Centrifugal casting

Centrifugal casting is a casting process for the production of rotationally symmetrical components (e.g. cast iron pipes for water and sewage). For this purpose, liquid metal (melt) or liquid plastic (e.g. cast polyamide) is poured into a mold ( chill mold ) rotating around its central axis . The melt is also set in rotation by friction-related shear forces and pressed against the mold wall by centrifugal force . The speed of the mold is selected so that high centrifugal forces act. Compared with static casting processes, the melt solidifies in this way to a structure with significantly fewer pores, voids , a higher degree of purity and higher strength . The outer contour of the component is determined by the internal geometry of the mold.

Horizontal casting

Centrifugal casting, horizontal mold.

With horizontal casting, the melt is poured into a horizontal mold that is closed with lids from the front and back. The mold, which is made of steel or cast iron, is often lined with a ceramic protective layer, the so-called sizing . For particularly long castings or for continuous centrifugal casting, the casting channel can be moved during the casting process. The outer contour is mostly smooth, but cast-on collars, a slightly conical contour or steps are also possible. Due to the centrifugal force, the inner surface is cylindrically smooth. The wall thickness results from the amount of melt poured in. In centrifugal casting with a movable casting channel, the wall thickness can also be influenced by the speed of rotation and the axial displacement of the casting channel.

The dimensions that can be produced are heavily dependent on the material and other technical restrictions. Inner diameters from 20 mm are feasible, but economically only from 40 mm. The wall thickness should, depending on the material, not be greater than the inner diameter. The outside diameter is almost unlimited, but in practice it is often limited to approx. 1200 mm. The length depends on the inner diameter and material. It is typically between one and a half inner diameters and max. 6 meters.

Vertical casting

With vertical casting, the casting takes place in a mold, the axis of which rotates vertically. Vertical casting is mainly used for conical or spherical outer contours. The speed of rotation is often significantly slower than with horizontal casting. As a result, a parabola forms on the inner surface through the combination of gravity and centrifugal force, the shape of which can be adapted via the speed. Furthermore, the inner contour can be controlled via the amount of material introduced.

Composite casting in centrifugal casting

Composite casting example: guide rollers for continuous casting plants

In composite casting , several layers of different materials are poured on top of one another, usually while the solidification is not yet complete. In horizontal casting, products can be created that combine different properties of different steels. In contrast to welded or applied layers, the composite casting allows the transition of the two materials to take place in fine gradations. The advantage of composite casting lies in the higher productivity compared to classic application processes and in the possibility of achieving large wall thicknesses of the outer layer.

Fields of application

Tube

Applications

Water supply , sanitation

Decanter

Applications

Pharmaceutical industry, biotechnical industry, sewage sludge and paint sludge treatment, cooling emulsion processing, plastic recycling, food industry

Components

Conical and cylindrical drum shells, screw bodies and screw body parts, sealing covers, end walls, seal carriers, pipes, bushings, rings

Engine construction

Applications

Valve seat rings in the inlet and outlet of large diesel engines. These diesel engines are used as drive units for ships and as stationary power generators.

Components

Inlet valve seat rings and exhaust valve seat rings

Rolling mills, metallurgical plants and steel mills

Applications

In continuous casting plants for: slab, bloom, beam blank and billet plants

Components

Strand guide rollers for all segments of the continuous casting plants, with or without internal cooling, in mono-cast or as a composite design ("bi-metal").

Mathematical description

rotational speed

Calculation of the necessary minimum speed to overcome the earth's gravity field ( rule of thumb according to Hurst ):

{\ displaystyle n \ approx {\ frac {7200} {\ rho \ cdot {\ sqrt {D}}}}}

${\ displaystyle n}$ : Speed in min ⁻¹
${\ displaystyle \ rho}$ : Density in g / cm ³
${\ displaystyle D}$ : Outer diameter of the casting in mm

In order to avoid voids and slag and to avoid compression, the machine operates at speeds of up to 4,000 rpm.

Casting pressure

The casting pressure is generated by centrifugal force and can be approximately calculated using Väth's formula:

{\ displaystyle p \ approx {\ frac {\ rho \ cdot \ omega ^ {2}} {3000 \ cdot g}} \ cdot \ left (R ^ {2} - {\ frac {r ^ {2}} { R ^ {2}}} \ right)}

p: casting pressure in bar
${\ displaystyle \ rho}$ : Density in g / cm ³
${\ displaystyle \ omega}$ : Angular velocity in rad / s
g: acceleration due to gravity = 981 cm / s ²
R: outer radius in cm
r: inner radius in cm

The casting pressure can (depending on the speed) rise to 50 bar. Compared to sand casting , this results in denser cast structures that are free of voids and inclusions .

Other areas of application

Centrifugal casting processes are also used in other areas of application, e.g. B. in the production of concrete sewer pipes (see concrete pipe ) and reinforced concrete masts, such as conical guy masts for tramway overhead lines. The company UFIP manufactures percussion cymbals using the “rotocasting” process .

Web links

Centrifugal casting process . Klaus Kuhn Edelstahlgießerei GmbH.
Centrifugal casting process . Example of centrifugal casting from non-ferrous metal, stainless steel and aluminum - LBI
Centrifugal Casting in Action . Gibson Centri Tech Limited (animation).

Individual evidence

↑ Bottenbruch Ludwig (Ed.): Technical thermoplastics. Polyamides . Hanser, Munich 1998, ISBN 978-3-4461-6486-4 , pp. 417-418.
^ ^A ^b Alfred Herbert Fritz: Manufacturing technology . Springer, 2008, ISBN 978-3-540-76696-4 , pp. 75–76 ( limited preview in Google Book search).

[1] Bottenbruch Ludwig (Ed.): Technical thermoplastics. Polyamides . Hanser, Munich 1998, ISBN 978-3-4461-6486-4 , pp. 417-418.

[Fritz_2008-2] A ^b Alfred Herbert Fritz: Manufacturing technology . Springer, 2008, ISBN 978-3-540-76696-4 , pp. 75–76 ( limited preview in Google Book search).