Laser cutting wood creates some very beautiful effects. One of the beauties of wood is its grain, and knots are a part of these growth patterns, but knots can present some production challenges too.
What is a knot?
Knots are found at the bases of side branches in trees. Lower branches often die. As the girth of a tree expands, the trunk envelopes them, forming the imperfections we know as knots.
Beautiful as these imperfections are, the wood in those areas is much denser than the surrounding wood. It’s this difference in density that can cause issues.
How do knots affect laser cutting?
Denser wood is harder to cut and needs a slower cutting speed to cut through cleanly. If I cut 3mm ply at my usual speed, this is fine for most of the sheet of ply, but if the laser beam hits a knot, the chances are it will be going too fast to cut through the knot effectively. This is clearly shown by the 9mm ply ampersand at the top. Two knots prevented a clean cut through to allow the middle piece to fall out cleanly.
This means that the cut through won’t be clean in the area of the knot, and the item won’t separate from the sheet and won’t be of sufficient quality to be sold. If the wood is solid like oak, you can see where all the knots are. In plywood, however, there are knots in the middle layers that you can’t see. You can see the star below made of 6mm ply had one of these, and the knot caused the telltale puff of black dirt on the surface of the wood that can be sanded off.
As a rule, the more knots there are in a piece of wood, the higher you can expect the failure rate of laser cut items to be.
How do they affect laser engraving?
Knots don’t cause so many problems with engraving. You can expect to see any engraving over them to be shallower than on the rest of the wood. Knots are denser so engraving depth is compromised, but the effect is still easily seen. The knot shown above is under the n and t of adventure,
If the artwork is vectorised, it’s possible that the sections over the knot can be engraved more times to achieve more depth to compensate.
Do you have a questions about laser cutting or engraving and how it might affect your project? Contact us with your questions and I’ll write a blog about it.