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5.1 INTRODUCTION Blow molding is a process for producing hollow objects, primarily from thermoplastic materials. Bottles and packaging are the primary applications of blow molded parts.

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5.1.1 Plastic Process To visualize the blow molding process, think of an inflatable plastic tube that is closed on both ends except at one point. If air enters at this point, the tube will expand, causing the walls of the tube to become thinner. It is much like blowing up a balloon. FIGURE 5.1(Page 305) Inflating an expandable tube. FIGURE 5.1(Page 305) Inflating an expandable tube.

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The blow molding process begins with a hot, soft plastic tube called a parison or preform. The parison is placed between two halves of a mold that has one or more hollow cavities (Figure 5.2). The mold halves are then clamped together. Air is blown into the parison, expanding it against the inside walls of the mold to form a hollow plastic part in the shape of the cavity. The mold is cooled, usually with water, and the hot plastic cools by contact with the mold. Once the part has cooled, the mold opens and the part is ejected. FIGURE 5.2(Page 305) Basic blow molding process. FIGURE 5.2(Page 305) Basic blow molding process.

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5.1.2 Brief History of Blow Molding TABLE 5.1(Page 307) Key Blow Molding Industry Events TABLE 5.1(Page 307) Key Blow Molding Industry Events Bottles are the primary application of blow-molded plastic parts; however, the blow molding industry also makes products used in the aircraft, automotive, building and construction, electronics, furniture, lawn and garden, medical, and recreation fields. 5.1.3 Typical Blow Molding Markets and Product Applications

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FIGURE 5.3 (a) Assorted industrial and consumer blow-molded parts; (b) intricate blowmolded case; (c) aerobic step; (d) dock floates, foam filled for buoyancy. Applications include balls, toys, bellows, and car bumpers (fenders). Many of the bottles and jars are used in the food industry to package both solid and liquid products, such as plastic containers for salt and sauces, and most food-packaging containers are blow molded.

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5.1.4 Blow Molding Materials 5.1.4.1 Materials Used Thermoplastic materials are often suitable for many applications without extensive material or component modification, but in other cases they are not suitable. One reason is that many plastics are permeable; that is, a liquid product or part of the product can migrate through the walls of the container, or something from the environment, such as oxygen, can enter the container through the plastic walls. For a given plastic material, there are several ways of overcoming this problem. They include (1)increasing the product thickness; (2)coating the product with an impermeable material such as glass, cross-linked PE, or more commonly, polyvinylidiene chloride (PVDC); (3)incorporation of a platelike filler (e.g., glass flake); (4)and using a combination of plastic materials. Such combinations, in which one parison is surrounded by another, are usually produced by co-extrusion. Orientation (stretching the parison) also yields improvements.

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5.1.4.2 Chemical End Use and Physical and Processing Requirements HDPE is impervious (unaffected by, or resistant) to water, water vapor, inorganic chemicals, and inorganic materials in aqueous (water-based) media, especially at the ambient temperatures normally encountered during storage and transport. Its high moisture resistance makes HDPE an ideal container material, keeping moisture out as well as in. However, with organic chemicals and fluids, HDPE must be used more selectively.

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For hydrocarbon fluids, oils, and solvents such as those listed below, absorption and permeation through HDPE become very important factors. Most of these compounds are flammable and would not normally be contained in HDPE containers without special consideration. Their use as an ingredient in any product being considered for packaging or storage in HDPE containers requires proper testing to determine whether or not excessive permeation and absorption will occur.

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5.2 STAGES AND TYPES OF BLOW MOLDING

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5.2.1 Stages of the Blow Molding Process 1. Plasticizing (melting) the resin 2. Production of the parison (extrusion) or preform (injection) 3. Inflation of the parison or preform followed by cooling in the mold (This step takes the most time and controls the machine cycle.) 4. Ejection of the part from the mold 5. Trimming or finishing the part (the trim step is frequently performed while the other four steps are cycling) It should be noted that when several pieces are made at one time in multiple molds on one machine, the first four steps may overlap.

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5.2.2 Types of Blow Molding 1)Extrusion 2)Injection 3)Stretch Figure 5.5-page 314 describes these types and subtypes. Figure 5.5-page 314 describes these types and subtypes.

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Blow Molding Extrusion 1)Continuous 2)Intermittent (Accumulator) Co-extrusion Three- Dimensional Extrusion 1)Suction 2)Vertical manipulation 3)Horizontal manipulation Injection 1)injection station 2)blow station 3)ejection station Stretch 1)Heating a molded and cooled preform 2)Closing blowing mold 3)Stretching

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5.2.3 Extrusion Blow Molding There are two basic types of extrusion blow molding: 1) Continuous Blow Molding 2) Intermittent (Accumulator) Method

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5.2.3.1 Continuous Blow Molding In the continuous method, the parison is extruded continuously from a head or die unit. The extruder produces an endless parison. In one variation of the continuous method, a shuttle press carries one or more molds. As soon as the mold closes, it moves away and an open mold moves into place as the parison or parisons continue to be extruded. When the mold closes, air is blown into the parison. Once the parison has expanded, air pressure holds it against the mold as the part(s) cool.

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5.2.3.2 Intermittent (Accumulator) Method In this method, the extruder runs continuously. A chamber, called an accumulator, accumulates a substantial volume of well- plasticized melt that is delivered by the extruder. Once a plunger or ram forces the melt out through the die head to form the parison (Figure 5.6).

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A benefit of the intermittent or accumulator method is that it allows the delivery rate of hot plastic from the die head to be independent of the delivery rate of the extruder. Accumulator capacity determines the maximum size of large blown parts. A large accumulator may hold enough plastic melt to make a parison for a 150-lb (68-kg) part. The advantages of an accumulator are: 1)It holds a large volume of melt for large items requiring very long molding and cooling cycles. 2) It permits high production rates. 3)It permits the fast extrusion of large parisons and consequently a short suspension time for the parison, allowing comparatively little sag and better control of wall thickness. 4)It provides more uniform shot size (weight of plastic), which minimizes waste. 5)It decreases idle mold time to a minimum.

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5.2.3.3 Co-extrusion Blow Molding Co-extrusion refers to the technology used to make products that contain multiple layers in their wall structures. Such products are said to be co-extruded. The layers may be made of the same or different materials, colored or uncolored material, or recycled and virgin materials. The main difference between multiple-layer and single-material extrusion blow molding is in the extrusion system. In co-extrusion, each material is extruded from its own extruder. Examples of products made from this process are ketchup bottles and automotive gas tanks.

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Arrangement of Extruders for Co-extrusion An arrangement of extruders to produce co-extruded, multilayer structures is illustrated in Figure 5.8. FIGURE 5.8-Page 316: Co-extrusion blow molding. FIGURE 5.8-Page 316: Co-extrusion blow molding.

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Multilayered Structures A co-extruded, multilayered structure (Figure 5.9a) may be created to provide one or more characteristics that cannot be provided by a single-layer product. for example, a better heat barrier or increased resistance to permeation. or that a costly color be used in only one layer of the structure instead of throughout the entire wall thickness.

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Co-extrusion Systems The extrusion system in co-extrusion blow molding must supply several streams of melted material to the die simultaneously. Some streams are smaller, by design, in volume than others, in order to produce thinner layers. The processing conditions may also differ from one material to another. See Figure 5.9b which shows a typical co-extrusion die head. See Figure 5.9c, which illustrates the multiple layers in a packaging container.

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FIGURE 5.9 (a) Multilayer structures; (b) multilayer extrusion die and manifold delivery system; (c) multiple layers used in packaging applications.

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5.2.4.1 Three-Dimensional Extrusion Processes Three types of three-dimensional technology methods are available: (1) the suction blow module, (2) vertical clamp parison manipulation, and (3) horizontal segmented mold parison manipulation. FIGURE 5.11(Page 319) Suction blow molding. Suction Blow Molding: FIGURE 5.11(Page 319) Suction blow molding. (1) The suction blow molding

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Parison Manipulation In the vertical clamp method the mold opens vertically, the lower half slides out, the parison is placed in the cavity, and the mold slides back and blows (Figure 5.12). This method is ideal for multilayer applications: for example, fuel filler pipes. Conventual mold halves provide a lower mold cost. The mold is also very accessible, and double-clamp stations can be used for higher volumes (Figure 5.13- Page 320). FIGURE 5.12(Page 319) Vertical clamp method. FIGURE 5.12(Page 319) Vertical clamp method. (2) Vertical clamp parison manipulation

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FIGURE 5.13 Integrated mounting. (Courtesy of SIG Kautex, Inc., North Branch, NJ.)

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(3) Horizontal segmented mold parison manipulation In the segmented mold/horizontal clamp process (figure 5.14) the parison is extruded, and as the mold begins to close, the parison is inflated. Then the parison is manipulated to the cavity configuration and the mold completes closure, cools, and opens with part ejection. The parison manipulation is accomplished with A sixaxis robot. Complex shapes can be produced by using segmented mold technology, with the incorporation of value-added design features through the use of A clamp function and pinch-off for twin tubes, brackets, and so on. The part quality is high due to the minimum contact between parts and mold. FIGURE 5.14(Page 320) Horizontal clamp. FIGURE 5.14(Page 320) Horizontal clamp.

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5.2.4.2 Double-Walled Parts and Containers A typical part is shown in Figure 5.15a.

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see page 321 for figures 5-15 The parison is usually produced on a continuous extrusion machine with a transfer parison into a mold or a shuttle press (Figure 5.15b). In Figure 5.15c the parison has been prepinched on the bottom. It has been preblown with low pressure to form a pillow. As the mold closes, the parison bulges and forms over the male form. The side and top of the parison begin to be trapped around the edges, essential for the successful double-walled part (Figure 5.15d). The mold closes further, with air preventing the walls from collapsing (Figure 5.15e). Cooling and high-pressure air are used to conform the mold core and cavity (Figure 5.15f ). The high-pressure air is usually blown through a hollow needle.(see page 321 for figures 5-15)

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5.2.5 Injection Blow Molding Injection blow molding is used to produce a molded parison called a preform. Injection blow molding is usually preferred over extrusion blow molding for making small parts that require high production volumes and closer control of dimensions. (Figure 5.16-Page322). The injection blow molding process occurs in two steps: (1) injection molding a preform onto a support pin or core, which provides neck threads that are already formed to their required dimensions; and (2) blowing the preform, still on the support(core) pin, to its final shape in a separate mold(Figure 5.16-Page322).

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5.2.5.1 Injection Blow Molding Machine In phase 1(injection station) of this process, the preforms are molded by injecting the plastic material into a matched metal mold, consisting of top and bottom split cavities, over a core that forms the inside of the tubular preform. The preforms are allowed to cool only long enough to hold their shape. In phase 2(blow station) the mold opens and the hot, semiviscous preforms are indexed to the next station (Figure 5.18), where split cavities in the shape of the part are closed over the preforms. Here, the hot preforms are blown to the shape of the cavities, then cooled. In phase 3(ejection station) the blow molds open and the parts are indexed to the next station for ejection. In this three-phase process, all three phases take place at the same time (Figure 5.19).

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FIGURE 5.18 Uniloy Milacron transfer turret, indexing injection-molded performs on cores to blowing station. FIGURE 5.18 Uniloy Milacron transfer turret, indexing injection-molded performs on cores to blowing station.

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FIGURE 5.19 Three-station injection blow molding machine. FIGURE 5.19 Three-station injection blow molding machine.

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The majority of machines are of the three-station type described, with 120 indexing and matching injection and blow molds. (Figure 5.20-page 324). Special four-station machines are indexed on 90 turns, with the fourth station being used for special conditioning of the parison core rod after stripping (pick-off) and before blowing (Figure 5.20-page 324). Four-station machines are often used for two-color or multicolor bottles.

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5.2.5.2 Stretch Blow Molding Stretch blow molding [referred to earlier as injection stretch blow molding (ISBM)] involves: 1)conditioning (heating) a molded and cooled preform to a specific temperature, 2)closing it in the blowing mold, 3)then very quickly stretching it in two directions, length and diameter. Often, a rod is used to stretch the hot preform in the axial direction, with air pressure then used to stretch it in the radial direction. Apllication of this type of Molding: improve impact strength, transparency, surface gloss, gas barrier, and stiffness properties.

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FIGURE 5.21: with the most common application for stretch blow molding being the soda bottles made of clear or tinted polyethylene terephthalate (PET)

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FIGURE 5.22 Injection stretch blow molding machine manufactured by Jomar Corporation, FIGURE 5.22 Injection stretch blow molding machine manufactured by Jomar Corporation,

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5.2.6 Control System FIGURE 5.23 Uniloy Milacron Carmac 486 process controller for extrusion blow molding machines.

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5.2.6.1 Reprogramming 5.2.6.2 Universality 5.2.6.3 Safety 5.2.6.4 Troubleshooting 5.2.6.5 Ancillary Control 5.2.6.6 Open- and Closed-Loop Control 5.2.6.7 Statistics 5.2.6.8 Automatic Quality Control

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5.2.7 Advantages of Extrusion and Injection Blow Molding Figures 5.24 to 5.27 The blow molding process is a natural production process for containers and hollow parts. It is the preferred process for high-volume containers for packaging applications such as food, personal care items, and household products, as well as for industrial high-strength applications such as automotive and agricultural tanks, pressure vessels, and air ducts Figures 5.24 to 5.27 show several examples of blow- molded parts. 5.2.7.1 Extrusion Blow Molding

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FIGURE 5.24 Cannondale bike bellows.

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FIGURE 5.25 Panels.

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FIGURE 5.26 Assorted bottles and containers.

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FIGURE 5.27 Medical prescription containers.

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5.2.7.2 Injection Blow Molding The main advantages of injection blow molding are: There is no scrap or flash to trim and reclaim. The neck finish and details are very accurate and of high quality. There is no process weight variation. Injection blow molding typically offers the lowest part cost for high-volume bottles weighing 37 g or less.

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5.2.8 Disadvantages of Extrusion and Injection Blow Molding A disadvantage of blow molding is uneven wall thickness. Although this may be minimized with programming, the wall is usually thicker at pinch-off areas and thinner in corners. The tendency of thicker walls to shrink more in the center of the part has to be compensated for in the product design. For example, rectangular containers usually have outwardlycurved surfaces. Another disadvantage is that close dimensional tolerances are difficult to achieve, the exception being bottle threads produced by injection blow molding. The accuracy of surface finishing details on extrusion blow molded products is also relatively low. 5.2.8.1 Extrusion Blow Molding

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5.2.8.2 Injection Blow Molding The disadvantages of the injection molding process are: The cost of tooling is higher than for extrusion blow molding. Bottle sizes and shapes are limited to an ovality ratio of 2 : 1 and a blow-up ratio no greater than 3 : 1. Slightly offset necks are possible with this process, but handles are not.