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The production of poly bags involves highly technical and scientifically engineered processes that are precisely planned to ensure high quality results. All forms of poly bags begin with high resin plastic pellets that produce the required type of film and gauge.
All poly producers rely on the heating and extruding of the resin pellets, which are made from ethylene or propylene that are extracted from natural gas. In ethylene and propylene are hydrocarbons that are used to make monomers from a cracking process. From the monomers comes the polymerization reaction to produce polymer resins for the manufacture of poly bags.
Resin or Plastic Pellets
The type of resin selected for the manufacture of a poly bag determines the durability, weight, and thickness of the bag. There are five types of resins used to produce plastic bags: high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), medium density polyethylene (MDPE), and polypropylene (PP). The characteristics and properties of these polymers are the major factors regarding a bag‘s strength and life expectancy.
The creation and manufacture of high quality poly bags begins with the selection of the correct bag materials.
Extrusion
Once the resin has been selected, it is loaded into the hopper of an extruder. The initial method for melting plastic pellets was to simply heat them in a vat or large barrel. The result was an uneven heating of the material with pellets closest to the heat source being hotter and more pliable.
This led to the development of the extruder, a screw mechanism that evenly heats the material at 350° F to 450° F such that all of the pellets are melted to the same temperature. An extruder is screw-like with a loading hopper at one end and a die at the other. The resin is fed into the hopper, which slowly drops the resin into the barrel of the extruder. As the screw turns, the pellets move along the length of the extruder, heated by heating units placed along the length of the barrel.
Inflation
The molten plastic resin exits the extruder die in the shape of a tube and is inflated and stretched to the needed size and shape of the poly bag.
Cooling Tower
Once the tube of plastic is fully extended, it remains in the tower until it cools. In the cooling tower, it has the appearance of a tall plastic vertical balloon. As it expands upward, it comes into contact with the walls of the tower and cools as it works its way up to the cutting rollers.
Nip Rollers
To make the plastic fabric, the tube passes through the collapsing frame before entering the nip rollers that flatten it into a lay flat form to be rolled into a large roll of plastic. Nip rollers provide high traction, isolate web tension zones, and improve winding onto a roller.
Rolling Film
Once the inflated film is flattened and passes through the collapsing frame and nip rollersl it is then rolled into a huge roll for processing. It is from these rolls that the poly bags will be printed, formed, cut to lengths, and also sealed.
Flexographic Printing
Poly bags come in different sizes, thicknesses, colors, and forms. Bags that are produced for stores, shops, and retail outlets usually have print identifying the store with some of the printing offering additional information. Printing on poly bags is done by a process known as flexography, a roll feed web printing process used to print on film, foils, paper, cardboard, and other materials.
The image to be transferred is imprinted on a polymer plate using ultraviolet light. The image is engineered and designed on a computer using design software such as CAD. The negative of the image is placed on a light sensitive polymer plate. When it is exposed to ultraviolet light, the image hardens while the material around it turns to a gummy substance that is washed away.
The completed plate is wrapped around a cylinder. As the poly material passes between two cylinders, one with the plate on it; the plate is then wetted with ink and prints the image on the bags. For single color printing, one plate and set of cylinders are used. When multiple colors are necessary, the poly bags will pass through a series of cylinders, each having a different portion of the image.
Cutting
During the printing process, the roll of film from the extruding and blowing process is unrolled as it passes through the flexographic printer. After the poly bags are printed, they are re-rolled in preparation for the cutting process.
Every manufacturer has their unique process for cutting poly bags, which includes the sealing of the bag. Handles may be cut and shaped in addition to cutting the bag to length.
Additionally, the cutting process has to allow for gussets that may have been added during the cooling of the plastic tube after extrusion.
Poly Bag Production
Though numbers vary, poly bag producers can process as many as 70,000 bags an hour, depending on the size, type of polymer, and length of the bag. Larger and longer bags are produced at 2,000 bags per hour.
Hourly production of poly bags is dependent on the size of the bag and the thickness of the polymer, which is significantly altered by thicker, longer, heavier bags.
Poly Bag Thicknesses
The ply of a poly bag refers to its strength, durability, and ability to protect materials. The strength of a poly bag is dependent on its thickness, which is determined by its mil score, an industry measurement for poly bag thickness or gauge with one mil equal to 0.001 of an inch. The majority of poly bags are 6 mil or less with the average thickness being 2 mil.
Poly bags are manufactured to 10% of their minimal thickness, which means that a 2 mil poly bag will be 1.8 mil or 0.0018 inch thick. When a customer orders a poly bag, they need to specify whether the bag will have minimal thickness or full gauge thickness.
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Common Mil Type Thicknesses and Their Use |
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Thickness in Mils |
Thickness in Inches |
Uses |
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<1 |
0.0005-0.00075 |
Meant for short-term use requiring little protection, such as produce bags or dry-cleaning bags. |
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1-2 |
0.001-0.002 |
Moderate protection for storing, shipping, or displaying items like food, clothing, or retail products. |
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4 |
0.004 |
Heavy-duty protection heavy or products. Can also be tear and scratch-resistant for added durability. Ideal for storing bulk items like screws or parts or large industrial uses such as pallet covers. |
There are three factors to consider when choosing the correct poly bag mil to fit a product.
- Product Weight and Size
- Level of Required Protection
- Necessary Durability and Shelf Life
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