How Penetration Defects Happen, and How to Prevent Them

Sept. 4, 2014
Metalcasters need to understand the realities of rheology to choose, apply and appreciate coatings’ influence Gaps in molds Making a coating effective Understanding flowability Defect influence factors
Figure 1: A view of a casting depicting a penetration defect without the use of a coating, and the results of casting the same design in a mold with a coating.
Q: What is the science behind penetration defects?  Although I’ve heard and read explanations to this question — the information always seems oversimplified.  I’d like to know the mechanics of penetration, how it happens and how to prevent it from happening.   A:A penetration defect is one of the more common imperfections discovered on a casting.  The fact that penetration is so common in metalcasting operations probably explains the “oversimplified” responses you apparently have had up until now.  Presumably, you’ve been told that gaps in the mold are responsible for the occurrence.  That’s actually true, however few people seem to understand why these gaps form or how to prevent them. 

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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