ROTATIONAL MOULDING

Rotational moulding is a manufacturing process for thermoplastic materials that allows hollow articles to be produced of any shape or size. It is a technology that is different from conventional injection and blown moulding processes, and is particularly indicated for use in the production of small series of articles, even geometrically complex ones, particularly of large size and hollow articles which are impossible to produce otherwise. The process began to be used typically for containers, taking advantage of the fact that hollow articles could be produced without the subsequent need to weld or assemble. Applications have multiplied, thanks too to the continuing development of research applied to the polymers used in the process: polyethylene, (LLDPE,LDPE, LHDPE, HDPE), reticulated polyethylene, polypropylene, E.V.A., nylon, polycarbonate, PVC, etc.

Here, in brief, are the main advantages of the process:

• maximum versatility: the possibility of obtaining articles of different shapes and sizes from the machine simultaneously;
• an unlimited design field for the article to be produced: just think of the shapes of any formed container or recipient;
• limited initial investment compared with the productive capacity available;
• complex shaped articles can be moulded in one piece maintaining the consistency of wall thickness;
• articles of differing thicknesses can be produced simultaneously;
• colours and materials can be changed rapidly;
• doubled walled articles can be produced;
• low pressures used and reduced speed of rotation minimise wear of the moulds and moving parts;
• possibility of differing the sizes of articles to achieve the rigidity and impact resistance desired;
• "automatic" inclusion of metal, plastic and rubber parts;
• limited wastage: an almost exact correspondence between the quantity of material required and that utilised. The residues of work originate in trimmings and removal of certain appendages;
• low cost of moulds and reduced management costs.

THE PROCESS

The basic process is straightforward. A powder or liquid polymer is introduced into the mould which, rotating simultaneously on two vertical axes, is first heated and then cooled. The shaping of the article occurs in different ways, according to whether the material used is powder or liquid. Powder (usually polyethylene), during the heating phase, initially forms a porous film on the internal surface of the mould and to which the rest of the material, which has gradually become fluid, then adheres to form a uniform layer which will solidify in the subsequent cooling phase.

Liquid material (normally PVC and PLASTISOL), on the other hand runs along the walls of the mould and is heated to the temperature at which the fluid solidifies, taking on the shape of the mould which will be cooled subsequently in a water bath or with blown air. The cooling phase is crucial for certain products because each material needs different timings in the lowering of the temperature, generally longer than that needed for polyethylene. The final phase is that of opening the mould to extract the article obtained and introduce fresh raw material for the subsequent cycle.

The most widespread system of rotational moulding uses a horizontal rotating unit commonly called a carousel equipped with 3, 4 or more mould carrying arms which are made to pass automatically through the various workstations (loading, baking, cooling, article extraction). Different types of heating are used though the forced hot-air system has proved to be the cleanest, cheapest and safest.

As it can be seen, the cycle is completely automated: only the operations of loading/unloading are manually performed and this is probably the price we have to pay for the great versatility of the process. The technology needed for complete automation already exists, but it is hardly ever used because it reduces flexibility considerably. For the moment, only automated systems for weighing and dosing the resins are economically applicable.

Even though the cooling phase is usually considered the least important, research has shown that correct control of the cooling cycle can provide dimensional balance, so improving the physical properties of the article. Depending on the chemical properties of the polymer, the cooling phase produces different effects on the physical properties of the article. In crystalline polymers it is to be hoped that minimum distortion of the crystals will take place, especially if good strength is required at low temperatures. In amorphous polymers on the other hand, cooling has a minimal effect on the physical properties of the product, as it is not necessary to keep an increase of the crystals under control.

THE PLANT

The range of machines available goes from small and simple types for explorative development and for use in laboratories with one arm and moulding chamber, to complex plants with independently controllable arms with a spherical diameter of up to 5,000 mm, capable of volume production and large articles.

Recently several moulding machines for rotational moulding with shuttles (Rock & Roll machines) have been designed and constructed for moulding larger articles. One of these machines currently produces containers of over lt. 40,000 capacity With this type of machine, the mould and the material are loaded on to a large independent shuttle, which runs along tracks for the move from oven to cooling chamber.

THE MOULDS

If they are compared to the equipment used in injection or blow moulding, the moulds for rotational moulding are certainly more economical. In their construction different types of metals are used. The most practical for the production of small or medium sized articles with multiple cavities are moulds in cast aluminium. Costs vary according to quality or the details requested for the finished product. The moulds in electro-formed nickel are used for the moulding of small articles in PVC and have the advantage of providing a good surface reproduction, without the problems of porosity that can afflict moulds in cast aluminium. Sheet steel is used normally for moulding prototypes and for producing large articles, generally cylindrical containers and other simple shapes.

APPLICATIONS

The articles that can be rotation moulded are increasing continually both because of innovations implemented in plant and research carried out on the materials. The articles most typically found are: containers for produce and foodstuffs, containers for chemical products, containers for water collection, office furnishings, household goods, garden articles, waste bins, lighting globes, boats, canoes, various types of toys, dolls etc. For a more detailed list, see the attached specification.

NEW SYSTEMS OF PROCESS CONTROL

In the rotational moulding industry too, electronic data processing applied to systems of supervision of the machine cycle has come in: very many machines are already in use which are controlled by microprocessors. The large quantity of data generated by the devices mounted on the installation is organised and rendered available by a PC linked to the PLC. It is thus possible to manage the different phases using a mouse and keyboard or set up completely automatic functioning. The system then generates all the data of the cycle (temperature, duration, rotations etc.) which are available for subsequent checking. The most advanced systems allow active management of the moulding recipes and so permit recovery of the correct mix for each article. Of particular utility is the automatic display system and locating of alarms.

APPLICATIONS SPECIFICATION FOR ROTATIONAL MOULDING

Containers for foodstuffs and agricultural produce:

• Storage tanks, vats and drums for dairies and the ice cream industry;
• Small silos for the preparation and packing of pasta, rice, spices etc.;
• Small cisterns, tubs and bag-in boxes for various drinks;
• Containers for the treatment of vegetables, olives and fruit;
• Marls and vats for sausages and butchery;
• Tanks for the transport of fry;
• Isothermal containers with double walls for ready to eat foodstuffs, coconut oil, shelled nuts, fish products and frozen foods;
• Small silos and cisterns for solid and liquid fertilisers;
• Hoppers for the distribution of animal feed.

Machine components and agricultural plant:

• Agricultural machines; fuel and lubricating oil tanks, cisterns and hoppers for products to be irrigated, hoods, dashboards, fairings, mudguards;
• Zootechny and irrigation; mangers, feed distributors, silos, boxes for programmed irrigation and feeding, channels and derivations;
• Gardening: tanks, frames for grass collectors and mower bodies, carts and manual rollers, flower pots and flower pot holders including those for hydroponics.

Energy sources, recovery and community services:

• Insulated cases for solar panels;
• Insulated hydraulic accumulators for heat exchangers and pumps;
• Urban rubbish bins;
• Glass collection bells;
• Small silos for plastic off-cuts;
• Containers for industrial residues;
• Small tanks for the washing of rubbish bins;
• Cisterns for emergency water reserves;
• Special containers for fire prevention;
• Various types of urban furniture.

Containers for the transport and storage of miscellaneous materials: (powders, granules, liquids, bulk, including aggressive chemicals)

• Small framed, palletised and stackable silos and cisterns;
• Tipping carts and palletised cases for bulk products;
• Spherical tanks for low-pressure release;
• Drums and pallets for special uses;
• Packing in expanded plastic for fragile and valuable objects;
• Underground storage tanks;
• Graduated cylindrical containers.

Installations and industrial services:

• Meters for water treatment;
• Frames for fans, turrets, aspirators, impellers, hoods;
• Surface treatment and miscellaneous work tanks;
• Tanks and hoods for floor cleaners and industrial vacuum cleaners, compressors, electric generators and welders;
• Piping and flanged connectors;
• Insulated elements for sound proofing plant and machinery.

General applications for various manufacturing industry:

• Boxes for NC machine tool programmers;
• Cabinets for terminals, electronic weighing machines, piece counters;
• Large diameter cable rollers;
• Cases for electric accumulators;
• Protective shielding for mechanical transmissions;
• Covers for various machines and plant;
• Shaped tanks for lubricating oils and miscellaneous liquids;
• Large capacity rotating barrels for the tanning industry;
• Baking containers for the dying industry;
• Jars for spinners;
• Frames for armchairs and sofas;
• Benches and the like for external furnishing.

Transport and road signs:

• Air conveyers and other under bonnet accessories;
• Arms and frames for seats;
• Tanks for water and sanitary installations in caravans and mobile homes;
• Luggage racks and trailers;
• Air deflectors and intakes, mudguards and grilles for industrial vehicles;
• Body parts for camper derived vehicles, ambulances etc.;
• Fairings for motorcycles, bobsleighs, sidecars etc.;
• Work platforms for mobile ladder vehicles;
• Gas bottle covers for caravans and mobile homes;
• Battery boxes;
• Road salt distributors;
• Miscellaneous elements for sign-posting and road safety, guard-rails, emergency telephone boxes.

Sailing and ports:

• Rowing boats and outboards;
• Rudders and catamarans;
• Windsurf;
• Canoes, floats, and fenders;
• Multiple life belts, and containers for life boats;
• Jetties, floating collars, rafts;
• Elements for beach cabins.

Toys and publicity materials:

• Rocking horses, toy vehicles;
• Slides and large size toys for community use;
• Miscellaneous elements for fairground amusements;
• Dummies and other publicity articles.