We perform painting, serigraphy, mechanical cleaning, grinding, sanding, brushing and polishing
Serigraphy or screen printing is an artistic technique for printing images and graphics on any materials or surfaces through the use of fabric (print fabric), by transfer of ink to a media through the free areas of the fabric. The term “serigraphy” comes from the Latin word “seri” (silk) and the Greek “grafein” (writing or drawing), given that the first fabrics that served as stencils were made of silk. After drawing or transferring the stencil to the frame (via various techniques), the frame is placed on the selected media, the ink is then placed on top of the frame and spread over the entire frame with a rubber spatula, including, of course, the areas open to the substrate. The ink passes through the open spaces and is deposited on the media (paper, canvas, or cloth, for example). The frame is then removed and the media is let to dry. The frame can be reused after cleaning.
A sanded, cleaned, and polished metallic surface is very important for decorative applications. Since an electroplated deposit is always the faithful image of its media, it goes without saying that, in order to be plated uniformly, the media must be as homogeneous as possible. It would be wrong to think that any scratches on a metal surface can be removed by electroplating, as it would happen by fire coating the said surface. Even for high electroplating thicknesses, the status of the base metal is always reproduced in all its details.
Hence, metal surfaces should be cleaned in order to obtain homogeneous, non-porous, glossy or semi-glossy surfaces. Only with this kind of surfaces it is possible to obtain excellent electrolytic coatings, especially as regards corrosion protection and appearance.
The cleaning, which is done by placing the piece against an abrasive means in motion, naturally depends on the initial state of the metal surfaces to be coated. In principle, to go from a raw material (rough), not classifiable among small parts, to a perfectly clean material (mirror polished), the necessary mechanical operations can be divided as follows:
- Brushing and polishing
It is carried out with hard discs consisting of natural or artificial agglomerated abrasives. Actual «grinders» can be used, as well as wooden discs covered with leather, or glued up hard felt disks for coarse grinding (level SO, 70, 60, 50 grinding according to the American scale). With this operation, the more irregular and macroscopic bumps due to earlier melting or mechanical treatments (burrs, lumps, growths, etc..) are removed.
It is performed gradually by means of increasingly thin abrasives; when using the coarser abrasives, it is called roughing. Various types of sanders are used. The most common type is the disc sander, edged with abrasive material, on which the piece to be sanded is pressed from the bottom. The size and hardness of the disc (felt) are chosen depending on the base metal and the shape of the object to be cleaned. The various steps are performed with the abrasive (emery) glued to the felt with carpenter glue.
Brushing and polishing
It is carried out with different types of brushes applied to cleaning machines, or used by hand. They may be made with vegetable fibre or metal wires (brass, stainless steel, nickel, etc.).
After the sanding, in order to prepare the ferrous material for glossy nickel plating, or to facilitate the reviving of matt-plated metals, it is very useful to proceed to brushing with vegetable fibre brush discs soaked with fine emery and mineral oil, or olein. The most commonly used vegetable fibre brushes are those made with tampico (fig. 179), a type of grass grown in Mexico. The aim of the treatment with tampico is to even the streaks left by previous sanders, directing them towards one way. The resulting uniformity facilitates subsequent polishing treatments. There are ring-shaped tampico brushes (fig. 179) as well as continuous spiral brushes (fig. 179 b), also known as strip brushes; the latter have a compact and uniform brushing surface.
Different satin and matt effects can be obtained by brushing the surfaces in certain directions with variously lubricated metal wire discs, with or without abrasives. A given surface appearance depends on the thickness of the wires.
Use of abrasive belts
Abrasive belts, or belts, of different kinds are increasingly used, mainly because their r2 and cold cutting power increases productivity. The belt runs in the cleaning machine between a wheel tightener and a contact wheel, against which the object to be sanded or polished is pressed; this means that the contact wheel is very important; it can be hard, very hard, soft, very soft, or ultra soft, depending on the operation to be performed on the surface of the object. Therefore, there are contact wheels made of aluminium or cast iron, cotton strips, rubber with helical grooves, compressed cotton discs, foam rubber, etc.. Between these wheels runs the abrasive belt, in turn with a media (paper, cloth, fibre, etc..), as well as some adhesives (based on synthetic resin or animal glues) chosen according to the aims to be pursued. Besides, particular oils based on chlorates, sulphur, and fatty substances dissolved in mineral oil, are applied; these form a lubricating film that withstands the highest pressure on the working point and the high temperature friction effects. When a high degree of finishing is required, certain fats can be used to reduce the penetration of the abrasive grit; these liquefy at the working point and increase the life of the belt.
This process, as well as vibratory finish, is particularly suitable for large quantities of small parts. It employs tumblers, also called drums or barrels, made of iron or steel, usually filled by three-quarters of the volume with the items to be cleaned, as well as with an abrasive (sand, quartz, silica, pumice, red dust, emery, etc..) and water, or lubricating materials; often, carbonate or sodium hydroxide water is also added. It is also carried out “dry” with sawdust, or leather and emery scraps. The cleaning carried out with such rotating devices can require many hours or a few days, depending on the state of the material.
The most used is wet roto-finishing (water), which allows to obtain various types of finishes (glossy, semi-glossy, satin, etc.). It employs devices basically consisting of an electric motor and a system for transmitting the rotary motion to the container, which usually is octagonal in shape. The width of the octagonal prism can vary from 30 to 150 cm.