Open Vs. Closed Mold Process For Fiberglass Fabrication

Fiberglass and other composite materials are renowned for their light weight and imperviousness to rust and corrosion, as well as their overall robustness when compared to metals and other materials. If you're interested in fabricating fiberglass parts on a small-scale or large-scale basis, then you'll want to look at the two most common processes used for fiberglass fabrication: the open mold and closed mold process. The following takes an in-depth look at both processes, along with their advantages and disadvantages.

Open Mold Process

Hand lay-up and spray-up are the two most common open mold processes. As the name implies, the hand lay-up process is done using only rudimentary hand tools, making it affordable for small-scale production and custom fabrication jobs. With the spray-up process, a specialized spray gun is used to deposit the fiber resin onto the mold surface. The spray gun cuts the fiberglass material into small strands and mixes the material with resin before spraying the material onto the mold surface.

Under most circumstances, fabricators can make fiberglass parts using the open mold process at a low cost while using as few specialized tools as possible. The simplicity and speed of open molding also makes it ideal for fabricating prototype parts. However, the relatively low cost and simplicity of open molding is offset by limitations in production speed. Most parts created through open molding require secondary trimming and other post-fabrication work, which is often a time-consuming process.

Closed Mold Process

The closed mold process is common among fiberglass fabricators interested in high-volume, large-scale production. There are several different closed mold techniques, but the following three are the most commonly used:

  • Light Resin Transfer Molding (LRTM) - This method involves pumping pre-mixed low-viscosity resin and fiberglass into a two-part closed mold made from metal or composite material. This method allows fiberglass parts to be produced without the need for an autoclave or a large amount of post-fabrication work.
  • Vacuum Infusion Process - The vacuum infusion process involves drawing resin into the mold via vacuum, producing an accurate and consistent product. Unlike LRTM, VIP can be performed using a one-sided mold as long as a laminate is used to contain the resin.
  • Reusable Bag Molding - This method can be considered as something of a hybrid, as it borrows certain attributes from the LRTM and vacuum infusion processes. With RBM, the fiberglass is contained within a specially-designed silicon bag. The bag itself is held inside of the mold under a vacuum while another vacuum draws the resin and fiberglass mix into the bag.

Unlike open molding, closed molding requires a considerable investment in specialized equipment, depending on the technique used. However, the end result is a consistent product that does not require secondary trimming or any other post-fabrication work. The closed mold process also cuts down on the amount of fiberglass material and resin wasted during fabrication. The process also allows fabricators to create a smoother, double-sided product -- something that is seldom achieved through open molding.

The closed mold process is also effective at preventing the emission of styrene, a volatile and potentially carcinogenic solvent that often becomes airborne during evaporation and atomization. Most closed molding techniques effectively contain the styrene, preventing it from escaping into the atmosphere during the molding process.

Which Process Works Best?

If your facility is strictly into prototyping or producing small batches of fiberglass components, then it's easier and more affordable to go with the open mold process. Open molding offers greater flexibility for crafting one-off parts in a short amount of time, giving manufacturers a faster product development cycle.

If you're looking forward to mass production of fiberglass components, however, the closed mold process allows for faster, higher-volume fabrication. Many aspects of the closed molding process can also be automated, resulting in faster production output without a loss in product quality. In the end, your choice between open and closed molding hinges on a number of variables, including your budget, production goals and the type of products you intend to fabricate.

For more information and options, talk with a professional fiberglass manufacturing company