Blow molding is the process of inflating a hot, hollow, thermoplastic preform or parison inside a closed mold, so its shape conforms to that of the mold cavity. A wide variety of blow molded hollow parts, including plastic bottles, can be produced from many different plastics using this process.
There are three main types of blow molding:
• Extrusion blow molding
• Injection blow molding
• Injection stretch blow molding
The main differences among them are the method of forming the parison; either by extrusion or injection molding, the size of the parison and the method of movement between the parison and blow molds; either stationary, shuttling, linear or rotary.
There are three specific types of blow molding machines, as outlined below:
Extrusion Blow Molding (EBM) – This method is used for extremely high production. Depending on the customer’s need, EBM can work in conjunction with other manufacturing processes, such as filling and labeling. With this method, plastic is melted and then extruded into a parison. Next, the parison is inserted into a metal mold that has been cooled, followed by air being blown into the parison to inflate it into the desired shape.
Injection Blow Molding (IBM) – For this method, the parison is made using a separate operation. This type of molding starts with molten plastic, which is injected into a preform mold that has been heated. The plastic then seeps around a hollow core rod. After all of the plastic is injected, it then transfers to a second station in the machine that blows the container. The third station is where the core rod is stripped away and the parts exit the machine. IBM is the method used for manufacturing single-serve and small plastic products, like medicine bottles.
Stretch Blow Molding (SBM) – This process is used to make parts that have a biaxial molecular alignment. With SBM, the parison is mechanically elongated in the mold and then significantly expanded using a blowing process. Compared to the other two methods, SBM requires a cooler temperature, thereby making the process somewhat challenging to control. However, not only is there less wasted material, the finished product is stronger.