During the twentieth century and even now , plastic material was engineered for specific purposes.
Two examples are: high impact plastics used for construction workers hard hats and flexible soft material used for heat valves. The application of blow moulding began with the use of Celluloid and celluloid nitrate, which is highly flammable, and had a limited use .
Later celluloid acetate was used for novelties and toys. The commercialisation came with the use of the first automatic blow moulding machine by Plax Corporation in the s, see Figure 1. By they had machinery, which could produce 25, bottles per day. This material was a factor in the winning of the Battle of Britain since it was used for radar co-axial cable. Continental Can purchased a continuous extrusion horizontal wheel technology from EE Mills. Later Professor Natta, of Italy, went further and polymerised both propylene and butylenes .
With the appearance of HDPE in the market place a virtual explosion of blow moulded products occurred in both Europe and North America. The one limitation of HDPE is its opaqueness. Thus, when clarity is required polyvinyl chloride is used. In the s European equipment became available in the United States for items other than bottles. Then in the s biaxially oriented polyethylene terephthalate PET was developed with the introduction of the two-step process in which the preform and bottles are produced on separate machines by Cincinnati Milacron, USA.
Practical Guide to Blow Moulding Multilayer blow moulding came to the USA with the introduction of the ketchup tomato sauce squeeze bottle.
Practical Extrusion Blow Molding (Plastics Engineering)
With the introduction and application of microprocessor resins, a wide range of material properties became available. Also the availability of larger more robust equipment and microprocessor technology led to the production of a range of industrial products such as automotive fuel tanks, arm rests air conditioners. Then from Japan and Germany complex shapes and irregular contours were possible with the introduction of 3-D blow moulding.
Figure 1. This process is used primarily for small pharmaceutical and personal product containers. The economics for such containers dictate multiple up to ten cavities - one set each for injection moulding and blow moulding. When tolerances in the neck and closure thread area are critical, injection blow moulding is applicable to larger bottles.
An example is pressurised carbonated beverage containers up to 2 litres. Practical Guide to Blow Moulding application is aerosol spray bottles, in which neck tolerances must provide sufficient thickness and taper to meet top load stresses from pressurised filling line equipment, and the closure thread or lip must conform to the twist cap or valve seal.
There is essentially no plastic waste or trim generated with the latest injection blow moulding machines, only improperly formed containers from start-up and occasional off-spec parts from production are candidates for regrind. This method is preferred over extrusion blow moulding for making small parts that require high production volumes and closer quality dimensions.
Injection blow moulding consists of injecting a thermoplastic material into a cavity and around a core rod producing a hollow test tube like shape preform. The moulded preform still on the core rod is transferred to the blow mould. The mould is clamped around the preform and air is blown to shape of the cavity. The preform is injected onto a support pin or core, which forms a neck with threads to their required dimensions. The preform is then blown against the cavity wall to its final shape. See Figure 1.
Injection of material into a mould can take place by direct extrusion into the mould, or by using the reciprocating screw as an injection plunger, or by a combination of the two.
When the screw serves as an injection plunger, this unit acts as a holding, measuring, and injection chamber see Figure 1. Bottle sizes and shapes are limited to an ovality ratio of and a blow-up ratio of no greater than Bi-axial stretch blow moulding applies to the method of producing a plastic container from a preform that is stretched in the hoop direction and the axial direction when the preform is blown into its final shape.
Stretch blow moulding consists of conditioning heating a moulded and cooled preform to a specific temperature.
Practical Guide To Injection Blow Molding
The preform is closed in the blowing mould and is stretched in length and diameter see Figure 1. A temperature conditioned preform is inserted into the blowmould cavity, then is rapidly stretched. Often a rod is used to stretch the preform in the axial direction with air pressure to stretch the preform in the radial direction. This includes the closure threads on bottles and, in some cases, handles and support lugs.
Containers produced by extrusion blow moulding must meet minimum stiffness requirements to undergo filling on automated lines and to avoid, or limit, unsightly bulging under weight of their contents, both alone or when stacked. They must also withstand normal impacts of handling, transport and accidental dropping.
Such impact must be absorbed by container walls, weld lines pinch-off and handle areas , and screw cap closure threads, often under extremes of temperature. HDPE provides most of the requirements noted previously at low cost, and is also chemically inert to many fluids used in personal products, food products, and household and industrial chemicals see Figure 1. For blowing purposes this is a shape from which the parison is cut.
There are several main parts to an extrusion blow moulding machine, which are: screw, barrel, hopper, feed section, compression section, metering section, screen pack, breaker plates, adapter, die head, core, mandrel, and die tip. In the various sections the resin is melted, plasticised, and delivered to a die or dies at the proper temperature at a uniform rate see Figure 1.
These layers may be different materials, coloured or not coloured, recycled or virgin. Practical Guide to Blow Moulding This process makes it possible to combine materials of various properties to create a final product to meet the requirements of a particular application. The first commercial application of this process was a Heinz Ketchup tomato sauce bottle by the American Can Company in Currently, the automotive fuel tank is a typical application of this method Figure 1. Combining layers in the die before finally extruding a parison creates the multilayered structure of co-extruded products.
See multi-layered head - Figure 1. Resin selection is not an easy job.
The problems must be clearly defined beforehand. Careful investigation of the end use properties must be made. Moreover, resin requirements vary with blow moulding procedure and the available equipment. Extrusion Method When the parison is extruded from the die it is subject to gravity pull.
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Thus the material wall will thin out at the top Figure 1. Such thinning is overcome by formulating the resin to provide body so that it will not drop to the floor before the mould closes. This factor is indicated by the melt index which is a measure of melt flow. Thinning is further compensated by moving the mandrel in the die, which gradually increases the wall thickness, and is known as programming.
Injection Method Since the resin is injected through a small orifice the melt flow of the material has to be rapid thus in the plastic state it needs to be liquid. The resin formation is also influenced by gate size, mould filling pattern, wall thickness and dimensional tolerances. The main influences are those exerted by heat or cooling and pressure, and the time and duration of these influences is essential. This is why good knowledge of the equipment and mould and their operation will permit its skilful use to obtain the highest possible output of high quality, uniform items.
These are discussed in the following chapters. These items represent currently the bulk of the blow moulding in North America market. For squeeze bottles and flexible items such as toys, lower-density polyethylene resins are more suitable. PET is the choice for soda bottles and personal care products [6, 7]. Recently the use PET has largely replaced the use of glass in such sectors such as personal care products. Chart of percentages of polymer usage by country is shown in Figure 1. Note: Materials selection is covered more extensively in Chapter 2 Design and Engineering. Source: Applied Market Information Ltd.
Rosato and D. The following considerations such as characteristics of materials, processing, assembly and finishing, life cycle and performance of the product all play a part in producing the ultimate product, thus it is important to have a team approach to the product design, engineering and development .
The new manufacturing enterprise wheel  Figure 2. Figure 2. Although one cannot be an expert in all aspects of the design arena, one can have an understanding of how the system should work. When designing for plastic blow moulded parts, one should take into account aesthetics, functional process, capabilities and the manufacturing constraints.