Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE
Improved Rotational Moulding Process
FIELD OF THE INVENTION
This invention relates to rotational moulding of pl~slics articles and in
s particuîar to the manufacture of moulded pl~stics articles having an outer
skin and an inner core.
BACKGROUND TO THE INVENTION
Rotational moulding of thermoplastics material is a convenient and
cost effective method for the manufacture of a wide variety of articles,
10 ranging in size from domestic col)tai"ers and toys to large liquid storage
tanks.
Rotational moulding processes are characterised by low cspital costs
for moulds and other apparatus when compared with injection moulding
processes for example. They are further characterised by relatively short
15 setup times. the capacity for economic low volume manufacturing and
immense flexibility in product size range.
Typical thermoplastics materials employed in rotational moulding
processes are the polyolefins, including polyethylene and polypropylene,
polyvinyl-chloride, polycarbonates, nylon, and acrylobutadiene-styrenes
20 (ABS). In this respect virtually any thermoplastic resins are suitabte for use
in rotational moulding, including copolymeric materials and mixtures of
compatible resins.
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Products produced by rotationai moulding are generally characterised
by good strength and structural integrity, abrasion resistance, weather
resistance including UV stability, wide colour range, selectable surface finish
from high gloss to matt and textured, chemical resistance and environmental
5 stress crack resistance (ESCR).
Although generally satisfactory for most purposes, in certain
applications rotationally moulded products lack stiffness, impact resistance
(particulariy at low temperatures), and load bearing capacity (particularly at
elevated temperatures). These deficiencies generally arise from the
10 relatively low wall thickness of products produced by rotational moulding
which are inherently due to thermal conductivity and cost constraints.
Growing envirol""ental concerns over the disposal of plastics waste
has focussed the need for recycling of plastics materials reclaimed
subsequent to use by domestic or industrial consumers. One of the major
15 difficulties in recycling such reclaimed plastics is contamination.
Contamination can arise from inadequate sorting of polymeric species which
is difficult to achieve by unskilled manual labour and virtually impossible to
automate.
Mixtures of plastics species are difficult to process due to differing
20 melt temperatures and viscosities as well as inherent chemical
incompatibility. Other sources of contamination, apart from dirt and dust,
are paper and plastics film labels, non-plastics container seals and
incompletely discharged contents of such containers.
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A method for manufacturing products using reclaimed plastics,
typically polyolefins, employs an injection extrusion process whereby
granulated plastics waste is subjected to working under high shear
conditions in an extruder. The compounded plastics issuing from the
5 extruder is pumped directly, under screw pressure, into a shaped hollow die
and allowed to cool, in a manner similar to injection moulding.
The injection extrusion of reclaimed plastics is generally used to
produce articles such as plank members for outdoor seating, tree and shrub
surrounds, garden edging, man-hole covers and pits for underground utilities
10 or shipping pallets. The articles are characterised by high mass, bulky
shape, limited colour range, poor chemical resistance, poor ESCR and UV
resistance along with inferior mechanical properties. The surface of such
articles is uneven or porous and thus difficult to keep clean. Textured
finishes, as required for slip resistance on man-hole covers, are diffcult to
15 reproduce.
These characteristics of reclaimed pl~stics products result from
contamination and chemical incompatibility problems and the use of a dark
pigment, usually black brown or grey, to mask unsightly flow marks from
inadequately blended plastics of differing colours and viscosities. A large
20 amount of expensive pigment must be added to achieve even a reasonable
colour finish.
The applicant's Australian Patent Application No. 40440/95 describes
a method for the manufacture of moulded plastics articles which combines
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certain advantages of conventional rotational moulding with certain
advantages of injected cores to selectively overcome disadvantages of each
process.
The applicant's earlier method comprises a two cycle method of first
5 rotationally moulding a thin walled hollow body and secondly introducing into
the core of said hollow body a flowable mass of plastics material, preferably
reclaimed plastics material, to occupy substantially the entire volume of the
core of said hollow body. This method is particularly suited to producing
relatively large thin walled objects such as buoyancy modules or liquid
10 storage tanks.
Whilst suitable for thin walled objects, the applicant's earlier method
consumes a large amount of reclaimed plastics in the production of articles
that have a core volume commensurate with the overall size of the article,
thus resulting in a high mass product.
Consequently there is a need for an improved method which
incorporates the benefits of the double skin structure of the prior art method
but utilises less material to produce articles that are lighter in weight whilst
maintaining their structural integrity. Furthermore it would be advantageous
if the method required only a single cycle rotational moulding process, rather
20 than the prior art two cycle method of forming the outer skin followed by
introducing the flowable reclaimed plastics material to form the core.
The improved method is suited to the production of bulky articles,
such as pallets, lids for industrial waste bins and tanks, covers for utility
.
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access ways, pits (eg. for underground cable or conduit systems), or
structural members such as wall, roof, or floor panels.
OBJECT OF THE INVENTION
It is an object of the present invention to provide an improved
5 rotational moulding method that overcomes at least some of the
disadvantages of prior art moulding methods.
It is a further object of the invention to provide an improved rotational
moulding method for producing composite products that have the
advantages of virgin thermoplastics without some of the disadvantages of
10 mixed reclaimed plastics incorporated therein.
Further objects will be evident from the following description.
DISCLOSURE OF THE INVENTION
In one form, although it need not be the only or indeed the broadest
form, the invention resides in a method for rotational moulding of articles,
15 including the step of rotationally moulding a mixture of particulate plastics
material COI"p,i~i"9 at least one the~"o~ slics material having a first particle
size and a second thermoplastics material having a larger particle size
wherein, in use, said first thermoplastic material fuses to form an outer skin
and subsequently said second thel",opl~stiGs material fuses to form an inner
20 skin bonded to the outer skin of a resultant moulded article.
The melt temperature of the second thermoplastics material may be
higher than the melt temperature of the first thermoplastics material.
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Preferably the second thermoplastics material is comprised of
reclaimed plastics material.
In preference the first thermoplastics material is homopolymeric.
The second thermoplastics material may be a mixture of polymeric
5 species.
Preferably the second thermoplastics material includes a quantity of
thermosetting polymeric material.
In preference the second thermoplastics material includes a quantity
of foaming agent.
The decomposition temperature of the foaming agent is preferably
greater than the melt temperature of the second thermoplastics material.
Preferably the second thermoplastics material includes a quantity of
filler material.
If required particles of the second thermopl ~stics material may include
15 an adhesion resistant coating to reduce adhesion between the second
thermopl~.stics material and the first thermopl~stics material during the fusing
of the outer skin.
In a further form of the invention the method may also include the step
of incorporating a quantity of rollable moulded reinforcing elements in the
20 mould with said first and second thermoplastics materials, wherein the
reinforcing elements are located in a spaced array thereby forming bridges
between opposed inner skin surfaces as they expand.
.
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Suitably, the reinforcing elements are larger than the particle size of
the second thermoplastics material.
The reinforcing elements may be generally spherical or generally
cylindrical.
Preferably the reinforcing elements are mechanically engaged
between the opposed inner skin surfaces.
In preference, the reinforcing elements are at least partially fused to
said opposed inner skin surfaces.
The reinforcing elements may include an adhesion resistant coating,
such as talc, to reduce adhesion between the reinforcing elements and the
first and second thermoplastics materials during the fusing of said skins.
If required, the rcinr~r~ii"g ele",enl~ may include an adhesion resistant
additive to reduce adhesion between the reinforcing elements and the first
and second thermoplastics materials during the fusing of said skins.
Suitably, the adhesion resistant additive may be selected from one
of a soap, such as calcium stearate or the like, a silicone oil or a suitable
plasticiser.
The reinforcing elements may be of a solid, hollow or foamed
construction.
Preferably the reinforcing elements may be comprised of virgin or
reclaimed plastics material.
If required, the reinforcing elements may be comprised of ceramics
material.
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In a yet further form of the invention the method may also include the
step of incorporating a reinforcement structure at predetermined locations in
the mould with said first and second thermoplastics materials.
The reinforcement structure may be constructed of plastics or metallic
5 material or a combination of both.
Preferably the reinforcement structure comprises steel or aluminium
mesh.
If required the reinforcing structure may comprise angle or channel
profiled members.
In a still further form, the invention resides in an article manufactured
using the improved rotational moulding method disclosed herein.
The article may colnprise a shipping pallet, man-hole cover, tank lid,
utility pit cover or the like.
The article may comprise a structural member, such as a wall panel,
15 floor panel, roof panel or the like.
In another form, the invention resides in a rotationally moulded
shipping pallet having a supporting surface and a series of leg means
depending therefrom, wherein said surface includes a plurality of recesses
which are arranged to locate pallets relative to one another when two or
20 more are stacked.
Preferably the leg means comprise rails depending from opposite
ends of the pallet and a central rail extending congruently to said end rails.
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Preferably the rails include a series of depending lugs which
cooperate with said plurality of recesses.
BRIEF DETAILS OF THE DRAWINGS
To assist in understanding the invention preferred embodiments will
5 now be described with reference to the following figures in which:
FIG 1 is a perspective view of a shipping pallet manufactured in
accordance with an embodiment of the invention.
FIG 2 is a partial elevationai cross-section of the shipping pallet
illustrated in FIG 1.
FIG 3 is a partial elevational cross-section of a structural panel made
in accordance with another embodiment of the invention.
FIG 4 is an isometric view of an alternative embodiment of a shipping
pallet manufactured in accordance with the invention.
FIG 5 is a front elevation of the shipping pallet shown in FIG 4.
DETAILED DESCRIPTION OF THE DRAWINGS
In the drawings, like reference numerals refer to like parts. Referring
to FIG 1, the shipping pallet 1 of the embodiment comprises a deck 2, and
rails 3 thereby providing a raised portion 4 facilitating insertion of the tines
of a fork-lift truck or similar transportation means.
FIG 2 is a schematic cross-section illustrating the construction of the
pallet 1, which com~rises an outer skin 5, a foamed inner skin 6, spaced
reinforcing members 7 in the form of foamed macro-spheres and residual
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voids 8. The random spaced array of reinforcing members forms a bridging
layer 9 between opposed inner surfaces of the inner skin 6.
In the embodiment the outer skin layer 5 is formed from a virgin
polyolefin, typically an opaque polyethylene, which can provide a smooth or
5 textured surface as required. The outer skin may also be coloured as
desired, of food handling grade and UV resistant. The inner foamed skin 6
is also formed from a polyolefin, but typically of mixed reclaimed plastic
which has been pre-extruded in granular form and incorporates a chemical
foaming or blowing agent.
The bridging layer 9 comprises rollable reinforcing elements in the
form of balls or macro-spheres. The rei"furl,i"g eleme"l~ are pre-formed by
rotational or injection moulding, typically also from reclaimed pl~slics. In the
embodiment the reinforcing elements are foamed polyolefin, but may
alternatively be of solid or hollow construction. The balls are of a diameter
15 appropriate to the gap between the opposed inner skin surfaces, in this case
around 50 mm. In other embodiments the reinforcing elements may be of
any appropriate rollable shape, such as cylindrical pellets, and alternatively
formed by extrusion or other suitable process.
The rotational moulding process uses a mould formed to the desired
20 outer shape of the pallet. Shipping pallets are generally 1200 mm square by
about 140 mm deep. The ingredients are initially introduced into the mould,
as follows:
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(a) approximately 5 kg of virgin polyethylene in the form of powder
or micro-pellets averaging 0.5 mm in diameter, the polyethylene
having a melting point of approximately 160 degC;
(b) approximately 10 kg of mixed reclaimed polyolefin plastics in
s the form of pre-extruded pellets averaging 3 mm in diameter, the
reclaimed plastics having a melting point in the approximate range of
165 to 220 degC; the foaming agent incorporated into the pellets
having a decomposition temperature of approximately 220 degC; and
(c) approximately 3 kg of pre-moulded balls of reclaimed foamed
polyolefin, 50 mm in diameter.
The rotaliollal moulding apparatus (not shown) can be any of the
known forms, but would preferably be an oven type machine with computer
process control providing, inter alia, precise control of the oven temperature
and the rotation and oscillation speeds of a two-axis mould drive rig. The
mould is sealed after the ingredients are loaded and the following process
steps effected by the rotational moulding apparatus, during which time the
mould is rotated and oscillated at pre-selected speeds:
(I) The oven is heated to a temperature of 300 degC and held for
a period of 5 minutes, so as to fuse the polyethylene powder and lay
down the outer skin on the inward facing walls of the mould. The finer
powder or micro-pellets separate from the larger inner skin foamable
pellets under the tumbling action within the slowly rotating mould.
This effect leads to the preferential laying down of the outer skin,
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which may be assisted by the addition of talc surface coating on the
pre-extruded pellets to slow the tendancy of the pellets to adhere to
the virgin plastics material whilst it melts onto the mould surface.
(Il) The oven is then elevated to a temperature of 320 degC and
held for a period of 5 minutes, so as to melt and fuse the reclaimed
plastics pellets thereby forming the inner skin on the inward facing
surface of the outer skin layer.
(Ill) During the preceding steps, the pre-moulded balls roll around
inside the mould while the outer and inner skin layers are laid down
in the mould. The balls are surface coated with talc in the
embodiment, to slow or minimise them adhering prematurely to either
skin.
(IV) The oven is further elevated to a temperature of 360 degC for
a period of 3 minutes to effect decomposition of the blowing agent,
which expands the volume of the inner skin. This expansion traps the
balls between, and in some instances partially embeds them in, the
inward facing surfaces of the inner skin.
(V) Cooling the mould to a temperature of at least 50 degC for a
period of 10 minutes, typically by quenching the mould with water to
cool and solidif,v the skins on the inside of the mould.
(Vl) The cooling step tends to effect a slight shrinkage of the
product from the inward facing walls of the mould, which eases
release when the mould is finally opened. The process produces a
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pallet having a double skinned wall and a spaced array of reinforcing
elements in the form of foamed balls.
It should be noted that the oven temperature refers to the ambient
temperature within the oven, whereas the temperature within the mould itself
5 will be somewhat lower due to heat transfer effects. However the
temperature gradient within the mould generally assists the process.
The selection of appropriately graded particle or granule size for the
plastics materials comprising the skins, so as to take advantage of the
separation effected by the mould tumbling action, allows all ingredients to be
10 loaded into the mould and the product manufactured using a single r~taliollal
moulding cycle.
This effect may be enhanced by appropriate selection of the melt
temperature and melt flow, as measured by the melt flow index (MFI), for the
plastics materials. See for example the melt temperatures set out above in
15 relation to the preferred embodiment. Similar enhancement may be
achieved by the selection of mould rotation speeds for each step.
The provision of a spaced array of reinforcing members, whether in
the form of balls or cylindrical pellets, provides improved strength and rigidity
for the finished product without the weight penalty associated with solid or
20 semi-solid cores. It is to be noted that, in other embodiments of the invention
where there is a product requirement for slimmer walls, the reinforcing
elements may be omitted because the plastics material for the inner skin can
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14
be chosen such that opposing surfaces of the inner skin touch one another
after foaming.
In embodiments which employ reclaimed plastics material forthe inner
skin avoid any need for adding pigments during the pre-extrusion process,
5 because it is hidden by the opaque outer skin. Similarly the reclaimed
plastics need not be pre-sorted by colour. Although principally constituted
from reclaimed polyolefins, granulated scrap plastics such as thermosetting
polymers may be incorporated into the inner skin ",aterial, because the pre-
extrusion process partially homogenises the reclaimed plastics. The second
10 thermoplastics material can include organic or inorganic filler material. An
example of a suitable inorganic filler material is talc, whilst the organic filler
material could be cellulosic including recycled newspapers, rice or peanut
hulls.
FIG 3 shows a schematic cross-section of a building panel made
15 using a further embodiment of the invention. In cases where panels require
additional strength for use in building applic~lions or the like, reinforcing
means in the form of metallic mesh, for example, may be incorporated into
the mould as part of the rotational moulding process.
The building panel 10 includes an outer skin 11, an inner skin 12, a
20 spaced array of macro-spheres 13 forming a bridging layer 14 between
inward facing surfaces of the inner skin. Substantially encapsulated in the
inner skin is a prefabricated aluminium mesh 15.
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During assembly of the mould for the building panel, the mesh
members are located therein and spaced therefrom by aluminium or
thermosetting plastics spacers 16. The spacing and quantity of reclaimed
- plastics material for the inner skin is chosen so as to substantially
5 encapsulate the mesh, thereby enhancing the mechanical properties of the
finished panel. In alternative embodiments requiring additional rigidity, the
rei"rur~ lg means may be comprised of angle or channel profiled members.
FIG 4 shows another embodiment of a shipping pallet 17, which
includes a deck 18 and, in addition to the end rails 19, a central support rail
10 20. The central rail provides additional support for the deck thereby resulting
in an improved load carrying capacity. In order to facilitate stacking of empty
pallets, the deck 18 includes a plurality of recesses 21 in its upper surface.
The recess are arranged to cooperate with lugs 22 disposed on the lower
surface of the rails, as depicted in FIG 5.
The recess and lug combination is but one arrangement which could
be used in this application. One alternative might include providing slots in
the upper surface of the deck, which slots are arranged to cooperate with the
end and central rails, omitting the lugs entirely. In another alternative, the
rails might be replaced by a series depending legs which cooperate with
20 enlarged recesses provided in the deck of the pallet.
The composite products constructed in accordance with the invention
exhibit the advantages of high grade polyolefins without the disadvantages
of reclaimed plastics materials.
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16
Throughout the specification the aim has been to describe the
preferred embodiments of the invention without limiting the invention to any
one embodiment or speciflc collection of features.
