The earliest blow moulding machines were designed to mould bottles from cellulose acetate which was expensive and, in that application, of limited technical performance. It wasn’t until the development of high density polyethylene and commercially available blow moulding machines during the late 1950s that the industry began to expand initially through the production of milk bottles and toys such as dolls’ bodies and skittles.
Today, the technique is still used mainly for the manufacture of polyethylene packaging, with modern very high molecular weight materials providing the required impact strength and other physical properties required for high performance, dangerous goods packaging.
However, designers and manufacturers have recently recognised the unique properties of blow moulded components and applied them to a new generation of ‘technical mouldings’ in HDPE, ABS, HIPS, polypropylene and PET. Successful projects have included automotive fuel tanks, spoilers and traffic light housings.
Blow moulding lends itself readily to the production of large, hollow components, as complex shapes can be made in a single process. By designing in moulded-in stiffeners and structural ribs, a high moulded stiffness to weight ratio can be achieved. Other key benefits include the unparalleled ability to produce a component to fit an awkward space. Blow moulded automotive fuel tanks are a case in point. Aesthetics can also be vastly improved Sega recently specified blow moulded side panels from ABS and HIPS for its arcade driving games to achieve a more futuristic profile than MDF could deliver.
The technology is often extremely cost effective too, particularly with high volume runs, and this has been proven over many years of blow moulded traffic light housing production. The use of interchangeable components within the blow mould means that modifications can be cheaper than those made on a steel stamping outline.
Where blow moulding delivers the ability to convert from metal to plastic, a whole range of benefits can be achieved. The chemical compatibilities of certain plastics are excellent and offer much improved resistance to a vast array of chemicals when compared to many metals, affected as they are by water, acids and alkaliis. Simultaneously a large reduction in component weights can be achieved.
The impact resistance of blow moulded articles often surprises designers and is well illustrated by the drop tests required for a 220 litre drum under current UN standards. These have to survive being filled with water and dropped, corner first, from 1.9m onto a steel plate fairly tangible evidence of their intrinsic strength. Even part of the oil industry a major user of the traditional steel 50 gallon drum has already converted to heavy duty, black, blow moulded HDPE drums in the US. Using the more durable blow moulded types, oil companies achieve more trips per drum and therefore far lower life cycle costs. A dented steel drum, if lacquer lined, will normally have to be discarded because the inner coating will have become damaged. HDPE drums, on the other hand, don’t dent!
Blow moulding has also enabled lighter and more durable versions of many household products, traditionally made from metal, to be produced in plastic – dustbins for example.
The basic blow moulding process involves a continuous or intermittently screw extruded parison (or tube) of material which is heated above its melting point but which still retains enough strength to hang under gravity. The parison, supported by air to prevent it welding together, is positioned between an open split mould and, after the mould has been closed, is expanded against the mould walls by air pressure. The mould is then opened and the component taken out.
Briefly, the variations on the basic theme include injection or stretch blow moulding where a pre-form (normally injection moulded) is stretched into the blow mould by a combination of a rod and air pressure. There is no flash (scrap) to trim from the preformed neck and therefore this method delivers a high quality neck finish. Large shopping centre light fittings have been produced using this method.
Co-extrusion blow moulding enables the incorporation of a physical or chemical quality into the moulding which the main polymer lacks. Basically you extrude a sandwich. A good example is the fuel tank which, because polyethylene is permeable to certain solvents leading to certain types of fuel vapourising through the skin of the tank, can be co-extruded with a barrier resistant material such as nylon or EVAL (ethylenevinyl alcohol). An alternative is fluorination of the inner surface. Tie layers are normally also required to bond the layers together, meaning the sandwich produced by the co-extrusion head consists of 5 layers. Consequently these heads are complex and expensive. However, over and above the ability to introduce a barrier layer to the moulding, other cost saving advantages include the ability to hide recycled material on the moulding’s inner surface or to pigment just the outer surface.
Foam blow moulding is a fairly new process which is currently being used to produce pallets for the food and pharmaceutical industries where hygiene is of paramount importance. This technique achieves a thickness and rigidity coupled with a much lower weight of polymer and is therefore ideal for structural components such as spoilers, bumpers etc. The foaming process is often combined with co-extrusion to ensure bubble-free inner and outer surfaces or where high insulation properties or buoyancy is required.
The blow moulding process is fast shedding its old fashioned image, and is providing a frequent solution for designers looking for flexibility of design coupled with a huge variety of material properties.