DE102018006329B3 - Process for the preparation of crucibles by stratification with thermal destruction of the crucible positives - Google Patents

Abstract

The invention relates to the production of crucibles by layer construction, which are constructed by alternately wetting the crucible positive with binder-containing, refractory molding materials in liquid form and then in powder form. All the problems that occur in the most commonly used process for the production of crucibles by layering for the melting of metals, based on the used mechanically and thermally sensitive material for crucible positive production and its elaborate removal by the melting out of the mold. According to the invention, the problem is solved by using, as crucible-positive, plastics from the blow-molding, which can be stretch-blown from the preforms into the appropriate crucible shape. This creates a simplified and improved process. This is characterized by the fact that when burning the green crucible, the crucible positive with the thermal destruction degrades to gaseous products and the crucible is simultaneously demolded. In addition, the crucible has very good mechanical properties and excellent surface qualities at the contact surfaces for the crucible positive.

Description

Technical area

The invention relates to the production of crucibles from binder-containing, refractory molded materials, so that they can be used for the melting of metals and subsequent casting of the final product. The invention is particularly well suited for the production of crucibles with layer structure by alternately wetting the crucible positive with binder-containing, refractory molding materials in liquid form and then in powder form.

State of the art

In the current melt crucible manufacturing process for metal sheet smelting, the main focus is on the lost wax process. Other materials that have a much lower melting point than the crucible material can also be used. In this process, crucible positives made of wax are first created using injection molding machines by using tools or by freehand molding. Also combinations of both methods can occur. In this process, a basic shape of the final crucible positive from wax is injection-molded using the mold and, after hardening, extended or modified with finer manually applied wax models. The wax molds produced in this way, which represent the crucible positives of the final crucible forms, are cleaned and dried so that the wax crucible positives can be released from the green crucibles more easily and without leaving any residue. A layer is applied by dipping the individual crucible positive from wax in liquefied, binder-containing, refractory molding material and then sprinkling with binder-containing, refractory molding material particles. Depending on the mixing ratio of liquid to solid molding material, each layer applied has to dry and harden for a certain time. These steps can be repeated until the desired green crucible thickness is achieved. Mostly, the primary cleaning of the wax crucible and the build up of the mold layers is automated because it guarantees more consistent results. For attaching, handling is required, which is injected with an excess of material into the crucible positive made of wax. This excess material is removed in the next step - usually with a cut at a predefined, favorable for the design of the crucible - and fixed the final shape of the crucible. The crucible positive made of wax, which is stuck in the molding material layers, is melted out by using, for example, an autoclave by means of a temperature increase, which must be above the melting point of the wax used or the crucible positive material used. Thus, the crucible positive is destroyed from wax and leaves depending on the applied layers of molding a negative whose surface forms the inside of the green crucible with a corresponding diameter. However, the individual layers of the green crucible are still porous and must be chemically consolidated and bonded by heating before use to melt a metal blank. As a result of the structure of the various layers, a fired crucible, the inner layer of which forms no chemical connection with the metal to be cast and whose outer layer is a temperature-resistant or resistant molding material, has been formed. Thereafter, the fired crucible protected by the inner layer without chemical reaction with the outer layer for melting and pouring metal may be used.

The state of the art is the international publication entitled "Crucible for melting and crystallizing a metal, a Hableiters or a metal alloy, component for a crucible base of a crucible and method for producing a component" with the file number WO002010105617A2 known. Furthermore, a "device and the method for producing a plastic bottle and its filling with a filling product" according to the publication with the file number DE102014104874A1 for stretch blow molding from a threaded preform. Also the patent document "Method for producing a ceramic casting mold by demoulding the positive and cast turbine blade" with the file reference DE102018003113B4 or the patent specification "Method for producing a ceramic casting mold by thermal destruction of the positive and cast turbine blade" with the file reference DE102018003125B4 have already been published and are given as state of the art.

issues

In the known processes for the production of crucibles for the melting of metals, the crucible positive can not be separated from the finished shape without melting out. Due to the high induced temperature differences, the green crucibles are burdened during melting with an internal pressure, which reaches its peak value by the expansion of the crucible positive shortly before melting, and thereby burst very easily. In addition, the temperature to be reached for melting out the crucible positive permanently negatively changes the mechanical properties of the green crucible. Also the Surface quality at the contact surfaces to the pot positive is affected by the thermal stress. In addition, it can not be ruled out that minimal residues of the crucible-positive material or melt residues remain on the surface of the green crucible and further reduce the quality of the surface. Another disadvantage is the mandatory process time to heat to Ausschmelztemperatur and to cool to room temperature. In order to avoid having to use high temperatures in the production of the crucible positive and during the melting and yet to use a well-shaped crucible positive material, materials with low melting points are used. This has the consequence that the crucible positive material is very sensitive to the smallest mechanical and thermal influences. For this reason, the crucible positives are usually processed in thermally sealed rooms and attached consuming on devices or workpiece carriers in order to minimize thermal and mechanical influences as possible, since most materials are very sensitive to changes of this kind.

solution

• • Preform-Material: Beeinflusst die Oberflächenqualität der Kontaktfläche des Schmelztiegels und ebenso die einfache Lösbarkeit des Tiegelpositivs aus der grünen Schmelztiegelform.
• • Preform-Verschluss: Setzt die Dimension für die Verbindung mit der Handlingseinheit fest. Für technische Anwendungen sollte idealerweise immer ein Gewindeverschluss mit Hals verwendet werden.
• • Preform-Dimension: Ist auszulegen nach der Menge des zu schmelzenden Metalls. Die Preform-Dimension legt auch das Volumen des Schmelztiegels und damit verbunden auch die Wanddicke und Gewicht des Tiegelpositivs fest.

kombiniert werden, um das optimale Tiegelpositiv für die Schmelztiegelherstellung zu erzeugen. Am besten eignen sich Verfahren, die bestrebt sind extrem dünnwandige und dabei stabile Kunststoffformen aus der Preform zu erzeugen. Diese können, nach dem Trennen des Handlings- bzw. Materialüberschusses, beim Brennen des grünen Schmelztiegels ohne weiteren Aufwand durch thermische Zerstörung der Tiegelpositive zu gasförmigen Produkten abgebaut werden. Folgende Verfahren können zum Einsatz zur Erzeugung des Tiegelpositivs aus den Preforms kommen:

• • Blasformen
• • Streckblasen
• • Extrusions-Blasformen

All of the above-mentioned problems which occur in the currently most commonly used method for the production of crucibles by layer formation for the melting of metals, based on the used mechanically and thermally sensitive material for crucible positive production and its elaborate removal by the melting out of the green crucible shape. According to the invention, the problem is solved when a plastic from the stretch blow molding or blow molding, which can be stretch blown or blow molded from preforms into the corresponding positive mold positive seal, is used as the seal positive. As a result, the above problems no longer occur and a simplified and improved method is invented. Blow molding plastics are temperature resistant up to approx. 180 degrees Celsius and can not be destroyed by simple mechanical stress or can hardly be finally removed from the mold. Due to the large selection of plastic preforms can have different properties

• • Preform material: influences the surface quality of the contact surface of the crucible and also the easy solubility of the crucible positive from the green crucible shape.
• • Preform closure: Sets the dimension for connection to the handling unit. For technical applications, ideally a threaded neck closure should always be used.
• • Preform dimension: to be interpreted according to the quantity of metal to be melted. The preform dimension also defines the volume of the crucible and, associated therewith, the wall thickness and weight of the crucible positive.

combined to produce the optimum crucible positive for crucible manufacturing. Most suitable are processes that endeavor to produce extremely thin-walled and yet stable plastic molds from the preform. These can, after the separation of the handling or excess material, be reduced during the firing of the green crucible without further effort by thermal destruction of the crucible positive to gaseous products. The following methods can be used to generate the crucible positive from the preforms:

• • Blow molding
• • Stretch blown
• • Extrusion blow molding

• • PET: Polyethylenterephthalat
• • PE: Polyethylen
• • PC: Polycarbonat
• • PP: Polypropylen
• • PS: Polystyrol
• • PVC: Polyvinylchlorid

The following typical plastics used in these processes can also be used for crucible positive production for crucibles:

• PET: polyethylene terephthalate
• • PE: polyethylene
• • PC: polycarbonate
• • PP: polypropylene
• • PS: polystyrene
• • PVC: polyvinyl chloride

Due to their properties, PE and especially PET are very well suited for the production of crucible positives. The above-described new approach eliminates or reduces most of the problems of current methods, and a simplified, improved method is invented which provides the following advantages.

advantages

• • Fehlteilkosten durch Sprengen der grünen Schmelztiegelformen beim Ausschmelzen entfallen komplett und Ausfälle der gebrannten Schmelztiegel beim Gießen werden durch die verbesserte Oberfläche bzw. bessere mechanische Eigenschaft drastisch reduziert.
• • Maschinenkosten für das Ausschmelzen durch den Autoklaven entfallen.
• • Energiekosten für das Betreiben des Autoklaven und Kühlung der Fertigungsräume entfallen.
• • Prozesszeiten für das Erhitzen und Abkühlen der grünen Schmelztiegelformen entfallen.
• • Indirekte Materialkosten für das Werkstückhandling werden durch die Nutzbarkeit des Preform-Gewindes und der höheren mechanischen Belastbarkeit des Tiegelpositivs gesenkt.

The crucible positive can remain in the finished green crucible mold and be degraded by thermal destruction during the burning out of the crucible to gaseous products. The crucible thus has better mechanical properties and is not exposed to the risk of bursting during melting, since the crucible positive is an extremely thin-walled mold. As a result, the crucible has excellent surface qualities at the contact surfaces for the crucible positive, since no residues of the crucible positive material can remain. In the complete process of crucible positive production from the plastic preform is a mechanical or temperature-induced deformation is not possible. As a result, no particularly careful treatment and tempered rooms are needed. In addition to the technical advantages, many economic benefits, which are briefly explained below, are also generated:

• • Missing parts costs due to blasting of the green crucible molds during melting are completely eliminated and breakdowns of the fired crucibles during casting are drastically reduced by the improved surface or better mechanical properties.
• • Machine costs for autoclave melting are eliminated.
• • Energy costs for operating the autoclave and cooling the production rooms are eliminated.
• • There are no process times for heating and cooling the green crucible molds.
• • Indirect material costs for workpiece handling are reduced by the usability of the preform thread and the higher mechanical load capacity of the pot positive.

The following embodiment shows an application possibility of the method for the production of crucibles by layer formation with thermal destruction of the crucible positive for the melting of metals.

embodiment

For the production of turbine blades made of titanium aluminide (TiAl), a ceramic crucible for melting the metal blank of TiAl is required per casting process, before the liquefied mass of about 1000 grams is poured into the casting mold of the turbine blade. For producing a PET crucible positive ( 3 ), the stretch blow molding method is used to produce a PET preform with closure as a thread ( 1 ) + ( 2 ) to produce a crucible positive with the capacity of about 500 milliliters ( 1 and 2 ). Multiple stretch blown PET crucible positives ( 3 ) are now using their threaded fasteners ( 1 ) is screwed to a handling aid by a 6-axis manipulator to be cleaned first. Thereafter, the PET crucible positives ( 3 ) Automatically wetted alternately with liquid yttria (Y ₂ O ₃ ) and then coated with yttria (Y ₂ O ₃ ) as a solid in powder form. After drying, this process is repeated until the desired layer thickness of Y ₂ O ₃ (6) is reached. The Y ₂ O ₃ coated PET crucible positives ( 5 ) are automated in the same way, wetted with liquid alumina (Al ₂ O ₃ ) and then coated with alumina (Al ₂ O ₃ ) as a solid in powder form and dried until the desired layer thickness of Al ₂ O ₃ (7) is reached ( 3 ). After the final drying process of the green crucible thus produced ( 4 ), these can be unscrewed from the handling aid. The ceramic crucibles can be distinguished from the material surpluses ( 8th ) consisting of ( 10 ) + ( 11 ) + ( 12 ), which were necessary for the handling and the stretch blow-molding process, are separated ( 4 ). The green crucible ( 9 ) consisting of ( 14 ) + ( 15 ), including crucible positive ( 13 ) to burn out. Burning out the green crucible ( 9 ), the crucible positive ( 13 ) under thermal destruction of the crucible positive ( 16 ) to gaseous degradation products ( 17 ) decomposes ( 5 ). By the thermal treatment, the layers of the green crucible ( 18 ) + ( 19 ) mechanically and chemically solidified and simultaneously demoulded. The result is a fired crucible with the layers ( 20 ) + ( 21 ). In the fired crucible, the TiAl metal blank ( 22 ) are placed and melted for casting the turbine blade ( 6 ).

• • 1: Preform für das Streckblasen oder Blasformen
• • 2: Streckgeblasener Tiegelpositiv
• • 3: Tiegelpositiv mit aufgetragenen verschiedenen Schichten
• • 4: Trennung von Schmelztiegel und Materialüberschuss
• • 5: Grüner Schmelztiegel mit Tiegelpositiv beim Ausbrennen zur thermischen Zerstörung des Tiegelpositivs zu gasförmigen Abbauprodukten
• • 6: Gebrannter Schmelztiegel mit Metallrohling zum Schmelzen

The figures show

• • 1 : Preform for stretch blow molding or blow molding
• • 2 : Stretch blown crucible positive
• • 3 : Crucible positive with applied different layers
• • 4 : Separation of crucible and excess material
• • 5 : Green crucible with crucible positive during burnout for the thermal destruction of the crucible positive to gaseous degradation products
• • 6 : Burnt crucible with metal blank for melting

1.

Preform-Verschluss als Gewindeausführung

2.

Preform-Körper

3.

Streckgeblasener Tiegelpositiv

4.

Kompletter grüner Schmelztiegel mit Materialüberschuss

5.

Streckgeblasener Tiegelpositiv nach der Schichtenbildung

6.

Formstoffschicht 1

7.

Formstoffschicht 2

8.

Grüner Schmelztiegel als Materialüberschuss

9.

Grüner Schmelztiegel mit Tiegelpositiv

10.

Tiegelpositivrest mit Gewindeverschluss als Materialüberschuss

11.

Formstoffschicht 1 als Materialüberschuss

12.

Formstoffschicht 2 als Materialüberschuss

13.

Tiegelpositiv als verlorene Form

14.

Ausgehärtete Formstoffschicht 1 als grüner Schmelztiegel

15.

Ausgehärtete Formstoffschicht 2 als grüner Schmelztiegel

16.

Tiegelpositiv im Prozess der thermischen Zerstörung beim Ausbrennen des grünen Schmelztiegels

17.

Gasförmige Abbauprodukte des thermisch zerstörten Tiegelpositivs

18.

Ausgehärtete Formstoffschicht 1 beim Ausbrennen

19.

Ausgehärtete Formstoffschicht 2 beim Ausbrennen

20.

Formstoffschicht 1 als ausgebrannter Schmelztiegel

21.

Formstoffschicht 2 als ausgebrannter Schmelztiegel

22.

Metallrohling zum Schmelzen

The positions show

1.

Preform closure as a threaded version

Second

Preform body

Third

Stretch blown crucible positive

4th

Complete green crucible with excess material

5th

Stretch blown crucible positive after stratification

6th

Molding material layer 1

7th

Molding material layer 2

8th.

Green crucible as material surplus

9th

Green crucible with crucible positive

10th

Crucible positive residue with threaded closure as surplus material

11th

Formstoffschicht 1 as excess material

12th

Formstoffschicht 2 as excess material

13th

Crucible positive as a lost form

14th

Cured molding material layer 1 as a green crucible

15th

Cured molding material layer 2 as a green crucible

16th

Crucible positive in the process of thermal destruction during burnout of the green crucible

17th

Gaseous decomposition products of the thermally destroyed pot positive

18th

Cured molding material layer 1 during burnout

19th

Cured molding material layer 2 during burnout

20th

Molding material layer 1 as a burnt-out crucible

21st

Molding material layer 2 as a burnt-out crucible

22nd

Metal blank for melting

Claims (3)

A method of making crucibles by layering with thermal destruction of the crucible positives using a lost crucible positive consisting of the steps a) producing a thin-walled, hollow crucible positive (3) with a thread (1) by stretch blow molding or blow molding a plastic blank (2) from a preform (1) + (2), b) mounting the crucible positive (3) to a workpiece carrier with thread suitable for the crucible positive (3), c) formation of successive layers by alternately wetting the crucible positive (5) with binder-containing, refractory molded materials in liquid form and then in powder form, d) repetition of step c) until the desired wall thicknesses of the binder-containing, refractory molding material (6) + (7) to the crucible assembly (4) is reached, e) curing the binder of the last layer to form a green crucible (4), f) separating the entire excess material of the green crucible (8), g) firing the green crucible (9) with thermal destruction of the crucible positive (16) to gaseous degradation products (17) to the burnt crucible. Method according to Claim 1 , characterized in that the plastic blank (2) or the preform (1) + (2) consists of PET. Melting crucible for the melting of metal with binder-containing refractory molding material layers, produced by a process according to one of the preceding claims.

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