Castings are a very powerful form of engineered metal component, and examples abound around us. From rivets and buttons in blue jeans to flight-critical turbine blades and airframe nodes, castings serve us well, worldwide. Castings can be formed from any alloy family in a myriad of mold types. How those mold types are formed now embodies the most cost-effective applications of additive and subtractive manufacturing.
Design engineering of excellent castings is difficult, and excellence in manufacturing engineering of the design to assure specified quality is critical. Software tools that encompass design engineering, durability analysis and manufacturing engineering are powerful, but disjointed, expensive and require specialized expertise. The SOLIDWORKS software suite can be used in a new and innovative way to make casting design and manufacturing engineering coherent, integrated and easier to accomplish exemplary results. This article will focus on how to develop the foundational castability geometry so that structural geometry can be overlaid in a castable result.
Status Quo versus A Fresh New IdeaExcellence in casting performance requires two engineering facets:
- Design engineering for function or structure, including durability analysis, if the design is structural
- Manufacturing engineering of the mold cavity-making process and its tooling design and construction.
Aspects of the Status Quo
Typically OEMs independently engineer the casting’s functional or structural geometry via solid model software, including analysis of structural durability via transformed stress simulation software. Later, metal casting supplier teams do the manufacturing engineering of the design geometry’s final net shape with different software but little or no authority to improve the casting’s geometry to better suit the chosen alloy. Consequently, casting designs resulting from this disjointed methodology are typically only fair-to-poor in their producibility, requiring extensive metal casting supplier team tooling and process engineering to achieve an acceptable result with respect to specifications. Fits, restarts and re-tooling shakes money out of everyone’s pockets.
The Fresh, New Idea
Without all the usual pitfalls, the OEM’s concept-to-production team can use the full SOLIDWORKS suite to encompass both design and manufacturing engineering of a new casting. The design handed over to the metal casting supplier teams can already be very good, only needing their tweaks to become excellent.
Any mold cavity-making process in the casting manufacturing realm is applicable, but our focus is on the most common one: gravity castings. The chosen alloy’s preference for the mold cavity shape that it flows into is the overriding influence on casting design excellence. A fundamental but little known fact about casting design geometry is the alloy’s “like” or “dislike” of the shape that it flows into. There are vast differences among metal casting alloy likes and dislikes about casting design geometry. The design engineer must know about the chosen alloy’s geometry preference before the development of the first solid model. The inset below defines the 5 alloy characteristics that classify castability geometry, the shape that an alloy would “like” to flow into and solidify with specified integrity.
Step by Step: How “The Better Way” Works
Within the spectrum of functional and structural casting designs, structural designs are the most powerful and challenging. We reached out to Michael Gwyn at NoRedesign for his expertise. Mike has authored/contributed to more than 60 book chapters, webinars, articles and technical papers for the American Foundry Society, Steel Founders’ Society of America, Society of Manufacturing Engineers and Society of Automotive Engineers. NoRedesign also has an extensive library of 32 instructional videos in 8 topical segments available for streaming, so if you are interested in learning more you can visit their website.
In the context of the engineering professions on the OEM’s Concept-to-Production Team, the following are NoRedesign’s casting design & manufacturing steps integrated into the stand-alone design and simulation capabilities of the SOLIDWORKS software suite:
Step 1: Design Engineer - Isometric Sketches
For significant reasons beyond our immediate scope, hand-drawn, isometric sketches, brainstormed one after another are the most powerful way to precede a casting design solid model. It is the castability geometry foundation that is the objective of the brainstormed sketches. The isometric sketches anticipate what the eventual structural geometry might look like.
Step 2: Design Engineer - Sketch to Model
Convert the best, brainstormed isometric sketch to front, top and right side 2D views of the most important solid model-building sketch planes. Use SOLIDWORKS Sketch Picture to import, trace over and scale the hand-drawn 2D sketches as the beginnings of the first solid model of the casting design.
Step 3: Design Engineer, Materials Engineer & Durability Analysis Engineer
Input the alloy from the metals library with mechanical properties closest to the chosen metal casting alloy into material on the casting solid model’s Features Tree. With input from the Materials Engineer, edit the material properties to align with the metal casting alloy. Most importantly, with the estimate from the Durability Analysis Engineer, input the allowable transformed stress as the yield stress value.
Step 4: Design Engineer & Durability Analysis Engineer
For a quick, yet insightful look at the design’s structural efficiency, use SOLIDWORKS SimulationXpress to see 3D color plots of the Transformed (Von Mises) Stress on the casting solid model.
Step 5: Design Engineer & Manufacturing Engineer - Thermal Flow
At this stage in the casting solid model’s development, SOLIDWORKS Flow Simulation with its thermal flow analysis capability is amazingly effective in evaluating the producibility of the casting. While remaining inside SOLIDWORKS 3D CAD parametrics, flow simulation and thermal flow enable evaluation of the design’s castability geometry by simulating the real time temperature distribution inside the solid model, as though it were a casting mold being filled. That temperature distribution, as it completes dynamically and begins to cool toward solidification statically, provides insight to solidification integrity, as specialized metal casting industry software does so well.
So, without having to leave the 3D CAD environment and without having to export the .sldprt file to a separate part as a .stl file, the Design Engineer and Manufacturing Engineer can evaluate the design’s castability geometry for themselves… still inside SOLIDWORKS 3D CAD parametrics.
Watch the on-demand webinar below where we talk with Mike specifically about thermal flow for casting designs using options within SOLIDWORKS Flow Simulation.
Step 6: Design Engineer & Manufacturing Engineer - Thermal Flow AnalysisHere is the set-up for the flow simulation thermal flow analysis:
- Place the solid model as a negative geometry in a block mold material. Depending on the gravity mold cavity-making process and the alloy choice, the mold material could be sand, a ceramic shell or metal.
- Orient the solid model cavity logically with respect to gravity. This doesn’t have to be the perfect orientation choice. It can be changed during analysis and iteration or it might ultimately be changed by whatever metal casting supplier team becomes the producer. (Perfection in mold cavity design isn’t the OEM concept-to-production team’s objective, but improving the castability geometry of the solid model is).
- On whatever side becomes the top of the solid model cavity, place hollow cylinders of adequate size and position to evacuate air from the cavity. Flow Simulation includes both a design fluid and air, so it is important to evacuate the air ahead of the molten metal front to prevent air from retarding the molten alloy’s flow throughout the simulated mold cavity.
- Define heat transfer coefficients around the perimeter of the solid model negative surface.
- Define the temperature of the surrounding ambient air.
- Define the alloy’s shear viscosity and density as a function of temperature.
- Define the ports where the liquid alloy would flow in along with a mass flow rate that is compatible with the alloy’s fluid life.
- Helpful, but not necessary: with a little experience the team can define non-cooling surfaces where connections for reservoirs of liquid feed metal would be attached to feed solidification shrinkage inside the simulated mold cavity. In addition, surfaces where a metal chill on the mold wall interface (or a cooling water line close to the mold wall interface) would accelerate cooling to aid the solidification integrity gradients. The heat transfer coefficients can be established by setting up defined surface areas on the solid model and assigning the proper heat transfer coefficients accordingly.
Step 7: Design Engineer & Manufacturing Engineer - Castability Geometry
Iterate back and forth between 3D CAD model revisions and Flow Simulation, all the while quickly and efficiently remaining inside SOLIDWORKS parametric environment. The final result in the foundational castability geometry doesn’t have to be perfect. Metal casting supplier teams who ultimately bid on the castabilty geometry that has been parametrically brainstormed and refined will recognize that geometry as very close to the geometry that their alloy likes to flow into and solidify with specified integrity. They will be happy to avoid their usual application of manufacturing engineering tricks to accommodate casting geometry problems. The bidding supplier teams will have requests, but they will be tweaks, not complaints.
There are further processes that can be explored on this topic, such as overlaying structural geometry on the foundational castability geometry without upsetting the benefits of temperature distribution patterns and temperature/mass gradients for solidification integrity. These will be the subject covered in upcoming articles.
Hopefully this article has enhanced your understanding of the casting design and manufacturing engineering process and demonstrated how the SOLIDWORKS suite of software tools can enable that process most efficiently and cost-effectively, all the while remaining inside the SOLIDWORKS 3D CAD parametrics environment.
For more information on the topics we covered and the ones we didn’t get to, you can check out NoRedesign’s website and course curriculum.
For more information about SOLIDWORKS Flow Simulation, we put together a comprehensive introductory webinar as part of our “Get to Know SOLIDWORKS” series you can watch for free.