Blowing the Parison

Blowing inside the neck sometimes is difficult. Small orifices may create 

A high volumeteric air flow at a low linear velocity is desired. A high volumetric flow gives the parison a minimum time to cool before coming in contact with the mold, and provides a more uniform rate of expansion. A low linear velocity is desirable to prevent a venturi effect (see above). Volumetric flow is controlled by the line pressure and the orifice diameter. Linear velocity is controlled by flow control valves close to the orifice. 

The blowing time differs from the cooling time, being much shorter than the time required to cool the thickest section to prevent distortion on ejection. The blow time for an item may be computed from Table 4-5 and the formula  

This is for free air but there will be a pressure buildup as the parison is inflated, so the blow rate has to be adjusted. The value of cu ft/s is obtained from Table 4-5, according to the line pressure and the orifice diameter. The final mold pressure is assumed to be the line pressure for purposes of calculation. Actually, the blow air is heated by the mold, raising its pressure. Calculations ignoring this heat effect will be satisfactory when blow times are under one second (for small to medium-size parts) but if blow times are longer, the air will have time to pick up heat, resulting in a more rapid pressure buildup and shorter than calculated blow times (2). 

Resins vary in cooling requirements. It is not usually necessary to postcool PVC it gives up its heat much more readily than the polyolefins (and thus is more appropriate for a dedicated operation than for custom blow molding). Also the bigger the part, the more cost-effective its cooling becomes. 

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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.

Before blowing the parison into the cavity, it can be mechanically stretched to orient molecules axially, Fig. 3.61 [25], The subsequent blowing operation introduces tangential orientation. A container with biaxial molecular orientation exhibits higher optical (clarity)... 

The blow molding process therefore involves essentially two properly synchronized operations parison formation from the plastic material and blowing the parison into the shape of the desired part. There are two techniques for plasticizing the resin for parison formation. These are extrusion blow molding (which is the most common method and which is characterized by scrap production) and injection blow molding. The latter process is versatile and scrap free and is beginning to be more understood and accepted by processors. 

Figure 1.4 Three Station Injection Blow Molding Machine [Miller, 1983]. The parison is injection molded on a core pin (instead of as a tube in free air, as with extrusion blow molding) at the preform mold station (1). The parison and neck finish of the container are formed there. The parison is then transferred on the core pin to the blow mold station (2) where air is introduced through the core pin to blow the parison into the shape of the blow mold. The blow container is then transferred to the stripper station (3) for removal.

About three-fourths of all blow-molding production is by extrusion blow molding. In this case, a hollow tube of molten or thermally softened polymers, called the parison, is extruded. The parison in this process is usually not supported. After leaving the die at a set parison length, a split cavity mold closes around the parison and crimps one end. A so-called blow pin (opposite the crimped or closed end) is used to inject compressed air, which blows the parison... 

Blow moulding is used to manufacture enclosed shapes (bottles, barrels, containers, etc.) where the shape is simple. Screw threads can be formed on the top, corrugated sections can be used to stiffen the product, and handles can be incorporated. Thinning will occur on the corners of the base and also on the corners in the sides of square bottles, but can be reduced by blowing a profiled parison. Some orientation is created in the blowing the parison, and is largely retained in the finished product (e.g. at a maximum in the vertical walls of a bottle). 

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. 

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. 

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. 

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. 

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. ... 

Example 5.10 During the blow moulding of polythene bottles the parison is 0.3 m long and is left hanging for 5 seconds. Estimate the amount of sagging which occurs. The density of polythene is 760 kg/m ... 

During the blow moulding of polypropylene bottles, the parison is extruded at a temperature of 230°C and the mould temperature is 50°C. If the wall thickness of the bottle is I mm and the bottles can be ejected at a temperature of I20°C estimate the cooling time in the mould. 

This technique is also called nonaxisym-metric blow molding. In conventional EBM the parison enters the mold rather in a straight tube. In 3-D BM the parison is laid or oriented in the mold prior to closing. It is manipulated in the tool cavity providing complex geometric products that can have... 

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

Great care is taken that we design blow molds to avoid thin or weak regions that could result in premature failure. Molds are designed to avoid excessive draw into corners that would result in locally thin areas. For this reason, blow molded products invariably have rounded corners. Another potential source of weakness is the pinch-off line. To compensate for this fact, it is common to program the parison to produce a thickened base. 

When a parison or preform is inflated, it displaces the air around it within the mold. If no provision is made to vent the mold, compression of the air around the parison or preform can raise its temperature to such an extent that it can scorch the surface of the product. To avoid this problem, we equip blow molds with vents. These can consist of slit vents at the parting line between mold halves, porous plugs of sintered metal, or small holes drilled into the cavity walls. 

The principle (Figure 5.6) is to produce a preform or parison by extrusion or injection, and then to blow it into a cooled mould to obtain the final hollow recipient. There are four steps fabrication of the parison... 

Extrusion blow moulding (the simplest), in which the parison is an extruded tube that is blown with air. Various types of machinery are marketed shuttle, reciprocating and wheel machines. 

Injection blow moulding, in which the parison is injected into a first mould and then blown in a second mould having the shape of the final recipient. This process is more expensive and the cycle time is a longer. The aspect and dimensional quality are better compared to extrusion blow moulding. 

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