Investment Casting Defects Analysis and Remedies
In the last article, we have introduced one of the biggest defects of investment casting—Crack. Besides it, some others defects happened during the production of investment precision castings. Here, we will introduce them, analyze the cause and provide the basic remedies solution for them. Foundry production is a complex multi-step process. It avoids less 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
The 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 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 are usually pear-shaped. The shape of the 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 blowhole | Blowholes, 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. |
Gas hole Causes and Analysis
| Poor venting of mold and cores |
| The preheating temperature of the casting mold is too low, and the cooling of liquid metal is too fast when it goes through the pouring system. |
| The bad coating material, poor exhaust, even volatile or break down the gas itself. |
| Holes and pits are excited in the surface of the mold cavity, when pouring liquid metal, the gas in the holes and pits are rapidly expanding, and compress gas-liquid metal, thus forming a choking hole. |
| The surface of the 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 |
Gas hole Remedies Method
| Mould should be fully preheated, the granularity of coatings (graphite) should not be too thin, permeability needs to be good. |
| Using tilt pouring way when casting. |
| Raw materials should be stored in a ventilated, dry place, and should be preheated when used. |
| Choose deoxidizer (magnesium) with better deoxidization effect. |
| The 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 the 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 that occurs during the solidification process.
The grains of the shrinkage cavity is big and thick. Often occur in the casting sprue, riser root, thick area, the connecting area between thin & wall thickness, etc. 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 properly 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 the 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 accelerates 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 a running (gate) system with risers, choose a 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 the 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 boxes 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 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. |
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 solidification 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 caused by pouring.
Cold shut
Cold shut is a type of surface crack with a 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 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 |
| The pouring speed is too slow. |
Prevention of cold shot
| The best way to prevent cold shuts is to increase the fluidity of the molten metal. |
| Using mechanical vibration metal mold for casting. |
| Using tilt pouring way. |
| Properly thicken the coating layer for thin wall thickness investment castings. |
| Correct design of 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 a 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 a smooth hole. The 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 the casting surface near the sprue. Sometimes thin sheets, or irregular cloud shapes with wrinkles, or form flake interlayers, or exist as a 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 that 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 a slag hole. |
Slag Prevention Methods
| Remove slag particles from the molten metal before pouring them 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
The 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 on 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 outer mold or is crushed when clamping 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. |
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 and quality standards 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 regular 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 a satisfactory answer in our investment casting foundry.
