Common Defects in Investment Casting

Below are some of the common defects:

1. Gas hole

Gas hole is the hole that exists in the casting surface or internal area. It can be round, oval, or irregular shape, sometimes is an air mass made up of multiple gas holes. Internal gas holes is usually pear-shaped. Shape of choking hole is irregular, and the surface is rough. Gas nest is casting dents on the surface, the surface is much more smooth. Surface gas holes can be found by appearance inspection, however, subsurface gas holes can ony be tested after machining.

Causes:

• Preheating temperature of casting mould is too low, and cooling of liquid metal is too fast when it goes throught the pouring system.
• The exhaust design of mould is not good, so that gas can not flow out.
• Bad coating material, poor exhaust, even volatile or break down the gas itself.
• Holes and pits are exsited in the surface of mold cavity, when pouring liquid metal, the gas in the holes and pits are rapidly expanding, and compress gas liquid metal, thus to form choking hole.
• Surface of mold cavity is rust, and did not clean up.
• Raw material (sand core) is improperly stored, without preheating before use.
• Poor deoxidizer, improper dosage or improper operation, etc.

Prevention Methods:

• Mould should be fully preheat, the granularity of coatings (graphite) shoulds not be too thin, permeability needs to be good.
• Using tilt pouring way when casting.
• Raw materials should be stored in ventilated, dry place, and should be preheated when used.
• Choose deoxidizer (magnesium) with better deoxidization effect.
• Pouring temperature should not be exorbitant.

2. Shrinkage Cavity (Shrinkage)

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 proper designed.
• Poured riser design did not meet fully the effect of feeding.

Prevention Methods:

• Improve mold temperature.
• Adjust the thickness of coating layer, even paint spraying, and avoid local coating accumulation phenomenon when coating paint falls off or filling.
• Local heating 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 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 suitable pouring temperature.

3. Slag hole (flux slag or metal oxide slag)

Slag hole is clear or unclear hole in the casting. It is fully or partially filled by slag, and the shape is irregular. Small shape flux slag is not easy to be found, after removing slag, it will present smooth hole, general is distributed at the bottom area of the pouring position, around the runner section, or near the corner. Oxide slag is distributed like meshed shape in casting surface near the sprue, sometimes thin sheet, or irregular cloud shape with wrinkles, or form flake interlayer, or exist as flocculent shape in the inside area of castings.

Causes:

Slag hole is mainly caused by alloy smelting process and pouring process (including improper design of gating system), the mold itself will not cause slag hole, and the metal mould is one of the effective methods to prevent slag inclusion.

Prevention Methods:

• Correct gating system design, or use cast fiber filter.
• Using tilt pouring way.
• Choose the flux, and strictly control the quality.

4. Crack (thermal crack, cold crack)

The appearance of the crack is linear or irregular curve. The surface of thermal crack is dark grey or black due to strong oxidation, no metallic luster. The fracture surface of cold crack is clean, with metallic luster. General casting crack can be seen directly, however, inside crack can only be seen using other methods.Cracks are often associated with defects such as shrinkage, slag inclusion. They are mostly occurred in the inside of casting angle, thickness of junction, and hot section between poured riser and castings.

Causes:

Metal mould casting is easy to produce crack defects, because the metal mould 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, ect. may all cause casting crack. Beside when there is a crack in the mold itself, it will also be easy to cause crack.

Prevention Methods:

• Attention should be paid on the casting structure, make 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 to form too large internal stress.
• The working temperature of the metal mould should be paid attention to and adjust the angle of mold.

5. Cold Shut

Cold shut is a kind of slot or surface crack with 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 postion 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.

6. 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:

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

Prevention Methods:

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