Defects in High Pressure Die Casting Processes
Excerpt from the instructional book by Edward J. Vinarcik
Casting processes are inherently complex due to the phase transformation from liquid to solid metal, which creates all geometric features as well as material properties. With such an intricate process, many potential defects may result. Potential defects related ot high integriy die casting are presented in this chapter. Defects encountered in conventional die casting will be introduced but will not be discussed in detail, as several texts are available that examine these issues. Defects unique to high integrity die casting processes will be discussed in greater detail.
11.2. Conventional Die Casting Defects
Many potential defects commonly found in conventional die casting can also occur in hight ingegrity die casting. often these defects are avoidable if basic guidelines are followed related to component design, equipment selection, die design, process design, raw material quality, process control, die maintenance, equipment upkeep, and material handling. Conventional die casting defects can be divided into three categories: surface defects, internal defects, and dimensional defects. Each category will be discussed separately below.
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 imperections 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 properly, 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 problem.
Cracks often occur in die casting 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 independent die components. In most cases, flash is the result of high metal temperatures, high metal intensification pressures during solidification, 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.
Laminations are a type of cold shut that occurs at the surface of the component. Due to the turbulent and complex flow patterns within a die during metal injection, a portion of the die may receive a small quantity of metal that freezes quickly to the die surface, forming a thin layer of skin. As the die continues to fill, a bond does not form between this think layer of solidified skin and the subsquent metal filling the cavity. The result is a thin, partially attached lamination.
A short shot is the incomplete filling of the die cavity caused by an undersized volume of metal being metered into the metal injection system. Such defects are easy to detect by examining the ejected component(s) and the runner system. Without enough metal in the injection system, the biscuit at the start of the runner system may be absent or is very small. In most cases, the component is not fully formed.
Sinks are surface defects caused by localized solidification shrinkage beneath the surface of the casting. Although this defect is common in conventional and vacuum die casting, this problem may be alleviated by using squeeze casting and semi-solid metalworking.
Repetitive thermal cycling of dies from the injection of metal followed by the application of lubricants results in find cracking of the die face. This phenomenon is referred to as heat checking. Once these cracks are present, metal will fill these fine cracks creating veins or fins on the manufactured component. Although die maintenance may delay the onset of heat checks, die cavities must be replaced to correct this issue. If aesthetics is not a concern, veins from heat checking may not render a component unfit for use.
Lubricants are typically applied to the surface of the die to avoid interaction between the die surface and the metal injected into the die. However, interactions may still occur resulting a metallurgical bond between the die and the injected metal. This phenomenon is known as soldering and occurs most often when injecting aluminum into ferrous dies.