Vacuum resin infusion: part 1
So what is vacuum resin infusion? Good question. A couple of years ago I had never heard of it, at least not in relation to flute making. I became familiar with vacuum chambers at the time because I was researching methods for speeding up the drying of wood. Any flute maker will tell you that the acquisition and storage of wood for flute making is a big part of the job. One can buy kiln dried wood, which is very nice, but often the nicest pieces of wood are sourced in such a way that they have never been through any commercial drying process and they need a considerable amount of time to settle and dry before they can be used. With some of those oily, dense tropical hardwoods it can take many years for this to happen naturally.
I had heard of the use of vacuum chambers for speeding up this process. These chambers can pull a fairly powerful vacuum that literally sucks the moisture out of the wood by accelerating the natural drying process. So that what might take months and years of drying can be done in days.
It sounds like magic, but there are down sides to this process. If done on very dense woods that are still quite green it can cause “checking”, which is another word for “cracking”. This is a danger with woods that air dry as well, but highly accelerated drying can exacerbate this. I’ve already had some experience with this phenomena. However, it is a small price to pay for the ability to make flute timbers available sooner rather than later, especially for a maker who is still in the process of building and organizing stock that is meant for use years down the road.
So I acquired a vacuum chamber initially in order to dry wood. But along the way I learned about how these chambers (if fitted correctly) can be used for something called resin infusion.
Another thing that any flute maker can tell you is that wood is not stable. Because it is an organic substance, even when it is dry it will shift, shrink and bend. When it is cut and shaped, inner grain tension is released and it can get pretty mobile. It will also shrink and swell in response to moisture, depending upon how well it is sealed. In essence, it can be a tricky thing to work with, especially when you are making a flute with joints. Flute makers who craft these types of instruments have learned how to season wood and how to work it in stages, letting it settle between stages so that this tension can be released along the way. This makes for a more stable finished product.
Despite these precautions, it is common for some types of woods to warp and shrink over time. Many of the woods favored by makers are chosen because of their inherent stability, their resistance to moisture and their workability. There are many woods that are truly beautiful but completely unsuited to making flutes. Many highly figured woods, burls and softwoods fall into this category. Some of my favorite woods for sheer beauty are Curly Redwood and Redwood Burl, but there are many others.
These particular woods are light, porous and fragile. They are difficult to work and highly susceptible to warping, so they are rarely used for flute making, especially among the makers of European style woodwinds.
Vacuum resin infusion changes all of that.
How it works is thus: billets of the wood are placed into a deep metal tray inside of the vacuum chamber. The pump is activated and the vacuum is pulled, sucking the air and any remaining moisture from the wood. This vacuum is left in place for a couple of days, typically, but longer if the wood is not completely dry. After the wood is totally free of moisture and air, a liquid resin is introduced. The chamber that I have comes equipped with injection ports that allow a liquid to be fed into the chamber while still under vacuum. This is a tricky business. I use a type of resin called Cactus Juice, which requires a heat cure (more on that later) but it has the virtue of being designed to penetrate the wood very effectively.
So the liquid resin is fed into
the chamber until the wood in the tray is totally submerged. Once that
is done, the feed line is clamped off and the vacuum is released.
Most of us have heard the phrase “nature abhors a vacuum”. This is very true. The billets of wood within the chamber have been forcibly emptied of air and moisture and then submerged in liquid. What happens when the vacuum is released is that they try to fill back up with air. But they can’t because they are submerged, so instead they drink the resin. They fill every air space inside with this liquid resin until they are saturated. You’d be surprised how much air space there is inside of a piece of dry curly redwood! I’ve put in a billet weighing 1lb. in its dry state and when I’ve taken it from the bath it weights 3.25lbs! Curly Redwood is one of the softest, lightest woods out there, and when full of resin it weighs as much as Bubinga (a very dense tropical hardwood).
At this point the process is only partly complete. The resin has filled the wood, but it is uncured (still liquid). It takes heat to harden the resin, so the wood has to be prepared for the cure oven.
Continued on next page…