6 parameters you must control for lost foam coating

Keywords：
  • lost foam coating
As we all know, as one of the most important raw materials for lost foam casting, coating is closely related to the final casting quality. At present, in the actual production process, the factory often only uses Baume to control the quality of coating, but the effect is not satisfactory. The problems of burning sand and slag holes are still one of the main reasons for the defects of lost foam casting. Coating thickness as a key parameter, can better translate part of the performance of coating, but due to poor consistency, at the same time, there is a fluctuation of artificial measurement, in the actual production, it is often difficult to control. So, by what way can we better control the quality of coating, this article listed 6 must control parameters, hope to help you in the actual production process

1. Gas Permeability

Coating permeability controls the back pressure at the metal front by allowing the gas generated by the pattern degradation to escape the casting cavity. Coating permeability is controlled by the size and number of pores in the coating. The pore size and number of pores is fixed by the coating supplier through his selection of ingredients used to formulate the coating. Coating permeability must be controlled within a narrow range to assure the metal velocity is consistently within the ‘window’ required for minimum casting defects. To achieve consistency coating permeability should be monitored on a schedule consisting of at least once every shift (8 hours) or any time additions are made to the coating tank. Benefits of coating permeability control can be seen in Figure 1. 

Figure 1-Effect of Coating Permeability on Weekly Scrap Rate


Not only does the basic coating permeability control the metal velocity but the coating thickness also has a dramatic effect on the ‘system permeability’ (Figure 2). In addition, the sand permeability and pattern permeability have an effect on the ‘system permeability’. 

Figure 2 - Effect of Coating Thickness on Coating Air Flow

2. Liquid Absorption

Liquid absorption capability of coatings closely follows gas permeability since the pore structure controls both properties. Normally coatings with high gas permeability also have high liquid absorption. Although this property is believed to be important in the filling process of Lost Foam Castings current data does not indicate a strong correlation between casting defects and liquid absorption. The current physical model of metal filling illustrates that liquid island are trapped between the coating and metal after the metal front has passed. These islands of liquid are heated by the metal until they are vaporized with the generated gases pass through the coating. Hence hot gas permeability should control the exodus of liquid components.

3. Thermal Conductivity

Coating thermal conductivity controls the rate of heat removal from the molten metal during the filling process. If the conductivity is high the metal will not flow through long pattern sections before it solidifies, resulting in a misrun. Thermal conductivity of coatings falls into two categories 1.) high and 2.) low. High conductivity coatings are made from granular particles of refractories such as Silica while low conductivity coatings are made from platelet refractories such as Mica (Figure 3). A 40% reduction in thermal conductivity can be achieved by choosing a Mica coating over a Silica coating, however the Mica coatings tend to be lower in gas permeability. Usually, a mixture of particulate and platelet refractories is used to customize coating permeability and thermal conductivity.
A secondary controlling factor in the ‘system’ thermal conductivity is coating thickness. A thick coating provides more insulation; however, the thicker coating also effects the ‘system’ permeability. Again, each casting has a velocity ‘window’ that produces a minimum of casting defects and coating thickness will dramatically affect the metal velocity.

Figure 3 - Thermal Conductivity of Lost Foam Coatings


Note: these coatings tend to fall into two groups. SK400 and 510 contain Mica while SK27 and 533 are granular silica.

4. Viscosity

Coating viscosity controls the coating thickness (Figure 4). For this reason, coating viscosity should be determined on a scheduled basis. Coating viscosity in the dip tank can change for several reasons 1.) mixing procedures 2.) water evaporation 3.) water removal by poorly fused patterns 4.) bacterial growth. Procedures for measuring viscosity of coatings have been developed and should be strictly followed. 

Figure 4 - Effect of Coating Viscosity on Deposited Thickness


5.Temperature

Since coating viscosity is controlled by temperature it is important to maintain the coating dip tank within a temperature range of 75 F to 85 F range. This will provide consistency in the coating viscosity.

6. Solids Content

Coating’s suppliers use Solids Content of coatings as an indicator of coating rheology. Small changes in Solids Content will not affect coating gas permeability; however, the deposited amount of coating on the pattern will be influenced. Normally when Solids Content increases the coating viscosity will increase. If the reverse trend occurs the coating is usually infected and should be treated or replaced. Solids Content measurements should be a scheduled quality control measure.

Coating Property Measurement Procedures

Permeability
Coating permeabilities should be measured at least once each shift or following an addition to tank.
Ⅰ. Prepare coating strips using a draw bar as shown in Figure 5. Coating should be supplied directly from coating dip tank. Record the coating temperature.
Ⅱ. Dry coating strips using the same procedure as patterns are dried.
Ⅲ. Cut coating disks as shown in Figure 6. Measure coating disk thickness.
Ⅳ. Place coating disk in disk holder, adjust the pressure and read the flowmeter (Figure 7).
Ⅴ. Calculate the air flow rate through the coating.

Figure 5 - Coating Strip Preparation


Figure 6- Coating Disk Preparation

 


Figure 7 - Coating permeability Apparatus


Viscosity and Viscosity Index (Figure 8)
Coating viscosity should be determined on a scheduled basis or following any addition. Viscosity is normally measured using a Brookfield Viscometer with a #3 spindle and rotated at 20 rpm for 30 seconds. Coating sample should be drawn from the dip tank and immediately measured. A temperature measurement should accompany the sample withdrawal. Coating container should be large enough to allow at least one inch from any surface to the spindle. Viscosity index is determined by measuring coating viscosity at two or three (preferred) spindle speeds (shear rate). Consult your coating supplier for the correct ratio of viscosity between the shear rates. 
Viscosity index is a positive indicator of the ability of a coating to gel and remain on the pattern.

Figure 8- The Influence of Shear Rate on Coating Viscosity


Coating Viscosity is usually measured at a spindle speed of 20 rpm. Amore precise quality control procedure includes viscosity measurements at 20, 10 and 5 rpm. Viscosity index is the ratio of viscosity at different spindle speeds. Consult your coating supplier for the correct ratio (usually about 2.0). The viscosity index is an indicator of how much coating will be deposited on the pattern and how well the coating will remain on the pattern (drip loss). 

Solids Content

This procedure includes weighing a sample of coating from the dip tank, drying the sample and weighing the dried sample.    
% Solids = (Dry Weight / Wet Weight) * 100

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