Authored by:

Michael Pasko
Application Engineer
Spirol International Corp.
Danielson, Conn.

Edited by Kenneth J. Korane
ken.korane@penton.com

Key points:
• Brass inserts’ high thermal conductivity means faster heating and insertion into plastic parts.

• Brass is easier to machine than stainless steel, which lowers manufacturing costs.

Resources:

Spirol International, www.spirol.com

Assemblies made of plastic or other soft materials often rely on threaded metal inserts to secure fasteners and reinforce joints. Though stainless steel may be appropriate for use in some applications, it is often unnecessary. Brass inserts can satisfy the majority of performance requirements while offering significant cost benefits.

Raw material costs for brass and stainless steel are typically similar but the cost of machining stainless steel is much higher. Machinability ratings indicate comparative speeds at which materials may be cut while maintaining a quality surface finish. Machinability ratings assign efficiency ratings to materials of similar composition, so generalities can be made. Free-machining stainless steel may only be 40 to 50% as efficient to machine as brass. Other austenitic stainless steels may be less than 40% as efficient to machine.

Stainless steel is also a poor conductor of heat when compared to brass. This elevates temperatures during machining operations and reduces the life of cutting tools.

Brass scrap generated during machining can be sold with a minimal loss in value when compared to the cost of incoming rod. In contrast, stainless-steel scrap retains little of its initial value, which constitutes a further reduction in overall efficiency.

Machined inserts can be installed using a number of methods, though postmold heat installation is the most common. Inserts can be heated several ways: charged directly using heated tips or chimneys, or indirectly with ultrasonic energy. Either method requires the insert reach a temperature approximately equivalent to the plastic’s melting point.

Brass offers much better thermal conductivity than stainless steel or carbon steel. In fact, it’s twice as conductive as carbon steel and 15 times more conductive than austenitic stainless steel. This allows more rapid heating and cooling when heat installing in thermoplastics, and this improves cycle time.

More rapid dissipation of heat also improves the positional accuracy of inserts. Rapid cooling prevents a condition commonly referred to as “float.” This condition exists when inserts remain hot after the tips or ultrasonic horn has been removed. Rapid cooling lets plastic set quickly and this fixes insert position. This maintains accurate installation depth parallelism.

Brass
Brass offers significant advantages when compared to stainless steel and is preferred for most industrial and agricultural applications. But there are instances in which stainless steel may be required. For example, brass and stainless steel both resist corrosion, but they react differently to various corrosive agents. Here’s a closer look at some of the advantages and limitations of each material. For example, brass:

• Is excellent for hot and cold-water industrial/residential systems, including those carrying potable water.

• Is suitable for use in some marine environments including brackish water and seawater with moderate currents. Avoid exposure to high-velocity currents. Brass exposed to marine atmospheres develops a protective green patina.

• Performs well in cryogenic applications, making it an alternative to 300 Series stainless steel in some environments.

• Typically handles exposure to mild alkaline solutions, though strong solutions such as hydroxides and cyanides should be avoided.

• Resists corrosion in nonoxidizing acids. Oxidizing acids should be avoided.

• Provides excellent corrosion resistance to petroleum products.

• Possesses good strength and actually overlaps the tensile strength of 12L14 low-carbon steel. When designers must make a threaded part stronger, a simple increase in thread length is often adequate — this can avoid the higher cost of stainless steel.

• Can be nickel plated to reduce tarnishing and corrosion or to simply provide a “silver” finish. Nickel finishes can also provide a hard wear surface on products such as gears, bearings, and plumbing fixtures.

Stainless steel
Stainless steel is available in many varieties, though 300 Series or austenitic stainless steel is most commonly used for inserts. The properties of stainless steel vary dramatically from one alloy or family to the next and it is difficult to make general statements regarding performance. For this reason it’s a good idea to consult the application engineers at a reputable insert manufacturer when considering stainless steel or other special materials. Among stainless steel’s advantages:

• Certain grades, such as 316, are superior to brass in more-aggressive marine environments such as fast-moving currents, but this does not apply to all grades. The most common turning stainless, free-machining 303, contains added sulfur that improves machinability in comparison to other 300 Series stainless steels. Conversely, sulfur significantly decreases corrosion resistance in seawater. Brackish or slow moving seawater may also increase crevice or pitting corrosion in various grades.

• Excellent resistance to petroleum products and many acids, and it can be passivated in either citric or nitric acid solutions. Avoid hydrochloric acids.

• Is typically stronger than brass, though actual comparisons depend on grade and alloy.

• Certain grades of austenitic stainless steel such as 302, 304, and 316 are FDA approved for food contact, and are, therefore, a good choice for food and beverage applications. Free-machining 303 stainless steel is not approved for use in contact with food.

• Austenitic stainless steel provides a higher service temperature than brass. It’s important to note that this offers limited benefit because the heat-deflection temperature of the plastic component is usually the limiting factor. The service temperatures for most plastic assemblies are within the acceptable service temperature for brass.