Rotational moulding

Rotational moulding is one of the plastic processing methods for the production of hollow plastic products. Plastic powder is charged and subjected to biaxial rotation in an oven at a temperature of 200- 00 °C. The plastic powder melts inside the mould by heat transferred via convection through the mould wall. All the powder melts and the mould is moved out of the oven while maintaining the biaxial orientation. Exposure to air, or using a fan or water shower are usually employed to cool the mould. 

Mould open and Rotational part removal moulded part 

8 Rotational Moulding and Transfer Moulding 6.8.1 Rotational Moulding 

Rotational moulding or rotomoulding is an extremely popular and much used process for producing items that are usually hollow. It is most often used for very large articles that are usually made in small quantities. Items such as children s toys, garden furniture, and road traffic bollards are manufactured by rotational moulding. 

When PE is subjected to rotational moulding, warpage can occur due to the operational conditions. Warpage is a complex process depending on mould size, material from which the mould is constructed, cooling mode, and presence or absence of release agent. 

XPE can be rotational moulded to produce automotive parts, toys, food handling systems, and chemical tanks. It also provides higher heat/chemical resistance and stiffness than normally achieved by rotational mouldable LLDPE. LDPE with an MER of 2-20 g/10 min and a density of 916-939 kg/m is recommended, followed by EVA copolymer and HOPE. HOPE is more difficnlt to melt than LDPE. It also shows a tendency to thermal degradation and more prononnced deformation on cooling, mainly for flat surfaces. PP is avoided because of its susceptibility to degradation. 

Apart from products with compact walls made of a single type of polyolefin, rotational monlding can be also used to obtain products with cellular and stratified walls made from PE/PP, PE/PA, etc. 

PP can be rotational moulded as an alternative to PA, polyester and crosslinked PE for such rotational mouldings as automotive parts, toys, food handling systems and chemical tanks [30]. It also provides higher heat/chemical resistance and stififiiess than normally achieved by rotational mouldable LLDPE. 

The process was originally developed in the 1940s for use with vinyl plas-tisols in liquid form. It was not until the 1950s that polyethylene powders were successfully moulded in this way. Nowadays a range of materials such as nylon, polycarbonate, ABS, high impact polystyrene and polypropylene can be moulded but by far the most common material is polyethylene. 

The process is attractive for a number of reasons. Firstly, since it is a low pressure process the moulds are generally simple and relatively inexpensive. Also the moulded articles can have a very uniform thickness, can contain reinforcement, are virtually strain free and their surface can be textured if desired. The use of this moulding method is growing steadily because although the cycle times are slow compared with injection or blow moulding, it can produce very large, thick walled articles which could not be produced economically by any other technique. Wall thicknesses of 10 mm are not a problem for rotationally moulded articles. 

The heating of rotational moulds may be achieved using infra-red, hot liquid, open gas flame or hot-air convection. However, the latter method is the most common. The oven temperature is usually in the range 250-450°C and since the mould is cool when it enters the oven it takes a certain time to get up to a temperature which will melt the plastic. This time may be estimated as follows. 

3 en the mould is placed in the heated oven, the heat input (or loss) per unit time must be equal to the change in internal energy of the material (in this case the mould). 

This equation suggests that there is an exponential rise in mould temperature when it enters the oven, and in practice this is often found to be the case. 

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Injection/compression moulding - Thermoplastics and thermosets Blow moulding - Thermoplastics Rotational moulding - Thermoplastics... 

Low molecular weight, narrow MWD polymer—used widely for injection moulding and rotational moulding. 

Although the above applications consume over 90% of the polyethylene produced there are a number of other important end-uses. Filament for ropes, fishing nets and fabrics are an important outlet for high-density polyethylene powdered polymers are used for dip coating, flame spraying, rotational moulding and other outlets, whilst fabricated sheet is important in chemical plant. 

One recent development is rotational moulding. This process has enabled large mouldings of polycarbonate to be made using reasonably simple and inexpensive equipment. 

Another area which is of considerable interest is the development of rotationally moulded products. These mouldings include air ducting housing and a 700-litre frozen food container, both of which are greater than 20 kg in weight. 

Fig. 4.61 illustrates typical temperature profiles during the rotational moulding of polyethylene. A ith typical values of oven temperatures and data for an aluminium mould... 

Fig. 4.61 illustrates that the mould temperature is quite different from the set oven temperature (330°C) or indeed the actual oven temperature, throughout the moulding cycle. An even more important observation is that in order to control the rotational moulding process it is desirable to monitor the temperature of the air inside the mould. This is possible because there is normally a vent tube through the mould wall in order to ensure equal pressures inside and outside the mould. This vent tube provides an easy access for a thermocouple to measure the internal air temperature. 

Developments in rotational moulding are continuing, with the ever increasing use of features such as... 

Crawford, R.J. Rotational Moulding of Plastics, Research Studies Press 2nd edition (1996). 

During a rotational moulding operation an aluminium mould with a uniform thickness of 3 mm is put into an oven at 300°C. If the initial temperature of the mould is 23°C, estimate the time taken for it to reach 250°C. The natural convection heat transfer coefficient is 28.4 J/m s. 

Additionally, plastisol ( paste ) type PVC compounds are in the form of a thick paste that can be applied by techniques such as coating, dipping, or rotational moulding. 

Report 71 Rotational Moulding, R.J. Crawford, The Queen s University of Belfast. 

A process for making hollow articles from latex. A heat-sensitised compounded latex is poured into a hollow non-porous mould which is then rotated about several axes until the latex has gelled on the surface of the mould. See Rotational Moulding. Similar to slip casting of ceramics. Ketones... 

A machine for determining the resistance of elastomers to flex cracking. It is used particularly in the evaluation of shoe soling materials. Rotational Moulding... 

A method of manufacturing articles from latex or from a plastisol. A heated metal mould is filled with the latex compound and, after an interval for gelation, the excess ungelled latex is poured out. Further heating sets the deposit, which is removed from the mould, dried and vulcanised. See Kaysam Process and Rotational Moulding. Smoked Blanket Crepe See Blanket Crepe. 

The essence of the early 1960s process was that one injection unit fed 4, 6, 8, 10 or 12 vertical presses on a rotating mould carrier. Sometimes two injection units were used at different mould stations, one injecting a soft upper compound for a bootee and the other a harder sole compound. Sometimes two different coloured compounds were used. 

Table 2.17 analyses some examples of the relative processing cost versus the annual production in units. The various technologies listed are not suitable for all materials or parts machining is used with numerous materials rotational moulding uses liquid resins vacuum forming uses thermoplastic sheets blow moulding uses special grades of thermoplastics... 

Units per annum Hand lay-up Machining Rotational moulding, Rotational moulding, Vacuum Blow Injection... 

Extrusion, injection, compression, blown film, blow moulding, rotational moulding, foam, coating, powdering, co-extrusion, for thin or thick parts, for crosslinking, electrowelding. .. 

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