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Blowholes is a term used in metallurgy and denotes a in the solidification of cast parts formed cavity . Voids occur during casting of molten individually or heaped on. The cavities, known as blowholes, are caused by the shrinkage of the material volume due to the cooling and solidification of the melt. If a casting has voids, it is called voids .

In a broader sense, unwanted cavities in materials other than metal, which can arise in various ways, are also referred to as blowholes. For more information, see the section on the extended definition of blowholes .


The origin is likely to be in the Germanic word lunk for "sink". Likewise, an origin of lunken is possible, which means "to become hollow" in the Rhenish dialect.

Causes of the formation of cavities

Micro-cavities in lutetium melted in an electric arc furnace , from the group of lanthanoids ( rare earth metals ). This 1 cm 3 cube was milled out of a melted blank .

The susceptibility to the formation of cavities depends on the material and can be influenced by its modification. A complete avoidance of voids is not possible for most cast parts. Instead, the cavity is delocalized. For this purpose, one or more risers are provided in the mold in addition to the sprue , which are positioned in such a way that the shrinkage in volume is counteracted with the availability of liquid melt.

Real voids are only created by a decrease in volume ( shrinkage ) when a molten metal solidifies and cools. The only determining factor is whether cavities that arise in the casting during the phase transition from liquid to solid can be filled by the supply of (still) liquid melt, the make-up. If this is the case, there are no cavities in the cast part, but in the riser, which is then removed. Otherwise, the Lunkerung phased, parallel to the volume shrinkage of the melt, a liquid contraction that the casting temperature T falls below the developed G starts and continues as long as the melt is still above the solidification temperature T S is located. When the solidus or solidification point T S is reached, a sudden solidification contraction occurs. Regardless of this, the shrinkage of the cooling casting continues until it has adjusted to room temperature.

The real shrinkage can vary in size and appearance.

A distinction is made between open and closed cavities.

Open blowholes

To the open cavities, also called Außenlunker and always counting on the Groblunkern that nevertheless does not imply in any case, casting defects, include "Sauglunker" at the sprue and cast piece conditionally placed feeders . In any case, they prove that a make-up took place to compensate for volume deficits during solidification. Careful separation from the casting shows whether the cavity is limited to the sprue - double-sided sprue is also possible - or extends into the casting and thus proves to be insufficiently dimensioned. The same applies to the feeders.

Another open, because visible, voids is the "pressure blowhole", technically also known as the "postscript". It shows up as a sink mark on a cast surface. It is caused by the fact that a shell began to form in the mold, but then liquid metal was sucked off into other areas and the existing shell sank into the cavity under it due to its own weight.

Cross-sectional transitions on the casting are also susceptible to open voids, because the stronger parts that have already solidified are not fed by the neighboring, weaker and already solidified parts.

Closed cavity

They are disadvantageous for the operational reject rate because they are only recognized during a leak test or the expensive machining of the casting. Depending on the composition of the melt, they occur in thick-walled parts of a casting that have not been adequately replenished, either as "coarse voids" (macro voids) - the "thread voids" are also assigned here - or the volume deficit that forms leads to an intergranular fine or “micro-cavities”, which can only be distinguished macroscopically from gas porosity. Alloys with a wide solidification interval tend to have this type of cavity. Eutectic alloys - such as AlSi 12 ( aluminum with 12% silicon ) - form coarse cavities if left untreated, without influencing the structure by sodium or strontium .

Prevention of cavities

The blowhole as a casting defect either causes scrap and a total loss of the processing costs accrued up to now for the casting (minus the pure material value), or it requires expensive rework, especially when it comes to large parts where the defect can be repaired without any loss of casting quality is possible. The measures to avoid cavities depend on their type. A previous casting simulation is extremely helpful, which shows problem areas of the casting and thus offers the way to a suitable remedy.

Nowadays there are a large number of casting simulators that show how a certain casting will solidify and where and to what extent volume deficits, which include not only the shrinkage of the melt but also the linear shrinkage , must be taken into account when solidifying in the mold. This is particularly true in view of mostly very different wall thicknesses and the resulting different solidification times.

Form technical measures

Pouring funnels require a large diameter. According to a rule of thumb, in the interests of rapid, powerful filling of the mold, the cross-section of the pouring funnel should be larger than the sum of all the “gates” that lead the melt into the casting and branch off from the “inlet”. A “starvation” of the casting is thus prevented in a first step.

Another rule is that the hottest metal fed into the mold is found in the sprue until the end and the coldest in the zone furthest away from it. In the ideal case and assuming a piece with the same wall thickness, this corresponds to a void-avoiding, directional solidification.

Thick-walled areas in the area away from the sprue without compensating for the volume shrinkage necessarily lead to voids. This can only be countered by making hot liquid metal available to the area that has become critical. In practice, “risers” with a thick cross-section are placed on the mold, in which the metal remains liquid longer than in the mold and can be “sucked” into the deficient area. Parts that are inaccessible to attached risers but that are at risk of cavities when solidifying due to their accumulation of material can attract liquid metal from molded-in blind risers (lost head) to compensate for a volume deficit.

One of the technical molding measures with the aim of supplying all areas of a casting with hot metal quickly and still without vortices is the increasing mold filling from bottom to top. Double pouring funnels, ring runs around the casting, with several gates, laterally tangential in opposite directions or designed as a horn gate are a prerequisite for this.

In practice, this also tries to distribute the hot metal when the mold is filled, in order to prevent the inlet zone from heating up in the case of larger pieces, which would favor voids. High cast parts are therefore cut on several levels.

Casting measures

The volume deficit leading to the formation of voids is a physical condition of the phase change from liquid to solid. Rapid solidification, also known as quenching, prevents coarse cavities and replaces them with micro-cavities embedded in the structure. Slow solidification, as is the case with sand casting, requires technical casting measures that relocate the blowholes from the casting into open feeders or blind feeders and the pouring system. So that liquid metal can be replenished from these points during the period of solidification and the associated shrinkage, they are isolated or additional heating is provided. The simple insulation makes use of little or no heat dissipating materials, which are either placed around the feeder and the sprue as a sleeve or shaped as an "insert". Another step is their production from aluminothermically reacting masses, which not only reduce heat losses, but even add heat. In addition, the feeder and pouring funnel can also be covered with an exothermic “blowhole powder”, or a lid made from it can be provided.

In order to avoid the occurrence of voids, a relevant auxiliary materials industry provides a wide range of options. The casting determines where insulation is appropriate, i.e. the insulating stones or rings used in cast steel, or the installation of cast-iron shock plates, cooling irons and other cooling elements, partly on a highly conductive copper basis (cooling cores, cooling nails, cooling fins), be it in sand molds or metallic ones Permanent molds (gravity die casting, die casting).

Void control

With regard to a variety of causes, voids can never be completely ruled out, even in castings manufactured with the help of aids to prevent voids. Inspection based on their design and the alloy used for parts that are sensitive to voids is therefore essential. For this purpose, methods that have been handed down over the years were predominantly used until the 20th century.

The minimum requirement is a visual inspection for external cavities that are not always immediately recognizable at cross-sectional transitions. In addition, the cast parts are knocked off with a hammer. The practiced can hear from the sound whether there are voids or cracks. Another method that still exists today as a density sample - albeit mainly for checking the hydrogen content and the resulting porosity - is density determination using the Archimedean principle.

In the case of series casting, it was common to subject a part taken from the belt at random or after a certain number of cast pieces to a destructive test and to check the critical points by means of cuts or breaks.

Since then, however - initially for safety-relevant parts in general - non-destructive material testing with its various methods has established itself. The ultrasound test with a (portable) pulse-echo sounder and data processing connected to it is practical to use . With the X-ray examination in a fluoroscopic chamber, a method associated with medicine was transferred to technical products for the first time. The further development leads to computed tomography (CT) with the possibility of three-dimensional representation of defects in the cast structure.

Extended concept of blowholes

Although mainly assigned to metallurgy , other areas of production also use the word blowholes. It can be found in construction and in high-voltage technology, but in both cases it is false blowholes, as they are not based on a temperature-dependent volume shrinkage of a melt flow, but rather on an avoidable manufacturing defect that leads to a local material deficit. In concrete construction , a blowhole is a cavity that is created by insufficient compression ( compression pore ). When masonry, a blowhole occurs as a result of insufficient mortar, which is particularly favored by the uneven stones used in quarry stone masonry. In electrical engineering , a cavity in an insulator is called a blowhole. The term blowholes is also used in welding technology as a type of undesirable irregularity .

A volume shrinkage as a prerequisite for the formation of voids can also be found outside of metallurgy in slowly solidifying mineral melts, such as glass (casting of large telescopic mirrors), ceramics , in nature with lava and in semi-crystalline thermoplastic materials.

False blowholes

False blowholes ("blow holes") in a metallic workpiece

A cavity in a casting is widely referred to as a blow hole, but the expression only applies to cavities that are caused by solidification shrinkage. Cavities that are caused by air inclusions or gas-forming factors such as air or water vapor from the mold moisture or from cores are called bubbles. Gas bubbles that can barely be seen with the naked eye are called pores .

Typical "non-blow holes" are the so-called "blow blow holes" regardless of the name. They arise locally when existing mold moisture turns into water vapor when the melt flows into the mold and this with its pressure of formation ( vapor pressure ) finds no other way out than to penetrate the not yet hardened shell of the casting and become enclosed as a bubble.

Another casting defect that occurs particularly with aluminum , but also with copper, is similar to a micro or fine hole. An exact statement is only possible under magnification, because even very fine bubbles are smooth-walled on the inside, whereas the wall of a blowhole has crystalline roughness . Smooth bubbles indicate hydrogen porosity, which is due to the fact that the melt contained dissolved hydrogen. It can adhere to the charge, as a thin layer of aluminum oxide mono- or trihydrate, or it can only arise when the melt comes into contact with moisture, in particular air moisture . The reduction of moisture - regardless of its origin - always leads to the formation of oxide ( aluminum oxide , copper oxide ) and atomic hydrogen in the melt .

Upon solidification of a melt is their ability to keep hydrogen in solution, very strong back - by aluminum of 1 cc / 100 g of aluminum to only 0.05 cc / 100 g. Some of the hydrogen that has become excess escapes from the melt. Appropriate melt treatment reduces it further. Remaining hydrogen shows up as porosity in the cooled casting due to its precipitation pressure . If these pores or bubbles are very small and the necessary inspection is not carried out, incorrect assignment as a cavity is not uncommon.

The water vapor porosity that occurs in zinc-free copper alloys is related to hydrogen porosity. The cause of this lies in the ability of hydrogen to reduce the copper oxide present in the melt. The resulting water vapor is very volatile (bp 100 ° C), but if it forms in a poured-off mold and is prevented from escaping, it sometimes leads to coarser pores or bubbles in the casting (see blowholes).


  • Foundry Lexicon. 19th edition. Schiele & Schön, Berlin, ISBN 3-7949-0606-3 .
  • Association of German Foundry Experts : Guß Fehler-Atlas. 2 volumes, Giesserei-Verlag, Düsseldorf 1955/1956.
  • Roland Irmann : Aluminum casting in sand and mold. Publishing house of Aluminum-Zentrale eV, Düsseldorf 1952.
  • VDG et al. (Hrsg.): The foundry in a common representation. edited by Hans Schmidt. 3rd revised and expanded edition. Foundry publishing house, Düsseldorf 1953.
  • W. Bergmann: Materials technology. Volume 2: Material production - material processing - material application. Carl Hanser Verlag, 2001, ISBN 3-446-21639-1 .

Web links

Wiktionary: Lunker  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. "The Great Brockhaus", 20th edition. 1996; Gerhard Truig, "German Dictionary", also Duden, Volume 1, "The German Spelling". Issues 1947 and 1991; less clearly in “Duden. Spelling of the German language and foreign words ”, 9th edition. 1924, under “Hüttenw.”: “Vacuum space within the solidified metal mass in the mold”. More also in the "Sprach-Brockhaus", Verlag E. Brockhaus, Wiesbaden, 6th edition. 1951.
  2. "elongated cavity along the thermal central axis" (according to Giesserei Lexikon).
  3. see as an example: P. Kainzinger, M. Wohlfahrt, W. Eichlseder: Makrolunker in cast iron with spheroidal graphite - prediction by simulation and its influence on the fatigue strength. In: VoeG Giesserei Rundschau. Jhg. 58, issue 5/6 2011, p. 106.
  4. to the risks: CA Rowe: Blind feeders, why they sometimes fail. In: Taschenbuch der Gießerei-Praxis 1993. Verlag Schiele & Schön, Berlin.
  5. Aids are a magnifying glass and scriber.
  6. From the Austrian Foundry Institute of the Association for Practical Foundry Research in Leoben, Activity Report 2008: Possibilities and Limits of Computed Tomography. In: VÖG Foundry - Rundschau. Issue 7/8 2009, p. 142.
  7. Saechtling, plastic Paperback , 30th edition, Carl Hanser Verlag, 2007, ISBN 3-446-40352-3 .
  8. ^ Definition according to the foundry lexicon. The literature also knows the term angle bubble, as a blow hole on over-moistened sand edges.
  9. in the case of zinc-containing copper alloys, the vapor pressure of the zinc (Kp 907 ° C) does not allow any hydrogen content.