An interior door leaf is mostly air. Two thin skins are held apart by an infill core, and the core is what stops the skins touching when the door is pushed. Paper honeycomb does that job at a low weight, which is why most hollow-core interior doors use it, and why the core specification is about flatness and cell stability rather than about raw strength.
The leaf is a sandwich, and the core keeps it apart
The skins carry the bending load; the core keeps them at a constant distance so the load path stays straight. If the core crushes locally, the skins touch, the leaf goes soft in that spot and the defect shows as a ripple in the paint or veneer.
That is why the core has to be flat and evenly expanded. An uneven cell field produces a leaf that looks fine on the bench and shows a shadow line after finishing.
Specification at a glance
Door cores are normally 25-30 mm board with a small cell, cut slightly oversize so the leaf frame can be fitted after the core is placed.
| Door leaf | Core thickness | Cell | Note |
|---|
| Standard hollow core | 25-30 mm | 4-6 mm | Solid lock block required |
| Flush door, painted | 30 mm | 4 mm | Flat core, even expansion |
| Veneered leaf | 30 mm | 4 mm | No shadow line after finishing |
| Lightweight interior | 25 mm | 6 mm | Lower weight, check hardware |
| Door with glazing | 30 mm | 4 mm | Solid framing around the opening |
Where a solid insert is still needed
The core does not take a hinge screw, a lock or a latch. Those need a solid lock block and, at the hinge edge, a solid rail or a denser insert. Any design that fixes hardware directly into a honeycomb core will fail in service, and no increase in board thickness changes that.
The door edge is the other place the core cannot be exposed. The stile closes it, and the core has to be cut accurately enough that the stile bonds to the skins rather than to crushed cells.
Handling, moisture and finishing
Core board is light and easy to damage at the cut edge before it is enclosed. Keep it flat and dry in store, because a core that has taken on moisture will not lay flat under the press and the finished leaf will cup.
Where the door will be used in a humid interior, discuss a moisture-resistant facing on the core with your door line. It is not a waterproofing measure, and it should not be presented as one.
Questions buyers ask before ordering
Can a hinge be fixed directly into the core?
No. A solid lock block and a hinge rail are required; the core has no screw holding power.
What cell size gives the flattest door?
A small cell, 4-6 mm, with even expansion. Flatness comes from even cells, not from thickness alone.
Is the core fire rated?
We do not make a fire-rated claim for the core. Fire performance depends on the complete door assembly and must be established by the door manufacturer.
How is the core supplied?
Cut to the leaf size, flat, bundled and wrapped. Oversize is normal so the frame can be fitted after placement.
Can you supply cores for a specific door thickness?
Yes, thickness is made to order from 10 mm upwards, subject to the door design.
Tell us the piece size, quantity per shipment and how it is handled at mona@hengnicepacking.com and we will advise on the board build.
