A plastic part can be coated by applying paint on its surface, a metalizing coating, and application of a film coating on its surface.
1. Several colours or a new colour can be produced on the part.
2. Additional protection against UV light and weather can be obtained.
3. Abrasion resistance of the surface can be increased.
4. Chemical resistance can be increased.
5. Electrical shielding can be obtained.
6. Defects like mould marks, swirl patterns etc can be concealed.
All types of paints cannot be used on all plastic material but there is at least one paint that is compatible with one plastic material.
TYPES OF PAINTS:
1. Lacquers: consists of resin dissolved in a solvent. They are usually air dried by the evaporation of solvent.
2. Enamels: solution of resin in solvent. But they dry or cure by polymerization induced by heat and/or oxygen.
3. Water based paints and others are developed due to the air pollution restriction and due to the power used for baking or drying.
4. Radiation cured paints: they include polyester, acrylic, epoxy and epoxy-acrylic based material. Radiation cured material have an advantage that the paints are dried within seconds, with no heat and very little or no solvents used. The paints can be cured by UV radiation or electron beam. The main disadvantage of UV curing paints is the fact that the clear coatings can be cured only up to a thickness. While the electron beam coating can be cured up to a thickness of 2-3 mils but it has a disadvantage of being costlier and special equipment is required to cure the paint. The coated Moulding pass into concrete enclosed radiation chamber, where nitrogen atmosphere prevents oxidation and improves colour control. In curing chamber the Mouldings pass under twin banks of electron accelerators which polymerize the coating. In general radiation cured coatings are best suited for a flat or nearly flat surfaces and will not be effective with round parts, internal surfaces etc.
5. Powdered coating: here no solvent is required. These materials are used in electrostatic system where the coating can be applied to all surfaces of the part, without rotation and with almost complete elimination of overspray. There use is confined to thermosets which can withstand the temperature range of 1200-1750C because the powder coating must be fused after application.
REMOVING MOULD RELEASE:
Mould releases should be avoided if possible when the parts are to be painted or coated. However, if a mould release must be used, the one which is least harmful to the paint adhesion like zinc stearates must be used. Silicones must always be avoided.
Lack of adhesion due to excessive mould release can be easily traced if the problem is observed in the same area on a number of parts. The usual areas are corners or the sharp areas where difficulty in releasing a part is encountered. Also, sometimes the operator finds difficulty in releasing every 5th or 8th part; hence he may use excessive mould release. The adhesion then becomes spotty.
1. The parts are washed to correct the mould release problem.
2. Alcohols like isopropanol or butanol do not attack solvent sensitive plastic; hence they are suitable for washing. Methanol is avoided because it can be absorbed through the skin in to the blood stream of the worker.
3. Equal parts blend of isopropanol and Naphtha is effective especially with oil on the parts.
4. Blended solvents, used to reduce the lacquers, can be used to wash the parts if and only if they do not attack the plastic.
5. The solvents must be changed frequently to avoid contamination of washing tanks with mould release otherwise an even film of mould release would be formed over the entire part.
The paint adhesion or the coatability of plastics is affected by the molecular weight, method of resin manufacture, processing condition and the form and shape of the mould. Most plastic parts require some type of surface treatment before being subjected to finishing. Usually this involves only cleaning the part of dirt or other residue. However, for some plastics especially for those in the polyolefin family, more involved treatment is required. The polyolefins have a wax like surface that is difficult to wet. Since number of factors affect the ability of coating to adhere the plastic, it is difficult to generalize the type of surface treatment and coating system which can be used. Some of the surface treatments are:
1. Solvent treatment: (hot or cold) this softens the substrate without causing surface deformation, crazing or cracking.
2. Etching: strong oxidising agents etch the plastic surface to improve adhesion.
3. Flame or heat treatment: this provides an oxidised surface without using any liquid agents.
4. Corona or Arc Discharge: this provides an oxidised surface using an ozone field.
5. Mechanical abrasion: abrasive belts or grit roughens the surface to improve adhesion and to give it ‘tooth’.
6. Prime coat: the base coatings are formulated to adhere to the plastic substrate and to provide a good bond for the top coat.
7. Gas-Plasma surface treatment: this treatment either cross links the surface molecule to create a tight, coherent skin permitting stronger adhesive bonds or forms free radical on the polymer surface providing strong chemical bonds to coatings.