Blow molding, also known as BrE molding, is a manufacturing process where hollow plastics are inflated with hot thermoplastic perform to produce a bottle or a container that conforms to the mold cavity. This process is also used in glass bottles production. Blow molding usually begins by melting down the plastic material into a parison. A parison is made up of tube-like plastic that has a hole where compressed passes through it.
After melting, the parison is clamped into a mold, and compressed air is passed through it pushing the plastic out that matches with the mold. When this plastic is cooled and then hardened, the mold will open, and the part gets ejected.
There are three main different molding processes namely:
• Injection blow molding
• Extrusion blow molding
• Stretch blow molding
Injection Blow Molding
In this molding process, the plastic material is melted down and then injected through a core pin before it is rotated. The hot plastic material is then indexed to a bolding station where the material is allowed to cool. When processing PET material, it is critical for the core pin to be cooled. After getting cooled, the bottled made is then indexed to a station where it is ejected. This process compared to extrusion molding process allows precise detail in the threaded (finish) area. There is minimal improvement in physical properties in the injection molding process since there is little orientation occurring. Moreover, the injection process is usually limited to manufacturing relatively small process. Eastapak polymer 1991 is usually implemented in injection and stretch blow molding processes.
Stretch Blow Molding
Stretch blow process involves the production of objects that are hollow in shape through the biaxial molecular orientation. This orientation provides an enabling environment where physical properties, gas barrier properties, and clarity are enhanced. Stretch blow process is very critical, especially when producing bottles meant for carbonated beverages.
There are two distinct techniques of stretch blow molding. One is a one-stage process where injection molding is carried out in perform and then conditioned with proper temperature before getting blown into bottles. This process is continuous and is most useful in making products such as wide-mouthed jars that do not require a high production rate.
On the other hand, the two-stage process involves injection molding of plastic material, storing it for a short time and then blowing it up into containers through a reheat-blow machine. This technique is best suited for producing high volume products due to the high cost of RHB equipment and molding.
Extrusion Blow Molding
This molding process begins by conventional extrusion of material through a parison (hollow tube) before it is indexed into a metal mold for cooling. Air is blown through the tube to inflate material into the shape of the mold then cooled sufficiently before it is open and ejected. The material is usually extruded between open blow molds until it reaches the required length. After the bottle is ejected, the excess plastic is trimmed either from the neck or bottom. In the extrusion process, different kinds of plastic can be processed including the PVC, PP, PC, HDPE, and PETG. It is suitable for the production of small units and requires relatively little capital.