Mold production

Production of casting molds using sand with the addition of binders (bentonite, resin, water glass) and various additives.

As a general rule, differentiation is made between molds for small, medium, and large castings; according to that, mold production is also classified in hand molding and machine molding. Today, conventional hand mold casting is not of importance any more. Even smallest batches are realized using other methods (e.g. Rapid prototyping). Meanwhile, large-part casting (using open or floor molds) has also been mechanized and partly automated (mold material slinger) so that this can no longer be referred to as hand molding process in a classical sense.   

A special case in hand molding is boxless molding using what is known as hinged boxes or snap flasks that are opened after stripping the mold from the pattern so that a boxless mold block is obtained. A special kind of hand and floor molding is strickling.

For machine molding, molding machines and automatic systems with appropriate pattern equipment are used. Differentiation is made between box molding methods and boxless molding (e.g. in disamatic systems).

The mold quality is characterized by:

  • Accuracy of contour rendering
  • Mold and contour stability
  • Isotropy of properties (homogeneous compacting)
  • Technological, casting-related condition of the mold

The most important influential factors with regard to mold quality taken as a basis for high casting quality are listed below: 

  • Mold material quality
  • Parameters of the mold-forming process
  • Design and profile of the castings 

For the process of mold production using bentonite-bonded mold materials, the defining parameters are density and strength capacity. Mold production, i.e. mapping and forming of the shape-defining interior mold cavity and the exterior mold cod dimensions is performed in three phases:

1. Introduction of the moist, pourable mold material into the mold box or the mold chamber. This partial process has significant influence on the mold quality finally obtained. Mold strength and density as well as homogeneousness of mold properties are considerably influenced by the rate and evenness of bulk density available prior to main compaction. For this reason, the process of “mold box filling” preferably ought to be referred to as “pre-forming of contours”. It is particularly uniform introduction of mold material into back drafts or undercuts of the pattern contours and in regions between the pattern and the mold box, as well as prevention of bridge structures that have great influence on the occurrence of casting defects (e.g. local penetration). Filling aids (such as aerators, screens, vibration, reduced pressure, among others) can be used to positively influence filling.

The pouring behavior characterizes the mold material properties in conditioned, loose state prior to compaction (s. Compaction). It has great influence, particularly with regard to design of complex pattern and mold contours. Good pouring and running behavior of mold materials ensures for complex pattern contours and back drafts, difficult mold cods and what is known as mold pockets to be filled with mold material and thus achieve accurate rendering of the pattern profile. 
Modern molding machinery requires molding materials with good pouring behavior since the filling process is very fast, great mold material quantities are metered and uniform distribution along the cross section and height of the mold box or mold chamber must be achieved. The goal of pre-forming of contours is to obtain as uniform a density of the molding material as possible throughout the entire mold volume, including the pattern contours.

2.  Solidification of the clay-bonded molding material through compaction (“contour solidification”) The goal of this section is to achieve good and uniform compaction throughout the entire mold. Type and intensity of compaction, mold material quality (strength, flowability, “smooth running"), compaction aids (squeeze head design, different squeezing variants, fluidization) are major influential factors.
Compaction of the loose, poured mold material causes formation and solidification of the mold contours.

3. The aim of Demolding behavior (separation of the compacted mold from the pattern) is to overall maintain the formed and solidified mold contour and the mold cod without defects. In this context, the mold material quality (moldability, plasticity, tensile strength) has a major influence in addition to technical parameters vibration lubricants, pattern design, mold inclinations).