Investment casting is a process for the production of cast parts ( molded casting ), by means of which even small and very small parts can be produced true to shape and with minimal post-processing. Investment casting is classified as a casting from lost models and lost molds, since both the model, usually made of wax or plastic, and the mold no longer exist after the casting.
Investment casting is the production of small to very small cast parts using the lost wax process . The castings are characterized by their level of detail, dimensional accuracy and surface quality. Machining can often be saved. Up until the middle of the 20th century, there was not always a clear distinction between injection molding and investment casting in the literature.Meanwhile, injection molding is widely used as a term for plastic parts manufactured in a variant of the die casting process, whereas investment casting is primarily made using the gravity process - as is common with chill casting - but also occasionally can be produced from almost any alloy as low-pressure casting or centrifugal casting.
The model is made from specially suitable waxes or similar thermoplastics or their mixtures, for example by injection molding . The models are initially injected in single or multiple molds. These tools are usually made of aluminum or steel . Depending on the total number of pieces, the shape of the casting and the type of model material, the appropriate injection mold is built. In order to introduce undercuts in the contour, preformed water-soluble or ceramic cores may be required, for which an additional tool is then required.
Another possibility for producing the physical templates is the use of rapid prototyping technologies . It is a matter of lost models made of a meltable, gasifiable or in a liquid soluble model material.
In the next step, the models are combined with the casting system to form so-called model clusters. This cluster of models is then dipped into a so-called slip. The slip is a ceramic mass for the production of a molded shell from fine refractory flour as a molding base and, for example, ethyl silicate as a binding agent. The grape wetted with slip is then sprinkled with sand or the grape is immersed in a bed of sand fluidized by compressed air. Dipping and sanding is repeated until the shell mold has achieved the stability required for casting. Special melting ovens (usually autoclaves) are used to melt the wax models at around 150 ° C, while the molds are fired at around 750 to 1200 ° C. The fired forms can now be poured directly. If the metallostatic pressure and the casting temperature of the melt are high, the molds can also be placed in a box and backfilled with dry sand as a filler. Pouring is mostly done in hot molds so that even narrow cross-sections and fine contours "flow out" cleanly, as the caster calls it. After casting and complete solidification of the melt, the casting is removed from the mold and the cast parts are separated from the casting system by means of a cutting disc, saw or vibration. The necessary reworking then takes place through cleaning, grinding, blasting as well as heat treatment and straightening, as well as the necessary tests of the cast parts.
With this process, castings with a mass of 0.001 up to several hundred kilograms can be cast. The linear tolerances are around ± 0.4 to ± 0.7% of the nominal dimension. For investment castings made of copper - for electrical engineering - a higher degree of shrinkage of 1.8% to 2.2% must be taken into account. In general, iron, aluminum, nickel, cobalt, titanium, copper, magnesium or zirconium-based steels and alloys can be cast in the investment casting process.
The advantage of the process is that the cast parts are usually ready for installation or require very little machining. As a rule of thumb for economic efficiency, investment casting is particularly beneficial if the component is complex and / or the more difficult it is to process. In addition, the method is particularly advantageous when additional add-on parts required in other methods can be integrated into the investment casting part.
Alternative to fine cast steels
The construction material silicon tombac is sometimes used as an alternative material for mechanically stressed components. The reason for this is the similar mechanical values of both materials. As a rule, however, silicon tombac is used in the die casting process.
- DKI Informationsdruck 1.004: Copper - occurrence, extraction, properties, processing, use. (see especially p. 9).
- Cast from copper alloys . Fachverlag Schiele & Schoen, 1986, ISBN 978-3-7949-0444-0 , p. 96 ff.
- HJ Müller, Handbuch der Schmelz- und Alierungspraxis für Leichtmetalle, Verlag Schiele & Schön, Berlin 1977, ISBN 3-7949-0247-5 , see p. 110 f.
- Andreas Kalweit, Christof Paul, Sascha Peters, Reiner Wallbaum: Handbook for Technical Product Design: Material and Production, Decision-Making Bases for Designers and Engineers. Springer-Verlag, 2011. Page 423f. ISBN 978-3-642-02642-3 .
- Stephan Hasse: Taschenbuch der Gießerei-Praxis 2010. Fachverlag Schiele & Schoen. ISBN 978-3-7949-0801-1 .