Friday, April 29, 2011

Miscellaneous metal plating techniques

BARREL PLATING:
The method was used to impart metallic finish to small parts before the electroplating method was developed. The method is still used.
Process:
A conductive coat of silver is precipitated on the plastic part from silver nitrate solution to make the m conductive. The n the part is plated in the rotating plating barrels containing acid copper plating solution and copper anodes. After the copper is deposited the articles are polished with a steel shot in a burnishing barrel. A plating of gold, silver or nickel is deposited on them by another barrel plating operation.
HOT STAMPING:
The conventional hot stamping process can be used to put a gold, silver or other metallic decoration on plastics. Now high lustre metalized silver foils or roll leaf are developed for exterior use.
SPRAY PLATING:
Here silver or copper plating is deposited by chemical reaction of water based solution on the substrate. The base coat and a top coat are usually incorporated.
Here the silver containing salts are deposited on the product through a dual nozzle gun. When copper is to be applied the silver is applied first and then the copper is applied on the silver coat. A typical system will consist of a spray booth with special bottom to recover silver, mixing vessels, demineraliser for water purification and four guns: one for water to clean the part after each processing step, one for the activator to treat surface of the part, a dual nozzle gun for silver and a dual nozzle gun for copper.

ELECTROPLATING

The coating can be quite thick and many different metals can be used like copper, nickel, gold, silver, chrome etc. The most popular substrate plastics are ABS and PP. Polysulfone, phenylene oxide based resins and polyethers are also used.
Process:
It involves a series of Preplating treatments followed by the electroplating operation.
Preplating treatment:
It consists usually of cleaning, etching, neutralizing, catalyzing, accelerating and electro less nickel or copper deposition.
a)Cleaning:
To remove soil, particles, fingerprints, etc. The cleaning solution consists of mild alkaline cleaners or detergent solution depending on the type of plastic.
b)Pre Etching:
Sometimes a conditioning or pre etching step is considered to make the plastic more receptive to the etching process or to overcome the thermal problem involved with the exposure to the heated etch bath. For ABS organic solvent is used as a pre etch step and for PP special silica filled PP is required.
c)Etching:
The etchant solution (solution of chromic acid and other strong oxidising agent) softens the surface of the material and then the etchant will selectively attack the butadiene domains on the surface. This produces porosity or micro etches and the surface is rendered hydrophilic. Any remaining foreign matter on the surface is also removed.
d)Catalyzing:
The catalysts are used to introduce acidic solution of tin and palladium salts. The tin compounds are absorbed in the surface of plastic and also carry with them the palladium ions which act as ‘seeds’ to catalyse the electro less deposition of nickel or copper.
e)Accelerating:
It removes excess of tin salts and the accelerators also intensify the effectiveness of the catalyst particles.
f)Electro less nickel or copper deposition:
A thin (0.0010 to 0.0025 mill inch) conductive metallic coating is deposited on the surface of the treated plastic to provide the electrical conductivity necessary for the electroplate operation. Generally the electro less metal is chemically reduced from solution on the catalysed surface. The parts at this stage should be handled with great care to prevent degradation of the extremely thin and fragile conductive coating otherwise it would be reflected in the final electroplated coating.
Electroplating Operation:
The Preplating plastic moves in a mild H2SO4 etch bath. The n the 1st electroplate layer of copper is deposited. This soft, ductile copper coating is deposited from an acidic copper solution. This soft copper provides a smooth coat for the succeeding coat and it acts as a stress (induced by the nickel and chrome deposit and thermal) absorbing layer for the next coating. The thermal stresses are induced by the differences in thermal coefficient of expansion of plastic and the metals. Nickel and then chrome is deposited on the plastic substrate which is electroplated with copper. Sometimes gold, silver or tin is deposited over the copper or nickel if the functional requirement of the application dictates. When more corrosion protection is required a semi bright nickel is used instead of a bright nickel. Sometimes a dual nickel system (stress free semi bright nickel and a bright nickel) or a triple nickel system (semi bright nickel, bright nickel and a thin deposit of a stress inducing nickel to provide additional protection between the nickel and the chromium plate) are used.
Design Guide:
a)Part should be designed for moulding in one piece because mechanical welds are difficult to plate.
b)Gates should be hidden on non critical surfaces.
c)Deep recesses or blind holes should be avoided because plating solution may build up in cavity.
d)Non critical surface areas should be designated and designed for cathode contacts because plating will not be as uniform or a bright in these areas.
e)Sufficient rigidity should be allowed to prevent part warpage under elevated temperature during plating.

Wednesday, April 27, 2011

METALLIC FINISHES

Significance:
It is applied to plastic parts for decorative purposes, to provide electrical conductivity, to minimize electromagnetic interference and to provide surface with improved solvent and/or abrasion resistance.
Methods of Metallic coating:
1. Vacuum metalizing
2. Electroplating
3. Barrel plating
4. Hot stamping
5. Spray plating
The choice of the process depends on the type of the part and the environment in which it is used. The first two techniques are the most popular techniques.
Vacuum Metalizing:
Definition:
The use of a vacuum chamber to deposit a metal coating on a part or on both surfaces of a plastic film or sheet is called vacuum metallization. The metal coating is usually very thin Aluminium.
Process: It is performed in three phase: A) The base coat application B) Metal deposition and C) Top coat application.
A)The base coat application:
A clear, high gloss material is applied to the substrate. It is specially formulated to have high adherence to the particular substrate, to smooth out any small surface imperfection and to act as a mechanical and molecular bonding surface for the vacuum applied metal. Improper base coats or improperly applied coating can cause low adhesion or failure of the metal coating under humid condition hence the parts before coating should be thoroughly cleaned and preferably be moulded without mould release agent. Sections of the part that are not to be metalized should be masked prior to the base coat application.
B)Metal deposition:
The small wires of metal to be deposited (usually Aluminium) is placed on the tungsten filament which is heated to a temperature that causes vaporisation of the wires. The metal vapours condense on the parts that are on rotating racks inside the vacuum chambers. Since the condensation is a line-of-sight process correct placement, rotation of the part and racking is very important for uniform metal deposition.
C)Top coat application:
It can be a clear, abrasion resistant coating; or it can be a tinted coating, so that the final colour may have a simulation of Gold, Copper, Brass or any other transparent shade. It is specially formulated for maximum adhesion to the deposited metal and not affecting the adhesion of the metal to the base coat. It protects the metal from wear, oxidation or any environmental effect.
Though any metal can be theoretically deposited by this process, Aluminium is the most popularly and particularly chosen because of its low cost, high reflectivity and brilliant surface. Since the coating is very thin (several millionth of an inch) the top coat is required. However the transparent top coat is required when the metal is deposited on the rear or 2nd surface (hence 2nd surface metallization process) of a transparent plastic or glass part and a final protective coating of an opaque, heavily pigmented lacquer may be used.
Design guide:
a)Sharp corners, edges and serrations should be avoided. The top coat will fail at these areas first when exposed to abrasion.
b)Moulds should be highly polished.
c)Gates should be adequate to reduce stress which may result in craze marks on the application of the base coat.
d)Mould release should not be used if possible. Internal lubricants should also be minimised because they interfere with good adhesion of the base coat or the metal.
e)Low density areas in the part, caused by too rapid mould filling and insufficient dwell, may cause irregularities in the base coat such as dullness and loss of adhesion.
Recent Development:
The most popularly used plastic substrates are Polystyrene, Acrylics, Phenolics, unplasticized Vinyls and Polycarbonate. Several formulations of base coat are developed that can be used with variety of substrates e.g. ABS, Phenylene oxide based plastic. Some base coats are suitable for Polystyrene and ABS.
Fast curve base coats are developed especially for the high volume conveyorized shapes. Vacuum metalizing base coats which can be applied to primer less Polypropylene.

Thursday, April 7, 2011

DESIGN GUIDE FOR HOT STAMPING

Factors to be considered to obtain maximum efficiency in the process are:
1. Use a uniform wall thickness, avoid stamping over ribs and bosses because they tend to cause shrinks in parts.
2. Raised patterns, numbers, or letters should be at least 0.5 mm high and preferably 0.75 mm or more to minimize over stamping in adjacent areas.
3. Edges of patterns and letters should be rounded rather than being sharp. This will minimize cutting of the silicone pad, thus insuring a longer life to the die.
4. Areas to be stamped should as flat and parallel as possible.
5. Dimensions must not vary from cavity to cavity in multi cavity moulds because variation of even 0.025mm can cause difficulty in stamping.
6. Very short moulding cycle, not allowing the part to cool properly, will result in excessive shrinkage and warp.
7. When several moulds are made by different moulders, the base pattern should be the same i.e. the hob used should be same.
8. In multi cavity moulds, the runners and gates should be positioned in such a way that all the parts are ejected from the mould at the same temperature.
9. Compatibility of the substrate with the hot stamping and heat transfer process should be considered while choosing the substrate.
10. Sufficient draft on the parts should be allowed to prevent distortion during ejection.
11. All the tool marks, in the areas to be decorated, should be polished because any imperfection will be magnified when high lustre metallics or colours are used.
12. When angular or spherical borders are coated, the angle from the horizontal should not exceed 400. A step surface of at least 1.25 mm from the adjacent area should be provided.
13. When raised areas are located in a deep recess, a minimum clearance of 1.5 mm from the edge of the wall of recess should be provided. This minimises the wrinkling of leaf and prevents the heated die from marring (spoiling or damaging) the side wall.

EQUIPMENTS FOR HOT STAMPING

There are basically two types of machine used in the process viz.: The Reciprocating Press and The Rotary machine.
The Reciprocating press:
It is used when the surface of the substrate is flat, concave, or convex. It is also used to mark the cylindrical shape. It is difficult to eliminate air pockets trapped between the die, roll leaf and the part with this type of press. Usually it is squeezed out by doming the die. The press basically consists of: a head ram--- moving up and down; a table ram--- which can be adjusted up and down according to the height of the plastic part and a frame--- containing the drive mechanism and it supports the pressure development and maintains alignment between the two rams. A heating head is attached to the head ram to supply and maintain heat to the die. An automatic roll feed is also attached to the head ram to advance the roll leaf or transfer. The head ram is driven by direct air cylinder, or a hydraulic or an air-hydraulic system.
Rotary type machine:
Here the rolling action of the rotating silicone coated roller pushes the entrapped air and gases ahead of it as it passes across a broad area. It also eliminates the need of extremely high pressure on broad area coverage because only a line contact is made between the roller and the plastic surface. The die can be cylindrical or flat. The item is placed on a support mandrel, to back up the impression. The mandrel, which accurately fits the item, is driven mechanically at the same speed as the rotating die, when designs or lettering are applied, but when smooth silicone pad is used to release the solid foil coating or a transfer design, the mandrel is not mechanically driven but is designed to turn as freely as possible. When the part with a flat surface is to be decorated, the part is mounted on a fixture and is transported, by a hydro-check table, beneath the roller. The speed of rotation of the roller is equal to the speed of movement of the table because tremendous heat is removed from the roller, it is usually heated both internally and externally and hence the roller is constantly rotated even if the object (substrate) is not in contact, otherwise the external heater can cause overheating and rapid decomposition of the rubber coating. On some rotaries, several rollers are used to assure complete transfer of the coating. Low tooling cost in short runs can be obtained because flat die is cheaper than a cylindrical shaped die.

HEAT TRANSFERS

A Multi colour heat transfer consists of a carrier strip printed with a specific design, pattern, picture etc. in several colours. Carrier strip is thicker than that used in a Roll leaf. It is usually a craft paper or a polyester film. Exact colour registration is guaranteed in a multicoloured item because the carrier is printed with design. While with Roll leaf process, the item is to be passed through the press for each individual colour. In multi colour Hot stamper, a turn table system feeding several presses is used. Most transfers are formulated with a sizing. However, recently the ink is formulated with the same plastic as that used in the substrate. Thus the design fuses when the heat is applied. The abrasion resistance is enough to eliminate the need for a protective top coat.
Effects that can be obtained by using the Roll leaf and Heat transfers:
In laid or de-bossed effect (for Roll leaf): Here a heated metal die is used to deposit a roll leaf coating below the surface of the plastic to be decorated.
Surface coating (for both): Here a heated silastic die or roller is used to transfer the coating on a smooth or slightly textured surface.
Embossed effect (for both): It is obtained when the coating is deposited on the surface of a raised section in the plastic. It is usually applied by using a silastic die.

THE ROLL LEAF

It consists of a coating or several layers of coatings which are released from a carrier film and deposited on a substrate.
The process of manufacture:
The carrier is a thin film (0.0125-0.025 mm) of cellulose acetate or polyester film.
First coat: Film is coated on one side with a release agent (usually wax) which melts on application of heat to the uncoated side of the carrier film.
Second coat: The carrier is passed through a colour or metallic coating process.
Third coat: Heat activated sizing to obtain acceptable adhesion is applied as third coat. The sizing is formulated to bond on the particular material that is decorated.
Fourth coat: When extreme abrasion and solvent resistance is required a protective coat between the release coat and the colour coat is applied. Dye is added usually to the protective coat. These protective coats are developed to protect the roll leaf from alcohol, salt spray, detergents etc.
Creativity: simulated wood grain roll leaf is used for wood graining radio/TV cabinets. In some wood grains 5 coats are applied:
1st coat is the release coat, 2nd is protective top coat, 3rd coat is the grain which is applied by a textured roller which is engraved to simulate the particular grain desired, 4th coat is the base coating which is a solid pigment colour to complete the two tone effect and the 5th coat is the size coating. Wood grain roll leaf is available with a gloss (lacquered) finish or with a hand rubbed finish. For hand rubbed finish an etched polyester carrier strip is used.
The roll leafs release at temperature from 2500-3500 F. But the foils releasing at lower temperature will lower the die heat and thus will reduce distortion of parts with thin walls. It will also lengthen the life of silastic dies and the soft metal dies.

HOT STAMPING

Definition:
The process where the decoration from a coloured film, printed or wood grained film or a metalized film is transferred to the surface of the part by heat, pressure and precisely controlled time interval. It is a completely dry process requiring no pre-treatment of the plastics. It requires a special machine and a heated die to apply the film, in roll form, to any thermoplastic or thermoset. Usually thermosets are not decorated in this way because melting of the surface is required for adhesion.
Process:
The heated metal or rubber die is brought into contact with the plastic under pressure. The coated film is mounted between the die and the plastic. After some time (normally less than one second) the coating releases from the carrier film and adheres itself to the plastic surface. When the die is removed, the part is completely dry and can be handled immediately.
Dies:
They are made of silicone rubber or a soft metal such as zinc, magnesium, copper or brass. Silicone rubber is frequently used because it can compensate small variation of thickness in the plastic and withstand the temperature ranging from 1200-1900C.
Films:
The decorative films used in the process are usually of two type viz.: The Roll leaf and The Heat transfers.

QA DOCUMENTATION

Checklist Documentation
Here are some of the important documents included in a checklist and the vital documentation procedures that a company has to fulfil
o Quality Manual – This document includes comprehensive details related to the quality management system of an organization. It defines each procedure, process, expectation and justification if there are exclusions for a procedure. It also describes how the quality assurance checklist can help the organization monitor its records and management reviews. Generally, the quality manual - that can be either in a hard copy or soft copy form – provides a guarantee that assurance of quality has been achieved.
o The complete list of documented activities – This document helps define the appropriate measures to create, implement, endorse and review the quality processes included in the system.
o Records regulation – The regulation or control of records in an organization is necessary to establish that it satisfactorily conforms to the defined procedures. Since it helps determine and confirm if the quality programs in QA checklist are still effective, records regulation is an important aspect of the checklist.
o Management responsibilities – This record is a declaration of the company commitment to quality as well as a reference to their role in implementing quality management policies. It includes planning, communication - both external and internal - and customer focus. It also describes the responsibility and authority rules established by the management.
o Human resources – This aspect of the checklist deals with the personnel who are involved in the quality programs of the organization. It also establishes the qualifications required by the personnel to be able to ensure the competence specified in QA checklist. This section of the checklist also details the necessary education, skills, training and work experience required of each employee in order to efficiently carry out quality improvement tasks.
o Product realization – This is another significant component of the checklist that details the processes involved in manufacturing a product. Since product realization must fulfil the criteria included in the quality manual, it is a very useful guideline during the design and manufacturing process. If any deviations are found, the product will be classified as non-compliant.
o Measuring the outcome – In this part of the checklist, the organization offers evidence of performance in QA checklist activities - including how they assess, analyze, amend and advance their quality processes. This analysis of the actual outcome also shows how much the company conforms to the quality management system. This document must be able to provide proof of compliance in an easy-to-follow format such as statistical data.
1. Finished Product Inspection Report
2. Report Date:
3. Vendor Name:
4. Style :
5. Inspector Name:
6. Factory Name/Address:
7. Description:
8. Order Due Date:
9. Est. Ship Date:
10. Colour:
11. City/ State/Prov. P.O. :
12. Ship method:
13. Country of Origin:
14. Order Quantity:
15. Report accepted by:
16. (Authorized factory manager)
17. Visual Product ( ) Pass ( ) Fail ( ) Waived ( )
18. Measurement ( ) Pass ( ) Fail ( ) Waived ( )
19. Packaging ( ) Pass ( ) Fail ( ) Waived ( )
20. Overall ( ) Pass ( ) Fail
21. Visual Inspection (Visual, Construction, and Packaging)
22. Sample Size:
23. Major Defects Allowed:
24. Minor Defects Allowed:
25. List Each Defect found
26. Defect Description: Critical( ) Major( ) Minor ( )
27. Corrective Action to be taken:
28. Authorised signatory

Quality Assurance Checklist

QA PROJECT PLAN CHECKLIST:
Quality control checklists are a practical means of measuring production in a factory setting, allowing factory managers to inspect the productivity of their workers and to improve the factory's finished product. Such checklists typically record the rate of defective products and material waste as well as the time required for the completion of different tasks in an assembly line setting. Once the numbers are gathered, quality control personnel and line managers can then conduct further investigations regarding the causes and origins of product defects or low productivity. In factories, quality control checklists serve as a means of improving product quality and saving money.


Quality Assurance Checklist:
1. Every company relies on quality assurance checklist to effectively track, examine and verify all quality enhancement programs before, during and after implementation. Such a checklist is meant to provide business owners, project designers and other employees with valuable guidance in determining and planning all quality monitoring activities within the company. The checklist includes quality checking activities to be implemented throughout the product lifecycle.
The basic objectives of quality checking programs include providing the management with the necessary data that informs about the product quality and verifying that the products conform to specified standards and requirements. To fulfil these objectives and other quality standards of the company, quality assurance checklist primarily involves the following activities
o Periodic review of the quality activities by the senior management
o Regular review of the quality activities and products by independent quality professionals
o Identifying, documenting and controlling deviations in quality activities

Quality Assurance Plan

• The objective of quality assurance plan is to develop and design the activities related to quality control project for the organization. It is a composite document containing all the information related to the quality control activities. It is used to schedule the reviews and audits for checking different business components and also to check the correctness of these testing procedures as defined in the plan. The quality management team is totally responsible to build up the primary design of the plan. To develop this plan, certain steps are followed, which are described below.
o Step 1: To define the quality goals for the processes. These goals will be accepted unconditionally by the developer and the customer, both. These objectives are to be clearly described in the plan, so that both the parties can understand easily the scope of the processes. The developers might also set a standard to define the goals. If possible, the plan can also describe the quality goals in terms of measurement. This will ultimately help to measure the performance of the processes in terms of gradation.
o Step 2: To define the organization and the roles and responsibilities of the participant activities. It should include the reporting system for the outcome of the quality reviews. The quality team should know where to submit the reports, directly to the developers or somebody else. In many cases, the reports are submitted to the project review team, who in turn delivers the report to the subsequent departments and keeps it in storage for records. Whatever is the process of reporting, it should be well defined in the plan to avoid disputes or complications in the submission process for reviews and audits.
o Step 3: The subsidiary quality assurance plan: It includes the list of other related plans describing project standards, which have references in any of the process. These subsidiary plans are related to the quality standards of several business components and how they are related to each other in achieving the collective qualitative objective. This information also helps to determine the different types of reviews to be done and how often they will be performed. Normally, the included referenced plans are identified below.
 Documentation Plan
 Measurement Plan
 Risk Measurement Plan
 Problem Resolution Plan
 Configuration Management Plan
 Product Development Plan
 Test Plan
 Subcontractor Management Plan etc.
o Step 4: To identify the task and activities of the quality control team. Generally, this will include following reviews.
 Reviewing project plans to ensure that the project abide by the defined process.
 Reviewing project to ensure the performance according to the plans.
 Endorsement of variation from the standard process.
 Assessing the improvement of the processes.
It is the responsibility of the quality manager, to fix the schedule for the reviews and audits to conduct quality control. This schedule is also documented within the plan, so that task control can be done at an individual level.
Thus, the entire process of quality control is documented within the plan. This helps as a guideline for the reviewers and developers, simultaneously. For any future reference, this could be used as a practical evidence of total quality control.

PRODUCT QUALITY CONTROL

INSPECTION OF FINISHED PRODUCT














Quality assurance project plan — a formal document describing in comprehensive detail the necessary quality assurance procedures, quality control activities, and other technical activities that need to be implemented to ensure that the results of the work performed will satisfy the stated performance or acceptance criteria. The quality control checklist is an assessment tool used for measuring the value of a product. This structured questionnaire facilitates the collection and analysis of the data necessary to determine strengths and weaknesses in work quality.
Record: A completed document that provides objective evidence of an item or process. Records may include photographs, drawings, magnetic tape, and other data recording media.

Monday, April 4, 2011

SILK SCREEN COATING

Here the coating is wiped through a pattern on a screen. Originally the screens were made of silk but now it is made of nylon fabric, polyester fabric or stainless steel mesh to get longer life and better reproducibility. The process is well suited for application of two or more colours and/or patterns depending on the number of heads added on the machine. It can be used with curved and flat surfaces.
Silk screen coating is now termed as screen printing because it is used to print bottles, moulded items, flat films and extruded shape. The basic components of the screen printing system are: 1. Screen, 2. Frame to hold the screen 3. Squeegee.
1. Screen: Nylon screens are preferable for plastic but with hot inks stainless steel must be used. Split screens are used for simultaneous printing of two colours.
2. Frames: Although most frames are made of wood, metal frames are desirable because the metal frames have longer life and they do not warp. Specially operated frames, stretching all the four sides of the screen simultaneously are available to increase the mesh life.
3. Squeegee: Its blades are best if they are made of resilient rubber or plastic. The shape of the printing edge of the squeegee determines the sharpness and deposit of ink. A double sided squeegee with a bevelled (slant) edge is used for direct printing on uneven surfaces, bottles or containers. Softer blades are recommended for rough surfaces and for depositing more colours. The blade should at least be 5 cm longer than the print. In high speed machine, the squeegee should print from either direction to prevent build up of ink on one side of the screen. Sometimes, a one way sweep is also necessary.
PROCESS:
The squeegee is pulled across the screen so that the ink is forced through the mesh to make the print. Either screen frame or the squeegee can be moved.
INKS:
Types of Ink:
1. Flame Dry Ink: It dries in seconds and do not require long dryers. They are under development and are used most widely in the dairy industry and are limited to PE.
2. Standard Inks or cold Inks: They are not to be heated before use. They are composed of a variety of material based on the type of material, gloss, product resistance and drying time. eg. PVC products require a vinyl base ink.
The ink fuses to the product in many cases; hence the adhesion in the screen printing is excellent.
Both PE and PP must be treated before being printed. Three methods are used:
1. Flame treatment: The most practical and widely used method.
2. Corona discharge and chemical methods are not widely used, but it has an advantage of not destroying the gloss of PP.
Auxiliary equipment:
1. Nest carriers: to hold objects to be printed in a special manner like printing up to the edge of the part. A vacuum system may also be used to hold the part throughout the printing and feeding.
2. Static eliminators: are placed after the feeding mechanism and ahead of the printing head. Tinsel and tinsel bars, electric static eliminator and remotely controlled ionized air units which are expensive and efficient mechanical equipments are used. Permanent and temporary anti static compounds may also be used either in the resin itself or as a coating on the product.
3. Dryers: for semi automatic unit, a flat bed oven with circulating air, heated by gas or electricity is usually used. In automatic unit oven with special nests carrying the products through an up and down configuration are desirable.
4. Un scramblers are attached to orient the bottles before placing them in the detector.
5. Automatic registration in the printing station requires a notch in the part to match the registration mark in the printing station.
6. Expandable mandrels to hold pieces with irregular inner dimensions and soft plastic or open tubes are also incorporated.
7. Automatic feed and take off and step less variable speed drive is also available.

Miscellaneous Paint Application Techniques

WIPING:
Paint wipe operation should be considered when a product with a fairly flat surface, a lower surface is to be painted. The wipe set up is an automatic operation. Its use is more expensive than the cost of a mask, but it can handle very large volume. It can also be used where very fine lettering or designs are involved, where designing a mask for the job is almost impossible. Manual wipe is only feasible where the labour cost is very low.
Automatic wipe: Here the product indexes from the paint station to a wipe station where an absorbent fabric passes over the top of the product taking the paint off the flat surface and leaving the paint in the indented parts. A light solvent spray can be used to aid in paint removal. Disposable soft paper is usually used as an absorbent fabric, however some processors also use reusable fabrics.
ROLLER COATING:
Here the raised surface of the part is coated with paint while the depressed surface of the part remains uncoated. No secondary operations are required and the process does not paint depressed surfaces when even slightly raised sections are present.
FLOW COATING:
Paint is applied from the overhead nozzles on the racked parts. The excess coating is drained off, after painting and is re circulated for use in the next cycle, thus resulting in very little waste. The process is not continuous, but many parts can be handled in a single cycle. Sharp corners should be avoided in the parts since it can result in excessive paint build up.
DIP COATING:
The part is dipped in the paint bath so as to minimize the wastage due to the overspray. Automatic system for raising and lowering the parts are used to control the uniformity of the coating thickness. Parts that entraps excessive air cannot be dip coated because they will require prolonged drain time.
FLOCKING:
It is a decorating technique also classified as a variation in painting. It involves coating of a plastic part with adhesive lacquers tinted to match the flock colour, and then the fibre (flock) is applied with a special flock spray gun. Flock imparts the appearance and feel of material such as felt (matted fibre), suede (soft or furry surface of cloth) or short napped mohair (Angora goat with long silky hair), to the plastic part.