Technical Information 1 - Coated Abrasives

Production of Coated Abrasives

General

A large proportion of grinding tasks in craft or industrial production

are performed by the machining of workpieces with coated

abrasives. These tasks cover a wide-range of application extending

from rough grinding to finishing and polishing. In addition to pregrinding,

intermediate grinding and re-grinding to produce defined

shapes and dimensions with given surface qualities, production

of the surface finish is an important application.

Grinding tools are used, among other things, for machining of

metal, wood, plastic, paint, ceramics, stone, rubber and

leather. The large number of possible applications necessitates

numerous tool shapes and dimensions as well as tool characteristics

adapted to the specific grinding task. These characteristics

are achieved by using a wide variety of raw materials in

various combinations. The most important structural components

of grinding tools are the abrasive, the bonding material and

the backing. Abrasive rolls in production width of up to 1620 mm

can be produced from these components at large manufacturing

installations.

Abrasives

Abrasives can basically be divided into natural (e.g. diamond,

natural corundum, garnet, emery, flint, quartz, pumice) and

synthetic (e.g. synthetic diamond, cubic boron nitride, fused

corundum, zirconic corundum, silicon carbide) abrasives.

Today, synthetic abrasive grain types such as fused corundum,

silicon carbide, zirconia alumina, cubic boron nitride and diamond

are practically the only types of technical importance.

Grinding grit developments have become known recently, in

which microcrystalline aluminum oxide is manufactured and

through a liquid phase by calcinating and sintering. The advantage

of such ceramic grits is an increase in thoughness whilst

maintaining a high degree of hardness. Because of its characteristic

light blue colour, the new grit developed in Hermes Research has

the name "Sapphire Blue".

Fused corundums differ with respect to their degree of purity.

Depending on the degree of contamination, there is black corundum

(70 to 75 % AL2O3), medium quality corundum (94 to 97% AL2O3),

semi-noble corundum (97 to 98% AL2O3) and noble corundum

(over 99 % AL2O3), whose colours are black, brown, pink and

white. The pink colour of noble corundum is due to a 0,5 to 2,0 %

component of chromium dioxide.

The chemical composition influences the hardness and toughness

of the grain material, thus noble corundum is harder than medium

quality corundum but is easier to fracture (not as tough). Of the

conventional abrasives, silicon carbide achieves the greatest

degree of hardness. Its toughness also depends on its purity.

Consequently, the black SiC (98,3%) normally used for abrasives

on backing is tougher than the green version (99,7%). However,

both are considerably more brittle than noble corundum.

Zirconia alumina has markedly better toughness properties,

when compared with fused corundum. This compound, consisting

of AL2O3 and approx. 40% Zr0, achieves hardness values

comparable with corundum.

Cubic boron nitride (CBN) and diamond are the hardest abrasive

grain types in this list. However, they have not yet achieved any

significance for abrasives on backings. Garnet, on the other hand

has a limited application field in woodworking due to its sharp

grain edges. Its hardness is below that of fused corundums.

Bonding Materials

Abrasive bonding materials have the task of securing the grain to

the backing until the end of the abrasives service life. Natural

glues, synthetic resins and lacquers are used predominantly.

Natural glues (hide glues) include top quality animal glues, for

example, which are extracted from water-enlarged collagen in

animal hides. Synthethic resins, phenolic resins, alkyd resins,

aminoaldehyde resins and aminoaldehyde furan resins, epoxy

resins and polyurethane resins are all utilized, depending on

application.

In accordance with the type of bonding, we distinguish between

pure glue or synthetic resin bonding as base or top bonding and

partial synthetic resin bonding with glue priming and synthetic

resin top bonding.

The advantages of the hide glues are good workability, good

adhesion to the backing material, simple drying and good elasticity.

Synthetic resin bonding materials offer advantages for higher

grinding demands, such as good grip, high impact resistance,

excellent adhesion properties, heat resistance and insensitivity to

water.

Backings

A characteristic feature of coated abrasives in the elasticity and

flexibility of the backing material, whereby materials with a wide

variety of properties can be used. Sodium and sulphate wood pulp

craft papers with different base weights are generally used for

paper backings. Vulcanized fibre backings consist of several

layers of parchment papers which are combined to the required

thickness in rolling presses under high pressure.

If increased demands are placed on the strength and flexibility of

the tools, backing made of cotton, synthetic fibre (polyester) or

mixed fabrics in linen, twill ore sateen weave are used. In addition,

knit fabrics, synthetic non-woven web and polyester film are

available as other backing systems.

Normally the warp yarns are interlaced for the woven fabrics. In

the case of knit fabrics the longitudinal and cross threads are

sewn with a third thread at the knotted points. This results in high

strength and less elongation in the longitudinal direction.

Non-woven webs are crosslinked textile fabrics consisting of

synthetic fibres where the abrasive grain is anchored on the fibre

surfaces by bonding agent droplets.

Because of their extreme uniformity, polyester film substrates are

used primarily with applications in final sanding with fine grits.

In addition to the abrasive and bonding materials, grinding tools

may also contain different types of additives in varying quantities

and compositions which act as active grinding components to

improve the grinding characteristics.

Preparation for Manufacture

Production of coated abrasives is generally a continuous process.

Raw materials are selected and prepared as required for each

production run. Woven and knitted backings are provided with a

filler so the required physical characteristics can be realized.

Flexibility and adherability of the bonding material for the abrasive

grain, sliding and adhesive properties of the backing for use with

fixed support elements, energy transfer to the drive rollers and

resistance to coolants are all examples of these characterics. The

processed raw backing products are then available as roll goods

in production width for the basic production process.

The abrasives are prepared on a job-specific basis with respect to

type, grain size, composition and quantity. An important

characteristic is the grain size classification. A distinction is made

here between macro and micro grains, whereby the macro range

lies between P 12 and P 220 and the micro range between P 240

and P 2500 for abrasives on backings. Grain size classification

takes place in sieve machines (macro grains) or sedimentation

(micro grains). Sieving or sedimentation is performed in

accordance with DIN 69 176 or FEPA 43-D (F, GB)-1984 in the

Federal Republic of Germany and many European countries, –

while ANSI/ASC B 74.18-1984 is used in the USA. Careful

monitoring ensures uniform grain quality.

The bonding agents are produced in a bonding compound

preparation. Here, we must distinguish between the base bond

(make coat) and top bond (size coat). In principle, pure hide glue,

partial synthetic resin or full synthetic resin bonding agents are

possible. Pure hide g lue bonding agents are frequently used

when lower loads are involved in the grinding process. The

preferred backings used in this process are paper and woven

fabrics.

Partially synthetic resin-bond coated abrasives are provided with

hide glue as a base bond and synthetic resin as a top bond. These

grinding tools are generally suitable for medium-duty grinding

tasks. If the demands placed by the grinding process are high

(high metal removal volume, high grinding forces), then the

abrasives are exclusively bonded in synthetic resin. The relatively

hard bonding creates strong cohesion between the backing and

abrasive grain.

Manufacturing Procedure

The backing, or processed roll, is now placed on the unwinding

station of the production installation and is fed to the stamping

machine by means of deflection rollers. The backing then passes

through the printing unit where it is stamped over the entire width

of the rear side with information concerning the manufacturer,

type, grain and the production number or additionally required

safety information (DSA No., maximum working speed).

The first bonding agent is applied in the base coating machine.

The base coat bonding agent is stored in a supply tank into which

a rubber-coated roller is immersed. This roller then applies the

bonding agent uniformly to the grain-side of the backing when it

rotates. A spring-loaded metering roller (counter- roller, squeezing

roller) controls the coated quantity by means of an adjustable gap

distance.

Tank filling and agent removal are monitored by automated

weighing devices. An oscillating spreading brush positioned

transversely with respect to the backing additionally improves the

homogeneity of the bonding agent coating.

The roll goods then pass through the spreading machine for

abrasive grain coating. Two coating methods are possible. In the

case of mechanical coating, the abrasive grain is applied to the

backing by free fall. If this method is used, the grains generally

have a flat slope with respect to the backing surface. The required

grain coating quantity is adjusted by varying the gap distance in

the spreading hopper and by changing the backing speed. Loose,

non-adhering abrasive grains fall into a collection hopper.

In the case of improved electrostatic coating, the abrasive grains

are accelerated by conveyor belt from the bottom to top backing

in an electric force field. Here, they align themselves in accordance

with their geometric longitudinal axis and stand on the base

coating with their sharp cutting edges perpendicular to the

backing. The advantages of this spreading method are a uniformly

and easily reproducible distribution of the abrasive grains,

improved grain engagement properties during grinding and longer

tool life.

If this method is used, excessive abrasive grains are also brought

into the electrostatic field. The strength of the force field and the

speed of the backing again determines the quantity of grain

applied. A downstream knocking roller causes the loose abrasive

grains to fall off in both coating methods. After coating the

backing passes through the drying section. The abrasive backings

are moved forward slowly in this predrying section while hanging

in loops. The length and spacing of the loops depends on the

backing quantity and the transport speed. The temperatures in the

drying section are between 30 and 45°C for hide glue products

and between 90 and 130°C for synthetic resin products.

After pre-drying, the abrasive backings are transported into the

coating machine for the top bonding agent coating. The bonding

material is coated in the same way as for the base coat. Rubber

rollers are used so grain edges are not damaged during coating.

The bonding agent and grain coating operations are constantly

monitored. The coating values at the measurement using the

irradiation method is used.

Both the process backings and the coating layers are measured;

the values are then compared in the computer with the

standards and corrected in the event of deviations. This ensures

a homogenous and reproducible production process over

the entire abrasive surface and guarantees uniform product

quality.

Final hardening of the product emerging from the post-glueing

machine takes place in a similar way to pre-drying in a drying

section which is 4 to 5 times as long as the predrying and

intermediate sections. Like the intermediate drying section, the

material enters the section in loops. The spacing and length of the

loops, dwell time in the drying section (which is divided into

individual and conditioning zones), drying temperature and

transport speed are adjusted in accordance with product

requirements. Towards the end of the hardening process, the

moisture of the abrasive backing which has evaporated as a result

of the heating process must be re-supplied by means of a

conditioning process in order to ensure a supple, flexible finished

basic product.

The wind-up-machine is located at the drying section exit. The

basic coated abrasive material is rolled up over its entire width,

wereby seams and defects are identified by tickets. When a roll

diameter of 800 to 1000 mm has heen reached, the roll ("Jumbo")

is taken from the roll-up machine and is supplied to the flexing

department or held for further processing depending on the

planned application. If necessary, additional drying is also possible

in chamber ovens.

In order to achieve the required and uniform flexibility of the basic

product, the next operation is the jumbo roll flexing. Here, the

hardened and rigid bond is partically broken in a controlled

manner in small area sections. In these sections, the abrasive

grain remains firmly anchored, so that premature breaking out of

the abrasive grains is avoided as a result of changing loads in the

grinding process. During this process (flexing), the abrasive

backing is guided diagonally and/or transversly over cylindrical

flexing rods of small diameter or over flexing knives and can be

deflected a number of times. The degree of flexibility of the

abrasives depends on the diameter, width and shape of the flexing

tool, tension and flexing angle.

Further processing of the abrasive products to produce endless

wide and narrow belts, rolls, drum covers, discs, sheets, flap

wheels and other design forms is performed on special machines

in the conversion departments. In addition to manual devices,

state of the art semi- and fully-automatic machines and installations

are utilized in the conversion department.