Quick connect couplings build materials: Plastics v. Metals

In order to correctly choose the best quick connect coupling, materials is only one part of the selection process. Performance capabilities of the quick connect, the application in which the quick connect fitting will be used, and the material from which the quick connect coupling will be made must also be considered. All of these qualities are equally important when looking to install or repair a quick connect.

Quick connect build materials

Build materials used in the construction of hydraulic quick connects, fluid quick connects, and pneumatic quick disconnects as well as other pipes, tubing, male fittings, female fittings, spacers, flanges and gaskets, include: Stainless Steel, Aluminum, Carbon Steel, Brass, Copper, Iron, Nitrile, Fluorocarbon, PTFE (polytetrafluoroethylene), ETFE, PVDF, PFA, FEP, PVC (polyvinyl), CPVC, PP (polypropylene), PE (Polyethylene), PEEK, PEK, Nylon, Vinyl, PCTFE and UHMW.

Quick connect styles and designs

Along with a multitude of quick connect build materials, there are also a large number of quick connect styles and designs. Each quick connect design works differently to provide safety and performance benefits for the hydraulic, oil, fluid, or gas system. Among the most largely-used quick connect designs are; Ball Lock Couplings, Roller Lock Couplings, Pin Lock Couplings, Flat Faced Couplings (ideal for limiting spills and drips), Bayonet Couplings, Keg Ball Lock Couplings, Ring lock Couplings, and Cam Lock Couplings. More general/universal terms may also be used, like; Single Shut-Off Couplings, Push-to-Connect Fittings, Poppet Couplings, or Quick Release Couplings.  

Determining the best material

In order to determine if a plastic or metal quick connect is the best material suited for your system, it is important to first understand the specs and ratings of the quick connect coupling. Compare these ratings and limitations to the parameters of the hydraulic, fluid, or pneumatic system. This includes ranges and limits for temperatures, pressures, flow rates, volume, pressure surges, and estimated connect/disconnection cycles. It is also important to understand the style/design of the quick connect, and why a particular design is the best choice to provide optimal safety and efficiency for your system. Typically replacing like couplings and valve designs is recommended by OEM’s. But as technology changes, we see higher pressures, increasing system temperatures, along with new and innovative fluids and gases; thus deeming it important to understand the reasons the incumbent quick connect coupling is still the best for your hydraulic, fluid, or compressed air application.

Application specific requirements

Once you match the flow rates, pressures, and temperature ratings of the system to the specs of the quick disconnect, you need to look more at the application the quick connect will be used. This will provide more information about what metals and/or plastics should be considered. Following are examples of applications that require specific designs and build materials to safely transfer/supply the air, gas, chemical, liquid, oil, or water through the fitting.

-         Chemical quick connect fittings - Chemical quick disconnect couplers must hold up against potentially caustic substances such as fuels, oils, solvents, acids, and strong bases. With chemicals, always check the chemical compatibility of the quick connects plastics and/or metals along with that of the fluid, oil, or gas to be used in the system.

-         Gas quick connect fittings – The integrity of a gas quick connect is critical. For systems handling flammable gases, the seals and fitment between quick connect fittings must adhere to the tightest of tolerances to ensure gas do not escape/leak from the fitting. Propane quick connect couplings, and natural gas couplings must be made to the highest safety and quality standards. To avoid gas leaks in the quick-connect, utilize high quality materials and seals to keep users and others safe.

-         Hydraulic quick connect fittings – A high quality hydraulic disconnect coupling will help make system maintenance and machine repairs easy while matching the flow rates and pressure demands of the rest of the hydraulic system. Watch for pressure ratings (hydraulic fittings generally require meeting high pressure ratings), and flow rates to ensure the coupling can safely handle the demands of the system. Also, consider the viscosity of the hydraulic fluids and the temperatures of the system and those of the environment in which the machinery will be operated. A strong metal quick connect is often the best option when picking a quick connect that is strong enough to hold up in extremely high-pressure hydraulic assemblies. (reference; ISO DIS 13031 – Earth moving machinery – quick couplers – safety, and BS ISO 5675 – Agricultural tractors and machinery – general purpose quick-action hydraulic couplers)

-         Pneumatic quick connect fittings – Flow rates are of the utmost importance for compressed air fittings. These are considered the work-horses of the air supply lines and will get more use than almost any other hose or fitting built into the compressed air supply lines. Pick a material that is durable enough to last in heavy-duty industrial/manufacturing environments.

-         Food-grade quick connect fittings - Food-grade quick couplers must retain the sanitary nature of the fluids, liquids and gas in applications such as the food, beverage, medical, and pharmaceutical industries. It is also important that the materials of the quick connect do not off gas, or otherwise introduce chemicals directly or via chemical reactions produced by contact, or flow or the fluid, or gases in the system.

-         Water quick connect fittings - Water quick connect couplers must keep potable water free of contaminants. Typically operated at lower pressures than some of the other systems listed, the water inside these systems will be used by industrial manufacturing, municipal water, and potentially consumed by the general public. For these reasons, the quick connects must be free of potentially dangerous chemicals, as well as foreign bodies and contaminants that could damage industrial equipment or products.

Of course, there are many subsets of the markets listed above. These smaller niche markets are numerous, with demands that fall far outside the standard quick connect fitting specs. In order to meet the demands of these varying systems, countless variations of metal quick connects/plastic quick connects are available. Among them are hybrids that fall into both the plastic and metal quick connect categories. Below are general insights on the metals and plastics most commonly used in the construction of quick connect fittings.

Metal Quick Connect Benefits Overview

Metal quick connect fittings are stronger than plastic, respond better to higher temperatures than plastic quick connect couplers, and can withstand increased pressures and pressure surges more than plastic quick connects and fittings.

Aluminum – Aluminum quick connect couplings are lightweight and resist corrosion. Aluminum fittings are generally more affordable than other metal quick-connect couplings, but are more susceptible to scratches, and wear when not alloyed with other metals like; zinc, copper, silicon, or manganese to improve strength and increase hardness.

Brass – Brass quick connect couplings (an alloy of copper and zinc) are strong, durable, resist corrosion, and won’t negatively react/interact with copper or zinc piping. Brass is highly conductive and remains durable at high temperatures. These properties make brass a very popular material for the manufacturing of quick connect coupling. Along with its ability to tolerate heat, it is also favored by manufacturers as it is easier to work with and machine than some other metals.

Bronze – Bronze quick connect couplers (alloyed copper and tin), are harder but more brittle than brass quick connect couplings. Bronze fittings are highly corrosion-resistant and remain tough and durable in high temperature hydraulic systems and hold against salt and in saltwater environments. Bronze (like brass) are preferred over stainless steel quick connect couplers when a solder or weld connection is to be used.

Cast Iron – Cast iron is less popular with quick connect manufacturers. It is far more common to find cast iron in complementary system components, such as fluid traps, flanges, pipes, vent tubing, and waste removal. Cast iron is beneficial in waste water systems due to its ability to resist wear from abrasive debris and solid waste / waste water.

Copper – Copper quick connect components offer superior corrosion resistant properties. Copper is favored by many for piping applications due to its pliability and workability. When used in quick connect couplings, copper is work hardened to make it rigid and less susceptible to wear than it would be if used in its softer state.

Steel and Carbon Steel – Steel fittings and Carbon steel quick connect couplings and fittings are extremely hard and resilient (made of a mix of iron and carbon). Stainless steel quick connects and carbon steel quick connects will resist damage better than softer metals like brass and copper. Steel quick connect fittings also have a wider working temperature range, making them more versatile for tougher applications. Carbon steel fittings are more susceptible to corrosion than stainless steel quick connect couplings and fittings. Outside of mixed steel alloys, there are also galvanized steel options that coat the steal in zinc to form a rust and chemical resistant coupling.

Stainless Steel – Stainless steel quick connect fittings are in high demand due to their resilience and durability when exposed to and handling caustic chemicals and other potentially corrosive fluids, oils, and gases. If you are looking for the best in pressure ratings, high temperature functionality and corrosion resistance, look to stainless steel quick connect fittings.

Plastic Quick Connect Benefits Overview

Plastics are lower priced materials and thus offer cheaper quick connect fitting options to the consumer.  Plastic quick connects and fittings will not rust, making them less prone to getting valves stuck open or closed. They are also lighter, thereby reducing shipping costs.

*Caution is strongly advised when installing a plastic quick connect coupler. Over tightening valves/piping can compromise the integrity of the plastic quick connect body, valves, flanges, and seals.

Teflon (PTFE) – PTFE quick connect couplers are chemically resistant with a smooth, non-stick surface that facilitate steady flow rates and help to eliminate media buildup. PTFE fittings have a low friction coefficient and good dielectric capability. PTFE fittings are chemically inert and work well with chemicals, corrosive fluids, oils, drinking water, and compressed gases.

PVDF (Kynar, Hylar, KF) (Fluororesins) – PVDF connections, piping, and fittings are an economical alternative to PTFE (Teflon), PFA, and FEP. Kynar fittings and connection options are used in low temperature fluid and gas applications. PVDF exhibit more acid and solvent resistant properties than polypropylene fittings. PVDF fittings are generally very resistant to degradation from an array of chemicals, and are capable of handling high temperature fluids and gases than other plastic fittings.

PFA (fluoropolymers) – PFA fittings, couplings, hoses, and pipes are popular for handling aggressive chemicals. They are used as a liner in chemical tanks to prevent erosion in gas scrubbers, reactors, containment tanks, and piping. PFA pipes and fittings can withstand prolonged contact with corrosive surfactants, and chemicals.

PVC (poly-vinyl-chloride) – PVC fittings and couplings are popular in chemical processing, industrial plating, chilled water distribution/transfer, deionized water lines, chemical drainage, and waste water treatment systems. PVC fittings are rigid and come in a variety of pressure ratings.

Polyethylene (PE) – PE is a strong thermoplastic. PE couplers, quick connects, fittings and pipe have outstanding electrical properties, but will not hold up to high temperatures. PE fittings have good chemical compatibility properties, but are prone to stress cracks. Other problems that plague PE fittings are poor UV resistance and poor barrier properties. PE fittings are popular for water transfer systems that do not experience pressure or temperature fluctuations.

Polypropylene (PP) – Quick connects made of a thermoplastic material called PP (Polypropylene) are good options for cold flow applications. They have similar properties to PVC, but are more versatile and durable. Unlike PVC, they can be used in exposed applications due to their resistance to UV, weathering, and ozone damage.

CONCLUSION: PLASTIC v. METAL QUICK CONNECT

In order to correctly choose the best quick connect coupling, materials is only one part of the selection process. Performance capabilities of the quick connect, the application in which the quick connect fitting will be used, and the material from which the quick connect coupling will be made must also be considered. All of these qualities are equally important when looking to install or repair a quick connect.

Make sure to invest in the long term. Integrating a cheap or inferior quick connect fitting could result in excess equipment repairs, unnecessary downtime, tool and machinery performance issues, as well as limited life spans for hydraulic, fluid, and pneumatic componentry.

So, are plastic or metal quick disconnect fittings superior? The answer is, “it depends”. Generally speaking, metal quick connect fittings have an advantage over plastic quick connects when it comes to handling high temperatures and high-pressure system demands. Plastic quick connect fittings can be used successfully for low temperature and lower pressure applications. Heavy duty plastic couplers and fittings are available, but the costs are a limiting factor for most applications. Plastic quick connects are typically best for non-critical application types in which a failure is more inconvenient than damaging. Always consider safety first.