This article gives comprehensive information about the blow molding process and its raw materials. Read further to learn more.
Blow molding is a manufacturing technique for creating hollow plastic products using thermoplastic materials. The process begins with heating a plastic tube known as a parison or preform, which is inflated inside a mold. The parison is positioned between two halves of a mold, defining the final product's shape. Compressed air is introduced to enlarge the tube, forcing the plastic to stretch and conform to the mold's contours. Once the molding is finished, the item is cooled, extracted from the mold, trimmed, and prepped for any further processing steps.
Blow molding is predominantly used in bottling and packaging, which represent nearly 49% of the global market demand. Other major uses include construction, consumer goods, and transportation. In 2019, the global blow molding industry was valued at approximately $78 billion and expects an annual growth rate of 2.8% from 2020 through 2027. Common materials employed in blow molding consist of polyethylene (PE), polyethylene terephthalate (PET), and polypropylene (PP).
The origins of plastic blow molding can be traced back to the ancient practice of glass blowing, a foundational technique in the evolution of modern manufacturing processes. Both molten glass and thermoplastics can be shaped by introducing pressurized air into the material, a principle that lies at the heart of today’s blow molding industry. The earliest method of molding glass, known as free-blowing, dates back to the 1st century BC and revolutionized glass container production by enabling complex, hollow shapes. Shortly after, around the 1st century AD, the glass mold-blowing technique emerged, allowing artisans to produce more uniform glass products. These early innovations laid the groundwork for future advancements in molding technology, inspiring methods that shape not only glass but also a wide range of polymers.
Following the pioneering work with glass, the earliest non-glass blow molding material was natural rubber. In the 1850s, inventor Samuel Armstrong patented a process for molding hollow rubber products, setting a precedent for the exploration of alternative materials in blow molding. The true transformation in plastic manufacturing came in the 20th century. A major technological leap occurred in the 1930s when Plax Corporation introduced the first plastic blow molding machine, utilizing cellulose acetate as a raw material — a development considered the genesis of commercial plastic blow molding.
A pivotal moment arrived in 1939 when Imperial Chemical Industries (ICI) from England developed low-density polyethylene (LDPE), a thermoplastic polymer renowned for its flexibility and strength. LDPE’s suitability for extrusion blow molding and its accessibility accelerated the adoption of the blow molding process in the plastics industry. By the 1950s, blow molding machines were extensively used for manufacturing hollow plastic containers, bottles, automotive components, and industrial packaging. This era marked the commercialization and rapid expansion of blow molded products, with high-density polyethylene (HDPE) and other polymers soon following suit for items requiring greater durability or specific properties.
Today, blow molding is a fundamental plastics processing method used worldwide for producing a vast range of items such as custom plastic bottles, containers for food and beverages, industrial drums, automotive parts, medical device housings, and consumer packaging. The three main types of blow molding — extrusion blow molding, injection blow molding, and stretch blow molding — address diverse industry requirements and product specifications. Modern advancements in computer-aided design, precision mold manufacturing, and sustainable bio-based resins have further revolutionized the field, enabling high-volume, cost-effective, and environmentally friendly production.
Understanding blow molding’s rich history is essential for manufacturers, engineers, product designers, and procurement specialists seeking reliable, efficient, and innovative solutions in plastic packaging and industrial manufacturing. For businesses aiming to optimize their product development cycles, reduce material costs, and achieve greater design flexibility, partnering with leading blow molding companies ensures access to cutting-edge technology and industry expertise.
The blow molding process is a foundational manufacturing technique used to produce hollow plastic parts, bottles, and containers at scale. This method is valued for its efficiency in shaping thermoplastic resins into durable, lightweight components, especially for packaging applications and industrial products. In this chapter, we provide a comprehensive overview of the basic blow molding steps, which consist of melting, homogenizing, extruding, molding (blowing), cooling, and ejection. Depending on the specific requirements—such as product type, material, shape, or strength—manufacturing plants may integrate further steps into the process. These additional steps can include specialized cooling or heating sequences, blending in additives to improve performance (such as UV stabilizers or colorants), or implementing advanced in-line quality control systems. Each adjustment is made to improve product performance, appearance, or compliance with industry-specific standards in markets like food and beverage, automotive, or pharmaceutical packaging.
The initial stage of the blow molding process is plastic resin handling and feeding. Thermoplastic resin pellets—such as high-density polyethylene (HDPE), polyethylene terephthalate (PET), polypropylene (PP), or polyvinyl chloride (PVC)—are conveyed automatically into the extruder hopper. Vacuum pumps and pneumatic conveying systems transfer pellets from bulk storage, like silos or big bags, to raw material hoppers, ensuring consistent supply and minimizing contamination. A rotary feeder at the base of the silo accurately controls the flow of resin into the plastifier or extruder, aiding in process stability and reducing material wastage. In modern extrusion blow molding systems, some plants achieve direct vacuum conveying from storage bins to the extruder, further streamlining workflow and reducing downtime.
Once in the extrusion machine, the plastic resin is plasticized—transformed from a solid to a homogeneous molten state—through mechanical shearing and external heat. Electric heating bands, cartridge heaters, and jacketed barrels uniformly raise the temperature along the extruder. The extruder screw is engineered with feed, compression, and metering zones, each performing critical mixing and pressurization tasks. This homogenization step ensures that colorants, impact modifiers, or other additives are thoroughly blended before the material is extruded, vital for uniform product quality and mechanical strength. Optimized melt flow is key for producing consistent wall thickness across molded products.
This critical step involves creating the parison (a tubular plastic form) or injecting molten plastic into a preform mold, depending on whether extrusion blow molding, injection blow molding, or stretch blow molding is used. In extrusion blow molding, the parison is extruded vertically, while in injection stretch blow molding, a preform is first molded and later reheated for blowing. Controlling parison dimensions directly impacts product uniformity, wall thickness distribution, and overall container performance. For a detailed breakdown of each process variation, see Chapter 3.
During inflation, compressed air is introduced into the enclosed parison or preform. The air pressure causes the heated plastic to expand outward, conforming perfectly to the contours of the mold cavity, forming the final product shape. The blow molding machine’s control over air pressure and timing results in products with precise dimensions and optimized mechanical properties—crucial for applications in packaging, automotive fuel tanks, and industrial drums.
In the cooling stage, the molded plastic rapidly loses heat upon contact with the chilled mold surfaces. Controlled cooling rates stabilize product dimensions, reduce internal stresses, and ensure uniform wall thickness. Once adequately cooled, the mold opens and the finished product is automatically ejected. Efficient mold cooling systems are critical in high-volume bottle manufacturing to enhance cycle times and maintain product consistency.
Trimming addresses “flashing”—excess plastic formed at the mold parting line—typically found on extrusion blow molded parts. Many blow molding machines feature automatic deflashing units that trim this material as the mold opens. For items like bottles and containers, residual flash at the neck or base may require rotary knife trimming or precision cutting systems. Advanced plants often reclaim this trimmed material through granulation and recycling, increasing operational sustainability and reducing raw material costs.
As part of stringent quality control, automatic leak detection assesses the integrity of each container. The process generates a vacuum or applies compressed air within the molded product, checking for any loss in pressure—indicative of holes or weak welds. Only leak-free bottles and containers proceed to subsequent stages, ensuring compliance with food safety, pharmaceutical, or industrial packaging standards. Defective products are automatically separated and reprocessed, ensuring minimal waste.
Once primary shaping is complete, secondary processes—such as labeling, printing, embossing, or laser marking—customize the container for branding, regulatory compliance, or traceability. Products then move into automated packaging lines for inspection, sorting, and shipment. Efficient packaging and palletizing systems ensure that blow molded containers arrive safely at manufacturers, distributors, and end-users, supporting logistics workflows for sectors ranging from consumer goods to chemical processing.
Understanding each step of the blow molding process is critical for optimizing production, minimizing defects, and ensuring product quality across various applications. By mastering resin selection, precise extrusion and mold temperature control, and integrating secondary operations, manufacturers achieve highly consistent, cost-effective plastic products. Whether producing PET water bottles, HDPE detergent containers, or custom technical components, leveraging modern advancements in blow molding equipment and process automation can yield superior results—reducing cycle times, improving material efficiency, and enhancing market competitiveness.
For further insights on blow molding machinery, process optimization, resin selection, quality assurance measures, and the latest industry trends, consult our in-depth resources or connect with experienced blow molding solution providers. Staying informed about new technologies and market requirements is essential to maintaining a competitive edge in the plastic manufacturing sector.
Numerous machines are designed to optimize the blow molding process. Below are five brands that manufacture blow molding machines used in the United States and Canada, including specific models and their distinctive features:
Description: Graham Engineering Corporation provides Rotary Wheel Blow Molding Systems, renowned for their high-speed production capabilities. These systems feature multiple molds mounted on a rotating wheel, allowing for continuous production and high output rates. Graham's machines are celebrated for their efficiency, accuracy, and versatility in manufacturing a broad array of blow-molded products.
Description: Bekum America Corporation specializes in blow molding machines, notably their H Series models. These machines are equipped with advanced features, including servo-driven extrusion units, precise parison control, and adaptable mold clamping systems. Bekum's H Series is recognized for its energy efficiency, rapid cycle times, and versatility in producing a wide range of blow-molded products.
Description: Kautex Machines, Inc. produces the KBB Series Blow Molding Machines, which are widely recognized in the industry. These machines feature multi-layer extrusion capability, accurate parison control, and high-speed performance. Kautex's machines are noted for their dependability, high productivity, and ability to manufacture intricate blow-molded products.
Description: Wilmington Machinery offers Blow Molding Systems tailored for diverse applications. Their machines are characterized by a compact design, energy efficiency, and precise control. Wilmington's systems are known for their durability, flexibility, and capability to deliver high-quality blow-molded products across various sectors.
Description: Milacron LLC's Extrusion Blow Molding Machines are celebrated for their efficiency and precision. These machines come equipped with features such as closed-loop parison control, high-speed operation, and adaptability to a wide range of materials. Milacron's machines are renowned for their robustness, accuracy, and consistent production of high-quality blow-molded items.
Keep in mind that the availability and specifications of individual models can change over time. For the latest and most accurate details regarding the blow molding machines offered by these manufacturers in the United States or Canada, it is advisable to reach out to the manufacturers directly or consult their most recent product catalogs and technical documentation.
There are two main types of blow molding: extrusion and injection blow molding. They differ according to the method of producing the preform or parison. Preform is the more general term used to describe the heated plastic tube while parison is commonly associated with extrusion blow molding. A third type, stretch blow molding, is basically a modification of the two main types that are used for creating biaxially oriented plastics. These processes have their pros and cons that aim to serve a particular application.
Extrusion Blow Molding: Extrusion blow molding involves the extrusion of a parison with a predetermined length which is held by a split die on its ends. The parison is sealed in one end while the other end is fitted to an air supply. Compressed air is introduced which inflates the parison. The dies are typically cold which cools the inflated molten plastic as it meets the die surface. When the dimensions of the product are stable, the mold is opened to eject the product.
There are two main types of extrusion blow molding categorized by the method of extruding the plastic to create the parison. These are continuous and intermittent blow molding.
Continuous Extrusion Blow Molding: In this method, the parison is continuously extruded from the extrusion machine. Polymer homogenizing is performed continuously. Once the parison is extruded, it is then cut to length by the closing mold halves. These mold halves can be configured into different types according to the mounting and movement of the mold. One configuration is the shuttle press type which can hold single or multiple parisons positioned horizontally next to each other. Shuttle press is cheaper than the other configurations but has a lower throughput.
Another type is the rotating wheel which can be horizontal or vertical. In this type, several molds are located at the periphery of a rotating wheel. Different sections of the rotating wheel correspond to the blow molding processes of parison clamping, cutting, inflating, cooling, and ejecting. The molds are continuously used as they revolve around the wheel axis. The rotating wheel type is suitable for high volume production.
Continuous blow molding is suitable for producing small to medium-sized plastic parts requiring lesser investment and less complex operation. However, to produce larger products, plastic with the right viscosity and good melt strength must be used. This is to prevent any sagging which can affect the quality of the product. Continuous blow molding is also used for blow molding heat-sensitive plastics such as PVC. This is due to the shorter cycle times that limit the degradation or unwanted curing of the polymer.
Intermittent Extrusion Blow Molding: In intermittent blow molding, the extrusion process starts when there is enough melt mass inside the extruder head or accumulator. Homogenizing and homogenizing is done intermittently inside the extrusion barrel. At the end of the barrel, the batch is held until sufficient volume is gathered and the previous batch is finished. Different types of machines are available for extruding the parison. The most popular are the reciprocating screw and accumulator head machines. The reciprocating screw type operates by retracting the screw as the melt collects in the extruder head. Once the batch is prepared, the screw moves longitudinally that compresses and extrudes the melt mass creating a parison. The accumulator head type, on the other hand, works by collecting the melt mass inside an accumulator head. Once the batch and mold are prepared, a ram pushes the melt to form the parison.
Intermittent blow molding is used for producing large products. The main advantage comes from the independent rates of accumulation and extrusion. This allows faster extrusion of large parisons. Faster extrusion means shorter suspension time and little sagging. Thus, products made from large parisons can be achieved even with plastics having low melt strength.
Extrusion blow molding can create a more sophisticated product with multiple layers by employing the coextrusion technology. Coextrusion blow molding involves the independent extrusion of plastics with different properties. The multilayered plastic melt is collected and prepared in a coextrusion head similar to an accumulator type. Multiple rams press on the coextrusion head forcing the melt through a die with several die cores. Once the multilayered parison is formed, the usual process of blow molding continues. Coextrusion is used in applications that require good barrier properties while at the same time provide structural rigidity and low cost. Examples of these are bottles for carbonated drinks and oils, chemical containers, and fuel tanks.
Injection Blow Molding: This process combines injection molding in forming the preform with blow molding. The injection blow molding process starts by melting and homogenizing the plastic. This is done in a plastifier and screw extruder which is typically a reciprocating screw. This operates with similar principles to that of the intermittent extrusion process. Its main difference is the use of a preform mold with a metal rod enclosed in the center. The typical mounting of these metal rods is a rotating table with three sections, each corresponds to a phase of the blow molding cycle.
The first phase is the melt injection. When the batch is complete, it is injected into a preform mold containing the metal rod. The preform mold consists of two dies; one is stationary while the other is movable. After injection, the temperature of the plastic is lowered until sufficient viscosity is attained to hold its shape. The second phase involves transferring the preform into another set of molds that contain the profile of the final product. The preforms are then blown to shape. After achieving the dimensions of the mold, the plastic is then cooled for ejection. The last phase is the ejection phase where the die is opened to release the product.
Some injection molding machines can have extra phases. The additional phases are used for additional heating or cooling cycles, preform conditioning, coating, and other secondary processes.
Stretch Blow Molding: Stretch blow molding, also known as injection stretch blow molding, is basically a modification of the injection blow molding process. This process also includes the three main phases with an additional step in the blow molding phase. In stretch blow molding, the preform is created by injecting the plastic melt into a die with a metal road at the center. The developed preform is then transferred to the product mold in preparation for the blow molding phase. Before inflating the preform, it is first stretched to a specific length by a stretch rod. After stretching, the plastic melt is then inflated to the shape of the final product. After molding, the product is then cooled and ejected.
Stretch blow molding is not limited to plastic injection. Stretching can also be achieved in an extrusion blow molding machine. This is done by partially extruding the parison and stretching it with a pull rod. It is then enclosed by a die and is inflated to shape.
The main objective of stretch blow molding is to create a product with a biaxially oriented plastic. Biaxial orientation is the process in which the plastic is stretched in two directions. In the longitudinal direction, the plastic is stretched by the stretch rod while in the lateral direction, the plastic is stretched by inflation. A biaxially oriented plastic has many desirable qualities including high impact, tensile, and tear strength, improved barrier properties, and excellent transparency. With the increased mechanical properties, bottles and packaging materials can be made with thinner walls. Note that not all plastics can be biaxially oriented. Moreover, the plastic resin must have the right properties and quality for it to stretch without tearing or producing specks or other defects. Plastics used in this application are PET, PP, and PVC. PET is a common biaxially oriented plastic that is used to produce clear plastic bottles.
There is a wide array of raw materials that can be used for blow molding. Blow molding materials are commonly thermoplastics that do not easily degrade upon heating. Blow molding is extensively used in the manufacture of plastic bottles which is why most raw materials have properties suitable for such application. Currently, blow molding is gaining momentum in the transportation, construction, and consumer goods industries. This makes engineering plastics such as polyamides and polycarbonates viable to be blown molded as well.
Polyethylene (PE): Polyethylene is the most widely used blow molding material. Polyethylene has many desirable characteristics such as easy processability, toughness, and flexibility which are retained even at low temperatures, odor and toxin free, excellent clarity, good water barrier properties, good electrical insulation properties, and low cost. It has two main types: high-density polyethylene (HDPE), and low-density polyethylene (LDPE).
Polyethylene Terephthalate (PET): PET, specifically biaxially oriented PET, is known for its low permeability to carbon dioxide. This makes the material desirable for producing bottles for carbonated beverages. The downside of using PET, however, is its affinity with water. It tends to absorb water which makes processing difficult as the resin needs to be dried before extrusion.
Nylon or Polyamide (PA): Polyamide is considered as an engineering plastic characterized by its high toughness, high impact strength, resistance to solvents, good abrasion resistance, and can be modified to have high heat resistance. PA production mostly goes into the manufacture of fibers. Only about 10% of PA production volume is used in plastic forming processes. The common applications of blow-molded PA are automotive parts and fuel tanks.
Cyclic Olefin Copolymer (COC): COC is a family of fully amorphous polymer resins that are desired because of its comparable properties with PVC without the negative effects. It is best suited for food packaging, medicine bottles, and vials production because of its low water vapor permeability. The properties of COC can be altered by developing specific blends of polyolefins. Some of the properties that can be modified are its elasticity, transparency, vapor transmission rate, and coefficient of friction.
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