Investment Casting Design Guide for Designers, Engineers & Buyers
The world of manufacturing has benefited immeasurably from the use of 3D design, modelling and simulation. In combination with FEA and now AI, designers can generate and evaluate components and assemblies in a virtual environment that can replicate a version of reality.
The challenge is then placed upon manufacturers to take the design and replicate it in the physical world. At Lestercast, our experts have worked closely with some of the most prestigious companies in the world, producing complex castings that would ordinarily be challenging. We would like to share some of our knowledge and offer guidance on what we consider to be best practical for casting design-for-manufacture.
Casting Design Principles
When designing a new component, it is relatively easy to decrease weight, increase strength and make it aesthetically pleasing. But have you considered how it is going to be manufactured? Not just the prototype, but later in production where you need repeatability, scaled volume production, and cost reductions. Has the component been “designed-for-manufacture”? What happens if the manufacturer can’t produce it as per your design, do you change to another manufacturer of the same process type? Or maybe choose another manufacturing process? Would you have to consider changing the design to suit? If so, you are going to have to go through the design loop again. All of these options are unnecessarily expensive and will waste valuable time.
So, let’s consider the basic features of a typical component;
Wall sections, thickness, shape and interconnections.
Try to avoid thin wall sections connecting directly to larger / thicker features. This can cause many structural issues such as distortion, cracking, and a general reduction in mechanical performance. It’s important to note that there are different types of defects that occur only when the metal is in it’s different states. In it’s solid state, the metal can distort and crack, and thermal / mechanical stresses can cause fatigue failure. In it’s liquid state the situation becomes more complex, as many types of defects can occur without robust process controls. Temperature is the main influence, where temperatures above the Liquidus (TL) generally ensure that the metal remains in it’s liquid state. Too high above the TL and gases can form either through metal/mold reactions, or from element losses. Below and/or close to the TL means that solids begin to form in the liquid. Excess solid % increases the viscosity of the liquid, and the closer to the Solidus (TS) the more viscous the liquid becomes. If the metal still hasn’t filled the mold but the temperature of the flow front is below the (TL), the metal itself becomes a mixture of partial liquid and partial solid. Depending on the metal type, it can resemble a mush or slurry which cannot easily flow through the mold.
This situation will have adverse effects on the metal quality. If the solid % becomes too high before the metal has filled the mold, then misrun and/or cold lap occurs.
Mis-run example
The outer surface of the metal cools down before it has completely filled the mold;
Cold Lap example
Two (or more) cold surfaces interact with each other as the metal flows through the mold, but because the temperature at the surface is below the Liquidus (TL) they cannot merge to form a cohesive structure.
Gas porosity risk becomes higher as the gas cannot easily pass through the solidifying structure, so holes are left in the solid state. During solidification, as the liquid changes to a solid there is also a risk of hot tearing, where the structure literally tears itself apart. This usually occurs when a thinner section has solidified and is cooling down, but it’s directly connected to a heavier/thicker section. The transition zone between the thinner and thicker section is still semi-solid and has not yet formed a crystalline structure, so does not have any strength. The zone then tears as the solidified areas contract and pulls against the semi crystalline structure.
Simply making the thin wall thicker can solve some of the issues, but thicker wall sections relative to the overall component size will cause porosity in those sections. Porosity (Gas or Shrinkage type) reduces mechanical strength, contributes to the formation of leak paths, and will cause parts to fail NDT approval. Thicker wall sections or features require a localised reservoir of metal (typically known as a feeder) to feed liquid metal into the solidifying structure. This can help to reduce shrinkage porosity but adds to the overall cost of manufacture, so it’s an important consideration that requires input from both the designer and the manufacturer.
A practical solution which takes advantage of the natural temperature gradients during metal solidification, is using tapered transition on a wall section. This basic casting design principle has been historically used to feed from heavier sections into thinner sections and can help to reduce the number of feeders required to produce an acceptable casting result.
This casting design can be changed to improve the transition between the two sections, and now only one feeder is required;
Radii and Fillets
One of the most fundamental Casting Design considerations is the removal of sharp corners and edges. A sharp corner or edge leads to many types of defects, directly and indirectly. Sand and ceramic edges can break off during mold filling, causing inclusions. Sharp corners can super-heat; extending the local solidification time and increasing the risk of shrinkage, deformation or sand burn-on. Localised thermal and/or mechanical stresses and strains can increase, leading to a structure that is under high tension or compression.
It’s important to note that excessively large internal corner fillet radius can also introduce problems if not sized correctly to the local geometry. If the radius added is too large, then this will add additional volume into the corner, which then creates a localised thermal hotspot. This area may then require a feeder to reduce the risk of shrinkage porosity, adding to the overall cast weight and a feeder that needs to be removed. So in this case, the larger fillet radius may solve the sharp corner problem, but because of it’s excessive size, has inadvertently created new and potentially much wider costlier issues.
Simply making the internal corner radius smaller can help, but the adjacent external radii needs to also be smaller in order to maintain a nominal wall thickness. Otherwise a thin section is created;
Sharp edges on the casting is also a hazard for operators during handling. Post-cast removal of the edges is of course possible, but this takes time & cost, and is considered to be a non-value added activity. A casting that has been designed to include a rounded edge rather than a sharp square one, means safer handling and less post-processing of the casting. A sharp edge can also become more easily damaged / dented, and could damage adjacent castings during transportation or collective processing. So, it’s best to avoid creating problems such as these, and design the component with rounded corners where possible.
Please see the example on the right, of geometry that has been modified to suit the casting process.
If you would like Lestercast to quote your project, please call us on (0)116 276 7284 or alternatively submit your enquiry using our quotation form.
