Extrusion blow-molding

The extrusion blow molding cycle is illustrated in Fig. 14.2. The extrusion component of the cycle is normally continuous. As soon as one length of parison has been captured by the mold, another length starts to form. To allow room for a new length of parison to emerge from the die, the mold moves aside as soon it has captured a parison and the knife has severed it. The mold is rapidly translated to a remote blowing station where inflation takes place. After the product is ejected, the open mold moves back under the die where it surrounds and captures another length of parison. 

We can generally extrude a parison much faster than we can inflate, cool, and eject the product When this is the case, ve employ more than one mold. If we are using two molds, they shuttle back and forth alternately between their individual blowng stations and the parison capture 

Considerable scrap is produced, especially with complex designs. Usually scrap can be reused in the process, but not always. Extrusion blow molding is also not very economical for very small bottles, so these are usually produced by injection blow molding. The resins used must have sufficient melt strength for the parison to remain unsupported until it is captured in the mold. A large variety of plastic bottles are produced by extrusion blow molding. 

To ensure that the least heat history (residence time) is developed during processing, the design of the accumulator should provide that the first melt in be the first to leave when the ram empties the chamber the aim should be to have the chamber totally emptied on each stroke. 

When the parison exits the die and reaches a preset length, a split cavity mold closes around it and pinches one end of it. Usually a blow pin is located opposite the pinched end of the tube. Compressed air inflates the parison against the pinched end of the 

Various techniques are used to introduce air. It can enter through the extrusion die mandrel, through a blow pin over which the end of a parison has dropped (see Fig. 7-57), or through blowing needles that pierce the parison. Tlie wall distribution and thickness of the blown part are usually controlled by parison programming, the blow ratio, and part configuration. 

Electronic parison programming is an effective way to control material usage and improve both quality and productivity. The most common method used is orifice modulation (see 

In regard to parison control, a compromise is necessary between the desired net weight and the need to maintain a sufficient safety margin over a set of minimum specifications, which include minimum wall thickness, drop speed, drop strength, dimensional stability, and fluctuations in net weight. Most of these parameters can be affected directly by the mold-er s ability to control the parison wall thickness. The most common and practical way of doing this has been to adjust the gap between the die and the mandrel (see Table 7-15). 

All bottle weights are finish weights (flash being considered as 100% reusable). 

When the parison exits the die and reaches a preset length, a split cavity mold closes around it and pinches one end of it. Usually a blow pin is located opposite the pinched end of the tube. Compressed air inflates the parison against the female cavity of the mold surfaces. Upon contact with the relatively cool mold surface, the blown parison cools and solidifies to the part shape. Next the mold opens, ejects the part, and then repeats the cycle by again closing around the parison, shaping it, and so on. 

The mold clamping methods are hydraulic and/or toggle, similar to, but less sophisticated than, those used with IM. Sufficient daylight is needed in the mold platen area to accommodate parison systems, ejection of blown 

Sustainable Plastics Environmental Assessments of Biobased, Biodegradable, and Recycled Plastics, First Edition. Joseph P. Greene. 

Processing conditions that produced PHA bottles are listed in Table C.l. The optimal conditions are rear temperature of 320°F, front temperature of 300°F, block temperature of 300°F, injection pressure of 1000 psi, blow pressure of 60 psi, mold temperature of 120°F, and mold close time of 60 seconds. 

FIGURE C.2 Rocheleau R4 extrusion blow molding machine. 

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Designers and processors to produce products at the lowest cost have unconsciously used the basic concept of the FALLO approach. This approach makes one aware that many steps are involved to be successful, all of which must be coordinated and interrelated. It starts with the design that involves specifying the plastic, and specifying the manufacturing process. The specific process (injection, extrusion, blow molding, thermoforming, and so forth) is an important part of the overall scheme and should not be problematic. 

Fig. 3-37 Example of an extrusion blow molded container with a living hinge.

Injection Molding Extrusion Blow Molding Thermoforming Reaction Injection Molding Rotational Molding Compression and Transfer Molding Matched Mold Spray-up... 

Standard Extrusion Blow-molding 2-Parison Head 4-Fold Stretch Blow Molding PVC (2) Single-parison Heads 4-Fold Stretch Blow Molding PET... 

An unwritten rule says that a mold or die should cost almost half the cost of the basic machine (injection, extrusion, blow molding, etc.). If it does not then something is wrong such as you probably have an oversized machine for the job for the lower cost mold or die. 

Figure 14.1 illustrates the general principles of extrusion blow molding. A molten tube of polymer, known as a parison, is extruded vertically downwards from an extrusion die, as shown in Fig. 14.1 a). The two halves of the blow mold surround and then close on the... 

Figure 14.1 Schematic diagram of extrusion blow molding ...

Figure 14.2 Simplified extrusion blow molding cyde...

Describe both the extrusion blow molding and injection blow molding processes. Why would one method be chosen over the other ... 

Figure 18.11 contains a sketch of an extrusion blow-molding scheme. Here a heat-softened hollow plastic tube, or parison, is forced against the walls of the mold by air pressure. The sequence of material introduction into the mold and subsequent rejection of the material from the mold is generally rapid and automated. Approximately 1 million tons of thermoplastics are produced by this technique annually. 

Extrusion blow molding that uses an extruded tube preform or parison... 

Stretch blow molding that employs an injection molded, extrusion blow-molded preform, or extruded tube preform... 

The major difference between injection and extrusion blow molding is the way the soft hollow tube (called a preform or parison) is made. In injection blow molding, two different molds are used. One mold forms the preform and the other mold is used in the actual blow-molding operation to give the final shaped article. In the molding process, the softened material preform, from the preform mold, is introduced into the blowing mold and blow-molded to... 

FIGURE 18.11 Steps employed in simple extrusion blow molding of a test tube. From left to right mold closed softened material introduced air or other gas injected forcing the softened thermoplastic against the walls of the mold and, after suitable cooling, the mold is opened giving the molded plastic test tube. 

In continuous extrusion blow molding, the preform is continuously produced at the same rate as the article is molded, cooled, and released. To avoid interference with the preform formation, the mold-clamping step must be rapid to capture the preform and move it to the blow mold station. There are various modifications of this that allow essentially continuous operation. 

Belcher, S. 1999. Practical Extrusion Blow Molding. Dekker, New York. 

Figure 7.81 Schematic diagram of rotating mold system used in continuous extrusion blow molding. Reprinted, by permission, from A. B. Strong, Plastics Materials and Processing, 2nd ed., p. 489. Copyright 2000 by Prentice Hall, Inc.

The multilayer film is formed by injection molding, co-injection blow molding, co-injection stretch-blow molding or co-extrusion blow molding techniques. 

Processing. SAN copolymers may be processed using the conventional fabrication methods of extrusion, blow molding, injection molding, thermo forming, and casting. 

The organic cellulose ester plastics are versatile materials and can be processed by almost any hot-processing technique used for thermoplastics. The principal techniques for all three plastics are injection molding and extrusion. Blow molding is also possible. Butyrate and propionate powder are used in tluidized-bed and electrostatic coating processes, as well as in the rotational molding process. 

Practical Extrusion Blow Molding, edited by Samuel L. Belcher... 

Secondary shaping operations such as extrusion blow molding, film blowing, and fiber spinning occur immediately after the extrusion profile emerges from the die. The thermo-... 

Figure 3.56 Schematic of the extrusion blow molding process.

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