Specifying Plastics for Electronics Design

Homi Ahmadi

Most electronic equipment uses some type of thermoplastic. It is important to understand the characteristics of plastics used in electronics equipment to determine which plastic is appropriate for a given application. These characteristics often affect the safety and reliability of the final product. This article examines many factors surrounding plastics selection that engineers should consider during a product's design stages.

Underwriters Laboratories (UL) has one of the most comprehensive materials databases available, and UL 94 ratings are widely accepted flammability performance standards for plastic materials. The UL 94 standard explains various flammability categories and describes the test methods used for each rating.

Classification

Each material tested can receive several ratings based on color and thickness. The amount and type of color additive can vary the flammability rating of a plastic. The UL plastic component directory normally specifies four colors: black, white, red, and natural. When specifying a material for an application, the UL rating should be applicable for the thickness used in the wall section of the plastic part. It is very important to remember that the thickness must always be reported with the UL rating to provide meaningful information about the material's characteristics. Ratings are categorized as follows:

• 94HB.
• 94V (subdivided into 94V-0, 94V-1, and 94V-2).
• 94-5V (subdivided into 94-5VA and 94-5VB).
• 94VTM (subdivided into 94VTM-0, 94VTM-1, and 94VTM-2).
• 94HBF.
• 94HF (subdivided into 94HF-1 and 94HF-2).

Ratings are differentiated primarily by the testing method. The classification depends on the following factors:

• Sample orientation (horizontal or vertical).
• Burn rate.
• Time to extinguish.
• Resistance to dripping.
• Drip flammability.

These parameters affect the end results, and hence the classification. With this in mind, each material tested could receive several ratings, depending on its color and thickness. Some ratings apply to specific product types. VTM, for example, refers to very thin material. HBF, HF-1, and HF-2 refer to foamed materials. These ratings, therefore, should not be compared to those in other categories. In other words, a vertically rated plastic material is better than a plastic that simply meets the HB requirements. In addition, a material accepted for a 5V rating must first comply with the vertical test requirements for V-0, V-1, or V-2. Depending on the end-product application, a designer could specify one or more ratings for a product.

Operating Temperature

Some engineers make the erroneous assumption that there is a direct correlation between a material's UL rating and its operating temperature. UL ratings relate only to a material's behavior when introduced to a flame source. How a material reacts when the flames come in direct contact with it determines its UL rating. For example, for a rating of 94V-0, a material must be self-extinguishing and must not drip or run while burning.

In the test, a sample of the material is held over a Bunsen burner, ignited, and allowed to burn. When the sample is removed from the flame, the fire must go out within 10 seconds, and the material must not have dripped from the burning sample. If the material continues to burn or if it drips and runs, it cannot be rated 94V-0. For this rating, operating temperature never comes into play.

Operating temperature is determined by establishing the point at which temperature causes an end product to cease to perform as it was intended. This premise applies to minimum as well as maximum temperatures. Most nylon materials, for example, have a maximum operating temperature of 250°F (120°C). However, the actual operating temperatures of finished goods vary depending on the mass (volume of material), temperature variations over time, and mold factor. A 94V-0 rating for a material does not necessarily mean that a finished product can withstand a high temperature.

Safety Standards and Plastics

Almost all product safety standards have clauses concerning flammability requirements for plastics in electronics. Requirements normally cover plastics that support live parts, such as a transformer's bobbin; enclosure of live parts, such as a monitor cover; and decorative parts, such as a lamp cover. EN 60950, for example, has guidelines specifying minimum requirements for plastic parts (see Table I).

Most electrical and electronic equipment has some type of enclosure. Enclosures are normally evaluated to meet one or more of the following requirements:

• A fire enclosure must prevent the spread of fire and flames.
• An electrical enclosure must prevent access to hazardous voltages or parts that carry hazardous energy.
• A mechanical enclosure must prevent injury from physical or mechanical hazards.

A product can have one or more enclosure types. Section 4 of EN 60950 requires that fire and electrical enclosures meet certain parameters in order to be considered effective. A summary of these requirements is presented here, but it is essential that designers refer to the standard for complete details.

The top and side openings of the enclosure must satisfy one of the following conditions: do not exceed 5 mm in any dimension; do not exceed 1 mm in width regardless of length; are constructed with louvers shaped so that they deflect external, vertically falling objects outward; are located so that objects, upon entering the enclosure, are unlikely to fall on bare parts at hazardous voltages.

If the end product is a stationary or movable equipment with a mass of 18 kg or greater, fire enclosures are considered to comply without test if, in the smallest thickness used, the material is of flammability class 5V.

The bottom of a fire enclosure—or individual barriers—must provide protection underneath all internal parts, including partially enclosed components or assemblies that could emit, under fault conditions, material likely to ignite the supporting surface.

If a hole is cut to fit a plastic window or a screen in a fire enclosure, then the window or screen must have a flammability rating of 5V. However, if a hole is cut to accomodate a fuseholder, a switch, or similar components, then there is no need for these components to meet 5V flammability requirements, provided that such components have appropriate approvals.

Design Tips

Resources. One source of valuable information is the UL Recognized Component Directory, also known as the UL Yellow Book. This directory provides names of companies authorized by UL to provide plastic components bearing a UL mark. It also provides technical information about various plastics. The book uses some important abbreviations and terms (see sidebar below).

Materials. Selecting the appropriate plastic material for a particular design is often the most difficult task a designer must face. Many factors—such as ability to mold or machine, weight, cost, thermal behavior, or flammability rating—affect the final decision. Because no plastic is likely to meet all of a designer's requirements for a particular application, some degree of compromise is almost always necessary in designing plastic parts for electronics.

Selecting a material cannot be based simply on a comparison of numbers from published data sheets. Values from data sheets often represent laboratory tests that may not duplicate real-life molding conditions. For example, it is a mistake to choose the most economical material for a part by comparing the cost per pound of various plastics. Some plastics weigh twice as much per cubic inch as others, and so it would then require twice as much material to fill a given cavity—and cost twice as much to ship.

The choice of any material should be based on the best combination of required properties. An ideal material will have a value for each required property just sufficient to perform properly and safely in a given application. A molded plastic part is significantly affected by processing factors such as direction of flow, pressure during molding, melting temperature, thermal degradation, cooling rate, and stress concentrations. A high value provided in a data sheet could be reduced considerably by processing conditions.

There is no simple procedure for selecting the best plastic for a new application. Understanding the behavior of a plastic under real-life conditions is critical to determining how the material will perform after it is molded. Successfully designing plastic parts that demonstrate optimal cost and performance characteristics requires learning as much as possible about many different plastics, and understanding the peculiarities of their processing.

Compound Selection. One of the first design considerations to establish is whether thermosetting materials or thermoplastics are appropriate. Thermosetting materials are initially soft but change irreversibly hard upon heating. Thermoplastics can be repeatedly softened by heating and hardened again by cooling. Designers must study the generic properties of different compounds to become familiar with their differences.

To make this determination, it is often helpful to consult molders and plastics manufacturers. However, such advice should be taken cautiously because these sources do not have access to internal factors such as production, engineering, purchasing, and marketing considerations. Molders can often detect and correct visible problems or readily measured factors such as color, surface condition, and dimensions. However, without extensive testing and quality control, less-apparent property changes may not show up until the molded parts are in service. Properties such as impact strength, toughness, and chemical resistance can be diminished by improper control of processing parameters. Molding processes can alter the published data-sheet properties, reducing strength as well as creating areas of stress concentrations.

Applications. Study similar existing applications to learn which materials, processes, and designs have worked successfully. Discuss the application with experienced molders, mold builders, and materials manufacturers for their recommendations. Finally, compare the relevant properties of each recommended material to determine the best plastic for the application.

Prototypes. By far the fastest and most economical way to produce plastic prototypes is to machine them from slab or bar stock. This is particularly true of products that are small in size. Unless a prototype part is molded in a production mold, however, it cannot duplicate the performance of an injection-molded production part. Most designers use machined prototypes for initial laboratory tests to ensure that as many issues as possible are addressed before a mold is made.

UL Recognition. Choosing a thermoplastic material that is not UL recognized could be risky. Such materials are subjected to various tests at UL annually. The results of the flammability and identification tests are compared with tests conducted during the original investigation. One test that UL conducts on nonrecognized plastics is a series of spectral analysis tests, which consist of thermogravimetric analysis, differential scanning calorimeter, and infrared analysis. These tests are lengthy and, therefore, costly. In addition to spectral analysis tests, UL also conducts tests such as flammability, ignition, mold stress, and drop.

If recognized materials are used, no annual follow-up testing of thermoplastics is required. Even with the use of recognized plastics, safety agencies such as UL rely exclusively on manufacturers and their plastic components fabricators or molders to establish a quality control program.

Plastics molders can apply to be part of the UL Recognized Fabricated Parts Program (also known as the molder's program). Under this program, UL ensures that a molder or fabricator has the necessary procedures and documentation in place to demonstrate the tractability of the material in each step of the molding process. It is important to note that plastic parts used in some industries, such as audio/video and the military, must come from a molder that participates in the UL molder program.

Conclusion

Plastics play an important role in the design of electronic products. It is crucial that engineers understand the characteristics of plastics in order to select the appropriate plastic for a given application. Many factors affect this decision, including the required properties and the molding process. Ultimately, selecting the right plastics can help ensure the safety and reliability of the final product.

Homi Ahmadi is approvals manager for Cortech Systems (Simi Valley, CA). He can be reached at hahmadi@cortechsys.com.