Developed in the late 1800s, blow moulding is a highly developed moulding technology. It is best suited for hollow parts (such as plastic bottles) with uniform wall thicknesses, where outer shape is a key issue.
The Process
The blow moulding process begins when plastic grains are fed into a 'hopper' where they are stored. A motor turns a large thread, pushing the granules through a heated segment, where they melt and become liquidised. In molten state, the plastic is fed into a mould, forming a hollow tube called a parison. Air enters the mould, forcing the parison to take its shape. The plastic is then cooled until it solidifies and can be removed.
The Process
Stretch blow molding is best known for producing PET bottles commonly used for water, juice and a variety of other products. Stretch blow molding has been used since the early 1970's especially for packaging detergent, and has grown in existence with the primary use for making carbonated beverage bottles.
One of the major advantages of stretch blow molding is the ability to stretch the perform in both the hoop direction and the axial direction. This biaxial stretching of material increases the tensile strength, barrier properties, drop impact, clarity, and top load in the container. With these increases it is usually possible to reduce the overall weight in a container by 10 to 15 percent less than when producing a container in another way.
Stretch blow molding is divided into two different categories single-stage and two-stage.
Single-stage uses the extruder to inject parison into a preform mold where the plastic is rapidly cooled to form the preform. The preform is then reheated and placed in the bottle mold. Then softened parison stretches to about twice its original length. Compressed air is then blown into the stretched parison to expand to the bottles mold. Once the bottle is cooled the mold is opened and the finished bottle is emptied from the mold cavity. This technique is most effective in specialty applications, such as wide mouthed jars, where very high production rates are not a requirement.
Two-stage stretch blow molding is the same as single-stage, except the preforms are already made. The single-stage process is usually done using one machine, where the two-stage process uses preforms that have already been made and cooled. In this process, the machinery involved injection molds a preform, which is then transferred within the machine to another station where it is blown and then ejected from the machine. This type of machinery is generally called injection stretch blow molding (ISBM) and usually requires large runs to justify the very large expense for the injection molds to create the preform and then the blow molds to finish the blowing of the container.
The Process
Injection moulding is one of the most common methods of shaping plastic. It is used to create many things from packaging and bottle caps to automotive dashboards, pocket combs and most other plastic products. It is ideal for producing high volumes of the same object and offers the advantages of repeatable high tolerances, the ability to use a wide range of materials, minimal scrap losses and little need to finish parts after molding.
