Water tanks are liquid storage containers, and we store water for our various needs like food and other household works in it. Well the use of water tanks is very old and there are instances that people were using water tanks in the early days of civilization although they were very different from our regular plastic water tanks. A plastic water tank provides for the storage of drinking water, irrigation agriculture, fire suppression, agricultural farming and livestock, chemical manufacturing, food preparation as well as many other possible solutions.
There are many materials which may used to create any plastic water tank for example plastics (polyethylene, polypropylene), fiberglass, concrete, stone, steel (welded or bolted, carbon or stainless).
Materials used in the manufacture of these water tanks must have following qualities –
Durability – Often plastic water tanks are one time investment and people do not want to change tanks very often therefore it is required that material used to manufacture plastic water tank must be durable. Polyethylene, polypropylene are such durable materials and average life of plastic water tanks is in decades.
Portability – The biggest disadvantage with concrete, stone, steel water tanks is their weight. They are often very heavy and moving them here to there is really a though task. While plastic water tanks are very light weight a even a single man can move plastic water tanks with ease.
Temperature control – Biggest advantage with plastic water tank is their temperature resistant material. Plastic is highly thermo resistant and water stored in plastic water tanks get less hot or cold as compared to other water tanks like steel and concrete water tanks. Making them ideal for sunny or winter season.
Contamination free – Plastic water tanks are contamination free, It has been observed that concrete water tanks develop microorganism and other water based plants very often while plastic has natural resistance to such contamination.
There are various types of plastic water tanks which basically depends over their use and water storage need for example there are rain water storage tanks, under ground water storage tanks, elevated water storage tanks and commercial water storage tanks. Water storing capacity of plastic water tanks also differs and could be any thing between 100 liter to thousands of liter water. Water has its density equals to one means that any area having one cubic meter of volume can have one thousand liter of water.
Areas where water supply is such that you get any amount of water by just opening the tap may not have essential need of plastic water tanks but in areas where the water supply is not so good like in rural areas extra storage of water is very essential. However in urban areas also there is a need of extra water storage in order to cope with any water issue. Rain water storage tanks are playing their important role in preserving nature as in many continents where water is a really scares resource people use to store rain water in such tanks for future use.
Plastic water tanks is not a alien thing to all of us but when we go out to purchase any such water tank we must take care of our material used to make water tank. There are few good companies in Australia producing highly durable, contamination free, easy to move and reliable water tanks.
How Rotational Moulding Work
Rotational moulding, known also as rotomolding or rotocasting, is a process for manufacturing hollow plastic products. For certain types of liquid vinyls, the term slush molding is also used. Rotational molding has particular advantages in terms of relatively low levels of residual stresses and inexpensive molds. Rotational molding also has few competitors for the production of large (> 2 m3) hollow objects in one piece. Rotational molding is best known for the manufacture of tanks but it can also be used to make complex medical products, toys, leisure craft, and highly aesthetic point-of-sale products.
The process begins with specially developed and uniquely applied resins. They can include plastisols, polyethylene, polycarbonate, acetate butyrate, polyamide, elastomers, polyurethane, polypropylene, ethylene vinyl acetate and fluorocarbons. Some of the most advanced work is being done with cross-linked polyethylene and even nylon.
The principle of rotational molding of plastics is simple. Basically the process consists of introducing a known amount of plastic in powder, granular, or viscous liquid form into a hollow, shell-like mold.7œ9 The mold is rotated and/ or rocked about two principal axes at relatively low speeds as it is heated so that the plastic enclosed in the mold adheres to, and forms a monolithic layer against, the mold surface. The mold rotation continues during the cooling phase so that the plastic retains its desired shape as it solidifies. When the plastic is sufficiently rigid, the cooling and mold rotation is stopped to allow the removal of the plastic product from the mold. At this stage, the cyclic process may be repeated. The basic steps of (a) mold charging, (b) mold heating, (c) mold cooling, and (d) part ejection
The main attractions of rotational moulding are:
- The end product is essentially stress-free
- The moulds are relatively inexpensive
- The lead time for the manufacture of a mould is relatively short
- Short production can be economically viable
- There is no material wastage in that all of the material is normally consumed in making the part
- It is possible to make multilayer products
- Different types of product can be moulded together on the one machine
- Inserts are relatively easy to mould in
- High quality graphics can be moulded in
Machinery for Rotational moulding comes in two basic categories. The most efficient method uses a heated oven. Machinery with ovens comes in different configurations.
Whether the cycle takes place in or out of the oven the rocking action that is necessary to cover the internal surface of the mould wall is usually described as either bi-axial or rock ‘n roll. Bi-axial rotation moves the mould through any combination of full rotations of two axis and is unlimited in the recipe that allows the part to mould properly. Rock ‘n roll is limited rotation on two axis and the angle of the rock is vital for long narrow parts to ensure the powder pool covers the entire inside surface.
Carousel type machines usually use three or four arms on which the mouldsare mounted. These arms rotate the parts bi-axially and move from station to station on the machine being loaded heated cooled unloaded and loaded again ready for another cycle. These machines are excellent for energy efficient high production manufacturing. Carousel machines usually require the largest amount of available floor space.
Shuttle type’s machines will usually have two carts on either side of the oven on which are mounted arms which carry the moulds.These carts will move into and out of the oven in turn. While one cart is in the oven heating the other cart can be unloaded and loaded again ready for the next cycle. Shuttle machines usually require a smaller amount of space in the factory but may not produce as many parts as carousel machines.
For certain applications open flame machinery can offer an effective return on investment. Cycle times are usually much longer on open flame equipment and parts can be more affected by ambient temperature. For some parts where placing the part in an oven would not allow even one more cycle per shift open flame equipment is a popular alternative.
Clam shell machines are usually single arm machines that run one part at a time. These machines are more efficient than open flame machines but may not produce as many parts as the multi arm machinery available. Clam shell machines need even less factory space than shuttles.
Although the rotational moulding process has numerous attractive features it is also limited in many ways. The most significant limitation is the dearth of suitable materials. This is primarily due to the severe time-temperature demand placed on the polymer but it is also due to the relatively small existing market for nonpolyolefins.
There special resins have been made available the material prices are high due to the development costs that are passed through to the user and the additional cost of small-scale grinding of the plastic.
Rotational moulders currently use several different materials for rotational moulding however most of the products made using the process use polyethylene in a powder or pellet form. In different parts of the world this material is supplied in either a pellet form which is then ground into powder coloured and prepared to specification by the rotational moulder or the powder is supplied to the moulder ready for manufacture. Apart from a huge range of colours powder may also include a flame retardant uv resistance or a range of other specific formulations.
Currently polyethylene in its many forms represents about 85% to 90% of all polymers that are rotationally moulded. Cross-linked grades of polyethylene are also commonly used in rotational moulding. PVC plastisols make up about 12% of the world consumption and polycarbonate nylon polypropylene and other materials make up the rest. PE for the process can be pulverised into a powder at room temperature. Normally powder particles vary from less than 150 microns to about 500 microns. The particle size distribution is vital to the appearance of the final product. PE has to flow easily during the process which is measured by a Melt Flow Index value. High MFI material will have a low viscosity and usually for rotomoulding an MFI in the range of 2-8 is desirable depending on the product specifications.
PE’s for rotational moulding come in various densities including Low Density (LDPE) Linear Low (LLDPE) Medium (MDPE) and High (HDPE). The density of the material will have an effect on strength stiffness chemical resistance and impact vulnerability.
The majority of moulds for rotational moulding are currently made from metal usually sheet steel or aluminium or caste aluminium. The moulds are quite thin shell like structures which are designed to allow the heat source to quickly transfer heat through the metal into the powder.
Large simple moulds are generally made from fabricated metals. For highly detailed parts where a high quality finish is necessary either cast metals or nickel electroformed moulds are used.
The process doesn’t use pressure to form the part so the mould itself needs only to have enough strength to support itself. As the material cools it naturally shrinks away from the surface of the mould; and mould design must take this into account to create an accurately size final part.
Most moulds are manufactured in two pieces however for complex designs three and four piece moulds can be used. The area where the pieces of the mould connect to each other is known as the parting line and it can include a complex curvature. The parting line is vital because the mould sections must remain tightly clamped together during the heating and cooling of the part. Careful handling of the mould is necessary to ensure the parting line is not damaged.
Moulds are usually mounted to frames in halves to allow them to be placed on the machine. In some cases multiple moulds will be placed on the same frame known as a spider. This can provide substantial savings in demoulding and charging the moulds reducing cycle times.
Moulds must be vented to allow heated air to flow freely in and out during the process. If the venting tube is too small or poorly designed its possible for the mould to be subject to pressure and either explode or implode. The vent tube must also be deep enough to project through the powder pool so material does not flow out of the pipe during the rotation. A good guideline is that the vent tube should be 13mm for each 1m3 of mould volume.
Moulds can also be treated on the surface to provide a range of different surfaces on the final products and the outside of the mould can be treated by various methods including adding pins roughening the outside and using black paint to increase cycle times.
- Art Installations
- Icons of Rotomolding
- Materials Handling
- Road & Safety
Rotomoulded Machine Manufacturers
M-Plast India Ltd
A-15, Sector-60, Phase-III, Noida-201307 (U.P.) INDIA
Phone: +91 120 2581087, 2581089, 2580339
Plot No. 27 Shreeram Industrial Estate,
Behind C.M.C.,Nr. Anup Engineering,Odhav, Ahmedabad – 382415
Tele Fax: +91 79 22 890 927