by Riley Phipps, Value Plastics, Inc., a Nordson Company
Industrial and laboratory OEM designers are constantly being challenged to come up with more versatile, user-friendly, lightweight and cost-effective product solutions for their equipment and instrumentation. An important component in this design process is the selection of tubing connectors for the transfer and management of fluids and gases, particularly for mission-critical applications.
However, choosing the right plastic connector can be in itself a challenge. Plastic connectors offer more options for material selection, user interface and customized design than metal connectors.
While plastic connectors effectively fill many roles, they may not be suitable for all laboratory and industrial uses. Although metal tubing connectors have been traditionally used in industrial and laboratory markets, plastic connectors continue to supplant many that were previously metal, not only because of their increased options for design flexibility, improved ergonomics, reduced weight and lower cost, but also for their ability to effectively meet stringent industry standards in diverse and harsh environments. Many features of plastic connectors are now integrated into a broad spectrum of industrial equipment and laboratory instrumentation.
Brass, aluminum, die-cast zinc and stainless steel connectors are designed for extreme durability and high-performance fluid handling, particularly when influenced by high pressures and high temperatures. Choosing the most ideal connector first requires a careful assessment of the application. These factors to be considered are listed:
a. Temperature Range – the minimum and maximum temperature tolerances connectors will need to function in. Depending on connector material, temperature tolerances can range from -40° F to 200° F and above.
b. Pressure Range –the minimum, maximum and working pressures that the connectors need to tolerate.
c. Flow Rate –the required volume per minute, and the effect of fluid pulsation, and modulations from connect and disconnect forces.
d. Media – the viscosity, sensitivity and corrosiveness of the fluid or gas moving through the connection.
e. Exposure – degree of impact from external or internal conditions, such as UV, wind, dust, vibration, radiation, gases, water submersion, chemicals or cleaning agents, and mechanical stress.
f. Specialized Environments – for food and pharmaceutical grade manufacturing, including wash-down, cleanroom and aseptic environments, and vacuums.
g. User Interface – level of human contact expected with the system and connectors.
h. Cycle Life – anticipated maintenance and changeability required to be performed on the system, and expected longevity of the system in operation.
The requirements of the application will then determine what materials will best suit the connectors.
Plastic connector materials
The material chosen for plastic connectors should be based on the mechanical requirements of the connector and the type of media that will be moving through the system. Mechanical properties such as toughness, ductility, impact strength, transparency, lubricity, temperature capability, ozone resistance and UV compatibility need to be assessed when selecting the most functional material for the application.
The type of fluid or gas flowing through a connection can affect the strength, surface appearance, color and performance of the connection. Conversely, the wrong material can adversely impact the media. Therefore, the media is critically important.
Careful consideration should be taken when choosing a chemical and the most appropriate plastic resin for an application. Made from high-purity resin materials, for use with aggressive chemicals found in most industrial and manufacturing plants, plastic connectors offer broad chemical compatibility. Many are manufactured from virgin resins, with accompanying certification of lot traceability.
A broad spectrum of plastic resins can be selected to produce connectors, each with different characteristics to match the needs of system designers. These plastic resins are commonly used for connectors:
Polyethylene – chemically resistant, translucent or opaque thermoplastic with low temperature impact.
Polycarbonate – hard, transparent thermoplastic with moderate chemical resistance. It provides good impact resistance and dimensional stability.
Polypropylene – soft thermoplastic that is highly resistant to chemical attack from solvents and chemicals in harsh environments.
Polyamide (Nylon) – versatile thermoplastic with good wear and chemical resistance, low permeability to gases and performs well at elevated temperatures.
ABS – tough thermoplastic with good stiffness and impact resistance even at lower temperatures, as well as good dimensional stability and high temperature resistance.
Acetal – strong and lightweight thermoplastic that provides high strength and rigidity over a wide range of temperatures.
PTFE – fluoropolymer resistant to most chemicals and solvents, with stability at high temperatures.
PVDF – thermoplastic that is mechanically strong with good ductility over a broad temperature range, as well as having excellent chemical resistance.
After the most appropriate material for the connector is determined, the type of connection best suited for laboratory or industrial needs may be assessed.
Many factors can reduce a tubes’ ability to perform under pressure including temperature, chemical degradation, mechanical stress, fluid pulsation, and the selection of connector type and barb design.
The latest generation of plastic connector technology affords designers and manufacturers latitude of flexibility to design and set-up applications that custom fit their specific needs. Some connector manufacturers have comprehensive design centers to help instrumentation and equipment manufacturers achieve the highest level of performance from their connectors. With good consultation, being up front on the designer’s application requirements and prospective off the shelf or custom solutions, pitfalls can be avoided and optimal designs can be executed.
Connectors are designed to accommodate tubing of varying hardness (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 and their respective application needs, different connector types are used, including 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 connectors, luers and quick connects. These basic connector styles can cover a range of liquid and air applications in laboratory and industrial environments.
Tube-to-Tube Connectors – a popular choice for applications that do not require the disconnection of equipment or parts at any point during production or use. Tubing connectors are available in many different configurations, sizes and material options to adapt different tube sizes or styles, reroute the flow direction without kinking, and act as a manifold.
Luer Connectors – delivery systems can use 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 adapting to 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.
Quick Connects – quick connects (quick disconnects) allow flexible tubing and equipment to be quickly and safely connected and disconnected. They 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 more versatile plastic quick connect solutions available for laboratory and industrial applications, the MQC Series, has an intuitive push-to-connect design. It has large, ergonomic buttons that deliver audible click on connection.
Many of the latest quick connect designs focus on the user interface, and are equipped with intuitively simplistic thumb latch and side latch mechanisms 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 and grips for easy handling with gloves.
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. A number of barb designs are available – each with unique characteristics to tailor connection performance to specific needs – for handling assembly forces, tensile resistance and blow-off resistance without the need for clamps.
Barbs holding capability is attributed to the 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 from lower profile and easier connections to a more aggressive interference, depending on the pressure and tensile pull requirements.
The selection of the barb style is 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, it should be designed with 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, which inhibits retention, thus not allowing the tube a chance to relax behind the barb, which results in poor tensile pull strength.
Multi-barbs are also relegated to a manufacturing process that 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. An optimally designed and properly injection molded connector will incorporate a singular barb with no parting line, a sharp bite and a clean sealing surface.
Compared to metal, plastic connectors provide a considerable reduction in weight, and improved flexibility. Distinctively equipped to do so, plastic quick connects allow rapid and easy servicing and maintenance of assembly line equipment, filling and packaging systems, which limits system downtime and speeds throughput. Color-coding on plastic connectors also makes for quick tube identification and reconnection.
The cost difference between metal and plastic connectors is a major motivating factor pushing instrumentation. Equipment and system designers further embrace plastic connectors in laboratory and industrial applications.
With project requirements and timelines becoming increasingly demanding, the need for precision fluid management solutions applicable to industrial processing and instrumentation design is critical to achieve a high efficiency ROI. Plastic connectors, particularly when custom designed for the application, are more frequently becoming the preferred solution in industrial and laboratory settings.
Contact Value Plastics at www.valueplastics.com
Filed Under: Design World articles, Connectors (electrical) • crimp technologies, Materials • advanced, Test + measurement • test equipment