Plastic bottles are used to contain fluids as various as milk and engine oil. A typical polyethylene bottle weighs about 30 grams and has a wall thickness of about 0.8 mm. The designer specifies a tolerance of 1 mm and a surface roughness of 10 μm. The shape is 3-D hollow. The batch size is large (1,000,000 bottles). What process could be used to make them? (a) List the function and constraints, leave the objective blank and enter "Choice of process" for the free variable (b) Use the process selection charts to identify possible processes to make the casing.

Answer explained

Explanation:

The best process used to make them is Blow Molding considering cost, quality and time.

There are several common molding methods for plastic containers - Extrusion Blow Molding (EBM), Injection Blow Molding (IBM), Stretch Blow Molding (SBM), etc.

The first stage in bottle manufacturing is stretch blow molding. The PET is heated and placed in a mold, where it assumes the shape of a long, thin tube. The tube of PET, now called a parison, is then transferred into a second, bottle-shaped mold. A thin steel rod, called a mandrel, is slid inside the parison where it fills the parison with highly pressurized air, and stretch blow molding begins: as a result of the pressurized air, heat and pressure, the parison is blown and stretched into the mold, assuming a bottle shape. To ensure that the bottom of the bottle retains a consistently flat shape, a separate component of plastic is simultaneously joined to the bottle during blow molding.

The mold must be cooled relatively quickly, so that that the newly formed component is set properly. There are several cooling methods, both direct and indirect, that can effectively cool the mold and the plastic. Water can be coursed through pipes surrounding the mold, which indirectly cools the mold and plastic. Direct methods include using pressurized air or carbon dioxide directly on the mold and plastic.

Once the bottle (or, in continuous manufacturing, bottles) has cooled and set, it is ready to be removed from the mold. If a continuous molding process has been used, the bottles will need to be separated by trimming the plastic in between them. If a non-continuous process has been used, sometimes excess plastic can seep through the mold during manufacturing and will require trimming. After removing the bottle from the mold and removing excess plastic, the bottles are ready for transportation.

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