Volumes of production

Depending on the size and the material, blow molding can be an extremely rapid form of production, with an output of from approximately 500 units per hour to more than a million units per 24-hour cycle. To get the most out of the process, in terms of cost savings, production should run into the hundreds of thousands.

Unit price vs. capital investment

Unit prices for most standard blow-molded parts are very low, which is best comprehended by looking at the volume of cheap products and packaging that is produced with this process. This economy of scale is counterbalanced, of course, by the extremely high tooling costs.

Speed

Small containers can be produced in multicavity molds to yield approximately 60,000 small (less than, say, 23 fluid ounces) polyethylene terephthalate (PET) bottles per hour.

Surface

Excellent finish, but parting lines remain down the length.

Types/complexity of shape

Depending on the specific process, blow-molded shapes are generally simple and rounded. Although products can be produced with no draft angles, manufacturers prefer a small draft.

Scale

From small cosmetics bottles to parts that weigh more than 55 pounds.

Relevant materials

The typically waxy, high-density polyethylene (HDPE) is one of the most common materials used for this process. Other materials include polypropylene, polyethylene, polyethylene terephthalate (PET), and polyvinyl chloride (PVC).

Typical products

The chances are that in the average household you will have at least one large cabinet full of different plastic containers that are blow molded. Basically, blow-molded items include everything from plastic milk cartons and shampoo bottles to toys, toothpaste tubes, detergent bottles, watering cans, and—outside the home—car fuel tanks.

Similar methods

Stretch molding, extrusion blow molding (p.132), injection blow molding, and co-extrusion blow molding (p.132).

Sustainability Issues

A highly automated process that as a result of very fast cycle times maximizes the use of heat and electrical energy to produce a very precise quantity of material and optimized use of energy. PET, which is one of the main materials for blow molding, is among the most widely recycled plastics.

Further information

www.rpc-group.com

www.bpf.co.uk

Injection Blow Molding

with injection stretch molding

Product	injection-molded pre-form (left) and blow-molded bottle (right)
Materials	polyethylene terephthalate (PET)
Country	Germany

This pre-form and the resulting blown bottle show, in very simple terms, how straightforward the process is that forms the billions of plastic bottles that litter our urban landscape. The advantage of using injection molding is demonstrated by the detailed thread that has been formed around the neck of the pre-form.

Product	Sparkling Chair
Designer	Marcel Wanders
Materials	polyethylene terephthalate (PET)
Manufacturer	Magis
Country	Italy
Date	2010

The Sparkling Chair is a great illustration of how a material and production method can be translated from packaging into a completely new area of furniture.

Injection blow molding is most easily described as being a subdivision of plastic blow molding (see p.127), the process that works on the same principle as blowing up a balloon, but into a mold that forms the shape.

As the name implies, this is a two-step molding process that offers a number of advantages over other forms of blow molding because it is possible to create far more complex shapes around the neck of the molded part. A hollow pre-form is made using injection molding (see p.196), which allows for the molding of a complex thread at the neck. The pre-form is placed into the mold cavity where it is blown with air, forcing the plastic against the mold cavity.

Using an injection-molded pre-form means that this method offers a greater degree of stability and control over the shape than extrusion blow molding (see p.132), although the choice of suitable materials is more limited.