INTRODUCTION 1f handbook die casting proceses edwadrd j virnacik 2003

158 DEFECTS IN HIGH PRESSURE CASTING PROCESSES

11.2.1 Surface Defects

Defects visually identified on the exterior of the component are grouped into the category of surface defects. In some cases, a surface defect does not render the component unfit for use unless aesthetics are a necessary requirement. Discarding a component because it ‘‘looks bad’’ when aesthetics is not a requirement does nothing but create waste. Cold shuts also known as cold laps or knit lines are imper- fections visible on the surface of the casting due to unsatisfactory fusion of partially solidified metal. During filling of the die, the convergence of two nearly solidified fill fronts may not knit prop- erly, resulting in this defect. Although cold shuts are characterized by their appearance on the surface of a component, this defect often extends into the bulk metal, creating a weak spot. Surface contamination commonly identified by discoloration may occur in die casting processes. Often this staining is caused by manufacturing lubricants. Altering the type of lubricants often resolves this issue. Cracks often occur in die cast components. In some cases, cracks may be caused by cold shuts or residual stresses that form in the component during solidification and cooling. More often than not, cracks are caused by poor material handling techniques. Due to the high production rates, components are ejected from the die near their solidification temperature. While in this fragile state, care must be taken in handling the components. Cracks may occur due to uneven ejection from the die or due to an impact if they are dropped immediately after ejection. Drags are the result of mechanical interference between the component and the die cavity during ejection. Severe drags may also cause distortion or cracking of the component as well. Proper die design and maintenance are a must to avoid this issue. Flash is the undesired formation of thin metal sections. Flash most often forms along the parting line and between other inde- pendent die components. In most cases, flash is the result of high metal temperatures, high metal intensification pressures during so- lidification, dimensional variations in the die, and general die wear. Regular die maintenance may eliminate or minimize the occurrence of flash. However, flash is a chronic problem in all die casting technologies. Secondary operations such as trimming or shot blasting are typically used to remove this material.