Sand molding process
The sand molding or sand casting process is a casting process for metal and other materials that works with molds made from sand . It works on the principle of the lost mold , which means that the mold is destroyed after a single use to demold the cast body.
Procedural principle
For casting in a sand mold, a foundry model is molded in sand, then removed and the remaining cavity is filled with the molten material. In order to compensate for any shrinkage that may occur during the cooling process, the molds must be 0.5–2.5% larger. The metal flows into the sand mold, a system of outlet channels in some cases facilitates the escape of the gases from the cavities. After the cast body has solidified, the sand is removed in order to remove the workpiece. The sand can be processed and processed into new shapes.
Foundry pattern materials
Depending on the economic or technological requirements (number of pieces, accuracy or tradition), a foundry model consists of different materials: conventionally from suitable woods or laminated wood materials, in some cases from fired clay. In today's large-scale production, foundry models made of CNC-milled polyurethane block materials or highly abrasion-resistant, high-impact polyurethane casting resins are used, which allow higher numbers of shots than foundry models, which can also be made from metals.
Example of sand casting mold production (mold with two mold halves)
- The lower foundry pattern half is placed / mounted on a pattern plate. A mold frame is placed on the plate. Core brands must be introduced.
- The molding frame is filled with molding sand and this is compacted until it sticks to the frame (the sand-filled frame is called a molding box).
- The box is lifted off the lower model plate and turned (the lower half of the model remains in the sand).
- The upper half of the model is placed on the lower half of the model that has remained in the sand, then another mold frame is placed and filled with sand, which is then compacted.
- Then the upper molding box is removed, the model halves are removed from both boxes and cores may be inserted.
- Now both molding boxes are put together to form a closed mold, inside of which there are cavities in the form of the models (as well as the pouring opening, etc.) and poured.
After casting, the sand and cores (also consisting of sand) are removed in a destructive manner, thus exposing the cast part inside.
Further processing of the freshly cast part
The cast part cannot be used after casting and is usually taken to the foundry department called the foundry cleaning shop, where it is freed of sand residues and the sprues, feeders, risers, casting branches and casting burrs that are still on the cast part are removed. Until the cast part is ready for use, further manufacturing steps are now used if necessary, e.g. B. machining (drilling, grinding, thread cutting, milling, turning, etc.), as well as painting, powder coating or galvanic treatment. Depending on the material, metallurgical processes are also used to upgrade and condition the metallic structure (e.g. aging, hardening, tempering, etc.)
Technological scope
With the appropriate knowledge and experience (specific to the respective material), the sand casting process is suitable for manufacturing parts with complicated geometries (including cavities) and different material cross-sections.
Further, in-depth explanations of the sand casting experience
In casting production, sand casting has been supplemented by casting processes for larger quantities with permanent mold casting and for rotationally symmetrical geometries with centrifugal casting . An exception is, for example, bell casting , in which the molds would be too large for centrifugal casting. Thanks to this technology, a vast variety of cast products of all sizes and materials is manufactured using sand casting, for example cylinder rollers, steam turbine housings, etc.
To stabilize the sand (basic molding material), a binding agent is required, for example clay ( bentonite ) with the appropriate amount of water ( wet casting or green sand casting ) or oil, water glass with CO 2 or cold resins ( furan ). The mixing of the basic molding material and additives takes place in the sand mixer .
Sand molds (with quartz , chromite , zirconium , corundum , etc.) are or have been used, among other things, for bell casting and artificial casting and are predominant in the entire foundry industry for the production of metal castings . The traditional term of sand has been replaced in the modern usage of foundry technology by the technical term molding material , since the materials used have little in common with naturally occurring raw materials for the production of casting molds.
If the sand mold was made with a model that burns (gasifies) in the mold when it is poured, it is referred to as full mold casting with a lost mold, since this sand mold must also be destroyed when the cold cast part is emptied.
If a cast part is to be hollow on the inside or if it should have complex geometries, so-called cores are used. These are bodies made of compacted, bound sand, which are placed in the mold at designated points and which fill the later desired cavity as a displacement body. In some cases, the mold can consist entirely of such cores: one then speaks of a core shape (example: multi-part sand casting molds for the water-cooled cylinder head of internal combustion engines; with complex cavities such as oil channels and the cooling water jacket, which could not be cast with solid metal molds) .
Further development, outlook
There are already areas of application in which the sand molds and cores without going through a foundry model z. B. CNC-milled directly from compacted sand blocks strengthened by means of resins, or the sand molds are produced directly with a 3D printing process (" Binder Jetting ").
Individual evidence
- ^ Ernst Brunhuber (ed.): Giesserei Lexikon. Fachverlag Schiele & Schön, 16th edition, Berlin 1994, ISBN 3-7949-0561-X , pp. 998-1004.