Rotational molding is a method for forming hollow plastic objects by placing finely divided particles in a hollow mold that is rotated about two axes, exposing it to heat at high temperature and then to cold condition at low pressure. The process involves a heated hollow mould which is filled with a charge / shot weight of material. It’s 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. Rotomolding is common in industries that manufacture tanks, leisure craft, toys and medical Products.
NEED FOR ROTATIONAL MOULDING
It’s technology is prevalent, making it one of the rapidly growing industries in the last decade. There are numerous advantages of rotational molding, from design versatility, reduction of environmental impact, use of lighter materials, and meeting of demand.
It has the advantage in terms of relatively low levels of residual stresses in the manufacturing process and hence relatively inexpensive moulds. low cost incurred is for machining the mold. Before heating, the mold requires finished pieces added to it like color plastics, internal pipes and metal threads.
High durability: Parts are molded as one solid piece, eliminating the need for joining techniques such as welding and joint fabrication which creates weak spots.
High stability: As the molding material isn’t exposed to external pressure, increasing its stability and reducing the risk of defects in the finished part.
Machine costs are low compared to other processes and the investment required is small. There is great production flexibility with the process. There is almost no limit to the size of products and several different products can be moulded at the same time.
Generally, products are made from polyethylene which are protected from the hazardous UV Rays from the sun. In fact, Ultraviolet rays from the sun make products to fade quickly and degrade with time.
Roto moulded products have good durability and their wall thickness is good enough to withstand mechanical stress and they are not easy to break.
It has minimum environmental impact as, no chemicals or dangerous toxins are released during the process . They are 100% recyclable hence simple to reduce carbon footprints. This means that for every plastic item that is recycled, fewer materials are mined from the earth, therefore, reducing its impact on the environment.
With the rotomolding process, it is simple to generate a variety of shapes, majorly because when hot plastics are placed on a mold, they take the shape of the mold. Many molding processes have few options to choose from, but with rotomolding, any form can be designed from the process. This makes it ideal to be used in many industries.
Unlike other molding processes that accommodate a few sizes, rotational molding allows for diverse sizes to choose from. At Roto Dynamics, we can manufacture small-sized tanks with capacity of 500ltrs , medium sizes of 1000ltrs and even the largest sizes upto 2000ltrs.
TYPES OF ROTATIONAL MOULDING
1) Carousel machine
This is the most common type of rotomoulding machine which has multiple-spindle with each having one large mould or multiple small moulds mounted on it. The arms may be fixed or independent.
Fixed Carousel Machine has fixed arms having relative moment to each other. There are 3 work stations – a) Loading & Unloading , b) Heating (oven) & c) Cooling. These 3 arms with mounted moulds are indexed in a proper sequence through 3 workstation all working together.
Independent Carousel machine has arms which are free to move independent of each other. The machine can have 3 or more arms , 3-5 work stations to speed up the process.
2) The shuttle or Swing type machine
It is actually a type of highly aerodynamic single arm turret machine that displaces the mold back and forth between the heating and cooling chambers in a linear direction. These avant-garde machines are generally equipped with environment friendly burners, which are suitable for LPG, CNG, OIL duel fuels. This independent indexing machine is highly preferred because of the fact that it is low in cost as compared to the the size of product it produces. The machine has mould mounted on a movable bed which can slide over a linear track placing mould IN & OUT of the heating chamber (OVEN). The charged mould is placed in oven and as cycle is complete is moved out in open for cooling cycle. There could be another mould at other end of oven which starts its heating cycle till the earlier mould cooling completed.
3) Clamshell machine
It has closed enclosure having one arm and all the 4 process steps of heating , cooling , loading & unloading are done in same place. A significant benefit of the clamshell machine is the low floor space requirement; the biaxial rotation is identical of that in a fixed turret machine while also having two support arms connected to the horizontal shaft. Molds are mounted as assemblies on the turntables through the shaft. Once the clamshell oven closes, the axle rotates moving the molds in a spherical motion while also rotating the molds about their axis. The required temperature is obtained through heating and circulating air through the machine. For the cooling cycle, cooled air is circulated and a mist of water is also dispersed. At the end of the cycle the clamshell oven opens and the molds are opened for part removal; the molds are charged with resin and the cycle begins again.
4) Rock and Roll rotational moulding machine
It’s a specialized single arm machine in which the mould rolls about 360°C in a single direction. At the same time, it tips and rocks the mould at 45° up & down horizontal in both ends. These kinds of machines are said to be best for long thin parts having long length-to-width ration. It also has small heating chambers that save heating cost.
5) Vertical or Up & over rotational moulding machine
It varies in sizes (small to medium) as compared to the sizes of other rotational machines. It is energy sufficient because of the machines compact heating and cooling chambers. It hsas the same features with the Horizontal Carousel Multi Arm rotational moulding machine. Only it takes up lesser space. Heating statkons or ovens are fired by natural gas and blowers used stream to distribute heat throughout the chamber. Proper insulation is ensured to avoid heat losses. Open flame burners, passing hot air streams, hot liquid flow within mould channel or sometimes infra-red radiation are also used to heat the chamber. Normal oven temperature are maintained at 270-425 deg.C. Cooling station may be closed or open. The rotating mould is cooled by any or combination of fans and blowers, spraying of air & water (mist) or water spray for even cooling.