Blow molding

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Blow molding, also known as blow forming, is a manufacturing process by which hollow plastic parts are formed. It is a process used to produce hollow objects from thermoplastic.

In general, there are three main types of blow molding: extrusion blow molding, injection blow molding, and stretch blow molding.

The blow molding process begins with melting down the plastic and forming it into a parison or preform. The parison is a tube-like piece of plastic with a hole in one end in which compressed air can pass through.

The basic process has two fundamental phases. First, a preform (or parison) of hot plastic resin in a somewhat tubular shape is created. Second, a pressurized gas, usually air, is used to expand the hot preform and press it against a mold cavity. The pressure is held until the plastic cools. This action identifies another common feature of blow molded articles. Part dimensional detail is better controlled on the outside than on the inside, where material wall thickness can alter the internal shape.^[1] Once the plastic has cooled and hardened the mold opens up and the part is ejected.

Contents 1 Background of blow molding 1.1 History 1.2 Products 2 Extrusion blow molding 2.1 Continuous Extrusion Blow Molding 2.2 Intermittent blow molding 2.3 Controlling Wall Distribution 3 Injection blow molding 4 Stretch blow molding 5 New Applications of Blow Molding Technology 5.1 Coextrusion Blow Molding of Large Parts 5.2 Three-Dimensional Blow Molding 5.3 Hard-Soft-Hard and Soft-Hard-Soft Technology 5.4 Long-Glass-Fiber-Reinforced Blow Molding 5.5 Blow Molding Foam Technology 6 Polymer characteristics relevant for blow molding 6.1 High-Density Polyethylene (HDPE) 6.2 Acrylonitrile-butadiene-styrene (ABS) 6.3 Polycarbonate (PC) 6.4 Polypropylene (PP) 6.5 Polyphenylene Oxide (PPO) 6.6 Polyethylene Terephthalate (PET) 7 Equipments of blow molding 7.1 Single Stage Blow Molding Machine 7.2 Two Stage Blow Molding Machine 7.3 Integrated Two Stage Blow Molding Machine 7.4 PET Blow Molding Machinery 7.5 Automatic PET Blow Molding Machine 7.6 Semi Automatic PET Blow Molding Machine 8 Blow molding markets (overall) 9 Decoration of Blow Molded Products 9.1 Finishing 10 References 10.1 Notes 10.2 Bibliography

Background of blow molding

History

The process of using air to blow hot material was first used by the Syrians. The method is attributed to Syrian glass workers in the first century BC, who realized that a glass bulb on the end of a blow pipe could be shaped into many useful hollow forms, with handles and feet and decorated adjuncts added at will^[2]. This involved placing a long tube, into a receptacle, which is located in a blast furnace containing liquid glass, and removing a blob of white hot liquid glass and spinning it and then blowing into a mouthpiece on the other end of the tube. This process was refined in Europe during the Middle Ages because of the demand for bottles to contain and ship products such as wine.^[3] Reliefs on the walls of Egyptian royal tombs record the art of glass blow molding. Egyptians further developed the art of blow molding in 1700-1600 B.C.^[1] Enoch Ferngren and William Kopitke were the first verified people who used the Blow Molding Process. In 1851, S.T. Armstrong, U.S. patent reference to blow molding a plastic material other than glass.^[1] In the mid 1930s, ICI (Imperial Chemical Industries) developed low-density polyethylene (LDPE), which was commercialized in 1939 and perfected in 1945 for squeeze bottles.^[3] In 1938, Ferngren and Kopitke produced a blow molding machine and sold it to Hartford Empire Company. This was the beginning of the commercial blow molding process. During the 1940s the variety and amount of products were still very limited and therefore blow molding did not take off until later. Once the variety and production rates went up the amount of products created followed soon thereafter. In 1950, Kautex Werke (Reinhold Hagen, Germany) developed and soon offered the first commercially available blow molding equipment. The design uses a rising mold technique with continuously extruded open ended parison.^[1] In 1953, high-density polyethylene (HDPE) was discovered by both Paul Hogan of the Phillips Petroleum Company in the United States and Professor Zieglar in Germany. Later, Professor Natta from Italy went further and polymerized both propylene and butylenes. With the appearance of HDPE in the marketplace, a virtual explosion of blow molded products occurred in both Europe and North America.^[3] In 1970, Coca-Cola test markets the world’s first plastic carbonated beverage bottle, a methacrylonitrile/styrene bottle by Monsant.^[1] In 1970s, biaxial oriented polyethylene terephthalate (PET) was developed with introduction of the two-step process in which the preform and the bottles are produced on separate machines by Cincinnati Milacron, USA.^[3] In 1977 Nisser, ASB Company (Japan), began to offer biaxial orientation of PET using blow molding equipment based on a one-step process.^[3] In the United States soft drink industry, the amount of plastic containers went from zero in 1977 to ten Billion in 1999. With the introduction and application of microprocessor resins, a wide range of material properties became available. Also the availability of larger, more robust, equipment and microprocessor technology led to the production of a range of industrial products such as automotive fuel tanks, armrests, and air conditioners. Then from Japan and Germany complex shapes and irregular contours were possible with the introduction of 3-D blow molding.^[4] Today, an even greater amount of products are blow molded, and it is expected to keep increasing.

Products

A blow molding renaissance is occurring in which engineers and designers are discovering and promoting blow molding for a wide variety of industrial or technical application. Toy wheels, automobile seat back, ductwork, surf boards, bellows, fuel tanks, flower pots, automobile bumpers, double- walled tool cases, and cabinet panels are just a few examples of the many creative design being developed.^[5]

Extrusion blow molding

In extrusion blow molding (EBM), plastic is melted and extruded into a hollow tube (a parison). Blow molding is the forming of a hollow object by “blowing” a thermoplastic molten tube called a parison in the shape of a mold cavity. Extrusion blow molding is the most widely used of many blow molding methods.^[6] This parison is then captured by closing it into a cooled metal mold. Air is then blown into the parison, inflating it into the shape of the hollow bottle, container or part. After the plastic has cooled sufficiently, the mold is opened and the part is ejected. There are two extrusion blow process: continuous and intermittent.^[7]

Continuous Extrusion Blow Molding

Continuous and Intermittent are two variations of Extrusion Blow Molding. In Continuous Extrusion Blow Molding the parison is extruded continuously and the individual parts are cut off by a suitable knife.

EBM processes may be either continuous (constant extrusion of the parison) or intermittent. Types of EBM equipment may be categorized as follows:

Continuous extrusion equipment

• rotary wheel blow molding systems
• shuttle machinery

Examples of parts made by the EBM process include dairy containers, shampoo bottles, hoses/pipes, and hollow industrial parts such as drums.

Intermittent blow molding

Intermittent extrusion may be also called shot extrusion. Parison shot extrusion is accomplished by means of a reciprocating screw almost identical to those used in injection molding machines. ^[8]

In Intermittent blow molding there are two processes: straight intermittent is similar to injection molding whereby the screw turns, then stops and pushes the melt out. With the accumulator method, an accumulator gathers melted plastic and when the previous mold has cooled and enough plastic has accumulated, a rod pushes the melted plastic and forms the parison. In this case the screw may turn continuously or intermittently.^[9]

Intermittent extrusion machinery

• reciprocating screw machinery
• accumulator head machinery

Controlling Wall Distribution

Control of wall distribution is the heart of blow molding. There are two primary techniques in extrusion blow molding for controlling wall distribution: Programming and die shaping. Programming is the control of the wall thickness, from top to bottom, of the parsion as it emerges from the die head tooling during extrusion. In die shaping, sectors of the die bushing or mandrel are machined to thicken the parison longitudinally in those areas where the part being formed requires greater thickness. The diameter of the die tooling is very important, for it determines the parison diameter. Too small a parison will rupture or “blow out” because of too much stretch. Too large a parison will result in too much flash, and cause trimming problems.^[10]

Injection blow molding

The injection molding phase consists of injection molding a thermoplastic material into a hollow, tube-shaped article called a preform. The preform is transferred on a metal shank, called the core rod, into a blow mold.^[11]

The process of injection blow molding (IBM) is used for the production of hollow glass and plastic objects in large quantities. In the IBM process, the polymer is injection molded onto a core pin; then the core pin is rotated to a blow molding station to be inflated and cooled. This is the least-used of the three blow molding processes, and is typically used to make small medical and single serve bottles. The process is divided into three steps: injection, blowing and ejection.

The injection blow molding machine is based on an extruder barrel and screw assembly which melts the polymer. The molten polymer is fed into a manifold where it is injected through nozzles into a hollow, heated preform mold. The preform mold forms the external shape and is clamped around a mandrel (the core rod) which forms the internal shape of the preform. The preform consists of a fully formed bottle/jar neck with a thick tube of polymer attached, which will form the body.

The preform mold opens and the core rod is rotated and clamped into the hollow, chilled blow mold. The core rod opens and allows compressed air into the preform, which inflates it to the finished article shape.

After a cooling period the blow mold opens and the core rod is rotated to the ejection position. The finished article is stripped off the core rod and leak-tested prior to packing. The preform and blow mold can have many cavities, typically three to sixteen depending on the article size and the required output. There are three sets of core rods, which allow concurrent preform injection, blow molding and ejection.

Each thermoplastic resin has its own set of tooling design parameters. Hot melt density, shrink factors, stretch ratios, blow pressure, venting criteria, and surface area of the tooling must all be known prior to designing any tooling.^[12]

Stretch blow molding

With the commercial introduction of containers, stretch blow molding became a common term in the blow molding industry. Stretch blow molding is the method of producing a plastic container from a preform or parison that is stretched in both the hoop direction and the axial direction when the preform is blown into its desired container shape. Stretch blow molding is possible for various thermoplastic materials such as acrylnitrile(AN), polystyrene (PS), Polyvinyl chloride (PVC), nylon, polycarbonate (PC), Polysulfone, acetal, polyarlyate, polypropylene (PP), surlyn, and polyethylene terephthalate (PET). Amphorous materials such as PET. Etc.^[13]

In the stretch blow molding (SBM) process, the plastic is first molded into a "preform" using the injection molding process. These preforms are produced with the necks of the bottles, including threads (the "finish") on one end. These preforms are packaged, and fed later (after cooling) into a reheat stretch blow molding machine. In the SBM process, the preforms are heated (typically using infrared heaters) above their glass transition temperature, then blown using high pressure air into bottles using metal blow molds. Usually the preform is stretched with a core rod as part of the process. In the single-stage process both preform manufacture and bottle blowing are performed in the same machine. The stretching of some polymers, such as PET (polyethylene terephthalate) results in strain hardening of the resin, allowing the bottles to resist deforming under the pressures formed by carbonated beverages, which typically approach 60 psi.The main applications are bottles, jars and other containers.

Advantages of blow molding include: low tool and die cost; fast production rates; ability to mold complex part; produces recyclable parts. Increase the material’s tensile strength, barrier properties, clarity. Reduce weight stretch blow molding produces a container from less raw material and with improved economics and bottle properties.^[13]

Disadvantages of blow molding include: limited to hollow parts, wall thickness is hard to control.

New Applications of Blow Molding Technology

Coextrusion Blow Molding of Large Parts

Coextrusion is used extensively in small bottles and containers, but increasingly also for large blow molded parts (18.92 liters or 5 gallons and larger). Its growth, however, is very healthy and will accelerate in the coming years because of the need for recycling of plastic and the application of this technology to manufacture of products requiring improved barrier properties.^[14]

Three-Dimensional Blow Molding

The three-dimensional blow molding concept was developed several years ago in Japan. The most successful of these technologies is the Placo X-Y machine which moves the mold under the head [2]. There are many advantages to three-dimensional blow molding, including minimal flash, seamless parts, and sequential extrusion. Many complex shapes can be easily produced using the three-dimensional blow molding process. At this time, process options for three-dimensional molding: X-Y process, suction blow molding, and curved blow molding are offered by Krupp (add a link to Krupp.^[15]

Hard-Soft-Hard and Soft-Hard-Soft Technology

It has been shown that a combination of various materials imparts specific properties to a part in extrusion blow molding. Here, the extruded parison is built up in a radial direction by several overlaying layers (traditional or radial coextrusion). New in practical application, however, is the use of various materials in the axial direction, that is, specific article sections may be provided with specific properties by choosing corresponding materials.^[16]

Long-Glass-Fiber-Reinforced Blow Molding

The use of short-glass-fiber-reinforced thermoplastics (mainly PA 6, PA 6.6, and PP) with medium fiber lengths between 0.2 and 0.4 mm (0.007 in. and 0.0156 in.) has been known for some time. Unfortunately it was impossible to realize improvements in tensile strength, modulus of elasticity, and deflection temperature under load in the order hoped for by using glass fibers. The reason was the insufficient length of the fibers, which was reduced substantially from its original length as a consequence of the shearing action in the extruder.^[17]

Blow Molding Foam Technology

Krupp Kautex, Borealis, and OBG Design introduced foam technology, based on a patent held by leading equipment manufacture Krupp Kautex (Patent Number US 4.874.649). A special-recipe mastermix from Borealis, a major polylefin producer, allows highly reliable processing and consistent results. ^[18]

Polymer characteristics relevant for blow molding

High-Density Polyethylene (HDPE)

• High impact strength
• Low temperature toughness,
• Excellent resistance to chemicals,
• Good electrical insulating properties, and
• Poor ultraviolet resistance.

Acrylonitrile-butadiene-styrene (ABS)

• ABS is a hard, tough material;
• Good impact resistance;
• Good electrical insulation properties; and
• Versatile additive, filler, and reinforcing agents acceptance;

Polycarbonate (PC)

• Excellent resistance to heat,
• Hard, tough material,
• Good impact resistance, and
• Excellent transparency.

Polypropylene (PP)

• Good impact strength (poor at cold temperatures),
• Good chemical resistance,
• High abrasion resistance,
• High melt strength.

Polyphenylene Oxide (PPO)

• Good flame retardancy,
• Good chemical resistance,
• Good impact resistance, and
• Retains mechanical properties in high heat environments.

Polyethylene Terephthalate (PET)

Used in injection blow molding, produces clear amorphous preforms. Since there is less orientation, the impact strength is reduced. ^[19]

Equipments of blow molding

There are six kinds of PET Blow Molding Machineries:

• Single Stage Blow Molding Machine
• Two Stage Blow Molding Machine
• Integrated Two Stage Blow Molding Machine
• PET Blow Molding Machines
• Automatic PET blow molding machine
• Semi Automatic PET blow molding machine

Single Stage Blow Molding Machine

All one-stage injection stretch blow molding machines derived from this original Stretch Blow design are referred to as classic one-stage machines, as the concept has long since been extended into other PET developments. The classic one-stage machines design is extremely versatile in that the same basic machine design can be used to make a wide variety of bottles and jars in all shapes and sizes, which was the standard single-stage machine in the early years, had eight cavities for 1.5 liter bottles.

Two Stage Blow Molding Machine

In the early developments, preforms were made by continuously extruding a PET Blow Molding tube. To make these preforms, a preform manufacturing machine that took a continuously extruded PET tube, heated and closed one end, and then heated the other and formed a thread finish by blow molding. This process had a fasted output rate, at 12000 preforms per hour, than the early injection molding routes of 8 and 16 cavity moulds. Being extruded, the preforms could be multilayered with barrier materials. The system was overtaken by injection molded preforms as the cavitations increased to 32 and beyond. The quality of the injection molded (IM) neck, adding for example vent slots, made the IM finish preferable. Moreover, IM technology is available from more than one company, giving customers greater technical and commercial choice. Two-stage technology machine with six blow moulds operating at around 4000 bottles per hour. Subsequent mould and cooling development increased the output to 6000 bottles per hour.

Two stage PET processing includes: Making preforms by Injection Molding Blowing bottles by Stretch Blow Molding

Because it is more flexible than one step processing, it is widely accepted in Plastic packaging market.

Integrated Two Stage Blow Molding Machine

In Integrated Two Stage approach the preforms were made by more conventional injection molding routes (with the number of cavities optimized to match the required output) and then, while still hot, were carried to a separate blowing machine with the optimized number of blow moulds to suit the required output. This was the first integrated approach to PET bottle making. Here the equipment had the same number of injection cavities as blowing molds. This was a more compact approach and proved ideal for small batch output with excellent glossy surfaces.

PET Blow Molding Machinery

PET Blow Molding Machineries are suitable for small and medium sized bottle makers and drinks manufacturers worldwide to produce their own bottle. The machine starts with the preform, heat it, and then stretch-blow the heated preform in the blow mold into oriented PET container. According to different requirement, PET Blow Molding Machine has the ability to produce PET bottle from narrow neck finishes to wide mouth finishes in the size from 5 ml to 20,000 ml. The wide applicability for both narrow and wide neck, reliable capability make PET Blow Molding Machine become popular and attractive to medium and small size bottle and drink manufacturer all over the world. PET bottles and containers are widely utilized in food and drink industry. PET has the advantages of virtually unbreakable, easy to handle and transport, low cost and easy to manufacture and recycle.

There are two type of Pet Blow Molding Machine:

• Automatic Pet Blow Molding Machine
• Semi-Automatic Pet Blow Molding Machine

Automatic PET Blow Molding Machine

Automatic PET Blow Molding Machine is a new generation of fully automatic and versatile machine for mass production of PET bottles and jars. Ideally suitable for large scale factories and for applications where stringent quality requirements have to be met. Substantial reduction in operating cost is achieved by saving of manpower.

There are two types of Automatic PET Blow Molding Machine

• One Stage Automatic PET Blow Molding Machine
• Two Stage Automatic PET Blow Molding Machine

Full-automatic PET stretch blow molding machine is the most stable two-step automatic stretch blow molding machine. It has one to four cavities. It can blow bottles in shapes such as carbonated, mineral, pesticide, cosmetics, wide-mouth, hot filling, and other packing containers, which is made of plastic of crystalline type such as PET and PP etc.

Features (Automatic PET Blow Molding Machine)

• Runs stably, whether voltage is 200V or 240V, or whether room temperature is 0C or 400C.
• Conveying preforms automatically with conveyor.
• Strong penetrability and good and swift distribution of the heat by letting the bottles rotate by itself and revolute in the rails simultaneously in the infrared preheater.
• Minimum parameter settings through touch LCD, normally only temperature settings should be regulated, no need to play with time settings; Multi-language interfaces
• Easy installation and starting
• Satisfaction with different atmospheric pressure for blowing and mechanical action by dividing the blowing and action into three parts in the air pressure diagram of the machine.
• More mechanical & synchronized design to reduce fault to minimum level.
• Low rejection rate
• Air recycling unit to save compressed air 20~25%, Reduced heating power, Driving is mostly done by motor directly, instead of compressed air.
• 200% more efficient than traditional one.
• Adopts Aluminum mold, lighter to be installed or unloaded; 10 molding data memories to make mold changeover quicker.
• High safeties with security automatic-locking apparatus in each mechanical action, which will make the procedures turn into a state of safety in case of a breakdown in certain procedure.
• Safe, reliable, and unique design of the position of valve to make the air pressure diagram of the machine easier to understand.

Semi Automatic PET Blow Molding Machine

SEMI-AUTO PET blow molding machines are the most cost-effective solution for mass production of jars and bottles for small and medium scale factories. The machines are also designed for the production of big jars and containers.

There are two types of Semi Automatic PET Blow Molding Machine

• One Stage Semi Automatic PET Blow Molding Machine
• Two Stage Semi Automatic PET Blow Molding Machine

Two Stage Semi Automatic PET Blow Molding Machines are fit to blow carbonated beverage bottles, mineral water bottles, cosmetics bottles and hot-filling bottles. With microcomputer controlling system, it controls various technical parameter needed more accurately and more steadily. It can operate easily without any special training and more safely.

Features

• Simple installation & startup adjustment. Two Stage Semi Automatic PET Blow Molding Machine
• Ensures extremely uniform heating for PET preforms, superior molding performance and minimum product defects.
• Offers two functions in a single machine by simply changing the mold from single to double cavity mold.
• Heating zones can be adjusted to meet production variations.
• Equipped with special designed air storage unit.
• Heating tubes are adjustable in any direction such as up or down, forward or backward to suit various preform sizes and shapes.
• Full-time stabilized heating system. Heating temperature and speed are adjustable according to sizes and features of preforms. Independent Temperature Control for different heating lamp to get ideal preform heating results.
• Steady operation, negligible trouble and easy maintenance.
• Operated easily without special training.
• Requires little base space, suitable for small to medium size factories.
• Negligible waste rate of finished bottle.^[20]

Blow molding markets (overall)

Blow molding is a multibillion dollar business. In the late 1980s annual worldwide consumption of plastic resin by blow molding processes was about ten billion pounds. Packaging is the primary application for this resin and was used to produce approximately seventy billion bottles, drums, tanks, and other containers.

Blow molding grade high-density polyethylene (HDPE) comprises about 69 percent of all materials that are blow molded. The use of polyethylene terephthelate (PET) has increased rapidly in recent years and in 1990 claims about 21 percent of the blow molding market.

About 25 percent of the total blow molding poundage goes into mild bottles. That is a tremendous market, but consumption for household chemical containers (bleach bottles, detergent bottles, etc.) is even larger: 33 percent. About 76 percent of the total resin usage goes into bottles, and the other 24 percent is in industrial items- gas tanks, bulk- goods drums, seats, toys, and other large items.

Blow Molding Applications

• Milk bottles
• Pharmaceutical bottles
• Antifreeze bottles
• Polypropylene bottles
• Coliseum seats
• One-piece chair
• Ice chests and coolers
• Double-wall player case
• Garbage cans
• Drums
• Fuel tanks

Decoration of Blow Molded Products

There are several ways to decorate blow molded products, they include:

• Labels- both post-molding and in-molding
• Decals
• Screen printing
• Hot stamping
• Pad printing
• Paint^[21]

Finishing

Finishing of a blow molded part should be considered in product design, mold engineering, and process planning stages.^[22]

References

This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (January 2009)

Notes

Bibliography

• Extrusion Blow Molding, Hanser-Gardner Publications, ISBN 1-56990-334-4
• Stretch Blow Molding, PETplanet Publisher GmbH, ISBN 3-9807497-2-X
• Plastic Blow Molding Handbook., Norman C. Lee, New York, United State. 1990
• New Applications of Blow Molding Technology, Norman C. Lee, USA. 2008