Folding boxboard or grey-back duplex?
Both fold, both print, both come off the same converting lines. The difference is what sits in the middle — and that is what decides whether the carton holds its shape when the contents are cold and wet.
What is actually different
Folding boxboard is built from virgin pulp. The outer plies are bleached chemical pulp for surface and strength; the middle is mechanical pulp, which is bulky and light. That middle is why FBB is stiff for its weight — you are paying for air held in place by fibre.
Grey-back duplex is built from recovered fibre with a coated white top. The reverse stays grey because it is not lined. It is heavier than FBB at the same stiffness, and cheaper per tonne.
Neither is a better board. They are answers to different questions.
The three questions that decide it
Does the reverse show? If the carton is opened in front of the customer, or the inside faces the shelf, the grey reverse is visible. White-back duplex exists for exactly this, at a price between the two.
How wet does it get? Condensation on a chilled or frozen pack softens board. Virgin fibre recovers better from a wet-and-dry cycle than recovered fibre does. This is the reason frozen food cartons drift towards FBB even when cost pushes the other way.
What is the print? Fine screen work and large solids show more on a coated virgin top. For a two-colour carton in a case that is never merchandised, duplex is usually the sensible answer.
Where people get it wrong
The common mistake is specifying by grammage alone. Two boards at 300 g/m² can differ by a third in caliper, and caliper is what the creasing rule is set to. Ask for grammage and caliper, and tell the mill which one is the constraint.
The second mistake is assuming grey-back always costs less delivered. At the stiffness a frozen pack needs, duplex has to be run heavier — and freight is charged by weight.
Questions
Is ivory board the same as folding boxboard?
In most trade conversations, yes. Ivory board, FBB and GC1/GC2 all describe virgin folding boxboard; the letters describe how the top is finished. Confirm the construction rather than the name.
Can duplex board be used for frozen food cartons?
It is used for them. Whether it works for your pack depends on how much condensation the carton sees and how long it sits in it. Send the pack format and the cold chain and it can be specified properly.
What weights do you supply these in?
It depends on the mill and the run. Send the specification, the quantity and the discharge port, and it is confirmed with the quotation.