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How Corrugated Boxes Are Made | PCA

How Corrugated Boxes Are Made

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3 Types of Plants.
3 Different Workflows.

Full-line plants have a corrugator and converting equipment, sheet plants only have converting equipment and sheet feeders only have a corrugator.

Linerboard, Medium, Flutes and Starch

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the combining stage — a parallel process

Combining is putting the layers of linerboard and medium together to form complete corrugated board.

Medium is engineered to form easily into flutes as it is fed between two large gear-like rollers on the Wet End of the corrugator.

Linerboard is engineered to remain flat as it adheres to the top and bottom of the fluted medium by means of starch-based adhesive.

When combined, these two materials become the amazingly strong structural material that is corrugated board.

close up image of a corrugator roll schematic of a corrugator using starch adhesive to combine containerboard into corrugated board
image showing the linerboard, medium and adhesive being combined
image of a corrugator
close-up image of the corrugator roll on the machine

equipment:

Corrugator

  • Combines linerboard and medium into corrugated board
  • Cuts sheets to overall job size
  • Can put straight-line scores across flutes
  • Can make variety of flute profiles
  • Can make variety of board types: singlewall, doublewall, etc.
Full-line or combining plants and sheet feeders have corrugators inhouse. Sheet plants do not.

Any folding creases (scores) added inline on the corrugator before the board is cut down into sheets are always parallel to the direction that the paper travels through the machine — crossing the flutes. These scores typically become the flap scores of a finished box.

At the Dry End, the combined board is cut to job-specific sheet sizes and stacked, awaiting conveyance to a converting line.

schematic of a corrugator sheet with flap scores with a real image next to it
stacks of work in process
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The combined board product coming off of the Dry End of the corrugator at a Full-Line plant is Work In Process (WIP). It will move on to the converting stations within the plant

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The product coming off of a Sheet Feeder’s corrugator is considered Finished Goods for that plant. It will not be converted into boxes on the premises.

Folds, Joints, Angles, Shapes and Graphics

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the converting stage — a perpendicular process

Once the corrugated board has been fully formed and is stacked in cut sheets at the Dry End of the corrugator, it’s ready to go on to converting stations within the plant. Which converting machine receives which cut sheets depends upon the type of box being run and what its attributes and requirements are.

The sheets travel through the converting equipment perpendicular to the direction they traveled through the corrugator.

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Not only do the cut sheets travel through converting equipment perpendicular to the direction of flow through the corrugating line, but in many Full-Line plants, entire converting production lines run perpendicular to the corrugating line.

This horizontal-to-vertical plan allows for logical, efficient distribution of Work In Process from the corrugator out to the various converting stations.

schematic of a box plant layout showing the perpendicular process

converting equipment is only in FULL-LINE and SHEET plants

Full-line or combining plants and sheet plants have converting equipment in house. Sheet plants do not.

equipment:

  • Makes boxes that have only straight-line cuts and scores and have a “manufacturer’s joint” or “glue joint”
  • Uses cutting and creasing “wheels” on a rotating shaft
  • Prints on the outside of the box (flexographic printing)
  • Folds the box
  • Glues the manufacturer’s joint
Flexo folder-gluers can make straight-line cuts and scores on corrugated. They can also print, fold and glue the box. They do not do complex cuts or scores.

All scores made by the Flexo Folder-Gluer run perpendicular to any scores made by the corrugator. These scores run in the same direction as the flutes, instead of across them, and will become the body scores of a typical box.

This single machine handles a complete cycle of functions, starting with flat combined sheets with corrugated scores for the flaps, and ending with finished boxes in Knocked Down Flat (KDF) format.

Image showing a man at the flexo folder-gluer control panel, and image of a glued manufacturer's joint.
schematic showing body scores, flap scores and glue flaps

Typical box types coming off of this machine are:

RSC — Regular Slotted Container

RSC schematic schematic of a set-up RSC

HSC — Half Slotted Container

HSC schematic schematic of a set-up HSC
  • Makes boxes that have only straight-line cuts and scores, with no manufacturer’s joint
  • Uses cutting and creasing “wheels” on a rotating shaft
  • Prints on the outside of the box (flexographic printing)
  • Does not fold or glue
Printer-slotters can make straight-line cuts and scores on corrugated, and they can also print. They do not fold and glue the box, and they do not do complex cuts or scores.

The Printer-Slotter is used to finish non-glued products such as trays and telescoping boxes.

design-style tray 2d and 3d schematics
  • Makes boxes that can’t be manufactured using straight-line cutting and creasing wheels
  • Uses a cutting die, either on a flat bed or affixed to a cylinder
  • Prints on the outside of the box
  • Does not fold or glue

Die cutters are used for products that require:

  • More complex cuts which may include angles, rounded edges, tabs, slots and cutouts
  • Scores that allow for folding into shapes that may include angles other than 90 degrees
closeup image of a rotary cutting die
Die cutters can make complex cuts and scores, along with straight-line cuts and scores. They can also print. They do not fold and glue the box. image of a flatbed die cutter
die-cut tray schematic schematic of a set-up die-cut tray
closeup image of a rotary die cutter
  • Used for gluing die-cut items
  • Does not cut, score or print
A folder-gluer only folds and glues a corrugated item. It does not do any cutting, scoring or printing.

Any corrugated products that have been cut and scored on a die cutter and also have a glued manufacturer’s joint typically travel flat from the die cutter to a second converting phase on the Folder-Gluer.

specialty gluing item schematic showing manufacturer's joint closeup image of glue being applied
image of a folder-gluer

Labelers — affix high-quality lithographic labels onto combined sheets

Litho-Laminators — laminate full-size preprinted sheets onto singleface corrugated board

Curtain Coaters — apply a thin film of polymer-wax to the surface of the combined board to serve as a moisture barrier

Wax Cascaders — thoroughly saturate the combined board with a protective wax moisture barrier

Stitchers — staples a manufacturer’s joint to enhance its strength

image collage of labeler, litho-laminator, wax cascader and stitched joint

Preparing to Leave the Plant

Unit loads move on to strapping (and sometimes wrapping and palletizing, depending on the customer’s request). The shipping department takes the loads from there, and they’re out the door.

bundles of finished boxes ready for unitizing bundles of boxes being assembled into a unit load
unit load of boxes on a conveyor
adding dunnage to a unit load before strapping unit loads going through a strapper
finished unit loads ready for shipping