While rotational moulding in plastics only became possible with the introduction of polyethylene in powder form about fifty years ago, the last ten years has seen major advances in the process.
Rotational moulding now competes with injection and blow moulding in the production and moulding of both simple and complex components or products, especially when low volumes are required.
Historically the method was recommended for applications where a hollow part was required but today it is surprising even its harshest critics with the many and varied shapes and forms it can produce.
Think of a product as small as an ear syringe, a ping pong ball or a face mask and then imagine a 28,000 litre tank weighing 700 kilograms or a complete body of a boat. This will give you some idea of the scope of rotational moulding.
With important advances in plastics technology combined with enormous progress in mould construction, almost every industry is now finding applications for the much-improved process. High-tech sectors such as the aircraft industry have adopted rotational moulding – no longer viewing it simply as a process for moulding agricultural products.
Many furniture designers are catching on. But one of the biggest growth areas is retail display. Some of the newest and most successful applications of rotational moulding in Australia is in point of sale and merchandising systems.
According to Leisa Donlan, of the Rotational Moulders Association of Australasia, rotational moulding is ideally suited to point of sale applications because it makes short production runs very cost effective and it doesn’t have the high labour charges associated with some composite processes.
Colour and graphics
While much of the progress in rotational moulding has been in automation, the process has also opened up opportunities for extensive use of colour and graphics.
Masterbatchers have now developed a much wider range of colours for rotational moulding. New colourful pigments are now ground with the polymer to ensure increased colour intensity.
Historically it has been difficult to apply graphics to rotational mouldings because polyethylene (PE) resists adhesives. But with new technical advances, graphics can now be applied in the moulding process and embedded directly to the product, or permanently fused to the outer surface of an already moulded part.
Both forms of graphic application are resistant to chemicals, solvents and extreme weather conditions.
Special effects such as sparkles or luminescence can be added or simple markings like barcodes or serial numbers are now being applied. Corporate logos and graphics are featuring on many of the products coming through the process.
A wide range of colours, textures and effects can also be sprayed directly onto mouldings.
Rotational Moulding Compound
By incorporating a new material known as RMC3 (Rotational Moulding Compound) – patented by Mold In Graphics Systems® in the US – moulders now have the ability to create solid plastic features on the inside or outside of hollow parts.
The moulder inserts a pliable form of PE inside a mould at the required position. This can also help to solve any poor internal flow or fill problems.
A mould is placed in a moulding machine that has loading, heating and cooling areas. Several moulds may be placed on the machine at the same time. Cycle times are typically 15-20 minutes.
Production rates are increased with multiple arm machines, and systems that are fully automated. These new multi-armed machines allow multiple moulds of different sizes and shapes to be run at the same time.
Pre-measured plastic resin is loaded into each mould (in powder, granular or liquid form). Moulds are then moved to an oven where they are slowly rotated biaxially.
The molten resin sticks to the mould and coats every surface evenly while the moulds continue to rotate during a cooling phase so that the parts retain a consistent wall thickness.
Speed, heating and cooling is closely monitored throughout the process. Once parts are cooled they are removed from the mould.
Some of the largest ovens now incorporate a 5.5 metre swing. Entire boat hulls and automotive bodies are being rotationally moulded in one piece. Complex geometry is easily incorporated into a one-piece product design.
The automation of mould opening and mould filling is also helping to reduce cycle times and improve consistency of moulded parts.
Rotationally moulded products are renowned for their strength and durability. As it is a ’no stress’ process, the material tends to collect in corners and ribs making these areas thicker and stronger.
The least expensive and lowest maintenance mould is one that has two pieces and one part line. The toy industry in the US has mastered the concept of the two-piece mould in product design, resulting in low cost tools used in high volume production.
Solid modelling and CNC tooling enables moulders to produce complex products. And rotational moulding can be used to cost effectively redesign products to reduce the number of components required and cut down on assembly costs.
Donlan says new additives are continually being developed.
Materials have to meet stringent food contact requirements. Other specifications include chemical resistance, anti-static properties, weather resistance and flame retardance.
Donlan says that masterbatchers are currently working on anti-fungicides, reassessing UV stabilisers and looking at adding odours.
Solid or foam multilayers of the same material are becoming popular with moulders worldwide. Sandwich or triple wall mouldings, with an inner and outer skin filled with a foam layer are used for their strength and buoyancy characteristics in boating design.
There are several ways to add foam to a product, according to Donlan: “Some systems simply use a liquid to fill the cavity after moulding, other systems have a balloon type arrangement which bursts when the temperature increases and blows the material which then forms an inner skin.”
Prototyping and tooling
Rotational moulding is recognised as being an economical option for prototyping. Comparatively low cost tooling can be produced from aluminium, stainless steel, mild steel and nickel. It can be produced quickly and cheaply because no internal cores are required and minor changes can be easily made to existing moulds.
• A hollow part can be made in one piece without weld lines or joints.
• The moulded part is essentially stress free.
• The moulds are relatively inexpensive.
• The lead time for manufacture of a mould is relatively short.
• Wall thickness can be quite uniform compared to other free surface moulding methods such as blow moulding.
• Wall thickness distribution can be altered without modifying the mould.
• Short production runs can be economically viable.
• No material wastage, the full ‘charge’ of material is normally consumed in making the part.
• It is possible to make multilayer, including foamed parts.
• Different types of mouldings can be moulded together on one machine.
• Inserts are easily moulded in.
• High quality graphics can be moulded in or applied after moulding.
• The manufacturing times are long.
• The choice of moulding materials is limited, polyethylene is the primary material used.
• The material costs can be relatively high when special additive packages are required and the material must be ground to a fine powder.
• Some geometrical features such as ribs, require the skills of an experienced manufacturer.
Clever Makita stands out
The Makita ‘point of sale’ pillar is one of the most impressive products to emerge from recent improvements in rotational moulding in Australia. The clever concept is the work of Advantage Line Pty Ltd, who designed the unit, and Precision Poly Pty Ltd, the rotational moulder, based in New South Wales.
Precision Poly’s Director, Malcolm Baird, says the unit demonstrates an important breakthrough in reducing components required and the labour costs and time taken to assemble merchandising systems.
“The main feature of the Makita pillar is that it is moulded in one single piece. It is moulded using brass thread inserts, eliminating the need for drilling holes. The moulded undercuts allow for light fittings and a removable insert for placement of the power cord.”
Baird says the brief from Advantage Line was to “push the manufacturing process to it’s limits and make it happen” and most importantly eliminate the need for any secondary operations, therefore reducing the cost.
“A team meeting involving the Advantage Line design team, the toolmaker and rotational moulder was held to optimise the design and the look of the pillar. A foam model of the unit and a wooden plug were produced. This helped to eliminate any unforeseen difficulties or improvements required in the tool.
“Rotational moulding allows for a more efficient moulding process and also enables small volumes to be produced at an economical rate.”
The Makita pillar is a large complicated shape and an added attraction of rotational moulding is its ability to produce a stronger and consistently high quality product.
According to Baird, the mould was produced with a colour and design shape to epitomise the power tool casing used by Makita. The Makita logo was moulded into the curved section as a reverse indent.
“The Makita pillar has been very well received within the hardware industry. The unit itself is adaptable and can be used in a number of ways within the merchandise system. Fittings can be attached to the pillars in the middle or end of a display, or side by side and back to back.”
Precision Poly attracts interest from a variety of industries, mostly for custom-made moulding, but also produces large and small items ranging from point of sale units to water tanks, filters and testing units to construction site toilets.