PROCESS QUALITY CONTROL:
Precision — a measure of agreement among repeated measurements of the same property under identical, of substantially similar, conditions; expressed generally in terms of the standard deviation.
Process — a set of interrelated resources and activities that transforms inputs into outputs. Examples of processes include analysis, design, data collection, operation, fabrication, and calculation.
Standard operating procedure — a document that details the method for an operation, analysis, or action with thoroughly prescribed techniques and steps to be followed. It is officially approved as the method for performing certain routine or repetitive tasks.
Manufacturing process control and quality assurance is the set of policies, procedures, and processes used to ensure the quality of a product or a service. It improves the quality of the products and services produced, thereby improving market share, sales growth, sales margins, and competitive advantage; it also helps in avoiding litigation. Quality control methods in industrial production lead to the production of more reliable goods, with fewer defects.
For process control and improvement to be effective, a basic understanding of the process must be obtained. A multi-disciplined team should be established to develop the process Control Plan. All the available information should be utilized to gain a better understanding of the process, such as:
process flow diagram
system/design/process failure mode and effects analysis
special characteristics
lessons learned from similar parts
team’s knowledge of the process
design reviews
The process control plan provides a documented “summary description” of the methods used to minimize process and product variation. It provides a structured approach for the design, selection and implementation of value added control methods. It is not intended to replace the detailed information contained in operator work instructions.
STEPS FOR PROCESS CONTROL:
Process Operation: process step being considered.
Machine, Tool or Device: equipment used to perform a particular process step or operation.
Control Characteristic: process parameter being controlled. In the case of plastic processing equipment this list can become quite lengthy.
Method: means used to control the particular process parameter.
Frequency: how often the process control takes place.
Control Method: manner in which evidence of the process control is recorded.
Reaction Plan: actions that will take place if the process control fails.
Performance Measurement: standard against which the success of the process control is measured.
Friday, March 25, 2011
Thursday, March 24, 2011
RAW MATERIAL QUALITY CONTROL and related terminology
Quality control sample — an uncontaminated sample matrix spiked with known amounts of analyses from a source independent of the calibration standards. Generally used to establish intra- laboratory or analyst-specific precision and bias or to assess the performance of all or a portion of the measurement system.
Quality management plan — a document that describes the quality system in terms of the organization’s structure, the functional responsibilities of management and staff, the lines of authority, and the interfaces for those planning, implementing, and assessing all activities conducted.
Quality system — a structured and documented management system describing the policies, objectives, principles, organizational authority, responsibilities, accountability, and implementation plan of an organization for ensuring quality in its work processes, products (items), and services. The quality system provides the framework for planning, implementing, and assessing work performed by the organization and for carrying out quality assurance procedures and quality control activities.
RAW MATERIAL QUALITY CONTROL:
High quality of raw material is the prerequisite of a good manufacturing practice. There is a possibility of major accidents due to contaminants in the raw materials. All raw materials, reactants, intermediates, products and by-products known and can be retained safely without a deterioration in plant integrity. Requirement of emergency procedures, systems and provisions in place, to deal with events, result from raw materials control failure.
Factors affecting raw material control/sampling:
1. Human factors: Human error during acceptance of delivery and sampling.
2. Poorly skilled work force: Incompetent quality control staff.
3. Unconscious and conscious incompetence: Failure of quality assurance procedures
4. Wrong material used in wrong place: Failure to understand the properties of substances handled.
5. Contaminated or out of specification material: Contaminant entering the plant for example, flammables in non-flameproof areas, oxidisers mixing with flammable solvents. Failure to identify all credible contaminants and resultant reaction pathways that could disrupt the integrity of the plant involved.
Remedy:
The staff should be sufficiently informed, instructed, trained and supervised to minimise a potential human failing during raw material delivery, test and storage.
Sufficient chemical inventory of all chemicals should be used on site.
The suitable quality assurance procedures should be observed in place of testing all incoming raw materials on site.
Validated quality control test methods and equipment should be employed to identify any potentially hazardous contaminants present within a raw material delivery. Prevent contamination during storage.
ELECTROSTATIC PAINTING
In this technique for painting, electrical attraction is created between the paint and the part. Thus the overspray and wastage of paint is minimized. Here the salts like hygroscopic ionised salt solution are applied on the part so as to make it conductive. These solutions can be applied by various methods ranging from conveyorized dip to rotation of the part on spindles. The paint is then oppositely charged so that the electrical attraction is created. Then high voltage charge is placed on the paint particles and the part is earthed so that the paint that is normally be wasted as an overspray will wrap around the part and be attached to the sides and back. 100% paint utilization is sometimes claimed by some system.
Limitation:
It cannot be used where mask painting is required (it is usually more applicable where the entire surface is to be painted).
Application: ABS grills and other automotive parts having surfaces at many angles from the gun.
Caution: The solvent system should be formulated to have the proper polarity to wrap around the part and should be slow enough to prevent dry spray in deep recessed areas and on the rear of the part.
Limitation:
It cannot be used where mask painting is required (it is usually more applicable where the entire surface is to be painted).
Application: ABS grills and other automotive parts having surfaces at many angles from the gun.
Caution: The solvent system should be formulated to have the proper polarity to wrap around the part and should be slow enough to prevent dry spray in deep recessed areas and on the rear of the part.
PAINT APPLICATION TECHNIQUES and CONVENTIONAL SPRAY PAINTING
PAINT APPLICATION TECHNIQUES:
There are number of different methods which can be used for the application of paint coating. The choice of the application technique depends on the number of parts to be painted and the type of parts. The following are the different paint application techniques:
1. Conventional spray painting
2. Electrostatic painting
3. Wiping
4. Roller coating
5. Flow coating
6. Dip coating
7. Flocking
8. Silk screen coating
CONVENTIONAL SPRAY PAINTING:
It is either done manually using the simple hand gun or automatically using highly automated system with automatic screen washer and elaborate masks for multiple colour decoration. Large complex parts or low cost parts are not suitable for the automatic operation either due to their complexity or economic reasons. Basic components of spray painting are: 1. Gun 2. Mask 3. Spray booth.
Types of Gun:
1. Rotary Guns: Usually two guns are mounted so that they rotate around a part to get at hard to reach spots.
2. Reciprocating Guns: They have a to and fro action and travel on a curved traverse to paint the sides of complex parts.
3. Spindle machines: Here the parts rotate on a spindle while the guns are stationary. It can paint small parts from top and bottom at the same time.
4. Combined motion: There are two types: (i) Rotating guns and reciprocating parts. Or (ii) Rotating parts and reciprocating guns.
Masks: Sections of a part which are not to be painted should be shielded off using masks which are usually made by electroforming. They vary widely in complexity. However they can be classified in following four categories:
1. Lip mask: It is used for painting a depressed name or design. Here a lip of metal extends down the vertical side wall of the depressed design entirely or partially depending on the result required. The lip is thin and strong. The centres of circular letters or designs (A, 0, o, 6, 8 etc.) are securely held in the place by bridges. The fit and the lip of the mask ensure a clean sharp paint line. The draft angle of the depressed design is at least 5o.
2. Cap mask: It is used when the embossed name or design is to be shielded. The lip of the metal covers the vertical side walls entirely till the bottom and thus protecting the embossing.
3. Plug mask: It is used for protecting the depressed design. It is usually used for protecting the depressed design. It is usually used with transparent articles and articles where vacuum plating is required. Positive fitting is accomplished by directly electroforming in to 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 to a frame so as to provide a unit which can be handled in production.
4. Block cut out mask: It is used in filling depressed letters and calibration marks. For masking small letters, the openings in the masks will be so small that they will be immediately filled with paint, hence a cut out is made to enclose each character/s, (distinctive mark, sign or letter) to confine the paint to the intermediate area. The excess on the surface is removed by wiping or buffing, so that the depressed characters are left filled with paint. The block cut out masks can be designed to accommodate the articles made in multiple cavity mould which do not have very uniform dimensions.
While designing a mould, the probability of masking should be considered. The masks should be regularly washed during its operation or an automatic washing system should be incorporated. Three to four masks should be supplied for each part to facilitate the best production efficiency.
Washing of masks:
Most masks were washed with solvents but the restrictions on the discharge of solvents in to atmosphere require the cleaning of the paint masks with water based material. These are non-toxic, non-flammable and cheaper. Here a water soluble film is applied to the mask surface and when the mask is to be cleaned, it is immersed in hot water or a heated solution in which the protective film dissolves, along with the paint. However the paint does not dissolve and can be taken off mechanically.
Spray Booths:
In some automated and semi automated operations, a closed chamber is used. Choice of the booth depends on the plastic to be sprayed, the method of application and the rate of production desired. The basic types are:
1. Water wash booth: It provides a continuous water fall in the back of the booth to wash out the paint. It is useful in continuous spray production lines.
2. Filter booth: Here a filtering device to trap over spray particles is provided. It is suitable for long runs where slow drying or light viscosity materials are used. OR For intermittent or short run application. Roll type dispensing units are developed to replace the filters regularly.
3. Baffle booths: These are used where exhaust air do not have to be free of paint particles. It is suitable for intermittent production with quick drying materials. Baffles assure an even air flow distribution through the work area of the spray booth.
There are number of different methods which can be used for the application of paint coating. The choice of the application technique depends on the number of parts to be painted and the type of parts. The following are the different paint application techniques:
1. Conventional spray painting
2. Electrostatic painting
3. Wiping
4. Roller coating
5. Flow coating
6. Dip coating
7. Flocking
8. Silk screen coating
CONVENTIONAL SPRAY PAINTING:
It is either done manually using the simple hand gun or automatically using highly automated system with automatic screen washer and elaborate masks for multiple colour decoration. Large complex parts or low cost parts are not suitable for the automatic operation either due to their complexity or economic reasons. Basic components of spray painting are: 1. Gun 2. Mask 3. Spray booth.
Types of Gun:
1. Rotary Guns: Usually two guns are mounted so that they rotate around a part to get at hard to reach spots.
2. Reciprocating Guns: They have a to and fro action and travel on a curved traverse to paint the sides of complex parts.
3. Spindle machines: Here the parts rotate on a spindle while the guns are stationary. It can paint small parts from top and bottom at the same time.
4. Combined motion: There are two types: (i) Rotating guns and reciprocating parts. Or (ii) Rotating parts and reciprocating guns.
Masks: Sections of a part which are not to be painted should be shielded off using masks which are usually made by electroforming. They vary widely in complexity. However they can be classified in following four categories:
1. Lip mask: It is used for painting a depressed name or design. Here a lip of metal extends down the vertical side wall of the depressed design entirely or partially depending on the result required. The lip is thin and strong. The centres of circular letters or designs (A, 0, o, 6, 8 etc.) are securely held in the place by bridges. The fit and the lip of the mask ensure a clean sharp paint line. The draft angle of the depressed design is at least 5o.
2. Cap mask: It is used when the embossed name or design is to be shielded. The lip of the metal covers the vertical side walls entirely till the bottom and thus protecting the embossing.
3. Plug mask: It is used for protecting the depressed design. It is usually used for protecting the depressed design. It is usually used with transparent articles and articles where vacuum plating is required. Positive fitting is accomplished by directly electroforming in to 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 to a frame so as to provide a unit which can be handled in production.
4. Block cut out mask: It is used in filling depressed letters and calibration marks. For masking small letters, the openings in the masks will be so small that they will be immediately filled with paint, hence a cut out is made to enclose each character/s, (distinctive mark, sign or letter) to confine the paint to the intermediate area. The excess on the surface is removed by wiping or buffing, so that the depressed characters are left filled with paint. The block cut out masks can be designed to accommodate the articles made in multiple cavity mould which do not have very uniform dimensions.
While designing a mould, the probability of masking should be considered. The masks should be regularly washed during its operation or an automatic washing system should be incorporated. Three to four masks should be supplied for each part to facilitate the best production efficiency.
Washing of masks:
Most masks were washed with solvents but the restrictions on the discharge of solvents in to atmosphere require the cleaning of the paint masks with water based material. These are non-toxic, non-flammable and cheaper. Here a water soluble film is applied to the mask surface and when the mask is to be cleaned, it is immersed in hot water or a heated solution in which the protective film dissolves, along with the paint. However the paint does not dissolve and can be taken off mechanically.
Spray Booths:
In some automated and semi automated operations, a closed chamber is used. Choice of the booth depends on the plastic to be sprayed, the method of application and the rate of production desired. The basic types are:
1. Water wash booth: It provides a continuous water fall in the back of the booth to wash out the paint. It is useful in continuous spray production lines.
2. Filter booth: Here a filtering device to trap over spray particles is provided. It is suitable for long runs where slow drying or light viscosity materials are used. OR For intermittent or short run application. Roll type dispensing units are developed to replace the filters regularly.
3. Baffle booths: These are used where exhaust air do not have to be free of paint particles. It is suitable for intermittent production with quick drying materials. Baffles assure an even air flow distribution through the work area of the spray booth.
PAINT and COATING
PAINT SELECTION:
Tight adherence of a coating depends on the nature of the plastic and matching of the paint system with the plastic.
Design guide for the paint selection:
1. Heat distortion point and heat resistance: This determines whether bake type paint can be used and if yes, the maximum baking temperature the plastic can tolerate.
2. Solvent resistance: The susceptibility of the plastic to solvent attack dictates the choice of paint system. Some softening of the substrate is desirable to improve adhesion, but a solvent that aggressively attacks the surface and results in cracking or crazing must be avoided.
3. Residual stress: Application of coating to the localized areas of stress may swell the plastic and cause crazing. Annealing of the part before coating will minimize or eliminate the problem. It can be completely avoided by careful design of the moulded part to prevent locked-in stress.
4. Mould release residues: Excessive mould release can cause lack of adhesion. The plastic surface must be rinsed or cleaned of the release agents to assure satisfactory adhesion.
5. Plasticizers and other additives: Most plastics are compounded with plasticizers and chemical additives which usually migrate to the surface; thus softening the coating by destroying the adhesion. Hence the coating should be checked for short and long term softening problem for the specified formulation on which it is to be used.
6. Other factors: Stiffness or rigidity, dimensional stability and coefficient of expansion of the plastic affect the long term adhesion of the coating. The physical properties of the paint film should be compatible with those of the plastic substrate.
LESS CONVENTIONAL TYPES OF COATING: These types of coatings are applied to different plastics for specific purpose.
1. Intumescent coatings: Fire resistant coating is applied to a part in a thickness of 50-60 mil. It foams up to a thickness of ½ to 1”, during a fire, to protect the part from the flame and high temperature.
2. Anti-static coating: It is applied to minimize the tendency of the part to acquire or hold a static charge which may attract dust or dirt. The coating absorbs moisture from the air for its function. It is not effective in very dry atmosphere.
3. Abrasion resistance coating: Transparent coatings are used on acrylic and PC to increase the abrasion resistance of the surface.
4. Electrically conductive coatings: Coatings usually containing metal particles (Usually Ag or Cu or graphite) renders the coating electrically conductive. Some with fairly high resistance are used only as static charge dissipaters, others with much lower electrical resistance are used to conduct current so they can be used as heating films, or as RF shields in electronic applications. Transparent, electrically conductive coatings are also available, but require special application techniques.
5. Organosol coatings: they are used where a thick coating, 50-6- mil is required (in spray or dip application) and where the coating is required to exhibit a slight amount of resilience.
Tight adherence of a coating depends on the nature of the plastic and matching of the paint system with the plastic.
Design guide for the paint selection:
1. Heat distortion point and heat resistance: This determines whether bake type paint can be used and if yes, the maximum baking temperature the plastic can tolerate.
2. Solvent resistance: The susceptibility of the plastic to solvent attack dictates the choice of paint system. Some softening of the substrate is desirable to improve adhesion, but a solvent that aggressively attacks the surface and results in cracking or crazing must be avoided.
3. Residual stress: Application of coating to the localized areas of stress may swell the plastic and cause crazing. Annealing of the part before coating will minimize or eliminate the problem. It can be completely avoided by careful design of the moulded part to prevent locked-in stress.
4. Mould release residues: Excessive mould release can cause lack of adhesion. The plastic surface must be rinsed or cleaned of the release agents to assure satisfactory adhesion.
5. Plasticizers and other additives: Most plastics are compounded with plasticizers and chemical additives which usually migrate to the surface; thus softening the coating by destroying the adhesion. Hence the coating should be checked for short and long term softening problem for the specified formulation on which it is to be used.
6. Other factors: Stiffness or rigidity, dimensional stability and coefficient of expansion of the plastic affect the long term adhesion of the coating. The physical properties of the paint film should be compatible with those of the plastic substrate.
LESS CONVENTIONAL TYPES OF COATING: These types of coatings are applied to different plastics for specific purpose.
1. Intumescent coatings: Fire resistant coating is applied to a part in a thickness of 50-60 mil. It foams up to a thickness of ½ to 1”, during a fire, to protect the part from the flame and high temperature.
2. Anti-static coating: It is applied to minimize the tendency of the part to acquire or hold a static charge which may attract dust or dirt. The coating absorbs moisture from the air for its function. It is not effective in very dry atmosphere.
3. Abrasion resistance coating: Transparent coatings are used on acrylic and PC to increase the abrasion resistance of the surface.
4. Electrically conductive coatings: Coatings usually containing metal particles (Usually Ag or Cu or graphite) renders the coating electrically conductive. Some with fairly high resistance are used only as static charge dissipaters, others with much lower electrical resistance are used to conduct current so they can be used as heating films, or as RF shields in electronic applications. Transparent, electrically conductive coatings are also available, but require special application techniques.
5. Organosol coatings: they are used where a thick coating, 50-6- mil is required (in spray or dip application) and where the coating is required to exhibit a slight amount of resilience.
Thursday, March 10, 2011
COATING AND SURFACE TREATMENT
COATING:
A plastic part can be coated by applying paint on its surface, a metalizing coating, and application of a film coating on its surface.
Significance:
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.
PAINT COATING:
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.
REMEDY:
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.
PREATREATMENT:
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.
A plastic part can be coated by applying paint on its surface, a metalizing coating, and application of a film coating on its surface.
Significance:
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.
PAINT COATING:
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.
REMEDY:
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.
PREATREATMENT:
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.
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