Warping, Bowing and Cupping of Moulding Batches

Expert input on what could make fresh runs of moulding move in unwanted ways. October 2, 2005

I run a lot of mouldings, such as crown, flute, base, and s-4-s. These are used on products like fireplace mantels/enclosures. Lately, I've run a lot to stock future orders and have noticed some bowing and cupping in the pieces (8' to 14' in length). Is there a better way to store these parts to prevent warp, especially the cupping that seems to cause the most difficulty in assembly?

Forum Responses
From contributor G:
Has your lumber been properly kiln dried? Are you checking end grain for area sawed from log? Has your lumber been sawed from trees that were leaning or from big limbs? There is much to consider. How about a few more facts?

From contributor R:
Is your product pine or very soft wood? If so, try stacking about 6-10 pieces in a bundle and shrink wrap them. This will help until you are ready to use them.

From Dave Rankin, forum technical advisor:
How is the wood dried? If the wood comes out of the moulder with bow, then it is most likely a drying problem. If it bows within a couple of days, still a drying problem. If it is a storing problem, consider a few things:
1. Humidity control.
2. Temperature range control.

3. Manner of stacking.
4. Wood specie.

From contributor F:
When I buy commercial lumber, I have no idea how much lean there was to the tree, but I do know by the way the lumber behaves going through the saw if it was dried improperly and has case hardening.

If it is casehardened, there isn't any cure. If the lumber saws okay and comes out of the molder still flat, do as suggested and tightly shrink wrap the pieces in a bundle. I have found that there is usually a way to nest pieces of the same profile together by flipping every other one end for end and face for face, etc.

Bundling them tightly together prevents air from freely circulating on them and they will stay flatter (especially flat s4s jamb stock and the like). Also, store the bundles on a flat, straight surface. If I wasn't quite certain a molding would be sold soon, I wouldn't stock pile it, as over time the moldings will degrade as far as flatness and straightness.

From contributor J:
We had to install a humidifier to solve that problem in our shop.

From contributor G:
Case hardening is a condition caused by drying lumber too fast. The surface dries too fast and does not allow the core to dry. Would probably fail oven dry or moisture test before it got to the consumer. This condition can usually be corrected by relieving the lumber out of the kiln for 30 days or so and putting it back in the kiln to finish drying.

Get a moisture meter and check the MC of your lumber when you receive it. If it is not between 6 to 8 percent, send it back. 6% is best. To keep lumber at 6% in your shop, you should maintain 38% ERH.

I think your problem is the lumber you are buying. It's either from poor logs or improper drying. Air dried lumber would not be suitable unless you live in the desert where the RH is very low.

From contributor F:
Casehardened lumber has sure slipped its way into my shop a few times, usually in maple that wasn't properly conditioned at the end of the drying cycle. I am not in the kiln drying business, so I can't fix it. When I said there was no cure, I meant after you have made molding with casehardened lumber and it cups.

I think that having to live in the desert to use air dried stock is false. I live in Oregon and use it all the time and it is very undesertlike here. I also did a lot of work in California with air dried stock without any problems. I think if I built a piece of furniture with stock that was air dried in Oregon and shipped it to someone who lived in the desert, I would then have a problem.

From Professor Gene Wengert:
There is some misinformation in the previous postings about casehardening (also called drying stress). First, it will, if present, cause immediate warp when machining. Second, it is pretty much a natural event when drying, so the final step in almost all drying is to relieve the stress. This is done by adding water back to the surface of the lumber at high temperatures (160 F or higher). In practice, usually the lumber is steamed for a few hours. Drying stress will not disappear with time. It cannot be relieved by letting the lumber sit around in a humid condition.

There are two types of casehardening - longitudinal (lengthwise), which causes pieces to warp lengthwise, and transverse (across the grain), which causes pieces to cup.

The archives here contain more info on the process of relieving the stress, but it is done in the kiln at the end of drying and not just before the lumber is planed or used.

Wood will warp (after drying) for only one reason and that reason is a change in MC. (One exception is drying stress or casehardening, mentioned above.) So, if you are experiencing warp a few hours to a few days later, then the wood was not at the correct MC when it was molded. Almost always, the wood is too wet for the air in which it is used. (Note: Tests made after the warp will always indicate dry MC. Measure the pieces before they warp to get the wet readings.)

Note: Do not wrap the lumber when you first get it as that will prevent the lumber from drying out to the correct MC. Keep your shop at about 35% RH in the wintertime. Once the pieces are run, do not wrap them, as they will potentially be too wet. If wrapped and then unwrapped at the customer's location, they will dry at that point and you will hear complaints. Let them dry in your shop so that you only ship good pieces.

Basically, you need to either get your supplier to dry the wood properly, put in your own kiln, or change suppliers to one who is doing it correctly.

Note: In a few rare cases where the wood is stored poorly after drying, it can pick up moisture and then warp later. This event would be evidenced by having the core MC, measured with a pin meter, being lower than the shell.

From contributor G:
Thanks Dr. Gene. It's always good to hear your expert advice. Looks like I have missed something in the case hardening department.

From contributor F:
Just to be clear, when I speak of shrink wrapping moldings, I am talking about securing a bundle of molding together with a wrap at each end (not covering the end grain) and one in the center,

just as a fastening device, not smothering the entire molding.