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Green Sand Metalcasting Foundry News

Article Review: "Avoiding (Green Sand Foundry) Coating Related Defects" - Part 2

Posted by Hill and Griffith Company on Jul 9, 2019 12:30:08 PM

Refractory coatings exist to improve casting quality, but when not properly applied, they can cause unwanted characteristics in the final casting.

By Jay Morrison, Carpenter Brothers (Mequon, Wisconsin)

Applying a refractory coating to molds and cores can be a useful step to improve surface finish, control the heat transfer characteristics and thus the microstructure in the casting, improve core venting and prevent certain defects in the casting. As with any process step, proper techniques in applying the coating are paramount in achieving the desired effects and avoiding the cause of new quality issues. Metalcasters can apply coatings to their sand molds in four main ways, all with varying advantages and disadvantages.

Green Sand Foundry Coatings 2

Brushing/swabbing: Brushing or swabbing coating on molds provides good, controlled coverage. This method makes it easy to apply multiple coats and can be used with multiple types of coatings. The method works the coating into the refractory surface well, which can be helpful when applying to corners or walls. However, it is a slow process that is labor-intensive and the application can vary depend­ing on the operator.

Spraying: 'This method is faster than brushing or swab­bing and covers a large area readily. It also doesn't leave brush marks. However, like brushing, the application of the coating can vary by operator. The operator may miss some areas with the spray or find it difficult to properly coat deep pockets. Over-spraying may also occur. Finally, spraying a solvent-based coating releases fugitive emissions.

Flow coating: Flow coating is fast, less labor-inten­sive then brushing or spraying, and easily automated. It applies good coverage and is optimal for large cores and molds. However, the system used to apply the coating brings a level of equipment upkeep that requires regular maintenance.

Dipping: Some foundries opt to dip their cores or molds in the coating. This process provides good coating coverage and can be mechanized. Automated dipping gives an even, consistent coating and is well suited for high production. Manual dipping is labor-intensive and slower.

No matter the method, after the refractory coating is applied, a period of drying must occur. Coatings need to dry because the water and solvents expand to many times the volume when heated. The methods of drying used depend on whether the coating is water or solvent-based. Some solvent-based coatings are designed to be lit with a flame, which then activates the elements within the coating.

Metalcasters have a wide choice of useful refractory coatings, but they are designed with different chemistries and formulas that will all require specific process parameters to do their job correctly. This includes the optimum thick­ness of application, drying time, viscosity, and dispersion of particles within the solution. Metalcasters should consult the manufacturer for recommendations on each coatings' use and follow them closely.

Below are 6 coating-related casting defects

1. Teardrop


Why is it Happening?

  • A drop of dried coating has adhered to the coated surface leading to an inclusion or sur­face dimple.
  • More often occurs with dipping and flow coating.
  • Too much coating is being applied in an area that it builds up more than it should.

Where's the fix?

  • Check the application param­eters are correct and adhered to.
  • Check the consistency of the coatings-does it align with the manufacturer's suggested fluid density?
  • If the application method is dipping, examine the dip and drain cycle for areas the coat­ing might be building up on the core or mold walls.

2. Improper Blow-Off


Why is it Happening?

  • When coatings are dried with an air current (blow­off), sometimes the blown air removes too much coat­ing, leaving a bare spot or areas with a layer of coating that is too thin. This can lead to burn-in, burn-on or penetration
  • The velocity of the air cur­rent could be too high, or the current is too close or poorly aimed.

Where's the fix?

  • Reduce the blow-off velocity.
  • Redirect the blow-off current.
  • Consider eliminating the blow-off step with a longer drip cycle.

3. Syneresis, Water-Break


Why is it Happening?

  • A thinly coated streak on the core surface causes localized burn-on, burn-in or penetration.
  • It can be caused by low operating solids in the coating, poor mixing, or poor
  • In water-based coat­ings, biological growth can taint the product.

Where's the fix?

  • Verify the viscosity, mud-balance, and fluid density of the coating align with the manufacturer's recom­mendations.
  • Check the coating is being mixed thoroughly before application.
  • Make sure the system applying the coating is clean and sterilized to avoid biological growth.
  • Many manufacturers now ship coatings in tote systems that can then be hooked up to the customer's pump system for flow coating to avoid the potential for biological growth.

4. Blows 


Why is it Happening?

  • Smoothed rounded irregular depressions can appear on the casting surface when the coating is not sufficiently dried before metal is poured into the mold.
  • They can also occur where there is poor venting or a core is coated, inserted into a mold, and the mold is closed with insuf­ficient venting.
  • Sometimes the wrong coating permeability can lead to blows. Permeability is affected by the refractory in the coating and the thickness of the coating.

Where's the fix?

  • Check the drying process and make sure proper pro­cedures and parameters such as drying time are in place.
  • Add more venting.
  • Verify the optimum coating formula is being used for the application.

5. Metal Penetration and Veining


Why is it Happening?

  • Inadequate coating penetra­tion in the mold or core has occurred, or inadequate thickness of the coating has been applied so a strong enough barrier between the mold and the metal is not occurring.
  • The mold metal encapsu­lates the sand grains and they adhere to the casting surface.
  • An irregular line of metal adheres to the casting sur­face (veining).

Where's the fix?

  • Check the viscosity and balance of solids in the solution.
  • Increase the coating thickness.
  • Select a special refractory blend coating.

6. Spalling


Why is it Happening?

  • The refractory coating has popped off the mold or core surface due to radiant heat.
  • The coating is too thick or poorly design, so its bond to the mold or core surface is inadequate.
  • After the coating comes off the mold or core surface, the flakes float to the cope surface and then travel throughout the mold, resulting in an irregular depression on the cope surface accompanied by an inclusion somewhere else on the casting.

Where's the fix?

  • Confirm the proper coating is being used. The refractory in the coating can have an effect. For instance, a zircon wash can handle higher heat than a graphite wash.
  • Verify application parameters.

(Originally printed in the April 2019 article of MODERN CASTING Magazine. "Refractory coatings exist to improve casting quality. But when not properly applied, they can cause unwanted character­istics in the final casting. Six main coating-relating defects, their likely causes and corrections are explained here." For the full article, here's a link.)

Green Sand Foundry News from MODERN CASTING Magazine

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Tags: American Foundry Society, Green Sand Casting Products, Green Sand Foundry Supply, Foundry Supply, Green Sand Release Agents, Foundry Supplies, Modern Casting Magazine

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