- Polymer Processing
- Blow Molding
- Injection Molding
- Metal Injection Molding
- Metal Casting
- Centrifugal Casting
- Die Casting
- Investment Casting
- Permanent Mold
- Sand Casting
- Shell Mold Casting
- Drill Size Chart
- Tap Size Chart
- Sheet Metal Fabrication
- Cutting with shear
- Cutting without shear
- Gauge Size Chart
- Additive Fabrication
- 3D Printing
- Inkjet Printing
- Jetted Photopolymer
- Case Studies
- Cost Analysis
- Part Redesign
- Product Development
- Curriculum Resources
Blow molding is a manufacturing
process that is used to create hollow plastic parts by
inflating a heated plastic tube until it fills a mold
and forms the desired shape. The raw material in this
process is a thermoplastic in the form of small
pellets or granules, which is first melted and formed
into a hollow tube, called the parison. There are
various ways of forming the parison, as explained below.
The parison is then clamped between two mold halves and
inflated by pressurized air until it conforms to the
inner shape of the mold cavity. Typical pressures are
25 to 150 psi, far less than for injection molding.
Lastly, after the part has cooled, the mold
halves are separated and the part is ejected.
Parts made from blow molding are plastic, hollow, and thin-walled, such as bottles and containers that are available in a variety of shapes and sizes. Small products may include bottles for water, liquid soap, shampoo, motor oil, and milk, while larger containers include plastic drums, tubs, and storage tanks. Blow molded parts can be formed from a variety of thermoplastic materials, including the following:
- Low Density Polyethylene (LDPE)
- High Density Polyethylene (HDPE)
- Polyethylene Terephtalate (PET)
- Polypropylene (PP)
- Polyvinyl Chloride (PVC)
As mentioned above, there are different methods used to form the parison which distinguish the following three forms of blow molding:
- Extrusion blow molding - An extruder uses a rotating screw to force the molten plastic through a die head that forms the parison around a blow pin. The parison is extruded vertically between the two open mold halves, so they can close on the parison and blow pin. Pressurized air flows through the blow pin to inflate the parison. This is the most common type of blow molding and is used to manufacture large quantities of relatively simple parts.
- Injection blow molding - The molten plastic is injection molded around a core inside a parison mold to form the hollow parison. When the parison mold opens, both the parison and core are transferred to the blow mold and securely clamped. The core then opens and allows pressurized air to inflate the parison. This is the least commonly used method because of the lower production rate, but is capable of forming more complicated parts with higher accuracy. Injection blow molding is often preferred for small, complex bottles, such as those in medical applications.
- Stretch blow molding - The parison is formed in the same way as injection blow molding. However, once transferred to the blow mold, it is heated and stretched downward by the core before being inflated. This stretching provides greater strength to the plastic. Stretch blow molding is typically used to create parts that must withstand some internal pressure or be very durable, such as soda bottles.
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Disclaimer: All process specifications reflect the approximate range of a process's capabilities and should be viewed only as a guide. Actual capabilities are dependent upon the manufacturer, equipment, material, and part requirements.
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