Overview of Construction
The manufacturing of coated abrasives contain multiple components and processes, such as an Adhesive Bond, Backing Materials, Grinding Aids, the Abrasive Grains and Post Production processes.
There are up to three adhesives used in the manufacturing process of abrasive materials. The Make Coat is applied directly to the backing. this first adhesive provides an anchor for the grain to bond to the backing. A Size coat is applied after the make coat has dried and is applied to securely anchor the grain. A Top coat (or "super size" coat) is applied after the top coat has cured and is used to carry a grinding aid or lubricant.
Originally, abrasives used animal hide glue to bond the abrasive to the backing. The animal hide adhesives tend to be used on lower cost abrasives for manual abrasive uses such as sheets and handy rolls. The glue softens under the heat of grinding which can act as a cushion and gives a lower cut rate, but more uniform finish in finer grits. Glue bonded products can only be used dry.
Urea and Phenolic Resins
To overcome the shortfalls of glue based adhesives, urea and phenol formaldehyde resins have been developed. Phenolic resins are more widely used as their final properties are more suited to high temperature and pressure operations. Urea resins perform better than glue, but do not have the high performance characteristics of phenolic resins Fillers can be added to the resin formulation which would increase bond strength and dissipate heat. If the filler has an active role in the abrasive, it is known as a grinding aid.
Paper is often used where low product cost is preferred over strength and toughness. Papers are made to tight weight tolerances gauged by the weight (lbs) per paper maker's ream: 480, 24 x 36 sheets.
Generally heavier papers are less flexible and more tear resistant and are suited to mechanical applications. The lighter papers are less tear resistant and more flexible and are more suited to hand sanding operations and finer finishing. Because the surface of the paper can be made to be extremely flat, paper is used for extremely fine grits where any surface imperfection in the substrate would cause inconsistent sanding patterns in the polishing applications. Paper can be made waterproof by the addition of latex and other water resistant resins.
Cloth substrates are more expensive than paper but are structurally stronger. Cloth products are therefore better suited to mechanized operations, such as belts discs and wheels.
The first two cloths in the US market were cotton drills (X-wt.) and cotton jeans (J-wt.), both use a twill weave. Drill uses less yarns per inch but each yarn is heavier. The next advancement was a heavier weight drill commonly known as Y-wt. More recently double Y or H drills have been used for heavy grinding applications.
||Light and flexible. Used where the abrasive must contour to the workpiece. Usually cotton yarns.
||Similar to "J" Weight with extra flexing.
||The most common weight backing. Typically cotton or polycotton. A relatively stiff yet strong backing.
||Similar to "X" Weight with extra flexing.
||Very high strength and low stretch for use on heavy belt grinding applications. Usually polyester, but sometimes polycotton.
|"YY / H" Weight
||The strongest of all cloth backings designed for long life and coarse grinding in heavy stock removal.
Sometimes an F is added as a suffix to denote extra flexing (e.g. JF and XF).
While cotton was the first cloth, synthetic cloths, polyester and polyester / cotton mixes have become more prominent as they tend to be stronger and more stable in use. Also the synthetic polyesters are inherently waterproof while the cotton products need treatment to be waterproof.
While the fabric weight is a good indication of the product stiffness and strength, back filling and flexing also affect stiffness and strength. Whatever yarn is used back filling is required to ensure adhesion between the fabric and the adhesive coats of the manufacturing process. Backfilling also fine tunes the abrasives' final properties.
Vulcanized Fiber is a product made from cotton rags and treated with harsh chemicals to form a gelatinous substance which is then layered into plies. The plies are heat vulcanized and calendered to give a strong and stiff base material. Due to its high stiffness, vulcanized fiber products can only be used in disc form such as Resin Fibre Discs. The recommended use is for heavy duty stock removal, typically in grits 80 and coarser.
If the variation in the thickness of any product backing is greater than the size of the grain particle, non-uniform grinding may occur. This is a problem for finer grits in tight tolerance, high performance grinding, such as cam shafts. Film backing is a recent development to solve this problem. Films can be made with exceptional flatness and with tight thickness tolerance making them most suitable for technical operations. Film has added advantages of being tear resistant and waterproof.
Sodium Cryolite and Potassium Perfluoroborate are commonly used grinding aids added to the size coat or super size coat. Both minerals decompose during the grinding process and take in heat from the surroundings as they do. This reduces the temperature of grinding. Stearates are lubricants which are included in the super size coat. They reduce friction between the abrasive and the work piece and they reduce the likelihood of loading.
The cutting power of an abrasive grain is dependent on shape, hardness, toughness and grain orientation. All four factors control the penetration of cut and the life of the grain.
The shape of the grain governs the cutting efficiency. Sharper grains produce a cleaner cut and tend to be cooler during use. Blocky grains give a rougher cut and create more friction.
|White Aluminum Oxide
| Pink Aluminum Oxide
|Brown Aluminum Oxide
|Zirconia Aluminum Oxide
| Ceramic Aluminum Oxide
Toughness is the resistance of the grain to fracturing or breaking down, also known as friability. While silicon carbide is extremely hard it is not very tough, it breaks down easily. Both zirconia and alumina are very tough.
Grain Orientation is governed by the grain application technique. In the early days of manufacturing abrasives the grain was sprinkled onto the substrate via gravity. This resulted in a random orientation for the abrasive particles. The most common technique today gives each grain an electrostatic charge. The charge then pulls the grain from a lower conveyor, against gravity upwards onto the resin coated substrate. This aligns the grain with the sharpest point standing upright.
Aluminum Oxide, AO, is the most common abrasive grain in use today. AO is a synthetic grain manufactured by a fusing process from bauxite in a Higgins furnace. It is extremely tough and durable but has no clearly defined crystal structure. This means that when the grain eventually wears it produces rounded which reduces its cutting power. Brown AO is the usual grain for coated abrasives products product but AO is also available in white pink and other variants. The white is a pure form of AO which has a more distinct crystal structure. The pink is a pure form with added chromium which also affects the crystalline structure. Increased crystallinity gives a grain which is more likely fracture rather than erode to a round edge. Fracturing is preferred as new cutting surface are produces as the mineral wears.
Zirconia Alumina, ZA, is best described as an alloy of aluminum oxide and zirconia. During manufacture the two components are quenched from the molten form which gives the grain a crystalline structure. The structure is meta-stable and under heat and/or pressure the structure fractures producing new cutting edges. This gives the grain a longer cutting life and is especially suited to coarser stock removal applications typically 24 to 80 grit. Â It is a more expensive grain but the improved performance usually outweighs the cost.
Unlike fused aluminum oxide, ceramic aluminum oxides are formed via a chemical method. This proprietary process, sometimes called seeded gel or sol gel is similar to the process for manufacturing industrial ceramics hence the various names for this product. The grain from this process comprises millions of smaller crystals which fracture off in use and continuously form a new cutting surface. This self sharpening process requires heavy pressure to ensure that the grains do not glaze.
Silicon Carbide, SiC, is the hardest of all common abrasive materials and also very sharp. But it is friable which means that it breaks down along the crystal cleavage plains leaving new sharp edges for cutting. For this reason it is fast cutting initially but short in life. It is ideal for use on woods, non ferrous metals (Aluminum, brass, bronze, magnesium and titanium) and plastics. Silicon carbide has sometimes been used as a final finishing grain for stainless steel to give a bright finish. This is because the sharp SiC crystal structure produces a fine straight edged grind lines that reflect light better than the rounded aluminum oxide grind lines.
Emery is a naturally occurring mineral that is blocky in shape. It is also slow cutting but has found use in manual sanding operations in the metal working environment. Garnet is a naturally occurring grain with very sharp edges. It is relatively soft so that it cannot compete with synthetic grains for durability, extensively used in woodworking. Flint is a natural abrasive from the quartz family. Like other natural abrasives it cannot compete for durability with the synthetic grains however it does have a unique property of being non conductive. This lends itself to electrical application where if any abrasive is left on the part it would not cause a short circuit. Crocus is a naturally occurring form of iron oxide. Very fine and relatively soft used as a final polish for precious metals.
Structured, or Compact Grains are synthetic grains that consist of smaller abrasive grains which have been bonded together to form a large abrasive grain. There are patented processes for making structured abrasives with a pyramidal cross section. Other products use an agglomerated grain. The advantage of these products is that the finish is still relatively fine yet the grain has a much longer life.
Open Coat & Closed Coat
The degree of spacing between grains on the coated abrasive determines the openness of the coated abrasive. If the grains are touching one another the abrasive is said to be closed. If they are more that one grain apart the abrasive is said to be open. Some manufacturers have a middle specification of Semi Open.
A closed coat product has a higher number of abrasive cutting points per square inch and results in higher stock removal rate and longer time before the abrasive wears out. Most abrasives are closed coat. Open coat abrasives are used when the debris from the grinding process is likely to stick to the abrasive product and fill the gaps between the abrasive points; an undesirable process known as loading. Loading can be reduced by using an open coat. The voids between the grains offer more flexibility and more resilience to loading. Open coats tend to wear faster and don't give as uniform a finish as closed coat.
Abrasive Grain Size
For grains 220 grit and coarser, abrasive grain is mechanically sifted and sieved through a series of wire screens. The screens are accurately made and the number of openings per linear inch corresponds to the grit size. I.E. a 60 grit has been sieved through a screen with 60 openings per linear inch. Grains 240 and finer are sized via hydraulic sedimentation as sieves cannot be made with such fine openings.
Post Curing and Humidification are carried out simultaneously in a static curing oven. The rolls of abrasive material are put on racks in the oven and left to cure for up to 24 hours. Steam is used in the oven to re-humidify the abrasive roll.
Flexing introduces predetermined patterns of cracks or weak points in the resin bond. This prevents uncontrolled breakdown of the resin layer during use. The flexing operation also provides flexibility appropriate for the products' intended use. For instance, a 'J' weight belt used in contouring applications receives more flexing than a 'X' weight PSA Disc used on a rigid backing plate. The operations can be 45° double flexing or 90° single flexing. Combinations of the two are also made. Flexing requires the abrasive to be drawn over a bar of small radius, grit side up, to crack and break the filler, make and top coat resins.