Many commonly machined plastics sharing similar characteristics fall into the category of engineering plastics. Understanding the similarities and differences of these materials helps both in machining and making selections for specific applications.

This article will focus on the engineering plastics available as stock shapes for machining. Since past issues of Plastics Machining & Fabricating have provided analysis of the performance characteristics of most of these materials, this article offers property comparisons.

A simple way to classify thermoplastics is as a triangle, grouping the materials according to structure and properties. From top to bottom, the materials are ranked by their relative heat resistance. The left to right division is based on polymer chain structure, either amorphous or crystalline. Amorphous materials have a random, spaghetti-like molecular structure. Semi-crystalline materials have areas of highly organized molecular chains.

When high resistance to wear, such as for sheaves, cams, gears and bearings, a wear-resistant nylon is often the material of choice.

In general, crystalline thermoplastics are stronger, stiffer, more dimensionally stable, and more resistant to heat, wear, chemicals and creep than amorphous materials. The advantages of amorphous materials are transparency, increased toughness at low temperatures and the ability to be thermoformed into parts.

Standard plastics are at the bottom of the triangle and are generally used for non-critical, low-stress applications, where temperatures do not exceed 150F.

Engineering plastics, however, have higher strength and can be used for applications where temperatures do not exceed 250F. These materials are widely used for general-purpose structural (amorphous) or bearing and wear (crystalline) applications.

Materials with heat resistance to temperatures between 250F and 450F can be grouped as advanced engineering plastics. In addition to heat resistance, these materials typically have better chemical resistance (crystalline) and steam resistance (amorphous) than the engineering plastics.

At the top of the triangle are the imidized plastics, which perform at temperatures up to 800F under extreme stress or wear conditions.

If the basic requirements of an application are known, the triangle concept can be used to narrow the appropriate materials to a few choices. To distinguish between materials within a category, specific properties should be evaluated.

Two temperatures generally are considered in evaluating heat resistance: continuous use temperature and heat deflection temperature.

Continuous use temperature (CUT) is the temperature above which the physical properties of a material degrade after prolonged exposure. It is the maximum temperature recommended for long-term service.

Heat deflection temperature (HDT) is an indication of the softening temperature of a material under a load usually 264 psi, according to the ASTM method. In moderately- to highly-stressed applications, materials should not be used above their HDT.

Of all the engineering plastics, polycarbonate (an amorphous material) is the most heat-resistant, with a CUT of 250F. Nylon, PBT and PET (crystalline materials) can be used continuously at over 200F. The other engineering plastics generally are not recommended for continuous use at over 180F, although there are some special heat-resistant grades that can be used at slightly higher temperatures. For temperatures greater than 250F, materials in the advanced engineering plastic range are recommended.

Strength is measured by the stress requirement to deform a material in tension, compression or flex. Tensile strength is the most common measurement used to evaluate strength. Stiffness is a measurement of how much a material will deform when a load is applied, indicated by tensile modulus, compressive modulus, or flexural modulus, also called elastic modulus.

As previously mentioned, crystalline materials generally have higher strength and stiffness than the amorphous materials. Nylon (PA), acetal (POM) and thermoplastic polyester (PET) have about the same level of strength and stiffness at room temperature, with the polyesters having a slight advantage. PET also remains stiffer at higher temperatures than the other engineering plastics, including PBT. Comparatively, ultra-high molecular weight polyethylene (UHMWPE) has low strength and stiffness.

On the amorphous side, polycarbonate (PC) and acrylic (PMMA) approach the crystalline materials in strength and stiffness and outperform polyphenylene oxide (PPO) and ABS. The tensile strength of polycarbonate is actually higher than that of nylon and acetal at temperatures between 200F and 250F.

Impact strength, sometimes referred to as toughness, is the ability of a material to withstand a suddenly applied load. There are several tests used to measure impact strength. No single test predicts the impact behavior of a material under the variety of conditions possible.

One of the most common tests is the Notched Izod Impact Test, which is designed to measure the effect of a sharp notch when a material is suddenly impacted. The Tensile Impact Test measures the toughness of an unnotched sample when subjected to a sudden tensile load. The Gardner Impact Test measures the energy required to break a sample when a shaped weight is dropped on it. A more sophisticated test is the Instrumented Impact Test, which includes measurements of deceleration and energy as a sample is impacted by a weight. The values in the table on this page are from the Notched Izod Test.

Many plastic materials are notch sensitive, but some, especially the amorphous materials, are more impact resistant than others. Polycarbonate, UHMWPE and ABS are much more impact resistant than the other engineering plastics, especially at low temperatures.

Nylon has relatively low impact strength, however cast nylon shapes are slightly tougher than extruded nylon shapes. In addition, nylons get tougher when they contain some moisture, but measurements are usually done on dry samples. Although impact test values for nylon are lower than those for acetal, when nylon contains some moisture, it is actually tougher than acetal.

Two measurements are used for wear comparisons: the limiting PV, which is the maximum pressure-velocity product at which the material can operate; and the k factor, which measures the rate of wear.

For bearing and wear applications, the properties of crystalline materials far exceed those of the amorphous materials. In general, amorphous materials are not recommended for wear applications, unless they are modified with a lubricating agent.

Among engineered plastics, PET has the best wear resistance, wet or dry. It offers three times greater wear life than acetal, based on the k factor, and as much as 15 percent greater wear life than nylon. Nylon outperforms acetal in dry environments, but acetal is a more effective wear material in wet environments.

The engineering plastics, like other materials, usually do not wear well against themselves. When designing bearing and wear parts, different materials should be chosen for mating parts in order to increase wear life and avoid seizing. The exception to this is PET.

Another property related to wear resistance is abrasion resistance, which indicates a material's ability to resist wear in a sand-slurry or other abrasive mixture. UHMWPE is the most abrasion-resistant engineering plastic, although nylon and PET also have relatively high abrasion resistance.

Various additives can greatly improve wear characteristics. For example, compared to standard nylon, enhanced grades of nylon are available with a better wear rate and the ability to handle a higher PV.

Two main factors influence dimensional stability: thermal expansion and moisture absorption.

Most engineering plastics have relatively low rates of water absorption and low coefficients of thermal expansion, and therefore have excellent dimensional stability. One exception is nylon, which has a tendency to absorb moisture, although certain types of nylon are more resistant to water absorption than others. ABS and UHMWPE are also less dimensionally stable than the other engineering plastics, due to their higher coefficients of thermal expansion.

For very tight tolerance parts, improved dimensional stability can be attained by rough machining, annealing and finish machining with a light cut. Annealing consists of slowly heating the part to a specific temperature, holding at that temperature for several hours, and slowly cooling the material. Material manufacturers can recommend specific temperatures for post-machining annealing.

When using chemicals, the compatibility with each material should be tested, but generally, the crystalline engineering plastic will perform well in industrial environments.

UHMWPE and PET have very good chemical resistance, although strong bases attack PET. The amorphous materials generally have limited chemical resistance. For aggressive chemical environments, the crystalline advanced engineering plastics, such as PPS or PEEK, should be considered.

Water also has an effect on some engineering plastics. Hot water and steam hydrolyze, or attack, both PBT and PET. Extended exposure to water above 150F should be avoided with these materials.

Nylons are not chemically attacked by water, but can absorb up to 7 percent water (by weight) under high humidity or when fully submerged. This absorption causes dimensional change up to 2 percent, but can be compensated for by proper part design. If dimensional stability is important in wet applications, acetal may be a more appropriate choice.

Another concern related to chemical resistance is stain resistance. Nylon stains easily due to its tendency to absorb moisture. PET is one of the most stain-resistant engineering plastics and, for that reason, is preferred in many food applications.

The materials compared in this article are standard, unfilled plastics without additives. Several additives can increase the properties of materials significantly, although machining materials with certain additives may be more difficult.

PTFE, silicone, graphite and other additives increase wear resistance and improve friction properties.

Glass and carbon fibers are often added to increase material strength, stiffness, dimensional stability and heat deflection temperature.

Antistats or carbon are added to make plastics electrically conductive or semi-conductive. Specific properties of these filled materials will depend on the combination and percentage of additives.

Most engineering plastics are considered easy to machine.

Polyesters are slightly more difficult to machine than the other engineering plastics due to their notch sensitivity. Design and fabrication procedures are critical when machining polyesters.

Polycarbonate and acrylic can also be difficult to machine. If these materials get too hot during machining, they begin to soften and stick to tooling. Feed rates should be carefully selected.

Some grades of acetal, while one of the most easily machined materials, can outgas formaldehyde when heated. Heat generation can be minimized by choosing appropriate feed rates and using sharp, carbide-tipped tools. Also, coolants should be used to reduce heat generation and dissipate any fumes created.

For optimum surface finishes and close tolerances, non-aromatic, water-soluble coolants are suggested with all engineering plastics. General-purpose, petroleum-based or aromatic-based cutting fluids are appropriate for crystalline plastics. Only water-based coolants should be used with amorphous plastics. Petroleum-based coolants may cause amorphous materials to stress crack or "craze."

As mentioned previously, plastics can be notch-sensitive and parts should be designed to avoid sharp corners and edges. It is recommended to radius corners and the bottom of tapped holes to prevent cracking during machining and in use. For drilling operations, coolants are strongly recommended.

Engineering plastics are all available in various types of stock shapes, including plate, rod, disc and tube. Nylon is available in the widest range and the largest sizes since it can be cast as well as extruded. Nylon parts weighing up to 800 pounds and measuring 6 feet in diameter have been cast in a single mold.

The cost of engineered plastic stock shapes is generally proportionate to the properties of each material. UHMWPE and acrylic are the lowest-cost engineering plastics. Nylon and ABS are about two times the cost of these materials, and acetal and PET are 10 to 20 percent more costly than nylon and ABS. Polycarbonate and PPO are the most expensive engineering plastics: they cost two times as much as nylon and ABS.

Often the cost of producing machined parts is increased due to the amount of material wasted when making unusual shapes. When the material is nylon, using custom-cast shapes can reduce this cost significantly. For instance, using a tubular bar to fabricate a bearing provides significant savings over using a plate or rod to produce the same part. In addition to tubular bar, more detailed parts are routinely cast from nylon.

Within the category of engineering plastics, each material will excel under different conditions. Selecting the best material is seldom based on just one outstanding property, but on which material offers the best combination of performance characteristics in use. Understanding the "material triangle" and the basic properties of materials can help you better select and fabricate engineering plastics.

Laura Pugliese is an application engineer at DSM Engineering Plastic Products, Reading, PA. She has a BS in Chemical Engineering from the University of Virginia.

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