Method for producing foamed, molded thermoplastic articles

Method and apparatus for producing a molded article of a foamed thermoplastic resin and articles produced thereby. A molten mass of an expandable thermoplastic resin is accumulated in an accumulator while it is prevented from foaming. A quantity of the accumulated foamable melt is extruded through a die orifice having a shape adapted to reflect the shape of the desired finished product. The extruded foamable melt commences foaming as it comes in contact with the atmosphere. As the extruded thermoplastic resin foams, it is pulled vertically downward from the die orifice by gravity. The downward-hanging, foamed, thermoplastic material is captured between the halves of a vertically-oriented mold before the foaming expansion of the foamable melt has been completed. The foamed thermoplastic material is compressed by the vertically-oriented mold into the desired shape. The foamed thermoplastic material may be formed to intricate articulations within the female mold portion by gases emitted from jets in the male mold portion or by venting or drawing off gases disposed in the female mold portion. The mold halves may also be configured to confine any foamed resin flash to the bottom portion of the molded article.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates, in general, to foamed, molded, uncrosslinked 
thermoplastic articles. More specifically, a method and apparatus for 
producing foamed, molded, thermoplastic shoe midsoles are disclosed. 
2. Description of the Prior Art 
In the production of foamed articles of thermoplastic resin, three methods 
are generally known (1) the bead molding method; (2) the injection molding 
method; and (3) the extrusion method. Under the bead molding method, 
pre-foamed particles are placed in a mold cavity and heated to encourage 
further expansion and fuse-bonding of the particles. The bead molding 
method is disadvantageous because it requires two or more processing steps 
and traces or marks of the beads are left in the resulting molded article. 
Under the injection molding method, a molten mass of a foamable resin is 
injected into a mold cavity and the injected resin is allowed to expand in 
the mold. This method requires a high injection pressure, however, and 
hence, a large and strong molding apparatus capable of withstanding the 
pressure. Moreover, the injection molding can only provide an expansion 
ratio (i.e., density reduction from resin to foam) of at most about 2. 
In the extrusion method, a molten, expandable resin is extruded through a 
die. The method may only be used to produce foamed articles of a simple 
shape such as sheet- or rod-like products, however. 
One application where the above-noted problems of the foaming/molding prior 
art are especially visible is in the production of athletic shoe midsoles 
(i.e., the material between the shoe upper and the ground-contacting outer 
sole). Shoe midsoles have been characterized as the most important portion 
of athletic footwear. The rather significant forces generated by runners 
as they run, especially in the ball, forefoot and heel regions of the 
foot, must be largely absorbed by the shoe midsole. Furthermore, the 
midsole preferentially is also capable of returning a significant portion 
of the runner's energy through his/her body as the shoe contacts the 
ground, creating a beneficial sensation of springiness. Athletic shoe 
midsoles must also be able to withstand the large number of compression 
and return cycles generated by, for example, long-distance runners, 
without jeopardizing the weight bearing and cushioning capacity of the 
shoe midsole. 
Specifically, material utilized for an athletic shoe midsole must exhibit 
the requisite levels of hardness, resiliency and compressive strength. 
Hardness is commonly measured by, for example, an ASKER C hardness tester 
(or durometer). The hardness tester calculates the hardness of a test 
specimen from the measured depth of penetration of an indentor of 
predetermined geometry into the specimen (once a state of balance is 
reached between the resistance force of the specimen and the force applied 
to the indentor). To be suitable for use as an athletic shoe midsole, 
thermoplastic foamed material must exhibit a hardness of 30 to 70 ASKER C, 
and preferably exhibits a hardness of about 40 to 55 ASKER C. 
The resiliency of a material may be quantified by measuring the material's 
energy return ratio. In general, the energy return ratio is obtained by 
dropping an object onto the material and measuring how high the object 
bounces back (e.g., a perfect spring would have an energy return ratio of 
1.00). The methodology of measuring a material's energy return ratio is 
discussed in detail in U.S. Pat. No. 4,984,376, which is hereby 
incorporated by reference, at column 10, lines 37 to 64. To be suitable 
for use as an athletic shoe midsole, a material should exhibit an energy 
return ratio of at least 0.20 (using the method of measuring energy return 
ratio disclosed in the ASTM bulletin number D-2632-79). By way of 
comparison, under this testing procedure, foamed thermosetting 
polyurethane exhibits a energy return ratio of about 0.25 to 0.30 and 
foamed HYTREL.RTM. (a polyester elastomer manufactured by E. I. du Pont de 
Nemours and Co. of Wilmington, Del.) exhibits an energy return ratio of 
about 0.50 or more. 
Compressive strength is measured by gradually compressing a flat sample of 
material (e.g., a cube with a 10 cm.times.10 cm (or 1 inch by 1 inch) top 
surface) and measuring the pressure needed to compress the sample a given 
proportion of its original height (e.g., 10%, 25% and 50%). Compressive 
strength is measured in kilo Pascals (kPa), or pounds per square inch 
(psi). Preferred materials for athletic shoe midsoles should exhibit a 
compressive strength of about 48 to 138 kPa (7 to 20 psi) at 10% 
compression, 117 to 207 kPa (17 to 30 psi) at 25% compression and 248 to 
379 kPa (36 to 55 psi) at 50% compression. 
Another important criteria which any proposed athletic shoe midsole 
material must meet is the material's specific gravity. Specific gravity 
relates to, and in some senses, grows out of the previously discussed 
properties. To be suitable for use as an athletic shoe midsole, a material 
must have a specific gravity of about 0.5 gm/cm.sup.3 or less. Preferred 
midsole materials have a specific gravity of about 0.3 gm/cm.sup.3 or 
less. This restriction, in turn, limits the methods which may be used to 
form the midsole. For example, injection molding may typically only be 
used with materials having higher specific gravities than those suitable 
for use as athletic shoe midsoles (e.g., about 0.8 gm/c.sup.3). If lower 
density materials are injection molded, the material will often not foam 
uniformly, thereby causing broken cells within the foamed product. 
In addressing these concerns, the athletic footwear industry has developed 
a variety of different solutions. For example, many shoe midsoles are 
currently made of crosslinked EVA (ethylene vinyl acetate). Crosslinked 
EVA exhibits good durability, but since it is a crosslinked material, EVA 
generates a large amount of non-recyclable waste material during 
processing. Furthermore, production of midsoles from crosslinked EVA 
normally requires several processing steps (see, e.g., U.S. Pat. No. 
4,900,490, which is hereby incorporated by reference). For example, after 
a plank of EVA is produced, the plank must be skived (i.e., cut along its 
height to form two or more separate, thinner planks). Thereafter, the 
plank is cut into plugs having the approximate configuration of the 
desired midsole. The plugs of EVA are then inserted into molds and 
compression molded. The plugs are purposely cut slightly oversize relative 
to the molds to encourage the material to adapt to any configurations 
present within the mold. 
The compression molding step also re-forms a skin over the open cells of 
material which were exposed when the plank of EVA was skived. This 
processing methodology is both multi-step and time-consuming (e.g., 5 to 
10 minutes per compression cycle--i.e., heating to seal the skived EVA and 
allowing the re-compressed EVA to cool). 
Other currently-available processes also exhibit several problems. For 
example, in producing shoe midsoles from thermoplastic material (e.g., 
polyester elastomer), multi-step processing is still the norm. For 
example, a large piece of thermoplastic material is extruded. Thereafter, 
the material is skived, die cut into plugs of approximately the desired 
size and the plugs of material are subjected to secondary compression 
molding to form designs in the material and to create a cell-enclosing 
skin over the cut areas of foam. 
When uncrosslinked thermoplastic materials are utilized, the large amounts 
of waste material generated by this type of process may at least be 
recycled (with crosslinked material, the excess material cannot be 
reprocessed and must be discarded), but even with uncrosslinked materials, 
these multiple processing steps still mandate that large amounts of labor 
be expended in producing each foamed article. Furthermore, since the 
thermoplastic material is normally fully foamed when it is subjected to 
the secondary, skin-forming compression molding, it is difficult to 
produce articles having intricate areas and/or shapes of raised material 
(e.g., company logos on the sides of athletic shoe midsoles). Conventional 
foaming methods also have difficulty producing foamed material having 
substantially uniform density and cell structure throughout the article 
(e.g., in the expansion process of U.S. Pat. No. 4,806,293, which is 
hereby incorporated by reference, the expanding material may fold over on 
itself, thereby forming YMP seams in the finished article). 
The present method and apparatus, on the other hand, favorably resolve 
these problems and suboptimizations inherent in the prior art by providing 
a one-step process for producing low-density foamed articles from 
uncrosslinked (and hence recyclable) thermoplastic material whereby 
uniform density is maintained, foam cell integrity is maintained and even 
intricate designs in the female mold section may be reproduced in the shoe 
innersole. 
SUMMARY OF THE INVENTION 
In general, any known thermoplastic resin which is capable of being foamed 
to low density (e.g., 0.5 gm/cm.sup.3 or less) may be used as a raw 
material in the process of the present invention. The foamability of 
suitable resins may alternatively be quantified by noting that the resin 
should be capable of producing a density reduction of at least 0.5 (i.e., 
comparing the unfoamed and foamed forms of the resin). Illustrative of 
suitable thermoplastic resins are olefins such as polyethylenes, 
polypropylenes and copolymers thereof; styrene resins such as 
polystyrenes; polycarbonate resins; and thermoplastic polyurethanes and 
copolyetherester elastomers. The method of the present invention may be 
used with a wide variety of materials, the choice of a particular 
thermoplastic resin for a particular application will depend upon the 
particular article or properties desired to be produced. 
The thermoplastic resin exemplified above is homogeneously commingled with 
a blowing agent at a temperature higher than the melting point of the 
resin under a pressurized condition to obtain a molten, expandable 
thermoplastic resin composition. Both decomposition-type blowing agents 
and solvent-type blowing agents may be used for the purpose of the present 
invention. Examples of the solvent-type blowing agents include 
cycloparaffins such as cyclobutane and cyclopentane; aliphatic 
hydrocarbons such as propane, butanes, pentanes, hexanes and heptanes; and 
halogenated hydrocarbons such a trichlorofluoromethane, 
dichlorodifluoromethane, methyl chloride, methylene chloride, 
dichlorotetrafluoroethane, tetrafluoroethane, tetrafluorochloroethane, 
trifluorochloroethane, pentafluoroethane, trifluoroethane, 
difluorochloroethane, ethyl chloride, trifluoropropane, difluoropropane 
and octafluoropropane. Examples of decomposition-type blowing agents 
include gypsum, hydrated aluminas, azodicarbonamide, mixtures of sodium 
bicarbonate and citric acid, and sodium borohydrate. In general, a wide 
variety of blowing agents may be used in the present invention, the 
optimal blowing agent for a particular application of the invention will 
depend upon the type of resin being utilized and the type of article being 
formed (and the desired article's performance parameters, e.g., hardness, 
resiliency, compressiveness and specific gravity). 
The molten, expandable resin composition may further contain nucleating 
agents, e.g., to regulate cell size within the foam. Suitable nucleating 
agents are known from the prior art. For example, talc, calcium carbonate, 
calcium sulfate, diatomaceous earth, magnesium carbonate, magnesium 
hydroxide, magnesium sulfate, clay and barium sulfate may be useful in 
particular applications. The nucleating agents are generally used in 
amounts of from 0.5 to 5 percent by weight, preferably from 0.5 to 2 
percent by weight, based on the weight of the resin. 
The molten, expandable resin composition may further contain conventional 
additives in the usual amounts, e.g., pigments, dyes, fillers, 
flameproofing agents, antistatic agents, stabilizers, lubricants, 
plasticizers and nucleating agents.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The general process for preparing suitable foamable thermoplastic resin is 
outlined schematically in FIG. 1. The raw materials, such as thermoplastic 
resin pellets and, optionally, an expansion aid introduced from a hopper 2 
are commingled with a blowing agent supplied through a line 2a in a mixing 
zone, designated generally as 1. The mixing is conducted at a temperature 
higher than the melting point of the thermoplastic resin and under a 
pressure to obtain a foamable melt. Customarily employed temperature and 
pressure conditions for mixing and melting thermoplastic resins (which are 
dependent upon the resin, blowing agent, additives, etc., used), are 
utilized in the present invention. The mixing is generally effected by 
means of an extruder 3 provided with a screw 4. Any extrusion system 
capable of producing foam may suitably be used. 
The foamable melt of the expandable resin composition thus prepared in the 
mixing zone 1 is fed to an accumulator 5 through a passage 6 and 
accumulated therein. With some foam compositions, it may be advantageous 
to feed the foamable melt into a cooling zone before feeding the material 
into the accumulator 5. For some compositions, pre-accumulator cooling is 
helpful in achieving the optimum foaming temperature. A set of tandem 
extruders (one for mixing and a second to allow for pre-accumulator 
cooling) may also be used. Alternatively, in addition to mixing and 
melting zones, a portion of the extruder 3 may be configured to cool the 
mixture. 
During the accumulation of the molten resin, the inside of the accumulator 
is maintained at a temperature and a pressure under which substantially no 
expansion of the molten resin is caused. Generally, the foamable melt is 
maintained at a temperature approximately the same as the melt exiting the 
extruder and lower than that in the mixing zone 1. For example, when a low 
density polyethylene is used, the accumulator is preferably maintained at 
a temperature of 88.degree. to 116.degree. C. (190.degree. to 240.degree. 
F.) and a pressure of 1034 to 6895 kPa (150 to 1000 psi). Particular 
temperature and pressure conditions will vary widely, however, depending 
upon the type of material being foamed and the type of article being 
produced. Heating and/or cooling units may be employed in the mixing 
and/or accumulator portions of the apparatus to assure that appropriate 
temperature conditions are maintained prior to foaming. 
When a predetermined amount of the foamable melt has accumulated in the 
accumulator 5, it is rapidly discharged from the accumulator 5, through a 
die 7, into an atmosphere maintained at a pressure lower than that in the 
accumulator 5 (generally ambient pressure) so that the extruded foamable 
melt 10 commences expanding. To expedite the discharge of the foamable 
melt, it is preferable to use a means for forcibly discharging the molten 
resin. Thus, in the embodiment shown in FIG. 1, the accumulator 5 has a 
cylindrical shape and is provided with a gate (not shown) for opening and 
closing the aperture of the die 7 and with a ram 8 reciprocally slidably 
disposed therewithin. The ram 8 is secured to a piston rod 9 of a 
hydraulic means (not shown). Rapid discharge of the accumulated foamable 
melt is preferred to assure that all portions of the extruded material are 
equally foamed and cooled (e.g., in an athletic shoe midsole, to assure 
that both the heel and the toe portions of the discharged material are 
equally foamed and cooled) and to preclude premature formation of a skin 
on the discharged material. Preferably, the accumulator 5 is capable of 
intermittently discharging material at a rate of about 500 Kg/hr or more. 
The ram 8 is preferably operated in the following manner. The molten resin 
prepared in the mixing zone 1 is continuously introduced under pressure 
into the accumulator 5 through the passage 6 while the aperture of the die 
7 is closed by the gate. The pressure is transmitted to the ram 8 so that 
the ram 8 is gradually slid as the molten resin is accumulated within the 
accumulator 5. When the foamable melt accumulated in the accumulator 5 
reaches a predetermined volume, the hydraulic means is actuated and, 
simultaneously, the gate is opened. Thus, the piston rod 9 is driven to 
advance the ram 8 toward the die 7, thereby ejecting the accumulated 
foamable melt through the die 7 into the atmosphere. Thereafter, the gate 
is closed and the pressure of the hydraulic means is released so as to 
decrease the pressing force of the ram 8 to a predetermined level. 
The pressure inside the accumulator 5 is controlled so as to remain within 
a range such that the foaming of the molten resin is prevented. This can 
be performed by controlling the pressure exerted by the hydraulic means 
and applied to the ram 8. 
Alternatively, the molten material may be ejected into the atmosphere 
directly from the extruder 3 (if sufficient discharge rates are attained 
whereby foaming of the material within the die is prevented and 
differential cooling of the extruded material is prevented--e.g., to 
prevent differential cooling in the heel and toe regions of a shoe 
midsole). In a second alternate embodiment, the material (after leaving 
the extruder 3) may be first passed through a second, cooling, extruder 
before being passed into the accumulator 5. 
A preferred example of the accumulator as described above is disclosed in 
U.S. Pat. No. 4,323,528, the disclosure of which is hereby incorporated by 
reference. FIG. 2 shows an alternate extruder/accumulator arrangement 
having a horizontal gate 7, whereby the material leaving the accumulator 5 
moves vertically. 
Upon being extruded from the accumulator 5, the thermoplastic material 10 
begins to foam. As shown in FIG. 5, the material is preferably allowed to 
hang down vertically from the die 7 in an unsupported fashion. In FIG. 5, 
the material 10 is shown being ejected from a pipe 11 (e.g., connected to 
an accumulator 5), but in appropriate situations, the thermoplastic 
material 10 could be extruded directly from the extruder 3. Furthermore, 
the pipe 11 may be horizontally disposed (as shown in FIGS. 5 and 7) or 
vertically disposed (e.g., or the material may be extruded 
vertically-downwardly from the accumulator 5 directly, e.g., FIG. 2). 
Since the thermoplastic material 10 is preferably being extruded between 
the mold halves of a vertically-oriented mold apparatus, however, the 
discharge pipe 11 is preferably vertically-oriented to limit the tendency 
of the ejected thermoplastic material 10 to curl (i.e., reflecting a flow 
direction memory of the non-vertical components of the movement of the 
material 10 within the pipe 11). 
Most thermoplastic materials 10 become almost fully foamed essentially 
immediately after being extruded from the accumulator 5. Therefore, no 
purposeful period of free-fall (i.e., between the pipe 11 and the mold 20) 
is normally necessary. Rather, in order to limit the amount of excess 
material being extruded (i.e., in excess of the amount of material 
required to fill the mold) the ejected material 10 is preferably captured 
by the male 21 and female 22 mold sections as soon as feasible after being 
extruded. Quickly capturing the ejected material within the mold 20 helps 
to preclude the formation of a foam cell-covering "skin" on the extruded 
material (since the skin may preclude the material from filling intricate 
shapes in the mold sections) and helps to maintain a homogenous level of 
expansion and solidification within the extruded material 10 before 
molding (e.g., from toe to heel of a shoe midsole). From a practical 
viewpoint, however, it may be best to allow some free-fall of the extruded 
material 10 before molding (i.e., distance between the pipe 11 and mold 
20) to help limit the fouling of the outside of the mold 20 by excess 
thermoplastic material (since uncrosslinked thermosetting material is 
preferably used with the inventive process, however, any excess material, 
e.g., on the outside of the mold, can at least be reprocessed and reused). 
In some situations, however, (e.g., with certain types of resins, or when 
making some types of articles) it may be best to allow the ejected 
material a longer period of free-fall before molding is commenced. 
The shape of the extruded material 10 may be largely controlled by 
utilization of an appropriately shaped die 7. For example, a rectangular 
die may be used to produce extruded material having a generally 
rectangular cross-section and a round die may be used to produce a 
generally cylindrically-shaped mass of extruded material. More preferably, 
the die shape and extrusion rate may be coordinated to produce an extruded 
mass of material 10 having the desired configuration. For example, if a 
round die is employed and the accumulator 5 is programmed to extrude the 
material 10 in a (comparatively-speaking) slow-fast-slow sequence, due to 
the fact that most thermoplastic materials foam more vigorously when 
extruded at higher rates, the material will form a narrow-wide-narrow 
cylindrical mass of material suitable for molding into a football. 
Different die configurations and extrusion rates may be used to produce 
extruded masses of material of widely-varying shapes. 
Once the requisite amount of foamable melt material has been ejected from 
the pipe 11 (or alternatively, from the accumulator 5 or extruder 3 
directly), e.g., about 100-150 grams for an average size shoe 
midsole--which is preferably accomplished in about two seconds or less of 
ejection time (the weight of the ejected material and preferred ejection 
time depending upon the density of the foamable melt material being used), 
the mold sections are closed around the ejected material 10 (often called 
a "parison" in the industry). As the male 21 and female 22 mold sections 
are moved together (e.g., hydraulically or by any other suitable method) 
they enclose therebetween a mold cavity 23 (e.g., having the form of the 
desired shoe midsole). As the mold sections close, they capture a portion 
of the parison of thermoplastic material 10 within the mold cavity 23. 
Alternatively, more than two mold sections may be used to constitute the 
mold cavity 23. 
As discussed above, with many thermoplastic materials, the material is 
nearly fully foamed upon ejection, but any minor amount of remaining 
foaming/expansion when the material is captured within the mold cavity 23 
may actually be helpful in conforming the shape of the thermoplastic 
material into the shape of the mold cavity 23, and especially, in filling 
in any depressions or void areas within the female mold section 22, e.g., 
representing company logos on the sides of the shoe midsoles. In part 
because of the short amount of time between ejection and capture of the 
parison of thermoplastic material, the material within the mold cavity is 
still easily formable when it is captured and may be somewhat compressed 
by the action of the mold sections to further facilitate the formation of 
intricately-shaped molded articles. 
In an alternate embodiment (see FIG. 5), the male mold section 21 is 
provided with a plurality of jets 24 for emitting gases (e.g., air) under 
pressure. The gas emitted from these jets 24 further helps to insure that 
the ejected thermoplastic material 10 fully fills any voids or depressed 
areas within the female mold section 22 by forcing the material into the 
female mold section 22. In order to allow the action of the jets 24 to 
effectively act upon (i.e., move) the material 10 within the mold cavity 
23, the mold cavity should include an air-tight seal (e.g., provided by 
rubber gaskets disposed between the mold sections). The jets 24 may also 
be helpful in limiting the formation of excess material (or "flash") 
around the molded article (which must be trimmed from the molded article 
after the process is complete). 
In a second alternate embodiment (see, FIG. 5, and especially, FIG. 6), one 
or both of the mold sections 21, 22 (preferably the female mold section 
22) is provided with a plurality of vents 25, whereby gases may escape/be 
drawn out of the mold cavity 23 as the parison is compressed by the two 
mold sections. Hence, in this embodiment, vents 25 also help to insure 
that the ejected thermoplastic material 10 fully fills any voids or 
depressed areas within the mold sections 21, 22. As noted above, although 
the vents 25 are preferably disposed within the female mold section 22, 
vents 25 may be disposed in either or both mold sections 21, 22. Vents 25 
should not be disposed in a mold section which also has jets 24, however. 
The foamed thermoplastic material 10 is preferably maintained within the 
mold cavity 23 until the thermoplastic material has solidified into the 
form of the desired article. If the pressure on the thermoplastic material 
is maintained for too little time, (i.e., the mold sections are separated 
too soon) post-release expansion of the material may occur (e.g., causing 
cells within the foam to pop, and thereafter, causing voids to form in the 
molded article). If pressure is maintained on the thermoplastic material 
10 for too long, however, the foam within some areas of the molded article 
may collapse (causing sink marks). The preferred amount of molding time 
will vary according to the material being utilized, the mass of material 
being molded and the type of article being formed. For example, in forming 
an athletic shoe midsole from HYTREL.RTM., the foamed material is 
preferably maintained within the mold for about 2 to 3 minutes. 
After the thermoplastic foam has been maintained within the mold cavity 23 
for an appropriate length of time, the two mold sections 21, 22 are 
separated and the molded foam material (e.g., shoe midsole) is removed. 
Since the parisons of material ejected from the pipe 11 (or accumulator 5, 
or extruder 3) will always be of slightly different sizes, there will 
always be some excess material which must be trimmed from the finished 
molded article (e.g., with a shoe midsole of desired finished weight of 
105-110 grams, the molded material present when the mold is opened 
commonly has a total weight of 135-139 grams). This excess material, e.g., 
around the edges of the molded article, is known in the industry as 
"flash" (e.g., in FIG. 8, flash 50). The flash may be easily trimmed from 
around the edges of the midsole without destroying the outer surface, or 
"skin" of the molded article. Preferably, the mold sections 21, 22 are 
configured to minimize the amount of flash generated during processing, 
however, to limit the amount of wasted material. The mold sections 21, 22 
are preferably configured to meet at, for example, the bottom portion of 
the molded article (e.g., shoe midsole), whereby the flash may be trimmed 
from only one side (i.e., the bottom) of the molded article (see, FIGS. 9 
and 10). In this way, for example, if jets 24 are used, any imperfections 
created in the surface of the molded article by the jets 24 may be 
eliminated when the flash is trimmed. More preferably, the mold sections 
21, 22 are configured to produce a flat bottom surface on the molded 
article whereby the flash may be trimmed off with a single horizontal cut 
(see, FIG. 10--both flash 50 and bottom imperfections, e.g., caused by 
jets 24, may be eliminated in one cut). Most preferably, the mold sections 
21, 22 are configured to form a sharp line of intersection (or "part" 
line) therebetween (e.g., through use of a male mold section having only a 
short ridge around the outline of the midsole--see, FIG. 5, thereby 
forming a wedge 51 between the midsole and the flash 50, facilitating 
separation of the midsole and flash) whereby, after the molded article is 
released from the mold cavity 23, the flash will be attached to the molded 
article by only a thin bridge of material which may be easily severed by 
hand by ripping the flash from the molded article (see FIG. 9). 
Furthermore, since the process of the present invention preferably 
utilizes uncrosslinked thermoplastic material, any flash produced may be 
reprocessed/remelted and incorporated into future molded products. 
Given the length of time the mold 20 must remain closed as the parison of 
ejected foamed thermoplastic material 10 solidifies therein (i.e., to the 
point where, respectively, both post-release expansion and collapse of 
foam cells are avoided--e.g., for a shoe midsole formed from HYTREL.RTM., 
about 2--3 minutes), the inventive apparatus is more preferably configured 
to allow for more continuous utilization of the extruder/accumulator, 
i.e., a plurality of parisons of thermoplastic material being ejected 
during any one molding cycle. A variety of apparatus configurations may 
prove beneficial in this regard. For example, FIGS. 3-5 show the mold 20 
mounted on rails 30, whereby after one parison of thermoplastic material 
has been ejected into one mold 20, the mold may be slid out from under the 
extruder/accumulator pipe 11 and another (perhaps differently-sized) mold 
20 may be positioned beneath the pipe 11 to receive a second parison 10 of 
thermoplastic material. The accumulator 5 may also be adjusted or 
preprogrammed to vary the quantity of material ejected per parison and/or 
the speed at which the material is ejected. In another preferred 
embodiment, a plurality of molds 20 may be mounted on a circular carousel 
(see, FIG. 7). For example, if a particular molded article has a mass such 
that the available accumulator can produce ten parisons of material of the 
requisite size to produce the article per minute and the articles require 
one minute of cooling time within the mold cavity to solidify, a carousel 
having ten molds may profitably be employed. Also, to insure consistent 
mold temperatures during the preferred continuous molding process, the 
mold 20 also has associated therewith a temperature control mechanism 40 
for heating or cooling the mold 20 as required. 
The inventive apparatus and process may be further understood through 
reference to the following non-limiting examples. 
EXAMPLES: FOAMED ATHLETIC SHOE MIDSOLES 
Materials 
A preferred thermoplastic multi-block copolymer elastomer used in some of 
the examples was a copolyetherester elastomer sold by DuPont under the 
trademark HYTREL.RTM., grade 4078W. Depending upon the desired properties, 
however, a variety of other elastomers may also be used with the apparatus 
and process of the present invention. For example, ethylene vinylacetate 
(EVA), SANTOPRENE.RTM. (a thermoplastic elastomer made by Monsanto, Co.), 
KRATON.RTM. (a styrene-butadiene elastomer made by Shell Chemical Co.), 
PELLETHANE (a thermoplastic polyurethane made by the Dow Chemical Co.), 
and a variety of other materials such as copolyetheramide ester may also 
be used in appropriate circumstances. As discussed above, however, 
regardless of the particular material being foamed, it is important that 
the requisite levels of hardness, resiliency, compressive strength and 
specific gravity be attained. For example, in the production of athletic 
shoe midsoles, the foamed material should exhibit at hardness of 30 to 70 
ASKER C (preferably about 40 to 55 ASKER C), an energy return ratio (i.e., 
resiliency) of at least 0.20 (under ASTM method D-2632-79), a compressive 
strength of 48 to 138 kPa (7 to 20 psi) at 10% compression, 117 to 207 kPa 
(17 to 30 psi) at 25% compression, and 248 to 379 kPa (36 to 55 psi) at 
50% compression. Finally, to be suitable as a midsole, a elastomer should 
be foamable to a specific gravity of about 0.5 gm/cm.sup.3 or less. 
Both decomposition-type blowing agents and solvent-type blowing agents may 
be used in the present examples. Traditional nucleating agents and other 
foaming materials are also employed as hereinafter described in further 
detail. 
Foam Preparation 
The foams described in the following examples were prepared in a 7.62 cm (3 
inch) diameter, 48:1 (length:diameter) extruder. The extruder was equipped 
with an apparatus for injecting therein foaming agents and the forward 
portion of the extruder barrel was jacketed for cooling using circulating 
water. The extruder was attached to a foam accumulator, e.g., as described 
in U.S. Pat. No. 4,323,528. The accumulator was equipped with a piston for 
ejecting (extruding) the foamable melt through a closable die. The speed 
of the piston was variable to provide various extrusion rates. The use of 
an accumulator is not necessary to produce foams of large cross-sections 
with a large extruder. However, its use was required with the relatively 
small foam extruder used in the examples, which, by itself, would be 
incapable of producing foams of large cross-section. The use of a 
relatively small extruder also conserved raw materials as the foamable 
melt was intermittently extruded at rates of about 454 to 2268 kg/hr (1000 
to 5000 lbs/hr) (preferably about 1134 kg/hr (2500 lbs/hour)) while the 
actual output rate of the extruder was about 54.4 kg/hr (120 lbs/hr). 
Within the accumulator, the foamable material is preferably maintained at 
about 3448 kPa (500 psi). 
Example 1 
The foam accumulator was equipped with a radially-configured die having an 
aperture with cord length of 3.42 cm (1.345 inches), an arc length of 3.81 
cm (1.50 inches) and a gap of 0.279 cm (0.110 inches) [the dimensions of 
the particular die orifice utilized will depend upon the article being 
manufactured, the type of material being foamed, the ejection rate of the 
material, etc.]. Because the thermoplastic resins utilized in the present 
invention are commonly hygroscopic, the resin (e.g., in the form of 
pellets) was desiccated before being fed into the extruder. This 
desiccation of the resin may be accomplished in the hopper or before the 
resin is introduced into the extruder. Normally, contacting the resin with 
air which has been exposed to a desiccant (e.g., silica gel) and heated 
for about 2 hours at 93.degree. C. (200.degree. F.) adequately desiccates 
the resin [with HYTREL.RTM., however, it is preferable to desiccate the 
resin with air which has been heated to 107.degree. C. (225.degree. F.)]. 
The thermoplastic multi-block copolymer elastomer HYTREL.RTM. 4078W was 
mixed into the hopper of a single-screw extruder. The elastomer was mixed 
with about 0.33% (by weight of total mix) of HYDROCEROL.TM. CF-40 (for 
cell size control--"Hydrocerol" is an encapsulated mixture of sodium 
bicarbonate, citric acid and citric acid salts which liberates carbon 
dioxide and water under elevated temperatures in the extruder) and about 
21/2% (by weight) white color masterbatch. The foaming agent, isobutane, 
was injected into the extruder at a rate of about 0.32 kg per hour (0.7 
lbs/hr). The output of the extruder was about 54.4 kg per hour (120 
lbs/hr). After the foaming agent was injected, it was mixed into the 
polymer and then the mixture was cooled to the proper foaming temperature, 
about 174.degree. C. (about 345.degree. F.). The foamable melt exiting the 
extruder was transferred under pressure to the accumulator where it was 
stored and released intermittently at a rate of 1134 kg per hour (2500 
lbs/hour). 
Upon opening of the accumulator gate, a parison of material was extruded. 
The amount of material extruded will depend upon the type of article being 
formed and how well tuned the apparatus is. In the case of a shoe midsole, 
it is expected that approximately 135 to 139 grams of material will be 
trapped between the mold halves of the apparatus (i.e., forming the molded 
article and the flash). Upon extrusion, the foamable melt commenced 
foaming and hung downward from the die orifice. The extruded material was 
captured between the male and female mold sections of a 
vertically-oriented mold. The male and female sections of the mold 
enclosed the extruded material within a mold cavity in the shape of a shoe 
midsole. The extruded material was maintained in the mold cavity for 23/4 
minutes (165 seconds) until the shoe midsole was formed. After the flash 
was trimmed, the midsole weighed about 105 to 110 grams. 
Example 2 
The same materials used in Example 1 were used in this example. The 
apparatus used to mold the ejected, foamed thermoplastic resin included, 
in the male mold section, a plurality of jets. The jets were used to emit 
pressurized air (e.g., at about 138 kPa (20 psi)) against the material 
captured within the mold cavity, thereby urging the material to fill even 
intricately-shaped recesses (e.g., brand logos) within the female mold 
section. The mold sections were gasketed with rubber to prevent gas from 
escaping from between the mold sections. The mold sections were configured 
to join near the bottom of the midsole, whereby the flash could be removed 
in a single, horizontal cut. Insofar as this horizontal cut shears cells 
within the molded foam material, it may even be beneficial in the 
production of athletic shoe midsoles since this horizontal surface is 
subsequently bonded to other materials (e.g., the shoe outsole) and the 
open cells may assist subsequent glue bonding. 
Example 3 
The same materials used in Example 1 were used in this example. The 
apparatus use to mold the ejected, foamed thermoplastic resin included, in 
the female mold section, a plurality of apertures for venting or 
drawing-off gases from the material captured within the mold cavity, 
thereby urging the material to fill even intricately-shaped recesses 
within the female mold section. 
The vents are preferably matched to a particular type of mold, whereby all 
excess gasses may be removed from the mold cavity. For example, in the 
case of forming an athletic shoe midsole, if the mold sections are 
configured to join together (or form a "part line") at the bottom of the 
midsole, vents may preferably be positioned on the top of the midsole and 
in any fins disposed in the midsole (e.g., on the sides of the midsole). 
Venting is especially helpful in indented areas of the female mold 
section, because air may otherwise easily become trapped in these areas 
and prevent the foamed material from filling the recessed areas, thereby 
causing gaps in the fins of the finished articles. On the other hand, for 
example, if the part line between the two mold sections is configured to 
be formed at the top of the midsole, it may be unnecessary to provide 
vents in all of the fins (i.e., since some of the air which would 
otherwise be trapped in the vents may escape the mold cavity through the 
part line). 
The positioning of the part line and the utilization of vents are 
preferably balanced to achieve the overall goal of minimizing the amount 
of gas which is trapped inside the mold cavity when the thermoplastic 
material is molded. In general, positioning the part line at the bottom of 
the midsole makes it easier to trim the flash from the midsole, whereas 
positioning the part line near the top of the midsole decreases the number 
of vents which must be used to evacuate the gas from the mold cavity 
(since the part line may vent much of the gas trapped in the midsoles 
indented fins). 
Example 4 
The same materials used in Example 1 were used in this example. In order to 
facilitate removal of the flash, the mold sections were configured to form 
only a very sharp part line just above the top of the shoe midsole, 
whereby the flash was only very tenuously attached to the molded midsole. 
The sharpness of the part line permitted the flash to be peeled away from 
the molded article by hand. 
Example 5 
The same equipment used in Example 1 was used in this example. The 
elastomer utilized was PELLETHANE.RTM., Series 2102-90a, a polyester 
polycaprolactone manufactured by The Dow Chemical Company of Midland, 
Mich. After the resin was desiccated by being contacted for two hours with 
air which had been heated to 93.degree. C. (200.degree. F.) and passed 
through a dessicant, the PELLETHANE.RTM. Series 2102-90a was mixed with 
the blowing agent (isobutane--which constituted about 11 percent by weight 
of the mix) and the nucleating agent (talc--which constituted about 0.5 
percent by weight of the mixture or less). The output of the extruder was 
about 50.8 kg per hour (112 lbs/hour) of material. After the blowing agent 
was injected, it was mixed into the polymer and then the mixture was 
cooled to the proper foaming temperature, about 204.degree. C. to 
207.degree. C. (400.degree. F. to 405.degree. F.). The foamable melt 
exiting the extruder was transferred under pressure to the accumulator 
where it was stored at a pressure of about 1551 to 1724 kPa (225 to 250 
psi) and released intermittently at a rate of about 19996 kPa (2900 
lbs/hour). 
A portion of the extruded thermoplastic material weighing approximately 115 
to 120 grams was captured between the male and female mold sections of a 
vertically-oriented mold. The mold sections enclosed the ejected material 
within a mold cavity in the shape of a shoe midsole. The ejected material 
was maintained in the mold cavity (the mold cavity of this example was the 
same size as that utilized in Example 1) for about 23/4 minutes (165 
seconds) until the midsole had solidified. After being released from the 
mold and having the flash trimmed, the midsole weighed about 85 to 90 
grams. 
Like the midsole produced in Example 1 (and like the midsoles produced by 
all of the other examples as well) the article of molded thermoplastic 
material produced by the inventive process in this example was capable of 
performing as an athletic shoe midsole (i.e., had sufficient hardness, 
resiliency, compressive strength and specific gravity). Relative to the 
midsole produced in Example 1, the midsole produced in this example had a 
lower density, a lower resiliency and a higher rigidity per mass of 
material. Hence, depending upon the desired properties of the finished 
article sought in a particular application, the apparatus and process of 
the present invention may profitably be utilized with a wide variety of 
materials.