Vacuum Resin Infusion

Shop Notes Blog

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 downsides 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 less ideal for 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 has changed that to some degree.

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). However, as I’ll explain later, density and hardness are not the same thing.

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.

For the resin to cure, it has to be heated to a temperature of 200 degrees for a minimum of an hour. That means that the inside of the billet has to reach this temperature and stay there. If it cools too soon or if the temperature fluctuates too low, the curing process is aborted and cannot be restarted. If that happens, you have to chuck out the billet and start again, so it is very important to get it right.

Now, I found out the hard way that if you put a longish piece of figured wood in an oven it does not behave itself! The first piece I did as an experiment warped to an absurd degree. I managed to salvage it, but it made it very obvious to me that I couldn’t just toss pieces of resin-soaked wood into the oven and hope for the best. Precautions are needed.

Normally, the curing process requires the wood to be wrapped in aluminum foil to prevent the resin bonding to the oven or whatever else it comes into contact with. So the billets come out of the resin bath, get wrapped nice and snug with aluminum foil, and then they are clamped between sections of square aluminum tube stock. These are hollow, rectangular bars of metal tubing that are very strong but quite lightweight. The billet is sandwiched from four directions between these bars and clamped. This was the method I used for a long time to do the infusion process, utilizing square billets of wood, and clamping them worked pretty well. I’ve since discovered that sections of wood that are already turned round behave themselves better (having already had some of their grain tension released) and do not require clamping, especially if the pieces are not too long.

Once the oven is hot, it usually takes several hours for the cure process to work, providing the temperature is maintained properly. Once cured, the billets are removed and unwrapped and what had been pieces of soft, porous or otherwise unstable wood are now heavier and considerably more stable.

Stability is the magic reward for all of this. A piece of wood that has been infused with resin is now going to be highly stable compared to an untreated piece. It won’t shift as much with changes in temperature and humidity and its workability is enhanced. Anyone who has worked with highly figured woods or any kind of burl will tell you that it can be tricky, and great care is needed to avoid tear-out (chunks of wood being ripped away unexpectedly). Once the wood has been stabilized, these tendencies are greatly reduced (but not eliminated). It is worth observing that while resin infusion increases wood density dramatically, it does not increase hardness in proportion. Curly Redwood (as an example) is a fragile wood that breaks easily. Being stabilized with resin makes it somewhat less fragile, but even though it makes it weigh as much as a hardwood, it does not impart a hardwoods resistance to stress and impact. It still needs to be handled with care, but many of its weaknesses are ameliorated to such a degree that what was once a beautiful wood that was not well suited to being a flute, can now be used in certain applications.

When I was researching the benefits of resin infusion, I kept asking myself why more flute makers weren’t doing it. It seemed like a great solution to all sorts of difficulties, but now I know the answer.

I’m delighted that I have this tool at my disposal because it allows me to to utilize many woods that would not, in other circumstances, be ideal. It allows me to make flutes in ways that would be impossible (or ill-advised) without the stabilization process. BUT…

It is expensive and it is messy, and the learning curve for mastering the equipment and the process is pretty steep. I have run into one obstacle after another, and I’ve had to invest more money and more time than I had imagined would be necessary. I suspect that this is part of the reason why few flute makers try it. I wouldn’t be surprised to find out that some other makers have experimented with this, but I would also not be surprised to find out that they gave up :-). I have to have a small barn dedicated to the operation because there is no way to manage it in a tidy fashion. Resin will be spilled. Pump oil will gush and dribble on the floor from time to time, and you go through a lot of pump oil because it becomes contaminated with moisture from the vacuum process, obliging it to be changed every single time a batch is done. And there is a distinct odor during the infusion process, so having this dedicated space is key.

Also, the resin is really expensive and some of the woods absorb quite a lot of it. I actually think that the price of the resin is quite reasonable, but the man who markets it targets craftspeople who are making small items like pen blanks or knife handles. Such an item would use a very modest amount of resin. A nice, porous flute billet that is about 1.5”x1.5”x30” is going to drink up about a quart of the stuff at $90 per gallon! And the equipment itself is costly and the process takes a lot of patience. In addition to all of this, there are not really that many woods that are well suited to the process. Oily tropical woods are completely unsuitable and the vacuum process can actually damage them. Many woods have a grain structure that does not lend itself to absorbing the resin, so the stabilization process only works marginally well. As I mentioned, some soft porous woods absorb resin well, but they do not have the inherent hardness or strength to make them as desireable in certain applications. In the end, the wood that I use the most of is hard maple. Maple is already a hard, strong wood, but it also absorbs resin beautifully. So it is the undisputed champion. I also use it on woods like black walnut, cherry, boxwood, hawthorne, and curly douglas fir. None of these woods absorb resin like maple does, but even if they only increase their density by 20-30%, that is enough when combined with the actual process of baking the wood to render them far more stable. I don’t use a lot of burl woods, though they are a favorite choice for stabilzing among woodworkers (but not necessarily a great choice for flutes).

In truth, I didn’t know what I was getting into when I started this, and if I’m being really honest I wouldn’t necessarily recommend it to my flute making friends without also making some qualifications. However, now that I have the set up, I’m finding that it brings a whole other dimension to my craft and for my purposes it has proven worthwhile.

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Published Friday, October 26, 2018