A Realistic Look at Hand Lay-Up Versus Vacuum Infusion
A lot of marketing effort goes into discussing how different fiberglass yachts get built. Whether hand lay-up is as good as other methods or inferior to them, and whether the latest techniques in “vacuum bagging” and “infusion” yield a much stronger, more durable product.
As you might expect, the facts are more complicated than many marketing copywriters would have you believe — but not so complicated that average guys like you and me can’t understand the issues.
Execution Is King
The basic fact to keep in mind is that the choice of a laminating technique is not, in and by itself, as important as the ultimately achieved quality of the manufactured product.
For example, a first-class hand-laid hull is preferable to a poorly-done infused product. That doesn’t mean vacuum infusion isn’t desirable, only that field results vary widely from boatyard to boatyard, and that you should not pre-judge which product is likely to be better based solely on which lay-up technique is used.
Yachts are self-propelled mobile structures that require motive power. Because greater weight generally requires increased motive power, one of the goals in FRP laminates is to optimize strength-to-weight ratio. This usually involves maximizing in the final product the ratio of contained reinforcing fabric to surrounding resin polymer matrix. Generally, the optimum ratio of reinforcing fabric to resin polymer is considered to be 70/30 by finished (post-cured) weight.
Hand lay-up involves placing pieces of precut reinforcing fabric into the female tool (or mold) and saturating them with catalyzed or co-reacted liquid polymer — polyester, vinyl ester, or epoxy resin.
Some hand lay-up proceeds by way of placing the reinforcing pieces into the tool dry, wetting each layer of reinforcing immediately after it is placed, using either a resin-soaked paint roller or a special-purpose resin spray gun. Sometimes pre-cut pieces of reinforcing fabric are pre-wet by soaking them in a resin bath, then run through adjustable pinch rollers to squeeze out excess resin, and placed wet, layer by layer into the mold.
As each layer of material is placed and wet out, it is compacted to the layer that preceded it, using squeegees and ridged aluminum rollers that work to force resin throughout the layer of reinforcing and squeeze excess resin to the top surface of the laminate, where it is scooped up by various methods and disposed of.
As you might expect, hand lay-up is highly labor-intensive. And about the best reinforcing-to-resin ratio you can achieve with it is, on average, 60/40. However, when executed by a top-rate, experienced team, hand lay-up is not the firemen’s drill many think it is — with resin and reinforcing materials flying all around and half-crazed laminators scurrying about helter-skelter in resin-soaked coveralls, gobs of wet glass fabric sticking to their shoes and other clothing.
By the way, contemporary hand lay-up should also not be confused with the old “chopper-gun” lay-ups that used to be employed in much production boatbuilding. The old chopper-gun laminates were built up quickly using a resin gun that simultaneously sprayed catalyzed resin while chopping up glass filaments and feeding them into the gun’s resin stream on the run. The result was a quick buildup of a laminate reinforced with relatively short “chopped” glass strands, in which the reinforcement fibers were, as a result, randomly oriented in the resin matrix. It was fast. It was cheap. And it was pretty much crap — significantly inferior in terms of strength to today’s almost universal laminates of long, continuous-filament, unidirectional and bi-axial fabric reinforcements.
When performed by a trained and experienced crew, today’s hand lay-up can be exceptionally well organized and orderly. And it can produce a high-quality product, albeit one that, all other factors equal, will be a bit heavier for a given strength than a laminate laid up using vacuum infusion techniques.
Vacuum Infused Lay-Up
In vacuum infusion, pre-cut pieces of reinforcing fabric are placed dry one after another in a ”stack” in the mold. Once all the dry layers in the stack are placed, the entire stack is saturated in one fell swoop with resin by drawing the resin through the reinforcing by means of a vacuum pump or pumps.
During infusion, the fabric stack is sealed over with air-tight heavy-duty polyethylene sheeting, So, as the vacuum draws the resin through the stack of reinforcing materials, the vacuum sucking on the plastic covering sheet also compacts the various fabric layers tightly together. If properly and well executed, infused lay-up achieves an optimized reinforcing-to-resin ratio of about 70/30. The result is a very strong structure at minimum weight.
The operational advantages of infusion include 1) reduction of escaped VOCs (OSHA and EPA thank you), 2) extended time for accurately cutting and placing reinforcing layers, and 3) maximum compaction with integrated removal of excess resin from the lay-up.
The downsides of infusion include the possibility of a failure to fully infuse due to a rupture of the bag seal and a resultant loss of vacuum during the process. They also include the possibility of an insufficiently complete wet-out of the reinforcing material that leaves dry spots in the laminate which must be cut out and patched post-infusion.
As well, infusion is generally more expensive than hand lay-up. Although the labor in vacuum infusion might be less, the materials and consumable items associated with the process are relatively quite expensive.
Busting a Common Myth
A commonly misconception abounds that hand-laid laminates are necessarily inferior in strength to infused laminates. Which is poppycock..
Provided all other factors are held equal — namely, that resins with equivalent post-cure mechanical properties are used in both cases, and very similar types, quantities, and placement of reinforcing materials are employed — a hand-laid laminate will be very close in strength to that of its vacuum infused version.
Whoa, you say, how can that be? Why do so many builders extol the virtues of their vacuum infused laminates?
The confusion enters because in traditional engineering the “strength” of a material is usually expressed in terms of unit load carried per square unit of cross-sectional area (for example, in pounds per square inch).
That works well for homogeneous materials such as steel and aluminum. But fiberglass laminate is a composite material made up of glass or other reinforcing fiber encased in a polymer resin matrix. So understanding the comparative strengths of various different laminates is a lot more complicated. Let me explain.
The absolute tensile and bending strengths of an FRP laminate depends almost entirely on the strength, quantity, and type of the contained glass or other fibers and their orientation in the cured final product. So if two almost identical hulls are laid up, one (call it Hull A) using hand-lay techniques and the other (call it Hull B) using vacuum infusion procedures, with identical configurations of fiber reinforcing, hull A will end up with more resin in its cured laminate than hull B. Consequently, hull A will be not only a bit heavier, but somewhat thicker.
However, since we’re assuming, for sake of explanation, that the absolute amount, type, and arrangement of reinforcing is the same in both hulls, and that, therefore, the total load each hull respectively can carry at failure is the same, the thicker hull (the hand-laid hull A) will carry less load at failure per unit of cross-sectional area. But since A has more cross-sectional area (because it is thicker) than B, the two end up having the same strength when considered on an absolute basis.
So then, you ask, what is the big deal and why all the touting of vacuum infusion over hand lay-up? The big deal is that good infused laminates generally achieve the same strength at less weight than an equally good hand lay-up. And in yachts, you will remember, weight reduction almost always makes a difference to performance and range.
The other plus on the infusion side of the ledger is that vacuum infused laminates commonly exhibit more uniformity and predictability than hand-laid laminates, in terms of their post-cured mechanical properties. And uniformity and predictability are key elements in being able to more accurately engineer the structures built up of those laminates.