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Bottle blowing process is an important step in the manufacture of bottles for liquid products such as water and other beverages. Basically, this process involves the use of the bottle blowing machine in molding thermoplastic polymer materials (bottle) into required shapes.

The process combines both the injection and blowing techniques. Let’s look at how the bottle blowing machineactually works.


Principle -- Injection and blowing process

Preform stretching and blowing phase:

Here, the stretching rod descends and stretches the preform, then the compressed air is injected through the nozzle.

First, the material is injected to form a preform also known as a "sock ", whose shape resembles a "test tube". The neck of the preform already has its final shape: it is the part used to hold the future bottle during the blowing operation, so it cannot itself be deformed. In addition, if a plug is to be used, the shape of the neck integrates the fixing system, for example the screw thread: the injection process is ideal for obtaining complex shapes.

The preform is stored and transported to the blowing site. It is then heated and blown to the desired shape.

For polymers, the material must be enormously deformed and must not crystallize at the body level, which requires working with a cooled mold; the preform therefore comes out relatively cold, which relativizes the expected heating economy;

Injection is a relatively slow operation (several seconds) while blowing is very fast (about one second), which poses the problem of tune the rates if the operations are sequenced.

Blowing phase:

Here, the preform is enclosed in a blowing mold with the desired shape. The mold consists of two half-shells and a removable bottom, this separation allowing the mold to be removed:

The removable bottom allows to have a concave bottom, which ensures a good stability of the bottle:

  • On one hand, a flat bottom would be curved outwards under the effect of the weight of the liquid, and possibly the pressure of carbon dioxide for soft drinks

  • And on the other hand a central recess allows to adapt to the defects of the table or the folds of the tablecloth, some bottles even have feet

  • The bottom often has ribs, also concave, which stiffen it, the most resistant bottoms have a shape called "petaloid "

  • It should also be noted that the bottom can be given the hollow shape of the cork, which makes it possible to stack the bottles

  • And finally, a concavity reduces the inner volume, which is sometimes used to give the customer the impression that the bottle has a larger capacity (the customer perceives the apparent outer volume of the bottle, but buys the content that has the inner volume);

The separation of the two half-shells allows complex shapes to be created to perform the following functions:

  • Stiffening of the bottle - avoid unsightly deformation during filling and gripping (soft bottle) -, by circumferential grooves, but which also reduce the crushing resistance (problem when stacking pallets),

  • Aesthetics of the bottle (brand identity),

  • Easy grip (central section at the waist of the hand, rough surface for better grip).

The blowing nozzle is applied to the neck, and compressed air (most often) is injected into the cavity through the preform opening to press the material against the impression. The blowing pressure is a few dozen bars. The polymer cools on contact with the mold, which freezes the part in its final shape.

In many cases, stretch-blowing is carried out: once the preform is introduced into the mold, a drawing rod stretches the material in the axis, and a pre-blowing is carried out at a few bars of pressure, which forms a bubble. Thus, the polymer chains are oriented both in the axis of the bottle and tangentially. This dual orientation improves the mechanical properties of the bottle; sometimes the acronym SBO is used for dual-directional blowing. The final blowing is then carried out.