Enamel slip properties

The consistent liquid enamel slip properties is a critical parameter at any automated wet application process.

Unfortunately enamel rheology is known to change easily under the influence of particle size, specific weight, temperature, milling additives and/or pretreatment chemicals.

Failure to maintain its properties within measurable tolerances, may result in inconsistent enamel layer thickness and eventually even enamel defects.

Do you really believe that you can “feel it” with your fingers as demonstrated on the picture below, or do you rather prefer a more precise testing method ?!

Measuring enamel slip characteristics by hand
Old fashion way
Rotational Viscometer with temperature sensor
Rotational Viscometer with temperature sensor

Wet enamel application

Porcelain enamel may be applied via a variety of wet application methods, such as dipping, flow-coating, slushing and spraying.

Originally application of porcelain enamel was done manually, as shown at the pictures below.

Manual wet enamel spraying of pan supportsManual enamel dipping of washing machine drums

With increasing labor cost and production volumes, manual application was replaced by automatic application. In order to reduce enameling cost and/or improve the coating quality, more advanced applications systems, such as electrostatic wet spraying, robot spraying and electrophoresis, where introduced in the 50’s and 60’s.

Old fashioned electrostatic wet enamel spraying system

Nowadays wet enamel is typically used for small production runs (of special colors), special enamels and/or when the product design isn’t suitable for electrostatic dry application.

Well-know liquid enamel application equipment producers include : Binks, Devilbiss, ECCO, EIC, Eisenmann, Fisher, Sames, VGT.

DTC may assist you to evaluate, which application method and/or equipment is most suitable for your products, and if required also help you with sourcing / procuring the appropriate machines.

Related topics :

  • Wet enamel preparation
  • Enamel slip testing
  • Enamel consumption calculation
  • Transfer efficiency & material utilization calculation
  • Pumps for enamel slip
  • Powder enamel application

Typical mill additions to enamels and their function

Mill addition Function
Aerosil Set-up
Aluminium oxide Matting agent and to increase heat resistance of ground coat
Aluminium hydroxide Equivalent to aluminium oxide
Aluminium powder Increase heat resistance in ground coat
Antimony trioxide Increase adherence of ground coat
Barium chloride Set-up in cover coat
Barium carbonate Set-up in cover coat
Bentonite Set-up
Borax Set-up in ground coat
Boric acid Set-up
Cadmium oxide Stabilize Cd-Se red enamels
Clay Provide bubble structure
Cupric chloride Self lustre enamel 1118
EK-1000 For continuous clean enamel
Feldspar For ground coat
Gumst Increase filmstrength
Magnesium chloride Set-up cover coat
Magnesium nitrate to increase suspension and to prevent fishscales
Opacifier UV Opacifier
Potash Set-up
Potassium choride Set-up cover coat
Potassium nitrate Oxidizing agent for aluminium enamels
Quartz to increase firing range and matting agent
Silicic acid Set-up
Silver sand For heat resistant ground coat
Soda ash Set-up in ground coat
Sodium aluminate Set-up in cover coat
Sodium nitrite Set-up in ground coat
Sodium pyrophosphate To decrease set-up
Sodium silico fluoride Set-up cover coat
Tinoxide Opacifier
Titanium dioxide Opacifier
Trisodium phosphate To prevent rusting in cast iron ground coat
Urea To prevent hairlines in cover coat
Zirconite Opacifier
Zircon oxide Opacifier

Wet enamel preparation & milling

Prior to the actual application, porcelain enamel slip need to be prepared in accordance with the application requirements, such as fineness, set and specific gravity.

The traditional wet enamel preparation process (“milling”) consists of the following seven sequential steps :

  1. weighting of various frits & additions in accordance with the milling formulation
  2. loading ball mills
  3. adding water
  4. wet milling
  5. discharging ball mills
  6. sieving & magnetic screening of slip
  7. storage

Continue reading “Wet enamel preparation & milling”