Duplex Board

Duplex is built in layers on a multi-cylinder board machine. The top layer is a whiter pulp that is then coated with a clay or mineral coating to give a smooth, ink-holding print surface; the middle and back layers are recycled fibre. The colour of the back gives the two common grades. Grey back duplex has a grey recycled reverse and is the cheaper, default grade. White back duplex has a lighter reverse and is used where the inside of the carton is seen or printed. In the DIN 19303 lettering, coated recycled board with a grey back is GD, and coated recycled board with a white back is GT (triplex).

Grammage is written in GSM and Indian suppliers commonly list duplex from about 200 to 450 GSM, sold in reels or sheets. Mono cartons for soap, toothpaste, tea and medicines generally use 250–400 GSM; blister backing cards use 300–450 GSM so that the card stays flat under the sealed blister. Coating is described as LWC or HWC — light or heavy weight coating — and heavier coating gives better print gloss at a higher price. Because carton erecting on a packing line depends on stiffness rather than weight, ask the supplier for stiffness and caliper as well as GSM when you change grade.

Duplex is not corrugated board. A corrugated or CFB box has a fluted core between liners, which is what carries stacking load and cushions impact; duplex has no flute and almost no compression strength. In practice the two work together: the duplex mono carton presents the product on the shelf, and a 3-ply or 5-ply corrugated shipper carries a number of mono cartons through transport. For e-commerce or heavier retail items, a printed E-flute or F-flute micro-flute box replaces duplex where the carton itself has to survive the courier.

It also differs from virgin boards. Solid bleached sulphate (SBS) is made entirely from bleached chemical pulp, is white right through and is chosen for premium cosmetics, spirits and healthcare cartons. Folding box board (FBB, grades GC1 and GC2) has a mechanical-pulp core between bleached chemical-pulp faces, giving high stiffness at low weight. Duplex is the recycled, economical option: good print on the face, adequate stiffness, lower cost per carton, and the grey reverse is the giveaway.

Two practical cautions for Indian conditions. Recycled board takes up moisture in the monsoon, so cartons can warp, lose crease memory and jam on automatic cartoners; store reels and sheets wrapped and acclimatise them before running. And for food contact, duplex made from mixed recycled fibre normally needs a barrier or an inner pack, since the back is not a food-grade surface. Corrugated board, duplex and paper sit outside the plastic EPR rules, which is one reason paper-based secondary packs remain common.