What Every Engineer Should Know About Composites
Bob Lacovarra,
Technical Director,
Composites Fabricators Association. (CFA)
Index
- Lesson #1
- Lesson #2
- Lesson #3
- Lesson #4
- Lesson #5
- Lesson #6
Acknowledgement
Non-composites engineers, designers, architects, or product specifiers often call CFA Technical Services asking questions such as, "What are the mechanical properties of fiberglass?" or "What is the tensile strength of a 1/8" laminate?" While these are very specific requests, usually they are not the questions to the answers for which they are looking.
Many times non-composites engineers are attempting to use the "handbook" approach to composites design. For example, if one were looking for a steel I-beam to span 20 feet and carry a 2000 pound load, you simply open a structural steel handbook and choose the proper beam thickness and flange width from a chart. Unfortunately, composites do not work that way. The performance characteristics of composites can be varied to a tremendous degree and there is no such thing as a "generic" composite. The very thing that makes composites a highly adaptable engineering material also makes them more difficult to specify and engineer than some other materials.
Here are a few important points that anyone dealing with composites will benefit from understanding:
Lesson #1
Composites (FRP or fiberglass) encompass a wide range of materials and performance characteristics.
Composites (for the sake of this discussion) are an engineering material that consists of a thermoset resin and a fiber reinforcement. The liquid resin is combined with the fiber reinforcement, in the molding process, and cures into a solid laminate. There are many types of composites resins and reinforcements, and each of these imports specific properties to the FRP product.
There are at least six major family groups of resin used in composites fabrication. These include: polyester, vinylester, modified acrylic, epoxy, phenolic, and urethane resin systems. The list goes on; however, the important point to note is that each of these resins has specific performance characteristics. If you want to go fast, buy a sports car. Use a truck to haul loads and to go off-road, drive a 4x4. It is the same with resins. For example, if corrosion resistance is an issue, a vinylester resin would be a candidate. If high strength is critical, an epoxy might be the resin of choice. If cost vs. performance is an issue, polyester resin is most commonly used. In the realm of polyester resins alone, specific formulations will be used if cosmetics are an issue, enhanced corrosion resistance is required, elevated temperatures will be encountered, or cost is an over-riding factor. The resin system is selected based on the functional and cost requirements of the product.
There are also a number of reinforcement fibers used in composites. Glass fiber is used in over 90% of all composites. However, if required, advanced fibers such as Kevlar® or carbon fiber offer high level performance at a price. In the realm of glass fiber, there are many 'styles' of reinforcement depending on the molding process and the strength requirements of the product. Glass fiber is available in random fiber orientation in the form of chopped strand mat. There are also lightweight textile fabrics, heavy woven materials, knitted fabrics, and unidirectional fabrics all of which serve specific purposes in composites design. To maximise the cost/benefit of composite products, the component materials must be custom tailored to the application.
Lesson #2
There are many different process methods used to produce composite products.
Composites are molded in an array of process methods that range from very simple and low cost, to complex and capital intensive. The selection of the molding process is usually based on two major factors: first, the required end-use properties of the product; and second, the volume of product to be manufactured. The open molding process is capable of producing small to medium quantities of parts. The capital investment in the open mold tooling is relatively low, while production labor costs are relatively high. At the other end of the scale are various closed molding options, where the capital investment is high and labor cost becomes progressively lower.
Occasionally a dilemma develops in the case of a new product, where sales volume is unpredictable. The rock is to start with a low volume process and spend more money to move to a higher volume process if sales require it. The hard spot is to invest in a higher end process up front and hope sales catch up. With any new product, a detailed analysis should be made examining lifecycle cost and long term processing options.
Lesson #3
Tight tolerances cost money.
In many cases the dimensional tolerances of the product are critical. The question is "How critical is critical?" There are very real costs associated with decreasing tolerance ranges. These costs begin at the pattern fabrication stage and are carried through to the manufacturing process. It is very frustrating for manufacturing engineers to struggle with an arbitrary tolerance which is irrelevant to the function of the product. Designers and engineers need to realistically assess tolerances and determine the maximum tolerance range acceptable in light of the cost of achieving that accuracy. The capabilities of the molding process must set the limitations on acceptable tolerances.
Lesson #4
Good tooling is a good investment.
The discussion sometimes goes like this, "We'll start with quick and dirty tooling and when sales get rolling, we'll build good production tooling." When it comes to composites tooling the Puralator principle applies - "you can pay me now or pay me later." Cheap tooling is almost never worth the cost. The quality of a molded product is linked directly to the tool in which it originates.
An all too typical scenario involving this approach is cheap tooling that produces a few acceptable parts and then develops problems. Then, due to production requirements, interim molds must be fabricated before permanent production tooling can be put in place. The overall cost, quality problems, and delays may easily overshadow the cost of building high caliber tooling at the onset of the project. With composites tooling, the old axiom "you get what you pay for" is always true.
Lesson #5
Don't mimic metal fabrication.
Replacing metal products with composites can take advantage of the inherent benefits composites offer. However, if composites are used to mimic the shape and structural design of metal components, many times the advantage is lost. Metals and composites are formed in very different ways. Metal fabrications may be formed in stamping presses or sheet metal breaks, which produce linear shapes and possibly tight corner radii. Many metal structures use stringer and panel design.
Composites have a wider range of possibilities. For example, composites are not limited to linear sheet metal type contours. On the other hand, very sharp small radii are less welcome in composites molding. Composites offer the capability of eliminating stringers or ribs in many cases through the use of sandwich structures. The maximum benefit from composites can be achieved, not as a replacement for metal, but as a product designed for composite construction.
Lesson #6
Involve the composites fabricator in the early stages of design.
The 1990s term is "integrated manufacturing". When the designer, engineer, and fabricator work jointly on a project, inefficiencies are less likely to occur. By using the team concept from the beginning of a project, all vital elements can be ironed out before irreversible problems arise. The designer must understand the capabilities of the molding process, while the fabricator must grasp the function of the product. The engineer needs to match the proper materials to the performance requirements and cost objectives.
Since the fabricator becomes the focus of producing a product that conforms to specifications and cost parameters, he or she is a vital source of information in the early design stages. The fabricator and designer can work through the critical balance of styling and manufacturability, for example, while the fabricator can guide the engineer to the most cost-effective molding process and materials for the project. History has shown that having the fabricator involved early in a project avoids costly mistakes on the part of designers and engineers.
These six lessons are a valuable education for non-composites engineers and designers involved with FRP products. By making use of composites industry resources, common pitfalls can be avoided in the job of developing and sourcing FRP products. By understanding these few critical issues, informed decisions can be made leading to a successful project.
Published in the May 1995 edition of Composites Fabrication.
Included herewith by kind permission of Composites Fabrication.
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High Strength to Weight Ratio
Corrosion/ Chemical Resistant
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Electomagnetic Transparency