Investment Casting Defect Analysis and Remedial Measures

Since investment casting involves a complicated process, there’s no shortage of opportunities for some things to go wrong. When certain undesirable irregularities in a metal casting process result in defects, the manufacturer can decide whether these defects are tolerable, repairable, or need to be eliminated altogether. However, before a decision is to be made, the nature of the defect must be clearly understood. 

Sand Hole

Sand hole is formed in the casting surface or internal, and is relatively regular. The shapes are consistent with the shape of the sand. When investment castings are taken out of the mold, we can see sand grains inlaid in the casting surface.

Causes:

• The surface strength of the sand core is not good, burning, or not completely cured.
• The size of the sand core is not consistent with the outer mold or is crushed when clamping the mold.
• The mold is polluted by graphite water and sand.
• The sand is rushed into the cavity.

Prevention Methods:

• Making sand core according to the process district.
• Keep the size of the sand core consistent with the outer mold.
• Timely clean graphite water.
• Clean the sand in the mold cavity when putting sand core.

Gas Cavity

There are obvious or inconspicuous holes with the smooth inner surfaces on the casting.

Causes:

• Poor air permeability of the mold shell makes it too late for the gas in the cavity to be discharged during pouring.
• The insufficient firing of the shell shape results in insufficient removal of mold material residues and gas-producing materials in the shell material.
• The metal liquid gas content is too high, causing poor deoxidation.
• The setting of the pouring system is unreasonable, causing human gas to be involved in pouring.

Prevention Methods:

• Improve the air permeability of the shell and add vents if necessary.
• Fully fired shell.
• Improve deoxidation methods.
• Improve the gating system.

Shrinkage Cavity

Shrinkage cavity is the surface rough hole in the casting surface or internal, slight shrinkage cavity is many scattered small shrinkage cavity, namely shrinkage. The grains of shrinkage cavity are big and thick. Often occur in the casting sprue, riser root, thick area, the connecting area between thin & wall thickness, ect.

Causes:

• Control of mold working temperature did not meet the requirements of directional solidification.Improper coating
• selection, the control of the coating layer thickness control in different areas is bad.
• The position of the investment casting part in the mold is not properly designed.
• Poured riser design did not meet fully the effect of feeding.

Prevention Methods:

• Improve mold temperature.
• Adjust the thickness of the coating layer, even paint spraying, and avoid the local coating accumulation phenomenon when coating paint falls off or fills.
• Local heating of the mold, or local heat preservation using thermal insulation material.
• Design the heat sink in the mold, or the cooling speed accelerate the cooling speed in the local area through water or else spray water outside the mold.
• Place detachable handling chill blocks in turn inside the cavity, to avoid sufficient cooling of blocks when continuous production.
• Design pressure device on the riser of mold.
• Accurately design the gating system, and choose a suitable pouring temperature.

Cracks (hot cracks, cold cracks)

The appearance of the crack is a straight line or an irregular curve. The surface of the hot crack is strongly oxidized to dark gray or black and has no metallic luster. The surface of the cold crack is clean and has a metallic luster. Generally, the outer crack of the casting can be seen directly, while the inner crack can only be seen by other methods.

Causes:

• Metal mold casting it easy to produce crack defects because the metal mold does not have deformability itself. High cooling speed will be easy to increase the casting internal stress. Too large or small pouring angle, too thin coating thickness, etc. may all cause casting crack. Besides, when there is a crack in the mold itself, it will also be easy to cause a crack.

Prevention Methods:

• Attention should be paid to the casting structure, and make the uniform transition of non-uniform wall thickness. And adopt the appropriate fillet size.
• Adjust the thickness of the coating, try to achieve the required cooling rate of each casting area, and avoid forming too large internal stress.
• The working temperature of the metal mold should be paid attention to and adjust the angle of the mold.

Sticky Sand

There is a sticky sand layer near the runner or inside the casting, and there are raised burrs or pits after sand blowing.

Causes:

• The impurity content of the refractory powder used in the coating surface layer is too high, and these impurities react with the molten metal to form a low melting point eutectic.
• The pouring temperature is too high, especially the steel grades containing Al, Ti and other elements, which causes a chemical reaction between the molten metal and the silica.
• The setting of the gating system is unreasonable, and a large amount of molten metal flows through the inner runner, causing local overheating.

Prevention Methods:

• Use fused steel jade or zircon sand instead of silica sand, and try not to use water glass as a binder.
• Appropriately lower the pouring temperature.
• Add an inner runner to adjust the heat balance and reduce local overheating.

Cold Shut

Cold shut is a kind of slot or surface crack with a round edge, the middle is separated by a scale, so it is not fully integrated. When the cold shot is serious, it will become “owe cast”. Cold shut often appears in the top wall of the casting, thin horizontal or vertical plane, or in the joint area of thin & thick wall thickness.

Causes:

• The venting design of metal mold is not reasonable.
• Too low working temperature.
• Bad quality or improper operation of coating.
• Improper riser position design
• Pouring speed is too slow.

Prevention Methods:

• Correct design the runner and exhaust system.
• Proper thicken the coating layer for thin wall thickness investment castings.
• Improve mold working temperature.
• Using tilt pouring way.
• Using mechanical vibration metal mould for casting.

Slag

There is slag or other debris in the inside or surface of the casting. Internal inclusions need to be detected by X-ray or magnetic inspection.

Causes:

• 1. The slag material is too thin and is not cleaned before tapping.
• 2. The debris on the tapping trough of the crucible was not cleaned up before tapping and was brought into the molten metal.
• 3. When casting, the slag retention is not good, and the slag enters the cavity with the molten metal.

Prevention Methods:

• Adjust the composition of the slag material before tapping to increase the viscosity of the slag material and make it easier to remove the slag.
• Clean up the debris in the front trough of the steel to avoid bringing impurities into the molten metal in the ladle.
• Before casting, the molten steel should be properly sedated to facilitate the slag floating.
• Take a teapot pouring ladle with a slag stopper or a ceramic filter.

Burrs

Burrs is one of the most frequently occurring casting defects and also a common injection molding defect. burrs is any unwanted and excess material attached to a cast. It’s typically a thin sheet of metal that forms at the parting faces. burrs is a waste material that turns into dross after being re-melted.

Causes:

• Burrs on the casting surface are due to a crack or gap on the core surface. Insufficient weight on the mold or improper clamping of the flask can lead to a gap.

Prevention Methods:

• Remedy this issue by reassembling the mold and cores. There should be enough weight on the top part of the mold so that the two parts fit together tightly.
• Burrs can vary from minor to very serious. If it’s not too serious, manufacturers can remove flash by breaking it off with a hammer or pliers and filing it down to the parting line. However, this can be a costly process.

Warping

Warping is an unwanted casting deformity that can occur over time. It results in a change in the dimensions of the final product. It can happen during or after solidification.

Causes:

• Warping is typically a result of different rates of solidification of different sections, which causes stress in adjoining walls. Large and flat sections are more prone to warping.

Prevention Methods:

• Normalizing heat treatment can remove residual stress in iron casting. A straightening between quench and aging processes might also be required for aluminum casting.

Conclusion

In this article, we introduce several common casting defects in investment casting and how to prevent them. We hope this content is helpful to you.

Besser is a professional investment casting company. We have a strict quality inspection system to ensure that defect-free investment castings are delivered to your hands. If you need to know about investment casting services, please contact us.