Model makers and other foundry workers refer to an undercut as a construction element that protrudes freely from the casting and can thus prevent it from being removed from its mold . The term is also used accordingly in the artistic production of profiles , reliefs or sculptures .
While an undercut is a major technical problem in casting, it is used in surface design to increase the shadow effect.
An undercut can e.g. B. be a cam protruding from a crankcase of an engine block . This form element prevents a model from being removed from a sand mold ( demolding ). This problem also occurs when working with permanent molds , but the cast part cannot be removed here.
When making a model, the model maker must take care to avoid undercuts. This can be achieved by changing the shape. In this case, the protruding part is drawn as a rib down to the level between the "upper box" and "lower box" (division level). Converting to a rib is also a solution when working with permanent shapes.
Change of division
If it is structurally not possible to prevent an undercut, the parting plane of the model can also be changed (made uneven). However, this can be very complex and not always possible. In the case of machine production with models or permanent molds, an “uneven” division of a mold or a model can be advantageous. Intermediate forms (so-called "sliders" ) can also be used.
When the casting mold is hand-made, especially as a sand mold , a loose part is attached to the model to produce an undercut (it is already taken into account when the model is produced). When the model is removed from the finished form, this part initially remains in the resulting form, so it detaches from the model. In a further work step, the loose part (s) must then be removed from the mold without damaging the mold itself. It is not uncommon for these parts of the model to be forgotten in the form or lost in other ways.
Pulling in the parts
In the case of machine production and large quantities, intervention by a worker is rather undesirable. Therefore, models with undercuts are made with pneumatic control. Before removing the model from the mold, the protruding parts of the model are pulled inwards, preventing damage to the mold. In the case of permanent molds, it is only possible to remove the cast part.
If the number of pieces is small and the molds are manufactured manually or if there are unavoidable undercuts in parts that are cast in large numbers, a lost model is created. The undercut does not play a role here for demolding. Lost models are often made of polystyrene , in artificial casting from wax .
The lost model will:
- either completely or largely destroyed during removal
- destroyed by burning the model in the mold
- evaporated by the casting process when the liquid metal fills the mold.
Avoidance in construction
Experienced designers allow the problem of undercuts to flow into the planning of a workpiece and avoid undercuts in the design phase.
However, modern 3-D printing processes with sand allow the production of complex models with countless undercuts. Recently, this technology has been attracting more and more attention in prototyping and small series. The big advantage over the classic design is the faster response time to changes to the model, which may be necessary in the further process. Since simply changing the 3-D data and creating a new core or form package can be completed within a day, this results in new perspectives for all areas.