Improved Rotational Moulding Process

FIELD OF THE INVENTION

This invention relates to rotational moulding of plastics articles and in

particular to the manufacture of moulded plastics 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,

ranging in size from domestic containers and toys to large liquid storage

tanks.

Rotational moulding processes are characterised by low capital costs

for moulds and other apparatus when compared with injection moulding

processes for example. They are further characterised by relatively short

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

(ABS). In this respect virtually any thermoplastic resins are suitable for use

in rotational moulding, including copolymeric materials and mixtures of

compatible resins.

Products produced by rotational 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

stress crack resistance (ESCR).

Although generally satisfactory for most purposes, in certain

applications rotationally moulded products lack stiffness, impact resistance

(particularly at low temperatures), and load bearing capacity (particularly at

elevated temperatures). These deficiencies generally arise from the

relatively low wall thickness of products produced by rotational moulding

which are inherently due to thermal conductivity and cost constraints.

Growing environmental 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

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

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.

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

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

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 difficult to

reproduce.

These characteristics of reclaimed plastics 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

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

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

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

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

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

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

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

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,

including the step of rotationally moulding a mixture of particulate plastics

material comprising at least one thermoplastics 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 thermoplastics material fuses to form an inner

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.

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

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 ofthe second thermoplastics material may include

an adhesion resistant coating to reduce adhesion between the second

thermoplastics material and the first thermoplastics material during the fusing

of the outer skin.

In a further form ofthe invention the method may also include the step

of incorporating a quantity of reliable moulded reinforcing elements in the

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.

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 reinforcing elements 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.

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

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 comprise 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,

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

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.

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

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 elevational 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 comprises an outer skin 5, a foamed inner skin 6, spaced

reinforcing members 7 in the form of foamed macro-spheres and residual

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

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 Tollable reinforcing elements in the

form of balls or macro-spheres. The reinforcing elements are pre-formed by

rotational or injection moulding, typically also from reclaimed plastics. In the

embodiment the reinforcing elements are foamed polyolefin, but may

alternatively be of solid or hollow construction. The balls are of a diameter

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 reliable 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

outer shape ofthe pallet. Shipping pallets are generally 1200 mm square by

about 140 mm deep. The ingredients are initially introduced into the mould,

as follows:

(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

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 rotational 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,

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.

(II) 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.

(III) 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

bails 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 solidify the skins on the inside of the mould.

(VI) 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

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

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

loaded into the mould and the product manufactured using a single rotational

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

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

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

be chosen such that opposing surfaces of the inner skin touch one another

after foaming.

In embodiments which employ reclaimed plastics material for the inner

skin avoid any need for adding pigments during the pre-extrusion process,

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 material, because the pre-

extrusion process partially homogenises the reclaimed plastics. The second

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

using a further embodiment of the invention. In cases where panels require

additional strength for use in building applications 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

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.

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

encapsulate the mesh, thereby enhancing the mechanical properties of the

finished panel. In alternative embodiments requiring additional rigidity, the

reinforcing 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

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

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.

Throughout the specification the aim has been to describe the

preferred embodiments of the invention without limiting the invention to any

one embodiment or specific collection of features.