casting failures and errors

what are some casting failures and errors?

1. rough casting

2. large nodule

3. small nodule

4. fins

5. incomplete casting

6. suck-back porosity

7. back-pressure porosity

8. marginal discrepancy

9. inclusion porosity

what causes rough castings?

• excessive use of the surfactant (wax surface tension reducer) or investing was done before complete dryness of the surfactant

• increase water/powder ratio of investment

• excessive burnout temperature

what causes large nodules?

• air trapped during investment procedure

• excessive vibration

what causes small nodules?

• inadequate vacuum during investing

• improper brush technique of investing

• lack of surfactant

what causes fins?

they are caused by cracks in the investment that have been filled with molten metals due to:

• improperly situated pattern (too close to the periphery of the casting ring)

• weak mix of investment (high water to powder ratio)

• too rapid heating

• dropped mold

what causes incomplete casting?

• inadequate heating of the metal

• incomplete wax elimination

• excessive cooling or freezing of the mold

• insufficient casting force

• not enough metal

what causes suck-back porosity?

occurs if the metal in the sprue solidifies before the metal in the mold, and can happen when:

• improper pattern position

• narrow, long sprue

what causes back-pressure porosity?

large concave depression on the inner side of the casting that is caused by inability of the air in the mold to escape through the pores of the investment

what causes marginal discrepancy?

• wax pattern distortion during removal of wax pattern

• uneven expansion of the mold

what causes inclusion porosity?

voids in the casting, especially in the margin area, may be caused by debris trapped in the mold, usually a particle of the investment undetected