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Choosing the Right Connections and Fittings for Manifolds

Engineering GREAT Solutions

02  A white paper - Right Connections and Fittings for Manifolds

Customized Manifolds

Designing a customized integrated manifold for a medical or life science device presents engineers with the challenge of selecting from many different components. A key consideration in developing any new fluidic control system is the type of connections used to tie different components together in the wetted path. Over the years, many types of fittings have been developed to meet application specific needs.

Commonly Used Fittings In Life Science applications where manifold fluidics systems are incorporated, serviceability of the manifold is a critical requirement. Push-to-connect style fittings allow a technician or assembler to quickly make fluidic connections without the need for tools or additional hardware. These fittings use a collet to grasp the tubing, and have an internal O-ring to form a radial seal. Sliding the tube into the collet expands the O-ring against an internal wall of the fitting and the outer diameter of the tube. Generally, hard wall tubing, such Shore D 85-95 Nylon or Polyurethane tubing, is recommended with

these types of fittings. Cartridge versions of these fittings intended for manifolds are readily available, and eliminate an additional leakage point between the fitting and manifold.

For low pressure applications using a softer durometer tubing, traditional barbed fittings work well. These are typically available in single or multi barb configurations, and provide good sealing characteristics when the appropriate tubing is used. A variety of barb designs exist to enhance sealing and tube retention properties, and a hose clamp can be added to support higher pressures.

High pressure lines typically will require a sturdier connection that compression fittings offer. These fittings consist of a compression nut, ferrule, and the main fitting body with a flared end. After sliding the ferrule over the tubing, the compression nut is slid on behind the ferrule and tightened, simultaneously clamping the ends of the ferrule around the tubing and expanding the center of the ferrule into the flared end of the fitting body. These fittings have excellent sealing characteristics, and can be used with metal tubing.

Hard wall tubing used with push-to-connect fittings Push-to-connect fitting Compression fitting assembly

Cartridge style push-to-connect fitting Compression fitting components

A white paper - Right Connections and Fittings for Manifolds  03

Push-to-connect fittings in an acrylic manifold

Fitting to Manifold Interface

Manufacturing Considerations

The method of sealing between the manifold and fitting itself is important to consider. Increasingly, life science fitting manufacturers are using flat bottom sealing style fittings for lower pressure applications. These fittings can reduce concern for damage to the sealing surfaces during assembly, as they are recessed within the port rather than on a main external face of the manifold.

Selecting a material for fittings in most life science programs is primarily driven by the intended media the device will be running. However, consideration should be given when using small plastic fittings, as they are more susceptible to damage during assembly, shipping or handling.

More traditional thread-in style fittings are common on many life science manifolds as well. Sealing is typically achieved through an O-ring or gasket on the fitting that is compressed once the fitting is fully threaded into the port. Tapered threads, such as an NPTF thread, are also used for certain applications. These fittings rely on contact between the mating threads to provide a seal without the need for a separate O-ring or elastomer, and require the manifold material to be able to withstand the hoop stress imparted during assembly. Often these fittings can be supplied with a dry thread sealant or thread locker to ensure proper sealing and eliminate an assembly operation.

Flat bottom sealing fittings

In addition to design and material requirements, an engineer should make note of the impact different fittings may make on both the assembly and testing of the fluidic device. With most life science equipment, leakage testing is performed to ensure no external leakage is occurring in normal operation at the final end-of-line verification test. It is desirable to complete this testing with fittings installed to ensure there is no leakage at the connection point between the manifold and fitting.

Cartridge, push-to-connect style fittings tend to be favorable during the assembly process, as they can simply be seated using an arbor press. Creating a fixture to interface into the ports of these fittings for purposes of production testing can be simple if the fluidic porting is located on a single or opposing faces. With porting on multiple faces, test operators can still make connections with tubing, but care will need to be taken to ensure the O-ring does not become damaged from improperly cut tubing.

Thread in barb fittings used in life science applications are often small due to the low flow rates and low sample volumes, and require delicate handling during assembly to ensure the fittings do not become damaged. Operator handling during production testing tends to be more labor intensive, as designing clamping fixtures to supply pressure to the manifold become difficult to develop.

Further, when using NPT style thread in fittings, fitting alignment can be very difficult. This is especially true when using a stainlesssteel fitting in a stainless-steel manifold, and in some cases, it can become impossible to align the fitting and achieve proper sealing.

In these types of applications, it is generally recommended to use either a swivel style fitting or a non-tapered thread style fitting.

There are a vast number of fittings available to today’s fluidic system design engineers. Sturdy connections and leak-tight performance of an integrated system is paramount. Ease of manufacturing the assembly is critical for repeatable performance and reliability. It is critical for engineers to be familiar with all the options available to them to ensure a robust fluidic system design.

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Due to our policy of continuous development, IMI Precision Engineering reserve the right to change specifications without prior notice. z8642WP en/02/18 Selected Images used under license from Shutterstock.com

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