Custom Fiberglass Molding Processes
Gel Coat Process
Many products made from composite materials require a cosmetic finish as a function of the finished product. The use of an in-mold coating called gel coat is a fundamental process to many composites/ fiberglass manufacturing methods. Gel coat is a specialized polyester or vinyl ester resin that is pigmented and sprayed into a mold as the first step in the molding process. Once the gel coat film cures, the laminate (consisting of fiber reinforcement and resin) is applied in layers. A typical molded product is built from the outside layers to the inside of the part.
Gel coat serves several different functions – It provides a visually appealing glossy surface available in a wide range of colors and it protects the underlying laminate from the effects of the environment, including UV exposure and water absorption.
Hand Lay-Up Process (Open Molded)
The hand lay-up process uses a single-sided mold where the fiber reinforcement, such as fiberglass, carbon fiber or Kevlar©, in roll-stock form, is manually placed in the mold. The fiber reinforcement is then saturated with a liquid thermoset polymer, most commonly polyester, vinyl ester, or epoxy resins. In the process of saturating the fiber the laminate is manually formed into the shape of the mold surface using specialized laminating tools. The designation of hand lay-up is derived from the method of manually placing the fiber reinforcement on the mold surface. The liquid resin can be applied either manually from a container, or by a spray gun that dispenses the resin and curing agent mixture. The molding operators then "roll-out" the laminate to form the materials into the mold contours, tightly consolidate the laminate, and produce the appropriate proportions of resin and fiber reinforcement.
If a finished cosmetic surface is required the first step in the process is to apply gel coat to the mold surface. The part is then fabricated from the outside to the inside, beginning with the gel coat finish and then progressing through various layers of the structural laminate. Depending on the requirements many layers of laminate can be built-up to produce a specified thickness that meets the structural requirements of the application.
Hand lay-up, while one of the most basic composites/fiberglass molding processes (FRP), is widely used to produce a variety of transportation, marine, and commercial products. This process can be adapted to meet a broad range of production demands and products.
Resin Transfer Molding (RTM Closed Molding)
Resin transfer molding is a closed molding process where liquid resin is injected into a mold cavity under pressure. The two-halves of the mold-set can be gel coated if a cosmetic surface is required, or the part can be molded without an in-mold coating if cosmetics are not an issue. The dry reinforcement fiber is loaded into the mold cavity and the two halves of the mold-set secured by perimeter clamps or in a press. The resin is then injected, saturating the fiber reinforcement and filling the mold cavity while air is ejected through strategically placed vents.
RTM is a very flexible process that can use a wide variety of tooling, ranging from low cost composite "soft" tooling to temperature controlled metal tooling. Low volume production RTM can be accomplished in basic room temperature molds, or the process can take place in more sophisticated temperature controlled molds capable of producing rapid cycle times.
In production, the complexity and size of the molded part plays a role in selection of the RTM process. As the geometry (shape) of a part becomes more complex the reinforcement loading sequence becomes longer. As the part size becomes larger the structural requirements for the tooling and clamping are amplified.
Spray-Up Process (Open Molding)
The spray-up method is an open molding technique that applies a laminate to a single-sided mold using a specialized spray gun. The "chopper gun" derives its name from cutting (or chopping) a continuous strand of fiberglass into short segments and depositing them in a spray stream of resin. The result is that the resin matrix and the fiber reinforcement are mixed and deposited on the mold at the same time. This process creates a laminate with random orientation fiber that is easily contoured into complex shapes. Once the saturated fiber is distributed across the mold surface the laminate is manually formed using specialized laminating tools. The operators "roll-out" the laminate to form the materials into the mold contours, tightly consolidate the laminate, and produce the appropriate proportions of resin and fiber reinforcement. The result is a high strength molded fiberglass part or, as it is commonly referred to, a fiberglass reinforced plastic part (FRP part).
If a finished cosmetic surface is required the first step in the process is to apply gel coat to the mold surface. The part is then fabricated from the outside to the inside, beginning with the gel coat finish and then progressing through various layers of the structural laminate. Depending on the requirements, many layers of laminate can be built-up to produce a specified thickness that meets the structural specifications of the molded component. In some cases spray-up application is combined with layers of hand lay-up applied laminate to produce a multi-ply laminate with increased mechanical properties.
Spray-up fiberglass molding, while one of the most basic composites molding processes, is widely used to produce a variety of transportation, marine, and commercial products. This process can be adapted to meet a broad range of production demands and products.
Cold Press Molding (Closed Molding)
This closed molding process uses a two-sided mold set in a press to form the molded component. It is sometimes referred to as Liquid Composites Molding (LCM). The fiber reinforcement (usually fiberglass) is manually placed in the mold and the liquid resin is poured across the surface. The mold-set is then squeezed together in a hydraulic or air activated press that evenly distributes the resin and forms the shape, resulting in two smooth surfaces.
While this process is referred to as "cold press" molding, in some cases the tooling (mold-set) is actually heated. The term is derived from the fact that this process operates at a cooler temperature than the matched-metal die compression molding process. Cold press molding can produce fairly high production volumes and is best suited for non-cosmetic components.
Vacuum Bag Molding (Closed Molding)
The vacuum bag molding process makes use of atmospheric pressure to squeeze, or consolidate, a wet composites laminate in a mold. In this process, a fiber reinforcement, such as fiberglass, carbon, or Kevlar© fiber, is placed in a single-sided mold. The fiber is then saturated with a polymer resin, followed by a plastic sheet covering of "bagging material". The flexible membrane of the bag is sealed around the perimeter of the mold and then a vacuum is applied to the mold cavity.
The evacuation of air from within the vacuum bag causes the bagging membrane to pull down evenly across the mold surface. Up to approximately 14 pounds per square inch of atmospheric pressure compresses the wet laminate, consolidating the reinforcement plies in the laminate and squeezing out excess resin. The resulting laminate can be stronger than an open molded laminate and has a lower void content. Vacuum bagging is used to produce high quality laminates where weight and performance are at a premium.
RTM Lite (Vacuum Infusion Processing)
The RTM lite process is a closed molding method that employs a rigid-mold on one-side and either a light-weight rigid "B-side" mold or a vacuum bag. While there are a number of variations of RTM lite and related processes there are common denominators shared by this process group. The primary distinction is that the resin is pulled into the mold cavity under vacuum. Whereas traditional RTM uses pressure as the motive force to move resin into the mold cavity, RTM lite (and the related vacuum infusion processes) uses vacuum to pull the resin into the mold cavity.
During the process the fiber reinforcement dry-loaded in the mold and the light-weight "B-side" mold is many times vacuum clamped to secure the mold-set. The vacuum ports are then opened and the resin flows from containers into the mold cavity, saturating the fiber reinforcement.
There are several advantages to RTM lite — Because only one rigid-reinforced mold half is required, and the mold operates under vacuum rather than pressure, the tooling cost is relatively low. Also, the fiber reinforcement is pre-compressed by vacuum before the resin is introduced into the mold cavity. This results in a thin laminate with a high proportion of fiber, creating a strong lightweight structure. Gel coat surfaces can be applied for cosmetic finishes.
Compression molding is a high-volume, high-pressure method of molding complex composite parts on a rapid cycle. There are several variations of compression molding that include the use of sheet molding compound (SMC) and bulk molding compound (BMC) as the molding materials. These materials are "pre-mixed" compounds of thermoset resin, chopped glass fiber, various additives and an initiator.
The matched metal mold set is mounted in a large hydraulic molding press and the molds are heated. A measured charge of molding compound is placed in the open mold and the press is closed bringing the two halves of the mold set together under pressure. The molding compound flows under heat and pressure filling the mold cavity and quickly curing. Depending on thickness, size, and shape of the part, curing cycles range from less than a minute to about nine minutes.
Compression molding produces fast molding cycles, high part uniformity, and the process can be automated. Because of the ability to reproduce detailed shapes, compression molding offers product design flexibility, and features such as inserts, ribs, bosses and attachments can be molded in. Good surface finishes are obtainable, contributing to lower part finishing cost. Subsequent trimming and machining operations are minimized in compression molding because of the capability of the tooling to produce finished edges. Additionally, compared to other molding methods, labor costs are low.
Closed Cavity Bag Molding (CCBM)
Closed cavity bag molding is a technical variation of the vacuum infusion process. The process is a closed molding method that employs a rigid-mold on one-side and flexible rubber-like membrane to form the second side of the mold. The membrane is referred to as a re-usable vacuum bag and may be made of latex, silicone or urethane materials.
During the process the fiber reinforcement is dry-loaded in the mold, the flexible second-side mold is positioned over the laminate and vacuum is applied to seal the mold-set. The vacuum ports are then opened and the resin flows into the mold cavity, saturating the fiber reinforcement. In one variation of CCBM the resin can be pumped into the mold at low pressure as the vacuum is pulling the liquid resin through the mold cavity. This increases the resin flow rate into the mold and decreases the molding cycle time.
There are several advantages to CCBM — Because only one rigid-reinforced mold half is required, and the mold operates under vacuum rather than pressure, the tooling cost is relatively low. The "re-usable" vacuum bag further reduces cost over traditional vacuum infusion processing. Additionally, the fabrication of the flexible rubber mold-half allows complex shapes and details to be captured and provides a better back-side surface than a standard vacuum bag. During the CCBM process the fiber reinforcement is pre-compressed by vacuum before the resin is introduced into the mold cavity. This results in a laminate with a high proportion of fiber, creating a strong lightweight structure. Gel coat surfaces can be applied for cosmetic finishes.