Rotational Moulding is a plastics moulding process for creating mostly seamless hollow parts. In this high temperature, ambient pressure, open moulding plastic forming process a hollow mould, in the shape of the part to be produced is filled with a charge of resin in powder form and then rotated biaxially. The rotation is at low RPM around 2 perpendicular axes, in a heated oven.
Polyethylenes, Cross linked polyethylene, Polypropylene, Polycarbonate, PVC, Plastisols and Nylon are some of the materials which are rotomoulded. During the process the mould rotates through a heating cycle where the powder melts slowly and sinters to stick to the mould wall, distributing the material throughout the mould surface. This follows with a cooling cycle when the mould keeps rotating biaxially. After cooling is complete, the mould is opened and the part is removed.
In rotomoulding it is possible to attain relatively good uniformity of wall thickness. Wall thickness can range from 0.8 mm to 25mm. Similar uniformity in wall thickness is also obtained in double wall construction by the process. Such uniformity of wall thickness is possible because of the smooth rotation of the mould containing a pre determined amount of powder ( usually ground to 35 mesh or 500 microns ).
Rotomoulded parts have thicker outer corners which enhance strength of the parts. Strength can also be reinforced by including ribs in the part. Kiss-offs can also be included & inserts can be moulded into the part. In the 1940s and early 50s the process was typically for products like beach balls, tiny dolls, dolls heads where vinyl plastisols were used as raw material. In the late 50s the development of low & high density polyethylene grades specially designed for the process allowed rotomoulders to enter markets where vinyl parts & processes could not compete.
Cross linked and other modified polyethylene grades which emerged in the 70s enabled the opening of new markets to the process especially of large size tanks. Linear Low density polyethylene which was developed in the late 70s ensured that raw material became more suitable for the process. Non polyethylene resins like Nylon, Polypropylene & Polycarbonate began to be used in rotomoulding in the 80s. The process made its appearance in India in the early 70s as it got increasingly used in the manufacture of overhead water storage tanks. In present days Rotational Moulding is growing rapidly as a plastic processing method along with development of many new moulding techniques.
Advantages of Rotational Moulding
- Low cost tooling, inexpensive process
- Moulds of quite different shapes and sizes can be run simultaneously on the same equipment
- Parts with undercuts and intricate contours are moulded easily. Special features such as double skinned moulding readily moulded
- More uniform wall thickness in rotomoulding than in blow moulding & thermoforming
- Lack of pressure ensures relatively stress free parts.
- No seams & no weld lines
- Ideal & most economical process for large hollow parts with small production runs.
- Often when parts are impossible to fabricate by any other process they can be rotomoulded.
- Extremely design friendly process; unlimited design possibilities; possible to mould a large number of fully or partially closed items.
- Possible to produce a large range of sizes; very small to very large.
- Excellent details for a variety of surface finishes are possible.
- Rotomoulded products are resistant to stress cracking & corrosion
- Rotational Moulding saves cost of secondary tooling in the absence of secondary processes
- Rotomoulded products are much lighter when compared to those in metals.
- Absence of runners & tailings minimizes scrap in production
Applications
The Growing range of products & applications in rotational moudling in India consists of :
- All kinds of Overhead, Surface level & Underground Tanks
- Agriculture Large silos for grain storage, storage & transportation of farm produce, chemical & fertilizer storage & dispensing, animal husbandry products, water & feeder troughs for dairies, fisheries etc.
- Automotive diesel fuel tanks; environmental control Urea-Ad Blue tanks for transportation trucks; water and hydraulic tanks, guards, bumpers etc.
- Chemical & Process Industry – storage, transportation & dispensing
- Consumer Products – baby strollers, child car seats.
- Furniture – Schools, school bus, stations, airports & public places.
- Toys, Playground equipment & Sporting equipment like golf carts etc
- In ground Manholes, Septic tanks, waste water , drainage systems
- In ground Junction boxes & chambers for power , telecommunications etc
- Materials Handling reusable & tailored pallets, storage & handling containers, insulated bo xes, specialty containers.
- Retail Sector – POS-POP displays, mannequins, signage, carts etc.
- RWH – Rain Water Harvesting - - rotomoulded in-ground collection storage tanks; this could also include collection, treatment and reuse.
- SWH – Solid Waste Management - ....rotomoulded products & solutions for solid waste collection, transportation and management of daily household & consumer waste.
- Road Furniture – barricades, bollards, bull noses etc
- Marine recreation – Kayaks, canoes etc
State of the industry
While the global rotomoulding industry is a valued at about US $ 5 billion, the Indian rotomoulding industry is worth around US$ 700 million.
Raw material consumption in the industry in India is around 150 KTA and the rate of growth of the industry is 15-20% per annum.
