The short answer
Most cardboard starts as paper. Rolls of strong paper, made from new or recycled fiber, are fed into a machine that shapes one of them into a wavy layer and glues it between flat layers. The resulting board is cut, scored and folded into boxes. Thinner, flat cardboard such as cereal-box board is made differently, by building up layers of pulp into a single thick sheet rather than corrugating it.
What is cardboard?
“Cardboard” is an everyday word for several thick, paper-based materials. Industry uses more precise names:
- Corrugated fiberboard, with a wavy layer between flat liners, used for shipping boxes.
- Paperboard (or boxboard), a flat, thin board used for cereal boxes, cartons and packaging.
- Cardstock, heavy paper used in crafts.
To see how these relate to ordinary paper, read whether cardboard and paper are the same thing.
Raw materials
Cardboard is made from cellulose fibers, usually wood pulp or recycled paper, or a mix of both. Strong boxes depend on reasonably long fibers, so mills often use some fresh fiber alongside recycled material. The paper used in corrugated board is commonly called kraft, referring to a strong paper produced by a particular pulping process, although recycled-fiber papers are widely used too. Starch-based adhesives are common for bonding layers.
How corrugated cardboard is made, step by step
1. Make the pulp
Wood chips or recycled paper are mixed with water and broken into fibers. Dirt, plastics and other contaminants are screened out.
2. Make the paper
The pulp is spread into a thin, wet sheet, pressed to remove water, dried on heated rollers and wound onto large rolls. Different grades of paper are produced for the flat outer layers (liners) and for the wavy middle layer (the fluting or medium).
3. Feed the rolls into the corrugator
The rolls are mounted at the start of a long machine called a corrugator. Paper is unwound and fed through the machine at speed.
4. Shape the wavy layer (fluting)
The middle paper is heated, often with steam, to make it flexible and is then pressed between corrugated rollers with ridged teeth. This crimps it into a series of waves, called flutes.
5. Glue the first liner
Adhesive is applied to the tips of the flutes and the wavy layer is joined to a flat liner sheet. This two-layer combination is known as single-face board.
6. Add the second liner
Adhesive is applied to the other side of the flutes and a second flat liner is pressed on top, forming the classic sandwich shape. Heat and pressure help set the glue.
7. Dry, cool and cut
The board passes through a heated section to dry and flatten, then is cut to width and length. Some producers add a coating or sealant to improve water resistance.
Why the wave makes it strong
The flutes act like many tiny arches and braces. They add stiffness and cushioning without making the board very heavy, which is why corrugated board is both light and sturdy. Flute size varies, and finer flutes give a smoother surface for printing while larger flutes provide more cushioning.
Single wall, double wall and beyond
- Single wall board has one wavy layer between two liners.
- Double wall board has two wavy layers and three liners, for heavier loads.
- Triple wall exists for very heavy-duty uses.
What about flat cardboard?
Solid paperboard, such as the board for cereal boxes, is typically formed in layers and then pressed and dried. It has no wavy layer, so it is thinner and often used for printed cartons. It can have a white or coated outer surface so graphics print well.
From board to box
Once the corrugated sheets are made, box makers:
- Cut the board to a pattern with blades or dies.
- Score the fold lines so it bends neatly.
- Print graphics or information if needed.
- Fold and join the box with glue, tape or staples.
- Bundle and ship it flat to save space until it is assembled.
Making a box from a sheet of cardboard at home
You can make a simple open-top box from a flat piece of cardboard.
- Measure a square or rectangle, then draw a border around it the height of the walls you want.
- Mark a square at each corner of the border and cut along the lines to remove those squares, or cut slits and tabs as in a net pattern.
- Score the fold lines lightly with the back of a blade or a ruler and bone folder, without cutting through.
- Fold the sides up and secure the corners with tape or glue.
- Strengthen with an extra layer or a strip along the rim if the box will carry weight.
A box that will hold anything heavy or unusual will need more strength, so use the right board and test before loading it.
How to make a cardboard box house
For a playhouse or pretend building:
- Use a large, clean box and tape the bottom flaps well.
- An adult should cut the door and window openings with a sharp blade on a safe surface, leaving a frame of cardboard around each.
- Fold some flaps up as shutters, and use extra flaps to build a roof.
- Decorate with paper, paint and markers that are labeled non-toxic.
- Smooth any rough edges with tape.
Safety
- Cutting thick cardboard needs a sharp blade, which can slip. Use a cutting mat, cut away from your body, and never leave blades where children can reach.
- Children should only use scissors and craft knives with an adult present.
- Check that structures are stable and not blocking exits if children will play inside.
- Use glues and paints according to the label and in a ventilated space.
- Keep small scraps away from toddlers and pets.
Environmental notes
Cardboard is widely collected and recycled, and many boxes contain recycled fiber, but results depend on the product and the local recycling system. Fibers can be recycled several times before they become too short, so fresh fiber is still part of the cycle. Look at local guidelines for what is accepted. Wet, greasy or plastic-coated cardboard may not be accepted.
FAQ
What is cardboard made of? Mostly paper fibers from wood or recycled paper, bonded with adhesives.
Is cardboard strong because of glue? The glue holds the layers together, but the strength comes mostly from the wavy structure.
How is tissue different? Tissue is a much thinner and softer product, made with a creping step; see how tissue paper is made.