Coating and Decorating

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    Sometimes a designer wants to add decorative effects to a plastic product that cannot be achieved through regular molding methods. For this reason, there are many secondary operations just for decorating and coating. These methods can be used principally for adding aesthetic value to the product but can also enhance performance, usually by protecting the material from unwanted environmental effects.

    Coatings are thin layers of materials that are applied to the surfaces of other materials (called the substrates) so that the thin-layer material adheres to the substrate. If no adhesion occurs, as when the surface of the substrate material is merely etched or embossed, the process is called decorating. This section considers the coating and decorating methods used when the substrate material is a plastic.

21.10.1. Painting

    Because of the ease of coloring plastics by simply adding pigment or dyes to the plastic material itself, a logical question to ask is, why would someone want to paint the surface of a plastic part? Although the answers may vary widely, the principal reason given is that painting imparts a more aesthetically pleasing and richer color than can be obtained with internal colorants. Another reason is that the plastic might be part of an assembly, perhaps with metal, and the manufacturer wants all the parts to be the same color (such as with automobile exterior panels). Painting is the method most commonly used to coat plastics. This section treats the case where the paint is a liquid that is applied to the surface of the plastic. A later section discusses the case where the paint is a powder.

    Most paints consist of a polymer dissolved in a solvent and solidify by evaporation of the solvent. The solvent can be either organic or water, and both types are in wide use. Other paints are liquid polymers that crosslink when mixed together and may or may not include a solvent.

    The plastic is painted along with the metal so that all the exterior parts have the same color intensity and shade. This requirement means that plastics used in the exterior panels of automobiles must be as smooth as the metal parts surrounding them and that their surfaces must accept the paint in much the same way that metals do. Some plastic materials do not meet these requirements and therefore are not used for automotive exterior panels or other applications that require this high level of paint compatibility. For instance, paint doesn't adhere readily to polyethylene, and so it is not used for these high-performance automotive applications. In general, plastics with a high surface energy do adhere to paints and can be used.

    The painting of polyethylene and other plastics resistant to coatings is often a problem. The paintability of these low-energy surfaces can be improved by activating the surface with chemicals, plasma, corona, or flame etching. Adding a primer coat helps protect the activated surface and promotes additional adhesion with the coating.

21.10.2. Liquid Coating

    Spraying is the most versatile and most common method of painting. Spraying can easily be automated and generally gives an excellent surface quality–full coverage, uniform thickness, few runs, and few imperfections. Of course, the technology needed to achieve these results has been developed over many years.

    Spray guns usually use air pressure mixed with the liquid stream or hydraulic pressure on the paint itself to create the spray. The distance from the part and the speed of application are important factors to be worked out for each situation. Masking of areas that are not to be painted or are to be painted at a different time (perhaps with a different color) is easily done, although usually with considerable manual labor. Overspray Is inherent in the spraying method, and so the process is usually conducted in booths so that the overspray can be confined and the solvent, if any, can be captured and not released to the atmosphere. 

    Other traditional painting methods include rolling, brushing, or wiping the paint onto the surface of the plastic. Although used less frequently than spraying, these techniques have some advantages, especially for small lots and for special effects that can be created. For instance, a woodgrain effect can be created by using a roller with an engraved surface or by wiping or brushing the surface after a suitable paint or ink has been applied to an already painted surface.

    Parts that are very irregular can be painted by dip-coating. This is done by immersing the parts into the paint. Spinning or rotating of the parts after dip-coating is often done to assist in eliminating bubbles and to promote uniform drainage of excess paint from the part. 

    Another common method of painting is silk-screening. In this method, a series of screens is prepared (assuming that several colors are to be printed on the part), with one screen for each color. Each screen masks a different portion of the overall picture to be painted. The part is passed under each screen in turn as paint is forced (usually with a roller) through the unmasked portion of the screen and onto the surface of the plastic part. Some drying time is needed between each paint application (each screen), but that time can be shortened by using photo curable inks and passing the part under a UV lamp after each screen.

    Manufacturers may have problems getting paint to adhere to the plastic, usually because the plastic part was not properly prepared prior to coating. The techniques already discussed for ensuring good adhesion also apply to painting. The presence of mold release is an especially troublesome problem. The mold release should be carefully removed before the part is painted.

21.10.3. Powder Coating and Electrostatic Coating

    Whereas painting applies materials to the surface of a substrate as liquids, powder coating applies a powdered solid coating (usually a plastic) to a substrate. The part to be coated is immersed in a fluidized bed of plastic particles, which adhere to the substrate, or it is sprayed with the powder much as in liquid painting. After the part is thoroughly coated, it is baked at a high-enough temperature to fuse the coating particles but not so hot that the part is damaged. Metals, ceramics, and plastic parts all can be coated using powder coating. With all types of materials, care must be taken to ensure that the surface of the substrate bonds well to the coating. When the part to be coated is a plastic, special care should be exercised that the fusion step does not cause degradation or softening that will deform the plastic part.

    Another method of applying a plastic coating to a substrate using powders is called electrostatic coating. In this method, the part to be coated is placed inside a chamber and connected to an electrode so that surface charge is developed on the part. The coating material is then sprayed into an evacuated chamber into which an electric field has been introduced by insertion of an electrode having the opposite charge to the part. The sprayed plastic particles pick up an electrostatic charge from the electric field inside the chamber and are directed toward the part to be coated. If the plastic part does not readily accept a charge, the surface of the plastic can be dipped in a metal solution to give a very light surface coating that attracts the charged particles. Electrostatic coatings are especially useful for applying a very thin coating to a plastic. Overspray is minimal (less than 5%) in most well-run electrostatic coating operations.

21.10.4. Metallizing

    In vacuum metallizing, a pure metal or metal alloy is evaporated at high temperature in a closed, vacuumized chamber and then allowed to condense on a plastic part in the chamber, which is usually at room temperature. The plastic parts can be coated in batches or in a continuous process that is especially appealing for coating plastic films. Some common parts that are metallized by the batch process include compact discs (CDs), toys (such as plastic cap pistols and race cars), car dash and grill parts, water-fountain buttons, reflective mirrors (such as for headlights), cosmetic cases, furniture, electrical capacitors, holiday decorations, sunglasses, and traffic signs.

    Metallized plastic films are a major plastic product that has found many applications, both because of the low permeability of the metal (such as balloons and potato-chip bags) and because of the reflective, electrical or thermal properties of the metal. Some other specific applications include greenhouse shades, magnetic tape, copying film, photograph film, optical filters, multilayer flexible circuit boards, agricultural films, and thermal protective gear (such as for firefighters and astronauts). Several types of plastic film are commonly metallized, including PET, PE, PP, PVC, PS, nylon, polyimide, and PC, but the most common, by far, is PET. The most common metal used for coating the films is aluminum, although selenium, cadmium, silver, copper gold, chromium, nickel chromium, palladium, and titanium are also deposited. Even some nonmetals, such as silicon monoxide and magnesium fluoride, are used for coating. Metallizing could also be done by laminating a thin metal foil and the plastic film.

    The deposited coating is typically about 1 to 2 micro inches (0.005 to 0.01 µm) thick; because the metal layer is so thin, the metal mimics the surface of the plastic. The coated film typically has 1,000 times more plastic than metal. The thickness and uniformity of the metal coating are determined by the rate of deposition of the metal, which is affected by the temperature of the evaporation electrode (how much metal is vaporized), the rate of movement of the film through the chamber, the distance between the metal electrode and the film, the angle between the film and the electrode, and the degree of vacuum in the chamber. The pressure also affects adherence of the film, with lower pressure being better. A common measure of coating thickness is optical density, which measures the amount of light that passes through the coated film. Therefore, optical scanning can detect differences in coating thickness (uniformity) as well as total amount deposited.

    The metallizing process can also be done in solution where the plastic is charged by one electrode and the other electrode carries an opposite charge. The metal ions can be in the solution. With the imposition of current through the electrochemical cell, the metal ions can be made to plate out on the plastic part, a process called electroplating.

    

21.10.5. Printing

    Printing–putting ink on a surface–is similar to painting except that the printing does not cover the entire surface. As with painting, printing requires that the surfaces of some plastics be activated for improved adhesion. 

    Polyethylene and polypropylene, for example, both need surface treatment. Printing can be done either with a roller or pad to mark portions of a plastic surface or with a spray (ink jet), as is often used for manufacturing codes or other short identifications. In these applications, the solvent content is kept quite low so that drying occurs rapidly. Applicators for the ink can be timed to the flow of product down a manufacturing line so that the product indexes with the printer.

    Two common methods when a roller or pad is used are simply applying the ink to a raised portion of the surface of the plastic part or transferring the ink from another surface onto the plastic. Generally, this transfer method uses a master roll on which the desired pattern has been engraved. Ink is then applied to the master roll so that the ink sits on only the raised portions of the roll. Then, a pad is brought against the master roll and the ink is transferred to the pad. The pad is then pressed against the surface of the plastic and the ink again transfers. The nature of the pad (usually a silicone) and the types of ink are important in achieving clean transfers.

21.10.6. Thermostatic Printing

    Two common methods when a roller or pad is used are simply applying the ink to a raised portion of the surface of the plastic part or transferring the ink from another surface onto the plastic. Generally, this transfer method uses a master roll on which the desired pattern has been engraved. Ink is then applied to the master roll so that the ink sits on only the raised portions of the roll. Then, a pad is brought against the master roll and the ink is transferred to the pad. The pad is then pressed against the surface of the plastic and the ink again transfers. The nature of the pad (usually a silicone) and the types of ink are important in achieving clean transfers.

    Thermostatic printing has found widespread application in applications requiring high durability–such as the numbers and letters on computer keyboards, appliance dials, and indicatorsand for sports equipment–such as skis and snowboards. One drawback to the process is that not all plastics can be printed. 

21.10.7. Hot Stamping (Appliques)

    Hot stamping is a system that imprints a permanent, high-quality image on the surface of a molded part using a heated die and pressure. The source of the image is a foil or roll leaf, which is typically a film onto which a metal or color layer has been coated. The hot stamping system is shown in Figure 21.15. The part to be marked is placed against a support fixture that lies beneath a heater and die. The coated film is fed into the gap between the part and the die and is stopped so that a fresh area of metal or color is above the part. The heated die is then pressed against the foil, which, in turn, is pressed against the part, causing the plastic backing of the coated foil to melt against the surface of the part and transfer the color or metal onto the surface of the part.

    One of the advantages of the hot stamping method is the ease of changing the heated die. A typical example is the stamping of trash containers with both the name of the city and an identification number that indicates the date of manufacture and identifies the container uniquely. The number is made by a moveable die that is simply changed (indexed) after each stamping. The primary advantage of hot stamping is the look of the finished decoration. A foil stamp image can have a clean, metallic look similar to gold leaf. It is the only method by which permanent gold and silver metallic graphics can be conveniently produced. Another advantage is that it is a dry process. There are no solvents or wet inks and no smearing, and parts can be handled immediately after decorating. Also, there is minimal training needed for an operator to do the actual transfer. The setup costs are not as high as for many of the other printing or coating processes and the cost of the film/foil is fairly moderate. Both thermoplastics and thermosets can be hot stamped, and the foil can be applied to contoured surfaces. Due to the thermal bonding involved in the process, the foil has good abrasion resistance as well.

    The primary disadvantage is the limited print quality, especially with multicolor jobs. Decal heat transfers look far better and have more application options, including multiple colors, scratch and fade resistance, and the ability to transfer to many substrate surfaces and textures. Hot stamping equipment is moderately expensive to purchase and there is some downtime while changing foils. Aside from contoured surfaces, complex three-dimensional surfaces are not a hot-stamping specialty.

Figure 21.15 Hot stamping

21.10.8. In-Mold Applications

    A process similar to hot stamping uses the pressure and the heat of the resin or of the mold to cause a printed film to adhere to the part. In this process, the film is laid into the mold and is located and held precisely. When the mold fills with resin, the film does not move. Then, with the heat of the resin and the pressure from the mold, the printed film bonds to the surface of the part. An alternative method of accomplishing in-mold applications is by printing directly onto the surface of the mold. This printing then transfers to the part because the adhesion for the part is higher than the adhesion to the mold.

21.10.9. Laminating and Related Processes

    Films of various types can be coated onto plastic parts by several processes, the most important of which is laminating. The coating film, usually a plastic, is brought into a roller assembly into which the material to be coated is also brought. Then with pressure and, usually, heat, the two materials are pressed together such that a bond forms between them and the coating is mated to the part. The heating can be done either at the roller assembly or before.

    Laminating is also useful for bonding several layers of plastic material together at one time. When several layers are laminated, some of the layers are almost always heated. The several layers may be useful for making barrier films and packaging materials. These may also be made by co extruding, but that process is not always possible. Laminating allows customized multilayer materials to be made without the high cost of coextrusion. Laminating has an advantage in that it is done without the use of adhesives. The materials can therefore move rapidly through the laminating process, usually much more quickly than if an adhesive is applied.

    Laminating can be used to transfer a film coating from one surface to another surface, a process called transfer coating. A film coating is placed onto a substrate to which a release material has been previously applied. The coating adherence to this substrate is slight, so care must be taken not to disturb the material by touching it or by mechanically working it. This coated substrate is then carefully rolled and pressed against the substrate to be coated using a lamination technique. When the two substrates are pressed together with the coating between them, the coating leaves the release material and bonds to the new substrate. Some pressure from the laminating rolls usually accomplishes this task, al­ though heating of the new substrate may also be required.

    Lamination can also be used to coat a liquid plastic onto a film or sheet substrate. The liquid plastic can be a polymer solution or a thermoset that is not fully cured. In this method, a doctor blade is used to spread the plastic liquid material over the surface of the substrate. Then after the coating is spread, the substrate and the coating are run through laminating rollers to ensure that the liquid coating is uniformly spread across the substrate. If the lamination process includes heating, the layer can be cured at the same time. If the liquid material is heated and applied to a substrate is a thermoplastic, the process is called extrusion coating. An extruder is placed so that its nozzle applies the resin to the substrate (which is usually a film or sheet). The substrate is then run through lamination rollers to spread the coating. In this case, the laminating rolls may be chilled so that the coating solidifies rapidly. In the coating of liquids onto substrates, the lamination process is somewhat slower than the application of a film onto the surface. The slower speed is required to ensure uniform distribution of the liquid on the substrate.

    Many products use lamination, including vinyl flooring and Formica countertops, where the plastic coats a paper substrate.

21.10.10. Decorating

    Decorating differs from coating in that only a portion of the substrate surface is covered. Masking or differentiating the surface of the substrate often is required to limit the region to which the decorating is applied. Perhaps the simplest decoration method is the application of decals. These printed plastic sheets can be applied to many plastic surfaces, often with only minor heating to bond them. The heat should not, of course, cause distortion of either the decal or the plastic substrate. The printing allows great flexibility in design and use of color. Moreover, with some care, many contoured and even complex surfaces can be decorated with decals. However, some plastics do not bond well to the decals, so surface treatment or heating must be done with great care to achieve a permanent bond.

    Texturizing the surface of a plastic part involves forming a pattern on the surface of the part without molding. The pattern can be either a repeat pattern or a singular pattern. When the pattern is a repeat, it is usually applied with a heated, embossed roller. As the roller moves across the surface, it melts the plastic slightly and creates the pattern. Singular patterns are usually stamped onto the plastic surface, although almost any method that melts the plastic surface can be used to create a pattern. After being textured by either method, the plastic surface can be printed (touching only the high parts of the pattern) or otherwise decorated to give emphasis to the texturing.