What are the design principles for risers in sand casting?

Question

Risers are added reservoirs used to feed liquid metal to solidifying casting. In the case of alloys that shrink during solidification (this is not the case of all alloys), one uses such reservoirs to make liquid metal available throughout solidification. Risers should be kept liquid throughout the solidification, which means the solidification must be directed toward the risers, which must solidify last.

What are the principles used to design such risers?

What I already know:

• Their volume should be sufficient to compensate the solidification shrinkage
• They should remain liquid last, so maximizing the volume/surface ratio seems appropriate

What I am looking for:

• How to ensure the riser can efficiently feed the rest of the cast part?
• Are there other rules (some kind of general algorithm maybe) for efficiently designing risers?

Answer 1

Casting terminology can vary a bit. My experience is in art casting in cast iron.

In my experience with sand casting risers specifically tend to be used as vents to allow gas to escape from the mould and tend to be small diameter and placed at high points.

I'm more used to seeing the term well used in the context of feeding metal. There is no reason not to use risers as feeders of course although this sometimes conflicts with the fact that the can be in awkward places where you want to keep them as thin as possible.

In general terms a casting will cool from the thinnest sections first so its the thicker bits which need feeding. In many cases it is also convenient to have a well which feds the mold indirectly form the side or below which helps to control the flow in the mould as a whole as it is filling from head pressure rather directly from a falling stream of metal straight from the ladle this also helps to settle out any air and solid debris which gets mixed up in the initial stream .

So in general

• You want to be feeding the thickest parts of the mould
• Transitions between feeders and the mould (gates) should be thinner than both but short, a 'fishtail' shape is often desirable with the flare on the pattern side.
• The typical section size of the feeder should be larger than the section it feeds.

In terms of an intuitive approach try to imagine the mould cooling from thin to thick and look for islands of thick sections of metal, it is these which will need feeding if they aren't already fed from the main well. You can think of this as the thinner sections 'sucking' metal from the rest of the mould as they solidify.

Also in pattern design try to avoid abrupt changes of section

Answer 2

I am a metal casting process researcher who has several past and ongoing collaborations with commercial foundry and tooling experts. In commercial steel sand casting, I have observed the terms "riser" and "feeder" to be used more-or-less interchangeably. In the literature, British commonwealth authors tend to prefer feeder, while American authors tend to prefer riser. I will use feeder because it is a more descriptive term. When I use the term "section" I mean a contiguous region of the casting. The term is left vague because it is not a particularly well-defined term in practice. When I use the term "hotspot" I mean the last part of a section or feeder to solidify.

The Seven Feeding Rules

John Campbell defines seven rules for feeder design in Complete Casting Handbook (located on p. 661-688 of 2004 edition).

1. Do not feed unless necessary — only attach feeders to locations that have demonstrated shrinkage problems. Demonstration could take the form of simulation results. Prefer to use thin sections which taper away from the gates so feeding is done by the filling system. Ignoring this requirement increases the cost of production by consuming more liquid metal per casting. It is worth noting that this is not always possible nor the most economical route.
2. Heat transfer requirement — The feeder must solidify later than the section it is intended to feed. Influenced by feeder size and shape, and feeder-mold interfacial heat transfer properties. Ignoring this requirement can result in a feeder freezing before the section, leaving the section without feed metal.
3. Mass transfer requirement — The feeder must have sufficient useful volume to meet or exceed the contraction of the section it is intended to feed. Useful volume means the amount of liquid available to feed the section that isn't consumed by direct solidification of the feeder itself, or by thermal contraction. Influenced by all of the same factors as (2.). Ignoring this section can result in the feeder freezing before it has supplied sufficient feed metal to the section. The geometry of the section determines which of rules (2.) and (3.) is a stricter requirement. Heat transfer (2.) tends to be more important for thick sections, and mass transfer (3.) for thin sections. The reason is that thin sections will rapidly solidify, well before attached risers, so that useful feeder volume becomes the dominant factor.
4. Junction requirement — The point of attachment between feeder and section can become a hotspot. If this occurs, porosity will form at the attachment point. What results is referred to as "under-riser shrinkage porosity". Increasing the size of the feeder, or changing its location relative to the section, can help resolve this issue. A common rule is to use a feeder whose diameter (via Heuvers' circles or spheres) or section modulus (care of Chvorinov, Wlodawer, et al.) is 1.0 to 1.2 times that of the section. For sufficiently thick sections as much as 2.0 times may be necessary to avoid junction issues.
5. Feed path requirement — There must be a clear feeding path available from the feeder to every point in the section intended to be fed. If there is a narrow feature between the feeder and some part of the overall section, then that narrow feature will solidify before the section, cutting off feeding. The rest of the section will be isolated. To achieve this, ensure there is a monotonically increasing gradient of modulus or section thickness from most extreme points back to the feeder. Attaching excess metal in the form of feedpads can assist with this requirement. Feeding distance is limited by both feeder geometry and section geometry. Tapering sections can increase the feeding distance. Attempting to feed too far can result in microscopic shrinkage called center-line shrink or microshrink.
6. Pressure gradient requirement The feeder must have higher metallostatic pressure than the section intended to be fed, or the porosity will form in the section by the action of gravity. Ensuring the feeder is located at a higher point than the attached section can help, but is not sufficient. Vented feeders exert more pressure because they are not working against atmospheric pressure – atmosphere supports up to 1.5 meters of steel. The metallostatic pressure gradients change over the course of solidification as sections become cut off from one another. All of these changes must be accounted for.
7. Pressure requirement The feeder must have sufficient metallostatic pressure to ensure that points in the attached section are above points in the feeder. If the liquid level in the feeder drops below an un-solidified part of the attached section, that part will experience shrinkage porosity. Whereas the pressure gradient requirement considers the entire course of solidification, the pressure requirement is concerned with a single feeder-section system once it has become isolated from the rest of the casting.

Additional considerations

Other literature sources give additional criteria. Some of these are obvious, some follow from the seven rules above, and a couple are less obvious.

• Locate feeders near thick sections
• Place side feeders on gates
• Place top feeders away from gates
• Do not cluster feeders (minimum distance)
• Blind feeders height:diameter in range of 1:1 and 3:1
• Feeders located on flat, accessible surfaces to ease removal and minimize risk of damage to the casting
• Select feeder shapes from a list of standards

These are sourced from a Master's Thesis by Jensen, 1998, who cited: - S Guleypoglu and J L Hill AFS Transactions, 1995 - J L Hill, et al., AFS Transactions, 1991 - J L Hill, et al., AFS Transactions, 1993 - B Ravi and M N Srinivasan, Computer-Aided Design 22(1), 1990.

Numerical models

A few mathematical and geometric models exist that can guide engineers toward better solutions. These models tend to center around requirements (2.) and (3.), with a little bit into (4.) and (5.).

Two models that are still widely used 70 years after their invention are Heuvers' circles and Chvorinov's rule. Heuvers' circles are used to ensure that the diameter of the largest inscribed circle or sphere in a feeder is larger than that of the attached section. Chvorinov's rule is based on experimental work that shows solidification time is proportional to $\left(\frac{V}{A}\right)^{n}$, where $n$ is often taken to be $2$. If the modulus of a feeder is larger than its attached section, then it should solidify later.

Wlodawer and Heine independently published work extending Chvorinov's rule to individual parallelpiped sections of castings. They each noticed that castings can be broken up into sections with shared boundary segments. The value of $A$ for each section can then be discounted by shared area since very little heat is transferred across shared boundaries.

Sorelmetal's Ductile Iron book (pdf link) and the Redbook Rules from Steel Founders' Society of America (pdf link) both deal directly with sizing feeders based on geometric considerations. The models are too involved to share here, but are based on practical and empirical considerations from experts in the field.

What about computer models?

I am unaware of any commercially available computational models that can accurately guide an engineer through every aspect of feeder design. For what it's worth, MAGMASoft is a casting process simulation software capable of both solidification simulation and parametric geometry optimization based on simulation results. So if you have a "close-enough" design, it can optimize the geometry to minimize defects and maximize process efficiency.

Getting that "close-enough" design is the part that still largely relies on human judgement and experiential knowledge. While there are a number of mathematical and geometric models (described above), none of them are perfect or cover all of the above requirements. Furthermore, manual calculations associated with these methods are time-consuming. They can also be error-prone if used by engineers unfamiliar with the effects of geometry on feedpaths. There is at least one open-source program for automatically placing feeder geometries based on (5.) (GitHub link). Full disclosure: I am the author of that program. The original intent with the software is to divide the casting into feed-able sections using a watershed approach and then determine feeder size based on section geometry. It works reasonably well, and when we have an open source solidification solver it will be that much more accurate.