In this process between the two parts of open moulds a hot extruded tube called parison is placed. The two valves of the moulds move close to each other so that the moulds closes over the tube.
The parison’s bottom end is sealed. The molten plastic is mould with the help of blowing compressed air and the tube gets pinched off and also gets welded at the bottom by closing the moulds. The air pressure is about 0.7 to 10kg/cm². This air pressure will force the tube against the walls of the mould. Then is component is cooled to room temperature, and the mould is opened to release the component. The bloe moulding method will vary from simple manual operation to complicated automatic ones. The different types of blow moulding process are
1. Injection blow moulding
2. Extrusion blow moulding
3. Multi blow moulding
In multilayer blow moulding, multi layer structure are generally used. Typical example for multilayer blow moulding are plastic packing for food and beverage.
It is used in making plastic bottles and toys.The hollow containers are produced by this process only. The multilayer blow moulding is used in cosmetics and pharmaceutical industries.
Generally the thin walled hollow parts will be manufactured using rotational moulding process. In this method, a measured amount of polymer will be placed in a thin walled metal mould. Then the mould will be closed. And then it will be rotated about two mutually perpendicular axes as it is heated.
The rotation will make the powder to settle upon the mould walls. After heating and sintering, the mould is cooled while it is still rotating. The cooling of the mould is generally done by spraying air and water. After this the rotation is stopped and the moulded component is removed.
In this type of moulding a thin walled metal mould is made of two pieces and is rotated in perpendicular axis. A measured quantity of powdered plastic material is placed inside the mould. Then the mould is heated and rotated. Most of the thermo plastics and some types of thermosets can be formed into large hollow parts by rotational moulding. Rotational moulding is also used for producing components with hollow shapes with a very less wall thickness of about 0.4mm. Larger parts in terms of 4m can also be produced using this process. The surface finish of the internal of the mould is the same as that of surface finish of the walls. In this process time and temperature plays a vital role.
It is generally used in making large containers of polyethylene. Manufacturing of buckets, housings, boat hulls are made easy by this process. It is used to produce tanks of various sizes. It is also used for making football bladder.