Parisons

Molecular mechanics strain energies have another use They can also be used to com pare the energies of models that share the same molecular formula that is models that are either stereoisomers or different conformations of a single molecule (allowed com parisons are shown here)... 

Paris Paris blue Paris Convention Paris green Paris Green Paris green Paris green [12002-03-8] Paris law Parison Parity Parkerizing Parkes crust... 

In blow and blow operations, the gob of glass (parison) is deUvered from the feeder to the blank mold. The gob drops through a guide funnel iato the blank mold ia the iaverted position. Air is appHed to settie the gob iato the finish, and air is blown ia to complete the parison shape. 

Press and blow operations are used to produce wide mouth and some narrow neck containers, including beer botties. The difference between the press and blow operation and the blow and blow operation is that the parison is pressed iato shape by a plunger that fills the complete void ia the parison. 

Conventional extmsion or coextrusion may be performed on vertical or horizontal rotary or shuttle mold configurations. In shuttie blow mol ding the extmder and die are in fixed horizontal and vertical position two or more molds shuttie into and out of position beneath the die. By reciprocating in two planes, the mold may remove a parison and permit the extmder to function continuously. 

Horizontal rotary machines employ multiple molds in a horizontal plane on a rotary turret. As each mold approaches the extmder die exit, it opens to accept the parison and then closes. The parison is then blown into the bottle shape. The extmder must extmde on an intermittent basis or be intermittently withdrawn to provide a parison for each passing mold. 

Vertical rotary molds also employ multiple molds on a turret but rotate in a vertical plane. As each mold reaches the die exit, it grasps the parison and closes. Because of the vertical spacing between molds, intermittent extmder action is not required. Vertical wheels are used commercially for high volume apphcations. 

Fig. 14. The H.E. IS blow-and-blow machine (85). The gob is deUvered into a blank mold, setded with compressed air, and then preformed with a counter-blow. The parison or preform is then inverted and transferred into the blow mold where it is finished by blowing.

Fig. 15. Three stages of blow molding (a—c) A, air line B, die C, mold and D, parison. See text.

As the parison is extmded, the melt is free to swell and sag. The process requires a viscous resin with consistent swell and sag melt properties. For a large container the machine is usually equipped with a cylinder and a piston called an accumulator. The accumulator is filled with melt from the extmder and emptied at a much faster rate to form a large parison this minimises the sag of the molten tube. 

With a simple parison, the large-diameter sections of the botde have a thin wall and the small-diameter sections have a thick wad. Certain modifications of the die can control the thickness of the parison wad along its length, which results in a bottle with improved wad thickness distribution and better strength. High density polyethylene (HDPE) is the most common blow mol ding resin used to produce containers ranging in size from 30 cm to 200 L. 

In injection blow mol ding, a parison is injection molded onto a core pin the parison is then rapidly transferred via the core pin to a blow mold, where it is blown by air into an article. This process is appHed to smad and intricate bottles. 

Blow Molding. Blow mol ding is a multistep fabrication process for manufacturing hoUow symmetrical objects. The granules are melted and a parison is obtained by extmsion or by injection mol ding. The parison is then enclosed by the mold, and pressure or vacuum is appHed to force the material to assume the contour of the mold. After sufficient cooling, the object is ejected. 

PS or copolymers are used extensively in injection blow mol ding. Tough and cra2e-resistant PS containers have been made by multiaxiaHy oriented injection-molded parisons (238). This process permits the design of blow-molded objects with a high degree of controlled orientation, independent of blow ratio or shape. 

Determine performanee at guaranteed point and eom-parison with quoted performanee. 

Many articles, bottles and containers in particular, are made by blow moulding techniques of which there are many variations. In one typical process a hollow tube is extruded vertically downwards on to a spigot. Two mould halves close on to the extrudate (known in this context as the parison ) and air is blown through the spigot to inflate the parison so that it takes up the shape of the mould. As in injection moulding, polymers of low, intermediate and high density each find use according to the flexibility required of the finished product. 

Initially a molten tube of plastic called the Parison is extruded through an annular die. A mould then closes round the parison and a jet of gas inflates it to take up the shape of the mould. This is illustrated in Fig. 4.21(a). Although this process is principally used for the production of bottles (for washing-up liquid, disinfectant, soft drinks, etc.) it is not restricted to small hollow articles. Domestic cold water storage tanks, large storage drums and 2(X)... 

The convention extrusion blow moulding process may be continuous or intermittent. In the former method the extruder continuously supplies molten polymer through the annular die. In most cases the mould assembly moves relative to the die. When the mould has closed around the parison, a hot knife separates the latter from the extruder and the mould moves away for inflation, cooling and ejection of the moulding. Meanwhile the next parison will have been produced and this mould may move back to collect it or, in multi-mould systems, this would have been picked up by another mould. Alternatively in some machines the mould assembly is fixed and the required length of parison is cut off and transported to the mould by a robot arm. 

In the intermittent processes, single or multiple parisons are extruded using a reciprocating screw or ram accumulator. In the former system the screw moves forward to extrude the parisons and then screws back to prepare the charge of molten plastic for the next shot. In the other system the screw extruder supplies a constant output to an accumulator. A ram then pushes melt from the accumulator to produce a parison as required. 

Now consider the situation where the parison is inflated to All a cylindrical die of diameter, D. Assuming constancy of volume and neglecting draw-down effects, then from Fig. 4.23... 

This expression therefore enables the thickness of the moulded article to be calculated from a knowledge of the die dimensions, the swelling ratio and the mould diameter. The following example illustrates the use of this analysis. A further example on blow moulding may be found towards the end of Chapter 5 where there is also an example to illustrate how the amount of sagging of the parison may be estimated. 

Example 4.4 A blow moulding die has an outside diameter of 30 mm and an inside diameter of 27 mm. The parison is inflated with a pressure of 0.4 MN/m to produce a plastic bottle of diameter 50 mm. If the extrusion rate used causes a thickness swelling ratio of 2, estimate the wall thickness of the bottle. Comment on the suitability of the production conditions if melt fracture occurs at a stress of 6 MN/m. ... 

The maximum stress in the inflated parison will be the hoop stress, ae, which is given by... 

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