Which defects occur due to similar causes
misrun and cold shut
misrun and penetration misrun and mould crack
cold shut and mould crack
Answers
Answer:
If you’re looking to brush up on information about what the various types of casting defects are, what causes them, and how to prevent them, this guide is here to give you the rundown on all of these areas. There are a lot of things that can go wrong with a casting, but it can be difficult to tell from the results what the problem is that caused it. In this guide, we’re breaking down six different areas of casting defects to help you pinpoint, fix, and avoid issues with your castings. Here’s what this guide will cover:
Gas Porosity
Pinholes
Blowholes
Open Holes
Shrinkage Defects
Open
Closed
Warping
Mold Material Defects
Cuts and Washes
Swells
Drops
Metal Penetration
Rat Tails
Fusion
Run Out
Pouring Metal Defects
Cold Shot
Cold Shut
Misrun
Metallurgical Defects
Hot Tears
Hot Spots
Slag inclusion
Casting Shape Defects
Mismatches
Flash
Gas Porosity
When cast metal solidifies in a mold, it can’t hold as much gas as it does in liquid form, so it releases it. That is part of the reason molds are permeable, to allow gas to escape. Several factors can cause impermeability, which can result in gas bubbles getting trapped inside the metal. These bubbles can include:
Pinholes
Also known as porosities, pinholes are smaller holes in the upper part of a casting. They’re usually in groups near or at the casting’s surface and visible to the naked eye.
Blowholes
Blowholes are larger holes that can appear in the inside of a cast piece. Invisible to the naked eye, interior blowholes are detectable by x-ray, harmonic, ultrasonic, or magnetic analysis. Blisters, a variant of blowholes, are thinly covered shallow holes.
Open Holes
A type of blowhole, open holes appear at the piece’s surface. These defects are caused by air getting trapped as metal is poured into the mold. There is also a shallow variant of open holes called a scar.
What Causes Gas Porosity?
Holes like these tend to show up in badly vented areas or where too much dampness is present. More specifically, they can be caused by sand with too much moisture, wet ladles, insufficiently gasified sand, or the by the molten metal’s overly high temperature, which leads to overabsorption of gas.
How to Prevent Gas Porosity
Avoid pinholes, blowholes, and open holes by making sure the molding sand is dry and permeable. It should be noted that the coarser sand is, the more permeable it is. Even using sand that is too fine can prevent optimal permeability. Additionally, sand molds that have been rammed too much lose their permeability, so it’s important not to overdo it. Make sure that molds and cores are dry before use and stored under dry conditions as well, and be sure that there’s enough venting in the molds to allow gases to escape.
You should also use good melting practices by melting the material in a vacuum, around low-solubility gases, or under a flux, which prevents air from touching the molten metal. If nothing else works, try pouring the molten metal at a lower temperature during the casting to keep it from absorbing as much gas.
Shrinkage Defects
Shrinkage defects appear because metal alloys shrink as they cool. It is normal for an alloy piece to shrink as it solidifies (which should be included in calculations when designing the mold). However, defects occur when the metal shrinks unevenly, causing it to either distort the shape of the final product or create interior holes. This can also stress the metal.
Open Shrinkage Defects
Open shrinkage shows up on the surface of a cast product either as a dip (also known as a caved surface) or a hole (also known as a pipe). When metal shrinks unevenly, it draws air inside the mold in that area to create these types of defects.
Closed Shrinkage Defects
Closed shrinkage appears in the form of holes inside of a casting, generally where a part of the liquid metal was hotter than the rest of the material. It can appear in macro or micro form. Micro shrinkage, also called shrinkage porosity, looks to the naked eye like jagged marks or lines. The holes, which look angular, can only be seen with a microscope.
Explanation:
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