By Ed Kammerer

There is a buffet of sorts for fuel site operators to choose from when they are selecting components—such as manhole covers, underground storage tank (UST) and dispenser sumps and covers, multiport access covers, top hats and skirts—that are constructed of fiberglass. A completely outfitted fueling site will likely contain components that have been manufactured via one of four main fiberglass-production methods. All of them are viable and commonly used today, depending on the component that is being built.

  • Open-Mold Spray Up: This method, also known as “chop and spray,” is the oldest. It utilizes a specialized spray gun with a bottle of resin to apply a pre-mixed, spooled fiberglass strand to a single-sided mold. In addition to its low cost, the main benefit of this method is that it is easier to produce parts that are of an irregular shape (not circular or square), as well as parts that are not meant to have an airtight or watertight seal or bear heavy loads and forces. The main drawback is that a part can only be produced with one smooth side, and the thickness can be less consistent than those produced by other methods. It can also lead to an inconsistent blend of resin and glass fibers, causing weak or porous parts. Hand rollers are run over the surface to squeeze out any air pockets which can weaken the finished product. This process is labor and time intensive.
  • Resin-Transfer Molding (RTM): This construction method, which has been available for about two decades, uses a composite of glass fibers and resin that is packed into a mold, with the glass providing tensile strength and the resin contributing compressive strength to the finished product. All of the fiberglass fibers must be fully saturated with the resin to ensure a smooth, consistent finish on both sides of the object. This saturation is accomplished by layering the glass sheets into a mold and then injecting the mold with resin. The two components are then compressed together by extreme forces in a hydraulic press that ensures a perfect blend of resin-saturated glass. RTM is effective in developing dense, compact layers of fiberglass for objects that need to bear heavy loads or repeated high-force events, like forecourt manhole covers. The drawbacks to this method come in the actual manufacturing process itself, which requires expensive tooling and a large hydraulic press to ensure that the two halves of the mold are bonded together strongly.
  • Vacuum-Assisted Resin Transfer Molding (Vac-RTM): This evolutionary stage in the RTM method uses a vacuum to pull, rather than push, the resin into the fiberglass. This creates a better seal without the need of a large, expensive hydraulic press; something as basic as a pair of vice-grip pliers can be used to securely clamp the two halves of the mold together. The finished walls of a Vac-RTM product are also smooth, which makes it easier to attach other components to it with no holes or pores that can adversely affect sealing capabilities. The biggest challenge with this method is that there is an absolute maximum amount of force that can be generated by a vacuum. This limits the amount of fiberglass that can be permeated by the resin, which restricts the load-bearing properties of the finished product and the overall range of different products it can be used in. This process makes it ideal for parts like containment chambers but not manhole covers.
  • Sheet-Molded Compound (SMC): This is the newest fiberglass-manufacturing innovation, and it combines the best of the older Open-Mold Spray Up and RTM methods. SMC starts with a putty-type mixture of glass fibers and resin that is formed into sheets that can be molded to meet the specific end-user needs for the end product’s length, shape and density. The sheets are cut to the size and shape needed, then placed in a mold and pressed into shape. The result is a part that has the highest level of consistency available, making it the premier way to construct a high number of consistently shaped and formed parts over a short period of time. The main drawback with SMC is found at the onset of the manufacturing process—a large (sometimes two stories tall) hydraulic press is needed to shape the fiberglass/resin sheets into the finished product. The other drawback is the high initial tooling cost and capital expense. The SMC process is ideal for high-volume parts requiring consistent and detailed end results.


Manufacturers of fueling components that offer a full portfolio of products will utilize all four types of fiberglass-manufacturing processes and continue to find ways to innovate. For example, some new dispenser sumps are constructed via the SMC method, making them one of the few dispenser sump models that is currently available with smooth interior and exterior walls, which allows entry fittings to bond or seal to them better, preventing leaks.

Having an array of choices is always good. The challenge for the site operator is identifying parts that are able to feature the best traits of the specific manufacturing method, from top hats and skirts that are still produced with the chop-and-spray method, to next-generation dispenser sumps that take advantage of the advanced SMC production process.


Ed Kammerer is a member of the Fuels Market News Editorial Council and the director of Global Product Management for OPW, based in Cincinnati, Ohio. He can be reached at OPW is leading the way in fueling solutions and innovations worldwide. OPW delivers product excellence and the most comprehensive line of fueling equipment and services to retail and commercial fueling operations around the globe. For more information on OPW, please go to