- THE MAGAZINE
Old-style faucets relied on natural or compounded rubber to seal the gate portion of the valve against the stem body. After a number of open/close cycles, the faucet would eventually begin to drip. Using two faucet handles to mix hot and cold water only compounded the problem. After the user repeatedly cranked down on the faucet to stop the drip, the seal would compress to a point where it would no longer seal at all. In addition, the tap seat suffered wear as a result of the flow of water, while the washer was negatively affected by the actual activity of forcing the washer onto the seat.
A dripping faucet wastes both water and money (especially if the tap is running hot water). Ceramic washers and discs offer a number of advantages. Not only are they very hard, but they withstand the erosion of water very well. An additional benefit is that the rotation of a ceramic disc has very low friction. In fact, one can literally turn a ceramic tap on or off with one’s little finger, which is very useful for those with a weak grip or an ailment such as arthritis.
Hot + ColdMixing hot and cold water has become a lot easier through the incorporation of a cartridge containing a number of ported ceramic disks that act as washers and mixing valves. The basic principle of operation is a cartridge that forms the core of a faucet fitting. Driven by the lever (faucet handle) of the fitting, it regulates the amount and temperature of the water running from the faucet.
A cartridge can be made up of approximately 20 individual components made of plastic, metal and rubber o-rings, one ceramic seal disc and one regulator disc. A suitable cartridge must be developed and installed depending on the shape and requirements of the style and type of faucet.
The crucial components of the cartridge are the seal and regulator discs, which are adjacent to each other inside the cartridge and remain in contact as they are rotated. These ceramic discs vary in their geometry and holes, depending on the faucet and cartridge. However, most have two inflow holes for hot and cold water, as well as a combined water hole through which the mixed hot and cold water flows out. Depending on how the disc holes are rotated via the fitting lever, the appropriate aperture angle is attained and the desired amount of water flows through and is mixed together, thereby maintaining the temperature of the water that exits the faucet.
Seal and regulator discs need to fulfill several functions simultaneously. First, they maintain water temperature and make the faucet watertight, preventing dripping. Second, they must be able to be rotated together effortlessly so the faucet can be operated without difficulty. Third, they need to withstand any solid particles in the water, such as the sand or calcium that are present to some degree in all water supplies, so their polished surfaces remain level (flat) and therefore watertight. When tested in the laboratory, faucets and cartridges are turned on and off under various water temperatures 1.5 million times, which corresponds to 50 years of usage in a typical four-person household.
Production ConsiderationsMany ceramic materials can be used to create seals and washers, but the one that most manufacturers turn to is alumina. Alumina offers very good performance in terms of wear resistance, corrosion resistance and strength at a reasonable price for a variety of applications, including armor, semiconductor processing equipment parts, faucet disc valves, washer seals, electronic substrates and industrial machine components.
Alumina ceramics are available in hundreds of different types, including boehmite, hydrated, tabular, activated, gamma, beta and alpha. Alpha alumina of various densities and percent compositions has been a mainstay of the ceramic seal and washer industry. Table 1 details some general material specifications of the types of alumina washers/cartridges commonly produced. The percent alumina content varies by each brand depending on the manufacturer and type of faucet.
Manufacturing techniques involved in making ceramic cartridges or washers vary and include injection molding, roll compacting, extruding, isostatic pressing, dry pressing, hot pressing, tape casting, slip casting, precision grinding and lapping, and alumina ceramic-to-metal brazing. Two of the more practical processes for producing alumina-based seals, washers or cartridges are dry pressing and injection molding.
Dry pressing is used to manufacture mass-produced precision ceramic products. Non-clumping granulated ceramic powder is compressed in steel dies designed appropriately for the part to be manufactured. The high cost for the dies (sometimes made of carbide) can only be justified for large runs. The most common type of dry press used for this application is a single-axis dry press, which features regions of different compression within the die, though a double-axis press may also be used. Part geometry usually dictates the type of press.
Suitable for both simple and complex geometries, dry pressing is one of the most economic manufacturing processes for large production runs. Depressions and holes are well suited for cartridges and discs. Depending on the design of the dry pressing machine, components ranging in size from tiles down to match heads can be produced. Small discs or plates can be pressed with thicknesses of around 0.8 or 1.0 mm.
Injection molding is also ideal for the high-volume production of complex, tight-tolerance components. The process offers significant advantages over conventional forming methods, including cost effectiveness for complex designs; the ability to produce net or near-net shape parts; very tight tolerance control; and low-cost, high-volume manufacturing runs.
Injection molding materials such as 99.8% alumina oxide, 96% alumina oxide and others provide for denser, harder, more abrasion-resistant parts than those made by other processes. Injection molding can speed up the manufacturing process and substantially reduce manufacturing costs. The resulting ceramic parts are superior in quality and lower in cost.
Some of the characteristics of an injection-molded part include flatness and parallelism to within .00005 in., surface finishes as fine as 2 micro-inch, high tolerance of ± .0001 in., hole diameters as small as .006 in., and both internal and external threading.
The compressed parts need to be fired to obtain the desired hardness, and various types of kilns are used to convert the compressed alumina powder part into a fully dense ceramic part. Temperatures in a kiln can reach 1500ºC or higher, depending on the type of ceramic and desired density of the finished part.
Finishing ProcessesOnce washers, seals or cartridges have been fired to their near-net shape, they need to be finished through grinding and polishing to obtain the desired properties. When we speak of grinding we are not speaking in the traditional sense. To work properly, ceramic faucet components need to be made flat, which is achieved much like the planarization process for the memory chips used in computers.
Flatness is accomplished by lapping (grinding) the two sides of the part. The lapping machine consists of two parallel plates, usually made of cast iron and grooved with a preferential geometry that best suits the desired end result. A slurry of diamond or boron carbide abrasive is used to infiltrate the plates with particles of a sufficient distribution to make the parts flat. A slurry mixture of a certain weight percent abrasive is pumped onto the surfaces to keep a fresh distribution of particles needed to complete the flatness step.
Some typical specifications for flatness on alumina seals or washers include Ra (average traverse roughness) at 0.6 µ and flatness to 10 µ (no warp or bow). An abrasive slurry distribution of 75 to 105 µ can be used to accomplish this step.
If we were to stop at this step and assemble the washer set, the faucet would either leak or drip. An average of 0.6 µ roughness means that some coarser patterns were produced through grinding or lapping on the finish. Under pressure, water would be forced through these patterns and cause a drip.
One might ask why not polish to less than a few tenths of a micron and be done with it? If the polish is to a high degree, the mating surfaces of the washers/discs will stick together in water. By using the right distribution and amount of abrasive slurry, however, one can achieve the desired roughness coefficient to make the discs glide smoothly against each other without letting water drip through.
The polishing step is much like the lapping step. Similar equipment is used, but the finishing specifications are much tighter. The cast iron plates may have different groove geometries to facilitate abrasive slurry flow and transit time across the parts being polished.
Typical average roughness for alumina-based washers/seals is a Ra of 0.1 to 0.3 µ and flatness of less than 0.6 µ, which is an order of magnitude different than the grinding step. Typical particle size distributions in mostly oil-based slurries are 4-8 µ or 3-5 µ. The size distribution varies somewhat based on the density and hardness of the alumina seal or part being polished. The number of carats per liter of slurry is determined by the number of parts that need to be completed to specification per unit time.
Modern faucets that incorporate ceramic seals and washers are easy to use, do not leak and-because of their cartridge-type structure-give us many new designs to choose from at our local DIY store.