F2. injection blow molding - The operation for injection blow molding is similar to extrusion blow molding except that the parison is made by injection molding instead of by extrusion. The parison 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. Air entering the blow nozzle expands the parison against the cool walls of the blow mold. Trimming of the molded part is normally not required. In the usual arrangement, a three-station, horizontal, indexing table is an essential part of the equipment. The injection molding of the parison takes place at one station, inflation of the parison at the second station, and ejection of the finished part at the third station. (Some machines have a fourth station for pre-inflation of the part, or for a post-molding operation such as label attachment.) The process is adaptable to hollow parts that have some special shaped portion. The neck and opening of bottles, including screw threads for the cap, are produced in the injection mold as part of the parison. They can be made to closer dimensional tolerances than with extrusion blow molding and the wall thickness can be set as needed and more accurately controlled. The injection blow molding process is used extensively for smaller bottles of household products. It is illustrated in Fig. 4F2.

Fig. 4F2 Injection blow molding with a three-station index table arrangement.

F3. In stretch blow molding - a center rod stretches the parison to about two times its length. This axial stretching, plus the circumferential stretching action of the inflation, produces a biaxial orientation of the molecules in the walls of the part, improving the strength, barrier properties, and clarity of the walls. The process has some complexities. The temperature of the workpiece during the stretching operation is critical, and that temperature must be essentially uniform throughout the wall of the part; the inflation air pressure must be somewhat high in order to achieve the benefits of stretching. There are two basic stretch blowing methods: the continuous or single-stage process in which the temperature conditioning and stretching take place immediately after the parison is molded, and the two-stage process in which temperature conditioning and stretching take place later.

The single-stage method involves the following steps: 1) injection molding of the parison 2) temperature conditioning, in which the parison is brought to the best temperature for stretching, with uniformity throughout, 3) stretching and inflation in the mold and, 4) cooling and ejection of the finished part. Though the single stage process has some simplicity, it is not as rapid, overall, since the individual operations do not require the same amount of time. The two-stage method requires the parisons to be reheated, but the slower operations can be done in multiples to balance the flow. The two-stage approach, though more capital intensive, is most appropriate for the mass production levels required for most applications. Plastic soft drink bottles constitute the major application. Small bottles for pills and vitamins is another important use. PET, used for soft drink bottles, is the prime material, though other applications often involve different thermoplastics.

F4. multilayer blow molding - is a blow-molding operation that utilizes co-extrusion (see 4I2) – or co-injection-molding (see 4C3e) - to provide two or more layers in the parisons and in the final blow molded products. Coinjection blow molding and coextrusion blow molding are terms also applied to this approach. These processes are used in the production of containers when it is important to provide barriers against permeation and odor escape, and when the container is to be used for solvents, gasoline, herbicides, cosmetics, or pharmaceuticals. Stretching operations, as described above, are also common in the production of multiple-layer blow-molded bottles.

F5. dip blow molding - uses plastic resin adhering to a core rod, instead of an injected or extruded piece, as a parison. The core rod, whose diameter is the same as the inside diameter of the finished part’s neck opening, is inserted through a narrow opening into a chamber holding molten plastic. The core rod is then withdrawn while, at the same time, a piston advances into the chamber from the opposite end, maintaining pressure in the chamber and insuring that material remains on the rod. The core rod, when withdrawn, then has a coating of hot plastic. The coated rod is transferred to the blow molding station where air is blown through it. The air expands the plastic coating into contact with the mold cavity walls. The product is then cooled and hardens into a hollow part.

F6. other blow molding processes - Labels are sometimes placed in the mold before the inflation phase to provide better adhesion and protection of the label. In high-production situations, when labels are inserted, automatic equipment picks up each label by vacuum and positions it in the blow-mold cavity. A vacuum source, drawing air through small perforations in the mold cavity, holds the label in place as the parison is inflated in the mold. This method provides very good label adhesion because the plastic material it contacts is almost in the molten state.

Some extrusion blow-molding dies are equipped with de-flashing jaws inside the mold to grab and tear off the bottom flash as the finished bottle is ejected.