Thermosets: Engineering Plastics For Demanding Applications
Thermosets are a group of engineering plastics which are particularly well suited to demanding requirements. For the engineer, they are materials offering outstanding performance which is well understood and proven through decades of success. For the manager, they represent substantial cost savings as a replacement for metals, and as an alternative to other plastics offering similar performance, but at greater cost.
The hallmark of these materials is the ability to withstand heat and pressure for long periods of time, without failure. Their dimensional stability, creep resistance, chemical resistance, stiffness, and high temperature capability make them excellent choices where reliability under adverse conditions is important. A basic understanding of thermosets, and their strengths, can open the door to new ideas, and prevent the costly error of using the wrong material when designing in plastic.
Thermosets are different from other plastics. All plastics are composed of long strings of molecules connected end to end. In most plastics, these strings are independent of each other. These materials are like tangled masses of spaghetti, and are called thermoplastics. Thermoplastics may be melted over and over again like a wax candle. Because of this property, thermoplastics are melted at a high temperature, and then formed in a cool mold where they solidify.
Thermosets are different because their molecules become interconnected in a chemical reaction called crosslinking. Crosslinking is an irreversible process which requires heat. For this reason, thermosets are "cured" in a hot mold. Once crosslinked, all the molecules are tied together so the thermoset cannot be re-melted. Take cement for example. Once the sand, water, and concrete have been mixed together and cured, it is impossible to soften the mixture again. The principal members of the thermoset group are Phenolic, Epoxy, Diallyl Phthalate (DAP), and Polyester. Others include Polyimide, Alkyd, Melamine, and Silicone materials.
In general, thermosets are easy to design with, and their performance is predictable and dependable. Even so, there are potential pitfalls which can lead to unproducible designs, or costly failures in the field. Fortunately, many of these potential traps are easily defined, and avoided.
The first important consideration is stiffness. Depending on the material type and grade, thermosets range from about one tenth to twice as stiff as aluminum. They are far more rigid and dimensionally stable than thermoplastic. For applications requiring precise dimensional tolerances and low deformation under load, thermosets are an excellent choice.
An another important design criterion is associated with cross section thickness. Thermosets can be molded into very thick and paper-thin cross sections, but certain limitations apply. Thick sections take longer to cure, increasing cycle time and cost. Usually, such sections should be cored out, or replaced with reinforcing ribs which provide structural integrity at lower cost.
At the other extreme, thermosets will readily flow into cross sections of just a few thousandths of an inch. These materials, however, are too brittle to provide useful functions at thicknesses less than about 0.015". It is best to consult an experienced thermoset molder when delicate sections are required.
Impact strength is another area requiring special attention. Unreinforced thermosets have low impact strengths, however fiber reinforced thermosets offer excellent impact resistance. Depending on the application, one of these reinforced materials may be a good choice. High impact thermosets must be compression molded to obtain optimum results.
Sink marks are one problem eliminated by thermosets. Many plastics exhibit surface depressions over uneven cross sections. Thermosets are not susceptible to this problem. Applications which require flat surfaces may best be molded from a thermoset. An example might be a part intended to seal against a gasket, where even slight sink marks could lead to leakage. Further, lack of sink marks contributes to an excellent finish, and most thermosets are moldable with a surface finish which is unsurpassed by any other plastic. Highly polished molds will produce parts with a mirror-smooth finish.
Because of their dimensional stability, high strength, and thermal resistance, thermosets are often chosen as an alternative to metals. Potential benefits include cost saving, weight reduction, and improved chemical and corrosion resistance.
Phenolics and Epoxies are typically selected for metal replacement. Because it is possible to mold these materials into complex configurations in a single-step process, significant cost reductions can be achieved over machined, extruded, and cast metals. This is particularly true when threaded mounting joints or other features which require secondary machining are included. Phenolics and Epoxies are readily molded around threaded inserts during the molding process, eliminating the need for secondary operations.
Very close tolerances can be achieved when using thermosets, and repeatability from part to part is excellent. Experience has shown that accuracy of 0.005" is achievable over a 12' dimension on a high production basis for complex designs. Decreased part size and complexity permit closer tolerances. It is possible to mold features accurate and repeatable to a few ten thousandths of an inch. Dimensional stability of the materials insures that close tolerances will be maintained under severe conditions and over long periods of time.
Elevated temperature applications are solid thermoset territory. While many materials claim to be high temperature polymers, few stand up in real life, and those which do tend to be costly. High temperature thermoplastics will deform excessively if load is applied at temperatures approaching their deflection temperature. At lower temperatures, many of these materials will creep substantially. Thermosets on the other hand will often provide dimensional stability and load bearing capability at temperatures exceeding their deflection temperature and over long time periods. In general, thermosets offer high temperature performance equal to or better than other plastics, at a fraction of the cost.
Because crosslinking is irreversible, thermosets do not begin to melt as temperature rises. For this reason, strength and shape are retained at temperatures which cause other plastics to weaken. Thus, a thermoset is usually the best choice in any application where creep, strength, dimensional stability, and reliability at elevated temperatures are primary design considerations.
The diversity of the thermoset family allows for a wide range of specialized properties. Epoxy/ carbon fiber composites have been developed which achieve incredibly high strength. Phenolic pushes temperature resistance over 450F. Silicone and Polyimide push it even higher. Grades of materials offering conductive properties are in use for static bleed and EMI attenuation. Part of this flexibility is due to the fact that thermosets will accept huge amounts of filler and reinforcing materials. Fillers lend special properties to the material, and reduce the amount of costly resin in the final product. It is possible to load thermoset with as much as 70 percent filler to 30 percent resin. Thermoplastics typically are limited to about 30 percent filler. The versatility of thermoset resins allows the use of plastics in areas never before considered.
Clearly no one material or process can be right for every application. One challenge of design, however, is to specify the best possible material that meets the job requirements. The term "best" refers to a balance of performance and cost criteria. Typically, one might isolate a number of candidate materials/processes which will meet the design specifications being considered, then apply cost criteria to select the "best" one.
Molded thermosets are competitive with other engineering plastics when evaluated on this basis. Cost per cubic inch of some engineering thermoplastics can be compared to an engineering grade of phenolic. All of the materials are about 30 percent glass reinforced, and are UL 94 V-0 rated. Clearly, the phenolic is significantly less expensive, although it will provide greater stiffness, dimensional stability, compressive strength, and far better high temperature performance than the others. This is why molded thermosets should be considered for applications requiring these types of properties. This is not to say that thermosets are better than engineering thermoplastics, only that they have strengths which make them preferable in certain applications. If your application needs properties similar to those discussed in this article, using the right thermoset can yield a substantial payback in improved performance, greater reliability, and reduced costs.
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