Gelcoat is the decorative surface found on fiberglass parts such as boats, bathtubs, and restaurant seats. This outer layer is needed for aesthetics and protection of the underlying laminate structure. Chemically it is unsaturated polyester/vinylester resin that is unreinforced but heavily filled with a complex variety of additives. These additives are used to determine its color, UV stability, and chemical resistance.
The manufacture of fiberglass parts typically requires a gelcoat layer to aid in the release of the parts from the mold. The parts that will be painted also are manufactured with gelcoat but it is a “sandable” variety. These parts are often automotive-related, and may include pickuptruck toppers, fiberglass kitcar bodies, and other aftermarket add-ons such as running boards or hood scoops.
The unsaturated polyester/vinylester gelcoat is directly compatible with resins and laminate of the same chemistry. Gelcoat can be used with epoxy resins but requires a tiecoat for adhesion because of the chemistry difference.
Gelcoat is applied at 18 to 25 mils (thousandths) and will lose thickness as it cures. Typically when it is dry on the part, it can be down to 12 to 18 mils. This is a lot thicker than a painted surface. One of the disadvantages of gelcoat is that it can be more difficult to repair, especially with color matching. The thickness typically allows for sanding out scratches and blemishes, but going through can be painful and will require a respray. The outer layers of gelcoat will likely discolor over time due to UV degradation, and sanding and buffing into the underlying gelcoat may cause discoloration as the layers are different colors depending upon the depth.
A lot of FRP materials are used in the chemical and industrial sectors of the economy. The non-corrosive and non-conductive properties of the fiberglass materials have many advantages over steel and other materials.
Large tanks, pipes, and structures are fabricated both off-site and on-site of the final installation. The best quality comes from the controlled environment of the factory. However, the physical size may necessitate that structures are built in pieces for transport and installation. They onsite workers can join the pieces to become one. There are several joining methods, including adhesive, mechanical, and chemical bonding (fiberglass tabbing).
There are specialized resins used for these corrosion applications. These resins are the best at resisting chemical interaction, and are made to have high impact strengths to resist failure of the overall structure.
Fiberglass resin and glass is normally non conductive, both with heat and electricity. This can be adventageous for negating concerns for electrocution. The flipside is that static electricity can build up and discharge randomly in a search for a grounding path. Heat conduction is poor so it can act as an insulator and heat sink.
Many of the pipes and tanks are manufactured using a process called filament winding. This is a process where a round mandrel acts as a mold and rotates on its center axis. Reinforcement is wetted out with resin and fed onto the spinning mandrel in specific orientations for strength requirements. The part is allowed to cure and the mandrel is removed.
The matching of these materials to their specific application, use, and environment is extremely important. And it can be an area where composites can shine.
Vacuum Infusion and Vacuum Bagging can be accomplished are not only reserved for industry and can be accomplished in a workshop setting. Relatively inexpensively as well. One of the major variables is the shape of the part trying to be built.
We will look at a flatstock for now. The desired outcome could be either a test panel or a piece needed for flat construction. The mold will be the difference between this and any other more complicated shape. The mold must be able to be sealed from the atmosphere and waxed for release of the finished part, so my favorite “mold” is either a flat sheet of steel or a sheet of plate glass depending upon desired surface finish. This must then be waxed up with some paste wax to enable part release.
I always like to start with creating the perimeter seal. The 1/8 or 3/16 inch thick by 1/2″ wide gray butyl tape is the best. This is basically sticky on both sides and has wax paper on one side. It is sold at infusion suppliers or online as well. My little secret is that it is also sold at building supply stores, as it is used on polebuilding construction projects when sheets of steel siding/roofing are sealed together with this stuff. So this is unrolled and pushed down onto the mold and the wax paper is left on and facing up. Then I like to use cover this with some 2 inch wide masking tape.
Now we can start the part construction by placing our layer of gelcoat if so desired. Once this is cured, it is followed one of two ways. For vacuum bagging, the catalyzed resin-wet layers of reinforcing materials (glass or carbon) are placed down, including any core material. Once this is satisfactory, the peel ply is placed over the laminate, followed by the breather material. This is typically quilting batting, and a couple of good layers will do well to hold the excess resin from the laminate. Then the bag will be installed and a vacuum will be pulled which will pull the resin up into the breather material.
For infusion, the gelcoat is followed by the dry layers of reinforcing glass or carbon and core. Then any resin runner strips are added with a separation from the laminate with peel ply. Two circuits are setup; one for vacuuming the air out, and one for bringing resin into the laminate. The Vacuum circuit is accomplished with spiral wire loom wrap around the perimeter of the part leading to a T connection which will poke thru the bag. A resin inlet T will also pass thru the bag and lead into the runner strip. Now a bag will be placed over the part and sealed down. The hoses need to be vacuum rated, and can be either pvc or polyethylene.
A resin trap will be needed between the vacuum pump and the laminate for both infusion and bagging. This can effectively be accomplished by using a PVC pipe sealed at both ends with endcaps. Then drill and tap plastic or brass fittings and seal well with pipe dope. The fittings need to be at the top so that the resin will drop out and into the pipe. The resin will cure in the pipe and can be disposed when solid and full.
The bagging operation is accomplished by using polyethylene plastic that is free from holes, and in the range of 4 to 6 mils of thickness. Thicker plastic can do the job, but can be difficult to work with. The plastic needs to be cut larger than the area to be covered, typically by around 20 percent. It needs to be placed on squarely, and pushed into the butyl tape without wrinkes to create a good seal. The excess plastic is artfully taken up with extra pieces of butyl tape in these troughs that are connected to the main tape line around the edge. This can be very frustrating and is difficult to describle. But the bag will be sealed to the mold to create an airtight cavity. The resin hose needs to be closed off with vice grips.
Now the vacuum will be turned on. For the vacuum bagged part, once the vacuum is on, the part is basically left to cure. For infusion, the laminate is left to the vacuum to remove air and moisture for 20 mins to 2 hrs depending upon size. Infusion requires at least 27.5 inches of mercury, and 29 would be awesome. Any leaks need to be identified and eliminated. This can be difficult, but is necessary.
Now we are ready to put resin into the laminate. The wet resin is mixed with catalyst in buckets where the resin hose from the part is placed. Resin hoses are opened up and the resin flows from the bucket into the part. Once the part is full, the resin line is clamped off and the part is left to cure under vacuum. I typically wait at least 12 hours before demolding the finished part.
Resin infusion processing offers several advantages over traditional open mold processing techniques. All of the reinforcements and core materials are placed in the mold without resin, so care can be taken for close fit and proper orientation. And it is a lot cleaner without the sticky resin. Resin waste is typically lower because all of the resin is added at once. VOC’s are reduced as well, and are only emitted from the open mixing containers. There is much less worker interface with messy and stick resin on people and tools so cleanup materials and personal protection equipment expenses are reduced. The laminate itself is typically more consolidated, uniform, and visually pleasing.
One of the considerations that needs to be taken into account is that the ratios are different and the glass and resin are more compacted by the process. Using the same layup schedule would result in a thinner laminate that is lighter weight and uses less resin. One drawback to this is that the cross sectional area is less, usually resulting in a loss of stiffness. This can be regained by increasing the core thickness to compensate for that loss and to restore overall panel thickness.
Infusion processing does require specialized equipment, consumables, and materials. The resins need to have much lower viscosity(flow like water). The core and glass need to have holes and channels for the resin to flow. A bag needs to be created to cover and seal off the laminate to the mold with out ANY air leaks. The mold needs to be able to be sealed to the bag and be airtight. A high vacuum needs to be able to be drawn on the bagged mold to move the resin. The vacuum pump or venturi needs to be able to achieve a minimum of 28 inches of mercury.
The basic process is that the mold is gelcoated abd skinned with a good, low-shrink resin and chopped strand mat. Then layers of dry glass are added. Coring is placed nice and tight and glued if needed. And the final layers of glass are placed. During the process, any gaps in coring or glass will “bridge” where the vacuum doesnt pull them down, and solid resin will fill them. So good fit is very important. The the bag is placed and sealed after auxililary runner strips are added and resin feed hoses are placed. The perimeter vacuum channel is plumbed to the vacuum pump.
Air is removed from the laminate for a good amount of time before the resin lines are opened up and resin flows into the part (do not forget to catalyze it). This is part of the art of flowing resin to fill the whole laminate with resin before an area gets cutoff from the vacuum or the resin begins to harden. There are some tricks and practice is key.
Resin is stopped just before fill and hopefully resin doesnt run into the vacuum line too far, but that is what resin traps are for. The vacuum is left running as the part hardens and gets to full cure in a couple of hours.
FRP Composites have their own special set of considerations in design and use. And we will discuss a few here. The traditional fiberglass (unsaturated polyester with glass reinforcement) uses gelcoat on the molded (decorated) side. This is really only one side of the part, as this is not a matched-mold process. So the back side needs to be hidden from view or covered with another material, etc.
A boat, for example has a Deck and a Hull that are mated together so that the back sides of the parts are hidden from view. The inside of the cabin is then upholstered and trimmed out for comfort and decoration.
I have worked with fiberglass tabletops before where the top side is gelcoated in a decorative finish and the edge wraps down and trimmed. All of the unsightly surfaces face the floor where they cannot be viewed.
There are a few closed-mold processes that can be implemented, but these really only achieve a Class B surface out of the mold. With rework and repair, a Class A surface can be accomplished. I know of some folks that make Boat Access Hatches where they repair the backside (bottom) and rework it so it looks pleasant when opened.
Molded-in features must be used with caution. Sharp edges must be avoided because the process hates having to get resin and reinforcement into them, and because they can be stress concentrators.
Parts also need to have a minimum of 2-5 degrees of draft or angle in the pull direction so they can be removed from the mold. Molded in undercuts and reverses with multipiece molds add extreme expense and difficulty. Molded-in holes are not general practice because the process does not allow for crisp edges on these details.
Temperature plays an important role in the curing process of the resins used in composites. Many of the resins are setup for room-temperature curing. This requires that the ambient room temperature is ideally set between 65 and 75 degrees. And that the resin itself is near this temperature. The old rule of thumb is that a drum of resin takes about 24 hours to get to room temp when moved in from shipping or storage. These room temperature cured resins have windows of open working time before the curing cycle begins to happen. Elevated temperatures in the summer can cause havoc, but can be managed with special mixtures and ingredients.
Some resins cure with time and elevated temperatures, which are achieved with the use of ovens. These allow for nearly unlimited open working time before cure. When things are satisfactorily placed, the temperatures are elevated to start the cure process.
UV Light is another curing mechanism that has special applications and takes the temperature consideration away. This has a big use with the infrastructure restoration industries working onsite and underground. It is much more of a specialized niche application.
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