Gaining a Fuller Understanding of the Core-Blowing Cycle

Q: How can I use simulation to help solve defects in my core make process?

A: It is impossible to produce good castings without molds and cores of high quality, and any defects in your cores will carry on right through to your casting. An estimated 10-15% of casting scrap is due to core defects. Simulation for metal flow and solidification in castings is now a widely used tool, however many foundries are not aware that simulating core blowing and gassing is possible, too.

Arena-flow® is a CPFD^® (computational particle fluid dynamics) simulation software that models the true physics of blowing and curing core sands. As you can imagine, sand and fluids flow quite differently, and CPFD is the best way to get an accurate and reliable picture of the complex flows that happen in the blink of an eye.  Analyzing the visualized core throughout blowing and curing provides insights into the process and can be used to identify and correct core defects.

One of the most common core defects is sponginess, or unfilled portions of the core cavity. This is the result of sand compacting early in the blow cycle, before the sand/air mixture has a chance to fill in areas far from the blow tubes. Vents that are beneath blow tubes (or near invests on vertical tooling) will steal air flow, preventing it from reaching other areas of the core. Blocking vents can be tested easily, but adding vents requires sending the tool to a pattern shop, which requires both time and resources, and may not even fix the right defect!

In complex cores, thin or rangy sections may be difficult to fill reliably due to sand compacting or bunching up near the entrances to them. Adding a washer or diffuser to your blow plate can change the flowability of the sand mixture, helping to achieve a denser fill and improve core quality, or it may slow down filling too much and do more harm than good.

The only way to avoid trial and error, and the burden of repeatedly dumping your magazine head is to simulate the effects ahead of time so it only has to be done once.

Another common type of blowing defect is seen on multi-cavity tools, or family boxes, where cavities toward the outside of the blow hood frequently will have issues, while cores in the center will be fine, quality-wise. It’s usually ideal to fit as many cavities as possible in a core box, but you can simulate the effects of different cavity layouts before tooling construction starts. By taking into account the geometry of the sand magazine and simulating the whole core machine during blowing, a full understanding of the cycle can be achieved. A partially filled core that has sand missing directly under a tube or invest area is evidence of a rat hole, which can be easily seen in the simulation and fixed in a variety of ways, such as sand savers and diffusers.

Gassing-related defects, such as rubbery or uncured cores, can be diagnosed, too. Portions of the core cavity that have stagnant air flow, or uneven curing-gas front movement can be visualized, quickly identifying problems. The effects of adding additional vents to deep pockets or slow-curing areas can be simulated to see cycle-time reductions, before costly physical changes are made to the tooling.

Additionally, Arena-flow^® Ver8 (Parallel) will be released later this year. This new parallel version retains all of the accuracy of the previous release, and unlocks the ability to across multiple CPU cores, significantly reducing calculation times. While this speed increase depends on the geometry of the core, boosts of 800 to 1,000% are typical, especially in head water-jackets and rotor cores. Answers about core-box layout changes now can be had in a day rather than waiting for several days, letting you test more designs, faster, and implement changes to your tooling sooner that you can be confident about.

By making use of Arena-flow to simulate your cores, you can easily visually match real-world defects in your core room to virtual ones, allowing you to be confident that the next round of changes you make to your core box will be the one that fixes your defects.

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