The Blow Moulding Process

Blow-moulding is the established technrque for producing bottles and other containers based on simple hollow shapes. 

Modern blow moulding is separated into two major sub-divisions -extrusion blow moulding and injection blow moulding. Extrusion blow moulding was formerly by far the most dominant technique. Injection blow moulding has in recent years emerged as a major section for the production of bottles for carbonated drinks, using especially poly(ethylene terephthalate), PET, polymer. 

The basic requirements of the extrusion blow-moulding process are to extrude a molten plastic tube (the parison), clamp it in a cold split mould, expand the parison by means of compressed air, cool the shaped moulding, remove from the mould and trim as necessary. Interest in the present work centres on the example of the Acitainer, which is produced by this process. 

There are a multitude of commercially available machines that will adequately perform these operations in a variety of ways, e.g. intermittent extrusion, continuous extrusion, horizontal or vertical extruder arrangement. 

Wall thickness control is usually brought about by mechanically raising and lowering the core of the die mandrel with respect to the die ring to give a controllable variation in the mandrel/die land clearance. The mandrel is actuated every cycle either pneumatically, hydraulically, or by cam mechanisms electronic programming usually controls these functions. Fig. 22.1. 

---

Figure 5.6. Schematic principle of the blow-moulding process...

The blow-moulding process is economically attractive where high output rates are not required and the product is not critical with regard to its inner surface or tolerances on inner dimensions. It is very difficult to hold centres with the blow moulding process although individual dimensions can often be held to 0.1 mm (0.004 in). 

Blow-moulded products are associated with certain inherent limitations concerning shapes and wall thickness. While the blow-moulding process normally yields one-piece, closed, hollow products, pairs of open, hollow products have been made by splitting closed mouldings. 

Parison the length of plastics tube which emerges from an extmder, and which is blown into shape in the blow moulding process. 

M. Prystay and A. Garcia-Rejon, Application of Thermography for the Optimization of the Blow Moulding Process, SPE ANTEC Proceeding, 225-232 (1996). 

Y-L Hsu, T-C Liul, F Thibault and B LanctoL Design optimization of the blow moulding process using a fuzzy optimization algorithm. Proc. Inst Mech. Eng, 218, B, J. Eng. Manufacture, 197-212 (2004). 

D. Laroche, K.K. Kabanemi, L.Pecora and R.W. Diraddo, Integrated Numerical Modelling of the blow moulding process. Polymer Engineering and Science, 39,7, 1223-1233 (1999). 

Software specifications can make the final result totally different as the blow moulding process is sensitive to every parameter. The more options and calculations are involved in the simulation software, the more accurate results can be obtained. 

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. 

During the blow moulding of polyethylene at 170°C the parison is 0.4 m long and is left hanging for 1 second. Estimate the natural time for the process and the amount of sagging which occurs. The density of the melt may be taken at 730 kg/m. ... 

In the method of producing a hollow foamed plastic body from polyethylene or polypropylene by means of an extrusion blow-moulding process using a single-screw... 

Fluorination of polyethylene surfaces leads to an increase in the surface energy, some degree of cross-linking and a reduction of the free volume of the polymer. All of these effects impart on the surface of the polymer a barrier that is very impermeable to hydrocarbon solvents. A blow-moulding process, in which a low concentration of fluorine in nitrogen is used as the blow-moulding gas, is used for the production of plastic fuel tanks for the automotive industry (Airopak , Air Products) [51]. Post-treatment of hydrocarbon surfaces with fluorine is an alternative technology and techniques for the surface fluorination of natural and synthetic rubber have been described [52]. 

Injection blow moulding is used for the production of hollow objects in large quantities. The main applications are botdes, jars and other containers. The Injection blow moulding process produces bottles of superior visual and dimensional quality compared to extrusion blow moulding. The process is ideal for both narrow and wide-mouthed containers and produces them fully finished with no flash. 

Injection blow moulding A blow moulding process in which the parison to be blown is formed by injection molding. 

In the stretch blow moulding process, which is the most common, the extruded parison is initially blown undersized, and then stretched and blown to its final shape in a second mould. This produces biaxial molecular orientation of the container walls, which improves impact resistance, rigidity and clarity. However, permeability is reduced. 

Fig. 20.4 The injection stretch blow moulding process (ISBM).

Among the latest techniques is the Japanese Cuius blow moulding process which produces a preclosed tube from an extrusion process. It is argued that the absence of a base pinch off eliminates the weakest part of a conventionally moulded bottle. The process is suitable for single layer and multi-layer bottles with a wide variety of shapes. 

Both employ a preprinted transfer (up to six colours). In the in-mould process the transfer is placed within the mould prior to bottle blowing or injection moulding. During the moulding cycle the printed transfer becomes fused to the container. The printing can be produced by either screen or gravure and can be line or half tone. 

In the blow moulding and thermoforming processes, solidification commences only when the melt contacts the cold mould wall, and flow stops. The time available for melt stress relaxation depend on the thickness of the product, and how close the melt is to the cold mould. There is likely to be high orientation in the thin side walls of a thermoformed disposable cup, made from a highly viscous glassy polymer that has solidified rapidly (hence the heat reversion shown in Fig. 1.16). Hence, if an empty cup is squeezed flat, vertical cracks tend to occur in the side walls, in the direction of orientation. 

Clarifying additives enable PP to compete for applications that were previously captive to PET or polycarbonate, both of which cost more. PP supplier Borealis and Milliken have developed a two-stage injection stretch blow moulding process, used for still water bottles and juice containers, that allows the rapid manufacture of clear PP bottles at up to 1500 bottles per hour from each mould cavity, using Milliken s clarifiers. 

---