6 Common Defects in Castings: Causes and Prevention Methods

Guide

I believe you have encountered these common casting defects. Why are there defects and how to prevent them? This article explains very clearly.

Stomata (Bubbles, Cavities, Air Locks)

Stomata

Features:

Stomata are holes that exist on the surface or inside the surface, and are round, oval, or irregular. Sometimes multiple pores form an air mass, which is generally pear-shaped under the surface. The shape of the countersinks is irregular and the surface is rough. Air pockets are recessed into the surface of the casting, and the surface is relatively smooth. The appearance of the open hole can be found, and the subcutaneous air holes can only be found after machining.

Reasons for Formation:

  • The mold preheating temperature is too low, and the liquid metal cools too quickly when passing through the pouring system.
  • The mold exhaust design is bad, and the gas cannot be discharged smoothly.
  • 3. The coating is not good, the exhaust gas itself is not good, and even the gas itself is volatilized or decomposed.
  • There are holes and pits on the surface of the mold cavity. After the liquid metal is injected, the gas in the holes and pits rapidly expands and compresses the liquid metal to form countersinks.
  • The surface of the mold cavity is rusted and not cleaned.
  • The raw materials (sand cores) are improperly stored and have not been preheated before use.
  • Poor deoxidizer, or insufficient dosage or improper operation.

Prevention Methods:

  • The mold should be fully preheated, the particle size of the coating (graphite) should not be too fine, and the air permeability should be good.
  • Use inclined pouring.
  • Raw materials should be stored in a ventilated and dry place, and preheated when used.
  • The pouring temperature should not be too high.

Shrinkage

Shrinkage

Features:

Shrinkage cavity is a kind of rough surface hole on the surface or inside of casting. Slight shrinkage is a lot of scattered small shrinkage, that is, shrinkage porosity. The grains at the shrinkage or shrinkage porosity are coarse. It often occurs near the runner in the casting, the root of the riser, the thick part, the thickness of the wall, and the thickness of the large plane.

Reasons for Formation:

  • The mold working temperature control does not meet the requirements of directional solidification.
  • Improper coating selection, poor control of coating thickness in different parts.
  • The position of the casting in the mold is not designed properly.
  • The design of the pouring riser fails to achieve the effect of full supplementation.
  • The pouring temperature is too low or too high.

Prevention Methods:

  • Increase the temperature of the grinding tool.
  • Adjust the thickness of the coating layer, spray the coating evenly.When the paint is peeled off and reapplied, local paint accumulation cannot be formed.
  • Locally heat the moldor use local insulation for thermal insulation.
  • Inlaid copper blocks at the hot section to chill the part.
  • Design cooling fins on the mold, or accelerate the cooling rate in local areas through water, etc., or spray wateroutside the mold.
  • Use detachable chillingblocks, which are placed in the cavity in turn, to avoid insufficient cooling of the chillingblocks themselves during continuous production.
  • A pressure device is designed on the mold riser.
  • The design of the pouring system should be accurate, and the appropriate pouring temperature should be selected.

Slag Hole (Flux Slag or Metal Oxide Slag)

Slag Hole

Features:

The slag holes are open or dark holes on the casting. The holes are completely or partially filled with slag, and the shape is irregular. It is not easy to find slag inclusion in small spot-shaped flux. After removing the slag, it appears smooth holes, which are generally distributed in the lower part of the pouring position, near the inner runner or at the dead corner of the casting. The oxide slag is mostly distributed on the casting surface near the inner runner in a network. Sometimes it is flaky, or wrinkled, irregular clouds, or formed into flaky interlayers, or exists inside the casting in the form of floccus. When breaking, it often breaks from the interlayer, and the oxide is one of the sources of cracks in the casting.

Reasons for Formation:

The slag holes are mainly caused by the alloy melting process and the pouring process (including the incorrect design of the pouring system). The mold itself does not cause slag holes, and metal molds are one of the effective methods to avoid slag holes.

Prevention Methods:

  • The pouring system is set up correctly or a cast fiber filter is used.
  • It adopts inclined pouring method.
  • Select flux and strictly control quality.

Cracks (Hot Cracks, Cold Cracks)

Cracks

Features:

The appearance of the crack is a straight or irregular curve. The surface of the hot cracked fracture surface is strongly oxidized to be dark gray or black without metal luster. The surface of the cold cracked fracture surface is clean and has a metallic luster. Generally, the external cracks of castings can be seen directly, while the internal cracks can only be seen with the help of other methods. Cracks are often associated with defects such as shrinkage and slag inclusion. It usually occurs at the inside of the sharp corner of the casting, where the thick and thin sections meet, and the hot junction area where the pouring riser is connected to the casting.

Reasons for Formation:

Metal mold casting is prone to crack defects, because the metal mold itself has no concession, and the cooling rate is fast, which easily causes the internal stress of the casting to increase. If the mold is opened too early or too late, the casting angle is too small or too large, the coating layer is too thin, etc., it is easy to cause cracking of the casting, and the mold cavity itself is also easy to cause cracks.

Prevention Methods:

  • Attention should be paid to the craftsmanship of the casting structure, so that the uneven thickness of the casting has a uniform transition, and the appropriate fillet size is used.
  • Adjust the thickness of the coating to make every part of the casting reach the required cooling rate as far as possible to avoid the formation of too much internal stress.
  • Pay attention to the working temperature of the metal mold, adjust the mold inclination, and timely pull the core to crack, take out the casting and slowly cool.

Cold Shut (Poor Fusion)

Cold Shut

Features:

The cold shut is a kind of open seam or a surface with a round edge. The middle is separated by oxide scale and is not completely integrated. When the cold shut is severe, it becomes “undercast”. Cold shut often appears on the top wall of the casting, a thin horizontal or vertical plane, a thick-thin wall connection or a thin auxiliary plate.

Reasons for Formations:

  • The metal mold exhaust design is unreasonable.
  • The working temperature is too low.
  • Poor paint quality (artificial, material).
  • Improper position of runner.
  • 5.Pouring speed is too slow.

Prevention Methods:

  • Design the runner and exhaust system correctly.
  • For large-area thin-walled castings, the coating should not be too thin. Proper thickening of the coating layer is conducive to molding.
  • Properly increase the working temperature of the mold.
  • Adopt inclined pouring method.
  • 5.Using mechanical vibration metal mold casting.

Trachoma (Sand Hole)

Trachoma

Features:

A relatively regular hole is formed on the surface or inside of the casting, and its shape is consistent with the shape of the sand grains. The sand grains embedded on the casting surface can be seen when the mold is just out of the mold, and the sand grains can be pulled out. When multiple trachomas exist at the same time, the casting surface is orange peel-shaped.

Reasons for Formation:

Holes are formed because the sand particles falling on the surface of the sand core are surrounded by the copper liquid and the casting surface.

  • The surface strength of the sand core is not good, and it is burnt or not completely cured.
  • The size of the sand core does not match the outer mold. The sand core is crushed when the mold is closed.
  • The mold is dipped in graphite water contaminated with sand.
  • The sand which is rubbed off by the ladle and the sand core at the runner is washed into the cavity with copper water.

Prevention Methods:

  • The sand core is produced strictly according to the process and remember to checkthe quality.
  • The size of the sand core matches the size of the outer mold.
  • Clean the ink in time.
  • Avoid friction between ladle and sand core.
  • Blow out the sand in the mold cavity when lowering the sand core.

Sharing is Awesome, Do It!

Share this post with your friends

Leave a Comment

Your email address will not be published. Required fields are marked *

Join 30,000+ Readers
To Get Our Lastest Articles!

Subscribe
Your information will never be shared
close-link
May I help you?
Please feel free to let us know what can we do for you.
close-link

Sharing is Awesome, Do It!

Share this post with your friends
error: Content is protected !!
Scroll to Top