The logic of combi
Each layer of plywood has a different job. Face layers are seen, worn, painted and screwed: they need hardness and clean grain. The core is invisible: that is where weight and price are saved. Combi plywood assigns the two jobs to two species — birch to the faces, poplar to the core.
In Turkey this is not a preference but a consequence of geography. Poplar grows on the Sakarya, Düzce and Kocaeli plains on a 10–15 year rotation and is peeled on local lines; there is no import leg. Birch does not grow at industrial scale in Turkey and is imported in every case — from Russia or Scandinavia. Combining the two in one panel means using the imported material only in the layer where it is needed.
Three builds, three birch shares
| Build | Ply sequence (9 plies, ~15 mm) | Birch share | Where it is used |
|---|---|---|---|
| Face combi | birch / poplar × 7 / birch | ~22% | furniture, packaging, interior panels |
| Reinforced | birch / birch / poplar × 5 / birch / birch | ~45% | door panels, light formwork |
| Alternating | birch and poplar in turn | ~50% | balance of strength and weight |
As the share changes, two things change with it: cost per m³ and the panel’s bending strength. A birch ply directly under the face stiffens the panel noticeably compared with face-only combi — the outer layers do most of the work under load.
How a veneer lot splits across the layers
The standard purchase for combi is a mixed 1–4 lot. The split happens by itself:
- Grade 1–2 — 22–27% of the lot — to the face layers. Clean grain, small knots, paintable surface.
- Grade 3–4 — to the crossbands directly under the face. Invisible, working only for strength.
- Poplar — from your own line — to the core.
This is why a mixed lot is usually more economical for a combi producer than buying grades separately: every part of the lot has a layer to go to. Buying grade 1–2 separately makes sense for plants that make faces only and build the crossband from poplar as well. The grade definitions themselves: grades.
Two veneers, one press: what to watch
Moisture. Birch ships at 5–8%. If your own poplar leaves the dryer at 8–10%, the gap means stress in the press — the layers move at different rates and the glue line is loaded. Two remedies: bring the poplar closer to the birch, or let the birch rest for a few days in the same environment as the poplar before pressing. Why 5–8% and neither lower nor higher: the moisture page.
Thickness tolerance. Veneer from two different lines goes into the same sandwich; deviations add up. Birch is 1.5 mm nominal, ±0.1. Pressing a birch sample onto your own poplar core and measuring the panel thickness is the cheapest check before ordering.
Density difference. Birch is 620–680 kg/m³, poplar 400–550. Under the same press pressure the two species compress differently; the press recipe usually settles after a few trial panels. That is another reason for a single trial container on the first order.
How much panel one container makes
A cubic metre of plywood takes about 1.1 m³ of raw veneer; the difference is pressing and sanding. Birch share:
| Build | Birch per m³ of panel | Panel from a 33 m³ container |
|---|---|---|
| Face combi (22%) | ~0.25 m³ | ~130 m³ |
| Reinforced (45%) | ~0.50 m³ | ~65 m³ |
Format changes how many m³ fit a container: 38 m³ of 1600 × 1600 against 33 m³ of 2600 × 1300. The trim calculation for your own panel size is on the container loading page, and the current list is on the price list.
Where combi is enough, and where it is not
Enough: furniture carcasses, door panels, packaging and pallets, interior wall panels, light formwork. The common thread: the surface must be hard and clean, but the panel is not fully structural.
Not enough: heavy formwork, load-bearing floors, screwed joints that are taken apart repeatedly. There the core must be birch too — a poplar core lowers screw holding and bending strength. Formwork has its own page: veneer for formwork plywood. The comparison of the two species themselves: birch or poplar.
How to start
A combi producer tests a sample differently: instead of examining the birch on its own, press it as a face onto your own poplar core. Three things to look for: whether a moisture gap shows at the glue line, whether the stacked thickness lands on the target panel, and where the tone of the pressed surface falls. Samples are free and go by courier.
Then a single trial container, 33 m³. The clauses to fix in the contract and the checks on arrival: first order.