Fiberglass Resin Infusion and Vacuum Bagging

Over the past few posts we have been discussing fiberglass construction methods and some of the issues and conditions that have arisen over the years as boats have begun to age and deteriorate to some degree. We discussed issues surrounding structures of both solid fiberglass and cored sandwich construction methods. We have seen how voids in can occur in laminates and how they manifest themselves into gel coat surface blisters. We have also seen how improperly installed through hull and deck penetrations have also caused problems over the years.
In this post we are going to discuss a couple of advances in fiberglass construction methods that have come into play over the years allowing boat builders to turn out increasingly higher quality structures.
The first that we will look at is a technique called “Vacuum Bagging” in which the entire layup including the gel coat and all layers of mat and roving are installed in the mold (with the resin still wet) and covered with a large plastic bag and sealed at all the edges. Then a vacuum pump is connected to the bag, (usually with multiple connections at the gunnels) the air is sucked out and the entire structure is put under a pressure of 14.7 pounds per square inch or 29.9 inches of mercury. This might not sound like much in reality the pressure equates to 2,116 pounds per square foot. The excess resin is sucked out and the part is allowed to cure under pressure forcing resin into all areas that are difficult to reach using conventional layup techniques. This greatly reduces or even eliminates the dreaded “voids” or air pockets that can occur when traditional methods are employed. The end result is a stronger, lighter component with fewer or no voids. As a result the likelihood of the part developing structural issues or surface blisters over time is greatly reduced. Also because excess and unneeded resin is forced out the laminated part becomes both lighter and stronger. In conventional layup techniques excess resin is almost always used in order to fully saturate the mat and roving. This makes the part heavy and possibly brittle if taken to extremes.
What I have described here is a simplified version of the process and in reality it is a little more involved. Because the plastic bag will stick to the wet resin as it cures a layer of release fabric must be installed below the bag and also a layer of breather fabric or mesh must be used to evenly distribute the pressure applied by the vacuum pump. Also when working with cored laminates this process may be repeated twice, once to squeeze the core into the exterior laminate and again with the inner laminate layers. The downside to all of this is that the bag, breather and release fabrics cannot be re-used and as a result considerable amount of waste is generated.
The next method of modern fiberglass construction that we will discuss is an evolution of the vacuum bag technique that has become known throughout the industry as “Resin Infusion”. The difference here, is that after the gel coat is sprayed into the mold and allowed to cure to a tacky state all of the exterior layers of mat and roving, the core material (if used) and the inner laminate layers are then installed into the mold dry, that is with no resin applied to any of the laminate layers. Only a light bonding adhesive is utilized to secure placement of the materials in the mold. At this point the only resin anywhere in the layup is the gel coat, which has already been allowed to cure to a tacky state. As with vacuum bagging the bag, breather and release fabrics are installed over the dry laminate and the vacuum pump is connected with a manifold type of affair at the perimeter of the layup usually, in the case of a boat hull at the gunnels. Then an additional set of tubes are installed in the bag at the center of the component, again with a boat hull at the keelson area. These tubes are then connected to a supply of catalysed resin and vacuum from the pump is applied to the laminate and the resin is drawn into and through the laminate saturating the layers of glass and mat. When the laminate layers are fully saturated the supply of resin is discontinued and the vacuum pump applies pressure to the component until the resin is fully cured.
Doing it this way has several advantages. First, because the laminate layers are installed dry there are fewer time constraints than there are in a conventional layup allowing workers to be less rushed resulting in a more precise fitting of the components. Second, since resin is drawn under pressure into all the dry areas of the structure this results in a complete filling of all the Kerfs in the core material. We’ve already seen the difference that this can make over the long term. Third, the core material achieves a superior bond with the laminate layers on both sides. Forth, as with conventional vacuum bagging since the excess resin is sucked out via the pump stronger, lighter (up to 30%) parts are produced.
Another point worth mentioning is that since all curing is done within a sealed environment (the bag) a significant reduction of harmful vapors and pollutants is achieved.
I just want to add that even though many builders have adopted these newer methods many are still using the tried and true “bucket and brush” manual construction techniques for fiberglass layup and continue to turn out quality structures as they have for many years.