Plastic injection molding, by definition, is the process of injecting plastic into a mold (or mould in European circles), cooling the plastic, ejecting it from the mold, and packing the finished product for sale to the customer. This sounds simple and in it's basic form it is, but the process involved with making all this happen is actually quite complicated. Plastic can be injected into the mold with low pressure, but typically is done under high pressure upwards of 30,000 PSI cavity pressure.
There are many diverse types of injection molding and as many different types of plastic injection molding machines to go with it. Some of the more common types of plastic molding are standard mold injection, insert molding, plastic extrusion, blow molding, multi-color or material molding, stack molding, and rotary molding just to name a few. There are custom plastics as well, from common every day plastic resins to the more exotic engineering grade resins used in most automobiles today. There are even injection molding grade "metals" being experimented with and used these days.
There are many types of injection molding machines all made for specific purposes. There are the standard horizontal injection molding machines which range in size from a few tons all the way up to 9000 tons and more. You could drive car between the platens of a press that large. Vertical injection molding machines are often used for insert molding (although this can also be done horizontally) and share a common cover with two ejection halves of the mold. This allows for de-molding and loading of inserts in one half, while the other half is producing the next shot of parts. There are also two and three shot injection molding machines for producing multi-color or multi-material parts such as lenses for tail lights and gear shift knobs with hard plastic inner core and a soft vinyl out covering. The newest machines are "all electric versions" which stray from the standard hydraulic injection machines used now for years. These injection molding machines are much more efficient and repeatable then their hydraulic cousins are.
The basic steps to producing a plastic injection molded part on a stand injection molding machine are:
Heating the plastic resin to the required range of the product being used.
Developing the shot size through use of a reciprocating screw which conveys the melted plastic to the front of the screw.
Injecting the plastic into the mold under pressure to fill the cavity of the mold.
Packing the plastic to create a full part inside the mold.
Cooling the plastic in the mold through the use of cooling channels most commonly with water.
Ejecting the cooled part from the mold.