Sintered Porous Bronze Fuel Filters, Part 3
Porous sintered bronze elements FAQ, applications beyond filtration plus references
Read Part 2 to find out more details about how to make sintered porous bronze filter elements and how they work >>
An FAQ section about sintering, porous sintered bronze fuel filter elements and depth filters
How does depth filtration work?
Fine particles suspended in a fluid passing through a depth filter may impact or run into the filter material and stick or adhere to it. This particle impact and capture is called adsorption.
Depth filters have complex, meandering and irregular interconnecting pores (tortuosity). This tortuosity increases the likelihood that depth filters can capture suspended particles in one or more ways (particle capture mechanisms).
Particle capture mechanisms are also called principles of filtration, mechanisms of filtration and mechanisms of particle capture.
The ways depth filters capture fine particles are (see the illustration)
- Diffusion interception (top)
- Direct interception (middle)
- Inertial impaction or interception (bottom)
Tiny particles suspended in liquids are bounced around by the molecules of the liquid. This random bouncing (Brownian Motion) increases the odds that particles bounce out of the fluid stream and bump into the filter material where they may stick.
Depth filter materials have tiny, winding, interconnecting passages or pores (tortuosity). Many of these pores or passages taper or interconnect with smaller passages.
Fluid can easily flow through narrower passages, but particles that are too large cannot pass through. When this happens, filter material directly captures these particles.
As the liquid passes through filter material (filter media), the flow follows the path with the lowest resistance to flow. Particles carried along in the fluid have mass and velocity. These combine to give moving particles momentum.
Momentum is the tendency for a moving object to keep moving in a straight line. Unless another force interacts with it, the particle does not change direction. Inertia is this resistance to changing direction.
The winding passages (tortuosity) inside depth filters cause liquids passing through it to change direction repeatedly. When this happens, the momentum and inertia of particles in the fluid cause them to try to keep going on in a straight line. This straight-line motion causes them to strike the filter material where they likely stick.
The technical aspects of depth filtration can be quite tricky. If you are interested in learning more about the mechanisms of particle capture by depth filters, check out this downloadable PDF reference resource:
What are filter elements?
Filter elements are the components inside a filter that do the actual filtering. Plastic inline fuel filters direct fuel flow through the filter element, capturing debris and particle contaminants.
Fuel filter elements for small inline filters tend to have a slightly conical or drum shape. These element shapes maximize the element surface areas and flow rate through the filters.
Flow direction matters for small inline filters, usually indicated by flow direction arrows molded into the filter body.
Filter elements are added to the fuel filter during their assembly because the elements arrive as separate parts. This interchangeability allows for variations in filter element material and design. Filter element design variations provide more options for micron rating and flow rate.
Types of fuel filter elements typically used in small plastic inline filters
- Metal mesh screen
- Porous sintered bronze
- Polymer treated cellulose (paper)
- Polymer (plastic) filament mesh screen
How do you clean a sintered bronze filter?A combination of ultrasonic cleaning and appropriate solvents can clean porous sintered bronze filter elements. This approach works best if the solvents flow in reverse through the filter (backflushing or backwashing).
Cleaning the sintered metal fuel elements inside small plastic inline fuel filters is not practical because
- Potentially damaging effects of solvents
- The use of ultrasonic welding to assemble plastic fuel filter bodies
Backwashing can extend the life of a sintered bronze fuel filter but, over time, fine particles build up and cause the filter to become plugged. Even before flow stops, restricted flow can make a filter unusable. The relatively low cost of plastic inline fuel filters means replacing them as needed is usually the best option.
What is a sintered filter?
Manufacturers use a powder metallurgy process to produce porous metal parts like sintered filter elements. In this process, metal powders are sintered or heated to a temperature that causes the powder particles to bond together.
Controlled sintering leaves interconnecting, winding passages or pores throughout the filter body.
The pores allow air or liquid to flow through the filter. They capture particles at the pore openings on the surface as well as down inside the pore passages.
How are sintered filters made?
For porous bronze filter elements, the manufacturing process used is called pressureless sintering or gravity sintering.
Pressureless sintering begins with pre-alloyed, spherical bronze powders poured into appropriately shaped dies or molds.
Metal powders pour freely, much like a liquid. Vibrating powder-filled dies ensure that metal powders fill the dies but do not leave any voids.
Dies filled with bronze powder are moved to a controlled-atmosphere, multi-stage furnace. The furnace heats the dies to the bronze's sintering temperature. Sintering temperatures are always lower than the melting point of the alloy and its primary metal.
Sintering heat causes the metal to diffuse between the powder particles where they touch. This diffusing metal creates narrow metal necks where the powder particles bond together to make mechanically strong parts.
For filter elements, applying sintering heat just long enough achieves the desired filter porosity.
The sintering temperature and die material are also chosen so that the bronze powder does not bond with or stick to the die surfaces. The dies are then allowed to cool.
After cooling, the final step is to remove the new filter elements from the dies.
Plastic fuel filter assembly combines the porous sintered filter element and filter's plastic body components. Ultrasonically welding the body components together holds the filter element tightly inside the finished fuel filter.
Precision and good manufacturing quality control ensure both leakproof welds and proper filter element placement.
What is sintering?
Sintering is a powder metallurgy process where metal powders are fused to create mechanically strong parts. The heat used to sinter a metal powder is less than the metal's melting point. But this heat is still high enough to cause the metal to flow together and bond powder particles together where they touch.
Sintering requires controlled-atmosphere, multi-stage furnaces.
What is the sintering process?
Sintering begins with filling a part die or mold with metal powder.
Pressing or compacting the powder into the die is the next step when making solid parts and porous self-lubricating bearings. The pressure causes the metal powder particles to stick together. While not strong, these "green" parts hold their shape after being released from the die.
For fuel filter elements, the sintering process begins a little differently.
Similar to making solid parts, the process for filter elements begins with pouring metal powder into dies. Vibrating the dies removes any voids or empty spaces. The metal powder remains loose in the dies until sintering.
After being shaped in dies, "green" metal parts or powder-filled dies are exposed to a precise sintering heat. Sintering requires controlled-atmosphere, multi-stage furnaces.
This heat is enough to cause the metal to diffuse between the particles (diffusion bonding). This diffusing metal creates narrow necks of metal that bond the particles together. Additional sintering causes these necks of metal to grow.
This "Sinter neck formation" micrograph (photograph taken through a microscope) shows the metal necks that form between spherical sintered metal powder particles.
Sinter neck formation
Enough additional sintering can produce solid parts. Additional heating causes more and more metal to diffuse between the metal particles. Space between particles shrinks to the point where the parts become almost completely solid (densification).
For porous parts, the right amount of diffusion bonding leaves a pattern of holes on the surface. The holes or pores on the surface connect to a complex winding matrix of interconnecting passages throughout the finished part.
Carefully controlled sintering time and temperature create the required porosity of finished filter elements.
Producing porous metal parts requires careful control of these variables:
- Alloys used
- Particle shape
- Sintering time
- Sintering temperature
- Sintering atmosphere
Metal powders used for making sintered porous fuel filter elements also need to be
- Prealloyed tin bronze
- Highly spherical in shape
- Very uniform in size (narrow particle size distribution)
Carefully controlled sintering creates fuel filter elements with just the right amount of particle fusion. The result is a highly porous metal with controlled pore size and good depth filtration.
What is particle size distribution?
Particle-size distribution (PSD) is a way of describing the relative amounts of different-sized particles contained in batches of powder or other granular material. PSD provides particle analysis as a relative percentage or fraction of the total batch of material.
Usually, this information comes in a chart that graphically illustrates both percentages and particle sizes. Special measurement tools and techniques provide the data for these charts.
Sorting batches of powder for very consistent particle distribution is possible using sieving and other techniques.
The particle size data generated from analyzing these powder batches create bell-curved particle-size distribution graphs (see the illustration). The more carefully the powder is sieved and sorted, the narrower the bell curve is.
Manufacturers use highly spherical bronze powders with narrow particle size distributions to make sintered filter elements. The ready availability of uniform bronze powders makes it economical to produce sintered filter elements with specific porosity and micron ratings.
These special bronze powders also make it economical to manufacture sintered bronze filter elements small enough for small plastic inline fuel filters.
What is diffusion bonding?
Diffusion bonding, also called solid-state diffusion bonding, is a metal bonding process that resembles welding. When exposed to sintering heat, metal diffuses between powder particles where they touch.
The heat of sintering dramatically speeds up a tendency for the metal in each particle to migrate towards and intermix with metal from other particles.
Metal blending and fusion bind the particles together by causing metal "necks" to form and grow where the particles touch. These necks link the metal powder particles together.
Over time, exposure to the sintering temperature causes metal necks to grow larger and become stronger. The formerly loose particles become solidly locked together to make a mechanically strong part.
What is the difference between sintering and melting?
Pressure sintering, solid-state sintering and pressureless sintering (gravity sintering) all use temperatures below the metal's melting point. This temperature is called the sintering temperature. Even though it is less than the metal's melting point, it is high enough to cause the metal to quickly diffuse between each particle (mass transport). This metal diffusion bonds the particles together (diffusion bonding).
At the metal's sintering temperature, heat bonds the powder particles together, creating mechanically strong parts.
Solid parts and densification
Pressure sintering and solid-state sintering techniques produce solid parts starting with metal powders.
These powder metallurgy processes use a combination of heat and pressure to cause the metal to diffuse between the metal powder particles where they touch. This bonds the particles together. The parts are then kept at the sintering temperature long enough (dwell time) to eliminate the empty spaces between the particles (densification or consolidation).
Pressureless sintering and porosity
Pressureless sintering is the specific type of sintering used to make porous fuel filter elements. Filter elements produced using pressureless sintering have a uniform porosity that runs throughout the metal. Pressureless sintering is a relatively economical and useful way to make metal filter elements.
All sintered parts require special furnaces
All sintering requires controlled-atmosphere, multi-stage furnaces. The controlled atmosphere protects metals from exposure to oxygen-rich air. Exposure to oxygen during sintering creates metal oxide films that inhibit particle fusion.
Controlling the atmosphere or gases inside sintering furnaces also provides better, more uniform heat transfer inside the different furnace stages.
What is the sintering temperature?
In powder metallurgy, sintering is the heat treatment used to fuse metal powder particles to make solid parts. Sintering temperatures are less than the melting point of both the alloy and its primary metal.
Sintering temperatures are lower than melting temperatures because sintering does not use melting to bond the particles together. Instead, the heat of sintering is enough to cause the metal to diffuse between the powder particles. Diffusing metal causes interconnecting metal necks to form, grow and bond the particles together where they make contact.
Porous sintered bronze filter elements used in small plastic inline filters are tin bronze, usually about 89-91% copper and 9-11% tin.
- 449 ºF (232 ºC) tin melting point
- 1983 ºF (1084 ºC) copper melting point
- 1364-1436 ºF (740-780 ºC) tin-bronze sintering temperature
- 1,570 °F (854 °C) tin-bronze melting point (C90300/SAE 620 high-tin bronze)
Why is sintering done?
Sintered parts made using suitable materials and sintering techniques can be mechanically strong while still being filled with tiny winding, interconnecting passages. Parts produced this way can be used for filtering, sound dampening, sparging, flame and spark arresting, self-lubricating bearings and more.
Sintering causes metal "necks" to form and grow between the tiny bronze spheres. Metal diffuses or flows together at the particle contact points to create solid metal bonds.
The controlled sintering of porous sintered parts provides parts with metal necks that are narrower than the sphere diameters. The result is tortuous or sinuous winding passages that run through the depth of the porous metal walls.
Because porous bronze filter elements start as highly spherical particles, sintering produces filter elements with controlled porosity and specific micron ratings.
For both porous and solid parts, sintering has advantages:
- Material waste is very low
- Lower secondary machining costs
- Economical for short production runs
- Energy efficient because no melting of the raw material
- Even relatively complex parts have high production rates
- Very good control of finished part properties like hardness
What are powder metallurgy's disadvantages?
Making porous sintered filter elements requires the use of powder metallurgy techniques. They have distinct advantages over other manufacturing techniques for making solid parts.
It is important to note the disadvantages of powder metallurgy processes:
- Maximum part sizes may be limited
- Not suitable for highly complex shapes
- Not as strong as parts produced by forging
- Requires specifying just the correct metal powder
- Metal powders require significant time and effort to produce
- Expensive process for low value and low melting point materials
- Not possible to form some more complex features that only machining can create
What is pressureless sintering?
Sintering is a powder metallurgy manufacturing technique. Pressureless sintering (gravity sintering) is just one of many types of sintering.
Steps in the pressureless sintering process
- Pour metal powders into dies or molds
- Vibrate the dies to make sure the powder leaves no voids
- Place the dies in a furnace for controlled exposure to the metal's sintering temperature
Sintering temperatures vary by alloy and are lower than the melting point of the primary metal in the alloy.
During sintering, the metal powder does not melt. The temperature is high enough to cause narrow, metal-to-metal bonds or necks to form where the metal powder particles touch.
The result is a fully hardened, mechanically strong part that has excellent porosity. Porosity is the intricate pattern of winding, interconnecting passages throughout the walls of the finished part.
Pressureless sintering is especially useful for producing filter elements, but the process requires careful control of
- The technical specifications of the metal powder used
- The sintering temperature the powder-filled dies are exposed to
- The length of time the furnace holds dies at the sintering temperature
A careful, precise manufacturing process provides filter elements with good porosity, which makes them very efficient.
Efficiency means porous sintered bronze filter elements are very good at capturing small particles without creating too much restriction or reduced flow through the filter.
What's unique about the bronze powders used to make fuel filter elements?
It is necessary to control both the filter walls' porosity and the filter element's micron rating when making porous sintered bronze filters. This kind of control requires bronze powder with specific qualities.
Bronze powder requirements for making filter elements
- Prealloyed bronze
- Deoxidized bronze alloying
- Controlled spherical particle size
- Highly regular, consistent, spherical shape
Deoxidizing gives these bronze metal powders a light gloss. Because of this, better-quality sintered bronze filter elements tend to be shiny.
How are spherical bronze powders made?
Gas atomization is the process most widely used to produce highly spherical metal powders.
It involves pouring the molten bronze through inert gas jets. The pressurized gas coming out of the jets breaks the molten metal into fine metal droplets. These molten droplets cool down and harden as they fall.
Metal powders made by gas-atomization tend to have a perfectly spherical shape and are very clean.
What about ethanol, methanol and E85 gasohol materials compatibility?
In general, material compatibility with methanol is more challenging than gasoline-ethanol blends, including E85 flex-fuel. Unless used as a racing fuel, most fuel filter exposure to methanol comes from fuel additives. Additives for treating water contamination or for preventing fuel line freezing sometimes contain methanol as an ingredient. Any filter that is methanol compatible is also E85 compatible.
Low-quality materials and fuel compatibility
Low-quality paper filter element filters may not hold up well when exposed to gasohol (E10, E15 and E85). Lower-quality filters frequently use adhesives to attach the filter paper to the paper filter element's cap or end. These adhesives usually are not compatible with ethanol. The adhesive bonds weaken over time, and adhesive failure allows fuel flow to bypass the filter paper altogether.
Filter body plastics also matter when it comes to compatibility with ethanol, methanol and gasohol blends. High-quality OE and OEM aftermarket fuel filters have plastic bodies made with materials with very good resistance to ethanol and gasohol blends and limited exposure to methanol.
A good maintenance rule of thumb for plastic inline fuel filters is to use high-quality OE replacement and OEM aftermarket plastic inline fuel filters made by reputable manufacturers like ITW Fastex Filtration and ITW Powertrain Components.
For both Powersports and Outdoor Power Equipment (OPE) applications, recommended best practice is to replace fuel filters at least once every year.
What are the effects of ethanol on gasoline?
Ethanol or ethyl alcohol is the alcohol added to gasoline to make gasohol blends like E10, E15 and E85. Ethanol absorbs water more quickly than straight gasoline does. When a small engine with only a partially full tank of a gasoline-ethanol blended fuel sits around for a while, moisture can build up in the fuel system and may separate.
Ethanol, gasohol and absorbed water
If water does settle out, it settles out at the bottom of the tank. This settling happens because water is heavier than both gasoline and ethanol. This settling can also occur in fuel lines and carburetor bowls.
Fuel containing water can create problems for plastic inline fuel filters that use paper filter elements if they are not compatible with gasohol. Water absorption can cause low-quality filter paper to swell and lose strength.
The paper filter elements used in low-quality filters are also more likely to be degraded by exposure to ethanol. This ethanol also causes low-quality filter papers to lose strength.
Filter elements like porous sintered bronze are not affected by water or ethanol in the fuel.
Make sure that the plastic used for the filter housing is ethanol compatible also. Most lower quality plastic inline fuel filters use plastics that are less able to stand up to prolonged exposure to gasohol.
Low-quality filters are also more likely to be assembled using adhesives that may not withstand gasohol.
High-quality OEM fuel filter manufacturers use filter elements, body materials and assembly techniques not degraded by water and ethanol.
Besides fuel and oil filtration, what other applications use porous sintered bronze elements?
Porous sintered bronze is mechanically strong and made with very controlled levels of porosity. These features make porous bronze parts and elements useful for a variety of applications:
- Liquid aeration
- Exhaust mufflers
- Condensation traps
- Flame suppressors
- Powder fluidization
- Pressurized fluid flow control
- Snubbers or pressure snubbers
Breathers filter exhaust gases or incoming air. In this way, they serve as oil reservoir vents, air exhaust vents, air intakes and vacuum pressure equalization ports. Breathers can help reduce the exhaust of tiny, suspended oil droplets, although they are not as efficient as dedicated oil mist filters.
Pressurized gas passed through porous sintered bronze elements comes out as fine bubbles when suspended in liquids. The gas in these tiny bubbles is absorbed or dissolved into the liquid. Used this way, porous bronze elements can gasify or aerate liquids (sparging).
Mufflers or exhaust mufflers dissipate the velocity and dampen the noise level of compressed air coming out of pneumatic valve exhaust ports, air cylinders, air tools and similar applications.
MS series speed control muffler cross-section
showing the porous sintered bronze element
The porosity of sintered porous bronze allows it to retain water vapor by causing water to condense inside the porous bronze. In this way, porous sintered bronze elements are used both for drying exhaust and for protecting devices from corrosion caused by moisture entering the system.
Flame suppressors or arrestors prevent a flame from entering or leaving equipment while also providing precise gas flow control.
Porous sintered bronze elements have both high thermal conductivity and porosity. These features allow the flow of combustible gasses while preventing ignition. The porous bronze element absorbs and dissipates the heat from a flame front or combustion wave and stops combustion.
Sintered bronze can also fluidize fine powder solids. Fluidization is provided by passing pressurized gas through porous bronze elements.
Fluidization makes it easier to manipulate powder solids by causing them to behave more like a liquid. It does this by increasing the solids-to-air ratio. The increased solids-to-air ratio reduces the density of the gas and powder mix. This change in density allows the particles to become suspended and flow more easily.
Pressurized fluid flow control
The porosity of sintered bronze elements can provide a very precise, constant and controlled pressure drop for pressurized fluid processes.
Snubbers or pressure snubbers
Snubbers protect gauges and pressure instruments from damaging shocks caused by sudden pressure surges and fluctuations in fluid or gas flows. As a beneficial side effect, they also help ensure gauge accuracy by smoothing out pressure transients (spikes).
SNUB series pressure snubber
Some additional reference resources about porous sintered bronze filters, powder metallurgy and sintering
About this three-part series on sintered porous bronze fuel filters
Sintered Porous Bronze Fuel Filters - Part 1: Benefits of porous sintered bronze filter elements and how they are made >>
An overview of porous bronze filter elements, why they are so effective in small plastic inline fuel filters and what it takes to produce high-quality sintered porous bronze fuel filters.
Sintered Porous Bronze Fuel Filters - Part 2: About depth filtration and the porous filter elements used in fuel filters >>
More detail about bronze, porous sintered bronze filter elements as depth filtration, how to make them, and their installation and use.
Sintered Porous Bronze Fuel Filters - Part 3: Porous sintered bronze elements FAQ, applications beyond filtration plus references >>
An FAQ about sintering, sintered porous bronze fuel filter elements and depth filters. It also includes an overview of other applications that use porous bronze filter elements, plus a list of references.
Selected ITW Fastex Filtration Visu-Filter fuel filters
ITW Fastex Filtration and ITW Powertrain Components Visu-Filter fuel filters are compliant with ANSI/OPEI B71.10-2018.
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Steven C. Williams, BS, is the technical writer and an inbound marketing specialist at Industrial Specialties Manufacturing (ISM), an ISO 9001-2015 supplier of miniature pneumatic, vacuum and fluid circuitry components to OEM's and distributors all over the world. He writes on technical topics related to miniature pneumatic and fluidic components as well as topics of general interest at ISM.