For fittings, the most common method used for manufacturing is injection moulding. Basically injection moulding is a process where a solid thermoplastic material is heated until it reaches a state of fluidity, in other words until is melted. It is then injected under pressure into a mould where it is cooled until it reaches the solid state once again. This way the thermoplastic will duplicate all the details of the mould, forming complex three-dimensional plastic parts. At the beginning of each production cycle there should be enough quantity of granules (raw material) in the feeder. Depending on the kind of thermoplastic used, the humidity and temperature of the granules may affect the quality characteristics of the parts produced. Each production cycle begins with the admission of granules. Simultaneously the screw inside the barrel starts rotating and moving back. The effort exerted by the screw is designated by back pressure. The rotation of the screw, the friction of the granules in the walls of the barrel and the heating elements placed outside it are responsible for melting the thermoplastic granules. When the barrel contains enough melted material to fulfil the mould, the screw goes forward, injecting the material into the mould cavity. Non return valves prevent the material from flowing backwards and ensure that it reaches then mould cavity. During this period of time the clamping unit is responsible for maintaining the mould tightly closed. When the mould is completely full, pressure is maintained for an extra period of time, not only in order to avoid backflow of material but also to maintain the quality standards of the parts. At this time a small amount of extra material is squeezed into the mould. This acts as a compensate for the shrinkage of material as it cools. This pressure is called holding or packing pressure. When the material inside the mould has cooled down to the required temperature the clamping unit opens the mould and the part is extracted. In order to accelerate this phase of the process the mould has cooling channels. During this period new material is fed into barrel.

Feeding granules into the injection machine Injecting material into the mould No return valves