Exam 2 - Coating

Question 1

Coating (in general)

Answer 1

• the application of one substance onto another

Question 2

Coatings are differentiated from other processes, in that…

Answer 2

the coating remains on the substrate

Question 3

Coatings are applied to materials for numerous reasons (4)

Answer 3

– Insulation
– Protection
– Aesthetic Appeal
– Lubrication

Question 4

Coating Processes (6)

Answer 4

• Extrusion Coating
• Calender Coating
• Powder Coating
• Knife or Roller Coating
• Dip Coating
• Spray Coating

Question 5

Extrusion Coating

Answer 5

• the application of a thin film of molten plastic to a substrate

Question 6

Types of extrusion coating substrates

Answer 6

• Substrate can be paper, fabric, or metal foil
– Material is usually forced between a nip with the substrate and cooled before winding or slitting
• Substrate can also be wire
– In this case, the process is similar to pipe extrusion only the liquid plastic flows around the wire.

Question 7

Extrusion Coating Advantages

Answer 7

– Multiple layers of plastic can be placed on substrate
– No need for solvents
– Can vary thickness of applied material
– Can achieve uniform thickness on wire and cable
– Can apply cellular coatings to substrate

Question 8

Extrusion Coating Disadvantages

Answer 8

– Extrudates are hot melts

– Equipment is expensive.

Question 9

Calendar Coating process

Answer 9

Hot film is squeezed onto a
substrate by the pressure of
heated gauging rollers.

Question 10

What materials can be calendar coated

Answer 10

• Paper, plastics and textiles

may be coated

Question 11

What characteristics are enhanced by calendar coating

Answer 11

Enhanced Characteristics 
– Appearance 
– Strength 
– Moisture resistance 
– Sealing ability

Question 12

Calendar Coating Advantages

Answer 12

– High-speed continuous process
– Precise thickness control
– Pressure and heat sensitive hot melts can be used
– Coatings are stress free
– Short runs are economical

Question 13

Calendar Coating Disadvantages

Answer 13

– Equipment costs are high

– Additional equipment needed for flat stock.

Question 14

Powder Coating

Answer 14

Coating which is relatively hard, abrasion resistant (depending on the specification) and tough.

Question 15

Characteristics of Powder Coating

Answer 15

• Thin powder coatings can be bent (recommended for exterior applications)
• The choice of colors and finishes is almost limitless, given enough time and money.
• Coatings can be applied over a wide range of
thicknesses.
• Ability to color match from batch to batch..

Question 16

Fluidized Bed Coating Process (just be familiar with it)

Answer 16

• Plastic powder Is fluidized by gently blowing air into the bottom of the tank. Once the air goes on, the bed rises about 30%, and looks like its boiling at the surface.
• The part to be coated is preheated in an oven, then
dipped into the fluidized bed. As powder particles come in contact with the hot part, they melt on it. The coating builds up quickly and uniformly.
• After the part is withdrawn, here may be enough heat left to completely fuse the coating and melt the surface smooth. If not, the part is returned to the oven for a very short time.
• Fluid bed coatings usually are in the thickness range of .007” - .020”. Thicker coatings require multiple dips.
• Since there is no sharply defined surface to a fluid
bed, parts cannot be dipped to a line. If a partial coating is required, trimming or masking is necessary. .

Question 17

Fluidized Bed Coating Advantages

Answer 17

– Uniform thickness
– Thermoplastics and thermosets can be used
– No solvents required

Question 18

Fluidized Bed Coating Disadvantages

Answer 18

– Substrate must be heated above melt temperature
– Primer may be needed
– Thin coatings are hard to control
– Hard to automate
– Post cure is needed
– Surface finish may be uneven.

Question 19

Electrostatic Bed Coating

Answer 19

• Fine cloud of negatively charged particles is sprayed and deposited on a positively charged object
• Covers all conductive surfaces of the object
• Cured by heat.

Question 20

Electrostatic Bed Coating Advantages

Answer 20

– Thin and even coats are easily applied
– No preheating usually needed
– Process can be automated
– Reduced overspray

Question 21

Electrostatic Bed Coating Disavantages

Answer 21

– Thick coatings may require preheating
– Small openings and tight angles hard to coat
– May need dust recovery system
– Only ionic resins can be used
– Post cure is needed
– Substrates may require special preparation.

Question 22

Electrostatic Powder Gun Coating

Answer 22

• The powder is applied to a part that is at ground potential (dry painting).

Question 23

Electrostatic Powder Gun Coating Process

Answer 23

Before the powder is sent to the gun it is fluidized.
– to separate the individual grains of powder and improve the electrostatic charge that can be applied to the powder so that the powder flows more easily to the gun
• Because the powder particles are electrostatically charged, the powder wraps around to the back of the part as it passes by towards the air off take system.
• The powder will remain attached to the part as long as some of the charge remains on the powder.
• To obtain the final coating, the powder coated items are placed in an oven and heated to 160 to 210 degrees C.
• The final coating is continuous and will vary from
high gloss to flat matte depending on the powder..

Question 24

Electrostatic Powder Gun Coating Advantages

Answer 24

– Thin, even coats are easily applied
– No preheating necessary
– Process is easily automated
– Short runs are economical
– Lower equipment costs than electrostatic-bed coating

Question 25

Electrostatic Powder Gun Coating Disadvantages

Answer 25

– Thick coatings require heating of substrate
– Only ionic resins or plastics can be used
– Post cure is needed
– High labor costs
– Thickness is hard to control.

Question 26

Knife or Roller Coating (types) (3)

Answer 26

Air Knife Coating, Knife Over Roll Coating, Reverse Roll Coating

Question 27

Air Knife Coating

Answer 27

The coating is applied to the substrate and the excess is
‘blown off’ by a powerful jet from the air knife.
• This procedure is typically used for aqueous coatings
and is particularly noisy..

Question 28

Knife Over Roll Coating

Answer 28

• A coating is applied to a substrate which passes through a ‘gap’ between a ‘knife’ and a support roller.
• As the coating and substrate pass through, the excess is scraped off by the knife.
• This process can be used for high viscosity coatings and very high coat weights, such as plastisols and rubber coatings..

Question 29

Reverse Roll Coating

Answer 29

• The coating material is 
measured onto the applicator 
roller by precision setting of the 
gap between the upper 
metering roller and the 
application roller below it. 
• The coating is 'wiped' off the 
application roller by the 
substrate as it passes around 
the support roller at the bottom. 
• The diagram illustrates a 3-roll 
reverse roll coating process, 
although 4-roll versions are 
common..

Question 30

Knife or Roller Coating Advantages

Answer 30

– High speed continuous process
– Excellent thickness control
– Thick coatings are possible

Question 31

Knife or Roller Coating Disadvantages

Answer 31

– Equipment set-up time is costly

– Short runs are not economically feasible.

Question 32

Dip Coating

Answer 32

Applying a coating to a part by simple dipping in a bath

Question 33

Dip Coating Process

Answer 33

• Metal parts are preheated, dipped, and then post heated.
• During the dip, heat in the parts gels the surrounding
material. The hotter the parts and the longer the dip, the
thicker the gelled coating.
• Prior to plastic coating, the object to be coated can be
“primed“ to make the coating adhere to the part.
• Parts can have strippable coatings to protect them
during shipping..

Question 34

Spray Coating

Answer 34

Dispersions, solvent solutions or molten powders are

atomized by air, gas or the pressure of the solution itself

Question 35

Spray Coating Process

Answer 35

Flame coating applies finely ground powders through a

specially designed heated nozzle

Question 36

Spray Coating Advantages

Answer 36

– Low equipment costs; adaptable process

Question 37

Spray Coating Disadvantages

Answer 37

Thickness is difficult to control; high labor costs.