There are basically four types of blow molding machine 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 molding is part injection molding and part blow moulding machine. 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.

   Injection blow molding is generally suitable for smaller containers and absolutely no handleware. Extrusion blow molding allows for a wide variety of container shapes, sizes and neck openings, as well as the production of handleware. Extrusion blown containers can also have their gram weights adjusted through an extremely wide range, whereas injection blown containers usually have a set gram weight which cannot be changed unless a whole new set of blow stems are built. Extrusion blow molds are generally much less expensive than injection blow molds and can be produced in a much shorter period of time.

Blow molding is a manufacturing method used in the plastics and polymers industries to create hollow but strong containers for their clients. Plastic beverage bottles and fuel tanks are commonly created through blow molding. A typical blow molding machine set-up uses an extruded plastic preform and compressed air to fill the chamber of a divided mold. The two halves of the mold separate and the finished container is released.

Although many products are manufactured using the blow molding process, it might be simpler to examine what happens to one particular product -- a standard two-liter soda bottle.

Plastics are actually chains of polymers held together by a strong but fluid bond. The reason a thin soda bottle is strong enough to withstand the pressure of carbonated liquids is a phenomenon called 'biaxial orientation.' The polymer chains in a plastic bottle form in two directions, creating a very strong webbing effect. The plastic itself can be stretched out without sacrificing strength. Blow molding experts takes advantage of this property to make thin but strong containers.

Blow molding a two-liter soda bottle requires a preformed piece called a parison. This parison is usually extruded from a plastic injection mold placed very close to the blow molding machinery. The warm parison looks like a upside-down plastic test tube, with a preformed collar and threads for the cap at the bottom. The parison is mechanically loaded onto a stand and two sides of a bottle-shaped metal mold come together around it.

Before the parison cools down, a hollow ramrod is injected into its center and pushed to the top of the mold, stretching out the warm plastic preform as it goes. Compressed air is then forced out in controlled low-pressure stages through the hollow ramrod. The plastic form is forced out to the sides of the mold. Because the stretching is performed evenly, the plastic remains uniformly thin and strong. The soda bottle assumes the shape of the mold and is dropped out of the blow moulding machine as the two mold halves separate. A new parison is extruded and the entire blow molding process begins again. The actual manufacture of a soda bottle takes only a few seconds.

There are other forms of blow molding, but the general principle is the same. The plastic acts much like a latex rubber balloon -- as long as the pressure from the compressed air is controlled, the material will expand uniformly and form the shape of the mold. This requires a fair amount of skill and experience on the part of the blow molding engineers who design new pieces of equipment.

Blow moulding a shape is a common industrial process. The example shown below is of the production of a plastic bottle. The plastics normally used in this process are; polythene, PVC and polypropylene.

After a cooling period the mould is opened and the final article is ejected.To speed production several identical moulds may be fed in cycle by the same extruder unit.

The process is similar to injection moulding and extrusion.

1. The plastic is fed in granular form into a 'hopper' that stores it.

2. A large thread is turned by a motor which feeds the granules through a heated section.

3. In this heated section the granules melt and become a liquid and the liquid is fed into a mould.

4. Air is forced into the mould which forces the plastic to the sides, giving the shape of the bottle.

5.  The mould is then cooled and is removed.

Blow molding, also known as blow forming, is a manufacturing process by which hollow plastic parts are formed. It is a process used to produce hollow objects from thermoplastic.

In general, there are three main types of blow molding machines: extrusion blow molding machine, injection molding machine, and stretch blow molding machine.

The blow molding process begins with melting down the plastic and forming it into a parison or preform. The parison is a tube-like piece of plastic with a hole in one end in which compressed air can pass through.

The basic process has two fundamental phases. First, a preform (or parison) of hot plastic resin in a somewhat tubular shape is created. Second, a pressurized gas, usually air, is used to expand the hot preform and press it against a mold cavity. The pressure is held until the plastic cools. This action identifies another common feature of blow molded articles. Part dimensional detail is better controlled on the outside than on the inside, where material wall thickness can alter the internal shape. Once the plastic has cooled and hardened the mold opens up and the part is ejected.

A blow molding renaissance is occurring in which engineers and designers are discovering and promoting blow molding for a wide variety of industrial or technical application. Toy wheels, automobile seat back, ductwork, surf boards, bellows, fuel tanks, flower pots, automobile bumpers, double- walled tool cases, and cabinet panels are just a few examples of the many creative design being developed.