Stretch Blow Molding: Stretching Your Containers Helps them Build Character

To follow up our previous article Blow Molding, The Black Art, we would like to continue our discussion of blow molding by telling you the difference between the three main types of blow molding: Extrusion Blow Molding, Injection Blow Molding, and Stretch Blow Molding. This article is an in depth discussion of stretch blow molding.

Stretch Blow Molding

The stretch blow molding process is usually used for producing PET bottles for juices, water, soda, and several other products. This process of has been around since the 1970’s and stretch blow molding is used widely to package detergent and soda bottles. There are two main types of stretch blow molding: single-stage stretch blow molding, and two-stage stretch blow molding.

Single-Stage Stretch Blow Molding

Single-stage stretch blow molding uses an extruder that injects parison into a rapid cooling preform mold. The preform is reheated and then placed in the container mold. The now hot and softened parison is able to stretch to double its original length. Air then fills the container and it is stretched to fit the mold until it cools and hardens. Once the container and mold have cooled, the container is removed from the mold. This process is best for wide-mouthed jars that do not require high rates of production.

Two-Stage Stretch Blow Molding

To preform two-stage stretch blow molding you follow the same steps as single-stage stretch blow molding, but you make the preforms previously. Usually with the single-stage process you use one machine and with the two-stage process you have separate machines, one to create preforms and the other to heat and expand the containers to fit the molds. This allows companies to either manufacture or buy preforms ahead of time. Because molding and RHB equipment is so expensive, this is usually the preferred method of production for high volume containers like soda bottles.

The machinery required is to finish the second step is called injection stretch blow molding (ISBM machines) and they are relatively expensive. Because of the high expense, large production runs are usually necessary to justify the costs of injection molds to create the preforms and injection molds to create the fina

l container. Because of this, two-step stretch blow molding is generally used only for jobs that require extremely high volume runs like: peanut butter jars, liquor bottles, narrow mouthed water bottles etc.

Advantages of Stretch Blow Molding

One of the main advantages of stretch blow molding is that it stretches the container in both the axel direction, and the hoop direction. This is called biaxial stretching. This increases the barrier properties, tolerable drop impact, clarity, tensile strength and top load in a container. These increases in strength allow producers to reduce the overall weight of a container by about 10-15% compared to other production methods. This saves money.

Disadvantages of Stretch Blow Molding

The machinery is relatively expensive. In order to justify the costs large runs are required, so small runs are not a profitable option.