Materials board GSM duplex SBS
Choosing board and GSM for your packaging box
Choose the board type by what the pack has to do — duplex where only the outside prints, SBS where the inside is seen or colour has to be exact, kraft where a natural look matters more than colour range, corrugated where the box ships on its own — and then choose the weight by proving rigidity on a physical sample rather than by picking a number. Board is the largest single input into the cost of most cartons, it sets what the print can look like, and it decides whether the box arrives intact. It deserves more attention than it usually gets, and this post is what we tell clients when they ask.
Start with what the pack has to do, not with a board name
Four questions settle most of it. Will anyone see the inside of the box? Does the product need protecting from crushing, or only from dust and handling? Does the pack ship on its own, or inside another case? And does it touch food, or have to satisfy a regulator? The answers point to a family of board before any weight or brand comes into it.
The mistake to avoid is choosing a board because it sounds good. “Art card” and “imported board” are not specifications. Coated white one side or both, virgin or recycled fibre, single-ply or fluted — those are.
Duplex: the retail workhorse
Duplex board is coated white on the printing side and grey on the reverse, because the reverse is recycled fibre. That grey back is what makes it economical, and for the very large number of cartons where nobody ever looks inside, the saving costs you nothing. The coated face prints well, holds a fine screen and takes every normal finish.
Its limit is the inside. If the carton will be opened and looked into, the grey shows; if you need to print inside, colour on a grey base looks muddy. For pharmaceutical and food secondary packaging where the inner face is never seen, and for most retail cartons, duplex is the right default.
SBS: white all the way through
SBS — solid bleached sulphate — is made from bleached virgin pulp, so it is white throughout. Print the inside and it looks as good as the outside; print a pale tint and it stays clean instead of picking up a grey cast; print a white and it is genuinely white.
It costs meaningfully more than duplex at the same weight. It earns that where the interior is visible — gift boxes, display cartons, anything with a large window — where the brand sits at a price point the pack has to justify, and where virgin fibre is preferred for a food or pharmaceutical application. On a long run of a pack whose inside nobody sees, it buys nothing.
Kraft: strong, natural, and a different printing job
Kraft is uncoated brown board with unusual strength for its weight and the natural look a lot of brands want. The catch is that an uncoated brown surface changes what printing can do: ink soaks in rather than sitting on top, light tones vanish into the board, and full-colour CMYK comes out muted and shifted.
Designed for kraft — one or two spot colours, solid line work, generous negative space, perhaps foil — it looks excellent. A full-colour design moved onto kraft to save money almost never does. White kraft exists as a middle path: the strength and texture, with a white base to print on.
Corrugated: when the box ships on its own
Corrugated board is a fluted layer glued between flat liners, and the arches of the flute are what give it crush resistance and cushioning at very little weight. It is the material for anything that travels through a courier network without an outer case — mailers, shippers, subscription boxes — and for heavy or fragile products in any channel.
The flute is the specification that matters. E flute is thin, prints well and folds neatly — the usual choice for branded mailers. B flute is taller, stronger and coarser, for heavier contents and rougher handling. F flute is finer than E, for small premium boxes where print quality outranks protection. Double-wall constructions exist for transit cases where protection is everything. Flute direction matters too: the board is much stronger along the flutes than across them, and how the blank is laid out on the sheet decides how the box stacks.
GSM and caliper: weight is not thickness
Board is specified by GSM — grams per square metre — and higher generally means heavier and stiffer. As rough orientation, folding cartons typically run from around 250 gsm for small light packs up past 400 for large or heavy ones; corrugated is described by flute and liner weights rather than a single figure.
But GSM measures weight, not thickness, and two boards at the same GSM can differ noticeably in caliper because one is denser than the other. Thickness is what the machinery interacts with: crease width, folder-gluer gap and the internal dimensions of the finished box all derive from it. When rigidity is the question, ask for the caliper as well as the GSM, and then — this is the part that actually settles it — hold a sample.
Going up a grade “to be safe” is one of the commonest ways to overpay. Heavier board costs more per sheet, adds shipping weight on every unit for the life of the product, needs wider creases, and past a point folds worse and cracks more. The right weight is the lightest one that is rigid enough, and a mock-up in two candidate weights answers that in a minute.
How caliper changes the dieline
This is the part that catches people who download a template and use it for whatever board they happen to order. A dieline is drawn for a board thickness. The board takes up space at every fold, so the internal dimensions of the finished box differ from the flat drawing by an amount that depends on caliper, and the crease width is set from caliper too.
Use a dieline drawn for 350 micron board at 500 microns and the creases are too narrow, the corners bind, and the inside is smaller than intended — sometimes enough that the product no longer fits. The drawing looks correct; the box is not. So specify the board before the dieline is finalised, and if the board changes later, expect the dieline to be redrawn. Where the die has already been cut, that is a new tool. Our dieline templates state the caliper they were drawn for, for exactly this reason.
Food contact and pharmaceutical work
Two applications add requirements on top of the structural ones.
For food, the question is how the food touches the pack: directly, through a liner, or not at all. Direct contact needs board, coating and ink specified for it — grease-resistant or barrier-coated stock where the product is oily or wet, and ink systems suited to the contact type. Food-contact compliance is the brand owner’s responsibility; what the printer does is specify the materials to the requirement given and document what went into the job. Our food packaging page sets out how we handle that.
For pharmaceutical cartons, the board has to do three extra things: carry small regulatory type legibly, present a surface on which a 2D code will grade, and — for virgin-fibre preferences common in the sector — avoid the questions recycled content raises. SBS is the usual answer, and the code area is kept clear of varnish and specified as part of the artwork rather than left to finishing.
A decision table
| Application | Start with | Because |
|---|---|---|
| Retail carton, inside never seen | Duplex | Economical; the grey back costs nothing here |
| Gift or display box, inside visible | SBS | White throughout, prints inside and out |
| Cosmetics or fragrance carton | SBS | Clean tints, exact colour, premium base for finishes |
| Pharmaceutical mono carton | SBS | Legible small type, a code-grading surface, virgin fibre |
| Natural or eco-positioned pack | Kraft, one or two spot colours | The look is the point; design for the board |
| E-commerce mailer | E flute corrugated | Prints well, folds neatly, survives a courier |
| Heavy product or rough route | B flute or double-wall | Crush resistance outranks print quality |
| Small premium box that ships | F flute | Finer surface for print, still protective |
| Rigid gift box | Greyboard core with a printed wrap | A different construction entirely — see below |
One construction sits outside the table. A rigid box is not folded from a printed blank at all: a greyboard tray is assembled first and a separately printed wrap is applied by hand. It is the most premium feel available and the longest lead time, and it is chosen for the experience of opening it rather than for any of the reasons above.
What custom packaging is, in the end
People search for “what is custom packaging” and get sold a template. Custom packaging is a box whose dieline was drawn to your product’s real dimensions and to the caliper of a board chosen for what the box has to do — and then proved as a physical sample before anything was printed. The board decision is where that starts. Every product page on this site lists the materials each construction can be made from, and the paper and board guide goes into each one in more depth.
Tell us the product and we will send swatches in the weights we would specify, so you can feel the difference before you decide.
