This article gives comprehensive information about the blow molding process and its raw materials. Read further to learn more.
- Overview of blow molding
- History of blow molding
- The blow molding process
- And much more…
Chapter One – Overview of Blow Molding
Blow molding is a type of plastic forming process for creating hollow plastic products made from thermoplastic materials. The process involves heating and inflating a plastic tube known as a parison or preform. The parison is placed between two dies that contain the desired shape of the product. Air is then supplied to expand the tube causing the walls to become thinner and conform to the shape of the mold. Once the blowing process is complete, the product is then cooled, ejected, trimmed, and prepared for the secondary processes.
Bottling and packaging are the main applications of blow molding which comprise about 49% of the global blow molding market share. This is followed by building and construction, consumer products, and transportation industries. The global market share of blow molding was estimated at around $78 billion in 2019 and is expected to grow annually by 2.8% from 2020 to 2027. Typical raw materials used are polyethylene (PE), polyethylene terephthalate (PET), and polypropylene (PP).
Chapter Two – History of Blow Molding
Plastic blow molding originated from the ancient process of glass blowing. Both glass and plastic in their molten state can be formed easily by introducing small amounts of air inside the molten material. The first injection molding process for glass, known as free-blowing, was created around the 1st century BC. Glass mold-blowing was later developed as an alternative process that came about around the 1st century AD.
The first blow molding material used other than glass was natural rubber. The process was patented by Samuel Armstrong in the 1850s. The next major advancement came around the 1930s with the development of the first blow molding machine by Plax Corporation. The process uses cellulose acetate as the raw material. Come 1939, low-density polyethylene (LDPE) was introduced by Imperial Chemical Industries from England. LDPE is more suitable for blow molding which then paved the way for further process development. By the 1950s, the commercialization of blow molding was completed, and the industry grew exponentially.
Chapter Three – The Blow Molding Process
This chapter discusses the general blow molding process. Blow molding is a fairly straightforward process that includes melting, homogenizing, extruding, molding (blowing), cooling, and ejection. Different manufacturing plants can employ additional processes such as additional cooling or heating cycles and additives and colorants compounding. These additional processes depend on the design and intended application of the final product.
Plastic Resin Feeding or Charging: The first step in the blow molding process is plastic feeding. This is done by conveying the plastic pellets into the extruder hopper. Vacuum pumps draw pellets from big bags or bulk containers and transfer them into the raw material silos or hoppers. A rotary feeder at the bottom of the silo controls the rate of feeding into the plastifier or extruder. Compressed air is then used to convey the stored pellets to the extruder hopper. In other systems, plastic pellets from big bags or bins can be vacuum conveyed directly to the extruder hopper without the need for a separate conveying air system.
- Plasticizing or Melting: As the plastic resin enters and goes through the extrusion machine, it is melted by continuous kneading and heating. Electric heating elements or heating bands are wrapped around the extruder barrel to provide heat for melting the polymer. The extruder screw has different sections that serve a specific purpose. These are feeding, compressing, and metering. The extruder screw is designed to provide sufficient shearing and compression to homogenize and extrude the plastic.
Parison Extrusion or Preform Injection: This process is the preparation of the parison or preform to be inflated. This is done by extruding the plastic through free extrusion or injection into a preform mold. The different processes in producing the preform are discussed in detail in Chapter 3.
- Sealing or Clamping: Sealing or clamping involves a split die that captures the preform. The ends of the parison (extrusion blow molding) are sealed except for one hole, typically the container opening, where the compressed air will be injected.
Inflation or Blow Molding: This step is where the plastic takes its form. Compressed air is introduced inside the preform. This inflates the preform until it is molded according to the profile of the die.
- Cooling and Ejecting: The next step is the cooling process. Typically, as the plastic touches the die, it cools at a predefined rate which stabilizes the dimensions of the product. After cooling, the mold opens and ejects the product.
- Trimming: Flashing is generally evident in extrusion blow molding. Most blow molding machines have auto deflashing features as the dies clamp the preform. But in some instances, flash is present at the top and bottom parts of the product, especially at the opening where compressed air is injected. This excess material is trimmed by a rotating knife. To minimize wastage, some systems collect the excess materials, grind them, and feed them back to the extrusion machine.
Leak Test: This is the typical quality control method used in manufacturing bottles or packaging materials. In this step, either vacuum or compression is generated inside the container. The machine will then check if air enters or escapes the container by monitoring the pressure. If a leak is sensed from the container, it is rejected and fed back into the system.
Other Secondary Processes and Packaging: Secondary processes include labeling, marking, and printing. After completing the secondary processes, the products are packed and distributed to manufacturers or end consumers.
Chapter Four – Types of Blow Molding
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.
- 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.
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.
Chapter Five – Raw Materials
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).
- High-density Polyethylene (HDPE): Among the types of polyethylene, HDPE is the more dominant raw material in terms of market share. Its molecular structure is linear with little branching resulting in higher intermolecular forces. This gives HDPE its high specific strength.
- Low-density Polyethylene (LDPE): LDPE has a branched polymer chain that has weak intermolecular forces. This results in lower tensile strength and barrier properties. Nevertheless, it has better impact strength and resilience than HDPE.
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.
- Polypropylene (PP): Polypropylene is a polymer that can have a wide range of properties that depends on its molecular weight, morphology, crystalline structure, additives, and copolymerization. It can be made into polymers with a high degree of crystallinity; thus, higher tensile strength and hardness which is comparable to HDPE. Moreover, they can withstand higher temperatures without loss of strength or degradation. The disadvantages of using PP are its susceptibility to UV degradation and oxidation.
- Polyvinyl Chloride (PVC): Polyvinyl Chloride is another versatile material that can be formulated with different stabilizers, plasticizers, impact modifiers, processing aids, and other additives. It can be made into rigid or flexible plastics by modifying the amount of plasticizers. Moreover, they offer better clarity than other versatile plastics such as PP. However, PVCs can release harmful pollutants, acids, and toxins during processing or degradation. Its compounding ingredients are now being regulated by FDA, EPA, and other organizations.
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.
- Polycarbonate (PC): Polycarbonate is easily processed by different molding methods, with injection molding and sheet extrusion being the most common. In blow molding, its usual application is bottle production. Polycarbonates are known for their high impact strength, heat resistance, good electrical insulation, transparency, good water barrier properties, and inherent flame retarding properties.
- Copolyester: Copolyesters are described as having very high thermal oxidation stability, inherent flame resistance, chemical inertness, excellent clarity, and good mechanical properties. It can be molded into thin-walled containers at high speeds. Moreover, it has intrinsic lubricity and non-stick properties making it viable for molding.
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.
- Acrylonitrile Butadiene Styrene (ABS): ABS is a common plastic that is characterized by having good hardness and rigidity while having some degree of toughness. When blow molded, they can have a very good surface finish making them suitable for the manufacture of consumer goods. Protective coatings are usually applied due to their poor resistance to UV and only adequate resistance to most acids and alkalis.
- Blow molding is a type of plastic forming process for creating hollow plastic products made from thermoplastic materials. The process involves heating and inflating a plastic tube known as a parison or preform.
- Plastic blow molding originated from the ancient process of glass blowing. Both glass and plastic in their molten state can be formed easily by introducing small amounts of air inside the molten material.
- Blow molding is a fairly straightforward process that includes melting, homogenizing, extruding, molding (blowing), cooling, and ejection.
- 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.