Definition of Composites
Composites are a combination of two or more materials yielding properties superior to those of the individual ingredients. One material is in the form of a particulate or fiber, called the reinforcement or discrete phase. The other is a formable solid, called the matrix or continuous phase. The region where the reinforcement and matrix meet is called the interface. Composite properties are determined by chemical and mechanical interaction of the combined materials. Wood and concrete are composites under this definition.

What are FRP/Composites?
Fiberglass reinforced plastic, commonly known as fiberglass, was developed commercially after World War II. Since that time, the use of fiberglass has grown rapidly.

The term "fiberglass" describes a thermoset plastic resin that is reinforced with glass fibers. In this manual, the more general terms Fiber Reinforced Plastic/Composites or FRP/Composites will be used to describe these extremely useful material systems. Plastic resins come in two different classes - thermosets and thermoplastics. From a practical perspective, it's easy to remember that thermosets maintain their molded shape at higher temperatures and cannot be melted and reshaped. Thermoplastics will melt at a given temperature and can be solidified into new shapes by cooling to ambient temperatures. Thermosets and thermoplastics are described with more detail in the Resin Systems section of this document. Reinforcing fibers include glass, carbon, aramid and other man-made and natural materials that are further described in the Reinforcement section of this document. These are used in a variety of forms and combinations to provide the required properties.

The plastic resin systems determine chemical, electrical, and thermal properties. Fibers provide strength, dimensional stability, and heat resistance. Additives provide color and determine surface finish, and affect many other properties such as weathering and flame retardance. Processing of FRP/Composites involves complex chemical reactions.

Final properties are determined by many factors including the type, amount, and composition of the resin systems and reinforcements. In addition, the use of additives can greatly affect the FRP/Composite properties.

Differences in LCM & SMC Properties

Fiber Reinforcement.
LCM laminate fiber content can be routinely varied from about 15% to 50% by weight with random fibers and up to 75% by weight with oriented fibers. The fiber content and orientation throughout the LCM part can vary as required by the application because the fiber reinforcement is pre-placed in the mold and does not move during the molding operation. Standard SMC will contain between about 10% and 50% fiber by weight with special formulations containing up to 75% fiber by weight. Fiber content throughout the SMC part is very consistent as a result of the compounding process and the coupled flow of resin and fiber during molding. For the same reason, fiber content throughout the SMC part cannot be tailored or optimized to meet application requirements. Fiber orientation in SMC is greatly influenced by compound flow as the mold closes and is therefore more difficult to control.

Fiber Length.
The most common fibers are E-glass rovings chopped to a length (or blend of lengths) from 1/8" to 4". There is a relationship between fiber length and strength - longer is stronger. For polyester resins the strength curve is asymptotic at about one inch. Since longer fibers do not flow as well, most SMC fibers are one inch long. LCM fibers are often longer because it improves the integrity and the permeability of the mat. Some types of impact performance and toughness are improved by the longer fibers. Both SMC and LCM can incorporate continuous fibers if required. Because the fibers do not flow, LCM can also incorporate fabrics and non woven reinforcements.

Fiber Orientation.
The orientation of reinforcing fiber in the LCM laminate is determined by its positive placement in the mold cavity. Since the L CM reinforcement does not move during mold closure, the fiber orientation is highly tailored and predictable. The fiber orientation in SMC laminate is more difficult to predict and control. Because the fiber flows with the resin, the fibers tend to orient themselves in the direction of flow. This phenomenon can be controlled to some extent by charge size and location but this practice generally causes performance trade-offs with other laminate properties.

Ribs and Bosses.
Because of the difficulty of pre-forming glass fibers to details such as ribs and bosses, LCM is limited to generally uniform cross section shapes. The pre-combined fibers in SMC on the other hand flow easily to conform to many complex shapes.

Material Density.
In order to properly flow during the molding process, SMC must have at least a given proportion of mineral filler. This restricts the density or specific gravity of SMC on the low end. LCM does not have such a restriction and therefore given identical part thickness, LCM can produce a lower density part.

Part Weight.
Under the same design stress conditions, LCM can produce a lighter weight part compared to SMC. This is the result of more consistent strength, higher impact toughness and the flexibility to design with lower density laminates. See Material Density and Mechanical Properties sections for further discussion.

Mechanical Properties.
LCM creates composite structures that provide higher design strength allowables than similar laminates made from SMC. Since 1974, the Molded Fiber Glass Companies (MFG) has periodically compared its modern LCM processes with state-of-the-art SMC formulations.

Recently, MFG completed its latest test runs on nearly 500 samples taken from truck hoods that were compression molded using the two forms. The current samples were taken from two current model truck hoods - one a three-piece hood of SMC, the other a one-piece hood of LCM. The LCM hood had an average glass content of about 20% by weight and the SMC hood was slightly higher at about 25% glass content by weight.

Similar sample maps were developed for each hood with specimens oriented in various important directions (longitudinal, cross-car, vertical and horizontal). ASTM D638 tensile and D790 flexural strength tests were performed on the samples.

Flexural Strength.
Flexural Strength describes the amount of force required to bend and break the material when a specific thickness test piece is bent. A test piece is supported at both ends, a force is applied to a small, concentrated area in the center and the force and amount of bending is measured. Results are reported in pounds per square inch (psi). See ASTM D790 for specifics.

The horizontal line near the center of each plot shows the sample mean, or average flexural strength. The box areas above and below the mean line each represent one quartile or 25% of the data samples. The whiskers above and below the boxed area each represent the remaining quartiles. The figure clearly shows that although the average flexural strength of the two forms is about the same, the LCM form produces much less variation than the SMC form. A closer look shows that as many as 35% of the SMC samples have flexural strength below the weakest LCM samples.

Although the average strength of both the LCM and SMC samples are about the same, the variation in flexural strength is much greater in SMC than in LCM. In fact, 6 of 14 SMC sample areas had average flexural strength below the lowest performing areas in the LCM part.

Tensile Strength.
Tensile strength describes the amount of force (tensile stress) required to break a sample of specific dimensions when the material is stretched to its breaking point. The tensile stress at failure is divided by the cross-sectional area of the sample and results are reported in pounds per square inch (psi). See ASTM D638 for details. The tensile strength comparison results are similar to the flexural strength in that the average tensile strength of the two forms is about the same but the variation in the LCM parts is much lower than the SMC parts. One particularly important result in the tensile strength test is that the highest frequency (or mode) of SMC strength samples is at the lowest end of the scale whereas the mode of LCM strength samples is above the average strength.

Although the current testing shows that some areas of SMC can have very high strength compared to LCM, it is the low end of the variation that causes parts to fail. In both the design process and end use, when any area of laminate is subjected to stress, it is the weaker locations that fail, similar to the weak links in a chain. The more consistent, controllable and predictable properties of the resulting laminate make LCM the process of choice when flexural and tensile strengths are important.

Impact Strength.
Figure 7 is a comparison of impact strength, or toughness, of LCM and SMC laminates as measured using the Dynatup testing method. This method measures the reactive load produced when a weighted dart is dropped onto the laminate surface from a controlled height. The important performance measurements are the energy absorbed (Max Load x Drop Height) and the height where penetration of the dart into the material occurs.

The LCM laminate performs about 25% better than SMC in the energy absorbed in this test. Also, penetration occurs in the SMC at the 24 inch drop height while the LCM required a drop height of 45 inches for penetration to start.

Molding Pressure.
The molding pressure for LCM is usually a fraction of that required for SMC. Therefore, the press tonnage required is less for LCM than for SMC for a given part area. This is because the viscosity of the LCM paste is very low compared with that of SMC and no reinforcing fibers are displaced in LCM, as they are in SMC. In practice, this allows compression molding of larger parts in LCM than in SMC for a given press tonnage and within the platen size limits of the press.

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