What is the pouring process

Casting (process)

The manufacturing process to water is one of the primary molding processes. It is probably the oldest shaping process. When pouring, a solid body of a certain shape is created from the liquid material after it has solidified. Workpieces are cast if their production using other manufacturing processes is uneconomical, not possible or special properties of the cast material are to be used. Casting processes can be used particularly advantageously for the mass production of components of all types, both simple and complex shapes. Cores can also be used to create cavities in the interior of the cast parts. The costs for model / mold construction are comparatively high, but are put into perspective with the number of castings; also depending on the respective casting process.

Due to global competition and increased production costs, especially for energy[1] Many foundries, especially iron foundries, were closed in German-speaking countries and mainly relocated to Eastern Europe and Asia.

training

The place of work is called the foundry, the worker is called the foundry. The official names[2] are:

  • Foundry mechanic (apprenticeship period 3.5 years) specializing in hand-molded casting, pressure and permanent mold casting as well as machine-molded casting.
  • Metal and bell caster (apprenticeship period 3 years) specializing in art and bell casting technology, metal casting technology and tin casting technology

Despite mechanical aids, relatively high physical demands are placed on the foundrymen and their helpers.

Cast materials

Cast materials are essentially iron-carbon compounds (cast iron, steel), non-ferrous metals such as copper, lead, tin, zinc, nickel and their alloys, and light metals such as aluminum, magnesium and titanium and their alloys. Furthermore, molded parts made of other castable materials or materials, such as plastics and clay, ceramics or plaster of paris, can be produced by casting.

Melting technology

To produce the liquid state, various melting units are used, which, depending on the type, are heated with gas, oil, coke or electrically. These include cupola furnaces (shaft furnaces), Siemens-Martin furnaces, flame furnaces as well as electric arc furnaces and induction furnaces.

Workers at an electric / induction melting furnace

Molding and casting technology

During casting - depending on the casting temperature, contour and mass of the casting - the molds are subjected to such high thermal and mechanical loads that changes in shape and dimensions can occur. This must be taken into account with foresight in molding and casting technology as well as in model construction. Furthermore, the specific shrinkage of the intended casting material, which occurs when it cools from solidification to room temperature, must be taken into account when producing casting models and metal casting molds.

The material properties of the castings depend on the chemical composition of the melt on the one hand and the structure in the solid state on the other. The resulting structure depends on the cooling rate and the conditions for nucleation and crystal growth that prevail during solidification. Different cooling speeds in a workpiece depend on the shape, in particular the wall thickness. In order to be able to ensure a homogeneous cast body structure with the best possible mechanical properties, a directional solidification in the mold towards the sprue and the feeders, the location of the last solidification, must be ensured. In practice, this is achieved with a series of complementary, freezing-directing measures, such as forced cooling, heating of mold sections and suitable gating and feeder technology. In this context, a casting-compatible design of the casting is of great importance. Points to be avoided in particular are areas with accumulations of material, large differences in wall thickness, sharp corners and edges and contours where stresses can occur during solidification. That is why close cooperation between the component designer and the casting specialist is necessary in advance.
Workers on the core insertion line in the machine molding at the Heunisch foundry

The manufacturing processes of casting are differentiated according to the pattern set-up, the molding materials, the mold production and the casting method. Here pouring is divided into two groups:

  • Cast in lost form
  • Casting in permanent molds.

Special processes are low pressure casting, composite casting, bell casting, art casting and casting.

Cast in lost form

Casting in lost form (upper and lower box)
Upper and lower box for sand casting with inserts and risers

Lost forms are mostly made from sand with suitable binders. A model is required for shaping, a pattern of the casting to be produced, which is used to compensate for the shrinkage of the cast material must be made with an allowance (shrinkage) when it cools down (see above). One or more feeders are used to compensate for the shrinkage in the liquid state and during solidification.

As with forms, a distinction is made between permanent models and lost models. A further distinction is made between nature models and core models. Natural models correspond in their shape to the part to be cast (larger by the amount of shrinkage), core models also have core marks that serve as bearings for the cores to be inserted. Cores are required when certain contours in the casting (mostly cavities) cannot be formed with the help of a natural model. Permanent models are made of plastic, wood or metal, depending on the requirements. They not only contain the image of the casting that is to be cast, but also the gate, these are the channels through which the casting material is filled and distributed in the mold and through which the air contained in the mold and the gases produced during casting are discharged. Permanent models are surrounded by molding sand, which is then compacted by shaking and pressing so that it is stable. As a rule, castings are formed on both sides. That is why the mold is made up of two molding boxes, the lower and upper boxes, so that the model can be removed again before casting. The model must therefore not have any undercuts and must have "draft angles" (drafts) so that the shape is not damaged when the model is removed. Then the upper and lower box are put together again precisely, secured with clamps or load irons and the liquid cast material is poured into the mold created in this way. The “parting line” of the mold can often be seen on the finished casting, as it was either subsequently processed there or still contains the rest of the “burr”.

The binding agents of the molding sand are selected in such a way that they are destroyed by the "casting heat" if possible and the mold disintegrates by itself, otherwise the mold has to be mechanically destroyed in order to be able to remove the casting. Molding materials with their binders are so well developed today that they can also be used and reused for higher-melting metals.

Upper and lower box for machine molding (sand mold)

Lost models, on the other hand, are not removed from the mold before casting, so it can be made in one piece. Undercuts can also be incorporated here and draft angles are not required. The models are destroyed after the casting material has been poured in, as they either evaporate, melt or decompose in some other way (e.g. polystyrene), or they are melted out before casting (wax, synthetic resins). However, you need a preliminary model or a processing step, since they are also produced using a shaping process. See also full mold casting.

According to the type of models, casting into lost forms is divided into:

  • Casting with permanent models
  • Casting with lost models
    • Investment casting
    • Full mold casting
    • Special technology in the fine art foundry
    • Lost wax casting.

Casting in permanent molds (chill molds)

Filling liquid cast iron into a transport container at a supplier for the automotive industry

A larger number of cast parts can be produced with the permanent molds, since the molds are not destroyed by the casting process and can therefore be used permanently. These forms usually consist of metallic, rarely of non-metallic materials. Here one does not subdivide into the type of molds and their production variants, but into the type of mold filling, whereby this can be achieved through the action of different forces (e.g. gravity, pressure, centrifugal force, etc.).

Due to the constantly recurring thermal and mechanical loads during casting, the mold material must have the following properties:

  • sufficiently high melting point
  • Temperature resistance
  • low thermal expansion
  • high resistance to temperature changes
  • maximum wear resistance
  • good temperature and thermal conductivity
  • good mechanical workability
  • resistant to tempering
  • not susceptible to hot cracks.

That is why cast iron, cast steel, steel or copper alloys are used. Cores are also used here. Permanent cores are used for simple internal casting contours and lost cores for more complex contours (except in the die-casting process).

Compared to manufacturing processes with lost molds, casting processes with permanent molds have the following advantages:

  • smaller production areas
  • increased labor productivity
  • increased yield
  • less core molding material and elimination of the molding material
  • reproducible dimensional accuracy and thus lower cleaning effort, lower reject rate and lower additions for mechanical processing
  • improved surface quality
  • rapid cooling and better mechanical properties of the casting.

On the other hand, there are the following disadvantages:

  • more expensive shapes because these are mostly made of metal
  • low compliance and gas permeability
  • Cast parts with a high probability of internal stress and hot cracking
  • Cast surfaces with high hardness values.

That is why casting in permanent molds is used for large-scale production so that the high manufacturing costs for the metallic molds are amortized.

The casting processes with permanent molds include die casting, permanent mold casting, injection molding (only for plastics), centrifugal casting and continuous casting. A special casting process for the production of small series from plastics or low-melting alloys is vacuum casting.

In permanent mold casting, the liquid metal is usually only introduced into the mold by the action of gravity. An exception is the low-pressure chill casting process, in which the metal is pressed from a closed holding furnace by means of excess pressure directly into the chill above it. The process is suitable for the automatic casting of high-quality castings in series production. Almost all castable non-ferrous metals are processed in permanent mold casting.

historical development

In contrast to forming processes such as forging, casting molds are required for all casting processes. Prehistoric cultures developed four main types of molds

  • 1. One-piece stone molds for castings that are flat on one side;
  • 2. Removable, reusable, double-sided molds for the production of castings molded on both sides;
  • 3. For the production of hollow metal tools, casting molds with a core in the cavity, so that the melt could only pour into the space between the outer mold and the core, so the inside remained hollow. In doing so, however, threw the attachment of the core. Certain problems inside the hollow mold.
  • 4. Casting molds for single use, especially for the production of complex shapes. The mold was made over a wax, lead or other type of model that was melted out. Lost wax casting. Incidentally, in prehistoric Europe every metal that was mined was used for casting, with the exception of iron.

In the course of series production, more productive processes emerged later. On the one hand, the use of models made of wood, the contours of which were transferred into divisible form (sand) boxes (by pressing in with high pressure), on the other hand, through permanent forms made of structural steel. These steel molds produced in mold construction are mainly used in injection molding (plastics processing), permanent mold casting and die casting (light metal such as aluminum, magnesium and zinc). Depending on the structure of the die casting machines, a distinction is made between hot chamber and cold chamber machines. The cold chamber machines are usually larger and nowadays (2002) achieve a pressing force of up to 50,000 kN.

Course of the casting process

After the molten material has been poured into the mold, the solidification process takes place. This solidification process can be straight-walled, rough-walled or spongy (exogenous solidification) or pulpy shell-forming (endogenous). When solidifying, the specific volume of the cast material decreases, which is subject to shrinkage depending on the falling temperature. This shrinkage must be taken into account when designing the model.

Aftercare

Cleaning is a post-treatment of castings in which pouring funnels, feeders, barrels, seams, the casting core and the adhering molding material are removed. This is done using sandblasting blowers, all of which work with steel shot or steel wire grain, water jet machines, welding and flame cutting equipment, cleaning drums and vibrating devices. The scope of the plastering work should be as small as possible. Plastering work is influenced by the shape division, number and position of the cuts, cores, molding materials and other factors. The work in the fettling shop also includes the removal of casting defects.

Casting process simulation

A powerful software for the simulation of casting processes offers possibilities for interactive or automated evaluation of results (here for example on mold filling and solidification, porosity and flow behavior).

The casting process simulation uses numerical methods to predict the entire casting process including mold filling, solidification and cooling of cast parts and also allows the quantitative prediction of mechanical properties, thermal stresses and warpage of the cast parts. By means of simulation, the quality of a cast part can be precisely described even before production begins, and the casting technology can be tailored to the desired part properties. This not only eliminates expensive test casts in development. The precise design of the entire casting system also saves energy, material and tool costs.

Software for the simulation of casting processes supports the user from the design of the component, the definition of the melting practice and casting technology, through model construction and mold production to heat treatment and post-processing. This means that costs can be consistently saved along the entire production chain.

The casting process simulation has been developed at universities, particularly in Europe and the USA, since the 1970s and is considered to be the most important innovation in foundry technology of the last 50 years. Commercial programs have been available since the late 1980s, giving foundries the first insight into casting technology, which was previously a black box.

Individual foundries

Individual foundries are or were among others

  • Bauersche Foundry, Frankfurt am Main (1837 to 1972)
  • Union foundry Königsberg (1828 to 1931)
  • Machine factory in Liezen and foundry
  • Torgelow iron foundry
  • Heinrichshütte foundry near Hamm (Sieg) in the Westerwald, Altenkirchen district (1650 to 1927)
  • Heunisch foundry
  • Cast steel Gröditz
  • Royal Foundry House, (1644–1806)
  • Knight’s Foundry and Shops, (1873 to 2000)
  • Howaldtsche Metallgießerei Industrial Museum, (1884 to 1980)
  • Fritz Winter iron foundry

literature

  • Paul Schimpke: Engineering materials technology; Stuttgart, 17th ed. 1968
  • Karl Stölzel: Foundry for thousands of years. German publishing house for basic industry, Leipzig 1978, without ISBN
  • G. Spur, Th. Stöferle: Manufacturing engineering manual. part 1 Archetypes. Carl Hanser Verlag, Munich Vienna 1981
  • A. Herbert Fritz, Günter Schulze: manufacturing engineering. Springer-Verlag, Berlin Heidelberg 2006, or ISBN 3540256237
  • Rolf Roller (Ed.): Expertise for foundry professions, New edition; o.O., 2007

See also

  • Lost foam process
  • Molding
  • Cores (process)
  • Gunmetal
  • Melt treatment
  • Foundry practice, magazine
  • Fine art foundry, (Art cast) Special form of bronze technology

Web links

credentials

  1. ↑ Ernst Brunhuber: Cast from copper alloys, Schiele & Schön 1986, p. 105, here online
  2. ↑ Federal Employment Agency, job descriptions at BERUFENET