Investment Casting Defects Analysis and Remedies

Investment Casting Defects Analysis and Remedies

  In last article,we have introduced one of the biggest defects of investment casting—Crack. Beside it,there is also others defects happened during the production of investment precision castings.Here,we will introduce them ,analysis cause and provide the basic remedies solution for them.Foundry production is a complex multi-step process.It is avoidless that some investment casting defects are occurred due to some wrong operations. Once happened, How could we prevent it from happening?

Here we list 9 big investment casting defects Analysis and Remedies methods

1. Gas hole

Gas hole is the hole that exists in the casting surface or internal area. When the casting cools and solidifies, bubbles form because the solid form of the metal cannot hold as much gas as the liquid form. These bubbles appear on a casting as rounded, circular cavities or holes. It can be round, oval, or irregular shape, sometimes is an air mass made up of many 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, but, subsurface gas holes can only be tested after machining.

Three types of casting defects related to gas hole.

Pinholes
  
Pinholes, also sometimes referred to as porosity, are very tiny holes (about 2 mm) usually found in the cope (upper) part of the mold, in poorly vented pockets.casting defects
They usually appear in large numbers together, either at the surface or just below the surface of the casting. They are always visible to the naked eye and don’t require equipment to identify.
Subsurface blowholeBlowholes, or simply blows, are larger cavities than pinholes.
A subsurface blowhole appears on the inside of a cast and usually isn’t visible until after machining.Subsurface blowholes can be difficult to detect before machining, requiring harmonic, ultrasonic, magnetic or x-ray analysis. 
Open holes
  
These blowholes appear on the surface of the cast and are easier to detect than subsurface blowholes.

Investment Casting Defect Causes and Analysis:

Poor venting of mold and cores
Preheating temperature of casting mould is too low, and cooling of liquid metal is too fast when it goes throught the pouring system.
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.
Insufficient drying of mold and cores

Investment Casting Defect Remedies Method

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.
Incorporate good fluxing and melting practices: melt metal in a vacuum, in an environment of low-solubility gases or under a flux that prevents contact with the air
Increase gas permeability of sand: coarser sands have a higher permeability
Increase permeability of mold and cores. Allow air and gas to escape from the mold cavity
Dry out molds and cores before use and store dry
Increase rate of solidification by reducing metal temperature during casting

2.Shrinkage

  Shrinkage occurs because metals are less dense as a liquid than a solid. Shrinkage cavity is the surface rough hole in the casting surface or internal, slight shrinkage cavity is many scattered small shrinkage cavity, namely shrinkage. It is a depression in a casting which occurs during the solidification process. The grains of shrinkage cavity are big and thick. Often occur in the casting spruce, riser root, thick area, the connecting area between thin & wall thickness, ect. Shrinkage porosity appears with angular edges, compared to the round surfaces of gas porosity. Cavities might also be paired with dendrite fractures or cracks. Large shrinkage cavities can undermine the integrity of the casting and may cause it to eventually break under stress.

Two types of Shrinkage in Investment casting defects.

Open shrinkage and Closed shrinkage

Open shrinkage These are open to the atmosphere. Air compensated as the shrinkage cavity forms. Caved surfaces are shallow, open shrinkage defects that form across the surface of the casting.
Closed shrinkage Also known as shrinkage porosity, closed shrinkage defects form within the casting. Macro shrinkage can be viewed with the naked eye, but micro shrinkage cannot. Closed shrinkage defects usually appear at the top of hot spots, or isolated pools of hot liquid.

Shrinkage 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.

Shrinkage Defect Prevention Methods:

Improve mold temperature.Local heating the mold, or local heat preservation using thermal insulation material.
Reduce casting temperature to limit the total volume deficit
Adjust the thickness of coating layer, even paint spraying, and avoid local coating accumulation phenomenon when coating paint falls off or filling.
Increase local heat dissipation by inserting internal chills, cooling ribs or cooling coils 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 a running (gate) system with risers ,choose suitable pouring temperature. ensure a continuous flow of molten metal
Factor a shrinkage allowance into the pattern design before casting.
Prevent shrinkage casting defects by improving the overall casting structure.

3.Casting Shape Defects

Casting defects are related to the overall shape of the final casting.It happened in producing or after finished.

Mold Shift

Mold shift is due to misalignment of upper (cope) and lower (drag) part of the mold. Mold shift is usually reflected as a horizontal displacement.Core shift is similar to mold shift, but it’s the core that is misaligned, not the mold. Core shift is usually reflected as a vertical displacement.

Causes of shift

Loose box pins
Inaccurate pattern dowel pins or
Carelessness in placing the cope on the drag, causing misalignment

Prevention of shift

Checking the match plate pattern mounting and alignment. Make sure to use proper molding box and closing pins.

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 of Burrs

Burrs on the casting surface is 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.

Remedies of Burrs

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 of warping

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

Remedies of warping

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

4.Pouring Casting Defects

  Pouring casting defects happened during the process of pouring metal into the mold. Here we list types of casting defects cased by pouring.

Cold shut

  Cold shut is a type of surface crack with round edge. The middle is separated by a scale.You’ll see a line or crack with a round edge on the casting surface.This defect is visible to the naked eye.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 of cold shut

The venting design of metal mold is not reasonable.When molten metal enters the mold from two gates, the streams will meet at a junction.
Too low working temperature.Low temperatures can prevent fusion at the junction, causing the streams to solidify before fusion, creating a cold shut.
Cold shut is usually a result of a lack of fluidity of the molten metal, or a poor design of the gating system.
Bad quality or improper operation of coating.
Improper riser position design
Pouring speed is too slow.

Prevention of cold shot

The best way to prevent cold shuts is to increase fluidity of the molten metal.
Using mechanical vibration metal mould for casting.
Using tilt pouring way.
Proper thicken the coating layer for thin wall thickness investment castings.
Correct design the runner and exhaust system.
Improve gas permeability of the mold (through coarser grain size, etc.)
Improve mold working temperature.Increase the pouring temperature to prevent premature solidification
Optimize gating system to minimize narrow cross-paths and ensure short flow paths

Slag (flux slag or metal oxide slag)

  Slag 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.

  These irregular metallic crusts are found on the casting surface. Scabs are typically only a few millimeters thick but can be seen by the naked eye. They usually have sharp edges are firmly bonded to the casting.

Causes of Slag

Mainly caused by alloy smelting process and pouring process (including improper design of gating system).Slag inclusion is caused when molten metal containing slag particles is poured into the mold cavities and solidifies.the mold itself will not cause slag hole.

Slag Prevention Methods

Remove slag particles from the molten metal before pouring it into the mold cavity.
Correct gating system design, or use cast fiber filter.
Using tilt pouring way
Choose the flux, and strictly control the quality

5.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 district.
Keep the size of sand core consistent with outer mold.
Timely clean graphite water.
Clean the sand in mold cavity when putting sand core.

Conclusion

  The Knowledge of casting defects and causes is key to managing casting quality. To prevent product defects in casting products is to set clear tolerances andquality standard between the supplier and manufacturer before producing.

  In our foundry,we will make a trial order sample as a standard and confirm with our customers.When all details were confirmed,we start producing mass production.Our precision casting quality control department is the strictest in the producing process.Their regularly standard is 0 defects investment castings.If you have any inquiries or orders,Please feel free to place an order with us.You will get satisfied answer in our investment casting foundry.

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