Testing of porcelain enamel powder properties

 To maintain the reproducability of your enameling process, it is important to monitor and control all major process parameters.

With regard to the electrostatic powder enamel application department, it is necessary to keep your ambient conditions, equipment settings and powder properties within strict tolerances.

This web page focus on the powder properties of porcelain enamels, which may be characteristed by the following parameters :
  • Adhesion
  • Application rate
  • Fluidity
  • Particle size distribution
  • Surface resistivity
    •

    Underneath you will find an outline of the available testing methods to measure these powder parameters, as described in the international enamel literature and/or used by major powder suppliers.


    Adhesion

    Adhesion checks are most probably the best indicator of the condition of the powder.

    The adhesion of a powder is usually tested by applying a normal coating on a panel at standard settings. After a certain time interval, the panel is subjected to an impact of a controlled force, which is reproducible.

    Adhesion = ( powder weight after impact / powder weight before impact ) x 100 %

    According to the theory, there are two different methods :
  • STPA - Short Time Powder Adhesion
  • LTPA - Long Time Powder Adhesion
    •

    The only difference between the STPA and LTPA method is the duration of the time interval between powder application and impact. (1 minute versus 15 minutes)

    Test result Conclusion according to DEV
    STPA > 75 % Very safe for the enamelling shop
    75 % > STPA > 65 % With risk for the enameling shop
    STPA < 65 % Insufficient for the enameling shop

    However, please note that in order to get reproducable results, it is necessary to keep the following test parameters constant :
  • Film thickness of applied powder
  • Gun settings like tension, flowrate and atomizing air
  • Pressure, temperature and humidity of compressed air
  • Ambient conditions between application and impact
  • Force of impact
    •

    Sources :

  • DEZ Test Regulations, English Edition 1994, page 89.
  • PEI-501, 1995, page 8.

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    Application Rate

    The application rate of powder is generally used to determine the rate of material transfer.

    It may be measured by determining the gun output per unit of time and then using that value as a denominator for the amount of powder applied to a standard target at a given time.

    Application rate = ( grams on target / grams output from gun ) x 100 %

    Sources :

  • PEI-501, 1995, page 7.

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    Particle size distribution

    The gain size spectrum has a considerable influence on its electrostatic charging and aerodynamic characteristics.

    The share of coarse particles in a powder enamel can be determined by means of the Bayer test sieve with for example a screen aperture of 0.04 mm. (= 40 mu = 16900 mesh/cm2)

    The residue of powder (RP) in percentage by weight is a measure for the frequency of coarse particles.

    Since the powder particle size distribution is a "normal" distribution, the one point check is usually satisfactory for determining the acceptability of a powder.

    However, please note that the gain size distribution may be altered if the powder recovery at the application system is defective.
    Sources :

  • DEZ Test Regulations, English Edition 1994, page 87.
  • PEI-501, 1995, page 9.

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    Fluidity of powder

    The fluidity of a powder (its ability to get whirled up in a gas stream) can be measured with a Sames fluidimeter.

    The average outflow of powder in grams per sec are a good indication of a powder's fluidity.

    Fluidity Outflow in gr/sec
    Very good 4.0 - 3.5
    Good 3.5 - 3.0
    Fair 3.0 - 2.0
    Insufficient 2.0 - 0.0

    However, please note that in order to get reproducable results with this test, it is necessary to keep the following test parameters constant :
  • Quantity of powder used
  • Temperature and relative humidity of compressed air
  • Pressure of fluidisation air
  • Stirring time before executing the test
    •

    The flowability of a powder may also be defined as the product of the following equation :

    Flowability R = M x h1 / h0

    where :

    M is the weight of fluidized powder expelled from the side port in 30 seconds.
    h1 is the height of the powder bed in the fluidized state.
    h0 is the height of the powder bed in repose.

    A flowability of 140 to 350 is generally recommended.

    Sources :
  • DEZ Test Regulations, English Edition 1994, page 88.
  • Pemco Technical Information 10, American Edition 1987, page 6.


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    This document is made by Ronald Ditmer and last saved on September 12, 2007.

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