Channel Design Ideas
Crafting channels in bonded plastic manifolds requires precision, creativity, and a keen eye for detail. We'll go over some key points regarding channels themselves and their intricacies as well as how to accommodate accumulators and the like within them.
Let's start creating fluidic designs that streamline the flow within manifolds.
Important Design Factors Concerning Fluidic Channels' Max & Mins
When designing channels within manifolds and components, a designer or engineer must keep a few things in mind for optimal fluid flow. To serve as a guideline, we have noted that most channel sizes typically fall between 0.5-1.5 mm (0.02-0.06 in) in diameter. While manufacturers usually don’t have a maximum limit to the channel diameters, teams must keep working pressure in mind when creating channels. We will further explain this in detail later, but in the case of some larger features, the designer may need to consider using other internal support components to aid in the bonding process to ensure that channels remain open and functional.
When considering minimum channel sizes, most manufacturers can mill channels down to 50 microns (0.003 inches). When bonding, those channels can come down to 100 microns (0.004 inches). Most reserve such small channels for use in microfluidics with square channels, being the easier to machine. However, this depends entirely on the material. Some can handle the bonding process while others are less suited for it.
Channel Shapes & Options
Several options exist when it comes to the design of fluid channels. We can produce full-round, square, or U/D-shaped channels. Each design has their own merits and drawbacks, and the right one for the manifold depends heavily on its end-purpose.
Key Facts About Full-Round Channels
We often recommend full round channels — especially for liquid manifolds. Full round channels have the lowest flow resistance with a full swept volume. This configuration is often easiest to bond with and has the most aesthetic appeal. Typically, we also recommend an alignment within .004” or better.
Here are some additional benefits for round channels:
- Lowest frictional resistance to flow
- Bonds best with great force translation thru 2 arches
- Full fluid swept volume without particle entrapment points
- Best surface finish through ball endmill tooling
- Best aesthetic appeal
- Smoothest transitions
- Better cross section consistency through bonding
- Best choice for liquid fluid control
- Insignificant additional cost over the other shapes
Key Facts About Square Channels
Machines mill these channels to form either square or rectangular-shaped tunnels within the manifold. Although their qualities don't count as high as full-round, below we list the key points for designers' consideration.
- Used for applications where imaging of the fluid is desired, such as cell sorting
- Commonly used in pneumatics
- Lowest aesthetic appeal
Key Facts About D-Shape, U-Shape, & Half-Round Channels
These channel shapes offer a sort of mid-range option between full-round and square channels. Some manufactures suggest that D-shape channels are the best choice for cost because milling time is slightly less for D-shape channels.
- Better flow characteristics than square channels, but not as good as full round
- Corners can entrap particles
- Carry-over possible from run to run
- Better aesthetic appeal
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About Tolerances & Intersections for Fluidic Channel Design
Our bonding process is a highly controlled precision operation. We are able to hold the +/- .005 tolerance through the bonding operation. There is no prebond versus post bond size adjustment.
Size: ±.005” (125 microns)
Bond alignment: ±.004” (100 microns) for full-round channels.
To create a flush and functional manifold, designers have to think about the finer details including the intersections where channels meet. This includes when running into challenges concerning sharp corners and channel bends. Some types of intersections facilitate flow better while others have unique advantages unto themselves. Below, we detail out the four most commonly used intersection types: drilled-drilled, full-round drilled, half-round drilled, and square drilled.
Drilled-Drilled Intersections in Manifolds
As the most common configuration for two intersecting drilled holes, a machine creates this intersection when two drilled holes meet. Standard practice uses a 135° drill bit placing the drill points within .005” (125 microns) from each other. While having some scalloping, the irregularities are small enough that the process minimizes particle entrapment. Expect no overshoot.
For additional cost, manufacturers can use a secondary tool to create a full spherical intersection as seen in the black and white drawing.
Full-Round Drilled Intersections in Manifolds
As default intersection of a bonded manifold, this intersection is created when a milled full round channel meets a drilled hole of the same diameter. Intersection is full spherical and smooth. Manufacturers use additional tooling to produce this intersection as an only-drilled manifold.
Half-Round Drilled Intersections in Manifolds
This intersection is created when a half round channel intersects with a drilled hole of the same diameter.
Square Drilled Intersections in Manifolds
This intersection is created when a milled square channel intersects with another drilled square hole of the same diameter.
Regarding Sharp Channel Bends
When possible, avoid sharp corners in channels. Round corners create a much lower flow resistance. Round corners in milled channels pose no additional difficulty or cost.
Incorporating Bubble Traps, Reservoirs, & Accumulators within Channel Design
Reservoirs & Accumulators
Reservoirs and accumulators within manifolds serve a few important uses, and those change from one industry to the next. When fluid or gas needs to gather in a particular spot within the manifold, reservoirs work towards that purpose. As large openings between or in channels, reservoirs can hold the contents before proceeding further down the channel.
Accumulators work as large openings in the manifold channel system where fluid accumulates. They store and regulate fluid/gas pressure (usually) within hydraulic or pneumatic systems to maintain system stability, absorb shock, and provide temporary fluid sourcing. They can act like an electronic capacitor and dampen the flow pulsations or even as an emergency power source in the event of power failure.
Accumulators offer other benefits as well:
- Pressure regulation
- Energy storage
- Volume compensation
- Enhance system efficiency
We recommend keeping the anticipated material in mind along with the necessary pressure requirements and chemical resistance, if the product needs these features. Some materials work better than others when it comes to durability.
Key Things To Know
- Pressure capability goes down when a large cavity is added to the plastic manifold.
- For some very large cavities, we recommend designing posts in the middle for support.
- Maximum size is very design dependent because of different pressure applications. If you are considering a large reservoir, consider the strength of the plastic carefully.
- Our manufacturing process requires us to clean the parts after they are finish machined. Each reservoir must have at least two ports (in and out) for water to flow through for cleaning. Single feed designs (accumulators) will require a clean out port which is plugged using a fitting.
Bubble traps work to capture bubbles in the fluid flow via a channel section where the volume expands and traps them. Manufacturers can combine these with a vent that allows for gas release but impervious to liquid.
Bubble Formation on Channel Surfaces
Bubbles can and do form on surfaces within a manifold. While bubbles in the liquid cause bubble formation within a plastic manifold, it also appears to form out of nothing. Many manufacturers are uncertain about exactly how or why these bubbles form, but we can say these bubbles form from plastic outgassing and/or gasses dissolved in the manifold's fluid.
We have found that designing round channels with spherical corners often solves this challenge easiest along with polished surface finishes within the channels. Smooth surfaces and corners reduce the likelihood for bubbles to stick to the surfaces.
If, after these processes and adjustments, bubbles still form, here are a few possible solutions.
While this is not a service we provide in house, we can coordinate our production with an outside service.
Degassing Liquids & Fluids
Degassing will reduce the amount of trapped gases in the liquid reducing the chances of bubble formation. Alternatively, create a bubble trap as part of the manifold. Gas porous membrane or expansion chamber to remove bubbles in the fluid flow.
Here are some of our recommendations when it comes to feature spacing within a plastic manifold. Certain elements like channels, holes (including threaded), and reservoirs need these spaces to maintain proper function.
- Channel - Channel: 0.04” (1mm) minimum
- Channel - Hole: 0.04” (1mm) minimum
- Channel - Threaded Hole: 0.08” (2mm) minimum
- Channel - Reservoir: 0.12” (3mm) minimum
- Anything - Outside: 0.12” (3mm) minimum
It is possible to reduce feature spacing to as low as 0.02 inches (0.5 mm). Please contact us to discuss your application further and give assistance.