Designing With Plastics

Nov. 15, 2002
Some degree of compromise is almost always necessary in designing plastic parts.

Some degree of compromise is almost always necessary in designing plastic parts. Arriving at the best compromise usually requires satisfying the mechanical, thermal, and electrical requirements of the part, utilizing the most economical resin or compound that will perform satisfactorily and be attractive, and choosing a manufacturing process compatible with the part design and material choice. Setting realistic requirements for each of these areas is of utmost importance.

Probably no plastic will provide 100% of the requirements for the desired performance, appearance, processibility, and price. Selecting the best qualified material is not based simply on comparing numbers on published data sheets; such values can be grossly misleading. For example, choosing the most economical material for a part by comparing the cost per pound of various plastics is a mistake. Some plastics weigh twice as much per cubic inch as others and so would require twice as much to fill a given cavity and cost twice as much to ship.

A more meaningful comparison is cost per cubic inch. But since most expensive plastics are far stronger than the cheaper ones, cost/strength values should be analyzed as well. Paying more per pound or per cubic inch is often more economical if less material can do the job.

Standard test data
There is an attractive simplicity in deciding on the material with the highest ASTM test values as listed in manufacturers' data sheets. Unfortunately, this method seldom results in the best choice. First, the choice of any material should be based on the best combination of properties, not only on one property. An ideal material will have a value for each strength property just sufficient to perform properly and safely in a given application and no higher. The best material choice usually represents a trade-off among satisfactory properties, ease of processing, and cost. It is seldom the plastic with highest values in any single category.

Second, values in published data sheets represent laboratory tests, which do not duplicate real-life molding conditions. Strength of a molded plastic part is significantly affected by such processing factors as direction of flow, pressure during molding, melt temperature, thermal degradation, cooling rate, and stress concentrations. As a result, a high value listed in the data sheets for a given property can be reduced considerably by processing conditions.

Because the molding variables are beyond their control, material suppliers have chosen to make simple, standardized ASTM laboratory tests that are easily reproduced. The tests are not material performance tests for predicting real-life results over a period of time, but are actually material quality tests. They are made for the purpose of assuring a buyer that the batch he buys today is the same formulation and quality he bought last month or last year. The ASTM tests were not intended to compare one material with another on the basis of strength after molding; different configurations and different molding processes can change the values significantly.

Third, ASTM tests are essentially short-term tests with all variables fixed except the one being measured. For example, testing for tensile, flexural, or impact strength is often done at a standard rate of loading. A short-time tensile test may show a high strength value, but if the rate of loading is reduced by 100 or 1,000 times, tensile strength can drop to as little as 20% of the short-time strength. If the loading rate is increased, the tested tensile-strength value may double or triple. The data sheets do not show this.

Plastics are also temperature sensitive; strength properties may vary by a factor of 10 over a temperature difference as little as 200°F. Since most ASTM strength tests are made at room temperature, behavior at other temperatures cannot be reliably predicted from these data. Nor can the behavior be predicted under combinations of temperature, varying loads, and changing environments outside the narrow test conditions.

Flammability tests
The Federal Aviation Administration, the Department of Transportation, and other regulatory agencies have mandated that design materials comply with specific flammability test requirements. Flame-retardancy requirements generally include limits on flame spread, burning time, dripping, and smoke emission.

By far the most stringent and also the most widely accepted of such tests is the Underwriters' Laboratory Standard for safety, UL 94, for electrical devices. The test, involving the burning of a specimen in the vertical position, is the one by which most flame-retardant (FR) plastics are rated. In this test, the best rating is UL 94V-0, which defines a flame duration of 0 to 5 sec, an afterglow of 0 to 25 sec, and the presence of no flaming drips that ignite the dry, absorbent cotton located below the test specimen.

Flame spread and dripping tendencies of test materials are also characterized in the ASTM D635 Standard, a horizontal test that reports average time of burning (ATB) and average extent of burning (AEB). In both the UL and the ASTM tests, the presence of short reinforcing-glass fibers has been found to inhibit dripping in test compounds.

A more quantitative measure of a material's resistance to burning is determined from ASTM D2863, which measures the minimum concentration of oxygen in an oxygen/nitrogen mixture that will support candlelike burning for 3 min or longer. Results are reported as a Limiting Oxygen Index (LOIs). Composites with LOIs over 28% are usually listed as 94V-0.

Smoke emission is measured in the air column above a burning specimen in a National Bureau of Standards smoke chamber. In the test, a specified area of plastic is exposed to heat under flaming conditions. Smoke measurements are reported as "specific optical density." This is a dimensionless value because it represents the optical density measured over unit path length within a chamber of unit volume produced from a test specimen of unit surface area. The optical density measurement, (Dmax), is based on the attenuation of a light beam by smoke accumulating within the closed chamber during flaming combustion. (For a reference, the Dmax for red oak is 76.)

Smoke generated during combustion consists of suspended soot particles that are formed between the pyrolysis zone and the flame front. These particles are molecules of highly condensed ring structures that are most readily formed by aromatic polymers (SAN, SMA, and polyphenylene ether). Polymers having aliphatic carbon backbones, such as polypropylene and nylon, tend to generate less smoke, but this effect is offset in the FR compounds by the increase in smoke caused by halogenated flame-retardant additives. Resins of higher thermal stability (PC, PSF, PES, PEEK, and PPS produce the least smoke of the available UL 94V-0 thermoplastics.

Experience needed
There is no simple procedure for selecting the best plastic for a new application. It must be done with direct experience and knowledge of the behavior of various plastics under the real-life conditions to be encountered by a particular part after it is molded.

Until this experience is acquired, a designer has little choice but to seek the advice of a reliable molder, materials manufacturer, or compounder. Even here, there is the danger that these sources may not be aware of the many compromises a company must make internally among production, engineering, purchasing, and marketing considerations to produce a product that will sell at a profit. Also, a molder might be inclined to recommend the material that works best in his equipment, rather than the best for the application. Thus, the successful design of plastic parts that have the optimum cost/performance characteristics require learning as much as possible about many different plastics and the peculiarities of their processing.

Designers used to take little interest in the molding of parts they designed. They sent the drawings to the molder in another department or another company and expected perfect parts to emerge. But design and processing have become so interrelated that this separation can no longer exist if products are to be consistently successful.

Molders can usually be relied upon to detect and correct visible problems or readily measured factors such as color, surface condition, and dimensions. However, less apparent property changes are another matter. These may not show up until the moldings are in service, unless extensive testing and quality control are used. Such properties as impact strength, toughness, and chemical resistance can be diminished by improper control of processing parameters. Close cooperation between designers and molders can eliminate disappointment and help ensure successful products.

After candidate materials are selected, the design should be tested under real-life conditions involving the temperatures, loading, and environment of the anticipated service. Ideally, the test part should be molded in the shape and from the material to be used in production. In the beginning, this is costly and time consuming, but as experience is acquired, accelerated tests can be developed on simpler shapes; testing will then be more economical but just as reliable.

Understanding the molding process that will produce the part is also necessary. The process directly affects material choice, shape, tolerances, and properties of the part. For example, a container or housing can be made by injection molding, blow molding, thermoforming, or rotational molding. But each process requires a markedly different design, would use a different grade of plastic or a different plastic entirely, and would produce a component with significantly different properties.

All molding processes alter the published data-sheet properties, reducing most strengths and often creating areas of stress concentrations. But each process may create stresses in different areas. Sometimes, processing conditions are so severe that there is no choice but to redesign the shape and change to a different plastic. Unfortunately, reliable data on molded strength properties may never be available because of the basic nature of plastics. Their characteristics are partly those of solids and partly those of viscous liquids, preventing the use of classical Hookean engineering formulas for calculating load distributions accurately.

A starting point
One way to begin is to study similar existing applications to learn which materials, processes, and designs have worked successfully. Next, discuss the application with experienced molders, mold builders, and materials manufacturers and compounders to get their recommendations. Finally, try to select the best plastic for the application by comparing the relevant properties of each material that has been recommended.

The brief summaries presented here indicate property highlights and characteristics of the most common families of thermoplastics and thermosets used in industrial and consumer products. Only general family characteristics are given; many properties can be changed significantly by compounding the base resins with fillers, plasticizers, or reinforcements, or by copolymerizing them with other monomers.

Fillers usually decrease cost, increase stiffness, improve dimensional stability and reduce shrinkage. Plasticizers increase flexibility and reduce most strength properties. Reinforcements (usually glass, carbon, or mineral fibers) improve strength, dimensional stability, and thermal endurance, but increase cost. Copolymers can have either higher or lower properties and cost depending on the monomers used and their proportions.

A prototype plastic part cannot duplicate exactly the performance of an injection-molded production part unless it is molded in a production environment in a production mold. This is normally impractical, so the best that can be expected is an approximation of a production part.

The fastest and most economical way to produce plastic prototypes is to machine them from slab or bar stock. But stock forms of plastics are made by extrusion, not injection molding. Besides not having the same flow orientation of a molded part, extrusions of a given material are usually made from a higher molecular-weight grade than that used for injection molding. Consequently, properties such as impact strength, creep resistance, and chemical resistance tend to be higher in an extruded material. These differences are particularly significant in the crystalline plastics such as nylon, acetal, and polyethylene. The variations are usually smaller in amorphous materials such as ABS, polycarbonate, and polystyrene, but even minor differences can be critical in some applications.

Molding, rather than machining, of prototypes generally provides a better approximation of a production part, but here too, a number of differences in conditions can cause misleading results. For example, if the prototype mold is made from epoxy resin, the molded part will cool at a much slower rate than it would in a production (steel) mold. And cooling rate can affect tensile and impact strength as well as heat and chemical resistance, elongation, and stiffness -- particularly in crystalline plastics.

Making prototypes in an aluminum mold improves their similarity to production parts, but this method also has drawbacks. Here, because of the high thermal conductivity of aluminum, faster cooling is the problem that alters properties from what they would be in a part made in a steel mold. Also, there is difficulty in getting the resin to flow into a mold that cools rapidly. This problem can be offset by higher injection pressure, but the greater density that results causes other variations.

The closest duplication of a production part is produced by injection-molding prototype parts in steel molds. A relatively soft steel can be used for prototypes, so that machining is not difficult. But even here, because certain shortcuts are usually made (in polishing surfaces or in simplifying cooling passages, for example), the quality and accuracy of the resultant moldings are something less than what would be expected in production moldings. Nevertheless, steel prototype molds produce parts that most nearly duplicate production parts. Although this is the most expensive prototyping method, it may be the most economical in that it provides the surest way to avoid expensive changes in production molds.

Computer-aided design
In order to reduce the number of prototypes required, designers are turning to finite-element analysis. FEA programs allow parts design and structural analysis before tooling is cut. Related software, such as moldfilling analysis and warpage programs, help to eliminate processing problems before they start. Many of these programs depend on materials databases that organize various properties into a manageable format. Integrating FEA and molding programs with a database often optimizes the use of capital-intensive CAD/CAM equipment. Databases also enable users to conduct a fast and efficient materials search.

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