Plastic connectors: Many factors weigh in when choosing the right ones
Metal connectors continue to be supplanted in many industrial and laboratory applications
Diverse challenges confront industrial and laboratory original equipment manufacturers (OEMs) in developing versatile, user-friendly, lightweight and cost-effective equipment and instrument solutions. One important, but not always fully appreciated, task is selecting tubing connectors for transfer and management of fluids and gases, particularly for mission-critical applications.
For many applications, plastic tubing connectors continue to supplant those made of metal, not only for design flexibility, improved ergonomics, reduced weight and lower cost, but also for their ability to meet stringent industry standards in diverse and harsh environments. Many plastic connector features, long proven in critical medical equipment applications, are being integrated into a broad spectrum of industrial equipment and laboratory instruments.
Choosing the right plastic connector can be a challenge, with more options for material selection, user interface and custom design than metal connectors. A basic understanding of the options available can support specification of the most optimized features, leading to peak equipment or instrument performance.
While plastic connectors fill many roles, they aren’t suitable for all laboratory and industrial uses. Brass, aluminum, die-cast zinc and stainless steel connectors are right for extreme durability and high-performance fluid handling, particularly when influenced by high pressures and temperatures. Choosing the ideal connector calls for a careful application assessment. Prime factors to consider include the following:
Temperature range – minimum and maximum temperature tolerances within which the connectors need to function. Depending on connector material, temperature tolerances can range from -40 F to 200 F and above;
Pressure range – determining minimum, maximum and working pressures the connectors are expected to tolerate;
Flow rate – required volume per minute, and the effect of fluid pulsation and modulations from connect and disconnect forces;
Media – the viscosity, sensitivity and corrosiveness of the fluid or gas moving through the connection;
Exposure – degree of impact from external or internal conditions, such as UV, wind, dust, vibration, radiation, gases, water submersion, chemicals, cleaning agents or mechanical stress;
Specialized environments – for food and pharmaceutical grade manufacturing, including wash-down, clean room, aseptic environments and vacuums;
User interface – level of human contact expected with the system and connectors; and
Cycle life – anticipated maintenance and changeability required for the system, and expected operational longevity.
Once identified, the application requirements then determine what materials are best suited for the connectors.
Plastic connector materials
Plastic connector material is chosen based on mechanical requirements and the media type moving through the system. Significant pros and cons need to be weighed. Properties such as toughness, ductility, impact strength, transparency, lubricity, temperature capability, ozone resistance and UV compatibility may all come into play.
The media is critically important, given that fluid or gas flowing through a connection can affect its strength, surface appearance, color and performance. And, conversely, the wrong connector material can adversely impact the media. Chemical compatibility need be considered and the most appropriate plastic resin chosen.
When made from high-purity resin materials for use with aggressive chemicals, plastic connectors offer broad compatibility. Many are manufactured from virgin resins, with accompanying certification of lot traceability. Plastic resins across a broad spectrum are used for connectors, each with characteristics to match specific application needs. Some thermoplastic resins commonly used for connectors include:
Polyethylene – chemically resistant, translucent or opaque, with low-temperature impact, to withstand a variety of application environments;
Polycarbonate – hard, transparent and with moderate chemical resistance, provides good impact resistance and superior dimensional stability;
Polypropylene – soft, highly resistant to chemical attack from solvents and chemicals in harsh environments;
Polyamide (Nylon) – versatile, with good wear and chemical resistance, low permeability to gases and performs well at elevated temperatures;
ABS – tough, with good stiffness and impact resistance even at lower temperatures, as well as good dimensional stability and high temperature resistance;
Acetal – strong, lightweight and provides high strength and rigidity over a wide range of temperatures; and
PVDF – strong, with good ductility over a broad temperature range, as well as excellent chemical resistance.
In addition, PTFE is fluoropolymer-resistant to most chemicals and solvents, with stability at high temperatures. Once the connector material is determined, the type of connection best suited to the need can be picked.
Connectors are meant to accommodate tubing of varying hardness, as would be measured by a durometer, from soft and flexible like PVC, silicone and C-flex, to semi-rigid types like polypropylene, polyethylene, polyurethane and ethylene vinyl acetate (EVA).
To facilitate these varying styles of tubing, different connector types include barbed connectors, check valves, luer connectors, quick connects, threaded luers and tube-to-tube connectors. Of these, the most commonly used tubing connectors are tube-to-tube, luers and quick connects. These basic styles cover a wide range of liquid and air applications in laboratories and plants.
Tube-to-tube connectors are popular for applications that don’t require equipment or parts disconnection at any point during use. Tubing connectors are available in many different configurations, sizes and material options to adapt to different tube sizes or styles, reroute the flow direction without kinking and act as a manifold.
Delivery systems can employ conical or taper seal connectors, called luers, to link various system components. The male and female components of luer connectors join together to create secure, yet detachable, leak-proof connections with no O-ring or gasket required.
Luer connectors come in a variety of configurations, including for tube connections, threaded connections (UNF, NPT and metric) and other luer or quick connect terminations. Some of those incorporate a tapered UNF thread, similar to a pipe thread, which can also seal on the thread due to interference on the pitch diameter, facilitating directional alignment with tees and elbows.
So-called “quick connects” allow flexible tubing and equipment to be quickly and safely connected and disconnected. This may be preferred over general connectors for fluid control because they can incorporate built-in shut-off valves that prevent spillage, allow multiple disconnections and faster servicing.
One of the newest and most versatile plastic quick-connect solutions available for laboratory and industrial applications, the MQC Series manufactured by Value Plastics, a Nordson Company, provides an intuitive push-to-connect design. With its large, ergonomic buttons providing an audible click on connection and grips for easy handling with gloves, combined with a wide selection of color-coding options, the MQC is unique for ease of handling and the prevention of misconnections.
Many of the latest quick connect designs focus on the user interface and are equipped with simple thumb-latch and side-latch mechanisms to make for easy handling in laboratory and industrial fluid management applications. Quick connects mitigate the prospect of accidental misconnections and create quicker and safer device connections.
One barb, or two?
Plastic barb-style connectors accommodate the widest possible range of tubing properties and application conditions, including a multitude of configurations such as tees, Ys, elbows and manifolds. Barb designs are available for handling assembly forces, tensile resistance and blow-off resistance, without the need for clamps.
Barbs derive their holding capability by expanding tubing above its nominal inside diameter (ID), creating some amount of interference for a secure seal and good mechanical retention. The tube expansion can vary dramatically, from lower profile, easier connections to much more aggressive interferences, depending on the pressure and tensile pull requirements.
The selection of the barb style is very important to the connector’s tube-holding capability. The cylindrical surface behind the barb should allow the tubing to relax against the connector. In choosing a barb style, users should be sure the barb has a sharp peak, allowing it to “bite” into the tubing for optimal retention.
Many plastic connectors and almost all metal connectors use a multi-barb, making for an inferior tube connection and seal. Multi-barbs cannot create a sharp bite on the tube, inhibiting retention, and do not allow the tube a chance to relax behind the barb, also resulting in poor tensile pull strength.
The multi-barb manufacturing process leaves a parting line on the sealing surface, creating a potential leak path. This is an inherent design flaw, yet all multi-barb connector designs, including metal connectors, display this liability. In fact, many inferiorly crafted single-barb plastic connectors are also afflicted with a parting line, reducing connector efficacy. A good injection-molded connector has a singular barb with no parting line, a sharp bite and a clean sealing surface.
Many factors reduce tubing’s ability to perform under pressure — including temperature, chemical degradation, mechanical stress, fluid pulsation, selection of connector type and barb design. The latest generation of plastic connector technology affords designers and manufacturers wide flexibility to design and set-up applications to fit their specific needs.
Compared to metal, plastic connectors offer a considerable reduction in weight and much improved flexibility with regard to the equipment they serve. Plastic quick connects allow rapid and easy servicing and maintenance of assembly-line equipment, filling systems and packaging systems. The cost difference between metal and plastic connectors is a major motivating factor in the embrace of plants and laboratories of plastic connectors.
Given demanding project requirements and timelines, precision fluid management solutions are critical to achieve a high efficiency ROI. Plastic connectors, particularly when custom designed for the application, are becoming the preferred solution in industrial and laboratory settings.
Value Plastics Inc., a Nordson Co., manufactures and markets fluid management components designed specifically for flexible tubing and instrumentation. Riley Phipps is technical and design services manager there.