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.
Warping
Warping can happen either during the metal’s solidification or afterward, changing the casting’s dimensions and shape. This stresses the metal and causes curving, especially in large and flat sections of castings.
What Causes Shrinkage Defects?
Shrinkage defects show up when the liquid metal in the mold is not all the same temperature. There are two possible causes for this: either the metal being poured into the mold is too hot or the metal is solidifying unevenly. Uneven solidification often happens when mold design ignores the rule of directional solidification. This rule states that the thinnest pieces of the molded shape should solidify first, and the thickest pieces solidify last, to ensure shrinkage doesn’t alter or damage the piece.
How to Prevent Shrinkage Defects
Problems with continuous flow at the right temperature (which contributes to uneven solidification) can be lessened by ensuring a continuous, even supply of liquid metal into the molds. This can be accomplished by using a runner and gate system with risers to supply the molten metal, which involves channels for the metal to flow through into the mold (the running system) and reservoirs of liquid metal on top of the mold (also known as risers) to fill in where the metal shrinks.
Additionally, molds can be given padding, which widens the ends of narrow pieces attached to thicker parts, as this helps ensure the thicker sections get enough liquid metal before the thin part solidifies. Some manufacturers also place cooling ribs, cooling coils, or chills inside the molds to disperse heat faster. Pouring the metal at a lower temperature may also help.
Warping specifically can be avoided by using heat treatment for iron alloys, which removes leftover stress. For aluminum alloys, straightening the casting between the quenching and aging processes avoids warpage. It can also prevent warping to ensure large areas have more of a ribbed shape, like corrugation, to make a piece cool more evenly.
Mold Material Defects
As suggested by the name, mold material defects are caused by the mold material, but they can also be caused by mold design problems. This makes up the largest category of casting defects, with seven basic types.
Cuts and Washes
Cuts and washes are caused when the liquid metal erodes away part of the mold when pouring in and the sand doesn’t have enough heat strength to resist it. Washes and cuts show up as low bulges along a surface of the finished product, tilted toward one end where the molten metal had more force as it flowed in.
Swells
Swells show up as a swollen area on the finished piece. These defects happen when the mold is too soft, so that the weight of the liquid metal pushes the sand outward.
Drops
Drops show up as irregular bumps on the tops of castings. They’re caused when the mold is weak and sand falls into the liquid metal. Drops also make the metal surface dirty.
Runout
Runout happens when the mold leaks, leaving an inadequate amount of metal to form the desired casting.
Fusion
Fusion shows up as a glassy looking crust on the surface of a cast piece. It happens when some of the sand in the mold melts and fuses with the casting.
Metal Penetration
When sand grains are both loose and large, molten metal can penetrate into the mold, creating a rough surface in the casting.
Rat Tails
Rat tails, also known as veins, are irregular lines along a casting’s surface caused when the heat of the molten metal makes the sand expand. Buckles are more extreme versions of rat tails.
What Causes Mold Material Defects?
As noted, causes can vary with the defect produced. However, most defects are caused either by molds that are too soft which need more ramming, or by pouring in metal when it’s still too hot.
How to Prevent Mold Material Defects
Ensuring molds receive enough ramming to hold the molten metal is an effective way to cut down on all of these problems, as well as making sure the metal being poured isn’t too hot. However, if that doesn’t work there are other methods of fixing these issues that vary from defect to defect.
? Cuts and washes might need redesigned gate systems or more binders added to the sand.
? Swells can be stopped by lessening the water content of the sand in the mold.
? Drops can be avoided by reinforcing mold projections with nails or gaggers.
? Runout happens with worn out molds that need replacements, or badly designed molds.
? Fusion can be avoided by ensuring the sand in the mold is resistant to the temperature of the molten metal.
? Metal penetration can be fixed by ensuring the insides are properly coated with mold wash and sand grains are sufficiently small and impermeable to molten metal.
? Rat tails can be stopped by redesigning the mold to have smaller flat surfaces or ensuring the mold is soft enough to expand properly from the heat.
Pouring Metal Defects
These defects appear when temperatures are too low during the pouring process. They fall into three categories:
Cold Shot
Cold shot is balls or drops loosely attached to the casting, left over from splatters of molten metal as it was being poured. Since they’re smaller, the splatters cool before the rest of the casting and become embedded or attached.
Cold Shut
Cold shuts occur when metal flows into a mold from two or more points, but it’s too cold to merge into a seamless piece. That’s when it creates a crack through the middle with rounded edges, which becomes a weak spot in the casting.
Misruns
Misruns, the close relatives of cold shuts, happen when the metal is so cool it solidifies before it can fill the entire mold. Misruns show up as castings with parts missing.
What Causes Pouring Metal Defects?
Defects from pouring the metal all stem from the overly cold temperature of the metal once it reaches the inside of the mold. These defects can be caused by simply pouring in metal when it’s not hot enough. However, even with metal originally poured at the optimum temperature, defects can be caused by poor gating systems that allow the metal to cool off too much before it gets into the mold, or by mold sections that are too thin. Thin sections can solidify and block off other mold sections due to their thin size.
How to Prevent Pouring Metal Defects
Pouring metal defects can, for the most part, be avoided by ensuring the molten metal stays warm enough to completely conform to the inside of the mold. The gating system can be redesigned to transport the molten metal faster, or to avoid sections that are thin enough to solidify before the metal can fill larger cavities beyond them. If that’s not feasible, the pouring temperature can simply be increased. Increasing the mold’s gas permeability can also make the gases evacuate faster, so that the metal has less time to cool while still flowing into the mold. Cold shot specifically can be caused by careless pouring, so ensuring the metal is poured smoothly can also help with this problem.
Metallurgical Defects
Metallurgical defects show up when there are problems in the metal of a casting. There are three types, which include:
Slag Inclusion
Slag inclusion happens when metal isn’t properly cleared of slag before being poured. It simply means the finished casting will have impurities or foreign material embedded in it. When the inclusion is dirt, castings will have a honeycomb or spongy look.
Hot Tears
Hot tears, also known as hot cracks, show up when the cooling metal contracts. In this state, when the metal is still weak, residual stress in the metal or poor mold design can cause the metal to pull apart, resulting in branching, irregular cracks. Sometimes these are difficult to see with the naked eye.
Hot Spots
Also known as hard spots, these defects occur when certain parts of a casting cool off faster than the areas around them, creating harder parts of the casting. These can wear out tools and interfere in the machining process.
What Causes Metallurgical Defects?
Metallurgical defects are caused by different factors depending on the defect.
? Slag inclusion happens before the metal makes it to the mold, when the metal isn’t purified properly or too much of the top of the metal (where the slag is) is included in the pour. When dirt or other foreign material apart from slag shows up, it can also be a sign that the molds weren’t clean.
? Hot tears are generally caused by badly designed molds, which don’t allow the cast part to shrink in all directions. However, they can also be caused when the metal doesn’t have enough tensile strength, the excess heat of the poured metal makes it shrink more than the mold is calculated to allow for, or the cooling is uneven, creating internal stresses. Hot tears can also be caused by cores or a mold that doesn’t deform properly in heat.
? Hot spots appear when improper cooling happens, thanks to the design of a mold, or when a metal’s chemical composition is off.
How to Prevent Metallurgical Defects
With hot spots and hot tears, the mold is the key. The chances of both can be greatly reduced by using well-designed molds that evenly cool the casting and that deform the proper way in hot conditions. Alternatively for hot spots, changing the chemical composition may help to solve the problem.
Slag must be removed before pouring. This can be done by adding elements to the liquid to make slag float to the top where it can be removed, or by using a ladle that pours metal from its bottom. Slag can also be kept out of castings by putting a ceramic filter into the gating system.
Casting Shape Defects
Defects in the shape of a casting appear as either mismatches or flash. Shape defects are some of the easiest to fix, since they generally involve readjusting existing molds.
Mismatches
Mismatches, also known as shifts, are when different parts of a casting come out misaligned with each other, usually horizontally. Similarly, core shift (also known as vertical displacement) happens with misaligned cores.
Flash
Also called casting fins or burrs, flash shows up as extra material attached to the casting, usually as a thin sheet that forms where parts of a mold meet. Flash is one of the most common casting defects.
What Causes Casting Shape Defects?
Mismatches happen when the upper and lower parts of a mold aren’t properly lined up before the metal is poured, or the flask (which contains the mold) is misaligned. Core shift, on the other hand, can happen when box or pattern dowel pins are loose or inaccurate. Flash comes from a mold that isn’t weighed down or clamped properly. It forms when there are gaps in the core or mold.
How to Prevent Casting Shape Defects
To prevent casting shape defects, check to make sure the plate pattern mounting and alignment are the same. This can help to see if there are inaccuracies in the pins. Sometimes reassembling the mold can fix the problem. Meanwhile minor flash on finished castings can be gotten rid of through breaking it off and filing it down, although this can get expensive.
ABOUT KEVA
Keva Casting is a professional casting company that focuses on Construction, Machinery, Automobile, Hardware, Pipeline and many other industries.
We provide top quality products with the best service based on our complete production chain including our own mold design, advanced production techniques, and full management of the supply chain. Keva Casting has two factories located in Zhucheng, China's Shandong province.
We use "shell molding", which is a new technique that has been developed in recent years.This new technique has simplified production procedures and reduced air pollution compared with traditional sand casting and investment casting. Shell making and core making are faster to ensure production in enormous quantities. The dimensions and surface finishing are satisfactory for all kinds of industrial requirements. Besides supplying our own formwork & scaffolding systems, our casting products also supply OEM services in various industries.