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Blow Moulding Machines
Blow molding was initially developed as a method for producing hollow plastic articles such as bottles and barrels and although this is still the largest application area for the process, nowadays a wide range of technical mouldings are also made by this method. Products made using this process usually have thin walls and range in size and shape from small, fancy bottles to automobile fuel tanks. The basic technique of blow molding is to stretch and form plastic material against a mold, usually by air pressure. Blow moulding is relatively a simple process in which a hollow molten preform or tube is placed in a female mold and inflated to conform to the shape of the mold. Thus blow moulding consists of two processing phases: Fabrication of the tubular form and Inflation of the tube.
BLOW MOULDING AND ITS TYPES
In the blow moulding process, a free-blown form shaped like a tube called as the parison made of a heated polymer is positioned in the cavity of a split mould. Hot compressed air is then injected through a needle into the parison, which expands to conform to the shape of the cavity. Based on the method of forming the parison, blow moulding can be sub-divided into two categories namely: Injection blow moulding and Extrusion blow moulding as the tube can be injection moulded allowing a thread for a lid or some other detail to be formed. It can also be extruded as a tube, pinched at one end, and again expanded to fill the cavity of a two part metal mould. There are also many variations on the basic concepts of blow moulding to make the process faster, and more efficient.
EXTRUSION BLOW MOULDING
In extrusion blow molding, a molten tube (parison) is extruded through an annular die, without pre-forming, the bottom of the parison is pinched of by the mold. In simple terms, the extrusion blow molding method extrudes fluid plastic into parison before molding the final product. The parison is directed between two halves of a mold and then air pressure expands the parison forcing it to conform to the contour of the mold. The conventional extrusion blow moulding process may be continuous or intermittent based on the supply of the molten plastic. In the continuous type, when the desired length of the parison is reached, the mould closes around the parison. The parison is cut above the mould and the mould is transferred to a separate station where the parison is blown by compressed air, cooled and finally the article is ejected out of the mould. But in the intermittent parison type, the compressed air is blown through the mould at the same station when the desired length of the parison is reached. The main disadvantage of this method is the less controllable wall thickness, though some basic control is possible.
INJECTION BLOW MOULDING
Injection blow molding is similar to extrusion blow moulding except that the parison is injection-molded rather than being extruded. It is a two-step process where a blank preform is first molded, and then transferred to the mold cavity where it is blown into shape. Injection blow molding consists of the following steps:
- Parison is injection molded around a blowing rod
- Injection mold is opened and parison is transferred to a blow mold
- Soft polymer is inflated to conform to the blow mold
- Blow mold is opened and the blown product is ejected out.
The advantages of injection blow molding are good control over wall thickness, elimination of bottom pinch off line and no scrap. But it requires two moulds and also involves the transfer time between injection and blow mould.
ADVANTAGES AND LIMITATIONS
Blow moulding is basically flash free and a core less process. It provides rapid production rate, good dimensional precision and excellent consistency between lot to lot. But it is not an environment friendly process. No other method is as fast and efficient as blow moulding for manufacturing hollow containers of thermoplastic polymers. But it is limited for producing only hollow and tubular parts. But, blow moulding is being continually researched upon to increase its capability of producing even more complex shapes. These include unsymmetrical geometries and double wall mouldings. Polyethylene Terephthalate, Polypropylene, Polyethylene, Polycarbonate and Polyvinyl Chloride are the polymers used for blow moulding process. It is important to note that it is not suitable for processing thermosetting polymers. Disposable containers for liquid consumer goods, large shipping drums for liquids and powders, large storage tanks, gasoline tanks, toys, and hulls for sail boards and small boats are some of the blow moulded products.
Some of the famous brands in the world which manufacture blow molding machine are: SIPA, KOSME, Urola, ROMI, HESTA, Fong Kee, SIDEL, KHS, Guangdong Jinming, BEKUM, Jomar, KRONES, Meccanoplastica, KAUTEX and TECHNE.