Blow moulding

Blow moulding is a widely used technique for producing hollow containers (usually bottles) in vast numbers, extremely cheaply. There are three major variants  

They have in common the formation of the precursor, a simple hollow tube known as a parison. One end of the parison is closed (for instance, simj nipped) so that it can be inflated in the heated, softened state. If inflates until it touches the walls of the cooled mould. The parison at once takes up the shape of the mould and cools. The mould is then opened and the bottle removed. 

45 Three-station rotary machine for tslow moulding (after Modem Plastics Encydopedia). 

Blow moulding is the third most commercially important process for plastics production after extrusion and injection moulding. It is used to produce a range of hollow articles, for example, bottles, fuel tanks and other large containers. There are two main variations, injection blow moulding and extrusion blow moulding. The process sequence for both is the same. 

Injection blow moulding is most commonly employed for the production of transparent soft drinks containers. However, extrusion blow moulding is the one most commonly employed for mouldings such as shampoos and detergent containers, plastic drums and milk bottles and is described below. 

The criteria for a blow moulding material are quite specific  

During inflation the material expands at a constant rate to give a constant wall thickness across the body of the moulding. 

The screw feeds the melt directly into the head-die assembly within a continuous extrusion blow moulding machine. The melt flows 

In a ram- or plunger-type machine, the extruder feeds the material to an accumulator/head device. The plunger pushes the material rapidly through the head-die assembly once the desired volume has accumulated. The transfer of material does not need to occur via a blowing station in the mould clamp mechanism. The part is blown, cooled and removed from the mould, and the next parison is only extruded after the part has been removed. 

When the desired length of the parison is reached, the mould is closed and the parison is inflated by internal air introduced via the die-head assembly. The mould walls are vented and a vacuum may be applied. The molten plastic is thus forced to conform to the shape of the mould cavity. The article is then cooled, solidified and ejected from the mould. 

The annular die can be designed to incorporate a hydraulic mechanism to vary or programme the annular gap size in both extrusion- and plunger-type blow moulding. With extrusion blow moulding, a specific wall thickness distribntion or controlled weight of the parison can be programmed. 

In this technique good reproduction of detail is achieved (a leathergrain pattern, for example) by a combination of vacuum on one side of the sheet, and pressure on the other. 

The polymers used in thermoforming must be of fairly high relative molecular mass since the heated sheet must be form stable. The significant property is melt elasticity. The elastic effects are produced by entropy-elastic forces between physical cross-links (molecular entanglements). The polymers used include polystyrene, ABS, acrylics, polycarbonate, PVC, polypropylene, and linear polyethylene. 

Sandwich construction is used to yield a surface with specific properties. For example in refrigerator panels there is a problem with stress corrosion cracking in the presence of milk, fats, and oils. One solution is a surface of expensive, glossy and oil-resistant high-acrylonitrile ABS, with a cheaper low acrylonitrile ABS centre. The sandwich is coextruded (7.N.7). Coextrusion is not the only method for upgrading performance laminates, heavily-filled and foamed polymers are also thermoformed. 

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Fig 19 9 Forming methods for gloss pressing, rolling, float-moulding and blow-moulding. 

Fig. 24.4. (a) Vacuum forming is good for making simple shapes out of sheet. (b) Blow moulding is used to make plastic containers. 

Injection/compression moulding - Thermoplastics and thermosets Blow moulding - Thermoplastics Rotational moulding - Thermoplastics... 

A blow-moulded container, cylindrical in shape but with one spherical end, is prepared from the polysulphone whose creep curves at 20°C are illustrated in Figure 9.9. The cylindrical part of the container has an outside diameter of 200 mm and is required to withstand a constant internal pressure of 7 MPa at 20°C. It is estimated that the required service lifetime of the part will be one year and the maximum allowable strain is 2%. What will be the minimum wall thickness for satisfactory operation ... 

By the mid-1990s capacity for polyethylene production was about 50 000 000 t.p.a, much greater than for any other type of plastics material. Of this capacity about 40% was for HDPE, 36% for LDPE and about 24% for LLDPE. Since then considerable extra capacity has been or is in the course of being built but at the time of writing financial and economic problems around the world make an accurate assessment of effective capacity both difficult and academic. It is, however, appeirent that the capacity data above is not reflected in consumption of the three main types of material where usage of LLDPE is now of the same order as the other two materials. Some 75% of the HDPE and LLDPE produced is used for film applications and about 60% of HDPE for injection and blow moulding. 

Melt elasticity is of considerable importance in understanding much of the behaviour of polyethylene when processing by film extrusion techniques and when blow moulding. The complex relationships observed experimentally here have been summarised by the author elsewhere. ... 

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. 

Fligh molecular weight, hroad MWD polymer—used primarily for blow moulding and pipe. 

High molecular weight HDPE (HMW-HDPE) is used for blown film and for demanding moulding and structural uses. Examples are blow moulded drums for packing dangerous chemicals and pressure piping. 

As a blow moulding material polypropylene has never enjoyed the success of HDPE. This is in large measure because moulding of the former requires more attention to equipment design and operating conditions. Many successful mouldings have nevertheless been made commercially including chairs, horticultural sprayers and motor car parts. 

With the advent of improved clarity grades in the 1990s there has been interest in replacing PVC for blow moulded containers for bottled water. 

Extrusion blow moulding of bottles has been successfully accomplished in reeent years by attention to the points mentioned above. It is to be noted here that UP VC has a much lower average specific heat between the proeessing temperature and room temperature than polyethylene and, being essentially amorphous, no latent heat of fusion. This leads to much less heat needing to be removed on cooling of mouldings and very short cycle times are possible. 

Substantial quantities of UPVC are also used for blow moulded containers for such diverse materials as consumable liquids such as fruit squashes, liquids for household use such as detergents and disinfectants, cosmetics and toiletries, and pharmaceuticals. For most of these applications UPVC is in competition with at least one other polymer, particularly poly(ethylene) terephthalate (Chapter 25), polyethylene (Chapter 10), polypropylene (Chapter 11) and, to a small extent, the nitrile resins (Chapter 15). The net result is that in recent years there has been some replacement of PPVC in these areas, in part because of problems of waste disposal. 

Uses reported include motor housings for portable fans, food blender bases, tape storage housings, tough, clear toys, injection blow moulded products and thermoformed packages. 

Polystyrene and closely related thermoplasties such as the ABS polymers may be proeessed by sueh techniques as injection moulding, extrusion and blow moulding. Of less importance is the processing in latex and solution form and the... 

The high melt strength, facilitating thermoforming and blow moulding techniques. 

Acetal resins may be processed without difficulty on conventional injection moulding, blow moulding and extrusion equipment. The main points to be considered are ... 

Amongst the many other applications for acetal resins should be mentioned links in conveyor belts, moulded sprockets and chains, blower wheels, cams, fan blades, check valves, pump impellers, carburettor bodies, blow-moulded aerosol containers and plumbing components such as valve stems and shower heads. 

Structurally viscous grades are based on branched polymers (branching being effected by the use of tri- or higher functional phenols). These polymers exhibit a sharp decrease in viscosity with increasing shear rate which makes them particularly suitable for extrusion and blow moulding and also, it is claimed, in reducing drip in case of fire. 

Modified polyphenylene oxides may be extruded, injection moulded and blow moulded without undue difficulty. Predrying of granules is normally only necessary where they have been stored under damp conditions or where an... 

For extrusion and blow moulding the polysulphones used are of higher molecular weight. Melt temperatures for blow moulding are of the order of 300-360°C with mould temperatures about 70-95°C. 

Dried granules may be injection moulded, blow moulded and extruded without undue difficulty with melt temperatures of 220-260°C. The moulding shrinkage (0.003-0.004 cm/cm) is very low and a somewhat greater than normal draft may be required in deep draw moulds. 

In the late 1970s several developments occurred causing renewed interest in poly(ethylene terephthalate) as a plastics material. These included the development of a new mouldable grade by ICI (Melinar) and the development of a blow moulding technique to produce biaxially oriented PET bottles. In addition there appeared a glass-fibre filled, ionomer nucleated, dibenzoate plasticised material by Du Pont (Rynite) (see Chapter 26). 

The initial use was as a blow moulded vessel for vegetable oil candles. However, because of its biodegradability it is of interest for applications where paper and plastics materials are used together and which can, after use, be sent into a standard paper recycling process. Instances include blister packaging (the compound is transparent up to 3 mm in thickness), envelopes with transparent windows and clothes point-of-sale packaging. 

Processing temperatures should not exceed 180°C, and the duration of time that the material is in the melt state should be kept to a minimum. At the end of a run the processing equipment should be purged with polyethylene. When blow moulding, the blow pin and mould should be at about 60°C to optimise crystallisation rates. Similarly, injection moulds are recommended to be held at 60 5 C. 

Safety at injection moulding machines Safety at blow moulding machines Safety at thermoformmg machines... 

Example 3.14 A blow moulded plastic bottle has its wall thickness made of three layers. The layers are ... 

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