Casting defect prevention

In our production process of a casted part, there is always the problem that the molds are not filled correctly (incomplete casting) or that cold run errors occur. What can I do to find the cause of incomplete casting and remedy it?

Our global experts recommend:

For an accurate assessment and evaluation of the defect, it is important to know whether the production of the casting was previously possible or whether the defect occurs with a completely new casting. For example, has the casting material or the riser/ gating  system been changed?

In the first step, we recommend that you take a basic look at whether there is sufficient metal available in your process, whether the sprue and gating system used is correctly designed and whether an optimum casting height (metalostatic pressure) is used.

The temperature factor is also very important: too low a pouring temperature always leads to a cold run error. Therefore, take a closer look at the temperature curve of the casting metal (liquidus + solidus temperature), as well as the temperature measuring points for correct function. Does the desired temperature balance remain within the specified tolerances even in the event of malfunctions? Are there any devices in your process, e.g. chills, which influence the flowability and/or the solidification behavior of the metal? Are the temperatures in the furnace (holding or casting furnace) and the ladle correct?

If you have checked these points, you should pay attention to whether the defect always occurs at the same mold cavity (several castings of the same type in one casting cluster) or always at the same point on the casting. Check whether there are strong differences in wall thickness in the local vicinity, strong differences due to different cross-section geometries (neck downs) or whether there is a loss of melt due to poorly sealed core marks (backflow of core marks).

In general, good venting of the mold and the cores is very important! Air pipes should be used which lead to better mold ventilation and thus prevent air inclusions and poor mold filling due to excessive dynamic pressure.

If the problem still exists after checking all the above points, please contact our ASK-Tech Service.

Our global experts recommend:

You describe a so-called veining defect. This manifests itself in the form of thin, metallic excesses on castings, mostly in angles, corners and edges. Leaf veins are caused by the expansion of the quartz sand at a certain temperature gradient (573°C for quartz sand). The molding material tears open and the incoming metal fills the resulting gap and forms a rib-like base. This effect is intensified by the faster decomposition of the binder at higher casting temperatures. 

Too fine sand, too high a proportion of fines, leads to a high packing density, which can be the reason for veining. The basic molding material used in each case must be considered according to its specific behavior. Furthermore, insufficient thermal resistance, too high casting temperature and casting height as well as too long casting times favor the formation of veined leaves. An unfavorable gating system can be a cause for so-called hot spots (thermal centers), which favor the casting defect.

Your process can be optimized for a number of areas.

On the core side, the use of a multigrain sand with coarser grain size (screen distribution) or alternatively a less expanding sand (chrome ore or zircon sand) leads to veining prevention. Sand regenerates or sand with a feldspar content of 5-6% as well as molding sand additives with good thermal conductivity also have a positive influence. The compaction stress caused by the quartz transition is buffered by the lower softening temperatures of these special sands and additives.

On the mold side, the bentonite content can be increased to promote wet tensile strength. Furthermore, a fine quartz reduction improves the gas permeability. The reduction of the amount of new sand and/or the use of less moist molding materials helps to reduce the gas potential and thus the gas pressure in the mold.

In addition, the use of a coating can help. If a suitable structure is selected for ceramic fillers, the gas pressure resistance is enhanced. Increasing the layer thickness leads to improved insulating properties. If the coating can dry slowly, this prevents cracking in the dry coating layer.

We also recommend checking whether a suitable gating and feeding system is used which avoids local overheating of molds and core parts and thus a hot spot effect. Further improvement can also be achieved by shortening the casting time.

If the problem persists after checking all mentioned points, please contact our ASK-Tech Service.

Our global experst recommend:

It could be a penetration defect in your case.

This defect can occur in all sand molded castings (preferably green sand), regardless of the material. Penetrations occur particularly in places where the molded parts heat up strongly, such as on edges or ingate areas, in places where the molding material is weakly compacted and in thick-walled castings. The casting defect often affects entire casting sections and can be seen on the casting with the naked eye, as in your case.

Possible causes for such a defect could be excessive moisture in the green sand ("free" water), an excessively high or unevenly compacted mold, excessive casting speed or insufficient removal of the mold gases from the mold cavity. We therefore recommend checking four different areas and optimizing them if necessary.

The first area is the molding material: Special care should be taken to use thermally stable bentonite with a high montmorillonite content, which leads to a reduction of inert fines share as well as the water requirement. A positive effect can also be achieved by reducing the dust recirculation. Furthermore, attention should be paid here to the degree of pre-treatment, regeneration of the green sand. It is possible to pre-humidify the reclaimed sand, accelerate the water absorption of the bentonite or possibly extend the mixing times. The use of finer core sand or finer new sand as circulation sand, in which the sand grain size is reduced, can help to avoid the defect described by you. Please note, however, that the AFS number of sand must never be less than 60; it is helpful to check the gas permeability of the molding sand mixture. Another possibility for improvement is the use of materials with lower coke but higher lustrous carbon formation.

Secondly, you should check especially at the molding line whether the compaction is absolutely uniform and as low as possible. You should also check the hardness of the mold and reduce the compaction pressure if necessary. For all this, the molding line should guarantee even sand filling into the molding boxes.

Thirdly, with the molds and cores used, a coating of the parts or an increase in the layer thickness with endangered contours of the molds and/or cores could be an effective measure.

Finally, you can influence the gating and casting technique by checking whether the distances between the patterns are too small. Increase the distances and avoid hot spots.

You can also try to provide the mold with air pipes for degassing. To avoid water condensation during wet casting, endangered mold parts can be sprayed with a water-resistant release agent. The mold filling can be optimized by changing the gating system by reducing the casting speed.

If the problem persists after checking all the above points, please contact our ASK-Tech Service.

Our global experts recommend:

The casting defect you describe, bubbles, mainly occurs as a result of a poor degassing process in the core or mould and is more common in lamellar graphite iron castings (GJL) than in spheroidal graphite iron castings (GJS). Cavities with round, smooth walls then usually occur over large areas. The reason for the rounded or elongated bubbles are gases enclosed by the solidifying metal on the surface of the casting, often associated with slag or oxides. Such defects usually occur in the upper box of a mould, in poorly vented pockets and/or undercuts.

We recommend that you check the causes of gas formation separately. 

Cores

The release of core gases can promote the formation of bubbles. Binder reduction or the use of gas shock retardant binders can be advantageous.
Note: the lower the binder content, the lower the gas potential.

Depending on the speed at which the mould is filled with the casting metal, the gas permeability of the moulding material coatings (coating) must be taken into account. In general: fast mould filling = low gas permeability, slow mold filling = high gas permeability.

Always ensure that the cores are well dried after the coating process!

When storing cores, care should be taken to ensure a dry environment (low relative humidity) so that moisture cannot be absorbed. Warm cores or cores stored hot tend to absorb more moisture!

Insufficient core ventilation also plays a major role. When using coatings, please ensure that the core marks are free of coating materials. In some cases it is advisable to subsequently drill core vent holes or connect core vent holes using ceramic inserts.

Molds

When producing molds, especially on automatic green sand molding machines, the compacting of the molding material must not be too high. A too low gas permeability of the molding material (sand-binder mixture) or a too high gas release, e.g. from the lustrous carbon binder used in bentonite-bonded molding material (green sand / wet casting), lead to an increased gas risk.

If sand that is too moist and/or too warm is used in the production of the mold, this can cause the mold to "boil" and thus lead to an increased steam pressure in the mold. You can also improve degassing from the mould side by reducing the fine dust content, using coarser sand grains, reducing carbon carrier content or using slow-reacting lustrous carbon formers, and using bentonite with a high montmorillonite content (high specific binding capacity) and high thermal stability. A continuous control of the molding material preparation is absolutely necessary. A reduction in compaction force and the resulting reduction in mold hardness also yields results; the molding sand should be uniformly compacted.

When checking gating and casting techniques, it is important to ensure sufficient mold ventilation (air whistles). Improvements can be achieved by increasing the casting height and extending casting times.

Melt

Make sure that the melt is sufficiently degassed, in particular the specific boiling temperature and the holding time (standing of the Fe-melt) must be observed. Use clean materials, e.g. stainless steels and broken cast iron, to reduce the oxides directly at the beginning of melting activities. When melting, the temperature range in which the melt absorbs more gas must be passed through quickly.

If the problem still persists after checking all the above points, please contact our ASK-Tech Service.

Our global experts recommend:

Problems in the core shooting process are caused by many factors. First of all, the suitability of the selected core shooting machine for the used corebox should be examined. A sufficient shot volume of the machine is important in relation to the tooling (core box volume). The shooting head must offer a sufficiently large shooting range. Check whether the shooting head offers a sufficient volume to fill the core box (wormholes) even if the shooting tube or core shooter size is not selected properly. Here guide plates, spare pieces or sand labyrinths in the shooting head could be helpful for an optimized filling of the core box.

Furthermore, all pressurized components, seals and valves of the core shooter, including the nozzle seals, should also be tested.

In a further step, the shot cross-sections should be examined, e.g. whether the sum of the shot cross-sections is sufficient to fill the core box in the planned time and / or whether the shots are in a position favorable for filling (e.g. core marks).

The same applies to the venting cross-sections: Check whether the sum of the ventilation cross-sections corresponds to ~50% of the sum of the shot cross-sections. Vents should be in a favorable position in the corebox for filling. The type and dimensioning of the vents including the exhaust air pipes behind them must be suitable to ensure sufficient air removal during the shot. With the cold-gas-curing PU-Cold Box system, the necessary flushing with the catalyst gas must also be ensured. The uniform curing is the main focus here.

If the problem still persists after checking all the above points, please contact our Technical Service Department.