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10 Steps to Plastic Bags |
Plastic bags, also called “poly bags,” are one of those modern conveniences we take
for granted. But 60 years ago,
no one ever considered the question, “Paper or plastic?”
Back then, people managed to get by without Ziploc® bags or dry cleaner bags.
This was because the technology for creating polyethylene film didn’t emerge until
the 1940s during World War II. And the methods to create plastic bags from polyethylene
film weren’t developed until the 1950s.The most common way to produce poly bags
is by blown film extrusion, also called the “tubular film process.” In a moment,
you’ll see how it got this name. Plastic bags are made from polyethylene. During
processing, the polyethylene might be treated with any number of additives, such
as:
• Anti-block—Prevents the plastic layers from sticking together (blocking).
• Slip—Helps the plastic bag open easier and allows items to slide in smoothly.
• Ultraviolet Inhibitor (UVI)—Protects the plastic from ultraviolet radiation, which
can weaken the plastic’s strength and fade its color.
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1. The machine operator pours the polyethylene resin into the hopper.
2. The hopper feeds the resin into the extruder.
3. Heating elements and the turning of the extruder screw melts the resin into molten
form and forces it through the extruder.
4. The molten poly flows evenly up and over the circular die.
5. As the molten poly emerges from the die, the machine operator:
a. Grabs it wearing protective gloves.
b. Pinches the molten poly together.
c. Ties a rope to the top of the molten poly. The rope leads upward to a
pulley system.
d. Pulls the other end of the rope to move the molten poly upwards. At the
same time, the air ring blows cool air upwards, which solidifies the molten poly.
6. As the tubular shape moves up, the machine operator inserts an air gun through
the poly film to blow in additional air. This step is repeated until the diameter
of the tubular poly film reaches the required bag size.
7. Along the length of the tower are guides to keep the tubular poly film from shifting.
As the film reaches the top of the cooling tower, the guides gradually flatten it
into a layflat form. If the plastic bag will have gussets, special frames (called
“gusset boards”) indent the sides of the tube before it is flattened.
8. At the top of the cooling tower, motorized nip rollers grab the solidified poly
film. The nip rollers now take over the job of moving the poly film up the cooling
tower.
9. The layflat film travels over a series of rollers. For simple bags, such as trash
bags or industrial bags, the film is contained to a single, in-line process. In
this case, the polyethylene film:
− Travels through a bag machine that seals the bottom of the plastic bag and
perforates it at the same time. The perforation allows the bags to be easily torn
from the roll.
− May also travel through a separator that breaks the perforations and stacks
the poly bags on top of each other for bulk packaging in a box.
10. For poly bags with complex features (for example, multi-colored printing, Zip-Loc®
closure, or door knob hang holes) the film is wound on a roll and then taken out
of line for further processing at a converting facility. A converting facility has
a variety of specialized equipment to create the diverse features found in plastic
bags today.
The blown film extrusion process for creating poly bags hasn’t changed much over
the years. But what has changed is how we use plastic bags in our daily lives – whether we carry home groceries in “T-shirt” plastic bags or seal an expensive cut
of meat into an air-tight freezer bag. What was once a novelty is now an everyday
convenience that makes our lives easier.
Source:
http://multipakusa.com/
10_Steps_To_Plastic_Bags_May06.pdf |
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