreme positions shown in broken lines at numerals 32' and 32". 
Accordingly, the position of frame 33 determines the length of web 
material that is present in the degassing apparatus, which, in turn, 
allows variation of the extent to which gas can diffuse out of the web and 
into the atmosphere between the die and the foaming station, which is 
discussed below. Although not shown, radiant heaters or the like can also 
be employed in the degassing control device, if desired, to selectively 
increase the rate at which the gas diffuses out of one or both faces or 
the web. However, because the fragility of the web increases as it is 
heated, it is preferable that it not be heated to near its foaming 
temperature any earlier than is necessary. 
Beyond the tension control roller, the web passes around a pair of drive 
rollers 37, which are driven by motor 38 in opposite directions as 
illustrated schematically by motor belt 39 and crossed belt 40. The 
tension control roller 31 is mounted by one or more arms 41 to a sensing 
unit 42 and is movable between the positions shown in solid and broken 
lines, as a function of the tension in the web. The speed of motor 38 is 
controlled by sensing unit 42, thus maintaining the web under a selected 
degree of tension. This schematically depicted control means is only 
representative of many devices that are well known in the tension control 
art and that could be employed in this environment. 
Beyond the drive rollers, the web travels upwardly into enclosure 43 of the 
foaming station, where it is guided by roller 44 between confronting 
radiant heaters 45 or equivalent heating devices, which bring it quickly 
to its desired foaming temperature to initialize foaming. The heated web, 
in the process of foaming and thereby expanding in all directions, is then 
guided around fixed rollers 46, movable rollers 47 and tension sensing 
roller 48, which are at approximately the same temperature as the web 
because of conduction of heat from the web to the rollers. The movable 
rollers 47 are carried by a frame 49 slideably supported on guide rods 50 
and adjustable by screw 51 driven by motor 52, to provide selective 
regulation of the length of the foaming path, in the same manner 
previously described in connection with the degassing device between the 
die and the foaming station. This arrangement allows control of the extent 
to which foaming has occurred before the web leaves enclosure 43 and 
encounters chill or quench rollers 53, which rapidly cool the web to below 
its foaming temperature, thus terminating foaming. 
The chill or quench rollers 53 are driven in opposite directions by motor 
54 and are internally cooled by circulating liquid to cool the web rapidly 
from both faces. Obviously, other cooling means could be employed in 
conjunction with or as alternatives to the liquid cooled rollers. Motor 
54, driving rollers 53, is controlled by sensing unit 55 and its tension 
sensing roller 48, in the same manner previously described in connection 
with sensing roller 31, and thereby maintains the hot and softened web 
under constant but very low tension between drive rollers 37 and chill 
rollers 53, while completely isolating that portion of the web from 
tension influences elsewhere in the apparatus. 
Because of the fragility of the expanding web in the foaming station, the 
rollers 44, 46, 47 and 48 are preferably driven by so-called tendency 
drives, which impart only enough torque through the rollers to maintain 
their required rotational velocity without significantly tending to either 
retard or accelerate the web. Although many ways of providing such a drive 
are well known, a convenient expedient is to support such a roller by 
antifriction bearings on a shaft that, in turn, is rotated just slightly 
faster than the desired roller speed, so that the antifriction bearings 
transmit enough torque to the roller to maintain its rotation but cannot 
transmit any significant forces from the shaft to the web or vice versa. 
In the case of the sensing roller 48, the roller shaft could be tendency 
driven in this manner by a flexible shaft or by a belt with its other 
pulley concentric with the pivot axis of the roller support arms. 
Alternatively, the web might be supported in the foaming station by air 
bearing means rather than by rollers. If heated rollers are employed 
instead of or supplementary to the radiant heaters 45, such rollers should 
also preferably be tendency driven. As previously mentioned, the tension 
sensing arrangement employing roller 48 is only representative and other 
analogous devices are well known that completely eliminate this concern 
and that impose substantially no tension on the web. For example, the web 
can simply be allowed to hang in a loop between two tendency driven 
rollers and the position of the loop can be monitored by one or more 
photodetectors. 
Below the chill rollers, the web is guided by rollers 56 and 58 through a 
final cooling station 60 in which cold air or other conventional cooling 
means are used to cool it to substantially ambient temperature. Thereupon, 
the web is looped under tension control roller 62 and is guided by rollers 
64 to a take-up roll 66, driven by motor 68. As previously described, in 
connection with motors 38 and 54, motor 68 is controlled by sensing unit 
70 associated with tension control roller 62 so that the web is wound on 
the take-up roll under substantially constant tension, independent of the 
web tension in other parts of the apparatus. Preferably the roll is wound 
relatively loosely so that any gas remaining in the web material will 
diffuse out of the plastic to the atmosphere, thereby insuring that no 
further foaming will occur if the web is reheated in a subsequent 
thermoforming operation. 
It should be understood that FIG. 1 is a schematic representation of means 
for practicing the present invention and that there is not necessarily any 
direct correlation between specific illustrated features and the 
corresponding features of an actual production apparatus. For example, the 
illustration may differ from an actual installation with regard to 
specific types and relative sizes and locations of components, the 
direction of web movement at various locations, and the relative lengths 
of web materials at various stages of the processing operation. Also, it 
should be apparent that, although not illustrated or specifically 
described, various sensing devices can be employed at different locations 
to sense conditions such as web temperature, web velocity, web tension, 
web expansion, web density, die pressures and temperatures, etc., and that 
data from such sensors can be used to provide continuous automatic control 
of various process parameters to produce a finished product having 
predetermined characteristics. 
The illustrative die 16 shown in FIG. 2 comprises two accurately machined 
die plates 82 and 84 that are bolted together along their top and edge 
margins by a plurality of bolts or cap screws 86. The die plate 82 is 
provided with an inlet port 88 into which is threaded a nozzle member 90 
that delivers gas-impregnated molten plastic to the die from an extruder. 
Opposite port 88, die plate 84 is recessed to define a transition cavity 
92 that directs the plastic material into an elongate web forming die 
passageway 94, which is provided by a shallow slot in die plate 84. 
Near the lower portion of the transition cavity, the die plates 82 and 84 
include respective thermal barrier slots 96 and 98, which serves to reduce 
thermal conductivity of the die plates between the portions of the die 
above and below those slots. Stiffening plates 100, connected to the die 
plates by bolts 102, span the thermal barrier slots to avoid undesirable 
weakening of the die by the slots. 
Above the thermal barrier slots, the die plates include passageways 104 
which are provided with electrical heating elements or cartridges 106 fr 
heating the upper portion of the die. Alternatively, such heating can be 
achieved by circulating hot oil or the like through these or similar 
passageways. In either event, the temperature of the heated portion of the 
die is controlled by heating control unit 18, shown in FIG. 1, to thereby 
control the viscosity of the plastic material in the transition cavity, so 
that it flows smoothly and evenly into the web forming passageway. Below 
the thermal barrier slots, the die plates are provided with coolant 
passageways 107, through which coolant of a controlled temperature and 
volume is circulated by cooling control unit 20, shown in FIG. 1, to cool 
the molten plastic to below the temperature at which foaming will occur, 
before it emerges from the die. Toward the bottom of the die, die plate 84 
is notched to receive an insert plate 108, that is held in place by 
mounting screws one of which is shown at numeral 110. The inner face of 
the insert plate is normally co-planar with the adjacent passageway face 
of die plate 84 and the insert plate is weakened by a transverse slot 112 
so that it can be deflected inwardly by adjusting screws, one of which is 
shown at numeral 114. This adjustment allows the thickness of the passage 
to be selectively reduced at its lower end to restrict the flow of coating 
fluid out of the die. 
To lubricate the web, to provide a heat transfer medium between the web and 
the die, and to provide a sealing medium, a coating fluid, such as 
glycerin, is introduced through fluid inlet passageways 116 and 118, which 
are connected to an appropriate external manifold or the like, not shown. 
The fluid is supplied from pump unit 22, shown schematically in FIG. 1, 
which includes a high pressure pump and temperature control means. 
Passageways 116 and 118 communicate respectively with distributor slots 
120 and 122 in the die plates to distribute the fluid transversely across 
the respective faces of the web. Because the glycerin or other cooling 
fluid is of substantially lower viscosity than the plastic material, we 
have found that the fluid tends to migrate laterally toward the edges of 
the moving plastic material, thus providing a scarcity or absence of fluid 
along the center portion of the web below the fluid inlet slots. To 
counteract this tendency, the distributor slots are made wider toward the 
center of the web so that more coating fluid is delivered to the center of 
the web then to its edges. If desired, supplemental coating fluid openings 
can be provided along the center of the passageway 94 to provide 
additional coating fluid, as needed. The coating fluid pressure is 
selectively controllable and is normally regulated to be slightly higher 
than the pressure of the plastic materical at the distributor slots, thus 
insuring positive introduction of the fluid into the die and onto the 
faces of that material. Typically, when the plastic material emerges from 
the transition cavity and begins to be cooled, it shrinks slightly in 
thickness and width, which, augmented by compression of the material by 
the coating fluid, provides room in the die passageway for the coating of 
fluid on the surfaces of the solidifying web. However, the passageway can 
be made slightly thicker and wider below the fluid inlet slots to increase 
the thickness of the coating fluid layer or the passageway can be tapered 
slightly, both in thickness and width, if such a profile proves 
advantageous. 
Because of the several functions performed by the coating fluid, the 
optimum performance by the extrusion process requires proper balancing of 
a number of parameters. Briefly, the primary factors that must be 
considered in this regard are: (1) Because in this type of die the plastic 
web material must be cooled below its foaming temperature before emerging 
from the die, it is substantially solidified within the die passageway 94. 
Accordingly, lack of lubrication in the lower die section can cause 
adherence of the material to the die and/or sufficient frictional 
resistance to overcome the injection pressure, thereby either completely 
or partially plugging the die; (2) If the solidifying web experiences 
depressurization within the lower passageway before it is adequately 
cooled, foaming can be initiated within the die; and (3) If the coating 
fluid layer is too thick, the fluid will emerge from the die at greater 
velocity than the web and, because of viscous drag, will tend to stretch 
or possibly tear the solidifying web within the die. The interrelation of 
these and other less apparent factors is obviously very complex; for 
example, the lubricating, sealing and viscous drag effects of the cooling 
fluid are related to its viscosity, which is a function of its 
temperature, which in turn is influenced by the initial temperature of the 
injected plastic material, by the injection rate, by the heating and 
cooling of the different portions of the die and by heat transfer 
consideration. However, we have found that there is substantial latitude 
in balancing these factors and that high-quality foam material can be 
produced, even without continuous automatic control means, which, of 
course, are desirable for actual production operations. 
To initiate the extrusion process, the upper portion of the die is heated 
but no coolant is employed in the lower portion so that the latter is 
warmed by conduction. Then, the glycerin or other coating fluid is 
introduced into the die and allowed to flow out the bottom before molten 
plastic material is injected into the die. Initially, either ordinary 
plastic particles or pellets are fed into the extruder or, if a gas 
injection system is used, it is not yet activated. After web material is 
emerging from the die, pellets that were previously impregnated with gas 
are fed into the hopper or the gas injection system is activated, 
whereupon the coolant system, the pump unit and the lower die insert are 
adjusted until a satisfactory web of unfoamed gas-impregnated material is 
being produced. Once these adjustments have been determined for a 
particular type of plastic, the start-up operation can be repeated with 
minimum inconvenience or delay. 
When the die is producing a satisfactory gas-impregnated web, the latter is 
cut off and threaded through the various stages of the complete system 
previously described, thus completing the start-up process. 
The illustrative die shown in FIG. 3 includes a heated upper portion 124, 
which is similar to the corresponding portion of the prevously described 
die, including a transition cavity 126 terminating in a passageway throat 
128 that defines the thickness and width of the extruded web. Toward the 
lower end of the heated upper die portion, and below throat 128, the 
passageway for the plastic material is enlarged so that the faces of the 
plastic web, shown in broken lines at numeral 24, are spaced from the 
confronting die faces by a much greater distance than in the corresponding 
region of the previously described die, thereby providing the opposed 
coolant chambers 130 and 132. At their lower ends, the two die plates 134 
and 136 include respective integral lips 138 and 140 that cooperate to 
define a flange by which the die plates are bolted to a sealing unit, 
described in further detail below, in which the web passageway closely 
conforms to the web to reduce leakage of fluid around the web and to 
control hydraulic forces exerted on it. 
The glycerin or other coating fluid is introduced into chambers 130 and 132 
through respective inlet ports 142 and 144, which are piped to the output 
of a pump and temperature control unit as depicted at numeral 22 in FIG. 
1. Below the heated die portion, corresponding outlet ports 146 and 148 
communicate with the respective chambers 130 and 132 and are piped to 
return fluid to unit 22. As previously described, slots 150, are provided 
in the die plates to reduce thermal conductivity between the heated and 
cooled portions of the die. As shown by numerals 130' and 132' the coolant 
chambers 130 and 132 extend above the outlet ports beyond respective ribs 
152 and 154 which partially restrict the passage of fluid between the 
upper or transition portions of the chambers above the ribs and the lower 
or positive flow portions of the chambers below the ribs. 
As the plastic material passes between the coolant chambers, glycerin or an 
equivalent coolant is pumped upwardly through the chambers under 
sufficient pressure to prevent foaming and at an appropriate temperature 
to provide the required cooling of the web. The corresponding portions of 
the die plates are provided with passageways 156 for a 
controlled-temperature flow of cooling fluid, from cooling control unit 
20, which cools the die plates and also contributes to cooling the coating 
fluid. It should be apparent, however, the the actual cooling of the 
plastic material is attributable primarily to the circulating fluid within 
the chambers, which, in this region, is functioning principally as a 
coolant, in the conventional sense of the word, rather than as a medium 
for transferring heat to cooled portions of the die. 
Because of the partial isolation of the upper portions 130' and 132' of the 
respective chambers 130 and 132, by ribs 152 and 154, the fluid therein is 
relatively static or stagnant, i.e., it is not being positively 
circulated. Accordingly, by absorbing heat from the plastic and the 
adjacent portions of the heated upper die unit, this fluid becomes 
considerably hotter than that in the lower portions of the chambers and 
thus prevents the throat region of the die from being cooled sufficiently 
to cause the plastic to solidify prematurely and thereby adhere to or plug 
that portion of the die. If necessary, the upper chamber portions 131' and 
132' can be provided with venting means to allow vapor emitted by the 
plastic material to be bled out of the die. 
The sealing unit, below the cooling chambers, serves to maintain the web 
material under pressure, to further cool it, to prevent excess leakage of 
the fluid and to control hydraulic forces exerted on the plastic material. 
These considerations are essentially analogous to those described in 
connection with FIG. 2, but, in this case, the force controlling 
requirements are more stringent because the web material above the seal 
unit is essentially unconstrained. This implies, for example, that the 
hydraulic forces exerted on the plastic within the seal, i.e. the piston 
effect of the cross-section of the plastic material and the viscous drag 
forces exerted by the coating fluid, must be controlled relative to the 
extrusion rate so that sufficient tension is applied to the unsupported 
material to draw it through the die, so that it does not accumulate in the 
chambers by buckling under compression, while also insuring that the 
material between the chambers is not torn or stretched excessively. Also, 
because the web is only partially solidified in the chamber region, it is 
difficult and inconvenient, during start-up, to deliver the leading end of 
the web into and through a seal passageway of fixed dimension, even if the 
web is formed of impregnated plastic. Accordingly, the illustrative 
sealing unit provides a sealing passageway of variable thickness 
dimension, that can be opened to facilitate initiating the extrusion 
process and then adjusted to achieve optimum performance. 
The sealing unit comprises a rectangular open-top box structure bolted to 
the flange at the lower ends of the die plates by bolts 158. The bottom 
plate 160 of the box structures includes a relatively wide exit slot 162 
and is bolted to the walls of the box structure by bolts 164. The web 
passageway 166, through the sealing unit, is defined between the 
confronting faces of plate 168, which is readily movable to vary the 
passageway thickness dimension and plate 170, which is capable ot being 
manually adjusted but which normally is not disturbed once its position 
has been established for a particular extrusion operation. The edges of 
plates 168 and 170 are closely fitted to the confronting internal surfaces 
of the box structure and the die plate flange to substantially preclude 
leakage of fluid between those surfaces. Coolant conduits 172 communicate 
with coolant passageways 174 in plates 168 and 170, which are 
interconnected by other plate passageways, not shown, to allow coolant to 
flow in a serpentine path through the plates. The conduits 172 are 
connected by flexible hoses or the like to the cooling control unit and 
extend slideably through seals or glands 176 in the sealing unit which 
allow the conduits to move axially to accommodate corresponding movement 
of the plates 168 and 170. 
Adjusting wedge 178 is maintained against sealing unit wall 180 by tongues, 
not shown, which are received in corresponding grooves 182 in the front 
and rear walls of the seal unit to allow the wedge to move upwardly and 
downwardly in contact with wall 180. Slots in the upper and lower edges of 
the wedge, indicated by broken lines 184, accommodate the corresponding 
coolant conduits to prevent them from interferring with the movement of 
the wedge. The tapered inner face of wedge 178 is maintained in contact 
with the correspondingly tapered outer face of plate 168 by L-shaped 
tongues 186 on plate 168, received in mating slots in the wedge, not 
shown, which are parallel to its tapered face. Because plate 168 cannot 
move upwardly or downwardly, such movement of wedge 178 causes plate 168 
to move toward or away from plate 170, to vary the passageway gap. In the 
illustrated embodiment, this adjustment is accomplished by means of an 
adjusting screw 188 passing through a thrust-bearing seal 190 and threaded 
into bushing 192 in the wedge member. The screw, in turn, is connected to 
a gear reduction motor or an equivalent device, as shown schematically at 
numeral 194, for example, by bevel gears 196. Thus, the passageway 
adjustment can be controlled from a remote location or can be effected by 
an automatic control device. 
To adjust the position of plate 170, bolts 198, located in two or more 
rows, extend through sealing unit wall 200 and are threaded into plate 170 
and similarly located bolts 202, are threaded through wall 200 and abut 
against plate 170. As shown at numeral 204, the shanks of both types of 
bolts are provided with sealing rings 204, to prevent leakage of the 
coating fluid. Accordingly, by appropriate adjustment of the bolts, plate 
170 can be moved toward or away from plate 168 and can be tilted slightly, 
if desired, to locate it parallel to that plate or to intentionally 
provide a slightly tapered passageway. 
Although not illustrated, additional coating fluid can be provided to 
plates 168 and 170 by appropriate conduits provided with packing glands 
corresponding to glands 176. It should be noted that a slight amount of 
leakage of the coating fluid can occur around the edges of plates 168 and 
170, but this is actually beneficial in that it hydraulically balances 
forces exerted on the opposite sides of those plates so that the flatness 
of the plates is substantially independent of any distortion of the 
box-like die housing. 
It should be noted that the illustrative sealing unit can also be used in 
connection with the previously described process for preparing 
microcellular foam by pressurizing a preformed sheet or web in a pressure 
vessel, in which case such a seal can be used in conjunction with 
appropriate coating fluid supply means for conducting a sheet or web into 
and/or out of such a vessel. 
The start-up procedure for this die construction is generally similar to 
that previously described in connection with the die shown in FIG. 2. 
After the passageway of the seal unit has been fully opened, sufficient 
coating fluid is admitted to coat the faces of the seal plates and molten 
plastic that is not impregnated with gas is then extruded out of the upper 
die unit to form a web that moves downwardly, assisted by gravity, and 
passes through the open seal unit. Thereupon, the sealing unit is 
gradually closed and the rate of flow of the coating fluid is 
simultaneously increased until the desired condition is achieved, 
whereupon the gas-impregnated pellets are fed to the extruder or the 
impregnating gas is introduced into the extruder and final adjustments are 
made to optimize the quality of the cooled gas-impregnated web emerging 
from the seal unit. 
The two die arrangements described above both operate on the principle of 
maintaining the gas-impregnated extrusion at a sufficient pressure to 
prevent foaming until after it has cooled to below foaming temperature, 
which is the same basic principle disclosed in the previously mentioned 
U.S. Pat. No. 4,473,665. As opposed to this principle, the embodiment 
shown in FIG. 4 relies on our discovery of an entirely different and 
unexpected phenomenon, namely that the gas-impregnated material, which is 
pressurized above its foaming pressure within the extrusion die, can be 
extruded into atmospheric pressure at its foaming temperature and can 
nevertheless be prevented from foaming if it is quickly cooled to below 
its minimum foaming temperature. Notwithstanding theoretical explanations 
of the mechanism by which the foaming takes plate, as described for 
example in the U.S. patent just mentioned, it is doubtful that this 
mechanism is fully understood. However, it seems reasonable to conclude 
that this process is possible because the finite time required for the 
internal gas to migrate to the nucleation sites after depressurization and 
to initiate foaming is of sufficient duration to allow the relatively thin 
web or the like to be cooled throughout to below its foaming temperature 
before foaming commences or, at least, before any significant degree of 
foaming has occurred. A corollary to this concept is the fact that the gas 
tends to diffuse out of the sheet very rapidly at the extrusion 
temperature, but the rate of such outgassing is greatly reduced as soon as 
the outermost layers are cooled sufficiently to provide a barrier to the 
escape of the gas. 
To carry out this process, the illustrative embodiment of the invention 
shown in FIG. 4 includes a die member 206, substantially identical to the 
upper die portions shown in FIGS. 2 and 3, comprising die plates 208 and 
210, heating passageways 212 and a transition cavity 214 terminating in an 
extrusion passageway or throat 216 that defines the desired cross section 
of the extruded material. Below die member 206, an insulating plate 218 is 
attached to the die member by bolts 220 and is provided with a tapered 
opening 222 communicating with extrusion passageway 216 but insulating the 
face of the die member surrounding that passageway. The insulating plate 
218 also serves to support a cooling or quenching unit 224, attached to 
plate 218 by bolts 228. The quenching unit comprises a rectangular inner 
tube 230, the walls of which are spaced from both the faces and the edges 
of the extruded material passing through the unit. The upper portion of 
tube 230 is provided with opposed chambers or manifolds 232, which receive 
refrigerated coolant, preferably water, at regulated temperature and 
pressure from the cooling control unit 20 shown in FIG. 1; through conduit 
234 and through slotted distribution tubes 236. As shown at numeral 236, 
the cooling or quenching unit is provided with an insulation coating to 
minimize heating of the unit by the surrounding atmosphere. Because only 
relatively low coolant pressures are encountered within the cooling unit, 
it need not be constructed to withstand high pressure, as in the case of 
the cooling portions of the previously disclosed embodiments. 
The walls 238 of the illustrative inner tube 230 are sloped to provide a 
tapered passageway and are provided with downwardly directed slots or 
nozzles 240 through which the refrigerated coolant is directed into 
contact with the corresponding faces of the plastic material. The size of 
the slots in relation to the coolant flow rate is such that the passageway 
is substantially filled with coolant, which flows freely out of the bottom 
end thereof, and the use of a plurality of slots or nozzles helps to 
agitate the coolant in contact with the plastic material to improve the 
rate of heat transfer so that the material is chilled throughout to below 
its foaming temperature as quickly as possible. 
As mentioned earlier, it is important that the surfaces of the die adjacent 
the extrusion opening not be in contact with the coolant to avoid 
contamination or plugging of the passageway. Accordingly, the insulating 
die plate is provided with drainage passageways 242, which, in cooperation 
with the downwardly directed coolant flow produced by the slots, or 
nozzles 240, prevents the coolant level from rising above the drainage 
passageways, thus insuring that there is always an air space adjacent the 
die. Any fluid passing through passageways 242 drains into coolant pan 
244, through appropriate conduits 246. 
Preferably, the plastic material is cooled to below its foaming temperature 
before emerging from tube 230 but it nevertheless may still be within a 
temperature range at which it is quite fragile or at which diffusion of 
gas out of its surface may take place more rapidly than desired. 
Consequently, the lower end of tube 230 extends slightly below the coolant 
level, shown by broken line 247 in pan 244. The plastic web is guided 
through the coolant in the pan by rollers 248 and 250 before it is 
directed upwardly out of the coolant beyond roller 250. If the coolant 
employed must be subsequently removed from cooled web, e.g. a coolant 
other than water which might be employed to lower the coolant temperature 
below 0.degree. C., the web is then directed through a cleaning station as 
previously described in connection with FIG. 1. However, if water is 
employed as a coolant, it can be removed simply by wiper blades 252 or by 
equivalent means and the washing station is not needed. 
Because the successful performance of this embodiment of the invention 
depends on the rapidity with which the self-insulting plastic material can 
be cooled to below its foaming temperature, its use is limited by the 
cross-section of the extruded material, but, with a web of high impact 
polystyrene approximately 10 mils thick, impregnated with Nitrogen and 
extruded at a temperature of approximately 420.degree. F. and an extrusion 
velocity of 6 to 8 inches per second, we have achieved good results by 
simply extruding the material into a bath of cold water (approximately 
50.degree. F.) with a distance between the die lips and the water bath of 
about 0.125 inches. By increasing the extrusion velocity which thereby 
correspondingly increases the length of the path available to cool the 
extruded material, it is apparent that substantially thicker material can 
be satisfactorily cooled. Also, it should be obvious that many known 
cooling techniques might be employed, alone or in combination to achieve 
faster heat transfer, e.g. ultrasonic agitation of a cooling fluid, 
gaseous cooling, electrostatic cooling as described in U.S. Pat. No. 
3,224,497, etc. Similarly, it should be recognized that the cooling means 
need not be integral with or directly connected to the die structure but 
that a conventional die could be used to extrude the plastic material into 
a quenching bath or other cooling device. Although a slight amount of 
foaming may occur in the core of the web, because of the temperature 
gradient of the cooling through the thickness of the web, this does not 
negate the concept of producing an "undisturbed" skin because the entire 
web is reheated to at least the softening point of the plastic during the 
foaming process, thus relieving whatever slight strain may be present as a 
result of such relatively insignificant initial foaming. 
The start-up procedure for the process just described does not require the 
initial extrusion of plastic material free of gas, but can be carried out 
simply by extruding the gas-impregnated material from the die into the 
cooling unit, which is supplied with coolant prior to commencement of the 
extrusion process. If desired, however, the previously described two-stage 
procedure may be employed to insure against premature foaming, e.g. 
because of low initial extrusion speed or high initial material 
temperature. 
With regard to the relationship between cooling rates and the material 
thickness, an interesting variation of this technique is its use in 
forming an insulating coating on a wire, e.g. as disclosed in the 
previously mentioned U.S. Pat. No. 3,988,404. In this variation, the wire 
fed through the annular die is itself precooled to a substantially low 
temperature and is insulated from the die by an insulating sleeve so that 
it absorbs heat inwardly from the plastic material extruded around it, 
which is also cooled externally by a cooling bath or the like, as 
discussed above. In this way, a relatively heavy layer of gas-impregnated 
plastic material can be coated on the wire and can subsequently be 
reheated to provide a correspondingly thick coating of foamed insulation 
with an external skin of desired thickness. 
As previously mentioned, rather than being directly extruded from a die, 
the gas-impregnated web can be provided by exposing a previously formed 
non-impregnated web to gas under high pressure for a sufficient period of 
time to allow the gas to impregnate the web. To carry out this procedure 
in a continuous production process, the web is feb through a pressurizing 
chamber which contains a suficient length of the web to insure that the 
desired degree of gas-impregnation occurs during the time required for an 
increment of the web to pass completely through the chamber. Such an 
arrangement is obviously much less economical than direct extrusion of the 
gas-impregnated web if the thickness of the web dictates a long exposure 
to the pressurized gas, and particularly if the web is relatively wide and 
moving relatively rapidly. However, such an arrangement is practical for 
use with relatively thin webs, particularly if narrow widths and 
relatively low production quantities are involved, and can simply be 
substituted for the die device depicted in FIG. 1. 
The illustrative pressuring device shown in FIG. 5 includes a pressure 
vessel having an upper unit 262 separably connected to a lower unit 264 by 
clamping means schematically shown at 266. The upper unit is supported by 
brackets 268 and the lower unit is supported by a hydraulic jack or the 
like, shown at 270; thus allowing the lower unit to be lowered away from 
the upper unit after the two units are released from one another. 
The lower unit includes a plurality of parallel fixed rollers 272 and the 
upper unit includes a plurality of parallel movable rollers 274 carried by 
a frame 276 slideably supported by guide rods 278. A screw 280, extending 
through a seal 282 and driven by motor 284 allows the movable rollers to 
be moved relative to the upper pressure vessel unit between the lower and 
upper positions respectively shown in broken lines at numerals 274' and 
274". 
A pair of opposed seals 286 and 288 are located at opposite sides of the 
lower unit and, although depicted schematically, in FIG. 5 are 
substantially identical to the adjustable sealing unit in FIG. 3. The 
previously gas free web 289 is fed into the pressure vessel from supply 
roll 290 through inlet seal 286 and is trained alternating around rollers 
272 and 274 to establish a long serpentine path of a desired length within 
the pressure chamber before the web emerges from the pressure chamber 
through outlet seal 288, whereupon it is selectively degassed, foamed and 
cooled, as previously described. Pressurized gas is delivered to the 
pressure chamber by a pump or the like 291 and a sealing and lubricant 
fluid, such as glycerine, is supplied to the lower unit by a pump 292 from 
a reservoir 294. During operation, the level of the sealing and lubricant 
fluid in the lower unit is maintained above the level of seals 286 and 
288, for example by a level sensing device 296 that controls the operation 
of pump 292, and the temperature of the fluid is controlled by a heater 
element 298. Accordingly, as previously described, some of the sealing and 
lubricating fluid escapes from both seals in this process of performing 
its sealing and lubricating functions. This escaping fluid is 
substantially removed from the web by wiper blades 300 and is caught in 
pans 302, from which it is returned to reservoir 294. Any fluid remaining 
on the outgoing web can be removed by the washing device 26, previously 
described. 
To initiate the pressurizing process, pump 292 is turned off to allow the 
fluid to drain out of the lower unit and to the reservoir through the 
seals, so that the fluid level is at the level of the seal passageways. 
The clamping means 266 are then released and the lower unit is moved 
downwardly by jack 270. Frame 276 is then moved to its lowermost position 
of motor 284, thus allowing the end of the web to be threaded through seal 
286, alternately under and over rollers 272 and 274, through seal 288, and 
then through the wash station 26 and remainder of the apparatus shown in 
FIG. 1. Thereupon, the lower unit is raised and sealed to the upper unit, 
pump 292 is energized to raise the fluid to its desired level, seals 286 
and 288 are adjusted as required and pump 290 pressurizes the pressure 
vessel. The frame 274 is then raised as required to provide the desired 
length of web within the pressure vessel and the web is drawn through the 
device of motor 38, shown in FIG. 1. 
Because the pressure in the pressure vessel tends to extrude the web 
outwardly through both seals, considerable tension may develop in the web, 
particularly if it is relatively thick and relatively high pressure is 
used. If such tension is a problem, drive rollers can be employed within 
the lower unit, driven through rotary seals, to isolate this tension to 
those portions of the web immediately adjacent the respective seals 286 
and 288. Also, the depicted device can be provided with means for 
detecting a splice in the web and for automatically opening each seal at 
the proper time to allow the splice to pass through. 
For experimental purposes, or if any limited amount of foamed web of a 
particular type is required, a pressurizing device similar to that shown 
in FIG. 5 can be adapted to receive a rotatably supported roll of plastic 
web material, which can be withdrawn from the pressure vessel through a 
seal similar to seal 288. The roll is preferably wound relatively loosely 
and is exposed to pressurized inert gas to impregnate the entire roll 
before the web is withdrawn and further processed as previously described. 
In a typical example of the invention, a continuous web of Monsanto 
4300-300, super high impact polystyrene, approximately 0.0150 inches thick 
with a material density of 1.06 grams per cubic centimeter, was formed by 
means of a conventional plasticating screw extruder extruding the polymer 
melt into a polished chrome three roll stack. 
The continuous web was wound into a roll and was then placed into a 
pressure vessel, which was then sealed and pressurized to 300 pounds per 
square inch with 99.8% pure argon gas at a temperature of 80.degree. F. 
The continuous web was held in this environment for 80 minutes, sufficient 
time for the web to absorb 99.9% of the saturated gas concentration. The 
web was then taken out of the pressurized environment by unwinding the 
roll through a seal similar to those previously described at 20.0 feet per 
minute, thus allowing the remaining wound web to remain in equilibrium 
with the high pressure environment. The sealant used was glycerine. 
The resulting web, which was supersaturated with gas at ambient conditions, 
was passed through a washing station and then threaded through a degassing 
device which accommodated 17.1 feet of web between the washing station and 
the foaming station. This length of web required 51 seconds to traverse 
the length of the degassing path, i.e. 17.1 feet. Upon entering the 
foaming station the web was S-wrapped about two 8 inch diameter oil heated 
rolls at a temperature of 236.degree. F. The seb contacted the oil heated 
roll surfaces for 24 inches, giving a total contact time of 6.0 seconds. 
The onset of foam growth was evident as the web departed the second heated 
roll; the web's surface temperature at this point being 228.degree. F. 
Beyond the heated rolls and prior to the cooling station, 12.4 feet of web 
are accommodated in the foaming station, allowing approximately 28 seconds 
for the desired foamed structure to develop. The interior of the foaming 
station contained circulating air at 264.degree. F. The circulating air 
heated the web at a slow rate giving very little temperature gradient in 
the web to allow for controllable foam growth. Coming out of the foaming 
station, the velocity of the web was 25.9 feet per minute, which reflects 
the lengthwise expansion of the web during foaming. Upon exiting the 
foaming station the foam growth was stopped by cooling the web. This was 
done by passing the web over two 6 inch diameter chill rolls at a 
temperature of 200.degree. F. and passing the web through ambient air. 
The resultant foamed web was 0.0177 inches thick with an overall material 
density of 0.60 grams per cubic centimeter. The web had unfoamed solid 
surface skins of 0.0011 inches on both surfaces and a 0.0155 inch 
uniformly foamed core. The range of cell size was 2 to 9 micron diameter. 
It should be recognized that the illustrative embodiments represent 
experimental equipment developed to produce relatively narrow webs at 
relatively slow extrusion rates and that various features and relative 
dimensions will be different in production apparatus. However, such 
differences are well understood by those skilled in the art and embody the 
same principles discussed above. Also, it should be apparent that the 
various specifically disclosed methods and apparatus can be adapted for 
producing material other than in the form of a web, e.g. strands, rods, 
tubes and materials of other cross-sectional configuration. For example, 
it is likely that for purposes of producing relatively wide webs, it may 
be preferable to extrude the material in the form of a partial tube or a 
complete tube which is then slit into a web during the production process, 
as is well known in the art of making conventional foamed or unfoamed 
plastic web material. Similarly, it should be understood that, while the 
invention has been described exclusively in the context of impregnating a 
plastic material with inert gas to provide foaming, the invention may also 
be useful in providing foamed material by combining the use of such gas 
impregnation with various nucleating agents and/or by means of organic 
foaming agents. provided that the occurrence of foaming is dependent upon 
a minimum foaming temperature, below which the foaming cannot occur. 
Although the invention has been described with particular reference to 
presently preferred embodiments thereof, it will be understood that 
variations and modifications can be effected within the spirit and scope 
of the invention described above and as defined in the appended claims.