Injection moulding

Injection moulding is a manufacturing process for producing parts by injecting material into a mould. Injection molding can be performed with a number of different materials, including metals, glasses and most commonly thermoplastic and thermosettingplastics. Material for the part is put into a heated barrel, mixed, and forced into a mould  where it cools and hardens to the shape of the mould. 
What is a 'jig' and what is the purpose of one?
A jig is a type of custom-made tool used to control the location and/or motion of another tool. A jig's main purpose is to provide a stable working envrionment for the product making them able to accurately repeat a product. A jig is often confused with a fixture; a fixture holds the work in a fixed location. A device that does both functions (holding the work and guiding a tool) is called a jig.