SURFACE TREATMENT OF PLASTICS PRIOR TO DECORATION
Introduction:
Some plastic, notably poly olefins and acetals, are high resistant to bonding other media to themselves. To overcome this deficiency treatments like flame treatment and electronic treatment such as corona discharge, plasma discharge and chemical treatments are commonly used.
Flame treatment consists of passing the object through an oxidizing gas flame. The flame causes an oxidation on the plastic surface and enables the adhesion of inks, enamels etc.
In corona discharge process high voltage current is used for the oxidation of the surface. In plasma process air at low pressure through an electric discharge is used to change the physical, chemical properties of plastics. In chemical treatment products are dipped in acids resulting in an etched surface which makes it receptive to paint.
2 Types of surface treatment
Different types of surface treatment of plastics are screen process printing, hot stamping, heat transfer decoration, in-mould decoration, two-colour moulding, electroplating and vacuum metalizing
2.1 Screen process printing:
The process of printing through a screen consists of forcing ink or paint through the interstices of stenciled screen of the required image with a squeeze. The screen material is often nylon mesh or other synthetic material. Metallic screens such as stainless steels are also in common use.
2.2 Hot stamping:
Hot stamping by the roll leaf process is a method of decorating plastic materials. The leaf or foil consists of a thermoplastic colour coat applied to an acetate or cellophane carrier film. When a heated die is pressed against the foil carrier film, the colour coat is released and adheres to the product placed beneath it.
2 3 Heat transfer decoration:
It consists of a paper carrier, a fusible release coat and a thermo plastic ink image. The ink becomes sticky when heated by a hot platen. When it is pressed against the product the ink bonds to the surface. Some of the fusible release coat carries over with the ink, providing the image with a glossy protective coating.
2.4 In - mould decoration:
In this method the decoration becomes an integral part of the product making it most durable and permanent. In compression moulding the mould is loaded with the resin in the usual manner and closed. After a partial cure it is opened, and the cycle is proceeded after keeping the foil consisting of a printed cellulose sheet and closing it again. The foil is fused to the product, during the moulding cycle and becomes a part of the moulding. For thermo plastic mouldings, the foil or overlay is placed in the cavity prior to the injections. The overlay is printed in a film of 0.05 - 0.10 mm thick of the same polymer of that of the product. The ink used to imprint the film must be heat resistant. The gate location in this case is very important to avoid shifting of the overlay during injection.
Similar methods can be adopted in decorating blow-moulded products also.
2 5 Two-colour moulding:
Two-colour moulding is an injection process where two colours are successively moulded to create their portion of the finished parts. The shell is moulded first and then it is transferred to another cavity where the second colour is injected to complete the product.
2 6 Electro plating of plastics.
Plating is done on many plastics to provide a hard wear and corrosion - resistant surface. Physical properties such as tensile strength, heat deflection temperature etc. are also enhanced in plated plastics. The surface to be plated is to be properly conditioned and sensitized. The proper choice of resin for products to be plated is also very important. Many parts like knobs, light reflectors, name plates etc. are made out of plastics and plated to give metallic appearance with light weight and corrosion resistance. Plating is done on many plastics including Phenolic, urea, acetyl. ABS, polycarbonate etc.
2.7 Vacuum metalizing of plastics:
It is process whereby a bright thin film of metal is deposited on the surface of a plastic moulded product or film while being subjected to a high vacuum. The deposited metal may be gold, silver or most generally aluminium. Deposition is the result of vaporization of small clips of the metal to be deposited attached to an electrically heated filament. When electrical energy is applied to the filament, the clips melt and coat the filament. Increased energy caused vaporization of this coating, and plating of the product takes place. The deposition is compiled at high vacuum, in order of 0.5 micron. The thickness of the resulting coating is about 1 micron.
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