Evolution of the Moulding Industry
In 1872 Hyatt patents the first injection molding machine in which the hot material (celluloid, thermosetting) was plasticized in a hot cylinder and subsequently injected with the motion of the plunger.
The introduction of a second cylinder allowed to separate the heating step from the injection one: after the cylinder injects into the mold material, a new charge can be performed on the secondary cylinder, reducing the cycle time. Subsequently, the heating cylinder is replaced by a screw extruder, that remains in the injection cylinder, as it reduces the breakage of the fibers.
With the thermoplastic polymers, such as polyamide (PA, commercially called Nylon), polyethylene (PE), polyethylene terephthalate (PET) and polycarbonate (PC),birth in the '30s -' 40s, there is the need to develop new machines for the production of replacements, spare parts, and components made of these new thermoplastic materials. the first molding press with dual-purpose screw (it carries both the plasticizing function to inject) has been designed in the early 50s and it is still used today. 
Traditional Molding Press scheme
A: accumulation of molten material, B: injection, C: retention / cooling, D: extraction.

A: , B : , C : D : .