decoration of plastics
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DESCRIPTIONSEMINOR ON DECORATION OF PLASTICS .
<ul><li><p>SUBMITTED BY:RAVI NARAIAN PANDEY.</p></li><li><p>Decorating whether for aesthetic purposes. (e.g. to improve resistance to wear, scratching, marring, light or heat etc.) is frequently required in the manufacture of articles from any plastic, whether thermoplastic or thermo set. Each presents its own characteristics and hence its own problems; thus we caution the reader to investigate the various techniques and their applicability to the particular plastic that is involved in any given job. </p></li><li><p>1. Determine the decorating technique that best suits the desired end effect.2.Make sure that the plastic being used is suitable for the technique selected. The choice can have a significant effect on the entire finishing line. Some plastics, like the polyolefin's and acetals, for example, may require pretreatment. These pretreatment systems ideally should be hooked into the production line-either just prior to decorating or just after molding. </p></li><li><p>Be certain that the design of the product does not place any limitations on the successful decoration of the part. Good product design is a key to case of decorating and for keeping costs in line. Reinforcing ribs, marks, caused by welding knitting of the material as it enters as it enters the mold, and even knock-out pin marks may show through, and cause poor adhesion of the coating or other decorative surface to the part, or otherwise interfere with the method of decoration. If this is the case, the part may have to be redesigned, or post-finishing operations such as sanding may be required. </p></li><li><p>Painting on plastics. Printing on plastics.Vacuum Metallizing Surface treatment Integral Coloring </p></li><li><p>Buy the best tools and equipment you can afford, especially paint brushes. Buy good quality paint and materials. Buying cheap paint is often a false economy as many more coats are usually required. Follow the manufacturers instructions on tins and other materials. don't always interpret too literally as there is usually a degree of flexibility, especially in diluting paints. Try to ensure that you have bought enough materials to complete the job. This is essential when buying wall-paper. </p></li><li><p>ROUGH GUIDE TO USING MATERIALS </p></li><li><p>Fillers come in two categories; ready to use and those you mix yourself. The only one that you would normally mix yourself is filler bought in powder form that you mix with water to form a paste. It is cheaper to mix this kind of filler yourself than to buy it ready-mixed. This kind of filler, e.g. Polyfill, is used to repair cracks in plaster, holes and other general repairs. Linseed oil Putty is only used to repair and fill holes in wood, glazing around wooden and metal frames and windows. Brummer or wood filler is used to repair holes in wood and dries rock hard so apply it as smoothly as possible.</p></li><li><p>Primers are used to apply first coat to bare wood and metal. For galvanized metal a special zinc primer must be used otherwise your subsequent coats will eventually flake off.</p></li><li><p>Sealers are used for preserving, waterproofing and preventing problems occurring to subsequent coats caused by chemicals and fungi seeping through from the underlying surface. Many different sealers are used to counteract different problems.</p></li><li><p>Undercoats/basecoats provide the surface for the following finish coat. This is by far the most important stage in getting a first class finish. If necessary a second or even third undercoat should be applied if needed.</p></li><li><p>Gloss, semi-gloss, eggshell and matt are the usual type of finishes used; a high-gloss finish being far the most common. It is often difficult to get an undercoat to match various stronger colours and manufacturers normally recommend two coats of finish.</p></li><li><p>These are paints, (which are water-based) for use on ceilings and walls. They are easy to use and dry quickly. They can be used on interior wood but lack the hard-wearing qualities of undercoat and gloss.</p></li><li><p>A very important part of decorating is preparing the surfaces to be painted. Sandpaper is the most commonly used material for rubbing-down. Sanding pads are useful too. Lots of materials can be used, e.g. Pumice stone.</p></li><li><p>The two most common thinners used are water for emulsions/latex and Turpentine Substitute or White Spirit( for solvent based paints). Special spirit thinners are needed for cellulose paints.</p></li><li><p>Just as with metal or wood, getting a tightly adhering coating depends on more than mere good luck. It requires carefully matching the paint system and the plastic, plus close attention to the nature of the plastic. Some of the guidelines to be followed are listed below and recommended paints for the various plastics are given in this table:</p></li><li><p>PlasticCoat ability Pretreatment Recommended coating typesABSGoodSolventVinyl, modified vinyl, polyurethane, acrylic. AcrylicGoodSolvent wipeAcrylic, vinyl, nitrocellulose Cellulose acetate and butyrate GoodSolvent wipeAcrylic, vinyl, polyurethane Phenolic and melamineDifficultSolvent wipeAlkyds, polyurethane, epoxy, acrylicPolycarbonateFairPrimerPolyurethane, acrylic, modified acrylic.Polyester GoodSolvent wipe Polyurethane, epoxyPolyolefinsDifficultFlame solvent or primerPolyurethane, nitrocellulose, modified acrylic. Polyurethane oxide-basedfairPrimerPolyurethane PolyurethaneGoodPrimerPolyurethanePolyvinyl chlorideFairSolvent wipe or primerPolyurethane, nitrocellulose </p></li><li><p>There are many techniques in painting process:-Conventional Spray Painting. Electrostatic painting WipingRoller CoatingFlow CoatingDip CoatingFlocking </p></li><li><p>It is still one of the more popular techniques for decorating plastics, and today it is being used as much for a functional finish as a decorative finish. ABS exterior trim and styrene structural foam exterior window shutters, for example, are painted not only for aesthetic purposes but to increase their resistance to UV exposure. </p></li><li><p>There are a number of different ways to apply paint to plastic. The most common is conventional spray painting, ranging from the simple hand gun system to highly automated systems with automatic screen washers and elaborate masks for multiple-color decoration. It should be noted, however, that not all parts are suited to an automated operation by virtue of size Or because the volume is not large enough to warrant the expenses of such an operations. </p></li><li><p>In a spray painting system, a key factor is the type of gun used. Various methods are available</p></li><li><p>Usually two guns mounted so that they rotate around a part to get at hard to reach sports. </p></li><li><p>It have a back-and-forth action and travel on a curved transverse to paint the sides of complex parts. </p></li><li><p> In which the parts rotate on a spindle while the guns are stationary (and can paint small part from the top and bottom at the same time).</p></li><li><p>Like rotating guns and reciprocating parts or rotating parts and reciprocating guns. </p></li><li><p> Also essential to most spray operations is the use of a mask to shield off those sections of a part which are not meant to be painted. These form-fitting masks are generally made by electroforming and while they vary considerably in complexity, they usually fall into one of these four categories:-</p></li><li><p>It is used for painting a depressed name or design. In this mask a lip of metal extends down the vertical side wall of the depressed design, all the way or only part way down the side wall, depending on the result desired. This lip must be thin, yet strong. The centers of letters and numbers such as O,A,6,8 must be securely held in place by bridges. The fit and the lip of the mask ensure a clean sharp paint line. The draft angle of the depressed design should be at least 5 degrees. </p></li><li><p>It is the reverse of the lip. It is used where the embossed name or design is to be kept clean while painting the background. The lip of metal must cover the vertical side walls all the way to the bottom. So as to protect the embossing completely. </p></li><li><p>It is used for protecting a depressed design while painting the background. Its principal use is with transparent articles such as automobile horn buttons and doors of refrigerator evaporators and where vacuum plating is required. Here again the positive fitting essential to prevent fogging on vertical side walls and bottoms of such designs is accomplished by electroforming directly into the design. The plugs are cut out, and finished with the proper radii and draft angles to facilitate painting. They are then suspended by fine wires, usually attached. In designing molds, the engineer must remember that the draft angles of the depressed design should be kept to a minimum, preferably not more that 5 to 7 degrees. </p></li><li><p>This type of masking is employed for high production dial filling of letters and numerals, when the width of the groove is less than 3/32" (.093). This method is frequently employed with sprayed "Wipe-In and "Dry-Wipe" dial filling materials. </p></li><li><p>When application of a design is required on a flat surface, without recessed or raised lettering, it is feasible by careful control of the painting operation to use an engraved type mask. Typical products are balls, scale model trains, and racing cars </p></li><li><p>Occasionally it is not feasible to cap small letters or numerals, due to size, on a large painted area. A wiping or highlighting mask is provided that permits the raised surfaces to protrude for secondary wiping or buffing.</p></li><li><p>The mold engineer should design the mold with thought to the problems of masking. Slight changes in the demarcation of colors which will not essentially change the design of the article may definitely facilitate the masking. </p></li><li><p>In operating a spray painting set-up, it is important that the masks be washed on a regular basis. Most set-ups today involve some type of automatic washing system. To achieve best production efficiency, whether the masks is on or off the machine, three or four masks are often required for each part. This gives at least one mask on the painting operation, one being washed and one being dried. </p></li><li><p>Until recently, most masks were washed with solvents. Government regulations concerning the discharge of solvents into the atmosphere, however has encouraged users to look in other directions. Typical: cleaning paint masks with water-based materials that are non-toxic nonflammable and lower cost. In one such system, a water-soluble protective film is applied to the mask surface. When the mask needs cleaning. It is immersed in hot water or a heated solution and the protective film dissolves, taking the paint alongwith it. In essence, the paint is not dissolved, but taken off mechanically. </p></li><li><p>Another key element in a spray painting operations is the spray booth itself. Selection of the right type of booth will depend on the plastic being sprayed, the methods of application, and the rate of production desired. There are three types on the market today:- </p></li><li><p>The water-wash booth incorporates a continuous waterfall in the back of the booth that literally washes paint out of the exhaust air. It is especially useful in continuous spray production lines. </p></li><li><p>In a filter booth, a filtering device traps over-spray particles. It is most suitable for long runs when using slow drying or light viscosity materials or for intermittent or short runs not matter what type of paint is used. Since filters must be replaced regularly, roll-type dispensing units have been developed. </p></li><li><p>Battle booths are used where exhaust air to the outside does not have to be free of paint particles. This type is suitable for intermittent production with quick drying materials. Baffles assure an even air flow distribution through the work area of the spay booth. </p></li><li><p>Unsatisfactory results in spray application may be due to a variety of causes, such as faulty technique on the part of the operative, or again, spraying at an incorrect pressure. In some instances they may arise from failure to keep the gun clean and in good condition; the following are those which occur </p></li><li><p>This may be due to a dirty or damaged air cap or from the tip not centering properly; to remedy, remove and clean the cap, paying special attention to the air ports. Should one of these be blocked with dried paint, the shape of the air stream and atomized paint will be irregular. If any parts are damaged, see that they are replaced. </p></li><li><p>This may result from too much atomizing pressure or from misalignment of tip and nozzle, or from an obstructed port. </p></li><li><p>This is probably due to the air valve not seating properly and, provided no actual damage has been sustained by the gun, can be remedied by cleaning. </p></li><li><p>In this case the needle may not be seating correctly in the fluid tip, due to dirt or other impurities in the tip. Alternatively, the needle may be bent or of an incorrect size for the tip. </p></li><li><p>This is probably caused by an air leakage through the needle-packing gland; the trouble can be rectified by means of a new packing washer. </p></li><li><p>This fault, in which the material is deposited on the surface too dry, as a kind of dust, is usually the result of spraying at too high an air pressure which causes the solvents in the finish to evaporate from the atomized particles before the latter reach the surface. </p></li><li><p>This may be due to the material itself not possessing sufficient flow or to the use of unsuitable thinners. Again, it may be caused by the use of too low an air pressure in relation to the viscosity of the material, with the result that atomization is imperfect. </p></li><li><p>These occur when too much paint is projected on to the surface in any one area, due to incorrect handling of the gun, as when there is too much overlapping in succeeding strokes. </p></li><li><p>Once the function of the spray plant and a general knowledge of the best uses to which it may be placed are known, the user will be in a position to carry out the various effects which are possible in spray painting. The scope of the operator will be enlarged considerably and ideas may be carried out which are completely outside the scope of the brush.The remainder of this chapter will describe how the various designs are to be obtained, and with a little practice and ingenuity many pleasing and effective patterns are obtainable. </p></li><li><p>In the early 1970s the government enacted various air pollution laws that restricted the emission of photochemical reactive solvents into the atmosphere. Included in this classification were the aromatic hydrocarbons like toluene and the branched chain ketones. As a result, a great deal of effort has been expended on creating water-based coatings that do not involve the use of solvents. These paints are based resins such as alkyds and acrylics and can be applied by the same methods used for solvent based coatings.</p></li><li><p>Water-based paints are advantageous for use on plastics because the absence of a solvent eliminates attack on the plastic substrate. However, such systems are higher in cost than solvent-based coatings and, at this writing, there are still limitations in terms of the systems failures in humidity or water soak. Water base coatings are available today for use on acrylic, ABS, polystyrene, and polycarbonate. </p></li><li><p>Another development to meet air pollution regulations is the high...</p></li></ul>
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