Simplified Understanding About Plastic Bag Making Process

Thứ năm - 04/07/2013 01:18
Automatic cutting machine
Plastic bag, also called "poly bag" made from polyethylene (HDPE: high-density polyethylene, LDPE: low-density polyethylene, MDPE: medium-density polyethylene) is popular everywhere as an economic & useful carrier product. After over a half of century of development, now it has a various range of designs to please users' requirement: t-shirt handle, block head, die cut, patch, flexi-loop, clip loop, rigid handle, duffer, bag on roll... The production technique for each bag is different but there is a simplified description about plastic bag making process with 4 steps: material blending, extruding, printing, finishing.

1. Material Blending
Polyethylene resin is mixed in a blending machine. Here, the resin will be mixed with some additives, such as:
- EPI, D2W (1-2%) to make the bag been degradable. 
- Anti-static (1%) to prevent the plastic layers from sticking together, especially to help thin LDPE bag to be opened easier. 
- Master-batch (4-7%) to make the color film: black, white, yellow, red, blue. 
- Ultra-violet Inhibitor (UVI) to protect the plastic from ultra-violet radiation. This additive is very useful in agricultural plastic cover.

2. Extruding
After blending, the mixture will be treated into molten form at high temperature. It goes out at the shape of a bubble full of cool air that comes from the air ring. This air stream helps to blow the bubble up & cool down the plastic. The step is very important because it can affect the strength, thickness consistence & transparency of the film. The bubble goes through cooling tower of 25 - 35 feet tall. At the top of the tower, it is guided to be flatten gradually to form flat shape. The lay-flat film travels over a number of rollers. With simple bag, it is often printed, sealed & perforated at the same time of extrusion. For complicated bag, it will be moved to next steps in the shape of rolls.

3. Printing
Depending on printing design, color numbers, the plastic bag can be treated by flexo- printing or gravure-printing. With natural printing design, it is best to use gravure printing. For simple, small quantity & food contact bag, it is better to use flexo-printing.

4. Finishing
This process includes 3 main steps: cutting, sealing & folding. For different types of bags, they use suitable cutting machines. In this step, side gussets, bottom gussets will be made. In this step, the heat temperature, the speed of machine will decide the strength & the appearance of the seal. The cut-out part of die cut bag, patch handle bag, t-shirt bag will be collected & recycled for garbage bag. For bags with handle: clip loop, rigid, soft-loop, they have one more step of handling. With shopping square bottom bag, users also require cardboard to make bag strong & in a good shape.

Author: Elis Hue - Vietnam Poly Bag Import Export JSC


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