TCI Application & Troubleshooting Guide

Chapter Eleven: Powder Curing Process

     
Baking Needs for Powder Coatings

Thermoplastic Powders:

  • Enough heat to liquefy and smooth out coating film

Thermoset Powders:

  • Sufficient heat to liquefy film
  • adequate time at the specified bake temperature to develop full design properties.

Although the bake/cure process generally follows after the coating is applied, there are instances when it’s preferable to heat the substrate first.

  • With cast iron / aluminum, preheating allows trapped gases to escape from the porous metal surfaces and reduces the tendency for blisters to form in the film.
  • When the mass of the part is sufficient to allow residual heat to adequately cure it.
  • If higher than normal film thickness (greater than 6 mils) or fast film deposition is required.
 
Factors Affecting Cure
  • Powder chemistry
  • Type of oven
  • Metal thickness
  • Temperature / voltage / wavelength
  • Bake time
  • Oven efficiency
  • Air velocity
Balancing time vs. temperature is important in the powder coating curing process

Recommended cure times are based on time at METAL TEMPERATURE.



Stages of Cure Development

Melt point:

  • Powder particles begin changing from a solid to semi-liquid state.

Flow stage:

  • Powder is fully liquefied, reaching lowest viscosity, allowing film to smooth out

Cross linking stage:

  • Sufficient, sustained heat triggers large scale reaction within film, initial steps to total chemical/physical change of product

Gel stage:

  • When sufficient crosslinking has occurred, for solidification of the film from a liquid to a solid

Cure development:

  • The final, and most critical stage when baking results in reaction of majority of crosslinking sites and development of full design properties



Typical Cure Development
% Cure Properties Typical cure development of powder coatings - powder coating troubleshooting guide
 
Cure Time (minutes)



Importance of Metal Thickness
Metal Temperature (°F) Importance of metal thickness when powder coating - powder coating troubleshooting guide
 
Oven Time (minutes)



Bake Oven Designs and Energy Consumption

 

  • Convection
  • Infrared
  • Ultraviolet
  • Electron Beam
  • Induction
Bake oven designs for curing powder coatings, energy consumption required for curing powder coatings

 

IR Curing Sources

  • Reflected energy: energy that bounces back.
  • Absorbed energy: energy absorbed by the coating (usually by the pigment).
  • Transmitted energy: energy that passes through the film where the substrate either absorbs or reflects the energy.

Different wave lengths have different curing properties to be considered.

  • Short wave: 0.76 to 2.3 micron wave length with a source temperature of 2000° to 5000° F.
  • Medium wave: 2.3 to 3.3 microns with a source temperature of 860° to 2000° F.
  • Long wave: 3.3 to 1,000 microns with a source temperature of 100° to 860° F.

Organic coatings with C-H and O-H bonds have a peak absorption in the 2.2 to 3.3 micron wave length range.
 

 

The Frequency Spectrum


The frequency spectrum - powder coating troubleshooting guide

The Frequency Spectrum


The frequency spectrum - powder coating troubleshooting guideThe frequency spectrum - powder coating troubleshooting guide

Optimum Infrared Wave Lengths


Optimum infrared wavelengths for powder coating - powder coating troubleshooting guide

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