TECHNICAL BACKGROUND

The present invention relates to a process of forming plastic products using powdered polymeric materials on the external surface of a mould. More specifically, the process is for producing a moulded product through an external powder moulding process akin to rotational moulding (also known as rotomoulding).

BACKGROUND ART

Many plastic processing methods are used to produce hollow plastic components including blow moulding, injection moulding, thermoforming, slush moulding and rotational moulding. The rotational moulding process, in particular, utilizes high temperatures, a thin- walled metal or composite mould, biaxial rotation in two perpendicular axes, finely divided or liquid polymers, a cooling means using air and/or water to finally produce hollow, seamless, low stress plastic products.

A conventional rotational moulding process has four basic steps described below:

(1) Loading

A pre-determined amount of powdered or liquid plastic is placed in one half of a thin-walled hollow metal or composite mould that is mounted on the arm of a moulding machine. The mould is then closed with the other half of the hollow mould using clamps or bolts.

(2) Heating

The mould is then rotated biaxially about perpendicular axes and moved into an oven where heat is applied. The temperature of the metal mould increases and heats the powder/liquid tumbling inside the mould. The hot material sticks to the inner surface of the mould in successive layers to form the moulded product.

(3) Cooling

When the material has melted and has been consolidated, the mould is moved to a cooling station where forced air, water mist or a combination of both is used to bring the temperature of the mould down to a point below the crystallization or solidification point of the material. Uniaxial or biaxial rotation continues to prevent the molten material from sagging.

(4) Unloading

Once the mould is cooled, the mould is moved to the unloading station where the final moulded product is removed. The mould is then ready to begin the process again.

The simplicity of the process belies the complex interaction of heat transfer and material distribution that occurs within the mould during the process. Traditionally, once a mould entered the oven, nothing more was known other than that the powder material melted and then cooled to form the final product until the cycle was completed. Recently, it is now possible to measure temperatures inside the mould during the cycle, scan the surface of the mould continuously for temperature readings or even place a video camera inside to view the formation of the part. Rotational moulding is unique among plastics processes in that heating, forming and cooling of the material take place within the mould without the use of pressure.

Two of the main limitations for a conventional rotational moulding are cycle time and energy efficiency. The process of heating the machine, mould and material to cure the material and then cooling the same once again to room temperature is time consuming and relatively inefficient in terms of energy utilization. Moreover, the biaxial rotation of the entire mould subjects the machine to large amounts of movement which is likely to cause mechanical failure of machine parts due to stress and wear over time. Such a machine will also require a large footprint in order to have adequate space for the biaxial rotation of the mould.

Other limitations include the ability to produce controlled variable wall thickness across the surface of a moulded part and to readily include multiple layers of materials and/or colours for utility or aesthetic purposes.

Hence, there is a need for a new moulding process that is able to reduce cycle time and utilize energy in a more efficient manner. The inventor is seeking to address some or all the issues described above with the invention of this application. SUMMARY OF THE INVENTION

In accordance with the invention, there is provided an external powder moulding process for producing a moulded product. The process comprises the following steps: (1) rotating a mould having an exterior surface that conforms to the internal surface of the moulded product to be produced;

(2) heating the exterior surface;

(3) dispensing at least one type of raw material onto the heated exterior surface;

(4) melting and distributing said raw material over the heated exterior surface to form the moulded product;

(5) cooling the moulded product; and

(6) stripping the moulded product from the mould.

By way of significant and advantageous difference from the prior art, the moulded product is formed on the exterior surface of the mould while the mould is rotating.

In an embodiment, the rotating step and heating step may be conducted concurrently or alternatively, the heating step may be conducted or initiated before the rotating step. In a further embodiment, the process may comprise a step of gathering any excess raw material. The process may also further comprise a step of recirculating the gathered excess material for re-use in the dispensing step.

In another embodiment, the rotating step may include rotating the mould about a horizontal axis and/or about a tilted axis. The rotating step may also include reversing the direction of rotation.

In an embodiment, the process may further comprise a step of pre-heating the raw material before being dispensed over the heated exterior surface.

In a further embodiment, the dispensing step may comprise controlling the dispensing of raw material so as to form at least one layer. In another embodiment, the dispensing step comprises sequentially dispensing at least two layers of raw material to form multiple layers.

In another aspect of the invention, there is provided a rotational moulding system for producing a moulded product comprising a rotatable mould having an exterior surface that conforms to the internal surface of the moulded product to be produced, a heating means for heating the exterior surface and at least one raw material distributor for dispensing raw material onto the heated exterior surface as the mould rotates such that the raw material melts and distributes over the heated exterior surface to form the moulded product on the exterior surface of the mould.

In an embodiment, the heating means may be provided externally and/or internally of the mould.

In another embodiment, the system may further comprise an excess material collector for gathering any excess raw material. The system may also further comprise a means for recirculating the gathered excess material to the distributor.

In a further embodiment, the system may further comprise a cooling means for cooling the moulded product on the mould. The cooling means may be provided externally and/or internally of the mould.

In an embodiment, the system may further comprise a stripping system for removing the moulded product from the mould.

In another embodiment, the system may be configurable to rotate the mould about a horizontal axis and/or about a tilted axis. The rotation of the mould may also be reversible.

In an embodiment, the system may further comprise a material temperature controller. The material temperature controller maybe provided externally and/or internally of said mould. The process and system of the present invention are able to significantly reduce cycle time of each batch due to a significant increase in energy efficiency of the process compared to the prior art. Also, the present invention allows for considerably better process control since the entire process is conducted on the exterior surface of the mould and is visible throughout.

The process and system provide for various advantages and will be further elaborated in the following pages.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated, although not limited, by the following description of embodiments made with reference to the accompanying drawings in which:

Figure 1 illustrates a side view of the rotational moulding system according to an embodiment of the present invention. Arrows depict the flow direction of the raw material when dispensed onto the surface of the mould. Figure 2 illustrates a front view of the rotation moulding system according to an embodiment of the present invention. Arrows depict the flow direction of the raw material when dispensed onto the surface of the mould.

Figure 3 illustrates a side view of the mould of the rotational moulding system.

Figure 4 illustrates a front view of the mould showing the position of the heating means according to an embodiment of the present invention.

Figure 5 illustrates a plan view of the mould showing the position of the heating means according to an embodiment of the present invention.

Figure 6 illustrates a side view of the mould with insulation installed according to an embodiment of the present invention. Figure 7 illustrates a side view of the mould showing the flow direction of cool air within the cavity of the mould according to an embodiment of the present invention.

Figure 8 illustrates a front view of the mould showing the baffles and airflow holes provided within the cavity of the mould according to an embodiment of the present invention.

Figure 9 illustrates a side view of the rotational moulding system when in use according to an embodiment of the present invention. Figure 10 illustrates a side view of the rotational moulding system according to an embodiment of the present invention when the mould is unloaded from the assembly.

DETAILED DESCRIPTION

The present invention is directed at an external powder moulding process for producing a moulded product and the system used in the process.

In an aspect of the present invention as illustrated in Figure 9, there is provided a moulding system for producing a moulded product. The system mainly comprises:

(1) a rotatable mould 1 having an exterior surface that conforms to the internal surface of the moulded product to be produced; (2) a heating means (7a, 7b, 7c) for heating the exterior surface and/or the interior of the mould 1; and

(3) at least one raw material distributor 4 for dispensing raw material onto the heated exterior surface as the mould 1 rotates. As the raw material is dispensed while the mould 1 rotates, the raw material melts and distributes over the heated exterior surface to form the moulded product on the exterior surface of the mould 1.

Each component will be described in detail in the following paragraphs.

First, a mould 1 is provided. The mould 1 provides a surface for consolidating raw material to form the moulded product. The mould 1 is preferably hollow having an exterior surface and an interior surface. The exterior surface of the mould 1 may be of any suitable shape that conforms to the internal surface of the moulded product to be produced. Raw material is dispensed over the exterior surface of the mould 1 which provides support to form the shape of the moulded product. The present system comprises a substantially cylindrically- shaped mould having a conical shape that is suitable for producing storage tanks but other non-symmetrical shapes can also be used. The mould 1 has one open end. An example of the mould of the present invention is illustrated in Figure 3.

As shown in Figure 1 and 2, at the open end of the mould 1, a flange 2 is provided. The flange 2 is to create the lip of the final moulded product. The flange 2 is shaped as a projecting flat rim extending orthogonal to the axis of the mould 1. Any other suitable shape may be used. As shown in Figure 7, a flange plate 10 may be provided to support the mould during rotation.

The mould 1 is preferably made of a rigid material that is able to withstand heat with temperatures of up to about 400°C. In an embodiment of the present invention, the mould 1 is made from steel. Other materials determined to be suitable such as composite or ceramic may be used or any other material that is able to withstand heat.

Insulation may be used to retain heat in the mould during and between moulding cycles. The insulation used is made of PTFE sheet or ceramic wool or another suitable insulator capable of withstanding the moulding temperatures used and preferably is ceramic wool. As shown in Figure 6, in an embodiment, the insulation may be attached to the internal surface 9 of the mould 1 as well as on the surface of the flange 8 facing away from the mould 1.

The mould 1 is rotated using a rotating means that is attached to the mould. The rotating means should preferably be capable of rotating the mould 1 with speeds of about 0.5 rpm to about 100 rpm. A single axis rotating arm driven by a motor 3 was used in the present system. Typical rotating speeds of the motor 3 are usually between about 2 rpm to about 8 rpm. In an embodiment of the present invention, the rotating means is capable of reversing the rotation of the mould 1 or stopping the rotation during the process to assist with material distribution on the exterior surface of the mould 1.

The mould 1 may be positioned to be rotated about a horizontal axis or about a tilted axis perpendicular to the plane of the view shown in Fig 1 This is to allow proper and even distribution of raw material throughout the exterior surface of the mould 1. The angle of rotational axis of the mould 1 may be between 0° and about 45°. Preferably, the angle of rotational axis is about 0°. In variations of the present invention, the angle of rotational axis of the mould 1 may be fixed throughout the entire process cycle or varied accordingly depending on the specific type of raw material being dispensed and its material properties. In an embodiment, the angle of rotational axis of the mould 1 may be continuously varied so as to create a rocking motion to assist with material distribution on the exterior surface of the mould 1. The mould 1 may optionally be mounted on a cart which is mounted on a track for ease of part removal or for moving the mould 1 to multiple stations for dispensing multiple types of raw material during the process.

Heat is applied to the mould 1 through a heating means. The heating means raises the temperature of the mould 1 past the melting point of the raw material so that the raw material melts and distributes over the exterior surface of the mould 1 while the mould 1 is rotating. As the process proceeds, the heating means melts layers of powder progressively added to the surface. The heating means may be provided externally from the mould 1 or internally inside the hollow portion of the mould 1. Alternatively, the heating means may be provided both externally and internally to hasten the heating process of the mould 1 to cut down cycle time. In the present embodiment, infra-red heaters, electrical heating or circulating hot air or a combination thereof may be used as the heating means. The heating means 7a, 7b, 7c are preferably positioned at the flange (7a), body (7b) and base (7c) of the mould 1. Any other suitable heating means may be used. Positions of the heating means according to the above embodiment are illustrated in Figures 4 and 5.

Illustrated in Figures 1 and 2, raw material is dispensed onto the mould 1 using a raw material distributor 4. The raw material distributor 4 stores the raw material before the process commences and subsequently dispenses the raw material at a predetermined fixed rate over the exterior surface of the mould 1 after the mould 1 is heated to the desired temperature. In the present embodiment, a gravity-fed hopper, brush dispenser or a vibratory feed system that is positioned above the mould 1 is used as a raw material distributor 4 that gradually dispenses the raw material over the exterior surface of the mould 1. Any other suitable means of storing and distributing the raw material over the exterior surface of the mould 1 may be used.

In a variation of the invention, the raw material may be dispensed at different rates along the mould 1 and/or at different times during the cycle to control the build-up of material across the moulded product. The final thickness distribution requirement will be determined by the final product design.

In a variation of the invention, multiple raw material distributors 4 may be used for dispensing different types of raw material onto the mould where upon completion of forming one layer, the mould 1 is moved to the next raw material distributor 4 for dispensing the next layer of raw material over the exterior surface and so forth.

In a variation of the present invention, a raw material deflector 5 may be provided. The deflector 5 is located at the end of the raw material distributor 4 at the closed end of the mould. The deflector 5 directs raw material onto the surface of the end of the mould and also prevents raw material from scattering away from the external surface of the mould. Any excess raw material would be contained and easily controlled to prevent wastage of material. The deflector 5 is preferably positioned vertically and is shaped substantially flat. However, any suitable position or shape may be used in which raw material can be contained and prevented from scattering.

Raw material used is preferably powdered plastic. The raw material powder used should have free flowing characteristics with powder flow values of about 10 to about 40 seconds/100 grams and particle sizes of about 100-1000 pm. Preferably, the raw material powder used has powder flow values of about 20 to about 25 seconds/100 grams and particle size below about 500pm. The raw material may be pre-coloured or in their natural form subject to the desired appearance of the moulded product. Alternatively, liquid plastic may be used as raw material.

Examples of specific powdered plastic material that can be used include polyethylene, polypropylene, nylon, polycarbonate, polyvinyl chloride (PVC), polystyrene, acrylonitrile butadiene styrene (ABS), polyphenylene sulphide (PPS), polylactic acid (PLA), ethylene- vinyl acetate (EVA), ethylene butyl-acetate (EBA), alloys of polyethylene, acetal (POM), polyether ether ketone (PEEK), polyvinylidene fluoride (PVDF), ethylene tetrafluoroethylene (ETFE), ethylene chlorotrifluoroethylene (ECTFE), perfluoroalkoxy alkane (PFA) or tetrafluoroethylene perfluoromethylvinylether (MFA). The type of material selected depends on the desired material to be used for the moulded product and the specific material properties required of the moulded product. A typical material most commonly used is polyethylene with a density range of about 0.920 to about 0.965 g/ml and a melt index range of about 1.5 to about 25 g/10 minutes.

Referring to Figures 1 and 2, as the raw material powder is dispensed over the exterior surface of the mould 1, the powder material particles melt as they come into contact with the heated exterior surface and subsequently stick to the surface. As the particles melt, they attract more material to adhere to the melted particles, eventually forming a continuous layer of melted material on the exterior surface of the mould 1 as the mould 1 rotates. The arrows in Figures 1 and 2 indicate the flow of raw material over the mould.

As the raw material melts and forms a continuous layer on the exterior surface, the thickness of the melted raw material layer must be controlled so that the desired thickness can be achieved with the minimum amount of raw material needed. The thickness of the melted raw material layer can be determined by weighing the residual powder being used, calculating time and temperature profiles or detected using ultrasonic sensors. Any other suitable detecting device or means may be used to monitor the thickness of the melted raw material layer.

Optionally, as shown in Figures 1 and 2, an excess material collector 6 may be provided to collect any excess raw material that fails to adhere to the mould 1, falling off the surface of the rotating mould 1 due to gravity. The excess material collector 6 is preferably located at the bottom of the mould 1 where falling material can be collected. In an embodiment, the excess material collector 6 is a hopper. Any other suitable means of collecting the falling material from the mould 1 may be used.

In another embodiment according to Figure 9, a means for recirculating the excess material collected is provided. This is to minimise wastage of raw material that has failed to adhere to the rotating mould 1. The recirculating means 14 channels the gathered excess material collected in the excess material collector 6 back to the raw material distributor 4. In an embodiment of the present invention, an auger or a vacuum loader was used as the recirculating means. Any other suitable means of recirculation may be used to effectively recirculate excess material back into the distributor 4.

In a variation of the present invention, the powdered raw material may be preheated in the distributor 4 prior to being dispensed over the mould 1. Preheating the raw material before dispensing accelerates the melting process as the material will melt within a shorter period of time upon contact with the heated exterior surface of the mould 1 and/or moulded product in formation. Preferably, the raw material is preheated to a temperature close to the melting point of the material. The temperature of the preheated raw material should not exceed the melting point to prevent the raw material from melting within the distributor 4. Heat is applied to the raw material through a heating means. In an embodiment, the heating means is an electrical jacket or a hot water jacket located around the surface of the distributor 4. Any other suitable heating means may be used such as hot air to fluidize the material in the distributor 4 or a hopper supplying material to the distributor 4.

During the process when the material is dispensed over the mould 1, it is important to monitor the surface temperature of the melted material. When the dispensing of raw material onto the mould 1 is complete, the heating process continues to allow the material to fully melt and for any bubbles trapped in the cross-section of the material to dissolve. This curing of the melted material allows the material surface to smooth into one continuous surface to achieve the desired material strength. Material surface temperature monitoring is conducted using a material temperature controller. In an embodiment, non- contact infra-red sensors provided externally of the mould 1 and/or embedded thermocouples provided internally of the mould 1 are used to control the temperature of the material. Any other suitable apparatus for monitoring surface temperature may be used.

Once the desired surface temperature is achieved, the process switches to a cooling stage. Heat is removed from the melted material using a cooling means. Cooling the melted material causes the material to solidify to form a rigid surface and eventually forming the final moulded product. The cooling means may be provided externally from the mould 1 or internally inside the hollow portion of the mould 1. Alternatively, the cooling means may be provided both externally and internally to balance the cooling rate on both sides of the material to prevent material defects due to cooling too rapidly. Cooling fans or atomized water spray may be used as the cooling means. Any other suitable cooling means may be used.

In an embodiment of the present invention, cooling fans 15 as shown in Figures 9 and 10 are used as the cooling means. The fans 15 push cool air over the external surface of the moulded product which has been formed on the surface of the mould - this direct cooling of the moulded part speeds up the moulding process as the part can solidify externally while the mould remains hot within. Cooling air may also be provided though the internal cavity of the mould 1, cooling the mould's external surface through heat transfer from the internal surface. Air flow through the mould's internal cavity is controlled using baffles 11 provided within the cavity as shown in Figures 7 and 8. Cool air enters the mould cavity though the flange plate 10. The flow of air is guided through airflow holes 12 so that cool air comes into contact with the internal surface of the mould 1, transferring heat and cooling the external surface. The heated air exits the mould cavity though the central baffle 11 located in the centre of the mould's cavity and finally exiting through the exit hole 13. The air flow within the mould's cavity is shown in Figure 7.

During the process when the material is being cooled, it is important to monitor the surface temperature of the material. The cooling rate is critical for reducing shrinkage and warpage in order to maximise the strength of the material. Material surface temperature monitoring is conducted using the material temperature controller described above.

Optionally, when the moulded product has adequately cooled to have sufficient strength for removal, a stripping system may be used to the remove moulded product from the mould 1. Variations for the stripping system such as an ejector provided inside the mould 1 or a ring around the external edge of the mould 1 or an air assist between the mould 1 and the moulded product may be used as a stripping system. Any other suitable means of removing the moulded product from the mould 1 may be used.

With the moulded product successfully removed from the mould 1, the system is checked for any flash to be removed and then prepared for the next cycle.

In another embodiment, the system may further comprise an insulated support plate 8 as shown in Figure 6. This plate acts as lid for guiding the formation of the end of the moulded product to be as close to the final desired shape of the moulded product. When the moulded product is stripped from the mould 1, there will be some excess material at the end of the moulded product which will be trimmed and removed in order to obtain a uniform edge for aesthetic reasons. This insulated support plate intends to minimise wastage due to trimming of excess material formed at the end of the moulded product by ensuring that the end of the moulded product does not have too much excess material that requires trimming. When compared to the prior art, the system of the present invention contains significantly less moving parts i.e. the only moving part is the rotating means and the mould 1 itself, thus, lowering equipment and design cost compared to those of the prior art. The prior art includes complex biaxial movement of the entire mould 1 and requires a significantly more complex equipment for performing such a movement.

Also, with significantly less moving parts required in the present invention, it is anticipated that the system will require a smaller footprint compared to those of the prior art. The prior art will require a significant amount of space for the biaxial rotation of the entire mould 1.

In another aspect of the present invention, there is provided an external powder moulding process for producing a moulded product. The process mainly comprises the following steps:

(1) rotating a mould 1 having an exterior surface that conforms to the internal surface of the moulded product to be produced;

(2) heating the exterior surface;

(3) dispensing at least one type of raw material onto the heated exterior surface;

(4) melting and distributing the raw material over the heated exterior surface to form the moulded product;

(5) cooling the moulded product; and

(6) stripping the moulded product from the mould 1

As a significant and advantageous difference from the prior art, the moulded product is formed on the exterior surface of the mould 1.

Each step of the process will be described in detail in the following paragraphs.

First, the external powder moulding system described above is provided. The process is initiated by first rotating the mould 1. The mould 1 is rotated at a rotational speed of between about 0.5 rpm to about 100 rpm. Preferably, the rotational speed is between about 2 rpm to about 8 rpm. Any other suitable rotational speed may be utilised depending on the type of raw material. Next, the mould 1 is heated to the desired temperature. Heat is applied to the exterior surface of the mould 1 using the heating means. The desired temperature should preferably be higher than the melting point of the raw material. In an exemplary embodiment of the present invention, linear-low density polymer polyethylene was selected as the raw material with a melting point of 123°C. Hence, the final heated temperature of the mould 1 should be at least 123°C and preferably above 150°C. In another embodiment, when there are two or more types of raw materials used, the final heated temperature of the mould 1 should be higher than the highest melting point of all the raw materials of the process. The heating process may be adjusted during a cycle comprising more than one layer if the melting point of the material being used for a particular layer varies.

In other variations of the present invention, the rotating step and heating step of the present process may be interchangeable where the heating step may be conducted or initiated before the rotating step or the rotating step and heating step may be conducted concurrently.

Once the exterior surface of the mould 1 is heated to the desired temperature, at least one type of raw material is dispensed onto the heated exterior surface. Raw material from the raw material distributor 4 is dispensed at a powder flow value of between about 10 to about 35 seconds/100 grams of raw material. Preferably, the powder flow value is between about 20 to about 25 seconds/100 grams of raw material. Any other powder flow value may be used depending on the type of raw material being used in the process.

The raw material is gradually dispensed from the raw material distributor 4 onto the exterior surface while the mould 1 is rotating. Upon contact with the heated surface, the raw material melts and consolidates until the entire exterior surface is coated with a layer of the melted raw material. Thickness of the layer is controlled by varying the amount of raw material dispensed on the exterior surface. Thickness can be varied along the length of the melted surface or in areas which require more or less thickness by varying the amount of heat applied or the amount of powder dispensed in that particular area. The thickness and build-up of the melted raw material on the exterior surface may be controlled by using thickness sensors positioned at predetermined locations along the surface of the part. Ideally the powder dispensing is controlled to synchronize the amount of energy being applied to the mould surface with the average thickness of powder deposited with each turn of the mould 1. If too much powder is applied at any given point on the surface and the powder falls off from the surface of mould 1, there is a lot of powder to be recirculated back into the hopper. A finer layer of powder allows more particles to adhere to a given point on the surface and the opportunity to melt the fine layer before that same point on the surface of the mould 1 returns to the dispensing point to receive another fine layer.

During the dispensing step, the mould 1 may be rotated about a horizontal axis or about a tilted axis. This is to assist in the even distribution of raw material throughout the exterior surface. Alternatively, the axis of rotation of the mould 1 may be alternated between a horizontal axis and a tilted axis so as to create a rocking motion that is effective in aiding in the even distribution of the raw material on the exterior surface. As and when required, the rotation of the mould 1 may be reversed. Any combination of rotation about a horizontal/tilted axis and/or reversing the direction of rotation and/or stopping rotation may be used depending on the type of raw material used.

When the desired thickness is achieved, dispensing of the raw material is halted. However, heat is continued to be applied and rotation continues. Continued heating allows the raw material to melt completely and allow for any bubbles trapped in the cross-section of the material to dissolve. Continued rotation prevents the material from flowing or deforming on the surface of the mould 1 while molten. This curing of the melted material allows the material surface to smooth into one continuous surface to achieve the desired material strength. Any suitable curing temperature may be used depending on the type of raw material being cured. In an exemplary embodiment of the present invention, polyethylene has a curing temperature of about 180°C to about 220°C.

In another embodiment of the present invention, it is possible to dispense multiple layers of different raw materials. Raw materials may differ in type of material and/or colour which will produce a moulded product having different features of hardness, stiffness, permeation resistance, foaming layers, insulation properties, strength, barrier properties, temperature requirements and cost. Multiple layers can be dispensed by subsequently dispensing a different material onto the previous melted material layer upon completion of curing. Each layer is dispensed in sequential steps. Two or more raw material distributors 4 may be used where upon completion of forming one layer, the mould 1 is subjected to the next raw material distributor 4 for dispensing the next layer of raw material over the exterior surface and so forth.

Examples of material combinations include different types and/or colours of polyethylene or different material combinations such as PE/polyamide, PE/PE foam/PE etc. Any other suitable material combination may be used depending on the desired physical requirements/appearance of the final moulded product.

Non-polymer materials may also be used in combination with polymer materials either to create aesthetic effects or to reduce cost or to add desirable physical features. Examples include powdered or crushed stone of various colours to create a stone-effect outer surface to replicate a natural stone exterior on the surface of the plastic part. Natural fibres, wood chips, sawdust or any other raw material may also be used as a layer in the final moulded part.

When all layers of raw material are dispensed and cured completely, the moulded product is subsequently cooled in a cooling step. The cooling step can be initiated immediately when the curing of the final layer is complete or delayed for a period to allow the temperature of the material on the mould 1 to achieve equilibrium. The cooling step assists in lowering the temperature of the moulded product so that the moulded product can be subsequently demoulded or stripped from the mould 1. The rate of cooling is crucial in controlling shrinkage and warpage to maximise material strength. The typical demoulding temperature will depend on the material's rigidity post-solidification. In an exemplary embodiment of the present invention, polyethylene has a demoulding temperature of between about 110°C and about 60°C. More preferably, the temperature is about 80°C. However, higher temperatures allow the part to be removed more quickly during the cycle. Any other suitable demoulding temperature may be used in the cooling step of the present invention depending on the type of material used and the material's rigidity properties.

Upon completion of the cooling step, the cooled moulded product can finally be stripped from the mould 1. In the present invention, the moulded product may be stripped using ejectors provided inside the mould 1 or a ring around the external edge of the mould 1 or an air assist between the mould 1 and the moulded product. Any suitable method of demoulding the moulded product may be used.

Once the moulded product is stripped from the mould 1, the mould 1 is inspected for any flash to be removed and prepared to repeat the process for the next cycle.

In other variants, the process may further comprise a step of gathering any excess raw material. Raw material that is dispensed over the exterior surface of the mould 1 may fail to adhere to the mould 1, falling off the surface of the rotating mould 1 due to gravity. An excess material collector 6 gathers any excess raw material that falls off the mould 1.

In another embodiment, the process may further comprise a step for recirculating the excess material collected from the excess material collector 6 described in the previous step to the raw material distributor 4. This recycling of the excess material is to minimise wastage of raw material that has failed to adhere to the rotating mould 1. In an embodiment of the present invention, an auger or a vacuum loading system was used for the recirculating step. Any other suitable means or apparatus of recirculation may be used to effectively recirculate excess material back into the distributor 4.

The process of the present invention is anticipated to reduce cycle time down to about 10 minutes or less as the heating and cooling is applied directly onto the exterior surface of the mould 1 where the raw material is situated. This is an improvement over the prior art where the heat has to be transferred through a layer of rigid material to the raw material located inside the mould 1. Also, the cooling step in the prior art is highly inefficient as the entire mould 1 has to be cooled rather than just cooling the moulded product directly as described in the present invention.

Also, forming the moulded product on the exterior surface of the mould 1 allows for better process control as the entire process is visible and any changes to the process controls can be made immediately where necessary. In the prior art, the melting process inside the mould 1 is difficult to monitor as it is not visible when the process is conducted. The entire process control has to rely entirely on sensors and/or visual cameras located inside the mould 1 which may not accurately detect any defects or faults that require immediate atention for rectification until the process is complete. The present invention eliminates this problem.

The prior art requires heating and cooling the entire mould 1 in order to produce the moulded product. Such indirect energy transfer through the mould's thick material requires a significant amount of energy. As the heating and cooling is applied directly to the raw material and moulded product in the present invention, the entire process is significantly more energy efficient compared to those of the prior art.

EXAMPLE

The following Example illustrates the various aspects, methods and steps of the process of this invention. This Example does not limit the invention, the scope of which is set out in the appended claims.

A mild steel water tank shaped mould approximately 800 mm in diameter at the top, 800 mm deep and 600 mm across the bottom was mounted on a uniaxially oriented rotating arm.

The mould was coated in a black ceramic paint to improve heat transfer.

The mould was made of 3 mm thick mild steel. The flange was made of 10 mm thick mild steel.

18 KW of electrically powered infra-red heating elements were arranged around the mould to provide heating at the flange, along the main length of the mould and at the bottom such that all surfaces were heated. Some areas received additional heating to ensure melting occurred, e.g. the thick flange of the mould.

The infra-red heating elements were tuned to a wavelength between the optimal value for steel and polyethylene. It may be possible to utilise elements which have been optimised for polyethylene alone to accelerate the melting process.

A series of hoppers were arranged above the mould to allow powder to flow freely onto the mould surface. These included deflectors to direct the powder onto the vertical faces of the flange and bottom of the mould.

Polyethylene powder ground to a commercial standard 35 mesh (500 microns) was used for testing. Several colours including black, green and red were tested - each colour has a different absorption rate for infra-red energy.

The mould was warmed using the heating elements for a period of approximately 10 minutes. The mould was rotated at 5 rpm. When the mould reached a temperature of approximately 150°C, the powder hoppers were opened to allow the powder to flow onto the mould surface. The hopper openings and deflector positions were adjusted to create a slow steady flow onto all surfaces of the mould.

Powder stuck to the surface and the process of recirculating powder which did not adhere was repeated until the desired thickness of the tank was estimated to have been achieved. In this test a wall thickness of approximately 3-4 mm with a tank weight of 6-7 kg was achieved.

When the thickness of the tank had reached an acceptable thickness, the mould continued to rotate allowing the material to melt and rise in temperature to approximately 200°C. This process required approximately 10 minutes.

As the material melted, the surface became smoother and the tank structure was consolidated.

Once the surface was acceptably smooth and the maximum temperature reached 200°C, the heating process was stopped and six cooling fans switched on at 20% capacity initially to prevent the molten plastic from being deformed. As the plastic solidified, the fan capacities were increased to 50%. The cooling process was fast as only the plastic needed to be cooled - the process took 3-5 minutes.

When the moulded part temperature had reached 100°C, air was applied via a port at the bottom end of the mould to force air between the part and the mould and push the part from the surface of the mould. The part was then removed from the mould and the cycle was complete.

The final part produced was a complete cylindrical shaped tank with a flange. The flange required minor trimming around the top edge to produce a final part.