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Coated, Bonded & Non-Woven Abrasives
A coated abrasive is a product that consists of a thin layer of abrasive grain attached to a backing material or substrate such as paper, cloth, etc. They are used primarily for wood and metal finishing and light-to-medium grinding. They are available in:
- Sanding Discs (PSA & Hook & Loop)
- Grinding Belts
- Polishing Belts
- Wide Belts
- File Belts
- Flap Discs
- Resin Fiber Discs
- Flap Wheels
- Quick Change Discs
- Shop Rolls
Backing Materials (Substrates)
Coated abrasives come with different backings include cloth, paper, vulcanized fiber or a combination of these. The backing must be strong enough to withstand grinding pressures and flexible enough to conform to contours, and they are described below.
Specialized technical papers are used as substrates for coated abrasives. The lighter the paper backing, the greater the degree of flexibility. The heavier the paper, the more tear resistant. Standard weights used in coated abrasives are 40, 70, 90, 130 and 165 lbs. and are indicated by a letter code.
- A-Weight (40 lbs.) – Used primarily for wet or dry hand finishing operations. Grits 80 and finer.
- C-Weight (70 lbs.) – Used for wet or dry hand sanding and for small portable power sanders. Intermediate through fine sanding. Grits 60 through 180.
- D-Weight (90 lbs.) –– Used for hand sanding and for small portable power sanders. Coarse through intermediate sanding. Grits 36 through 80.
- E-Weight (130 lbs.) – Used primarily on roll, belt and disc applications where high resistance to tearing is needed.
- F-Weight (165 lbs.) – The strongest, least flexible paper backing used.
Cloth backings are more durable than paper backings and offer greater resistance to tearing and tolerate continual bending and flexing during use. There are several types of cloth backings: cotton (i.e., Egyptian), polyester and polyester-cotton blends. The standard cloth weights and flexibility are indicated by a letter code which appears immediately after the grit size:
- “F” weight (J-Flex) is light and highly flexible. This lighter weight is suitable for cleaning, finishing, and polishing.
- “J” weight (Jeans) is highly flexible and suitable for cleaning, finishing, and polishing of curved surfaces.
- “X” weight (Drills) is medium to heavy weight for more strength and durability. It has low flexibility - used on products designed for coarse grit stock removal through fine grit finishing and polishing.
- “H” weight (Heavy Duty) is a heavier weight than “X” weight. It has low flexibility and is suitable for heavy duty grinding and deburring applications. Excellent edge stability. Used on coarse grit products designed for extreme pressure and heavy stock removal operations and used exclusively with zirconia alumina abrasive.
Fiber (Resin Fiber Discs or RFD)
These fiber backings are made from multiple layers of impregnated paper. They are very hard and strong, yet have sufficient flexibility for their intended application. They are primarily used for medium to heavy metal removal.
Fiber backing is a tough vulcanized material made from rag stock. This backing is usually used for abrasive fiber discs.
Combination backing is laminated paper and cloth, and is very sturdy and shock resistant. They are used for a wide range of grits and mounting techniques.
Other Backing Materials
There are also a variety of other substrates such as nylon fiber or screens that can be coated for special applications. Non-woven nylon impregnated with abrasive grain is another substrate that can be used for cleaning, polishing, or blending.
All coated abrasive products are constructed with a two-stage bonding process. The first or “maker” coat is the adhesive base between the abrasive and the backing. After drying or curing, the second or “sizer” coat is applied over the abrasive and the adhesive coat to anchor the abrasive and provide the desired strength to the finished product.
Glue, urea resin, and phenolic resin are the three basic bonding agents most commonly used. There are many layering combinations, such as glue over glue, urea over glue, and resin over resin. Glue over glue is the most flexible. Resin over resin is moisture-resistant, heat-resistant, has superior grain retention, but it is less flexible.
Glue has a tendency to soften or melt from the normal heat of grinding. Glue bonded products usually produce a more uniform, smoother finish which, if using finer grits, can be easily buffed out.
Resins offer greater heat resistance and are more durable for use in heavy stock removal operations. Resin bonded products are the best all-around coated abrasives. However, they have a tendency to produce a harsher, more scratchy finish which, even with the finer grits, is more difficult to buff out.
Flexing is a controlled breaking of the adhesive that holds the abrasive grain to the backing with the intention of varying the flexibility and aggressiveness of a specific product. The direction, spacing and severity of the breaks must be closely controlled if the product is to meet the requirements of the intended application.
The materials used in the manufacture of coated abrasives are selected on the basis of their hardness, toughness, inertness, resistance to heat, fracture characteristics and particle shape. The ability to penetrate the surface being abraded depends to a large extent upon the hardness and shape of the abrasive. The ability to resist breakdown and dulling under the stress of the grinding operation reflects its toughness. The best abrasive grain offers maximum resistance to point wear, yet fractures before serious dulling occurs, thereby fulfilling both stock removal and finishing requirements. The most common abrasives used are: aluminum oxide, zirconium, ceramic, silicon carbide and garnet. The crude materials are crushed and separated into sizes, called grit sizes, using calibrated screens. Grits range from 12 (very coarse) to 1200 (very fine). Once the grains are separated into sizes, they are attached to a backing material using various bond techniques. Below are descriptions of the most common abrasive grains:
A very tough, wedge shaped, man-made grain used for high speed grinding and finishing of metals, wood, and other high tensile strength materials such as carbon steel, alloy steels, tough bronze and hard woods without excessive fracturing or shedding. Wherever the ability to resist fracturing is the main consideration, aluminum oxide will outperform all other coated abrasive grains.
Silicon carbide cuts faster under light pressure than any other grain used in coated abrasives. They are dark gray to black in color, are man-made and the hardest and sharpest of the minerals used in coated abrasives. This makes it the ideal abrasive for sanding non-ferrous metals (aluminum, brass, bronze, magnesium, titanium, etc.), rubber, glass, plastics, fibrous woods, enamel, marble and other relatively soft materials.
A very fine, dense, man-made crystalline grain which can be used for aggressive stock removal. Zirconium is a very dense material with a unique self-sharpening characteristic giving it long life on heavy stock removal operations. Zirconia alumina is well suited for heavy grinding of metals and wood because the controlled fracturing of the grain continuously produces new sharp abrading points.
Ceramic grain, also known as Seeded-Gel or Sol-Gel, is the costliest abrasive grain. It is a premium, long-lasting grain which provides more consistent finishes. It is often blended with standard grains and engineered for specific applications. This grain is better for use on steel and thin stainless materials where burning or discoloration may be a concern.
Garnet is made of natural aluminum oxide which is a relatively sharp, but very weak bonding structure which is very inconsistent when compared to synthetics. It is used primarily in woodworking as garnet dulls too quickly to be used in metalworking. Garnet grains, red in color, are made from crushing semi-precious garnet material. Not as hard or durable as synthetic abrasives, garnet fractures along the cleavage lines of the crude crystals, therefore the resulting grains have very sharp edges.
Emery grains, black in color, are a natural composite of corundum and iron oxide. The particles are blocky in shape and tend to cut slowly, which polishes the material being abraded. Used for general maintenance and polishing of metals and, in very fine grits, for highly technical polishing such as preparing metallurgical specimens requiring very close tolerances.
Crocus consists of iron oxide particles in natural or synthetic form. Used mostly for cleaning corroded surfaces of polished metals where a minimum of stock removal is desired, crocus is also used for polishing gold and other soft metals.
Abrasive Grain or Grit Size
After the crude abrasives have been crushed, the grains are separated (graded) into standard particle sizes with screens. The grit number (mesh number) appearing on the coated abrasive backing represents the approximate number of openings per linear inch. Coated abrasives use grain sizes graded from 12 to 600, however not all products are available in the complete range. The following is an approximate comparison between grit size and mesh numbers:
- Extra Fine – 600 to 320
- Fine – 180 to 120
- Coarse – 60 to 50
- Very Fine – 280 to 220
- Medium – 100 to 60
- Very Coarse – 36 to 25
- Extra Coarse – 20 to 12
Abrasive Grain Treatments
Coated abrasives are generally manufactured in two levels of abrasive grain surface density:
With a closed coat, 90% to 100% of the surface is covered with abrasive grain, with almost no voids between the particles. The greater density of abrading points causes faster stock removal and produces a better finish than open-coat. Closed-coat is recommended for very hard materials where loading is not a significant problem.
With an open coat, 50% to 75% of the coated abrasive surface is covered by abrasive grain. Open-coat abrasives are the best choice where clogging or filling of the abrasive surface would be a problem. They offer greater flexibility, but are harsher cutting and do not provide as good a finish as closed-coat. Wood, soft metals and fiberglass sanding are typical applications for open-coat products.
Some products are given additional special treatments in order to make them even more suitable for specific applications. Such treatments include No-File and PSA.
In order to provide even better resistance to loading, some open-coat paper products are given a special treatment of zinc stearate. These products are ideal for applications where conventional abrasive products fail early due to loading, such as:
- Sanding between sealer coats on furniture
- Sanding after primer coats on automobiles
- Removing varnish from wood
PSA (Pressure-Sensitive Adhesive) Treatment
Some paper and cloth products have a special PSA coating applied to the reverse side of the backing. This offers peel-and-stick convenience and eliminates slippage between the sander back-up pad and disc.
With proper storage, coated abrasives will retain their efficacy and usefulness over longer periods. Poor storage can cause problems such as shedding, curl, brittleness and breakage. Temperature and humidity are the two most important factors influencing coated abrasive longevity. Backings and adhesives are sensitive to humidity which can cause them to gain or lose moisture.
Excessive moisture can soften some types of adhesive bonds, causing quick loading and cloging during use.
Excessive dryness may cause brittleness and distortion. As moisture increases or decreases, dimensional changes can occur. When the backing changes more than the adhesive bond, it causes cupping. High and low humidity can cause concave and convex cupping respectively.
- Constant levels of humidity and temperature should be maintained. Stockroom temperature should be 60°F to 80°F and relative humidity between 35% and 50%.
- Cartons should be kept away from damp or cold walls and floors where they may absorb moisture.
- Store coated abrasives away from heat sources such as radiators, hot air inlets, heat ducts, furnaces or ovens.
- Keep products in their original packages. These packages facilitate handling and practical, convenient stacking.
- Bulk rolls should not be stored on their edges after they have been unpacked. Store flat on shelves or pallets.
- Polishing belts or sanding belts that have been removed from the packing case should be rolled up and stored on edge on a clean shelf. Belts may be draped over a large cylinder such as a gallon can, brake drum, or flanged hanger of the type used for garden hose. Never hang a belt from a nail; the backing will crease and the abrasive may crack.
Bonded abrasives are natural or synthetic abrasive grains "bonded" into a solid form. They are a mixture of abrasive grains, fillers and bonding materials. The bonding agent (matrix) can be resin, rubber, glass, shellac, epoxy, magnesite or a range of plastic bonding materials. Many developments have been made using new types of abrasive grain and bond developments to increase the performance of the bonded abrasive product, and each product is designed to give a specific finish, speed of cutting and stock removal or cutting requirements.
Bonded abrasives are designed to grind, cut, polish, finish or sharpen. Products include:
- grinding wheels
- cutting wheels
- mounted points
- hone/sharpening stones
- grinding segments
- cut-off wheels
- snagging wheels
These products can have different bonding agents and abrasives which gives them different functions, and they can be used to cut-off and to grind on different hand-held machines such as: angle grinders, petrol saws, straight grinder, and vertical grinder. They work great for removing surface materials such as metal, ceramics, glass, plastics, and paint.
Common bonded abrasive materials include: aluminum oxide, silicon carbide, boron nitride, zirconia alumina and ceramic aluminum oxide. An ideal grinding abrasive has the ability to fracture before serious dulling occurs and offers maximum resistance to point wear. Each abrasive has a special crystal structure and fracture characteristics, making it suitable for operations on specific materials.
Aluminum Oxide - Most Common.
A tough, sharp grain produced under controlled conditions and especially adapted to grinding or cutting materials of high tensile strength, such as alloy steel, high speed steel, annealed, malleable iron, and tough bronze. This particular abrasive comes in a variety of styles that work with different materials.
Ceramic Alumina - Top of Line
A superior performing grain for high temperature alloys such as Inconel, Waspaloy and Hastelloy, hardened tool steels, spray metal or Stellite and other hardened steels. The consistent break down of the abrasive grains to form a sharp cutting surface improves a abrasion. Ceramic aluminum oxide, has a fracture rate which is controlled down to the sub-micron level. While using, the grains break down yet their surface remains just as sharp. Breaking the grains provides brand new cutting points making ceramic aluminum oxide the abrasive of choice when working with the hardest metals and needing precision.
Zirconia Alumina - Rough Grinding
A tough, sharp abrasive designed for optimum performance in cut-off wheels, snagging wheels and depressed center wheels for grinding or cutting cast steel, alloy steel, and malleable and ductile iron. This abrasive uses a mix of aluminum oxide and zirconium oxide to create a durable surface that works great in cut-off applications for steel and steel alloys
Silicon Carbide – Non-Ferrous Grinding
This is a very hard, super sharp abrasive grain used in the grinding of tungsten carbide and low tensile strength materials such as cast iron, bronze, aluminum, copper, and non-metallic materials such as plastics, glass, marble, granite and stone. It works well on non-ferrous materials like rubber, stone, plastic, and glass, and also cuts through cast iron, soft bronze or aluminum. You will replace this abrasive more frequently than normal. It is very sharp, but can also be brittle. It doesn’t last as long as an aluminum oxide.
Characteristics of Bonded Abrasives
The grade is the strength of the bonding of a grinding wheel, and is frequently referred to as its hardness. Grade is designated by a letter - the higher the letter, the stronger the bond. Grade is not a measure of the hardness of the abrasive material, but it is a measure of the ability of the bond to hold the abrasive grains together. A soft grade wheel can be made of the hardest abrasives, and a hard grade wheel can be made of the softest abrasives. A simple rule to follow is to use a hard wheel for soft materials and a soft wheel for hard materials.
Wheel structure refers to the spacing between grain particles. it is the wheel's density. There are fewer abrasive grains in an open-structure wheel than in a closed-structure wheel. A number from 1 to 15 designates the structure of a wheel. It varies with different grinding operations, depending upon the area of contact, the type of material being ground, the rate of stock removal and the finish required.
The purpose of the bonding material is to hold the abrasive grain particles in place.
In resinoid bonds, resins are used as binders, into which fillers are mixed with abrasive grains. They are more shock resistant than vitrified bonds and are generally operated at higher peripheral speeds. Resinoid-bonded grinding wheels are characterized by having a good cutting performance and cool grinding. Most resinoid bond wheels are used for fast stock removal in the metal fabrication and foundry markets.
Vitrified wheels have a glass bond composition. The bond is much harder that organic bonds but softer that metal bonds. Vitrified wheels hold form extremely well and produce a high ratio of stock removal to wheel use. Vitrified bonded wheels are commonly used for precision grinding in the tool and die market.
Porosity is one of the most important properties of a grinding wheel when it comes to positively influencing the performance. The pores support the supply of coolants into the grinding zone, and they facilitate the removal of chips and residues. Also, due to the thinner bond bridges in porous systems, the interaction between bond and workpiece is significantly reduced, resulting in a less heat. Vitrified wheels can be manufactured with induced porosity. All vitrified bonds that end in P (VLP, V40P, VPP) indicate induced porosity. This porosity allows excellent coolant flow and chip clearance. For materials that load or for wide surface area contact, this provides a cooler cutting action, less loading and less chance of burn on the workpiece.
Coated abrasives have layers of abrasive on the belt, disc or sheet. Bonded abrasives have the abrasives in a resin or binding agent to form a wheel or stone. Non-woven abrasives are used when coated or bonded products cannot achieve the required result. The low aggressive characteristics of the polyamide fibers and the positive effect of the non-woven materials create very smooth finished surfaces. Non-woven abrasives are waterproof, and can be rinsed out. They do not clog up, leave no rust on surfaces and do not conduct electricity.
To understand the finishing action of nonwovens, imagine the abrasive held by a spring. The pressure applied to a nonwoven abrasive is not applied directly to the workpiece. Some of the force is absorbed in the spring. Nonwovens are less aggressive than coated or bonded products and tend to give a finer finish.
As a non-woven product runs over an irregular surface the “springs” compress and expand allowing the grain to be constantly contacting the surface. Other products would skip over the valleys and remove excess material on the peaks.
This feature is especially important during a finishing process. The conventional method for removing deep scratches is to use a progression of coated abrasives, e.g. 80, 120 and 180 grit, each giving a progressively finer finish while removing the previous deeper scratch. The conforming nature of a non-woven provides a fine finish while removing deeper scratches. You experience major cost savings by reducing the number of steps in the process
There are several uses for Non-Woven Abrasives:
- Cleaning and Finishing
- Surface Conditioning Material (SCM)
- Convolute and Unitized Wheels
The grit rating of non-woven abrasives does not utilize a number coded like coated abrasives. It offers these ratings: course, medium, fine and very fine.
These abrasives can be used for deburring or cleaning and are excellent for surface work on: aluminum, brass, copper, nickel, stainless steel (INOX) and titanium. They are also good for work on materials which are hard to grind such as: ceramic, glass and plastic. They can be used for wet or dry grinding.