Method for making an expanded polystyrene article

A mold assembly and method for molding a substantially hemispherical construct from pre-expanded polystyrene beads. The mold assembly includes a female half mold; a mating male half mold; a first male mold insert mountable on the male half mold and having a continuous, peripheral, beveled edge; and a second male mold insert mountable on the first male mold insert. For forming a construct having an impact-resistant exterior layer and an energy-absorbing interior layer, such as a motorcyclist safety helmet, the beads are initially molded under heat and compression between the female half mold and the first insert to form a first intermediate product of a first volume having a continuous, peripheral, beveled edge; said product is thereafter molded under heat and compression between the female half mold and the second insert to a second, reduced volume to form a compressed, second intermediate product, the beveled edge of the first intermediate product serving to uniformly distribute the force applied thereto. The second intermediate product is then remolded to the same volume to improve volume stability, and thereafter backfilled between the male half mold and the female half mold with additional pre-expanded polystyrene beads to form an energy-absorbing inner layer.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates generally to expanded polystyrene constructs having
 a high-impact resistance and strength without an unacceptable increase in
 brittleness, and a method for making the same. More particularly, this
 invention relates to such constructs that are created by a molding process
 that includes, as an intermediary step, the imparting of a cam or beveled
 edge surface to the construct preparatory to compressing the construct to
 high density. The beveled edge surface imparted to the construct serves to
 uniformly distribute the forces applied during the compression step even
 over relatively large angular deviations from the mold parting line,
 thereby permitting the molding of a single large construct that formerly,
 prior to this invention, would have required the molding and combining of
 several smaller, partial constructs. Thus, the disclosed method is
 particularly useful in the molding of polystyrene motorcyclist and
 bicyclist safety helmets and polystyrene shipping boxes such as those
 commonly used for shipping iced fish. This invention also provides molding
 apparatus adapted for molding constructs of the type described.
 2. Background Art
 A method for molding an expanded, highly impact-resistant, polystyrene
 construct from polystyrene beads was disclosed in U.S. Pat. No. 5,718,968
 to P. W. Cutler et al., which by this reference is incorporated herein.
 The polystyrene beads include a thin outer shell of polystyrene and a
 hollow interior that includes a blowing agent, such as pentane, for
 example. According to the method, the beads were first pre-expanded by
 application of heat through hot air or steam, which caused the blowing
 agent to vaporize and expand the beads. The beads were then cooled, which
 caused the beads to have reduced internal pressure. The pre-expanded beads
 were next placed into a mold assembly where they were subjected to heat
 and pressure for molding to a first volume. The molded article was then
 rapidly cooled in the mold assembly, thereby causing the vaporized blowing
 agent within the beads to condense and create a pressure less than
 atmospheric inside the beads. Thereafter, the molded product with beads
 having low internal pressure was immediately subjected to compression
 within the mold assembly to a second volume, less than the first volume.
 This resulted in a single layer construct that had memory shape and was
 particularly suited for acoustic and thermal insulation. Alternatively,
 additional layers were molded within the mold assembly by adding beads to
 the first molding step and then molding together the originally-molded
 layer with the additional beads. This yielded a layered construct wherein
 each layer had a different density than the other layers and each layer
 had a density higher than the density of the beads from which it was
 molded, except when a last layer was a backfill layer of density equal to
 that of the beads from which it was molded. The layered construct was
 volume stable, high density, high strength, and highly impact resistant.
 When this method was used to mold a safety helmet for a motorcyclist or
 bicyclist, the outermost layer had the highest density, while inner layers
 had lower densities to absorb impact forces to minimize transmission of
 these forces through the construct. Such a helmet, being substantially
 hemispherical and having a 360 degree circumference, was thought to
 require molding in at least five parts--crown, front, back, and two
 sides--which thereafter were assembled together in a single mold and
 finally molded together. This was because it had been found that the
 compression process was less effective when the pressure is applied at
 angles between 46 degrees and 89 degrees to the parting line of the mold.
 It was theorized that compression pressure was diffused so that the
 compression ratio rapidly diminished with pressure applied at angles more
 than 45 degrees with respect to the parting line of the mold.
 There remains a need, therefore, for a method and mold assembly for molding
 pre-expanded polystyrene beads that permits molding a substantially
 hemispherical polystyrene construct as a whole, in a single, integrated
 molding process, thereby eliminating the need for first molding two or
 more individual component parts thereof and the combining of the parts
 through an additional molding step. There also remains a need for expanded
 polystyrene constructs that are produced by the method and mold assembly
 described herein, which feature high impact resistance and strength
 without an unacceptable increase in brittleness, and which can be molded
 in substantially hemispherical shapes in a single, integrated molding
 process. These needs are fulfilled by the present invention.
 SUMMARY OF THE INVENTION
 A method and apparatus are provided for molding an expanded polystyrene
 bead construct within a mold assembly. The mold assembly includes a female
 half mold having a first, continuous, peripheral edge surface and an
 interior surface configured to impart a desired exterior surface to the
 construct. The mold assembly further includes a male half mold having a
 second, peripheral edge surface in mating alignment with the first
 peripheral edge surface when the female half mold and the male half mold
 are aligned on an alignment axis. The mold assembly also includes a first
 male mold insert that is mountable on the male half mold. When the male
 half mold, with the first male mold insert mounted thereon, is inserted
 into the female half mold, the space between the female half mold and the
 first male insert defines a first volume. The mold assembly also includes
 a second male mold insert, mountable on the first male mold insert, such
 that when the male half mold, with the first and second male mold inserts
 mounted thereon, is inserted into the female half mold, the space between
 the female half mold and the second male mold insert defines a second
 volume, which is less than the first volume. The first male mold insert
 has a third, beveled, continuous edge surface for engaging an outer,
 peripheral portion of the pre-expanded polystyrene beads that are to be
 molded within the mold assembly, and to which portion it imparts a beveled
 edge. Preferably the beveled edge surface of the first male mold insert is
 deviated at least 45 degrees, but not more than 60 degrees, from a mold
 parting line normal to the alignment axis. In a preferred embodiment, for
 mounting the second male mold insert over the first male mold insert, the
 first male mold insert has a plurality of upstanding alignment pins and
 the second male mold insert has a plurality of second apertures in mating
 alignment with said alignment pins, and the female half mold similarly has
 a plurality of third apertures in mating alignment with said alignment
 pins.
 In a first embodiment, the method includes filling the female half mold
 with pre-expanded polystyrene beads of a predetermined density; mounting
 the first male mold insert on the male half mold; molding the beads under
 applied heat and pressure by forcing the first male mold insert against
 the beads to fuse the beads together to form a first intermediate product
 of a first volume having a continuous, beveled edge surface formed between
 the beveled edge surface of the first male mold insert and the peripheral
 edge surface of the female half mold; rapidly cooling the fused first
 intermediate product to within a subplasticizing temperature temperature
 (usually between 160 and 180 degrees F.) to cause lower than atmospheric
 pressure within the beads of the first intermediate product; mounting the
 second male mold insert on the first male mold insert; and compressing the
 cooled first intermediate product between the second male mold insert and
 the female half mold to a second, reduced volume, before the low pressure
 conditions within the beads equilibrate to ambient conditions, to produce
 a single-layer, fused, expanded bead body of at least 200 percent higher
 density than the predetermined density of the pre-expanded beads. The
 reason for rapidly cooling the first intermediate product to wthin a
 plasticizing temperature range is so that the temperature of the
 intermediate product will be low enough to escape the post expansion that
 would follow if the termperature is too high and, at the same time, avoid
 the cell wall damage that would occur during the subsequent compression if
 the temperature is too low.
 In a second embodiment, for molding a more volume-stabilized, single-layer,
 polystyrene bead construct, the method includes the preceding series of
 acts followed by the additional act of remolding the cooled intermediate
 product under heat and compression between the second male mold insert and
 the female half mold to a second, reduced volume, before the low pressure
 conditions within the beads equilibrate to ambient conditions, to produce
 a fused, expanded bead body of at least 200 percent higher density than
 the predetermined density of the pre-expanded beads.
 In a third embodiment, for molding a layered, expanded polystyrene bead
 construct, the method includes filling the female half mold with
 pre-expanded polystyrene beads of a predetermined density; mounting the
 first male mold insert on the male half mold; molding the beads under
 applied heat and pressure by forcing the first male mold insert against
 the beads to fuse the beads together to form a first intermediate product
 of a first volume having a continuous, beveled edge surface formed between
 the beveled edge surface of the first male mold insert and the peripheral
 edge surface of the female half mold; rapidly cooling the fused first
 intermediate product to within a subplasticizing temperature range to
 cause lower than atmospheric pressure within the beads of the first
 intermediate product; mounting the second male mold insert on the first
 male mold insert; and compressing the cooled first intermediate product
 between the second male mold insert and the female half mold to a second,
 reduced volume, before the low pressure conditions within the beads
 equilibrate to ambient conditions, to produce a single-layer, fused,
 expanded bead body of at least 200 percent higher density than the
 predetermined density of the pre-expanded beads; removing the second male
 mold insert from the first male mold insert; and adding pre-expanded
 polystyrene beads to the mold assembly and remolding the added beads
 between the female half mold and the male half mold under heat and
 compression to cause fusion of the added beads at an interface to the
 expanded bead body to form a layered construct comprising at least two
 layers of beads wherein the beads have different densities. In a fourth
 embodiment, a backfilled, layered construct is molded by a method that
 includes filling the female half mold with pre-expanded polystyrene beads
 of a predetermined density; mounting the first male mold insert on the
 male half mold; molding the beads under applied heat and pressure by
 forcing the first male mold insert against the beads to fuse the beads
 together to form a first intermediate product of a first volume having a
 continuous, beveled edge surface formed between the beveled edge surface
 of the first male mold insert and the peripheral edge surface of the
 female half mold; rapidly cooling the fused first intermediate product to
 within a subplasticizing temperature range to cause lower than atmospheric
 pressure within the beads of the first intermediate product; mounting the
 second male mold insert on the first male mold insert; and compressing the
 cooled first intermediate product between the second male mold insert and
 the female half mold to a second, reduced volume, before the low pressure
 conditions within the beads equilibrate to ambient conditions, to produce
 a single layer, fused, expanded bead body of at least 200 percent higher
 density than the predetermined density of the pre-expanded beads;
 remolding the cooled intermediate product under heat and compression
 between the second male mold insert and the female half mold to a second,
 reduced volume, before the low pressure conditions within the beads
 equilibrate to ambient conditions, to produce a fused, expanded bead body
 of at least 200 percent higher density than the predetermined density of
 the pre-expanded beads; removing the second male mold insert from the
 first male mold insert; and adding pre-expanded polystyrene beads to the
 mold assembly and remolding the added beads between the female half mold
 and the male half mold under heat and compression to cause fusion of the
 added beads at an interface to the expanded bead body to form a layered
 construct comprising at least two layers of beads wherein the layers have
 different densities. The latter embodiment of the method is particularly
 suited for molding substantially hemispherical constructs having a 360
 degree circumferential edge, such as bicyclist and motorcyclist safety
 helmets, and packing crates.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 Referring to FIG. 1, an illustrative embodiment of four component parts of
 a mold assembly, denoted generally by the numeral 10, for molding a
 motorcyclist safety helmet 12, are shown in exploded, perspective
 view--namely, a female half mold 14, a male half mold 16, a first male
 mold insert 18 placed over the male half mold 16; a second male mold
 insert 20, which is also part of the mold assembly 10, is insertable over
 the first male mold insert 18, as described below. All four components are
 aligned on an axis A--A. The female half mold 14 includes a flat plate
 portion 14P normal to axis A--A and a centrally disposed, substantially
 hemispherical portion 14H that extends axially away from the male half
 mold 16. The hemispherical portion 14H has an interior surface that is
 configured to impart a desired exterior surface to a mold construct for a
 motorcyclist safety helmet 12. The male half mold 16 has a flat plate
 portion 16P normal to axis A--A and a centrally disposed, substantially
 hemispherical portion 16H that extends axially towards the female half
 mold 14. A mold parting line P-P' is defined as any straight line normal
 to axis A--A and lying in a plane that contains apposed, engaged surfaces
 of plate portions 14P, 16P when the mold assembly 10 is in a closed
 position with the substantially hemispherical portion 16H of the male half
 mold 16 inserted inside of the substantially hemispherical portion 14H of
 the female half mold 14.
 Referring now to FIG. 2, the male half mold 16 is shown in enlarged,
 perspective view, removed from the remainder of the mold assembly 10. It
 may be seen that the male half mold 16 includes a convex, crown portion
 16C integral with, and surrounded by, a peripheral, relatively narrow,
 upturned flange portion 16F, the narrow space between the flange portion
 16F and the crown portion 16C defining a peripheral well 16W. The flange
 portion 16F is somewhat raised at the front relative to the sides and rear
 of the flange portion 16F, corresponding to the brow portion 12B of a
 motorcyclist safety helmet 12, depicted in FIG. 15, as molded on the
 illustrated mold assembly 10.
 Referring to FIG. 3, the first male mold insert 18 has a convex, crown
 portion 18C integral with, and surrounded by, a beveled, continuous
 peripheral edge 18E. Four alignment pins 18P, extend axially from the
 crown portion 18C and are configured for insertion into mating apertures
 20A, 14A in the second male insert 20 and in the female half mold 14,
 respectively. The crown portion 18C and the edge 18E are sized and
 configured such that the insert 18 mounts snugly on the male half mold 16
 with the edge 18E overlying the well 16W of the male half mold 16. As
 shown in FIG. 4, when the first male mold insert 18 is so mounted on the
 male half mold 16, the beveled edge 18E extends radially inward from the
 flange 16, and axially away from the mold parting line P-P' by angle R
 defined by a line OX tangent to the beveled edge 18E and the parting line
 P-P', where O is a peripherally-disposed point of intersection of said
 tangent line with the parting line P-P'. Preferably, R is between 45 and
 60 degrees.
 FIG. 5 shows that the second mold insert 20 is substantially hemispherical
 and, unlike the male half mold 16 and the first male insert 16, has no
 peripheral flange or beveled edge. As shown in FIG. 6, the second mold
 insert 20 is sized and configured for mounting atop the first male mold
 insert 18 (shown in phantom). Thus, as shown in FIG. 7, when the mold
 assembly 10 is closed, the distance D.sub.1, and hence the volume, between
 the substantially hemispherical portion 14H of the female half mold 14 and
 the second male mold insert is least; when the second mold insert 20 is
 removed and only the first male mold insert is mounted on the male half
 mold, the distance D.sub.2, and hence the volume between portion 14H and
 the first male mold insert 18 is somewhat larger; and when both inserts
 18, 20 are removed, the distance D.sub.3, and hence the volume, between
 portion 14H and the male half mold 16 is the largest.
 In use, for molding a motorcyclist safety helmet 12, the substantially
 hemispherical portion 14H of the female half mold 14 is filled with
 pre-expanded polystyrene beads (not shown) of a predetermined density. The
 first male mold insert 18 is mounted on the male half mold 16. Once
 filled, the mold is clamped shut. The beads are molded under applied heat
 and pressure by forcing the male mold insert against the beads to fuse the
 beads together to form a first intermediate product 32 of a first volume,
 depicted in FIGS. 8 and 11. Preferably, the molding is carried out at a
 pressure from about 9 to about 12 psi, and the product being molded is
 allowed to dwell at a temperature of from about 100 degrees C. to about
 110 degrees C. for from about 15 to about 30 seconds. The first
 intermediate product 30 is then rapidly cooled by the introduction of
 water into the mold assembly. The first intermediate product 30 is allowed
 to dwell with the cooling water for a sufficient time to enable the
 blowing agent within the beads to once again liquefy, typically, less than
 2 minutes, depending on the thickness of the molded part. The pressure is
 then relieved.
 With the first intermediate product 30 still within the female half mold
 14, the second male mold insert 20 is mounted over the first male insert
 18 with the alignment pins 18P inserted through the alignment apertures
 20A, and the mold assembly 10 is closed. The first intermediate product 30
 is substantially hemispherical, but has a continuous, beveled edge 30B, as
 may best be seen in FIG. 11. When the mold assembly 10 is in closed
 position, the peripheral edge 20E of the second mold insert 20 engages the
 beveled edge 30B of the first intermediate product 30. Thus, when next
 pressure is applied to the mold assembly 10 to compress the cooled first
 intermediate product 30 from a first volume to a second, lesser volume,
 the beveled edge 30B serves to uniformly distribute the force applied by
 the peripheral edge 20E. The mold assembly 10 is clamped shut during this
 step for at least 3 seconds. This operation produces a relatively thin,
 second intermediate product 32, depicted in FIGS. 9 and 12, which is a
 fused expanded bead body of at least 200 percent higher density than the
 predetermined density of the pre-expanded beads.
 Next, the cooled, second intermediate product 32 is remolded under heat and
 compression between the second male insert 20 and the female half mold 14
 to a second, reduced volume before the low pressure conditions within the
 beads equilibrate to ambient conditions; the remolded product is then
 cooled as before. This operation significantly increases the volume
 stability of the molded construct and results in a construct consisting of
 two layers.
 The second male mold insert 20 and the first male mold insert 18 are then
 removed from the male half mold 16. The female half mold 14 is again
 filled with pre-expanded polystyrene beads for remolding the added beads
 between the female half mold 14 and the male half mold 16 under heat and
 compression to cause fusion of the added beads at an interface to the
 expanded bead body to form a backfilled, layered construct 34, here
 comprising two layers, l.sub.1 (high density) and l.sub.2 (back fill, low
 density), as shown in FIG. 14. The backfilled, layered construct 34 is
 cooled as before and removed from the mold assembly 10. A suitable
 exterior finish is thereafter applied to the backfilled, layered construct
 34 in making the helmet 12. In this manner, a substantially hemispherical
 construct 34 can be molded without the necessity for first molding
 individual components thereof (crown, front, back and two sides) and the
 concomitant necessity thereafter for combining them in an additional
 molding operation.
 It will be understood that the above-described process for molding a
 multilayer polystyrene construct for use in a motorcyclist safety helmet
 can be adapted to the making of a variety of other objects by appropriate
 adjustments to the shapes and sizes of the component parts of the mold
 assembly. Such other objects include, for example, bicyclist safety
 helmets, packing crates, corner protectors, motor vehicle dashboard
 components, and, in general, any object in which a sturdy,
 impact-resistant, expanded polystyrene exterior layer is to be combined
 with a relatively soft, energy absorbing, interior layer. For example, a a
 packing crate 100, including a chest 100C and a lid 100L, depicted in FIG.
 16, an edge protector 110 (three-layered), useful for shipping furniture,
 for instance, and depicted in FIG. 17, and a corner protector 112
 (two-layered), for protecting a window during shipping and installation,
 depicted in FIG. 18, were each made according to the above-described
 process. In the case of the packing crate chest 100C, a high density layer
 100K surrounds a low density, back-filled layer 100J.
 Moreover, variations on the above-described molding process yield expanded
 polystyrene constructs having differing characteristics and a wide variety
 of uses. That is, instead of following all of the foregoing steps of the
 illustrated embodiment, one can terminate the molding process upon
 producing the first intermediate product, which is high density, unfixed
 (i.e., it spontaneously and partially expands somewhat to its original
 volume), with high memory (i.e., high resistance to being crushed under
 compression). Such constructs are suitable for molded shape parts for
 supporting weight, such as as a base under a uni-molded shower stall or a
 motor vehicle dashboard. The first intermediate product is also useful as
 acoustical and thermal insulation, wherein high impact resistance is
 ordinarily not required, but the ability to mold a substantially
 hemispherical shape by a single, integrated molding process may
 nevertheless be highly desirable. Alternatively, one can go on to remold
 the first intermediate product, which yields a high density, fixed
 construct with low memory. Such a construct is suitable for molded shape
 parts that 778require great structural strength; examples include surf
 board cores and industrial in-line assembly trays. One can proceed further
 to back fill the contruct, yielding a combination high density layer and
 back-filled low density layer; such constructs are suitable for
 energy-absorbing molded shape parts, such as motorcyle safety helmet
 liners and corner pads. One can proceed still farther to produce a
 combination high density, fixed layer with a back-filled, low density
 layer; such constructs are suitable, for example, for bicycle safety
 helmets, box-less clamshell packaging, and for the energy-absorbing
 component of a DRIVET.RTM. system for stucco-covered wall construction.
 Thus, while the preferred embodiment of the invention has been illustrated
 and described, it is accordingly intended that the disclosure be taken as
 illustrative only and not limiting in scope, and that the scope of the
 invention be defined by the following claims.