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?
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?
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.
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.
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.
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.