Top-quality Hand Lay-up Beats Second-rate Infusion Any Day

MUCH OF THE DISCUSSION OVER INFUSION IN COMPOSITE CONSTRUCTION MAKES THE UNWARRANTED ASSUMPTION THAT NEW TECHNOLOGY IS ALWAYS BETTER

A lot of boat-related marketing these days promotes the myth that infusion techniques produce as significantly stronger and more durable laminate. Well, as you might expect, the facts are more complicated than marketing copywriters would have you believe — although not so complicated that an average guys like you or me can’t see to the heart of the matter.

The basic fact to keep in mind is that the choice of a laminating technique is not, in and by itself, as important ultimately as the 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 nominally.

Beyond that, comparisons of “strength” don’t mean much unless related to a common unit denominator. That is, strength in lbs-load / square inch of cross-sectional area (PSI) or load in lbs carried / lb of structural weight.

Since yachts are self-propelled mobile structures that require motive power, more structural weight requires increased motive power. Therefore, one of the goals in FRP laminates is to optimize strength-to-weight ratio. This usually involves maximizing the laminate’s ratio of reinforcing fabric to surrounding resin polymer matrix. The optimum ratio of reinforcing fabric to resin polymer is generally considered to be 70/30 by finished (post-cured) weight.

Hand lay-up involves placing pieces of pre-cut reinforcing fabric into the female tool (or mold) and saturating them with catalyzed or co-reacted liquid polymer — polyester, vinylester, or epoxy resin.

Some hand lay-up proceeds by way of placing the reinforcing pieces into the tool (mold) dry, then 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 tool.

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 through and through the layer(s) of reinforcing and then squeeze excess resin to the top surface of the laminate, where it is picked up and disposed of by one or more of several methods.

As you might expect, hand lay-up is quite labor-intensive. Just as important, about the best reinforcing-to-resin ratio one can achieve with hand layup is, on average, 60/40.

However, when executed by a first-rate team, hand lay-up is not the firemen’s drill many picture it to be — with resin and reinforcing materials flying all around and half-crazed laminators scurrying about, helter-skelter, like a corps of Keystone Cops in resin-soaked coveralls with 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, on the run, into the gun’s resin stream. 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.

Build-up by chopper gun was fast and cheap. The problem with it was the ultimate quality of the cured laminate depended on the skill of the chopper gun operator. Expert operator, good result. Mediocre operator, truly terrible results. Poor to bad chopper gun laminates are significantly inferior in terms of strength and durability to today’s almost universal laminates, which are cmposed of long, continuous-filament, unidirectional and bi-axial fabric reinforcements.

But leaving “chopped” laminate aside, hand-laid laminate 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.

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 reinforcing fabric “stack” is sealed over with air-tight heavy-duty polyethylene sheeting, Then, as a vacuum pump draws resin through the stack of reinforcing materials, the plastic covering sheet pulled down onto the reinsorcing material by vacuum suction also compacts the various fabric layers tightly together. If properly and well executed, infusion achieves an optimized reinforcing-to-resin ratio of about 70/30. Which results in a very strong structure at minimum weight.

The operational advantages of infusion include 1) reduction of escaped VOCs (OSHA and the EPA thank you), 2) extended time for accurately cutting and placing reinforcing layers to a precisely engineered specification, 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.

And beyond that, infusion is definitely 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. In other words, during the infusion process, a lot of onetime-use materials are thrown away.

The common misconception that hand-laid laminates are necessarily inferior in strength to infused laminates is poppycock.

Provided all other factors are held constant — 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, albeit somewhat thicker and heavier.

Then why, you ask, do so many builders extol the virtues of their vacuum infused laminates and, in particular, their greater strength.

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).

This practice works well for homogeneous materials such as steel and aluminum. But a composite material such as fiberglass laminate is made up of glass or other reinforcing fiber encased in a polymer resin matrix. So understanding the comparative strengths of various different laminates is more complicated. Let me explain.

The absolute tensile and bending strengths of an FRP laminate depend almost entirely on the strength, quantity, and type of the contained glass or other fibers, as well as 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 process, with identical configurations of fiber reinforcing, hull A will end up with more resin in its cured laminate than hull B. As a result, hull A will be not only a bit heavier, but somewhat thicker.

Since we’re assuming for sake of this discussion that the absolute amount, type, and arrangement of reinforcing are the same in both hulls A and B, we thereby also assume that the total absolute load each hull respectively can carry at failure is the same. The hand-laid hull A, however, will be thicker and thus will carry less load at failure per unit of cross-sectional area because the thicker skin has more cross-sectional area than B. But since A has more cross-sectional area (because it is thicker) than B, and the two hulls (A and B) end up having the same strength when considered on an absolute basis, hull A ends up carrying less load per unit of cross sectional area.

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, a significant weight reduction always makes a measurable 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 finely engineer the structures built up of those laminates. — Phil Friedman

Copyright © 2023 by Phil Friedman — All Rights Reserved

Author’s Note: An earlier version of this article was originally published on the author's LinkedIn channel.