The document discusses mold and core coatings used in metal casting. It provides information on: 1) The physical and chemical effects that can occur between liquid metal and sand molds without a coating, such as metal penetration and sand burn-on. 2) The key characteristics an effective coating should possess, such as refractory properties and good adhesion. 3) The typical components of foundry coatings, including refractory fillers, liquid carriers, binders, and rheology control systems. 4) Common application methods for coatings like brushing, dipping, spraying, and overpouring. Equipment used in mixing and applying coatings is also discussed.

1. Mold and Core coating Constituents
Prepared by
Asfand shahid

3. The need for a coating
 When liquid metal is cast into a sand mold or against a core, there may be a
physical effect and a chemical reaction at the sand/metal interface. Either may
result in surface defects on the finished casting

4. Effect on
sand
Physical
effect
Metal
penetration
Sand
expansion
Chemical
effect
Burn on

5. Physical effect
Metal penetration
liquid metal penetrates the pores of the sand mould giving a rough surface to the casting
Dependence
Metallostatic pressure Dynamic pressure
penetration in lower part of casting more severe where the metal stream
impact on the mold surface
in high density metals iron or steel
Sand expansion
Stresses can form in bonded sand due to the thermal expansion this can lead to cracks

6. Chemical effect
Burn-on
A chemical reaction occurs between sand and metal. It occurs with
impure sand
some impurities, particularly alkalis, reduce the refractoriness of the sand some binders
mostly those based on sodium silicate which have poor refractoriness so that liquid
phases are produced at temperatures as low as 900°C.
Carbonaceous defects
Organic mold and core binders degrade at high metal pouring temperatures, forming
carbon bearing gases which can lead to carbon pick-up or surface pock marks due to
lustrous carbon.

7. Refractory coating
A refractory coating on the mold or core can reduce or eliminate metal
penetration and sand burn-on. For many years, foundries made their own
coatings, usually ‘blackings’, based on coke dust or graphite.
Ferrous and aluminum alloys show metal penetration, burn-on due to the high
casting temperature and high density.

8. Coating selection
An effective coating should have the following characteristics:
 Sufficiently refractory properties to resist the metal being poured.
 Good adhesion to the substrate.
 No blistering, cracking or scaling tendency on drying.
 Good suspension and remixing properties.
 Good stability in storage.
 Good covering power.
 Good application properties.

9. Choice of coating and form of supply
Applica
tion
method
Drying
method
Equipme
nt
required
Carrier
Mode
of
supply
Performa
nce
required

10. Types of coating
Powder coating
The attraction of a powder coating is that it costs less
to transport than a liquid, it is easy to store.
 The characteristics of a coating prepared from a
powder pre-mix are highly dependent upon the
mixing equipment used and the procedure
adopted.
 If simple paddle mixers are used, long mixing
times followed by ageing up to 24 hours may be
required to develop the correct suspension and
rheology.
 It is preferable to use special high shear mixers
to develop the liquid coating and avoid the need
for ageing
Cream and ready-for-use coating
 A ‘ready-for-use’ paint is the easiest to use.
So-called ready-for-use paints may also be
supplied as light creams or slurries which
require a small water addition to allow the
user to control the final consistency of the
coating to suit his particular application.

11. Components of a
coating
Foundry coatings consist of:
 Refractory filler
 Liquid carrier
 Binder
 Rheology control system

12. The refractory filler
 This is the most important constituent since its properties decide the efficiency
of the coating.
 The filler may be either a single material or a blend selected for specific
applications.
 Fillers are chosen for their particle size and shape, sintering point, melting point,
thermal conductivity, thermal expansion and reactivity towards the metal being
cast and the molding material on which it is being used.
 Zircon, being highly refractory, is used for the most demanding applications such
as steel or iron castings where section thickness is great.
 Graphite and coke are commonly used for lighter iron castings, often in
combination with zircon, silica and tar.

13. Liquid carrier
 The liquid carrier is the vehicle for the total coating composition and serves
 to transport the filler onto the sand substrate.
 It must be removed before casting takes place.
 The most commonly used carriers are water and isopropanol, although other
solvents such as methanol, ethanol, hydrocarbons and chlorinated hydrocarbons
have also been used.
 The use of water based coatings on cores and molds made with chemically
bonded sand can effect the strength of the bond, and may give rise to casting
defects due to surface friability of the core or mold.

14. • Fast air drying
• Obsolete due to
environmental
concern
• For large mold
• Dries faster than
water
• Removed by
burning
• RDT(Rapid drying
technology)
• Shortening drying
time
• Cheap
• Drying in oven
• Readily available
Water Foseco
Chlorinated
hydrocarbons
Isopropanol

15. Binder
 The function of the binder is to bond together the filler particles and to
 provide adhesion to the mold or core.
 The binder often interacts with the whole of the coating composition and
therefore cannot be considered in isolation.
 Silicate binders are particularly badly affected by water based coatings and spirit
based coatings should be used instead.
 Coldbox-with this method, sand is coated with one of several binders, such as
liquid sodium silicate or phenolic urethane, and catalyzed by a gas (such as
carbon dioxide or sulfur dioxide) passing through the sand.

16. The rheology control system
 This provides the suspension system that prevents the filler from separating out
during storage of the coating over extended periods.
 It ensures that the coating is homogeneous and ready for application with the
minimum of agitation.
 It also controls the flow properties of the coating and is designed to suit the
application method that is used.

17. Application methods for coatings
Brushing
Dipping
Swabbing
Spraying
Overpouring

18. Application methods
Dipping
 Submerging the core in the liquid coating
 It is the fastest method of application
 The core is dipped manually or mechanically
Dependence
 Consistency of the coating
 Rheological properties
 Uniform thickness of coating
Decrease in viscosity upon the application of
shear and regained by the removal of core
Brushing
 Simple technique One or more layered apply
 Allows brush marks to flow out after
application
 The coating can be of almost jelly-like
consistency in the can but will flow readily
when brushed
 The viscosity decreases slowly upon the action
of shear and regains quickly but not
immediately
 Extremely good suspension of the filler

19. Application methods
Swabbing
 Implement used has long
 Easier to penetrate with long blisters and apply
a thick smooth coating with minimum of
dripping
 less tendency to mold damage or residual
brush marks in the finished coating
 Slow process and require skilled operator to
achieve best result
Saprying
 Much faster technique than brushing but
penetration and coverage is not as good as
with brushing
 Sprayed coatings are of much lower viscosity
than brushing or dipping varieties
 Skill is required to ensure that deep pockets
and re-entrant angles are fully coated
 They are applied under pressure from a spray
gun delivering a spray of finely divided
droplets on to the surface to be coated

20. Application methods
Overpouring/Flow-coating
 It is particularly suitable for larger cores and molds
 it consists of a pumping system and a specially designed nozzle.
Process
 The coating is continuously pumped through the nozzle which directs it over the
core, the excess runs back into a reservoir to be recirculated through the nozzle
once more.
 The gel system should allow an even deposition of coating without the risk of
residual runs and tear marks.

21. Baumé
hydrometer
flow cup
Testing
coating
suspensions

22. Equipment for mixing and applying
coatings
 Dip tanks should be fitted with a paddle stirrer to maintain the coating in
suspension without excessive movement, which could affect the rheology of the
coating
 overpouring or flowcoating system consists of a method of pumping a
continuous flow of coating over the core, allowing the surplus to run off the core
to a catchment tray from which it is returned to the storage tank.
 Larger cores and molds are usually coated at a fixed station having a coating
storage tank of about 100 litre capacity allowing about 4 minutes uninterrupted
coating time.
 Small portable systems may be used, in which a portable catchment tray is
placed under the work and the coating pumped over the core through a coating
gun by displacement with low pressure compressed air.

23. Drying oven
 Cores coated with water based coatings must be thoroughly dried before casting.
 Warm air, infra-red and de-humidifying ovens may be used
 Good air circulation is needed to carry away the moisture
 Air temperatures up to 100°C may be used
 Large mold dried using portable warm air blowers, or a gas torch