The Sampling Process
Now that you have selected a foundry and procured the pattern and core boxes necessary to make your casting, what comes next? You are at a particularly important stage in the manufacturing process—the production of your first sample castings. As an industry standard, a sample production run (or runs) needs to be produced, evaluated and approved before serial production can begin. To be sure the samples will accurately represent what you will be receiving in production, consider the following points.
Don’t shorten the time allotted for the sampling process. Projects often need to conform to a timeline, and you may encounter the need to condense the timeframe required to bring a part into production. However, cutting the time allotted for producing sample castings should be avoided. Think about the basic premise for running samples: a short run of parts is being produced for conformance evaluation. There is often the tendency to have foundry personnel walk the samples through each step of the process to make sure that everything is done correctly and without delay. Doing so does not represent what could potentially happen in normal production. The sample castings should go through the same day-to-day environment and variation that a normal production run of castings would encounter so potential problems can be uncovered early in the process and eliminated.
Specify which evaluations are to be performed. You need to determine what testing and evaluation need to be performed by the foundry (or by an authorized third-party inspection service) to satisfy your engineering requirements. What are the typical evaluations? For castings, the most common sample inspections are metallurgical, soundness and dimensional (often called the Big 3).
Metallurgical evaluations will include chemical composition, mechanical properties (tensile, yield, elongation and hardness), and microstructure. Additional testing may be required depending on the application of your casting and the material specification or heat treat requirement. An example of an additional test could be Charpy impact testing for a structural casting.
Soundness evaluations determine what levels of internal porosity are present in the casting. ASTM standards have five levels of porosity ranging from Level 1 (no internal defects––nuclear grade parts) to Level 5 (gross internal defects). Your casting team will need to determine what level is acceptable to meet your casting design criteria. X-ray radiography is the preferred method to determine internal porosity levels as the films and reader sheets create a permanent record. However, saw cut sections at regular intervals through the casting can be acceptable. Surface porosity and visual defects also should be evaluated at this time for conformance to specifications. Keep in mind sometimes the gating or risering may need to be slightly adjusted to produce a sound casting. Using solidification and flow modeling greatly improves the first pass success rate in producing sound, defect-free castings.
Dimensional inspection validates the casting is within the drawing specifications. You need to determine what needs to be checked—anything from just the critical dimensions up to a full layout of every dimension shown on the drawing. Also, specify what type of dimensional reports you want to receive from the foundry; scribed castings with documented dimensions, full CMM reports and 3D scans are all possibilities.
Process information is vital. All the completed forms and paperwork from the testing discussed above are important and need to be submitted for approval, but do not forget that process information should be part of the sample submission. Most OEMs have adopted the AIAG PPAP (Production Part Approval Process) specification. Control plans, process FMEAs, and process flow diagrams are just a few of the PPAP documents available. Your engineering team will need to determine what level of PPAP will be required and which specific documentation you want to review and approve.
The steps taken during the sampling process are an important part of the validation of the overall production process and should not be viewed as a hinderance to getting production castings. Clear and concise communication between the buyer and the foundry at this stage is critical for successful casting development and production. CS
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