Blow molding or Blow Forming is a manufacturing process in which hollow plastics are formed. Blow moulded products are formed by various thermoplastic materials like LDPE, HDPE, PET, PP, and PVC. There are quite a few types of molding technology are discussed below-
1.Extrusion Blow Moulding:
It consists of a machine with a screw conveyor, on top is a hooper in which all the raw plastic ingredients is poured. It also consists of a heating element to melt the plastic ingredients.
Mechanism- Firstly, the raw plastic pellets are placed in the hooper, which by gravitational action comes down in the machine and is pushed forward by the Screw conveyor into the heating region where it melts. Finally, we have molten plastic/Parison; the molten material is pushed forward through a small orifice into the mold. The air is blown into the soft parison followed by a curing process; the part is then cooled and removed from the mould. Parison is a preform tube that is hollow, and it has a hole at one end.
2. Injection Blow Moulding:
It is a non-continuous cyclic process consisting of 2 phases-
- Parison is molded by injecting melted plastic into a steel mold cavity where it is kept hot and conditioned.
- The parison is metered into a mold where the final curing operation takes place to form the final part.
Mechanism: It consists of a machine with hooper on the top. On the left-hand side is the Archimedean Screw to push the material in the forward direction. In the middle, there is a heating part that melts the raw material. On the right-hand side is the mold cavity formed by the molding parts maintained in cold condition. Thus, the plastic ingredients are taken into a hooper and then the Archimedean screw is rotated to push the material in the heating region, where it melts, and at the nozzle the temperature increases to such an extent that it quickly enters the mold cavity where curing is done and finally with the help of extraction pin the article is taken out.
- Stretch-Blow Molding :
First, a test-tube like preform is made using injection molding or a similar process. The preforms’ neck is fully finished, but the diameter and length of the body portion are much smaller than the final product. The preform then undergoes a stretch-and-blow process. It is the most preferred technology for making PET bottles.
When looking at molding options for production purposes, there are a few different methods available. Blow molding and injection molding stand as the two most common. However, before a company decides what to go with, it is important to understand the difference between the two.
- Blow molding is designed to produce hollow, singular containers, such as bottles. On the other hand, injection molding is used to produce solid pieces, such as plastic products like helmet.
- Air is the worst enemy of injection molding. If air is present during the manufacturing process, it can create air pockets or bubbles. These abnormalities create weak spots in the product, ultimately leaving it defective and, likely tossed out. With the blowing method, the air is particularly important. Blow molding revolves around air forced into the mold to push out and expand the product.
- Stretch blow molding is best for PET bottles.