lecture - 15 decorating plastics

8/10/2019 Lecture - 15 Decorating Plastics

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Decoration of Plastics


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Decorating Methods

i. Painting

ii. Printing

iii. Vacuum coating techniques

iv. Plating

v. Dyeing

vi. Labels & Decals

vii.Mechanical surface changes


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Painting

Printing

Painting involves application of a

liquid coating onto the plasticsurface. The coating is dried or

otherwise solidified after it has

been applied to the surface in the

required manner and at the required

thickness.

It is the application of a

liquid coating over a

relatively small surfacearea. It is identical to the

painting process except that

different application

techniques are used.

Decorating Methods


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Vacuum coating Techniques

Plating

Where a coating, usually metallic,

is applied as a vapor or as small

particles to the surface.

Decorating Methods

Plating, which involves deposition

of metal by either electrolytic or

electroless processes


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Dyeing

Labels and Decals

Decorating Methods

Dyeing, where dye molecules

penetrate below the surface

Labels and decals, which involves

affixing of a predecorated material

to the plastic surface.

Mechanical surface changes

Mechanical surface changes, such

as polishing, grinding and

embossing.


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Coatability & coating adhesion

Good adhesion between the coating or printing ink and theplastic substrate is required in order to obtain a lasting

product. Coating adhesion is often tested by the tape test.

Tape Test, in which an adhesive tape is

applied over the coating and peeled away at

a sharp angle.Poorly adhered coatings are

removed by the tape. This test is a usefuland simple qualitative method to eliminate

poorly adhered coatings


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Coatability & coating adhesion

Several quantitative methods are also

used to test the coating adhesion. These

tests involve adhering a probe to thecoating surface and then measuring the

force required to pull away the probe, or

employing a probe to scratch the

surface of the coating.

In order to obtain a good coating adhesion

between the coating and the substrate, thecoating must wet the substrate.Wettability

is determined by the difference between the

surface tension of the liquid coating and the

surface energy of the solid substrate.

The coatingviscosity must be low enough to

allow the coating to flow. Usually the coating is

applied by some mechanical means, and thus is

spread over the surface by the forces acting

during coating.


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Surface Treatment

A plastic surface might require cleaning if it has become contaminated

during processing. The removal of release agents used in molds mightbe necessary. Surface cleaning treatment may be carried out by various

means as listed below

Water washing

Solvent cleaning and etching

Mechanical abrasion

Chemical etching

Priming

Flame treatment

Plasma treatment

Ultraviolet and other irradiation

Priming is a deposition of a thin coating of

another polymer between the plastic surface

and the paint to improve thepaintsadhesion.

Chemical Etching:Treatment of plastic surface

by exposure to solutions of reactive chemical

compounds, usually oxidizers. It may cause

chemical change of the plastic surface.


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Coated Products

Many of the molded plastics used in automobileindustry are painted or metallized.

Structural foam is used very widely for electronic

equipment housing and similar applications. The

surface of such rigid foams often inhibits swirl and

other imperfections and requires extensivefinishing including priming and painting.


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Coating Techniques

Spraying

Dip coating

Flow coating

Curtain coating

Roll coating

The choice of the most suitable technique is determined by the

geometry of the plastic part (the irregularly shaped part require

different coating methods form those used for flat films or sheets)


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Roll Coating

In roll coating the coating reservoir

lies between the smaller doctoring

roll and the application roll, the

sheets are run between the applicator

and the supporting roll.

Roll coating is suitable for low

viscosity fluids and for the

application of relatively light weight

coatings.

Roll Coater

Doctoring

Roll

Supporting

Roll

Application

Roll


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Printing

Printing is a variation of the coating process in which ink or paint is

deposited in small selected areas over the plastic surface.

Screen printing

Flexographic printing

Gravure printing

Pad printing

Methods of printing


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Screen Printing

Screen printing is widely used for plastics decorating forseveral reasons such as

The equipment is simple and inexpensive

Good quality multicolor printing is possible

Includes coverage of large surface area

However the process is slow and mainly suited for short

runs.


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Screen Printing

The screen, which is the most important component of the screen printing equipment,

consists of a stencil bonded to a fine weave fabric and tensioned in a frame.

Ink is pushed through the screen openings by a squeegee and deposited onto the

plastic surface.

The squeegee is simply a shaped flexible blade in a wood or metal holder.

Polyurethane elastomers are most often used to make squeegees.

Mechanics of the screen printing process


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Screen Printing

Mechanics of the screen printing process

Screen printers can be flat bed, rotary or cylinder

configurations and they can be simple, manually operated

units or completely automated machines. An automatedmachine can print above 8000 items per shift.


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Screen Printing

Automation may include automatic feed and take off,

nest carriers to hold objects to be printed, and otheraccessories.

Various ink types are used. Screen inks are specially

formulated to behave as a solid and not drop when placed

on the screen. The ink should flow easily through the

screen when a force is applied by the squeegee and itshould level well after its deposition.

Solvent borne inks require drying and the printing unit

usually is followed by a dryer unless air drying is possible.

UV- radiation curable inks also used for screen printing,because of their rapid cure.

Screen printing may be applied over flat or cylindrical

surfaces, but it is not suitable for the printing of irregularly

shaped surfaces.


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In-mold Decoration

Plastic parts may be decorated by various in

mold processes that eliminating a separate

decoration step.

In mold decorating may consists of decorating

the mold wall prior to molding or incorporating

the pre - printed insert or label.


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Vacuum Metallizing

Vacuum metalizing of plastics is a process

widely used to deposit a thin aluminium layer

over the surface of many different plastic

products like

Jewelry components, automotiveinterior parts, bottle caps, plumbing

accessories, trophies, etc


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Vacuum Metallizing

A metallic coating can be deposited on an

exposed surface or it can be deposited on thesecond surface of a transparent plastic item. In

the later case, the metallic coating is protected

by a layer of plastic.

In order to obtain a smooth and glossy

coating, the plastic surface may be undercoatedwith a lacquer.

It also may be over coated with a transparent

top coating to improve its abrasion resistance.

The metallic coating is usually aluminium but

other metals also can be deposited by

evaporation.


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Vacuum Metallizing

The metallization takes place invacuumin order to allow the metal vapor

to reach the plastic surface before the

vapor is oxidized by the oxygen in air.

The metal is heated to the point at which

it will vaporize under vacuum conditions.

The substrate is at a sufficiently low

temperature to allow the condensation of

metal vapor.

Parts to be metabolized are held in anappropriate fixture that might rotate in

order to expose all surfaces to be

metalized.

Schematic diagram of a vacuum evaporation system


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Vacuum Metallizing

Several metal evaporation sources are used.

Resistance heated tungsten filaments are used

most often for applying decorative coatings.

Plastics films are vacuum metalized for

packaging uses as well as for hot stamped foils.


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lecture - 15 decorating plastics

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