Mumbai University > Mechanical Engineering > Sem 3 > Production Process 1

Marks: 5M

Year: May 2016

• Blow molding is a molding process in which air pressure is used to inflate soft plastic into a mold cavity.
• It is an important industrial process for making one-piece hollow plastic parts with thin walls, such as bottles and similar containers.
• Since many of these items are used for consumer beverages for mass markets, production is typically organized for very high quantities.
• The technology is borrowed from the glass industry with which plastics compete in the disposable or recyclable bottle market.
• Blow molding is accomplished in two steps: (1) fabrication of a starting tube of molten plastic, called a parison (same as in glass-blowing); and (2) inflation of the tube to the desired final shape.
• Forming the parison is accomplished by either of two processes: extrusion or injection molding.

Extrusion Blow Molding.

• This form of blow molding consists of the cycle illustrated below.
• In most cases, the process is organized as a very high production operation for making plastic bottles.
• The sequence is automated and usually integrated with downstream operations such as bottle filling and labeling.
• It is usually a requirement that the blown container be rigid, and rigidity depends on wall thickness among other factors.
• Extrusion blow molding: (1) extrusion of parison; (2) parison is pinches at the top and sealed at the bottom around a metal blow pin as the two halves of the mold come together; (3) the tube is inflated so that it takes the shape of the mold cavity; and (4) mold is opened to remove the solidified part.

Injection Blow Molding.

• In this process, the starting parison is injection molded rather than extruded.
• A simplified sequence is outlined in below. Compared to its extrusion-based competitor, the injection blow-molding process has a lower production rate, which explains why it is less widely used.

• Injection blow molding: (1) parison is injection molded around a blowing rod; (2) injection mold is opened and parison is transferred to a blow mold; (3) soft polymer is inflated to conform to a blow mold; and (4) blow mold is opened and blown product is removed.