Plastic Bottle Manufacturing: Everything You Need to Know
Plastic bottles are an important aspect of everyday life – yet the plastic bottle manufacturing process is rarely thought of.
When we reach for a drink in the grocery store or from a vending machine, we’re usually thinking about what’s inside, not the container holding it.
But an understanding of how plastic bottles are manufactured provides a window into the way everyday items come into being.
Follow along for the journey that iced tea bottle took from the factory to your hand.
The Formation of Plastic Bottle Manufacturing: Polyethylene Terephthalate
In order to fully appreciate plastic bottle manufacturing, it’s important to understand what they’re made of.
Bottling companies have a challenge to meet when choosing the material for their product.
It must resist crushing while still being light enough to ship economically and function practically for consumers.
Concrete, for example, is a strong and durable material, but not exactly practical for joggers on the go.
The manufacturers’ solution is polyethylene terephthalate, or PET.
It’s a synthetic resin used in the production of a wide rage of commercial, medical, and military applications—everything from earrings to boat sails to polyester clothes.
If you look on the bottom of any plastic container and see a number 1 surrounded by “recycling” arrows, it is made of PET.
- A chemical reaction is then synthesized between this acid and form of alcohol to produce petroleum hydrocarbons.
- Chains of the petroleum hydrocarbon molecules are then polymerized, or networked, to form the PET.
PET is known as a “thermoplastic polymer,” which means that its manufacturers can alter its chemical formula to produce either a transparent or color material.
It is also easily molded and highly recyclable. Once this versatile material is produced, plastic bottle manufacturing can begin.
PET is shipped to manufacturers in the form of small whitish-clear pellets that resemble the innards of a bean bag, or packing material.
These are then poured into a heated container which contains a reciprocating screw.
The resin pellets are melted to about 500-600 degrees Fahrenheit, and the screw shoves the now-liquid PET into a mold.
This happens under extremely high pressure and fairly quickly, usually several times a minute.
In order to make the process more consistent and efficient, the molds are cooled to just below half that of the heating temperature — between 190-230 degrees Fahrenheit, depending on the thickness of the mold.
As the PET’s temperature drops, it becomes a solid once more, and begins to take the same shape of the mold.
In order to ensure strength and a stable product, the cooling process takes place even more quickly than the melting procedure.
The PET is now in the form of an elongated, hollow tube, anywhere between .75 and 6 millimeters thick.
Stretch Blow Molding
The former PET pellets are now ready to become more recognizable as a plastic bottle. The thin tubes are known as a “parison” or a “preform.”
They resemble wide test tubes with screw tops awaiting a cap. The parisons are open at one endin order to allow for a process known as stretch blow molding.
1. Non-traditional Bottle Shapes
In the event the manufacturer requires non-traditional bottle shapes, there’s an extra step at this point in the process. The parisons are given a free ride on a conveyor belt on their way to a pre-warming session.
Technicians at the controls of infrared oven heat lamps, depending on the type of bottle needed, adjust the shape and thickness of the plastic.
2. Traditional Bottle Shapes
Otherwise, for this second molding process, the parisons are placed into a second, bottle-shaped mold.
Then it’s time to warm up again: The parisons are heated once more, but not enough to melt—just enough to become malleable.
A mandrel, or a steel rod a few millimeters thick, is inserted lengthwise inside the tube of PET. Highly pressurized air is then shot into the parison.
This air, combined with outside pressure and high heat, stretches the parison into a bottle shape. It happens in seconds.
3. Choosing Bottle Shapes
The shape the bottle takes depends on what the manufacturer needs. Sometimes the molds are set for small, squat, single-serve containers; sometimes the preforms become large two liter bottles.
Some of the versatility and strength of plastic bottles lies in the fact that they do not have a seam at the bottom, which can lead to leaks.
This flat bottom is achieved by fusing a separate plastic component to the preform as stretch blow molding takes place.
The bottle shape is formed in a flash. Then, inside the mold, cool water rapidly circulates in order to set the plastic in its new form.
In some methods of stretch blow molding, carbon dioxide or cold pressurized air take the place of the circulating water.
The entire stretch blow molding procedure, start to finish, is completed in seconds. Some machines can churn out over ten thousand bottles in an hour.
Removal, Trimming, and Testing
Now that the bottle has been warmed, stretched, and cooled, it’s prettied up for the next step in the plastic bottle manufacturing process.
The bottle is taken from the mold.
What happens next depends on the type of machine performing the stretch blow molding:
- The bottle may be separated from other bottles which were formed at the same time (continuous molding).
- If it were fashioned individually (non continuous molding), it could be checked for PET which has seeped from the mold during the pressurization process.
Either way, the bottles are trimmed for uniformity.
No matter the rate of output, technicians pull random samples of the bottles off the line to test for quality control.
Weight, dimensions, and plastic thickness are all measured, and the bottle is subjected to a crush test to measure its strength.
The test bottle’s capacity is also measured, and containers destined to hold carbonated beverages are evaluated to ensure the plastic is strong enough to withstand the pressure of its contents.
Packaging and Shipping
The finished bottles are now given a conveyor belt ride to the final stage in their processing: Packaging and shipping.
They are sent to their new homes to be filled and labeled.
These are the simple steps to plastic bottle manufacturing.
There are many benefits of utilizing PET plastics – and this is why.
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