Manufacturing technique to create and assemble hollow plastic components.

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Blow molding (or moulding) is a specialized manufacturing process for creating hollow plastic items. This method is also applicable to forming glass bottles and other hollow structures.

Generally, blow molding is categorized into three principal types: extrusion blow molding, injection blow molding, and injection stretch blow molding.

The blow molding process commences by heating and softening a preform or parison, which is a tube-like piece of plastic with an opening at one end for compressed air to enter.

Once softened, the plastic piece is clamped inside a mold and inflated by blowing air into it. The inflated plastic conforms to the mold shape. Upon cooling and hardening, the mold opens, and the formed part is ejected. Cooling is assisted by water channels within the mold.

Inspired by the ancient practice of glassblowing, Enoch Ferngren and William Kopitke engineered a blow molding machine, which they sold to Hartford Empire Company in 1938. This marked the beginning of commercial blow molding. The initial variety and volume of products were limited throughout the 1940s, preventing the process from gaining immediate traction. However, as variety and production rates surged, the number of manufactured products quickly followed suit.

Early on, the techniques required for producing hollow items through blowing were well-defined. Given that glass is highly breakable, plastic began to replace glass in some applications following plastic’s introduction. Mass plastic bottle production commenced in America in 1939, while Germany adopted the technology slightly later and has since become a leading producer of blow molding machinery.

In the U.S., plastic container usage in the soft drink industry skyrocketed from zero in 1977 to ten billion pieces by 1999. Today, the production of blow-molded items continues to grow, showing a consistent upward trend.

Recently, blow molding for amorphous metals, also known as bulk metallic glasses, has been demonstrated under conditions similar to those for plastic blow molding.

Extrusion Blow Molding

In extrusion blow molding, plastic is melted and extruded into a parison—a hollow tube with one open end. The parison is captured by a cooled metal mold, into which air is blown, inflating the plastic to mimic the mold's shape. After cooling, the mold opens, and the part is ejected.

Continuous extrusion blow molding channels plastic forward similarly to injection molding using an Archimedean screw, which moves the plastic through a heated tube. The screw then pushes the molten plastic out. The accumulator method gathers melted plastic, which a rod subsequently pushes, forming the parison. This method enables precise control over wall thickness by adjusting the die gap with a parison programming device.

Continuous extrusion equipment includes rotary wheel blow molding systems and shuttle machinery, whereas intermittent extrusion machinery includes reciprocating screw machinery and accumulator head machinery.

Spin Trimming in Blow Molding

Excess material is often found on containers like jars due to the molding process. This excess is trimmed using a spinning blade, which recycles the leftover plastic to create new moldings. Spin trimmers handle materials such as PVC, HDPE, and PE+LDPE, each with unique trimming characteristics. Hard-to-trim amorphous materials, like PET, benefit from using titanium nitride-coated blades, which last up to 30 times longer than steel blades.

Injection Blow Molding Process

The Injection Blow Molding (IBM) process is primarily employed to produce hollow glass and plastic items in mass quantities. Here, polymer is injection molded onto a core pin, which then moves to a blow molding station for inflation and cooling. Typically used for small medical and single-serve bottles, this method is split into injection, blowing, and ejection stages.

An injection blow molding machine comprises an extruder barrel, screw assembly, a hot runner manifold, heated cavity, and core pin. Once the preform—featuring a bottle/jar neck—is formed, it transfers to a blow mold to be inflated into the desired shape.

Injection Stretch Blow Molding: Methods and Applications

Injection Stretch Blow Molding (ISBM) encompasses Single-stage and Double-stage methods, with Single-stage further divided into 3-station and 4-station operations. In Single-stage, both preform creation and bottle blowing occur within one machine. Single-stage eliminates the reheat phase to reduce costs, unlike the older 4-station process.

Double-stage ISBM first molds plastic into preforms, later reheated and blown into bottles using high-pressure air. Preforms retain their original threads and necks, making ISBM suitable for carbonated drink bottles, thanks to the improved barrier strength of biaxially stretched plastics.

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Exploring Blow Molding

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Introduction

This article presents in-depth details on the blow molding process and the materials involved. Continue reading to explore comprehensive information.

• Overview of blow molding
• History of blow molding
• The blow molding process
• And much more...

Overview of Blow Molding

Blow molding is a technique to form hollow plastic goods using thermoplastic materials. Initially, the process heats and inflates a plastic tube called a parison or preform. This parison is placed between dies that shape the product. Air inflates the parison to fit the mold, followed by cooling, ejecting, trimming, and secondary processing steps.

Bottling and packaging form nearly 49% of the global blow molding market. The 2019 market valuation was about $78 billion, with an anticipated annual growth rate of 2.8% from 2020 to 2027. Common raw materials include polyethylene (PE), polyethylene terephthalate (PET), and polypropylene (PP).

History of Blow Molding

Blow molding traces back to ancient glassblowing techniques where air was used to shape molten glass or plastic. Samuel Armstrong patented the first rubber blow molding process in the 1850s, followed by Plax Corporation's first blow molding machine in the 1930s, utilizing cellulose acetate. The introduction of low-density polyethylene (LDPE) by Imperial Chemical Industries in 1939 further advanced the process. By the 1950s, blow molding had gone commercial, fostering industry growth.

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The Blow Molding Process

This section explores the essential blow molding steps: melting, homogenizing, extruding, molding (blowing), cooling, and ejection. Additional processes like extra cooling or heating cycles and additive compounding exist, depending on the product design and application.

• Plastic Resin Feeding: Plastic pellets are transferred to the extruder hopper from silos via vacuum pumps and rotary feeders.
• Plasticizing: In the extruder, pellets are melted through kneading and heaters, then homogenized by the feeding, compressing, and metering screw sections.
• Parison Extrusion: The parison is prepared via free extrusion or injection into a preform mold.
• Sealing/Clamping: The preform is clamped in the mold, with one end open for air inflation.
• Inflation: Compressed air inflates the preform to conform to the mold.
• Cooling/Ejecting: The product cools and stabilizes within the mold, which then opens to eject the molded part.
• Trimming: Excess material, especially around openings, is removed using rotating blades and recycled.
• Leak Test: Containers undergo pressure/vacuum tests for quality control before further processing.
• Secondary Processes: Additional processes such as labeling and packaging follow before distribution.

Leading Blow Molding Machines

Several leading brands offer machines integral to the blow molding process in the U.S. and Canada:

Graham Engineering Corporation

Model: Graham Rotary Wheel Blow Molding Systems

Graham's Rotary Wheel systems excel in high-speed production, enabling continuous and efficient output with precision.

Bekum America Corporation

Model: Bekum H Series Blow Molding Machines

Bekum's H Series features advanced servo-driven units, precise parison control, and flexible clamping systems for versatile production.

Kautex Machines, Inc.

Model: Kautex KBB Series Blow Molding Machines

Kautex's KBB series offers multi-layer extrusion, precise control, and high-speed operation, suitable for complex products.

Wilmington Machinery

Model: Wilmington Blow Molding Systems

Wilmington's machines are known for their energy efficiency and precision control, providing high-quality production across industries.

Milacron LLC

Model: Milacron Extrusion Blow Molding Machines

Milacron's extrusion blow molding machines are praised for durability, accuracy, and high-speed operation across various materials.

Types of Blow Molding

Blow molding includes two primary methods: extrusion and injection blow molding, with a third type being stretch blow molding. Each method is tailored for specific applications based on the preform production process.

• Extrusion Blow Molding: This involves forming a parison captured by a die and inflated to the mold shape. It has two subtypes—continuous and intermittent.

  • Continuous Extrusion Blow Molding: Continuously extrudes parison, utilizing molds such as shuttle presses and rotating wheels for high-volume production.

  • Intermittent Extrusion Blow Molding: Gathers plastic batches in the extruder head for timely extrusion and forming large parisons, suitable for large products.

• Injection Blow Molding: Combines injection molding for preform creation and blow molding to form the product, ideal for small, precision products.

• Stretch Blow Molding: Enhances strength through biaxial stretching, extending and inflating preforms, often used for PET bottles.

Raw Materials in Blow Molding

Blow molding employs diverse thermoplastics, avoiding degradation with heat use. Common materials include polyethylene (PE), polyethylene terephthalate (PET), and polypropylene (PP), plus advanced engineering plastics for specific applications.

• Polyethylene (PE): Distinguished by high tensile strength (HDPE) and flexibility (LDPE), widely used for its processability and cost-effectiveness.

• Polyethylene Terephthalate (PET): Valued for low carbon dioxide permeability, perfect for carbonated beverage containers.

• Polypropylene (PP): Offers varying properties based on its structure, ideal for high-temperature and high-strength applications.

• Polyvinyl Chloride (PVC): Versatile but regulated due to potential harmful emissions during processing.

• Nylon or Polyamide (PA): Known for toughness, impact resistance, and solvent resistance, used in automotive parts.

• Polycarbonate (PC): Offers high impact strength, clarity, and heat resistance, often used for bottles.

• Copolyester: High thermal stability and clarity, suitable for thin-walled containers.

• Cyclic Olefin Copolymer (COC): Comparable to PVC, desirable for food and medical container production owing to its low water permeability.

• Acrylonitrile Butadiene Styrene (ABS): Combines hardness and toughness, suitable for consumer goods with a good surface finish.

Conclusion

• Blow molding creates hollow plastic goods by heating and inflating a plastic tube.
• The process has ancient origins in glassblowing, applicable to both glass and plastic.
• Blow molding involves several steps, including melting, extruding, molding, cooling, and ejection.
• Main types are extrusion and injection blow molding, each suited to specific methods and applications.

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