With the commercial introduction of containers, stretch blow molding became a common term in the blow molding industry.
Stretch blow molding is the method of producing a plastic container from a preform or parison that is stretched in both the hoop direction and the axial direction when the preform is blown into its desired container shape. Stretch blow molding is possible for various thermoplastic materials such as acrylnitrile(AN), polystyrene (PS), Polyvinyl chloride (PVC), polyamide (PA), polycarbonate (PC), Polysulfone, acetal, polyarlyate, polypropylene (PP), surlyn, and polyethylene terephthalate (PET). Amphorous materials such as PET. Etc.
In the stretch blow molding (SBM) process, the plastic is first molded into a “preform” using the injection molding process. These preforms are produced with the necks of the bottles, including threads (the “finish”) on one end. These preforms are packaged, and fed later (after cooling) into a reheat stretch blow molding machine. In the SBM process, the preforms are heated (typically using infrared heaters) above their glass transition temperature, then blown using high pressure air into bottles using metal blow molds.
Usually the preform is stretched with a core rod as part of the process. In the single-stage process both preform manufacture and bottle blowing are performed in the same machine. The stretching of some polymers, such as PET (polyethylene terephthalate) results in strain hardening of the resin, allowing the bottles to resist deforming under the pressures formed by carbonated beverages, which typically approach 60 psi.The main applications are bottles, jars and other containers.
Advantages of blow molding include: low tool and die cost; fast production rates; ability to mold complex part; produces recyclable parts. Increase the material’s tensile strength, barrier properties, clarity. Reduce weight stretch blow molding produces a container from less raw material and with improved economics and bottle properties.
Disadvantages of blow molding include: limited to hollow parts, wall thickness is hard to control.
Stretch blow molding is a two-stage process similar to that of injection blow molding. First, a test-tube like preform is made using injection molding or a similar process. The neck of the preforms is fully finished but the diameter and length of the body portion are much smaller than the final product.
The official application of blow moulding began with the use of celluloid and celluloid nitrate that were used to develop a limited number of products as the celluloid is highly flammable. The process was commercialised with the first automatic blow moulding machine by the Plax Corporation in the 1930s. By 1939, this corporation had a machine capable of producing 25,000 bottles per day, and in 1945, introduced the use of the recently developed low-density polyethylene (LDPE) to squeeze bottles.
In 1953, the market for blow moulding products exploded with the appearance of high-density polyethylene (HDPE). The introduction of polyethylene terephthalate (PET) in the 1970s also allowed the development of more efficient processes. The production of other diverse resins with time allowed a wide range of other material properties to become available, the appearance of larger and more robust equipment, that lead to the production of a large range of industrial and technical applications