GB2311245A - Rotational moulding of articles having cellular interiors - Google Patents

Abstract

Plastics articles having a solid skin and cellular interior are formed by rotational moulding in a process which comprises charging a mould with a first plastics material in fine powder form and heating at a first temperature while rotating to form the skin then charging the mould with a second plastics material containing a blowing agent and in the form of partially expanded, comparatively large granules and heating at a second temperature under rotation to form the cellular interior. Pelletised and partially blown plastics materials and processes for their preparation are also described as are a process in which the cellular interior is formed by the use of a comparatively large granular plastics material containing a blowing agent and a further process in which the fine powder material and granular material containing a blowing agent are charged together to the mould and after rotation at a first temperature to form the skin, mould and its contents are further heated to a second temperature at which the granular material is expanded to form the cellular interior.

Description

ROTATIONAL MOULDING This invention relates to rotational moulding and other casting processes. More specifically, but not exclusively, the invention relates to improved methods of producing a lightweight rigid, foamed or cellular thermoplastic article by the process known as rotational moulding, roto-moulding or rotational casting. The invention includes aspects relating both to one-shot and two-shot rotational moulding methods.

Using known rotational moulding techniques, moulded articles can be produced having a solid or unfoamed "skin" or envelope and a cellular interior, the latter having a density in the region of 0.15 g/cm3. The composite structure is rigid and of lower overall density than many common woods, and can be used for the manufacture of chemically resistant pallets, boxes, containers and similar goods.

Typical exmples of plastics materials used in the production of hollow articles having cellular interiors produced by a rotational moulding technique are polyethylene as the wall, envelope or skin material of the article, and polyurethane as the foam. Polyethylene has useful chemical resistance and toughness and polyurethane foam is rigid and provides the desired degree of stiffness.

TWO-SHOT PROCESSES The conventional technique for producing a foam-filled rotationally moulded article by a two-shot process comprises initially charging the mould with a plastics material to produce the envelope of the article, then opening a port hole in the mould, forming an opening in the already-formed skin or envelope of the pre-moulded article, delivering through the opening a charge of a second plastics material comprising a blowing agent, closing the mould and re-commencing the rotational moulding technique while blowing (i.e. foaming) of the second plastics material by the blowing agent takes place.

Conventionally also, the two-stage rotational moulding process employs finely divided particles of ground powder for the plastics material of the envelope these being usually in a size range of 500 to 600 microns. Such a particle size is used so that during the initial heating period of the production cycle, the powdered thermoplastics material is rapidly melted. Also, the small particle size has been found to be conducive to the production of a smooth and well-formed moulded surface on the finished article.

Conventionally also, the charge of second plastics material comprising a blowing or foaming agent is likewise in the form of a finely-divided powder of ground particles having a particulate size range comparable to that of the first plastics material, when it is introduced into the mould.

A problem that arises in such two-shot rotational moulding techniques is that there is an imperfect distribution of the second plastics material and its associated blowing or foaming agent within the moulded article so that there are portions of the article which are imperfectly filled with foam, and this has associated consequences for the mechanical strength and other properties of the article.

An object of this first aspect of the present invention is to provide a method for rotational moulding using a two-shot process wherein improvements are provided in relation to the distribution of the second plastics material and its associated blowing or foaming agent within the initially pre-formed or moulded article in the rotational moulding technique.

Conventionally, the approach to obtaining a proper distribution of the second plastics material and its associated blowing agent has been to provide it in finely divided form whereby a rapid heat-up step is achieved immediately upon entry into the mould, thereby minimising the thermal shock effect upon entry into the hot mould, and thereby likewise maximising opportunity for the particulate material to flow freely within the mould.

We have discovered that, on the contrary, the use of finely divided or relatively finely divided particulate material as the foam-forming material in a two-shot rotational moulding technique is itself the very factor which leads to the problems of imperfect distribution of the foam, bridging across narrow channels, and like defects, and in accordance with this first aspect of the invention we provide that the second plastics material and its associated blowing or foaming agent is used in the form of comparatively large granules which are preferably in the form of pellets, though other particles (such as a granulate) of a comparable size, if available, can be used likewise.

Accordingly, in accordance with this first aspect of the invention there is provided a method of producing a plastics article having a cellular interior by a rotational moulding process as defined in claim 11 of the accompanying claims.

By providing a method wherein the step of producing the cellular material within the mould employs comparatively large granules, preferably pellets, of the foam-producing material the result is achieved that these pellets are able, by virtue of their comparatively large size, to roll around within the hollow interior of the partly-moulded article, and indeed to pass through relatively narrow restrictions inherent in the shape thereof, so as to achieve a more uniform distribution than has hitherto been possible. This more uniform distribution is due, not least, to the fact that the larger particle size has the result that a slower melting and slower corresponding increase in tackiness of the particles or pellets is produced, whereby a lower tendency of these to bridge or otherwise block portions of the internal surface of the moulded article results.

In an embodiment, the shortcomings of the prior art use of a powder-form compounded plastics material and foaming agent are greatly reduced. These advantages include the difficulties of pouring a fine powder into a narrow gap in the mould using a funnel device. The pourability of, for example, polyethylene powders deteriorates as the temperature rises, the powder becoming sticky and being likely to stop or slow its flow through the filling funnel as the hot air in the mould rises and enters the funnel spout. Also, once the powder has entered the mould, the small particles melt too readily and can block narrow internal passages. Additionally, there is the cost of grinding the material into powder form.

In contrast, the embodiments of the invention, because of the greater mass of the pellets (for example the pellets usually being of about 4 to 5mm maximum dimension) are easily poured into the mould without being affected by the rise of hot air from the mould cavity. Moreover, the pellets are large enough to travel during pouring and subsequent mould rotation to the internal extremities of the mould without blocking or bridging across narrow internal channels or passages. Additionally, the costly grinding process to produce the powder-form product has been eliminated.

Turning now to a second aspect of the invention, this concerns the rate of foam or cellular material formation within the mould during the foam production stage of the moulding process.

There is a need to provide means for increasing the rate of foam formation since very appreciable time spans can be required to complete the foam formation stage. Thus, for example, periods in the region of half an hour or more may have to be contemplated for the production of the foam material within the mould and clearly if such periods could be kept to a minimum, this would lead to important advances in the efficiency of the process.

An object of this aspect of the present invention is to provide a method of producing a plastics article having a cellular interior offering improvements in relation to one or more of these latter factors, or generally.

In accordance with this aspect of the present invention there is provided a method of moulding a plastics article as defined in claim 1 of the accompanying claims.

In an embodiment, by the use of a plastics material, such as a polyolefin, e.g. polyethylene or polypropylene, comprising a blowing agent which has been caused to partially blow before entry into the mould, there is provided the unexpected advantage that the process of foam formation proceeds more rapidly utilising a thus partially pre-blown material than is the case with a totally un-blown material or indeed a fully pre-blown material.

The reason for this improvement in foaming rate has not been exhaustively studied but a possible mechanism concerns the conductivity of the gas produced by foaming as compared with the conductivity of the polymer itself.

Despite the low conductivity of, for example, nitrogen gas, this nevertheless is higher than that of the base material of polyethylene foam and accordingly one possible reason for such an improvement in foaming rate arises directly from the improved thermal conductivity which results from the partial foaming step.

In the embodiment, at least part of the partial expansion of the blowing agent in the pellets of plastics material and blowing agent occurs at the extrusion die at which the compounded plastics material and blowing agent is extruded prior to a pelletising cutting step.

In the embodiment, careful control of the compounding conditions enables any loss of ability to foam by the foamable material to be kept to a minimum. The small amount that is lost can easily be made-up by a small increase of the initial dose of foaming agent.

The partial expansion of the compounded pellets can be controlled by the temperature and shear rate of the compounding equipment. The degree of expansion of the pellets is measured during production so that the manufactured product is reproducible. Whilst the density of individual pellets is varied by the blowing or expansion process, the testing of pellets individually is not practical. A simple method of control testing is to measure the bulk density of a sample of pellets by pouring them into a one litre measuring cylinder and weighing the whole sample that occupies this volume.

By this method, the bulk density (BD) can be established.

The optimum BD, which relates to the degree of expansion of the pellets, has been found to be in the region of 0.30 g/cc. Solid, non-expanded pellets have a BD of approximately 0.50 to 0.60 g/cc. The poor thermal conductivity of the polymer of unexpanded pellets results in very slow heating of these. A degree of expansion of the pellets increases the rate of their rise in temperature in use, and in consequence increases their expansion rate. However, over-expansion of the pellets results in some loss of subsequent expansion in the moulding process.

Figure 1 of the accompanying drawings shows a plot of bulk density (BD) against foam rise (in millimetres) achieved in a fixed period of 20 minutes. There is a marked increase in the rate of foam rise with increasing pre-expansion of the foamable material up to a maximum at a bulk density of about 0.270 g/cc.

It can be seen that the chart shows results not only for foamable material comprising 2% foaming agent, but also 3% foaming agent. The results in both cases are parallel to each other, pointing to an optimum bulk density at around the mid-point region between the ends of the scale at 0.52 and 0.165 g/cc.

Because of their faster expansion, the use of partially expanded pellets results in faster heating cycles as full expansion is achieved sooner.

In this latter respect, reference is directed to Figure 2 of the accompanying drawings where the heating time is plotted against foam rise in millimetres with results being shown for pellets of varying bulk density (BD). As is readily apparent, pellets of lower BD achieved full expansion after 20 minutes whilst the higher BD sample had not reached this state after 30 minutes.

The degree of expansion of pellets produced by compounding the plastics material and the blowing agent can also be affected by the chemical composition of the compound.

Expansion can be accelerated by the presence of activators or catalysts for the blowing process, such as zinc stearate. Such a compound also reduces the operating time and temperature for the subsequent moulding cycle needed to expand the pellets in the mould.

This aspect of the invention, concerned with the use of a partially expanded second plastics material and associated blowing agent, is applicable mainly to a two-shot rotational moulding process. Accordingly, in the definition of the invention the references t6 a second stage or phase in which the mould contents are heated refers to the carrying out of these stages or phases in sequence, one after the other, rather than, effectively, simultaneously or in a time-overlapping manner as is the case for a one-shot process.

However, generally, in the definition of the broader aspects of the invention it is to be understood that the references to first and second times and first and second plastics materials do not exclude the possibility that the times may be overlapping and the plastics materials may be the same material.

Considering now features of the invention which are applicable both to that aspect of the invention concerned with the use of comparatively large granules, particularly pellets, of the plastics material in which a blowing agent is incorporated, and that aspect concerned with the provision in the particles, whether large or small, of such material as a partially blown blowing agent, we now deal with features applicable to both aspects, as follows.

In accordance with these aspects of the invention, the speed of rotation of the mould will generally be in the range of 5 - 20 rpm.

Preferably, the first plastics material, namely the material used for the envelope of the product, has a melt flow index (MFI) of 2 to 5 units (grams per 10 minutes), a particle size of 400 to 600 microns, usually 500 microns, and (for polyethylene) a density in the range of 0.920 to 0.945 g/cm3, and even from 0.85 to 0.96 g/cm3.

The first and second plastics materials may each be a polyolefin such as polyethylene or polypropylene, and copolymers thereof.

The second plastics material, which is mixed with a blowing agent, may be in the form of pellets produced by extrusion and pelletising of the compounded material, the shape of the extruded section being circular, or generally curved, or being a polygon, or any other comparable shape, and the length of each pellet being within the range of 0.5 to 2.0 times the maximum width dimension of the extruded section The first indicated oven temperature, of initial heating of the plastics material in the mould to make the envelope may be in the range of 180 to 2500C and preferably 190 to 210 cm, with the corresponding internal mould temperatures being in the range 190 to 2300C.

The second indicated oven temperature, for foam production, may be in the range 200 to 350"C, preferably 220 to 2500C. The corresponding internal mould temperatures depend on the thermal efficiency of the oven and mould and lie in the range 200 to 2500C and preferably 210 to 2200C.

The first predetermined time for forming the substantially impervious skin on the surface of the mould in the pre-moulding step is generally in the range of 15 to 40 minutes, the time depending upon the skin thickness, the longer the time taken, the greater the skin thickness.

The second predetermined time for causing blowing of the polymer providing the cellular interior may lie in the range of 10 to 30 minutes, and preferably 15 to 25 minutes. Usually, foaming continues after withdrawal of the mould from the oven (during cooling).

The method may further comprise the final steps of cooling the mould during continued rotation thereof, and withdrawing the moulded article from the mould.

An example of a blowing agent for use in embodiments of the present invention is azodicarbonamide. This decomposes at a temperature of 160 to 2300C to liberate gases, predominantly nitrogen, ammonia and carbon monoxide. Other blowing agents may be used.

In the embodiments, the use of comparatively large particles of the second plastics material incorporating the blowing agent allows the particles to enter the mould cavity easily and without the particles themselves softening and thus adhering to the molten surface of the skin or envelope of the first plastics material therein during the filling operation. Accordingly, an additional cooling stage after the primary moulding of the envelope is unnecessary. Moreover, the increased mass of the comparatively large particles or pellets eases their travel within the mould cavity to the extremities thereof before they absorb sufficient heat, become molten and thus have their progress impeded. Accordingly, improved distribution of the foamable material throughout the moulded article is achieved. Additionally, the subsequent foaming step of the comparatively large pre-expanded pellets is facilitated by the enhanced thermal conductivity of the expanded particles. The use of pellets of foamable material contributes to uniformity of foaming due to the uniformity of pellet size,- thereby avoiding the variations of foam density caused in the prior art by variation of particle size.

The compounded granules containing the chemical blowing agent should be as large as possible consistent with practical production methods. A diameter (in the case of spheroidal or cylindrical granules) or a side dimension (in the case of cuboidal granules, or polygonal-section pellets) of approximately 2 to 5mm and preferably approximately 3mm has been found to be useful. The pellets are made by extrusion compounding or other melt compounding methods, and the extruded laces are cut to granules by pelletising equipment, either at the extruder die face and cooled immediately, or they are drawn through a water bath and subsequently pelletised by the use of rotating cutters. Either method is suitable, but the dies should be adjusted if necessary to allow large pellets to be made. If pellets larger than 2mm are made, the advantages previously discussed can be readily achieved.

The pellets thus produced are used in pellet form and not ground to a powder in accordance with conventional techniques. The more even the shape of the particles or pellets, the better will be the filling and distribution within the mould.

According to the present invention there is further provided a compounded plastics material and blowing agent as defined by claim 7 for use in a method of rotationally moulding a plastics article having a foamed interior within an external envelope.

As mentioned above, the rate of expansion of the compounded plastics material is increased by the fact that it is partially pre-blown.

SINGLE SHOT PROCESS In view of the difficulties associated with the above-described two-shot process in which there is provided a preliminary step of charging the mould and pre-moulding the envelope of plastics material before the introduction of the foamable material, followed by the step of introducing that foamable material and causing it to be foamed, it would be desirable to provide means whereby a single charge of plastics materials could achieve the necessary moulding and foaming functions without the need to create an opening in the mould during the moulding process and introduce a second charge of plastics material into the hot mould and its internal pre-moulded plastics material.

There is disclosed in CA 983226 (Du Pont) a method of rotational moulding of polyolefin articles having a foamed inner layer and a substantially solid skin. Moulded articles are made in a one-step rotational moulding process using a mixture of powdered non-foamable ethylene polymer together with a foamable ethylene polymer in pellet form.

In the process of the Canadian patent, the particle size difference as between the foamable pellets and the powdered non-foamable polymer enables a separation of these materials to be achieved during the rotational moulding process so that an acceptable separation is achieved and the moulded product has the necessary solid outer envelope and associated foamed lining.

We have conducted tests of the process described in the Canadian patent and these show clearly that in practice only very thin and uneven skins or envelopes are achieved when using the conditions described in the Canadian patent. These very thin skins have occasional thick spots, but the inadequacy of the skin thickness is such that the process is not really practical, and this is maybe the reason why this one-shot process has never been commercially exploited. Further, the foamed material does not completely fill the moulded product and instead only forms a relatively thin lining on the interior surface of the skin, which limits the effectiveness of the material.

We have found that in a one-shot rotational moulding process it is not sufficient merely to provide the two main components of the mould charge with differing particle sizes. While such difference in particle size can achieve the requisite separation of the components to enable formation of the impermeable outer envelope and the foamed material within it, in the absence of other contributory factors as defined below, the process as described in the Canadian patent does not achieve an acceptable wall or envelope thickness.

To achieve such an acceptable wall or envelope thickness, we have ascertained that it is necessary to provide two distinct temperature stages in the process, for the materials within the mould. Firstly, there needs to be an initial phase in which the in-mould temperature is high enough to melt the powder of the plastics material which forms the skin or envelope, but is not sufficient to melt the large particles or pellets.

A further contributory factor in this initial phase, which assists in achieving good results is to increase the rate of rotation of the mould (as compared with the second phase of the process) so that the large particles or pellets do not stay in one position of the mould long enough to adhere to the melting powder.

When the skin or envelope has formed in the above-described initial phase, the mould temperature can be increased to that needed to cause the pellets or particles offoamable material to melt and expand so as to fill or partially fill the envelope within the mould.

Our experiments have shown that without the initial lower-temperature envelope-formation phase in the moulding operation, an envelope could not be obtained which had a consistent thickness greater than lmm. For most applications of rotational moulding, such as the production of pallets, floats, tote boxes, fish bins etc., a minimum envelope thickness of 3mm is required and thicknesses up to 6mm are usually needed for the more mechanically demanding applications.

In accordance with this aspect of the invention there is provided a method of producing a plastics article having a cellular interior by a rotational moulding process as defined in claim 16 of the accompanying claims.

By providing a process wherein the initial envelope formation stage is conducted at a lower temperature than the subsequent foaming stage, the embodiments are able to provide a moulded product having a commercially acceptable envelope or wall thickness.

Our test work has shown that the temperature required in the initial envelope or skin formation stage is important and should be in the range defined by the melting point of the plastics material which forms the skin or envelope and a temperature of 100C above that melting point. For example, in the case of polyethylenes, the melting points are around 1200C. Our tests show that if the temperature rises significantiy beyond this range, there will be a reduction in skin thickness.

At this lower temperature, the skin or envelope forms typically at a rate of approximately lmm of thickness per 10 minutes of moulding time. It appears that for a particular particle size and material, the rate of skin thickness formation cannot be readily increased as it is dependent upon the melting point of the polyethylene and the thermal conductivity thereof. Varying rates of fusion and skin thickness formation are caused by varying the particle size of the powder of the plastics material which forms the skin or envelope, and varying its Melt Flow Index and/or density.

Our work shows that the density of the foamable material is important in this one-shot process and the particles used should not have undergone any preliminary foaming or expansion since there is a critical bulk density (BD) for the foamable material below which it will not adequately separate from the other plastics material during the moulding process. This BD is around 0.5gm/cc. The higher this BD figure is, the better will be the separation from the envelope-forming powder, which has a bulk density of approximately 0.3gm/cc. In embodiments of the invention the difference between the BDs of the two plastics materials used in the one-shot process is kept as large as possible.

Reverting to the feature of using an initial lower temperature for the envelope-formation phase of the moulding process, followed by a higher foam-forming phase temperature, the temperatures selected in any given case need to take account of the following additional factors in relation to the mechanism of skin formation. Thus, the temperature in the initial envelope-formation phase should be such as to avoid as far as possible the likelihood of the particles of the envelope-forming plastics material melting right through from one side to the other on contacting the hot mould surface, and thereby forming sites at which the larger particles of the foamable second plastics material can adhere thereto and thus be trapped in or on the envelope or skin so that when the subsequent higher temperature foam-forming phase commences, there is produced a blowhole in the envelope of the moulded article, which will render it commercially unacceptable.

In a further embodiment, the above one-shot principles are applied to an adapted process in which, in place of a foamable material for providing a cellular structure within the envelope or skin of the moulded article, there is employed a second polymeric material which is unfoamed, for example nylon.

Thus, in this embodiment, pellets of nylon replace the pellets of foamable material in the one-shot process, and a good separation of the two plastics material is achieved, with a layer of nylon forming on the inner surface of the envelope or skin of the moulded article, which is of polyethylene. By use of a polyethylene compatibilizer such as is used in Capron 1517 (obtainable from Allied Signal) an improved bond between the nylon and the polyethylene may be achieved. Alternative compatibilizer systems could also be employed.

Such a rotationally moulded article has significant commercial applications, for example for providing polyethylene moulded articles having improved impermeability to hydrocarbon fuels, whereby rotary-moulded fuel tanks can be manufactured for use with petrol or gasolene, these currently being restricted to use with diesel fuels, except where they are separately treated with barrier systems.

Embodiments of the invention will now be described in the following non-limiting examples, in which plastics material 1 is polyethylene and plastics material 2 is polyethylene with a blowing agent.

EXAMPLE 1 - "TWO-SHOT" PROCESS A rotationally moulded polyolefin article having a foamed interior and a substantially solid skin was produced in a "two-shot system as follows: MOULD Block 300mm x 450mm x 45mm PROCESS Polyethylene powder was poured into the open mould.

After heating, with appropiate rotation, a port in the mould was opened and, while still hot, a second charge of partially expanded polyethylene pellets was poured into the mould through the port which was subsequently closed.

The mould was then heated a second time after which it was removed from the oven for cooling.

MATERIALS Polyethylene Particle size Resin Density BD MFI Charge weight (9Icc) (glcc) (@ 190C) (g) 1. Powder 500 0.935 0.30 4 1250 2 PEP* 3mm 0.940 0.33 2.5 850 PROCESS CONDITIONS Temperature Time Rotation Speed (-C) (mins) (RPM) First Heat Stage 210 10 5 Second Heat Stage 250 25 5 Cooling 40 5 PRODUCT The properties of the resultant product were: Average skin thickness 4 mm Cellular foam density 0.18 glcc Note * PEP = Partially expanded pellets.

EXAMPLE 2 - "TWO-SHOT" PROCESS A rotationally moulded polyolefin article having a foamed interior and a substantially solid skin was produced in a "two-shot" system as follows: MOULD Block 300mm x 450mm x 45mm PROCESS Polyethylene powder was poured into the open mould.

After heating, with appropiate rotation, a port in the mould was opened and, while still hot, a second charge of partially expanded polyethylene pellets was poured into the mould through the port which was subsequently closed.

The mould was then heated a second time after which it was removed from the oven for cooling.

MATERIALS Polyethylene Particle size Resin Density BD MFI Charge weight <RT

EXAMPLE 3 - "ONE-SHOT" PROCESS A rotationally moulded polyolefin article having a foamed interior and a substantially solid skin was produced in a òne-shot" system as follows: MOULD Block 300mm x 450mm x 45mm PROCESS Two polyethylene materials one powder the other pellets1 vçere poured into the open mould without prior blending. The mould was closed and heated with appropriate rotation. After heating for a suitable time the mould was removed from the oven for cooling.

MATERIALS Polyethylene Particle size Resin Density BD MFI Charge weight (glcc) (glcc) (e 190C) (g) 1. Powder 500cm 0.935 0.30 2 1250 2 Pellets 3mm 0.950 0.52 4 750 PROCESS CONDITIONS Temperature Time Rotation Speed ( C) (mins) (RPM) First Heat Stage 140 34 15 Second Heat Stage 280 6 5 Cooling 40 5 PRODUCT The properties of the resultant product were: Average skin thickness 4 mm Cellular foam density 0.15 glcc SEE ILLUSTRATION OF "EXAMPLE 3" Example 3 Photographic cross section of rotational moulding employing one shot cellular foam process

Density of cellular structure = 0.15 gm/ce Minimum outer skin thickness = 4 mm Hole created by mould vent tube EXAMPLE 4 - "ONE-SHOT" PROCESS A rotationally moulded polyolefin article having a foamed interior and a substantially solid skin was produced in a "one-shot" system as follows: MOULD Block 300mm x 450mm x 45mm PROCESS Two polyethylene materials1 one powder the other pellets, were poured into the open mould without prior blending. The mould was closed and heated with appropriate rotation. After heating for a suitable time the mould was removed from the oven for cooling.

MATERIALS Polyethylene Particle size Resin Density BD MFI Charge weight (g)cc) (glcc) (&commat;190C) (g) 1. Powder 500cm 0.925 0.30 1.5 1250 2 Pellets 3mm 0.950 0.52 4 1000 PROCESS CONDITIONS Temperature Time Rotation Speed ('c) (mins) (RPM) First Heat Stage 130 34 15 Second Heat Stage 280 10 5 Cooling 44 5 PRODUCT The properties of the resultant product were Average skin thickness 4 mm Cellular foam density 0.20 g/ce SEE ILLUSTRATION OF "EXAMPLE 4" Example 4 Photographic cross section of rotational moulding employing one shot cellular foam process

Density of cellular structure = 0.20 gm/cc Minimum outer skin thickness = 4 mm In further embodiments of the invention, not detailed above, examples were tested in which the particle size of the envelope or skin-forming first plastics material was modified from the 500 microns employed in the above examples to 800 and 1200 microns for use in a single-shot process in accordance with Example 3 and 4 above.

Contrary to the expectation that an increase in particle size of the envelope forming plastics material would tend to inhibit effective separation of it from the foamable plastics material, on the contrary we have found that good results are obtained in relation to the depth and rate of thickness formation of the envelope or skin.

It is thought that these enhanced performance characteristics can be attributed to the increased particle size of the envelope-forming plastics material enabling a more rapid build-up of envelope or skin thickness, coupled with a greater resistance to any tendency to melt-through from one side to the other of the particles on contacting the hot mould, thereby inhibiting any capture tendency of the particles in relation to entrapment of particles of the blowable material.

With the successful use of larger particles of powder it was apparent that the skin forming material itself could be in a pellet form provided the pellets were small enough to separate from the larger pellets. Micro pellets (down to 0.5mm in diameter) made by a die face cutter system was used instead of powder. The examples made suffered from a degree of entrapped air in the skin but otherwise functioned as expected.

In the embodiments, where pellets of the foamable material are employed, and likewise in relation to comparably sized particles of this material, the usual maximum range of particle or pellet sizes is from 0.5 to 6mm with a preferred range of 3 to 5mm, these dimensions referring to the maximum dimension of a pellet or particle.

Claims (20)

1. A method of producing a plastics article having a cellular interior by a rotational moulding process, the method comprising the steps of charging a mould with a first plastics material in comparatively fine powder form, heating the mould at a first temperature during rotation for a first predetermined time to form a substantially impervious skin on the surface of the mould, charging the mould with second plastics material containing a blowing agent in comparatively large granular form, and heating the mould at a second temperature during rotation for a second predetermined time to cause blowing and coalescence of said granules within the skin to form said cellular interior, characterised by the step of using said second plastics material in a form in which said blowing agent has been caused to partially blow or expand said second plastics material before said step of charging said second plastics material into the mould.

2. A method of producing a moulded plastics article having a cellular interior by a rotational moulding process characterised by the step of employing, to produce said cellular interior, a plastics material containing a blowing agent which has been partially blown.

3. A method according to claim 1, characterised by said granules of said second plastics material having bulk densities lying in the range of 0.2 to 0.45 g/cc in said partially expanded form of said granules.

4. A method according to claim 1 or claim 3, characterised by said step of partially blowing said second plastics material being carried out during compounding of said blowing agent with said second plastics material.

5. A method according to claim 4, characterised by the step of using said granules comprising said second plastics material in the form of pellets wherein said blowing agent has been caused to partially blow or expand during an extrusion step in the production of said pellets.

6. A method according to any one of claims 1 to 5, characterised by the step of including extra blowing agent with said second plastics material to offset the amount of blowing agent which has been lost in said partial blowing step.

7. For use in a method of rotationally moulding a plastics article having a foamed interior within an external envelope, a compounded plastics material and blowing agent in pelletised form, characterised by: a) said compounded plastics material having said blowing agent in partially expanded form; and b) said blowing agent being only partially expanded whereby the bulk density of the compounded plastics material lies in the range of 30 to 70% of the bulk density of the unblown plastics material.

8. For use in a method of moulding plastics articles comprising a foamed plastics material, a compounded plastics material and blowing agent characterised by said blowing agent being in partially expanded form

9. A method of making pelleted compounded plastics material according to claim 7 comprising compounding and extruding and pelletising said plastics material and said blowing agent and causing said blowing agent to become partially expanded during said compounding and/or extrusion steps, characterised by the step of controlling the degree of expansion of said blowing agent by temperature control of said compounding and/or extrusion steps and/or by controlling the temperature of the products thereof.

10. A method according to claim 9 characterised by the step of monitoring said degree of expansion of said blowing agent by monitoring the bulk density of particles of said compounded plastics material and blowing agent.

11. A method of producing a plastics article having a cellular interior by a rotational moulding process, the method comprising the steps of charging a mould with a first plastics material in comparatively fine powder form, heating the mould at a first temperature during rotation for a first predetermined time to form a substantially impervious skin on the surface of the mould, charging the mould with a second plastics material containing a blowing agent, and heating the mould at a second temperature during rotation for a second predetermined time to cause blowing of said second plastics material within the skin to form a cellular interior therein, characterised by said step of charging the mould with said second plastics material comprising charging the mould with said second plastics material in the form of comparatively large granules.

12. A method of producing a plastics article having a cellular interior by a rotational moulding process characterised by the step of charging the mould with comparatively large granules of a plastics material comprising a blowing agent, and causing said granules to produce said cellular interior.

13. A method according to claim 11 or claim 12, characterised by said step of charging the mould with said second plastics material comprising charging the mould with said granules in the form of pellets.

14. A method according to claim 13 characterised by using said pellets with dimensions such that, on average, the ratio of the maximum to the minimum overall dimension of a pellet is not great than 2.

15. A method according to claim 13 or claim 14 characterised by using said pellets with a maximum dimension of 4 to 5mm

16. A method of producing a plastics article having a cellular interior by a rotational moulding process, the method comprising the steps of charging a mould with a first plastics material in comparatively fine powder form, heating the mould at a first temperture during rotation for a first predetermined time to form a substantially impervious skin on the surface of the mould, charging the mould with a second plastics material comprising a blowing agent in the form of comparatively large granules, said charging step with said second plastics material being carried out substantially simultaneously with said charging step with said first plastics material, and said difference in particle size between said first and second plastics materials causing separation of said plastics materials during said rotation of said mould, and said method comprising causing said blowing agent to produce blowing of said granules within said skin to form said cellular interior, characterised by the step of raising the temperature of the materials within said mould from said first temperature to a second temperature to cause said blowing of said granules.

17. A method of producing a plastics article having a cellular interior by a one-shot rotational moulding process, characterised by the step of employing a first temperature in the region of the melting point of the plastics material of the skin or envelope of the plastics article during an initial moulding phase of the method, and a subsequently higher temperature to produce blowing of a second plastics material which has been simultaneously charged into said mould with said first plastics material, to produce said cellular interior.

18. A method according to claim 16 or claim 17, characterised by said first temperature being up to 50C above the melting point of said first plastics material, and said second temperature being higher and sufficient to produce blowing of said second plastics material by said blowing agent.

19. A method of producing a plastics article having a cellular interior by a rotational moulding process, the method being substantially as described herein with reference to and as illustrated by the accompanying figures.

20. For use in a method of rotationally moulding a plastics article having a foamed interior within an external envelope, a compounded plastics material substantially as described herein with reference to and as illustrated by the accompanying figures.