Injection stretch molding is a method used for high-end products (such as bottles) made from polyethylene terephthalate (pet) which uses a rod to stretch a pre-form into the mold before blowing.

–  Very low unit price.

–  Exceptionally fast rates of production.

–  Suited to small containers.

–  Allows greater control over neck design, weight, and wall thickness than other blow-molding methods.

–  Higher tooling costs than in extrusion blow molding (see p.132).

–  Demands high volumes.

–  Limited to fairly simple hollow forms.

Volumes of production

Injection blow molding is ideally suited to high-volume production, which often runs into millions of units.

Unit price vs. capital investment

Costly tooling, for both the injecting and blowing parts of the process, as well as substantial setup charges. However, unit prices can be extremely low because of the volumes produced, and this justifies the high initial costs.

Speed

The various forms of blow molding are difficult to pin down in terms of speed of production due to variables such as part size and the number of mold cavities in operation. A typical 5-fluid-ounce bottle, however, can be produced by injection blow molding in an eight-cavity mold at the rate of 2,400 units per hour.

Surface

Excellent finish.

Types/complexity of shape

Injection blow molding is suited to fairly simple shapes, which have a large radius and consistent wall thicknesses over the whole product.

Scale

Typically used for containers of less than 8 fluid ounces.

Relevant materials

Compared with extrusion blow molding (see p.132), this method is suited to more rigid materials such as polycarbonate (PC) and polyethylene terephthalate (PET). It is, however, often used for nonrigid materials, such as polyethylene (PE).

Typical products

Small shampoo, detergent, and other bottles.

Similar methods

Extrusion blow molding (p.132) for plastic, and press and blow molding (p.124) for glass.

Sustainability issues

As opposed to other forms of plastic blow molding, the pre-form is heated twice: first during production of the pre-form and then when it is blown into the final product, thus doubling on energy use. Virtually no waste is created during injection blow molding and cycle times are very fast with optimized use of material and energy. The process is often used to produce disposable PET packaging; as one of the most widely recycled plastics PET can be reprocessed to avoid landfill.

Further information

www.rpc-group.com

www.bpf.co.uk

Extrusion Blow Molding

with co-extrusion blow molding

Extrusion blow molding is part of the plastic blow molding group of processes (see p.127). In this particular method, the plastic is extruded (see extrusion, p.96) into a sausage shape known as a “lug” and pinched into short lengths as it drops into the mold cavity. Here, it is blown with air, forcing the plastic against the mold cavity to form the final shape. The process leaves excess “pinched” material (a “tail”), which must be removed—though evidence of it can be seen in the finished product, on the underside of any shampoo bottle, for example.

In co-extrusion blow molding different materials are combined to form a multilayered product.

–  Very low unit price.

–  Fast rates of production.

–  Suited to large containers of more than 20 fluid ounces.

–  Compared with injection blow molding (see p.129), extrusion blow molding is capable of producing more complex shapes.

–  Lower tooling costs than for injection blow molding.

–  Demands high volumes.