Production Process of PET Bottles
July 25, 2016
Plastic bottles are very much in demand, mainly because of their durability and low cost. We all use it in our daily life and it is very much safe and acts as a container for various liquids. It has numerous uses and is one of the greatest discoveries in the recent times.
What is PET Bottle?
Polyethylene terepthalate is the most common plastic that we use these days, which in short is called PET. It is not made by one single process, but with a reaction of two chemicals known as Purified trephthalic acid (PAT) and ethylene glycol (EG). These two chemicals when combined form Polyethylene terephthalate along with other types of plastic, which is used in variety of applications such as engineering, nylon production etc.
Some properties of PET are
- Density: 1.38~1.40g/mm3
- Melting Temperature: 254-256°C
- Crystallinity: >=45%
Glass Temperature: 82°C
PET is a long chained molecule with the repeating unit, resin, a bluish white substance and is made from poly condensation of terephthalic acid and ethylene glycol. Resin manufacturers supply PET in the form of small pellets, each weigh about 0.05 grams. Being transparent, PET can reach a tensile strength of about 1/3~1/2 of Steel’s. Rge number of PET repeating units per chain used in the bottle industry to make plastic bottles is about 100 to 155, usually labeled in terms of resin IV.
Uses of Polyethylene Terephthalate
Also known as PET, is a thermoplastic which is one the types of plastic amongst million others. By far the most useful type of plastic, PET bottles are used mostly in the beverage industry, where the bottles are filled with a particular beverage and supplied throughout the world. It could be water, juice, carbonated drinks, health drinks etc. Some of the most common uses would be bottling of mineral water, edible oil, juice, cosmetics, pharmaceutical products etc.
Main Advantages of PET
- Pure – PET does not involve any reaction with the food or beverage that is contained in it. Therefore PET complies with international food safety regulations.
- Lightweight – It is 10 times lighter than glass pack, therefore it reduces shipping costs by 30%. Even if a glass bottle and a PET bottle has the same size, PET will store more because of thinner walls. Also PET is durable and doesn’t crack like glass bottles.
- No Leakage – Due to the absence of a weld line in the bottom of the PET bottles, leakages do not occur.
- Design flexibility – can be designed in various shapes, size, designs and color.
- Recyclable – Since it is one single material, it can be washed and crushed into flakes and remolded into shape. After recycling it can also be used for strapping and making carpets etc.
Production of PET bottles
Usually PET bottles are manufactured using 2 different methods.
These are :-
- One-Step “hot perform” method.
- Two-Step “cold perform” method.
Step-one “hot perform” method
This method is just one step, from PET granule to finished bottles, all the processors are done in one integrated machine. That is from the injection mold of the preform to the removal of the preform from the injection cavity, and to stretch blow the preform to a bottle all in one machine, while the temperature remain constantly high for the whole process as injection molding and blow molding, the material should in an elastic form. This method saves a lot of energy as the material only has to be heated only once, that is when it is injected to the cavity to produce the preform. The One-Step method is commonly used in small or medium PET production companies.
One-Step “hot perform” method Integrated Machine.
Step-two”cold perform” method
This method has 2 distinct machines to perform the injecting and blow moldings. The preform is injected into shape on the first machine, and then the preform is reheated as the it isn’t brought directly to the blow molding machine and blown on the second machine. This method is not very efficient as the heat lost is considered as a loss of energy. But this machines are fully automated.This method is used in most medium to large scale PET production companies.This machine is 200% more efficient than the one-step machine.
Two-Step “cold perform” method, Fully automated machine.
Usually the Two-Step Method is used in production of PET bottles.
Driying and Dehumidifying of PET
PET is a type of plastic that absorbs moisture with an average water content of 0.05%. To increase the quality of the preform in transparency and physical performance the PET is dried to reduce the water content of the PET resin to less than or equal to 0.005%. The following drying methods are used.
- Oven Dry – The drying is done at a temperature of 110°C for a maximum of 8 hours after which PET will start to fail. The trays with PET inserted into the oven are about 50mm thick. The oven isn’t connected to the injection machine ant the dried PET is moved manually.
- Hopper dryer– The widely used method of drying PET as it very economical and works along with an autoloader, therefore the process is continuous and automatic.
- A Dehumidifier – Similar to the hopper dryer this also words together with an autoloader, but it is the most efficient method in drying. The PET dries in at a temperature of about -32°C, thereby increasing the durability and the final finish of the PET bottle.
The Optimum drying temperature would be about 150~170°C. The best duration for drying the PET would be about 4 hours. The drying time should not surpass 7~8 as will deteriorate the PET.
Injection Molding of the PET Preform
The above picture shows an injection molding machine with the autoloader and dryer other essential equipment needed in the process.
Schematic illustration of the injection molding process.
The process is simple, the heated PET which is in the molten form is injected into the injection chamber (2),by the rotation of screw barrel (1) within the machine, until the chamber is full.
Once the chamber is full, the screw pushed forward (3) to fill the injection cavity with molten plastic through the nozzle into the mold.
The temperature within the injection molding machine are different, at the rear end of the machine the temperature should be around 275°C, at the middle of the machine about 282°C, at the nozzle about 280°C. And at the runner the temperature should be around 270°C for the bottle formed to have optimum specifications. (These temperature may vary a little depending on the design of the mold)
Preform being Heated
Subsequently the molten plastic within the mold,called the preform, should be cooled. The cooling water used to cool the perform should be in the temperature range between 15~20°C, the pressure also has to be controlled, therefore a water chiller is used. The pressure should be kept around 500,000 Pa or 5Bar.
The PET preform is ejected (4) from the mold. Now the preformis reheated to the temperature until it is suitable for the blowing process.
Stretch Blow Molding of the PET Preform to Container
The Following process in production of PET is a type of Blow molding called Stretch Blow molding, as air is blown into the preheated PET Preform.
Possible Defects while production of PET Bottles
- OH- or COOH- end groups doesn’t becomes non-reactive.
- Amount of poly-functional macromolecules increases.
- Hydroxide and peroxide groups build up at the surface of the PET and etc.
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