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.
Friday, April 29, 2011
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.
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.
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.
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.
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.
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.
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.
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