Designing with Fiberglass »

Fiberglass and Composite Material Design Guide

This design guide will provide some general information and specifications on fiberglass and composite materials and how to design products with composites. If you have more specific questions, please contact our engineers at Performance Composites and they will gladly assist you.

Composites materials are made by combining two materials where one of the materials is reinforcement (fiber) and the other material is a matrix (resin). The combination of the fiber and matrix provide characteristics superior to either of the materials alone.

Composites are structures that are made up of diverse elements, with the principle being that the sum of the whole is greater than the sum of its component parts (i.e. 1+1=3). Primitive man used the basic materials that were available to him such as animal dung, clay, straw and sticks to form composite structures that were literally the first building blocks of civilization. Even the biblical Noah's Ark was allegedly made of coal-tar pitch and straw, which could perhaps be the first reported construction of a reinforced composites boat!

Composites are made up of individual materials referred to as constituent materials. There are two categories of constituent materials: matrix and reinforcement. At least one portion of each type is required. The matrix material surrounds and supports the reinforcement materials by maintaining their relative positions. The reinforcements impart their special mechanical and physical properties to enhance the matrix properties. A synergism produces material properties unavailable from the individual constituent materials, while the wide variety of matrix and strengthening materials allows the designer of the product or structure to choose an optimum combination.

Engineered composite materials must be formed to shape. The matrix material can be introduced to the reinforcement before or after the reinforcement material is placed into the mold cavity or onto the mold surface. The matrix material experiences a melding event, after which the part shape is essentially set. Depending upon the nature of the matrix material, this melding event can occur in various ways such as chemical polymerization or solidification from the melted state.

A variety of molding methods can be used according to the end-item design requirements. The principal factors impacting the methodology are the natures of the chosen matrix and reinforcement materials. Another important factor is the gross quantity of material to be produced. Large quantities can be used to justify high capital expenditures for rapid and automated manufacturing technology. Small production quantities are accommodated with lower capital expenditures but higher labor and tooling costs at a correspondingly slower rate.

Most commercially produced composites use a polymer matrix material often called a resin solution. There are many different polymers available depending upon the starting raw ingredients. There are several broad categories, each with numerous variations. The most common are known as polyester, vinyl ester, epoxy, phenolic, and others. The reinforcement materials are often fibers but also commonly ground minerals. The various methods described below have been developed to reduce the resin content of the final product, or the fiber content is increased. As a rule of thumb, lay up results in a product containing 70% resin and 30% fiber, whereas vacuum infusion gives a final product with 40% resin and 60% fiber content. The strength of the product is greatly dependent on this ratio.

Some examples of composite materials are plywood, reinforced concrete, fiberglass & polyester resin, and graphite & epoxy resin.


                                         Fiberglass & polyester Graphite & epoxy Wood (Douglas fir) Aluminum 6061 T-6 Steel, Mild
Material Cost $/lb $1.80 $8.00 $0.75 $3 $.50
Strength, yield (psi) 30,000 60,000 2,400 35,000 60,000
Stiffness (psi) 1.2 x 10 6 8 x 106 1.8 x 106 10 x 106 30 x 106
Density (lb/in3) .055 .065 .02 .10 .30

Manufacturing process

In general, the reinforcing and matrix materials are combined, compacted and processed to undergo a melding event. After the melding event, the part shape is essentially set, although it can deform under certain process conditions. For a thermoset polymeric matrix material, the melding event is a curing reaction that is initiated by the application of additional heat or chemical reactivity such as organic peroxide. For a thermoplastic polymeric matrix material, the melding event is solidification from the melted state. For a metal matrix material such as titanium foil, the melding event is a fusing at high pressure and a temperature near the melt point.

For many molding methods, it is convenient to refer to one mold piece as a "lower" mold and another mold piece as an "upper" mold. Lower and upper refer to the different faces of the molded panel, not the mold's configuration in space. In this convention, there is always a lower mold, and sometimes an upper mold. Part construction begins by applying materials to the lower mold. Lower mold and upper mold are more generalized descriptors than more common and specific terms such as male side, female side, a-side, b-side, tool side, bowl, hat, mandrel, etc.

The molded product is often referred to as a panel. For certain geometries and material combinations, it can be referred to as a casting. For certain continuous processes, it can be referred to as a profile. Applied with a pressure roller, a spray device or manually. This process is generally done at ambient temperature and atmospheric pressure. Two variations of open molding are Hand Layup and Spray-up.

Open Mold – Wet Layup

The most common manufacturing process for fiberglass is the wet lay-up process using an open mold. The shape of the part is determined by the shape of the mold, and the mold surface is typically in contact with the exterior of the part. Mold release is first applied to the mold to prevent the fiberglass part from adhering to the mold. Then gel coat, which is pigmented resin, is applied to the mold to give the part color. Fiberglass and resin are then deposited on to the mold and the fiberglass is compressed by rollers, which evenly distributes the resin and removes air pockets. Multiple layers of fiberglass are deposited until the desired thickness is achieved. When the resin is cured, the part is removed from the mold. Excess material is trimmed off, and the part is ready for paint and assembly. There are also closed mold processes for making fiberglass parts.


Vacuum Bagging Process

A process using a two-sided mold set that shapes both surfaces of the panel. On the lower side is a rigid mold and on the upper side is a flexible membrane or vacuum bag. The flexible membrane can be a reusable material or an extruded film. Then, vacuum is applied to the mold cavity. This process can be performed at either ambient or elevated temperature with ambient atmospheric pressure acting upon the vacuum bag.

A vacuum bag is a bag made of a modified latex material or a polymer film used to bond or laminate materials. In some applications the bag encloses the entire material, or in other applications a mold is used to form one face of the laminate with the bag being single sided to seal the outer face of the laminate to the mold. The open end is sealed and the air is drawn out of the bag through a nipple using a vacuum pump. As a result, uniform pressure approaching one atmosphere is applied to the surfaces of the object inside the bag, holding parts together while the matrix material cures. The entire bag may be placed in a temperature-controlled oven to accelerate curing.

Vacuum bagging is widely used in the composites industry. Carbon fiber fabric and fiberglass, along with resins and epoxies are common materials laminated together with a vacuum bag operation.

Prepreg Process

A process using a two-sided mold set that forms both surfaces of the panel. On the lower side is a rigid mold and on the upper side is a flexible membrane made from silicone, modified latex or an extruded polymer film such as nylon. Reinforcement materials can be placed manually or robotically. They include continuous fiber forms fashioned into textile constructions. They are pre-impregnated with the resin in the form of prepreg fabrics or unidirectional tapes. The upper mold is installed and vacuum is applied to the mold cavity. The assembly is placed into an oven or autoclave. This process is generally performed at both elevated pressure and elevated temperature. The use of elevated pressure facilitates a high fiber volume fraction and low void content for maximum structural efficiency.

Resin transfer molding (RTM)

A process using a two-sided mold set that forms both surfaces of the panel. The lower side is a rigid mold. The upper side can be a rigid or flexible mold. Flexible molds can be made from composite materials, silicone, modified latex or extruded polymer films such as nylon. The two sides fit together to produce a mold cavity. The distinguishing feature of resin transfer molding is that the reinforcement materials are placed into this cavity and the mold set is closed prior to the introduction of matrix material. Resin transfer molding includes numerous varieties which differ in the mechanics of how the resin is introduced to the reinforcement in the mold cavity. These variations include everything from vacuum infusion to vacuum assisted resin transfer molding (VARTM). This process can be performed at either ambient or elevated temperature.


Other types of molding include press molding, transfer molding, pultrusion molding, filament winding, casting, centrifugal casting and continuous casting.

Design Information

Like any material, fiberglass has advantages and disadvantages, but in applications such as corrosion, low to mid volume production, very large parts, contoured or rounded parts and parts needing high specific strength, fiberglass is the material of choice. Fiberglass is a designer's material, because the parts can be tailored to have strength and or stiffness in the directions and locations that are necessary by strategically placing materials and orienting fiber direction. Also the design and manufacturing flexibility that fiberglass offers, provides opportunities to consolidate parts and to incorporate many features into the part to further reduce the total part price. Some general design guidelines are listed below:

Material thickness          Typically range from 1/16" to 1/2". Can use sandwich construction to achieve lighter and stiffer parts.
Corner radius Recommend 1/8" or larger
Shape Will duplicate the shape of the mold.  Can be heavily contoured
Dimensional tolerance Tool side can be + .010" of the tool
Non Tool Side  + .030"
Surface finish Tool side can be class A
Non Tool side will be rough, but can be smoothed out
Can be gel coated painted, or use any other surface coating

.002 in/in

Electrical properties RF Transparent
Excellent insulating characteristics
Can provide EMI shielding through conductive coating
Fire retarding Resins available in fire retardant applications meeting various ASTM classes & smoke generation requirements
Corrosion Resins available for corrosion applications, especially for hot brine, most acids, caustics, & chlorine gases


The physical properties of composite materials are generally not isotropic (independent of direction of applied force) in nature, but rather are typically orthotropic (different depending on the direction of the applied force or load). For instance, the stiffness of a composite panel will often depend upon the orientation of the applied forces and/or moments. Panel stiffness is also dependent on the design of the panel. For instance, the fiber reinforcement and matrix used, the method of panel build, thermoset versus thermoplastic, type of weave, and orientation of fiber axis to the primary force.

In contrast, isotropic materials (for example, aluminum or steel), in standard wrought forms, typically have the same stiffness regardless of the directional orientation of the applied forces and/or moments.

The relationship between forces/moments and strains/curvatures for an isotropic material can be described with the following material properties: Young's Modulus, the shear Modulus and the Poisson's ratio, in relatively simple mathematical relationships. For the anisotropic material, it requires the mathematics of a second order tensor and up to 21 material property constants. For the special case of orthogonal isotropy, there are three different material property constants for each of Young's Modulus, Shear Modulus and Poisson's ratio—a total of 9 constants to describe the relationship between forces/moments and strains/curvatures.


Most common composite materials, including fiberglass, carbon fiber, and Kevlar, include at least two parts, the substrate and the resin.

Polyester resin tends to have yellowish tint, and is suitable for most projects. Its weaknesses are that it is UV sensitive and can tend to degrade over time, and thus generally is also coated to help preserve it. It is often used in the making of surfboards and for marine applications. Its hardener is a MEKP catalyst. When MEKP is mixed with the resin, the resulting chemical reaction causes heat to build up and cure or harden the resin.

Vinyl ester resin tends to have a purplish to bluish to greenish tint. This resin has lower viscosity than polyester resin, and is more transparent. This resin tends to be more resistant over time to degradation than polyester resin, and is more flexible. It uses the same hardener as polyester resin (at the same mix ratio) and the cost is approximately the same.

Epoxy resin is almost totally transparent when cured. In the aerospace industry, epoxy is used as a structural matrix material or as structural glue.

Categories of fiber-reinforced composite materials

Fiber reinforced composite materials can be divided into two main categories normally referred to as short fiber-reinforced materials and continuous fiber reinforced materials. Continuous reinforced materials will often constitute a layered or laminated structure. The woven and continuous fiber styles are typically available in a variety of forms, being pre-impregnated with the given matrix (resin), dry, uni-directional tapes of various widths, plain weave, harness satins, braided, and stitched.

The short and long fibers are typically employed in compression molding and sheet molding operations. These come in the form of flakes, chips, and random mate (which can also be made from a continuous fiber laid in random fashion until the desired thickness of the ply / laminate is achieved).


Shock, impact, or repeated cyclic stresses can cause the laminate to separate at the interface between two layers, a condition known as delamination. Individual fibers can separate from the matrix e.g. fiber pull-out.

Composites can fail on the microscopic or macroscopic scale. Compression failures can occur at both the macro scale or at each individual reinforcing fiber in compression buckling. Tension failures can be net section failures of the part or degradation of the composite at a microscopic scale where one or more of the layers in the composite fail in tension of the matrix or failure the bond between the matrix and fibers.

Some composites are brittle and have little reserve strength beyond the initial onset of failure while others may have large deformations and have reserve energy absorbing capacity past the onset of damage. The variations in fibers and matrices that are available and the mixtures that can be made with blends leave a very broad range of properties that can be designed into a composite structure.

The best known failure of a brittle ceramic matrix composite occurred when the carbon-carbon composite tile on the leading edge of the wing of the Space Shuttle Columbia fractured when impacted during take-off. It led to catastrophic break-up of the vehicle when it re-entered the Earth's atmosphere on 1 February 2003.


Tooling or molds are used to define the shape of the fiberglass parts. The fiberglass part will pick up all shapes and features of the molds; therefore the quality of the part is heavily influenced by the quality of the mold. The molds can be either male or female. The female molds are the most common and they will produce a part with a smooth exterior surface while a male mold will produce a smooth interior surface (please see drawing below).



For very short production runs (less than 10 parts), temporary molds can be made from wood, foam, clay or plaster. These molds are economical and can be fabricated quickly, which will allow inexpensive prototype parts to be fabricated. For larger volume production, molds are typically made with fiberglass. These molds have a life expectancy of 10+ years and 1000+ cycles. Fiberglass molds are inexpensive and usually cost 5 to 10 times the price of the part.

The mold is a mirror image of the part. To create a mold, a master (plug) is required. The master can be an actual part, or can be fabricated out of wood, foam, plaster, or clay. The exact shape and finish of the master will be transferred to the mold. Once the master is completed, it is polished, waxed and the mold is built up on the master. The fabrication technique of the mold is similar to fabricating a fiberglass part except that tooling materials (gel coat, resins, and cloth) are used to provide a durable mold that has low shrinkage and good dimensional stability. Once the mold is laminated, it is reinforced with wood, fiberglass or metal structure to ensure that it retains the proper shape. Then the mold is removed from the master and put into production.

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Products/Industries Served

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