A well-known preventative step to address penetration defects is to use a coating (Figure 1), though undoubtedly you’ve heard this before too. Yet, what makes a good coating? Rheology – a fancy R&D term – explains what separates and distinguishes a coating. In short, rheology describes the measurement/determination of a liquid/solid matter’s flowability.What is flowability? This term is affected by the deformation of the liquid, as well as solid components. The deformation of matter is essentially a reaction at the impact of a specific force. Surely this all makes sense, right? Clearly I’m kidding. However, just know this, the effects of poor rheology will include: insufficient layer thickness, teardrops, runs, prolonged handling times, inhomogeneity, and problems with core prints. Ultimately, it’s very important to trust a supplier with reliable R&D resources and proven technical services. That way you’ll have the assurance of knowing you’re receiving a coating with the ideal rheology characteristics for your requirements.
Now, let’s address the science behind a penetration defect.
Penetrations: Defect Pattern & Causes — Fundamental causes for real (mechanical/physical) penetration are metallostatic pressure, dynamic pressure during casting, and crystallization pressure during solidification.Thus, penetration can occur as a function of the following influence factors: • The grain size of the mold material is too large and the grain particle size distribution is too broad; • The proportions of binder are too low.
The proportion of materials that form lustrous carbon is too low;
• Unfavorable chemical composition of the casting material in combination with casting temperatures and metallostatic pressure that are too high;
• Inadequate and uneven compaction of the molds or cores; and
• Inadequate gating system and therefore excessive overheating of molds and core components
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