blow-molded_plastic_parts.pdf

BLOW-MOLDED PLASTIC PARTS

59. BLOW-MOLDED PLASTIC PARTS 59.1. THE PROCESS Blow molding is a means of forming hollow thermoplastic objects (see Fig. 6.5.1). Air pressure applied inside a small hollow and heated plastic piece (called a parison ) expands it like a balloon and forces it against the walls of a mold cavity, whose shape it assumes. There it cools and hardens. The mold opens, and the part is ejected.

Figure 6.5.1. Collection of typical blow-molded articles. (Courtesy AirLock Plastics, Inc.)

Figure 6.5.2. Schematic diagram of the extrusion-blow-molding process. (From Ronald D. Beck, Plastic Product Design, © Litton Educational Publishing, Inc., Van Nostrand Reinhold, New York, 1970.) In extrusion blow molding, the parison is extruded as a tube. This is inserted in the blow-molding die with one end engaging a blow pin or needle. As the die is closed, the tube is pinched at both ends. After the pinched-off tube is expanded and the part is formed and after the die opens and the part is ejected, the surplus material adjacent to the pinched-off areas is removed. Figure 6.5.2 illustrates the extrusion-blow-molding process. In injection blow molding, the operation is similar except that the parison is made by injection molding instead of extrusion. It is molded over a mandrel to provide the hollow shape, and this mandrel transfers the hot parison to the blow-molding die and then functions as the blow nozzle. In stretch blow molding, a center rod stretches the parison to about two times its length. This, plus the stretching action of the inflation, produces a biaxial orientation of the molecules in the walls of the part, improving the strength and clarity of the walls. Sometimes coextrusion or coinjection molding of more than one material is used to provide an improved barrier in blow-molded containers. Labels are also sometimes incorporated in the mold before the inflation phase to provide better adhesion and protection of the label.

59.2. TYPICAL CHARACTERISTICS AND APPLICATIONS Blow molding produces thin-walled hollow or tubular objects. Containers for liquids and other items used in the household are the most common application. Watering cans and bottles for laundry detergent and bleach,

cooking oil, shampoo, oatmeal, and various cosmetics and medicines are typical examples. The size range of blow-molded parts ranges from small pharmaceutical containers to large objects of 50 lb or more in weight. Another large market is in blow-molded toys, ranging from simple balls and lightweight baseball bats to elaborate dolls and animal toys, large play equipment, tricycles, and rideable play vehicles. Specialized containers such as carrying cases for instruments and tools, vehicle fuel tanks, 55-gallon drums, automobile glove compartments, center consoles, bumper systems, ducts, and even modern-design desks are also made by blow molding. Wall thicknesses for commercial household containers range from 0.4 mm (0.015 in) to about 3 mm (1/8 in). The wall thickness in the neck area of a container is normal ly greater than in the body to provide secure sealing surfaces for caps or dispenser devices. Walls can thin considerably below nominal values in other areas where the material stretches to fill the mold.

Figure 6.5.3. Comparison of the common size and shapes of containers made by injection blow molding and extrusion blow molding. (From Plastics Design & Processing.) Extrusion blow molding and injection blow molding have somewhat different capabilities when it comes to the size and shape of containers molded. Injection blow molding is more suitable for smaller containers, with fairly regular shapes, made with rigid materials. It also can produce more accurate dimensions around the neck of a container if this is necessary for the container’s closure or dispenser system. Injection-blow-molded parts normally do not have any “tail” or extraneous material after molding and therefore do not require a trimming operation. Close control over parison weight, shape, and wall thickness is inherent in the process, providing a

more nearly uniform and accurate blow-molded part. Extrusion blow molding is better for larger containers and those with irregular shapes. Figure 6.5.3 and Table 6.5.1 illustrate the advantageous applications for each process.

Table 6.5.1. Merits of Blowing Methods Extrusion blow molding 1.

More competitive for most containers over 16 oz.

2.

Lower initial mold cost.

3.

Can readily produce bottles with handles.

4.

More effective with rigid PVC.

5.

Can produce extremely irregular containers (such as ovals with maximum width-to-thickness ratios).

Injection blow molding 1.

More competitive for containers under 12 oz.

2.

Readily produces specialty neck finishes (safety closures).

3.

Closer tolerances on critical dimensions and weight.

4.

No finishing operations required.

5.

Improved yield (no waste).

6.

More efficient with rigid materials (polystyrene, polycarbonate).

7.

More efficient with wide-mouth containers over 43 mm.

Source: From Plastics Design & Processing.

59.3. ECONOMIC PRODUCTION QUANTITIES Injection blow molding is a high-volume process most commonly used for production in the millions. When less than 1 million units are to be produced, the extrusion-blow-molding process usually has the advantage, but for runs in excess of 2 million (and when part volume is less than 12 oz), injection blow molding definitely has an economic advantage. The fact that tooling for

injection blow molding can be costly places a lower limit on production quantities with which the process can be competitive. Extrusion blow molds are lower in cost than injection blow molds by about one-third for the same part but still are costly. Blow-molding operations, however, can be highly mechanized, with automatic loading of the parison into the blow mold and automatic trimming if that is required. Production cycles with such equipment may be very short. For example, 175-mL (6-oz) containers can be molded in a 12-s cycle even with a multiple-cavity mold. An eight-cavity mold for such a part can produce 2400 pieces per hour. Despite the very high production runs that are normal for blow moldings, the process can be suitable for much shorter runs in many cases. Production quantities on the order of 10,000 units are often feasible with the extrusionblow-molding process.

59.4. SUITABLE MATERIALS High-density polyethylene is by far the most common material used for blowmolded parts. It is used for household product containers and a wide variety of the large components listed above. Polyethylene processes well in blow molding and has good resistance to attack by solvents and corrosive materials. Polyethylene terephthalate (PET) is the second most commonly blow-molded material, with its major use being bottles for soft drinks. Other frequently used plastics for blow-molded components are polypropylene, polystyrene, polyvinyl chloride (PVC), and cellulosics. Styrene acrylonitrile (SAN) and acrylonitrile butadiene styrene (ABS) are two additional materials, as are the following engineering plastics: polycarbonate, polyphenylene ether blends, and amorphous nylon. PVC is used for containers for edible oils, cosmetics, and household chemicals. PVC is better processed by extrusion blow molding than by injection blow molding, as are the nitrile-based materials.

59.5. DESIGN RECOMMENDATIONS 59.5.1. Wall Thickness The wall thickness should be as nearly uniform as possible to ensure more

rapid molding cycles, conserve material, and avoid distortion due to uneven cooling. (Thinner sections cool more quickly.) If the neck portion of a bottle must have a thicker wall than the body to provide a rigid sealing structure, the ratio of wall thickness between the neck and body should not exceed 2:1. (See Fig. 6.5.4.)

Figure 6.5.4. When the neck portion of a bottle must have a thicker wall to provide a rigid sealing structure, the ratio of wall thickness between the neck and the body should not exceed 2:1.

59.5.2. Draft The part design should permit portions of the blow mold that are perpendicular to the parting plane to have a draft. This is preferred to a square configuration in ensuring that the molded part can be removed from the mold. (See Fig. 6.5.5.)

Figure 6.5.5. Provide a draft on surfaces perpendicular to the moldparting line.

59.5.3. Corners Generously rounded corners and large fillets are necessary in blow-molded

parts to maintain more nearly uniform wall thickness and ensure easy molding and maximum strength of the molded part.

Figure 6.5.6. Design limitations of injection-blow-molded containers. (From Modern Plastics Encyclopedia, McGraw-Hill, New York, 1970.)

59.5.4. Preferred Shapes Although somewhat intricate shapes can be blow-molded, regular, wellrounded shapes are more easily molded, are more economical, and should be specified when there is a choice. Extrusion blow molding is capable of producing containers with integral handles and other irregularities not feasible with injection blow molding. Figure 6.5.6 illustrates the practical limits of size and shape for injection-blow-molded parts. 59.5.5. Undercuts Although undercuts are a complicating factor and should be avoided if not needed, they can be incorporated into blow-molded parts. Undercuts considerably deeper than those feasible with injection molding can be incorporated in blow-molded parts without difficulty.

Figure 6.5.7. Standards for screw-threaded cap closures for blow-

molded bottles (1 1/2 screw threads). (From J. Harry Dubois and F. W. John, Plastics, 4th ed., Reinhold, New York, 1967.)

59.5.6. Standard Closures The use of standard container-closure designs helps promote manufacturing economy. The blow-molded container industry has adopted glass bottle industry standards for comparable plastic containers. Figure 6.5.7 illustrates the standards for a screw-cap closure having 1 1/2 threads. Citation EXPORT

James G.Bralla: Design for Manufacturability Handbook, Second Edition. BLOWMOLDED PLASTIC PARTS, Chapter (McGraw-Hill Professional, 1999, 1986), AccessEngineering

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