Castabilityn Analysis

Castability, a critical aspect of casting process, is the ease of producing a casting with minimum costs, defect and lead-time.

Castability analysis is a very important process, which guides in achieving the optimal product design of the desired quality. Castability analysis involves three stages, namely, which are discussed in detail below,

  • Casting Simulation
  • Casting Cost Estimation
  • Castability Checks

Casting Simulation This involves analysis of processes such as mold filling, grain structure, stress analysis, distortion and solidification. It also takes in account models of product and tool equipment like cores, parting, mold layout, feeders, gates etc., along with temperature regulated properties of mold and part materials and finally the process parameters such as pouring rate, temperature etc. The results from these studies are applied in predicting casting defects like hard spots, blowholes, cold shuts, shrinkage porosity, cracks and distortion.

This method requires expert handling of the inputs, which are normally not easily accessible to the product designers. However, best results can be achieved by concurrent engineering approach wherein the foundry and tooling engineers actively participate in the product design phase and simultaneously evolve the product, tooling and process design thereby ensuring mutual compatibility.

Cost Estimation Cost estimation is very vital in the entire casting process. The cost factor is directly proportional numerous aspects of the product such as weight, part metal, shape complexity, number of holes, type of parting line, feeders involved, yield, metal to sand ratio, quality requirements, production quantity, tooling material, elements in the mold like feed aids, filters etc and finally the desired lead time. Taking in account these factors the cost coefficients can be determined by regression analysis of past data. Past data refers to data extracted from sufficient cases of complete and correct process details. These coefficients are updated regularly and employed in assessing the process design and also applied in the “what-if” design analysis to lower the costs.

The drawback of this approach is that it can be utilized adequately only in case of castings which are similar to previous cases on which they can be correlated with respect to part metal, casting process and geometry.

Castability Checks Castability checks are very relevant and there are many guidelines documented to handle the common casting related issues. The figures illustrated below reflect some on the common problem scenarios and their recommended solutions.

Figure 1: Guidelines for parting and undercuts
Figure 2: Guidelines for solidification and feeding
solidification and feeding

Figure 3: Guidelines for mold filling and gating
mold filling and gating

Figure 4: Guidelines for cores
Guidelines for cores

Usually geometric details of casting features, such as base features (rectangular, spherical, torus etc), form features (pocket, slot, boss, rib, hole etc), tooling features (undercut, parting line, core, feeder etc), feature modifier (taper, fillet, chamfer), are collectively employed for initiating detection of potential problems or “health-checks” in the process. This is carried out by analyzing information about the each features and assessing their relation with each other. Generally such Castability health checks are formulated as a criterion to be maximized. For instance, a rib feature can be assessed for its thickness and correlated to the section where it is connected. In such instances the coefficients in the criterion equation largely depend on the metal process combination, which are determined from past data. The criteria can be weighted for signify their relevance and finally the Castability index of a particular casting design is evaluated as a weighted sum of all the assessment criteria.

Casting simulation and Costing methods are indicative of the desired quality and costs requiring inputs from product, tooling and process and thus can be used only on completion of designs. The feature based Castability checks are indicative of potential issues in the process and can be easily set up during concurrent design of product, tooling and process. Thus feature-based checks are used in the initial assessment of a good design and then verified further by process simulation to derive a zero defect casting after being subjected to final parametric cost estimation to ensure meeting the targeted expense for the casting.

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