Die Casting Defects - Causes and Solutions
The fluidity can be increased by changing the chemical composition of the metal or by increasing the pouring temperature. Another possible cause is back pressure from improperly vented mold cavities. Misruns and cold shuts are closely related and both involve the material freezing before it completely fills the mold cavity. These types of defects are serious because the area surrounding the defect is significantly weaker than intended. Fluidity affects the minimum section thickness that can be cast, the maximum length of thin sections, fineness of feasibly cast details, and the accuracy of filling mold extremities.
There are various ways of measuring the fluidity of a material, although it usually involves using a standard mould shape and measuring the distance the material flows. Fluidity is affected by the composition of the material, freezing temperature or range, surface tension of oxide films, and, most importantly, the pouring temperature. The higher the pouring temperature, the greater the fluidity; however, excessive temperatures can be detrimental, leading to a reaction between the material and the mold; in casting processes that use a porous mould material the material may even penetrate the mould material.
The point at which the material cannot flow is called the coherency point. The point is difficult to predict in mold design because it is dependent on the solid fraction, the structure of the solidified particles, and the local shear strain rate of the fluid. Usually this value ranges from 0. An inclusion is a metal contamination of dross , if solid, or slag , if liquid.
These usually are impurities in the pour metal generally oxides , less frequently nitrides , carbides , or sulfides , material that is eroded from furnace or ladle linings, or contaminates from the mold. In the specific case of aluminium alloys, it is important to control the concentration of inclusions by measuring them in the liquid aluminium and taking actions to keep them to the required level.
There are a number of ways to reduce the concentration of inclusions. In order to reduce oxide formation the metal can be melted with a flux , in a vacuum , or in an inert atmosphere. Other ingredients can be added to the mixture to cause the dross to float to the top where it can be skimmed off before the metal is poured into the mold. If this is not practical, then a special ladle that pours the metal from the bottom can be used.
Another option is to install ceramic filters into the gating system. Otherwise swirl gates can be formed which swirl the liquid metal as it is poured in, forcing the lighter inclusions to the center and keeping them out of the casting.
There are two defects in this category: Hot tears, also known as hot cracking , [21] are failures in the casting that occur as the casting cools. This happens because the metal is weak when it is hot and the residual stresses in the material can cause the casting to fail as it cools. Proper mold design prevents this type of defect. Hot spots are areas on the surface of casting that become very hard because they cooled more quickly than the surrounding material. This type of defect can be avoided by proper cooling practices or by changing the chemical composition of the metal.
In die casting the most common defects are misruns and cold shuts. These defects can be caused by cold dies, low metal temperature, dirty metal, lack of venting, or too much lubricant. Other possible defects are gas porosity, shrinkage porosity, hot tears, and flow marks. Flow marks are marks left on the surface of the casting due to poor gating, sharp corners, or excessive lubricant.
aluminum die casting defects
A longitudinal facial crack is a specialized type of defect that only occurs in continuous casting processes. This defect is caused by uneven cooling, both primary cooling and secondary cooling , and includes molten steel qualities, such as the chemical composition being out of specification, cleanliness of the material, and homogeneity. Sand casting has many defects that can occur due to the mold failing. The mold usually fails because of one of two reasons: The first type is mold erosion , which is the wearing away of the mold as the liquid metal fills the mold.
This type of defect usually only occurs in sand castings because most other casting processes have more robust molds. The castings produced have rough spots and excess material. The molding sand becomes incorporated into the casting metal and decreases the ductility , fatigue strength , and fracture toughness of the casting. This can be caused by a sand with too little strength or a pouring velocity that is too fast. The pouring velocity can be reduced by redesigning the gating system to use larger runners or multiple gates.
This also occurs when the mold is not properly rammed. The second type of defect is metal penetration , which occurs when the liquid metal penetrates into the molding sand. This causes a rough surface finish. This is caused by sand particles which are too coarse, lack of mold wash , or pouring temperatures that are too high. If the pouring temperature is too high or a sand of low melting point is used then the sand can fuse to the casting. When this happens the surface of the casting produced has a brittle, glassy appearance.
What is Casting Defects - Types, Causes and Remedies? - Mechanical Booster
A run out occurs when the liquid metal leaks out of the mold because of a faulty mold or flask. Scab s are a thin layer of metal that sits proud of the casting. They are easy to remove and always reveal a buckle underneath, which is an indentation in the casting surface.
Rattail s are similar to buckles, except they are thin line indentations and not associated with scabs. Another similar defect is pulldown s , which are buckles that occur in the cope of sand castings. All of these defects are visual in nature and are no reason to scrap the workpiece.
A swell occurs when the mold wall gives way across a whole face, and is caused by an improperly rammed mold. Burn-on occurs when metallic oxides interact with impurities in silica sands. The result is sand particles embedded in the surface of the finished casting. This defect can be avoided by reducing the temperature of the liquid metal, by using a mold wash, and by using various additives in the sand mixture. From Wikipedia, the free encyclopedia.
Types of Chips in Metal Cutting. Mechanical Properties of Materials.
It is a type of surface defects and a line on the surface can be seen. When the molten metal enters into the mold from two gates and when these two streams of molten metal meet at a junction with low temperatures than they do not fuse with each other and solidifies creating a cold shut appear as line on the casting.
Navigation menu
It looks like a crack with round edge. When the molten metal solidifies before completely filling the mold cavity and leaves a space in the mold called as misrun. This defect is caused when the molten metal containing slag particles is poured in the mold cavity and it gets solidifies. The failure of casting in this case is looks like cracks and called as hot tears or hot cracking. Hot spot defects occur when an area on the casting cools more rapidly than the surrounding materials.
Hot spot are areas on the casting which is harder than the surrounding area. It is also called as hard spot. It is the holes created on the external surface or inside the casting. It occurs when loose sand washes into the mold cavity and fuses into the interior of the casting or rapid pouring of the molten metal. It is an external defect in which there is a number of small cavities in close proximity present in the metal casting. It is an accidental and unwanted deformation in the casting that happens during or after solidification. Due to this defect, the dimension of the final product changes.
Working With a professional diecast company to Avoid DieCasting Defects
This induces stresses in adjoining walls and result in warpage. A thin projection of metal, not considered as a part of casting is called as fins or fin. It is usually occurs at the parting of the mold or core section. Here we have discussed about what is casting defects — types, causes and remedies to eliminate it. If you find any query regarding this post feel free to ask. Your email address will not be published. June 18, Pankaj Mishra 0. April 8, Pankaj Mishra 0. Leave a Reply Cancel reply Your email address will not be published.