Making polymers is a fantastic science.   Then there is the matter of shaping the plastic into useful objects....another fantastic science.  One of the most common methods of shaping plastic resins is a process called injection molding.  Injection molding is accomplished by large machines called injection molding machines or a blow molding machine.

Resin is fed to the machine through the hopper.   Colorants are usually fed to the machine directly after the hopper.  The resins enter the injection barrel by gravity though the feed throat.  Upon entrance into the barrel, the resin is heated to the appropriate melting temperature. 

The resin is injected into the mold by a reciprocating screw or a ram injector.  The reciprocating screw apparatus is shown above.  The reciprocating screw offers the advantage of being able to inject a smaller percentage of the total shot (amount of melted resin in the barrel).  The ram injector must typically inject at least 20% of the total shot while a screw injector can inject as little as 5% of the total shot.  Essentially, the screw injector is better suited for producing smaller parts.
    The mold is the part of the machine that receives the plastic and shapes it appropriately.  The mold is cooled constantly to a temperature that allows the resin to solidify and be cool to the touch.  The mold plates are held together by hydraulic or mechanical force.  The clamping force is defined as the injection pressure multiplied by the total cavity projected area.  Typically molds are overdesigned depending on the resin to be used.  Each resin has a calculated shrinkage value associated with in.

A brief introduction to the development of blow molding process history

Blow molding dates back to at least 1890, when it was used to produce celluloid baby rattles. From that time forward, many companies have tried numerous means to produce blow molded parts in a variety of materials. The first polyethylene bottle was blown in December of 1942. The rest is history: the U.S. currently produces 30 to 40 billion plastic bottles per year, with the number constantly growing. For an excellent history of the plastics industry through 1972, those reading this article may wish to consult Plastics History U.S.A. by Harry Dubois, published by Cahners Books, Boston, Mass., ISBN 0-8436-1203-7.

There are basically four types of blow moulding used in the production of plastic bottles, jugs and jars. These four types are: extrusion blow molding, injection blow molding, stretch blow molding and reheat and blow molding. Extrusion blow molding is perhaps the simplest type of blow molding, whereby a hot tube of plastic material is dropped from an extruder and captured in a water cooled mold. Once the molds are closed, air is injected through the top or the neck of the container; just as if one were blowing up a balloon. When the hot plastic material is blown up and touches the walls of the mold the material "freezes" and the container now maintains its rigid shape. There are various types of shuttle, reciprocating and wheel style machines for the production of extrusion blown bottles. Shuttle or reciprocating type machines can be used for small, medium and high volume production with wheel machines being the most efficient for huge volume production of certain resins.

Injection blow moulding is part injection molding and part blow molding. With injection blow molding, the hot plastic material is first injected into a cavity where it encircles the blow stem, which is used to create the neck and establish the gram weight. The injected material is then carried to the next station on the machine, where it is blown up into the finished container as in the extrusion blow molding process above.