How Envelopes Are Manufactured
Envelopes are packaging items used for transferring pieces of paper, like letters or cards. While different types of envelopes exist, the most common type is the windowed envelope. And although we’ve all used them, not many people understand how envelopes are manufactured. Ultimately, they go through a series of industrial processes starting with wood pulp and ending with the finished product.
Making the Wood Pulp
- Chemicals extract the pulp from wood. Kraft pulping is the most common method, in which wood chips go into a large, sealed container called a digester. The digester is full of an alkaline solution made from sodium hydroxide and sodium sulfide. The mixture heats to between 320 to 356 degrees Fahrenheit at a pressure of 116 pounds per square inch. This usually takes 30 minutes to two hours.
- The pulp then gets bleached, which removes lignin, a wood pigment that gives it its brown color. The pulp mixes with different oxidizing chemicals that react with the lignin. Once this is completed, the pulp is washed with an alkaline solution to remove the lignin.
- Fillers and other chemicals mix with the pulp solution to improve the paper’s brightness, opacity, and smoothness. Various starches or gums are added as sizers to make the paper less absorbent so that the ink doesn’t run.
Making the Paper
- Water is added to the pulp to form a diluted slurry, which allows the paper to have an even density. The slurry pumps through a moving mesh screen made up of fine metal or plastic wires. The water drains through these openings to form a sheet of wet web material. Rapidly spinning industrial rollers create suction to remove extra water from the mixture.
- From there, the web material moves across a felt belt with wool and synthetic fibers to absorb more water and protecting the sheet from roller damage. The sheet then moves across a series of steam-heated rollers to completely dry the sheet.
- Like any paper-based web handling process, the dried sheets are wound on a large industrial reel using different guiding and spooling components. It remains uncoated.
- The paper rolls, weighing as much as 220 pounds each, arrive at the envelope factory. Once there, the paper is cut before it enters an automated envelope machine. Sharp blades cut the sheets into their proper size, and the new sheets stack into piles for further cutting. These piles are then cut into blank envelopes shaped like diamonds. The reels can also feed directly into the machine. When cut directly from the reels, these sheets are cut directly into the diamond shape.
- These machines also perform the other necessary applications, like window cutting and the adhesive border. A strong glue is placed around transparent coverings for the windows and elsewhere to hold the envelope together. A weaker adhesive is placed on the flap for the consumer to seal. The machines then fold the blank paper to form an envelope. This machine may also apply additional printing or fasteners if needed. Once completed, the envelopes fill into cardboard boxes and ship to retailers.
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