Exam 2 - Coating Flashcards
Coating (in general)
- the application of one substance onto another
Coatings are differentiated from other processes, in that…
the coating remains on the substrate
Coatings are applied to materials for numerous reasons (4)
– Insulation
– Protection
– Aesthetic Appeal
– Lubrication
Coating Processes (6)
- Extrusion Coating
- Calender Coating
- Powder Coating
- Knife or Roller Coating
- Dip Coating
- Spray Coating
Extrusion Coating
- the application of a thin film of molten plastic to a substrate
Types of extrusion coating substrates
• 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.
Extrusion Coating Advantages
– 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
Extrusion Coating Disadvantages
– Extrudates are hot melts
– Equipment is expensive.
Calendar Coating process
Hot film is squeezed onto a
substrate by the pressure of
heated gauging rollers.
What materials can be calendar coated
• Paper, plastics and textiles
may be coated
What characteristics are enhanced by calendar coating
Enhanced Characteristics – Appearance – Strength – Moisture resistance – Sealing ability
Calendar Coating Advantages
– High-speed continuous process – Precise thickness control – Pressure and heat sensitive hot melts can be used – Coatings are stress free – Short runs are economical
Calendar Coating Disadvantages
– Equipment costs are high
– Additional equipment needed for flat stock.
Powder Coating
Coating which is relatively hard, abrasion resistant (depending on the specification) and tough.
Characteristics of Powder Coating
• 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..
Fluidized Bed Coating Process (just be familiar with it)
• 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. .
Fluidized Bed Coating Advantages
– Uniform thickness
– Thermoplastics and thermosets can be used
– No solvents required
Fluidized Bed Coating Disadvantages
– 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.
Electrostatic Bed Coating
- 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.
Electrostatic Bed Coating Advantages
– Thin and even coats are easily applied
– No preheating usually needed
– Process can be automated
– Reduced overspray
Electrostatic Bed Coating Disavantages
– 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.
Electrostatic Powder Gun Coating
• The powder is applied to a part that is at ground potential (dry painting).
Electrostatic Powder Gun Coating Process
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..
Electrostatic Powder Gun Coating Advantages
– Thin, even coats are easily applied – No preheating necessary – Process is easily automated – Short runs are economical – Lower equipment costs than electrostatic-bed coating
Electrostatic Powder Gun Coating Disadvantages
– 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.
Knife or Roller Coating (types) (3)
Air Knife Coating, Knife Over Roll Coating, Reverse Roll Coating
Air Knife Coating
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..
Knife Over Roll Coating
- 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..
Reverse Roll Coating
• 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..
Knife or Roller Coating Advantages
– High speed continuous process
– Excellent thickness control
– Thick coatings are possible
Knife or Roller Coating Disadvantages
– Equipment set-up time is costly
– Short runs are not economically feasible.
Dip Coating
Applying a coating to a part by simple dipping in a bath
Dip Coating Process
• 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..
Spray Coating
Dispersions, solvent solutions or molten powders are
atomized by air, gas or the pressure of the solution itself
Spray Coating Process
Flame coating applies finely ground powders through a
specially designed heated nozzle
Spray Coating Advantages
– Low equipment costs; adaptable process
Spray Coating Disadvantages
Thickness is difficult to control; high labor costs.
