Polishing is the term used to describe the slight removal of material. The goal is to smooth the surface.
For this purpose, polishing grains are drawn over the surface with a certain pressure. The polishing grains are contained in a paste, the polish. The actual process corresponds to grinding , but as little material as possible should be removed during polishing. Removing material next to scratches makes them invisible. The reduction in material thickness is a necessary side effect.
Polishing can serve two purposes. On the one hand, a smooth surface has less friction, which can be important for mechanical components. On the other hand, a smooth surface reflects light incident on it evenly, which is perceived as gloss. When industrial polishing of z. B. brass, polishing is used to produce surfaces that are as uniform as possible so that the workpiece is ideal for downstream processes such as B. the electroplating is prepared.
So-called thermoplastic processes are used for polishing special plastics and polymers (e.g. PMMA). The roughness peaks of the surface structure are plastically and partially plastically deformed and thus leveled (also press polishing ; English burnishing ). It is carried out depending on the type of polishing a smallest to less material removal ( smoothing ) as well as filling of wells. The surface achieved in this way is often shiny due to its smoothness .
In woodworking , polishing is also understood as the application of a polish made from shellac , oils or resins . This is usually done in several layers with the help of a polishing pad made of textile or cotton wool. Wood polishes can also contain pumice and other fillers and abrasives that smooth the surface and close pores. However, this is often not the case.
The polishing process
Through the polishing process, the state of the polished surface is polished from a matt surface with only low light reflection to a shiny surface with strong light reflection.
The polishing agents contain polishing minerals that level the surface roughness.
The polishing process often includes several stages. The polishing time is primarily a result of the initial condition (pre-grinding), the performance of the polishing agent used, the selected process parameters such as B. Contact pressure and number of revolutions as well as the desired end surface. The longer the polishing time, the higher the risk of a surface defect.
The following factors are decisive for the quality of polished surfaces: material, pretreatment and polishing parameters. The optimal polishing process depends on the fine-tuning of the polishing tools and polishing pastes used.
The polishing process can be carried out automatically by machine, machine or manually. In automated polishing, robots and rotary tables are used to polish the material. In the case of machine polishing, polishing is used with rotary and eccentric polishing machines. The surface can also be polished manually with hand polishing pads or microfiber cloths.
Manual polishing means rubbing objects or surfaces with a soft cloth or a soft brush after cleaning , often with the help of chemical additives, so that they shine. This applies, for example, to wooden floors, furniture or leather shoes with shoe polish . These applications are mostly carried out in the home environment.
In industrial polishing, polishing is largely automated. In automated polishing, robots and rotary indexing tables are used with the help of which the workpieces are polished.
Polishes and polishes
Polishing pastes consist of a combination of polishing minerals and a carrier . The polishing minerals influence the effect of the polishing agent. On the one hand, the carrier is necessary in order to be able to use the polishing minerals correctly and, on the other hand, it ensures the necessary cooling and adhesion to the polishing disc or the polishing pad. It forms a film between the surface and the pane. As with lapping , the abrasive grain moves within this film during polishing. The polishing minerals are either industrially produced or have a natural origin. The carrier defines the consistency of the polishing agent.
A distinction is made between solid and liquid carriers. It usually consists of the following raw materials: fats , waxes , hydrocarbons , emulsifiers , additives and water. Solid carriers consist only of fats and waxes.
Polishing agents are further divided into solid pastes , liquid emulsions , ointments and polishes . Solid pastes and emulsions are used to polish metals, lacquered woods and precious metals. varnished woods can also be polished with ointments. The polish is used on clearcoat in the automotive sector.
The polishing pastes of one type are assigned to different polishing levels. There are highly abrasive pastes (Heavy Cut), medium abrasive pastes (Medium Cut), finish pastes and sealants (Protection).
With conventional polishing, the surface is removed and smoothed with extremely fine polishing agents such as polishing red cake , polishing slate , whipped chalk , tin ash or cerium oxide , which are usually bound in a paste or liquid, sometimes called polish . The means are bound in a polish carrier , such as a cloth , felt , rubber , pitch or leather disc for rotating processes or a cloth for vibrating work, or applied to the surface.
The processing carried out at metal usually machined with rotating polishing wheels of fabric, felt or leather. The actual polishing agent is applied to the disk, the so-called polishing agent carrier, either as a suspension or as a so-called solid paste. The polishing agent consists of various fats, oils and the actual polishing agent. This can e.g. B. alumina, aluminum oxide , chromium (III) oxide or the like.
The electrolytic polishing allows metal parts (eg. As stainless steel, brass, aluminum alloys u. A.) To clean and deburr to bring shine. The metal is immersed in chemical baths (electrolytes) and exposed to a direct current.
Lacquered surfaces / wood
Painted surfaces are sensitive materials. The grinding process must therefore be set up very carefully. It is particularly important that heat is dissipated and that the final sanding is not too rough. Even wood is counted to painted surfaces. Because it is not the wood that is polished, but the lacquer with which it is protected and refined.
The wood is first watered and sanded and then coated with a varnish. After painting, several intermediate sanding steps and a renewed application of paint are carried out in order to close the pores and achieve a smooth surface. At the end of this process, the actual polishing is carried out by means of a rapidly rotating buffing disk or on the long belt sander with the aid of a buffing belt that moves quickly over the surface.
If shellac is used for the polish , it is also called a shellac polish , which is applied with a ball. Horsetail or pumice powder is preferred as an abrasive , which also fills the pores. Since the handling of shellac is complex and very labor-intensive, it is only used for the restoration of old furniture and for very high-quality musical instruments.
Another traditional method is oil polishing . However, it was only used briefly in furniture production. Because of this, and also as a result of incorrect restoration concepts, almost no objects have survived. Here, linseed oil is applied as a coating in several layers and with various abrasives, thus achieving a smooth surface.
Further recommendations for polishing painted surfaces:
- Polishing speeds between 6 and 24 m / s
The polishing process for painted surfaces is used in the following areas: furniture , kitchens , boat and yacht building, violin and piano making, automotive interiors, handicrafts and picture frames.
To polish flat stone surfaces, wooden frames or wooden plates are used that are covered with leather or felt and weighted with lead weights. While the stone slab to be processed is fixed in a plaster bed, the grinding frame placed on the stone is moved back and forth, with quartz sand being flushed between the workpiece and the grinding surface until the stone has a smooth surface. Then you continue to sand with continuously finer sand or emery .
Further stages of finer grinding are carried out with the addition of slurried tin ash or triple , a clay-like sediment imported from Africa, as well as the finest rock powder and water. A matt shine is achieved by polishing with white wool, and finally a reflective shine through the finest, dry pewter ash. Small works of art and jewelry are processed on lathes and small grinding tables.
Plastics are difficult to polish. If the workpieces are heated too much, the surface will burn or melt. There is also the risk that any paste residues will be worked into the surface. Precise pre-grinding and short polishing times are prerequisites for very good results. The use of thicker pastes ensures cooling during polishing.
Plastics such as polymethyl methacrylate (Plexiglas) are usually polished using various fine emery papers and the subsequent treatment with so-called polishing paste or different polishing waxes. The easiest and fastest way to polish small areas such as B. cut edges, is stroking along with an acetylene flame . The flame should burn as richly as possible in order to avoid soot formation. The heat melts the material on the surface and cools down again on a smooth surface. The surface is polished. The disadvantage of this method is that there are strong internal tensions in the material, which after a long time can become noticeable as fine cracks. In addition, the formation of cracks is promoted by cleaning agents containing alcohol .
When polishing automotive clearcoat, the polishing minerals that are in the polishing agent level out the roughness. During the polishing process, the polishing minerals become smaller. As a result, the abrasiveness decreases. The tasks of polishing differ depending on whether the polishing process was preceded by a grinding process. With the previous sanding process, the gloss level of the matt sanding point is adapted to the surrounding paint. If there is no previous grinding process, minor environmental damage is removed.
Automotive clearcoat can be processed in one or more stages.
When polishing aluminum , overheating of the surface must be avoided. In order to protect the material, it is recommended that the pre-sanding is not too rough. Fatter pastes are suitable for harder alloys. For softer alloys, drier pastes are used. For soft surfaces, polishing grains of the same size should always be used.
Other recommendations for polishing aluminum:
- Polishing speeds of 50–60 m / s
- Avoid oversized grains in polishing products
Aluminum waste produced during polishing can cause spontaneous combustion .
Aluminum is polished for the following areas of application: automotive trim strips, wheel rims, technical components, door handles , picture frames , lamps , chair frames, sheet metal, flat sheets, household goods , door and window frames.
When polishing brass , short polishing times are particularly important. Pre-grinding plays a major role here. If it is uneven, no equalization can be achieved by polishing. Strong polishing lines should be avoided. The polishing pattern is only visible after electroplating.
Other recommendations for polishing brass:
- Polishing speeds between 30 and 50 m / s
Polishing stainless steel requires an even and fine pre-grinding. Otherwise surface defects can no longer be corrected by polishing. Here you should pay attention to short polishing times and avoid excessive heating of the sheet metal, as the polishing piece can warp.
Other recommendations for polishing stainless steel:
- Polishing speeds between 30 and 35 m / s
Stainless steel is polished in the following areas of application: kitchen sinks, pots and cutlery , automotive attachments, medical instruments, handicrafts , mold making, vehicle construction , tools , furniture and lamps , machine components, flat sheets and endless belts.
The polishing of glass is primarily necessary when using optical glasses for lenses , mirrors or glasses . As when cutting glass with a glass cutter , cracks and fissures occur during the grinding process, which ideally do not extend through the entire material thickness. The grinding of the glass with bonded abrasives, which is also known as glass milling and makes the glass surface appear matt, is followed by processing with loose abrasives, otherwise the glass surfaces to be polished are still too rough. The polishing of glass is a complex process that can be dealt with early on. a. Robert Hooke , Isaac Newton and Lord Rayleigh studied and developed theories of the reaction of materials in the process of polishing glass. While grinding removes material by breaking out and jumping out and a surface layer with the strength of several light wavelengths remains constantly broken up, polishing is firstly a gentle removal of this layer, which is furrowed by valleys and cracks, and secondly, the subsequent smoothing of the surface up to Fractions of a wavelength. The so-called Beilby layer is created . Polished glass appears mirror-like to the human eye, only fine grooves and furrows can be seen from a magnification of 4000 times. The glasses are tested with a test glass and are related to the wavelength of the light; the possible deviations are in the range of 1/10000 mm.
- Walter Burkart: Handbook for grinding and polishing . 6th edition. Eugen G. Leuze Verlag, Saulgau 1991, ISBN 3-87480-021-0 .
- Walter Burkart: Handbook for grinding and polishing p. 94.