 About Blow Molding
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| Blow molded plastic is any hollow plastic product, the most common of which is bottles, |
| that is a result of the blow molding process. Blow molding uses a continuously running |
| extruder to which a tuned die head is attached, forming a extruded |
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| parison. The hot, hollow thermoplastic tube, which is either or re- |
| heated, is set within the cavity of a mold and held in place with |
| clamps. Using a blow pin, air pressure is introduced through the |
| inside of the parison, forcing it to assume the shape of the mold. |
| The end product is cooled by conduction or the evaporation of volatile fluids in the |
| container. The purpose of this method is to form a uniform finished product that does not |
| need to be joined together in any way. |
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| There are three basic methods for producing blow molded plastic: extrusion, injection and |
| stretch blow molding. Extrusion blow molding is the simplest type that allows for a wide |
| variety of container shapes, sizes and neck openings, along with handleware of many |
| kinds. The parison is dropped from an extruder and captured in a water-cooled mold |
| where it is blown against the mold and frozen into shape. Shuttle and reciprocating |
| extrusion machines are used for small to high volume production, while wheel extrusion |
| machines are the most efficient for huge volume production of certain resins. Injection |
| blow molding is a hybrid of the injection and blow molding processes and is suitable for |
| smaller containers but not for handleware of any kind. In this process, hot plastic |
| material is injected into a cavity. A neck is created as the material encircles the blow |
| stem, which also establishes the gram weight. The injected material is moved to the next |
| station on the machine where it is subjected to the extrusion blow molding process. |
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| Two types of stretch blow molding are injection stretch blow molding (ISBM) and reheat |
| and blow (RHB) molding, both of which are typically used to manufacture P.E.T bottles |
| for juice, water, etc. In the ISBM process, a preform is injection molded and then |
| transferred to another station on the machine, where it is blown. It is then ejected. |
| Because ISBM is so costly, it generally requires extremely high volume (in the multi- |
| millions) runs and is used for items such as wide-mouth peanut butter jars, narrow |
| mouth water bottles, liquor bottles and more. In RHB, a preform is purchased from a |
| vendor that has already put it through an injection molding process. It is then reheated |
| in a relatively simple machine in order to prepare it for blowing. RHB is advantageous |
| because the purchaser does not have to invest in expensive injection molding machinery |
| but still has access to a large catalog of existing preforms. |
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| There are distinct advantages to the blow molding process over other plastic manufactur- |
| ing processes. This process is the most well suited method for forming hollow parts. It |
| has much lower mold costs than injection molding. Blow molding has a faster cycle time |
| than that of rotational molding. In many cases, one blow molded plastic piece can take |
| the place of many individual parts. However, there is the problem of the scattering of |
| mold thickness, due to swell and drawdown. Swell refers to the rapid increase of the |
| thickness of the parison as the polymer melts leave the die and are released into the air. |
| In drawdown, the parison is stretched by its weight, reducing its thickness. |
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