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The Steps That are Involved in the Synthesis of Plastic Bags

Komal B. Patil Apr 19, 2019
Plastic being versatile can be put to many uses. This property allows it to be molded into various structures and range of hardness, to meet our needs. The most common example of plastic used in everyday life is that of plastic bags.

Did You Know?

Approximately 1 trillion plastic bags are used annually, worldwide.
The word "plastic" originates from the Greek word "plastikos" that refers to the ability of being shaped or molded. The most common plastic is polyethylene, also known as polythene.
It is primarily used to produce plastic bags, that are thin and flexible, and are bonded together via the use of adhesives or by heat sealing. This synthetic compound is a polymer that exhibits the chemical formula (C2H4)nH2.
Depending on the density and branching pattern of the polymers, polythene is classified into 11 types. They are:
  • Ultra-high-molecular-weight polyethylene (UHMWPE)
  • Ultra-low-molecular-weight polyethylene (ULMWPE or PE-WAX)
  • High-molecular-weight polyethylene (HMWPE)
  • High-density polyethylene (HDPE)
  • High-density cross-linked polyethylene (HDXLPE)
  • Cross-linked polyethylene (PEX or XLPE)
  • Medium-density polyethylene (MDPE)
  • Linear low-density polyethylene (LLDPE)
  • Low-density polyethylene (LDPE)
  • Very-low-density polyethylene (VLDPE)
  • Chlorinated polyethylene (CPE)
Out of all the kinds of plastics, the most important and widely used variants are HDPE, LLDPE and LDPE. These varieties are used in defined ratios to produce the raw material for making various types of plastic bags.

Steps Involved in the Production of Plastic Bags

Blending → Extrusion → Printing → Processing

Step 1: Blending

The raw material is in the form of pellets of clear LDPE and HDPE. These pellets are fed, in a specific ratio, into a blending machine, where they are melted down to produce a blend of molten plastic, called resin.
The additives added to the resin, vary according to the properties required in the finished product. An anti-static agent is added to prevent the layers of the bag from sticking to each other. Addition of a UV (ultra-violet) inhibitor leads to maintenance of the plastic's color and strength, on exposure to UV light.
Pigments can also be added at this stage if a colored end product is desired. In case of biodegradable plastics, another additive, EPI-D2W, is added. This additive imparts plastic the ability to biodegrade via oxidative and cellular processes. The resin is mixed with the additives till they are uniformly mixed. This blend is then fed into the extruder.

Step 2: Extrusion

The blend is heated at 380°F to create a homogeneous liquid, which is then fed into the extruding machine. The extruder, by using a screw method, pushes the blend through a circular aperture, resulting in a tube like shape. Air is then blown into this tube to inflate it, such that the tube expands without the sides touching each other.
During this process, the air causes the blown plastic to cool down gradually. The blowing process takes place in a vertical column of around 20 feet.
At the end of the extruder, the plastic is sufficiently cooled and then flattened and wound into rolls.

Step 3: Printing

This is optional, and is carried out if the end product needs to have something printed on it. The rolls of plastic are fed into flexographic stacked presses, which print the desired message onto the plastic films. These presses can print in 5 different colors. Once the films are printed, they are then further processed to give them the desired appearance.

Step 4: Processing

The roll of printed bags is fed into the cutting machine, where hot knives are used to cut the films into the desired shape and size. The use of hot knives not only results in a clean cut but also seals the cut ends together. This cut film is then fed into a bag shaping machine, which forms the crimps and seals of the bag, along with the handles of the bag.
Any special additions like Ziploc, and side folds can be added by the use of different types of machines. The scrap generated from these processes is later added in the next batch at the blending stage.
After the desired style and shape of bag is achieved, the bags are stacked on top of each other and packaged, to be shipped or mailed out to the required destination. The popular and high usage of these plastic bags has caused the plastic industry to become the 3rd largest manufacturing industry in the US, with more than 16,200 facilities nationwide.