Stretch blow molding is a manufacturing process used to create PET plastic bottles and jars. Small pellets of plastic resin are pushed into an extruder and get melted.  Then they are injected into a steel mold that shapes them into plastic tubes. These preforms are the starting point for the plastic bottles. By injecting air into this heated preform, the bottle shape is formed to the desired mold shape.

There are two types of stretch blow molding techniques: single stage and the two-stage process. In the single stage process, preforms are created and then blown into bottles in one continuous process, by one machine. This process is split into three stations: first melted plastic is injected into the preform mold.  It is switched to the second station for conditioning. And finally, at the third station, air is blown inside the heated preform stretching the plastic into the mold and your desired bottle shape. Each station takes the same amount of time since it is all part of one continuous process, on one machine. This technique is often used in applications that call for more rigid quality specifications or specialized shapes such as wide mouth jar openings.

In the two-stage process the three stages of molding are split into two machines that work independently allowing more efficient throughput and flexibility. Preforms can be produced by one machine and then warehoused for later production. Then, a batch of preforms could be loaded into another machine called a reheat blow, or RHB, machine, which is designed for blowing preforms into bottles in high quantities and faster production rates with the potential to lower piece cost.

For more information on which process is right for you, please contact one of our Packaging Specialists.

Choosing the Right Stretch Blow Molding Process

When deciding between single-stage and two-stage stretch blow molding, manufacturers should weigh a few key factors to determine the best fit for their production needs:

• Production volume: Two-stage stretch blow molding suits high-volume runs, while single-stage works well for smaller, specialized batches.
• Bottle design complexity: Applications needing wide mouth jar openings or rigid quality specifications often favor the single stage process.
• Equipment and cost: A reheat blow machine used in the two-stage process can lower piece cost at scale, but may require greater upfront investment across two machines.
• Flexibility: The two-stage process allows preforms to be produced and warehoused separately from bottle blowing, offering more scheduling flexibility for PET plastic bottles production.