Platinum casting defects are often very difficult to characterize due to the intrinsic complexity of the investment casting process. All defects will fall into one or more of the established seven categories of casting defects. The proper identification of a particular defect is the prerequisite to correcting and controlling the quality of platinum castings.  The nature of a casting defect can only be determined by correctly categorizing the shape, appearance, location and dimensions of the defect.  Once appropriately classified, the possible causes can be examined and the corrective action can be taken.

Casting defects in a controlled process will generally fall into one of two categories.  Defects can be chronic or sporadic.  Sporadic defects are probably the most troublesome for casters. It is the purpose of this article to examine only sporadic platinum casting defects.

Defect Characterization
Two distinct journeys must be taken to correct sporadic defects. They are "the diagnostic journey from symptom to cause and the remedial journey from cause to remedy."  There is a temptation to attempt to diagnose a defect by the possible causes; but, an incorrect diagnosis of the root cause can lead to an incomplete or incorrect remedy of the problem. It is important to correctly identify the defect symptoms prior to assigning the cause to the problem. 

In general, a casting defect is defined as an observable and unplanned variation of a specification. The identity of a particular casting defect is based upon the specific shape, appearance, location and dimension or profile of the anomaly. The proper identification of a specific defect is the prerequisite to correcting and controlling the quality of castings.  There are only seven categories of casting defects, which have been established.  These defects are:

A. Metallic Projections
B. Cavities
C. Discontinuities
D. Defective Surface
E. Incomplete Casting
F. Incorrect Dimensions or Shape
G. Inclusions or Structural Anomalies

At this point the original paper explains these defects in detail.

Defect Analysis
Once the defect has been properly identified, all the causes must be examined in order to pinpoint the true root cause of the problem. It is generally true that "there is usually enough skill in a company to identify and diagnose the technical aspect of the main problems."  This should be a team effort that involves the quality techniques of brainstorming to uncover all possible contributions to the defect and the fishbone diagram to lead the way to corrective action.

Corrective Action
The correction of defects will vary depending upon the root cause. With regard to sporadic defects, "the cause and effect relationship is often simple and localized." The corrective action for these defects will either involve making some change in the process to avoid the defect; or, corrective action will involve controls to keep the process from deviating from its proven path again.

The observation of one defective piece does not inevitably imply that the process is out of control. It has been well established that "predictable performance is not necessarily the same as desirable performance." With all the process variables moving within their natural state of control, a defect can occur that can only be explained by a slight drift of a key control characteristic. If a control limit is marginal, i.e., sometimes results in a defect and sometimes not; then, the control limit will have to be made artificially tighter to disallow the potential for problems. 

If the cause of a defect is related to quasi-controlled variables such as gating, investment, firing or casting techniques; then, the ideal corrective action would be to improve the application of these methods for the type of patterns which cause the difficulty. For instance, if a design that has fine detail along with thick sections does not cast well with standard gating and casting parameters, amend the gating to accommodate the appropriate casting method.  After a few such occurrences, an established process is developed for complex geometry patterns.

Of course, the defect analysis may uncover that the special cause of the problem is due to an avoidable process error. If the defect was caused by violation of control limits or improper application of established process variables, the corrective action is to insure that the system will not allow such excursions to take place again. This can be done by:

1. Elimination: Redesigning the process so the task with the potential error is no longer necessary.
2. Replacement: Substituting a more reliable process for the employee.
3. Facilitation: Making the work easier to perform, e.g., color-coding, posted instructions, etc.
4. Detection: Detecting an error before further processing, e.g., sequential checklist, observer, software, etc.
5. Mitigation: Minimizing the effect of the error.

In any case, corrective action must include some documentation of all the important information derived from the identification and analysis of the defect. The course of action must be communicated to all process participants and the changes to correct the defect must be integrated immediately into the process.

In conclusion, defects observed in the output of the casting process need to be identified, analyzed and corrected.  It is unavoidable that a defect will occur even in a tightly controlled process. Preventing that defect from recurrence is the only way to ensure that a designer's vision will be realized in a casting. Platinum casters need to pursue the elimination of defects as an unrelenting detective pursues a suspect in order to unlock the secrets in the investment casting process. The mystery of sporadic defects must be solved to insure that a casting customer get a superior product and not repetitious excuses. The only way to attain this goal is with a well-controlled and monitored process.
 

V5N2

Characterization and Correction
of Casting Defects
Timothy L. Donohue
Dr. Helmut F. Frye, Ph.D.
TechForm -
Advanced Casting Technology, L.L.C.

This is an abbreviated version of the original work. For full technical details, please consult the original paper.