Good slurry adhesion during the application of the primary coat of a ceramic shell is critical so for achieving excellent surface finish of precision investment castings it is important that Residual mould release should be washed off properly or can lead to casting defects such as lifting or buckling of the prime coat, and subsequent bulging on the surface of castings. Consistency and cleanliness of wax pattern surfaces can be achieved through good pattern wash practices and maintenance of the solvent.
Types of Pattern Wash Which Investment Casters Use.
Current foundry practices for cleaning and etching wax patterns, for the removal of silicone oil or other release agents before the primary ceramic coat is applied, include the use of:
- The use of high flash point petroleum based hydrocarbon and citrus-based solvents.
- Detergent solutions.
- A mixture of both.
(1) Petroleum – based hydrocarbon and citrus-based solvents
Pattern washes of this type have replaced the dangerous low flash point solvents such as mineral spirits, methyl ethyl ketone, acetone, toluene, xylene, and chlorinated solvents, although, still pose a potential harm to operators.
Some pattern wash solventsact as etchants on the surface of a wax pattern because of complex mixtures containing other chemicals. Now a day’s formulations are water soluble, diluteable with deionized water (for better economy) and recyclable.
The solubility of polymers is based on chemical bonds which are present in the backbone chain & its compatibility with the solvents. The Hildebrand solubility parameter is commonly used to evaluate polymer solubility. The closer the value of the parameters, the more likely dissolution will occur.
(2) Detergent / Detergent Emulsions
Foundries are gradually moving to be a safe workplace, so the use of hydrocarbon pattern washes has declined over the past 20 years. That is a good sign, now a solution of soap and water are viewed as a less risky option, even though its efficacy is less than a hydrocarbon solvent.
(3)Mixture of both
This type of pattern wash is generally known for overcoming health and safety concerns of straight solvents and low effectiveness of detergents. Usually comprising a proprietary mixture of light oil solvents and dissolved detergents.
The wax patterns are immersed in the pattern wash, the parts are cleaned in water and the detergent rapidly emulsifies the solvent, dissolved silicone oil, and grime in the manner described in above.
Composite pattern washes reduce the issues of harmful emissionwhile enhancing the effectiveness of a detergent – based system.