Simple Solutions That Work! Issue 13

DEFECT PREVENTION 45 Continued on next page • Varying from published casting procedures for the casting • Accidental A good method for preventing defects is to: • Identify the defect/state the problem • Get the facts • Research for missing facts • Test a trial solution • Document and communicate the findings • Develop a solution/take action • Document and communicate the results This process allows foundry engineers to use critical analysis to determine the cause and a solution for the defect. Defect prevention is not just the responsibility of the foundry engineer. Prevention activities should be planned into the responsibilities of each person in the casting process. Identify the defect/state the problem – A correct, concise, complete statement of the defect/ problem is mandatory for reducing the defect occurrence. For example, part number 123 has a consistent misrun in cavity two. Get the facts – The facts or data should come from the job process documentation and production logs. Always ask Where? When? How? How often? Why? Who? Data acquisition software is the preferred method for getting the facts as it eliminates human error. At minimum the following variables should be documented: • Metal temperature • Die temperature • Die shut time • Die open time • Total cycle time • Tilt speed • Hydrogen level in metal • Mold coating thickness • Alloy composition • Metal cleanliness Research for missing facts – look for areas that are not in the production log or in the molders head. Quite often the machine operator knows what caused the defect. Test a trial solution – many foundry engineers start the defect reduction process at this step and attempt to solve the problem without knowing the exact reason for the casting defect. Only change one casting parameter at a time. If the foundry engineer changes two or more parameters of the process and the defect is eliminated one cannot be sure which of the changes had the desired effect. Restate the problem/Take action – Once you have done your research and tested a trial solution it is possible to restate the problem in a way that will lead to a solution. Some foundry engineers skip all the preceding steps and skip directly to take action. This can be very expensive. Making a change in a process is the last step in process control, not the first. Remember, process control is an engineering discipline that deals with the mechanisms and algorithms for maintaining the output of the casting process within a desired range. The foundry engineer must communicate to the casting buyer what the capabilities of the permanent mold process are. They must both understand in advance what defects are acceptable and what justifies rejection. Methodology for process control: • Understand the process – before attempting to control the casting process the foundry engineer must understand the process and how it works. • Identify operating parameters – once the process is understood, operating parameters (see list above) and other variables specific to the process must be identified for its control. • Identify hazardous conditions – tilt pour permanent mold casting machines move in many axes and at extremely high pressure. A thorough risk assessment must be a part of the process design. • Identify measurables (see list above) • Identify points of measurement – once the measurables are identified, it is important to locate where they will be measured so that the system can be properly