Simple Solutions That Work! Issue 13

DEFECT PREVENTION 33 can impact coating deposit, coating penetration and the resultant barrier layer that is established at the mold metal interface. When dipping cores, it is important to maintain a consistent drain cycle. Many foundries have incorporated robotic dipping into the process where the casting mix will allow it. Operator training is critical where robotics are not possible. The elimination of tear drops, runs and drips, and heavy coating deposits will reduce the resulting casting defects associated with these issues. Baume, viscosity, and mixing controls are essential. Prevention measure should also be in place to minimize sand contamination in the coating dip tanks. This will result in sand inclusion defects and rough surfaces on the castings. The spray process presents different challenges. Properly adjusted spray equipment will help control the deposit on the core or mold surface. Most operators will adjust the equipment to their touch but the deposit must stay the same. Over atomization of the coating, too low a pump pressure, and spray distance can all have adverse effects on performance. In this application you are relying on the gun pressure, gun distance and coating consistency (Baume) to work the coating into the surface and form the desired barrier. Standardizing these factors is crucial to consistent casting results. Two of the main issues associated with brush application are inconsistent coating thickness and brush marks. Inconsistent coating thickness can result in potential metal penetration, erosion, and gas defects. Brush marks can result in poor surface finish and rough surfaces on the casting. Using a good quality brush or swab with proper viscosity coating will fix these issues. Also avoid re-brushing an area after the initial coat has matted but not completely dried. The final application method is flow coating. Just like in the previous applications, coating consistency and properties are important and will control how the coating flows over the core or mold surface. It is important to create a smooth flow from the wand tip. Coat from top to bottom and move at such a pace to get a good consistent deposit in one pass. If you have to go back over missed areas the deposit across the face you are coating will not be consistent. This can cause multiple casting issues at the mold metal interface: penetration, surface finish, core set issues, heat transfer and others. Sand contamination in catch basins is also an issue with this application technique resulting in inclusion type defects. Many variables can affect the application. Regardless of application type, best practice is to document the standard process for coating application. A standardized process with ensure consistent performance of your coating. KEY #5: PROPER DRYING OF COATING DEPOSIT All coatings have a carrier component that is designed to get the refractory component onto the surface and in between the sand gains. Once this is accomplished, the water or solvent carrier must be removed from the process by some type of drying method. There are time, temperature, and air movement components to this process and all are equally important. Drying oven temperatures must be controlled along with air flow through the ovens. Dwell time in the ovens adequate to remove all the water is a must. Timing of light off is important when using alcohol coatings. Lighting off too early can result in blistering and poor casting finish. Lighting off too late can result in residual alcohol left in the core/mold or too deep of coating penetration. Do not just assume that dry is dry and how you get there is not important. SUMMARY Refractory Coatings are one of the products that we utilize to produce quality castings. We do that by getting the right amount of properly prepared coating on the core or mold every time. Following these 5 keys will help us accomplish that task. • Selection of Coating • Coating Preparation • Quality Control • Proper Application Techniques • Proper Drying of Coating Deposit Better controlled processes lead to better understanding of what is changing in our foundries. Partnering with a quality coating supplier with open communication about issues will also be beneficial. This enables us to work together to address issues with sound judgement, factual data and proven methods to correct the casting defects as they arise - and they will! Control your process – don’t let it control you. Key Application Process Parameters Contact: STANLEY FOREHAND [email protected]