Valve Magazine - Summer 2009

WEIGHT AND DESIGN

A key advantage of engineering plastics versus metals is light weight. A quick calculation reveals that a 33% glass-filled nylon has a specific gravity of 1. 40 and is 84% lighter than brass (specific gravity of 8. 4) and 83% lighter than bronze ( 8. 1 specific gravity). Of course, a 1: 1 comparison in terms of weight isn’t the sole criteria when evaluating parts design. But engineering plastics also provide corrosion resistance, thermal insulation, no heavy metals and resistance to mineral buildup.

The primary processing method for production of plastic valves is the injec-tion-molding process. Because it’s highly repeatable, this process is especially suited for high-volume applications. It offers highly efficient and complex part design, parts consolidation, elimination of secondary steps and assembly, and lower overall finished part costs. With this confluence of material and design benefits, it is easy to understand how engineering plastics have made steady inroads.

In particular, engineering plastics such as nylon, polysulfone and PEEK have emerged as materials of choice for a range of demanding valve applications. But the same characteristics that make engineering plastics so useful and versatile can also present significant challenges to design engineers whose experience is mostly with metals.

Valve manufacturers are increasingly considering plastic a replacement for other materials such as quarter-turn valves (right) due to its light weight and corrosion resistance. Complex part design is possible with the use of glass-reinforced semi-aromatic nylon in an injection-molded multiport valve body/manifold (left). Such nylon also provides high strength and corrosion resistance for an injection-molded water inlet valve body (middle).

Also, understanding basic chemical composition is only one consideration when evaluating engineering plastics for valves. This understanding, along with standard data sheet properties, can provide an initial understanding, but such data gives only an indication of how test bars will behave for specific stresses at room temperature. Evaluation of properties that aren’t visible on the data sheet is what will lead to successful choices of materials.

KEY MATERIAL CRITERIA

Among the key reasons for choosing a certain material for a valve application is the need for strength and corrosion resistance. Engineers can find selecting the proper engineering plastic challenging when they don’t know what to seek. Professionals new to plastics should take time to understand their unique nature, their properties and the testing methods used. Only then can an engineer properly evaluate and understand the best way to use plastics.

One such characteristic is that, unlike metals, the mechanical properties of engineering plastics are time- and temperature-dependent. Engineers must

carefully consider how these and other operating conditions, such as chemical environment, pressure and mechanical loads, will affect the long-term properties critical for valve use.

Engineering plastics are generally grouped into two different classes: amorphous and semi-crystalline. One of the major differences between the two is how they respond to heat. Amorphous plastics do not have a defined melting point, but have ranges at which they soften, which are called the glass transition temperatures (Tg). Examples of amorphous plastics include polysulfone, polycarbonate and polystyrene. Each has vastly different thermal and physical properties.

Semi-crystalline plastics, on the other hand, have a glass transition temperature as well as a defined melting temperature (Tm). Examples include conventional nylons, semi-aromatic nylons and PEEK. Again, each has vastly different thermal and physical properties.

Understanding the differences helps in selecting a material. For example, when using amorphous plastics, the Tg should be significantly higher than the valve operating temperature because such plastics have no useful properties above this point. Another example is that those using semi-crystalline materials, which are typically glass-reinforced, need to be aware these materials maintain useful properties above the Tg, and are molten above the Tm. Figure 2 (page 28) shows typical changes in modulus as temperature changes for amorphous and semi-crystalline materials.

HEAT AND OPERATING
ENVIRONMENT

The effects of temperature and atmosphere on metal are well known. However, they are not so well known for engineering plastics. Because of this, dealing with these plastics requires looking beyond the data sheet properties. Since it always is extremely important to define the operating environment for short-term or long-term use when making material selections for valves, we need a way to do this for plastics. For thermoplastics, Underwriters Laboratories (UL) has designed a test that helps predict a Relative Thermal Index (RTI) for the material. This standard is known as UL 746B, Polymeric Materials, Long Term Property Evaluation.

UL developed the RTI to test deterioration of insulation materials in electrical devices over time, but it is also a useful tool in material selection. The RTI is the aging temperature that a material can endure for 100,000 hours while retaining at least half the initial property value being measured.