Connection Port Options For Plastic Manifolds
How to connect to a manifold is one of the more important decisions in manifold design. There are a lot of options possible, but in practice we only see a few common approaches.
We have a list of recommended vendors at the bottom of this page for your reference.
Threaded Ports & Fitting Connections
The connection type choices depend on a few factors. These include the style of tubing, expected lifecycle of tubing, cost, and use-case. In medical and life science applications, often the tubing style is the most critical factor that dictates the connection type. For example, if thick-walled flexible tubing is required a barb-style fitting will be best. In contrast, if thin-walled highly chemically inert connections are required, one might choose flared tubing and connection type to ensure there is the lowest possibility of carry-over or fluidic contact with a metal ferrule.
Typically flared, or sometimes referred to as flanged, most consider these kinds of tubing assemblies more cost efficient as they have a less expensive component, i.e. the O-ring/washer vs ferrule. However, this means that the tubing length cannot be adjusted as easily because the O-ring and fitting are installed prior to cutting and flaring the tubing. When using a ferruled connection, users can cut and recut the tubing as needed with the ferrule plus fitting installed without specialized equipment to create a tubing flare.
Ferrules tubing assemblies, or often referred to as flangeless, provide some benefits and drawbacks as well. Ferrules are easier to replace as special tools to create a flare are not required. They are also “free-spinning”, which can reduce strain on tubing depending on the application.
Just as the type of connection is critical, same is true for the fitting material. Standard options are Polypropylene (lower cost), PEEK (most resistant to wear), and PVDF (less constrictive than PEEK).
We recommend that designers also consider another mechanism in which the fitting is installed in the manifold and “head style”. These can be finger-tight, headless, hex head, or torque-limiting. Below are some visual examples
There are a variety of torque-limiting technologies available for fitting connections. They can reduce the risk of manifold cracking due to over-torque or under-torque. However, these technologies add significant cost to a design if engineers call for many within the design. Yet if the tubing assembly is installed correctly, these are typically not necessary for optimal function.
Design Suggestions For Ports
While following the IDEX/Upchurch design will most often ensure a successful product, designers need to keep two important considerations in mind.
Some fittings have an extended nose, meaning the thread starts further back from the end of the fitting. When installed in a standard thread length port, the thread engagement falls short in providing a flush design. Without the appropriate sealing torque, stress cracks will develop. We suggest increasing the thread length where possible to avoid product function failure.
Regarding port design, this depends on the manufacturer's experience and skill with port creation itself. Leaks can develop if the flat bottom surface finish is too course or has burrs, creating opportunities for liquid or gas to escape. We specify a 32 finish as an important aspect for great port function and sealing. However, measuring and testing this with a profilometer is not possible without some harm or damage to the product, which can render it nonfunctional. Usually, QA/QC can perform a visual check with either industry or company standards in mind to ensure that a product has a flush seal to prevent any leakage. You can see the differences between a "clean" surface finish and one that requires work with these two photos.
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Considerations with Straight Tubing
Medical grade stainless steel tubes slip fit into a counterbore in the manifold for connecting flexible tubing. This tubing “grabs” the fitting with either one of the connection types: a ferrule, a barb, or thermoforming a flare on the end of it (the connection type will dictate the fitting vendor/type). Then the machinist secures the tubing using a potting adhesive, either epoxy or UV cured resin.
As with many other products, designers must weigh what they truly need versus what the product can go without. Here are some pros and cons to using metal tubing within a manifold.
Pros
- Simple
- Inexpensive
- Stable
- Compact
Cons
- Permanent
- Usually for low pressure applications
We can see via the diagram how a metal tube would fit into a manifold. The metal tube inserted into the machined hole would use some sort of strong adhesive to make it airtight and free from contaminants. Metal tubing would provide strength, durability, and (in most cases) longevity to the manifold and its overall function.
We can then look at the physical example here of a micro-manifold that uses metal tubing in its makeup. The pencil tip gives a sense of scale to the micro-machined manifold. While other types of manifolds exist meant for higher pressure applications, the one seen here is primarily used for low pressure applications.
For low pressure applications, the #1 choice for a tubing connection is the IDEX/Upchurch flat bottom port. They eliminate the dead volume in the threaded ports and the contamination that can build up. The fittings are available in a multitude of configurations that are easily disconnected and replaced. Thread sizes are 6-40, 10-32, ¼-28, 5/16-24, ½-20, & M6; the ¼-28 is most often specified. As the most common threaded port, these are good for both liquid and pneumatic applications. It typically has a flat bottom, but for high pressure applications, it has a tapered bottom.
However, we must note one concern about these. As previously stated for other plastics, stress-cracking can occur from short thread engagement, hence torqueing the fitting properly is important. The connection is designed for maximum number thread engagement with successful sealing, if done properly.
Barb Fittings
These are usually metal or plastic fittings screwed or adhesively bonded into manifold with barbs for attaching tubes. While they have use in several situations, others exist where this does not serve the product's function the best.
Pros
- Relatively inexpensive and simple. Fittings are replaceable.
- Available with O-rings or flat gaskets to seal against the face.
- Available with a heavy chamfer to seal against a mating chamfer.
Cons
- Not great for high pressure applications (though there are some options) or use with rigid tubing.
- The design for these types of fitting can trap bubbles and render results inconclusive in some cases.
There are a few considerations that need to be taken into account for a proper barb-tubing connection. In order for the barb to “grip” tubing properly, there must be sufficient friction between the edge of the barb(s) and the tubing.
For example, a ridged tubing will be more difficult for the bar to compress into the edge and grip the tubing. A combination of barb type, style, and material will ensure reliable connections.
In the diagram here, we can see a few different options for barbed fittings. Some will work better than others depending on the product's ultimate function and purpose. Due to this, we ask designers to make careful considerations when choosing this options for port fittings.
About Fully Machined Barbs
Instead of a separate component, most manufacturers can machine a complete barb directly into the manifold. This creates a more flush and reliable product.
Fully Machined Barbs In Manifolds Advantages
- Saves space
- Lower cost
- Does not introduce another material as the barb is made from the same material as the manifold
Disadvantages of Fully Machined Barbs in Manifolds
- Higher risk of system failure
- Needs higher level of care
About Push-To-Connect Style Barbs
We recommend using push-to-connect style fitting when the product needs to have an easy way to use fittings that allow tubes to attach and detach when needed.
PTC Barbs In Manifolds Advantages
- High level of versatility
- Great for applications where tubes need to be replaced or moved often
- Good pressure rating
PTC Barbs in Manifolds Disadvantages
- Higher upfront production costs than other options
About Compression Fittings
The types of fittings are generally easy to use. They screw a locking ferrule tight onto a tube.
Advantages of Compression Fittings In Manifolds
- Simple design
- Creates a strong seal
- Strong against high pressure and otherwise harmful chemicals
Disadvantages of Compression Fittings in Manifolds
- Needs at least somewhat hard tubing
- Can become a less cost efficient option
To the left, we can notice a few different aspects about compression fittings. The combination of those three elements helps to add to the overall mechanism's strength and capacity for more demanding applications. However, these types of fittings can quickly become a less cost efficient option. For those that need to stay within strict budgetary concerns may need to seek another option.
Photo courtesy of McMaster Carr
About NTP & Pipe Threading
Photo courtesy of McMaster Carr
While NTP has its merits, we recommend avoiding a few matters where these are concerned. It would be in a designer's best interest to avoid NPT ports — if possible — for clear materials, such as acrylic. The wedge shape puts unnecessary stress on the plastic and can lead to stress cracking. Due to this as well, we recommend avoiding placing NPT ports on the bond line.
If NPT ports are still a valid option, it is best practice that those should be at least 2 diameters from the edge of the manifold for the same reasons cited above.
Alternatively, for larger ports, consider using a fitting with an O-ring face seal. These avoid the negative consequences of the tapered fitting while installing to the same height.
Recommended Vendors
Fittings
- IDEX Finger-tight Fittings
- IDEX Flangeless Fittings
- DIBA Click n' Seal
- DIBA Omni-lok
- VICI Jour Finger-tight & Hex Head
Barb Fittings
- Nordson/Value Plastics
- Beswick
- MEM-CO
- Okeefe
- Okeefe makes a barb fitting that is meant to be pressed or bonded in. This is very simple to install and works like any other barb but will have lower pressure capabilities.
- Colder Products
- Valve Plastics
- Eldon James
PTC Fittings
*Also recommend Beswick and Legris for their compression fittings as well