DE102012217939A1 - Process for the production of cores for the casting production of workpieces - Google Patents

Abstract

The present invention relates to a method for producing cores for the casting production of workpieces, in particular of annular cores for the formation of cooling channels in casting pistons piston or piston parts of internal combustion engines, wherein the cores are produced by casting a molten salt used as a casting material with high Flow rate and limited pressure in a the core to be produced or the cores to be produced corresponding cavity of a mold assembly is introduced, the flow rate and the temperatures of the molten salt and the moldings are adjusted so that a solidification of the molten salt occurs only after complete filling of the cavity, and the pressure is limited to a maximum value at which a deformation of the moldings remains below a tolerance value.

Description

The invention relates to a method for the production of cores for the casting production of workpieces, in particular of cores for the formation of cooling channels in casting by hand produced piston or piston parts of internal combustion engines.

Pistons of internal combustion engines, regardless of the nature of their production near the edge of the piston crown must have cooling ducts into which oil can be injected during operation of the internal combustion engine from its crank chamber in order to prevent thermal overloading. In this connection, it is particularly desirable to form cooling ducts in the form of annular channels with ring planes parallel to the piston crown in casting pistons or piston parts which can then be traversed by an oil flow via inflows and outflows open to the crank chamber of the internal combustion engine.

In a casting production of the cooling channels having pistons or piston parts then correspondingly annular cores are required. In the casting technique, it is generally known to use salt materials for the production of cores which can be obtained from the finished workpiece, e.g. can be washed out with relatively little effort by means of water or weak acids.

In the EP 0 700 454 B1 For example, a process for producing a polycrystalline salt body will be described. In this case, a molten salt is first produced by melting a starting material in a crucible inert against the molten salt, then the molten salt is placed in a mold whose bottom is maintained at a temperature 1 to 50 ° C below the solidification point of the molten salt, so that a solid salt crust forms on the ground. Thereafter, the mold is maintained at a temperature below the solidification point of the molten salt until it is completely solidified into a polycrystalline salt body, which is then cooled. In particular, large-area polycrystalline salt bodies are to be produced in this way.

From the DE 10 2008 004 929 A1 It is known to use for the production of salt cores for foundry purposes, a mixture of water-soluble salts to which a binder is added which contains a protein combination with inorganic constituents. This is to ensure that the prepared salt core has a good resistance to the metal to be cast and allows only a small penetration of the liquid metal.

Another method for producing a salt core for metal casting processes is described in US Pat DE 10 2009 015 984 A1 described. Here it is provided to produce the salt core in a die-casting process, wherein a salt melt intended for the production of the salt core is pressed with a die casting machine into the associated printing forme. In order to ensure that the salt core can be easily released from the die, surface areas of the die casting machine, as far as they are exposed to molten salt, can be exposed to a lubricant, which may contain a metal bromide and / or graphite. The molten salt is preferably sodium chloride and / or potassium chloride and / or potassium carbonate.

The EP 2 425 910 A1 is related to the production of salt cores for the casting production of cylinder blocks or the like of internal combustion engines. In this case, a molten salt is first prepared, which is subsequently introduced into a mold whose temperature is 0.52 to 0.7 T _m , where T _{m is} the measured in degrees Kelvin melting temperature of the molten salt. Thereafter, the molten salt solidifies in the mold so that the desired salt core is available after demolding.

From the DE 10 2008 028 197 A1 It is known to provide cooling channels in cast pistons with a groove structure and to press suitable cores of salts. However, the achievable surface quality was not satisfactory.

All the methods described above have in common that the salt cores produced are relatively compact and accordingly have a comparatively small core surface in comparison to the core volume, so that the heat flow from the molten salt into the respective shape takes place comparatively slowly.

In order to allow optimum heat dissipation, however, the cooling channel should have relatively large walls in comparison to the size of its cross section, in particular slot-like cooling channels with extremely long oval cross sections are desired. As a result, the cores necessary for the casting-technical production of such cooling channels must have a very filigree thin-walled structure.

Desirable may be cores having smooth-surfaced peripheral sides to allow cooling channels with corresponding smooth-surfaced walls and low flow resistance. Alternatively, however, salt cores having a predetermined uneven surface structure may also be desired. The advantage of this method is that the cooling channel already through the cast Salt core can be provided in high accuracy, for example, with a defined roughness, with grooves or other surface structures, which are for the properties of the cooling channel and the heat transfer from the piston to the coolant of great importance. Such rough surfaces can be generated with pressed salt cores only with considerable Nachbearbeitungsaufwand and little repeat accuracy.

This is where the invention starts.

The object of the invention is to provide a method for the production of cores for the formation of cooling channels in casting pistons piston and piston parts, on the one hand to ensure that the cores can be easily removed from the finished workpiece, and on the other hand a high precision is ensured in the formation of the cores.

This object is achieved by a method according to claim 1. Advantageous embodiments are the subject of the dependent claims.

The invention makes use of the knowledge that molten salts are well suited for the casting production of filigree workpieces. On the other hand, the invention recognizes the finding that well suitable die casting processes in connection with large workpieces are problematic for the production of very precise workpieces, because the molds have unavoidable flexibility, in particular elasticity, with the result that thick-walled molds due to their high wall thicknesses should be sufficiently pressure-resistant, lead to increased deviations from the nominal dimensions of the workpiece to be produced. Such is reliably prevented in the invention by limiting the pressure with which the molten salt is introduced into the cavity of the molds. On the other hand, the inventively provided high flow velocity of the molten salt at the introduction into the cavity ensures complete filling of the cavity, at the same time the advantage is achieved that a rapid heat dissipation of the molten salt in the molds can be made possible, as it is for fast cycle times the production of the cores is desired.

With regard to preferred features of the invention, reference is made to the claims and the following explanation of the drawing, on the basis of a particularly preferred embodiment of the invention is explained in detail.

Protection is claimed not only for illustrated or specified combinations of features, but also for any combination of the specified or illustrated individual features in principle.

In the drawing shows

1 1 is a schematic section of a cast piston with a cooling channel which has been produced by means of a salt core according to the invention,

2 a perspective view of a core according to the invention with a foot gate,

3 a perspective view of a core according to the invention with a Tangentialanschnitt, and

4 a perspective view of a core according to the invention with a fan gullet.

1 shows a first embodiment of a piston according to the invention 10 , The piston 10 has a piston head 11 and an axially adjacent piston stem 12 on. The piston head 11 has a combustion bowl 13 as well as a revolving firestep 14 and a circumferential ring part 15 with annular grooves for receiving piston rings (not shown). The piston 10 In the exemplary embodiment, a one-piece cast piston made of any suitable metallic material. However, the present invention also includes a two-piece piston having a piston head and a piston stem, wherein at least the piston head is molded of any suitable metallic material.

In the piston head 11 is equal to the ring part 15 a circumferential cooling channel 16 intended. The cooling channel 16 has an elongated oval cross-section, the long axis is inclined to the piston axis, whereby an effective cooling esp. The piston crown including the radially outer regions of the combustion bowl 13 is possible. The cooling channel 16 also has a known manner in its in the axial direction of the piston 10 lower wall portion (not shown) inlet or outlet channels for coolant.

The in the 2 to 4 represented cores 20 . 30 . 40 are formed as funnel-like rings, wherein the funnel wall has a cross section in the form of an extreme Langovals. Typically, the small diameter of the Langoval is of the order of 2 mm, while the large diameter is of the order of ten times. This information is not intended to be limiting, but merely to make it clear that the two diameters of the oval cross-section can have extremely different values. The ring diameter can on the order of magnitude between 50 and 200 mm. The funnel shape of the core is usually adapted to the shape of the piston crown, which the in 1 shown combustion bowl 13 may have, to which the proposed cooling channel is arranged substantially concentric. If the combustion bowl is flatter than in 1 is formed, the funnel wall of the cooling channel 16 have a correspondingly adapted opening angle.

Advantageously, closed rings are used as cores, as they are in the 2 - 4 are shown by way of example. By virtue of a substantially rotationally symmetrical structure and a concentric arrangement in the piston, the cooling channel thus formed permits a substantially uniform cooling of the piston crown along its circumference. But it can also be used not closed cores, which have a mostly short interruption between the later inlet and outlet of the cooling channel in the circumferential direction. Alternatively, two or more non-contiguous cores could be used, with which separate cooling channels are formed in the piston.

For the production of the illustrated cores 20 . 30 . 40 a not shown, preferably multi-part mold is used, the mold parts between them one of the shape of the cores corresponding cavity. This cavity is filled with a molten salt consisting essentially of sodium chloride (NaCl), to which, for example, further chlorides or carbonates, for example of potassium, may be mixed in order to lower the melting temperature. The aim is a salt mixture whose melting temperature is on the order of 600 ° C. Such a melt is introduced into the cavity of the mold via fan-shaped feed lines, tangential cuts or axial foot cuts with a larger cross-section compared to fan sections with a high flow rate of the order of 100 m / s, wherein the feed pressure to values of less than 100 bar, typically about 70 bar is limited. The limited supply pressure is essential insofar as the material of the molds has an unavoidable volume elasticity, so that at excessively high supply pressure more or less pronounced deviations from the respective nominal shape of the annular cores to be formed would have to be accepted, it being surprisingly found that the deviations from the desired shape increase with increasing wall thickness of the molds.

The channels forming the gate can be formed in individual dies, provided that this geometry still permits demoulding. Alternatively, the channels that supply the melt are formed at the interface between two molds.

2 shows as an embodiment of a foot gate, in which the melt is introduced into the cavity of the mold substantially in the axial direction parallel to the later position of the piston axis.

In contrast, shows 3 as another embodiment, a tangential section in which the melt enters the cavity of the mold substantially in a tangential direction through a channel whose long oval cross section corresponds approximately to that of the later core. This arrangement does not require any significant deflection of the melt at the inlet and thereby facilitates a rapid and uniform filling of the cavity in the circumferential direction at flow speeds of about 100 m / s.

4 shows a further embodiment with a fan cut, in which the melt is introduced in a substantially radial direction through a channel with a long oval cross-section in the cavity. The long axis of the oval cross-section is aligned perpendicular to the piston axis and extends radially inwardly toward the piston axis.

The protrusions originating from the incisions and having their respective shape 21 . 31 . 41 on the finished core are preferably removed prior to casting the piston. In some cases, however, they may be retained in whole or in part. In particular, the foot section according to 2 can during casting of the piston an axially extending projection 21 of the core 20 a channel for receiving an oil jet or for the discharge of oil from the cooling channel 16 form and replace a designated, later to be attached hole in whole or in part. In addition, the lead can 21 for fixing the position of the core 20 in the casting mold during casting of the piston contribute. Such foot cuts prove to be advantageous, for example, in broken or multi-part cores at their respective ends in the circumferential direction.

When the molten salt is introduced into the cavity of the mold, the molds have an initial temperature which is on the order of magnitude about 300 ° C. below the temperature of the molten salt, so that, after filling the mold due to the large peripheral surfaces of the cores to be produced, they rapidly respond accordingly to the temperature of the molds cools and solidifies. By inventively provided high flow rate of the molten salt during their import into the cavity of the mold premature solidification is reliably avoided.

LIST OF REFERENCE NUMBERS

10

 piston

11

 piston head

12

 piston shaft

13

 combustion bowl

14

 top land

15

 ring belt

16

 cooling channel

20

 core

21

 head Start

30

 core

31

 head Start

40

 core

41

 head Start

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 0700454 B1 [0004]
• DE 102008004929 A1 [0005]
• DE 102009015984 A1 [0006]
• EP 2425910 A1 [0007]
• DE 102008028197 A1 [0008]

Claims (12)

Method for producing cores ( 20 . 30 . 40 ) for the casting production of workpieces, in particular of annular cores for the formation of cooling channels ( 16 ) in casting technique produced piston ( 10 ) or piston parts of internal combustion engines, wherein the cores are produced by casting a molten salt used as a casting material at high flow rate and limited pressure in a corresponding to the core or cores to be produced cavity of a mold assembly is introduced, wherein the flow rate and the temperatures of the molten salt and the mold parts are adjusted so that a solidification of the molten salt occurs only after complete filling of the cavity, and the pressure is limited to a maximum value at which a deformation of the moldings remains below a tolerance value. A method according to claim 1, characterized in that the molten salt is introduced into the mold assembly at a pressure which is less than 100 bar. A method according to claim 1 or 2, characterized in that the temperature of the molten salt is up to about 10% above the melting temperature. Method according to one of claims 1 to 3, characterized in that the moldings of the mold assembly are brought to a temperature before introduction of the molten salt, which is about 150 to 300 ° C lower than the melting temperature of the salt material. Method according to one of claims 1 to 4, characterized in that a toroidal core ( 20 . 30 . 40 ) is cast with a pronounced long oval cross-section. Method according to one of claims 1 to 5, characterized in that the smaller diameter of the long oval cross section is of the order of 2 mm. Method according to one of claims 1 to 6, characterized in that a funnel-like torus shape is produced. Method according to one of claims 1 to 7, characterized in that the molten salt is introduced by fan-shaped arranged leads into the cavity of the mold assembly. Method according to one of claims 1 to 7, characterized in that the molten salt is introduced into the cavity substantially tangentially. Method according to one of claims 1 to 7, characterized in that the molten salt is introduced through axially arranged supply lines into the cavity. Method according to one of claims 1 to 10, characterized in that a molten salt is used, which consists essentially of sodium chloride. A method according to claim 11, characterized in that a molten salt containing additions of potassium chloride and potassium carbonate is used.

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