Rotational Molding (BrE moulding) involves a heated hollow mold which is filled with a charge or shot weight of material. It is then slowly rotated (usually around two perpendicular axes) causing the softened material to disperse and stick to the walls of the mold. In order to maintain even thickness throughout the part, the mold continues to rotate at all times during the heating phase and to avoid sagging or deformation also during the cooling phase. The process was applied to plastics in the 1940s but in the early years was little used because it was a slow process restricted to a small number of plastics. Over the past two decades, improvements in process control and developments with plastic powders have resulted in a significant increase in usage. Rotocasting (also known as rotacasting), by comparison, uses self-curing resins in an unheated mould, but shares slow rotational speeds in common with rotational molding. Spin casting should not be confused with either, utilizing self-curing resins or white metal in a high-speed centrifugal casting machine.
It is possible to mould very thick wall sections using the rotational moulding process. Large capacity tanks have been moulded with wall thicknesses of up to 25mm. For most products a wall thickness of 4-6mm is considered normal
The most commonly moulded plastic is linear polyethylene. This is available in Linear Low Density (LLDPE), Linear Medium Density (LMDPE) and High Density (HDPE) forms. Crosslinked Polyethylene (XLPE) is used where exceptional low temperature impact strength is required in moulded parts. Ultra Low Density Polyethylene (ULDPE) can be used for parts requiring extreme flexibility and low temperature performance. Other materials include Polypropylene (PP), Nylon (PA) and Fluoropolymers – Poly Vinyl Difluoride (PVDF), Ethylene Chlorotrifluoroethylene ECTFE. These are more expensive materials than polyethylene. Fluoropolymers are typically 30-60 times the price of polyethylene and are used only where extremely high chemical and mechanical performance is required.
A standard colour range but any colour can be formulated if a specific colour match is required. Special effects are also possible including stone effect and metallic effect. With customised mould textures, and post mould finishing, wood effects and rock effects can be reproduced.
The best method of colouring rotomouldable polymers is compounding. This involves melting the polymer granules, adding a pigment Masterbatch powder, extruding the molten mixture, cooling the extrudate and pelletising.The coloured pellets are then ground to powder for moulding. Compounding provides a homogenous mix with pigment well dispersed and encapsulated. The pigment when added to the polymer in this way has little or no effect on the base properties of the polymer.Alternatively, natural powder can be ‘dry-blended’ directly with coloured pigment, usually in a high speed mixer. This can be slightly cheaper than compounded colour, but reduces the need for stock of many colours of compound. However, compared to compound colour, the individual particles of dry-blended colour produce a weaker product and one that has a colour that is less light-fast. For these reasons, Francis Ward never recommends the use of dry-blend in rotomoulding. Francis Ward commonly uses a compound blend technique which involves the addition of a percentage of compounded powder to natural powder in the mould. Mixing is achieved during moulding and the resultant parts have good colour with good mechanical properties, and the cost of coloured parts is reduced.
The use of correct radiuses on the corners of rotomoulded parts is critical. Sharp corners with tight radiuses are stress concentration points in moulded products and commonly are impact fracture propagation points. The correct radius to adopt will depend on the part to be moulded. Large parts may require greater radiuses. Preferably a minimum of 5mm should be used. This is particularly important on any concave internal corners in the product.
The life expectancy of moulds depends on the quality of manufacture, gauge of metal used, weld effectiveness etc. Francis Ward has been able to produce more than 100,000 parts from some of its smaller steel moulds. Larger moulds, which experience greater moulding and de-moulding stresses, will produce far less parts before the need for repair or refurbishment. A 1000-litre capacity steel mould may produce up to 10,000 parts before needing significant attention. Aluminium moulds are not subject to such stresses but they are formed from a softer metal that can be scored or suffer abrasion, so maintenance of aluminium moulds is usually more frequently carried out than that carried out on steel moulds. They also require care when cleaning, particularly on mating surfaces.
It is very hard to produce large flat areas in rotationally moulded parts. Distortion during part shrinkage and cooling is common and difficult to prevent. Some jigging of parts and control of cooling will reduce distortion, but it is recommended that flat areas are not designed into parts. Straight sides can be achieved by using ribs, but it is always best to consult your rotomoulder on how to establish the best shape for your product
There are a number of ways to ensure your parts are as strong as possible. Your part design should have correct radiuses and wall thicknesses. It is possible to use computer modelling, such as Finite Element Analysis (FEA) to assess the stress characteristics of a design and adjust where necessary. Select the best material to achieve the performance you need. LLDPE is tougher than LMDPE which is tougher than HDPE. Rigidity increases with density but toughness reduces. If the parts are coloured, make sure that you are using the best colouring methods. Optimise processing conditions. Undercooked and overcooked mouldings will have reduced mechanical and impact strength. Use a competent and experienced rotational moulder who will be able to guide you through all of the above.
Due to the capabilities and machinery available at Francis Ward, we are able to create moulds that range from 0.5L to 40,000L capacity.
Rotomoulding is more environmentally friendly in a number of ways. For example: There is minimal waste as the raw material is used up in each moulding. The most commonly used material - polyethylene - is the most environmentally friendly in terms of manufacture. It can also be recycled / processed to make new rotational mouldings or it can be used to generate heat through incineration.
We have over 50 years of experience in rotational moulding so are happy to assist with the production of design concepts based on your requirements. We also work with partners to create 3D CAD drawings and can then manage the entire process end-to-end from management of tool making, sampling and prototyping, in-house fabrication and production.
There are a number of factors that make a roto moulded product ideal for the storage or transportation of liquids: Moulds are biaxially rotated at low rotation speed to produce one piece hollow mouldings. Due to the low pressures involved, and the rotation of the powder as it melts and shapes, you can achieve containers that have no seams or joints, and effectively no built-in stress. Wide range of materials to select from to give product optimum resistance to liquids, solids, chemicals and weather conditions
This depends on the type of material used in the production of the rotomoulded part, and also depends on other factors such as length of time exposed to heat and any pressure that is also being applied. As a guide however, polyethylene is suitable up to approx 100°C, polypropylene to about 300°C..
This very much depends on the application, however due to the strength and resilience of rotomoulded products some can last over 40 years.