Blow molding, also known as blow moulding in the United Kingdom, is a molding process in which heated plastic is blown into a mold cavity to create a hollow object. The defining characteristic of a blow molding is that it’s used to create hollow objects. Raw plastic is first heated, after which it’s formed into a parison. Next, the plastic parison is secured to the top of the mold. Finally, air blown down onto the plastic parison, thereby stretching it across the interior walls of the mold cavity.
Blow molding follows the same principle as glassblowing. With glassblowing, a glassblower blows air across heated glass, thereby creating a hollow glass object. With blow molding, a machine blows air across heated plastic that’s placed on top of a mold cavity. The air forces the heated plastic to expand across the interior walls of the mold cavity.
Extrusion blow molding: In this kind of blow molding, the plastic is melted and then poured or extruded into a parison (also called pare). This parison is then captured and closed into a chilled metal rod. After this, the air is circulated into the paring which inflates the inside of the hollow container. Once the plastic has sufficiently chilled down, the mould cracks open and the part is separated. There are two different versions of extrusion blow molding. They are:
In this kind of extrusion molding, the pare is continuously extruded and each part is cut with the help of a knife.
Intermittent: This kind of extrusion molding again comprises two separate processes namely, accumulator method and straight intermittent method. In the former, an accumulator collects all the melted plastic and when the mould cools down sufficiently, a rod is passed through the melted plastic. This forms the parison. In the straight intermittent method is somewhat similar to the injection molding process where the screw moves intermittently and pushes out the melted plastic. In both these forms of intermittent blow molding, the reciprocating screw or the hydraulic accumulator mainly use the hydraulic system to quickly push the pare out and reduce its weight. They allow for systematic control over the thickness of the wall by adjusting the gap with the help of the parison programming equipment. In both these forms of intermittent extrusion molding, the screw may turn continuously or intermittently. In case of the former, the weight of the pare drags it along and makes calibration difficult.
Injection blow molding: This is another variant of the blow molding process. It is mainly used in manufacturing hollow plastic and glass bottles or objects in huge quantities. This is a simple process whereby the polymer is injected and moulded on to a core pin. This core pin is then rotated to a molding station where it is left to be chilled and inflated. This is the least used process of blow molding and is used mainly for manufacturing small bottles or medical syringes.
Injection stretch molding: This process again tales two forms namely, single-stage and double-stage molding process. In both these stages, the plastic is first melted and shaped into a preform with the help of the injection blow molding process. The preforms are made with the bottlenecks which include threads at one of their ends. The preforms are then packaged and fed into a heat stretch blow molding machine. In this process of blow molding, the preforms are exposed to infrared heaters at temperatures above the glass transition temperature. They are then blown using very high-pressure air into the bottles with the help of metal blow moulds.
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