Knowing When to Switch to Casting
When you are designing a part to be put to market quickly, sometimes the quickest or most familiar manufacturing method is chosen. But time and volumes might begin to highlight deficiencies in the original manufacturing method of choice. At this point, it is time to consider moving to a better optimized manufacturing process.
The casting process has many advantages, but it might require more tooling and engineering time up front than a weldment or machined component. However, making those investments in tooling and engineering can pay dividends not just in reduced cost, but also improved performance, reduced weight, and even shorter lead-times.
Where do you start? Metalcasting facilities well-versed in redesigning parts for casting are experts at pinpointing what makes good candidates for casting conversion. And the examples of successful case studies are bountiful.
Read on for signs that a new design is needed and how to know when casting could be the process of choice.
Signs a part needs a new method:
1. Insufficient dimensional stability.
A casting’s dimensional tolerances are generally superior to welded and fabricated assemblies. A rotational and lift control post weldment for the oil market was experiencing distortion and stresses during fabrication that led to problems in assembly and in service. The cast version produced by Midwest Metal Products (Winona, Minnesota) eliminated the distortion and stress problems while also reducing cost by 38%. In cycle testing, the cast part demonstrated more than five times a longer life than the welded part.
2. Costly to manufacture and keep track of inventory.
Pier Foundry (St. Paul, Minnesota), uses onsite visits to help customers identify candidates for castings. In one instance, the metalcaster and customer pinpointed a six-piece universal disc leveler pivot arm as a promising candidate. Using finite element analysis and casting process modeling, the engineers revised the part’s design to be optimized for casting. Converting the weldment to a casting freed up 71 minutes per unit of shop capacity and allowed the customer to focus its efforts on other products in their lineup. It greatly reduced the need for surge capacity in the spring. The part also saw a cost reduction of 60%.
3. Production volumes have grown, or a family of parts has grown so the number of parts and subcomponents has become unwieldy.
Monarch Industries (Winnipeg, Manitoba, Canada) worked with a customer to redesign the mounting structure for tillage equipment. Each weldment contained an average of eight pieces of square and round tubing and plasma/laser cut plates. As the customer designed new pieces or new sizes and variations, the family of weldments grew to 17 different part numbers with a total of about 136 subcomponents. Each part number had to be kept in inventory for production and service requirements. Monarch worked with its customer to reduce the number of parts, optimize part strength and lower tooling and part costs. In the end, the customer converted its family of weldments into three castings used across equipment such as field cultivators, solid finishers, chisel plows and disc chisels. This provided an average reduction to cost of 30%, greater dimensional stability, improved aesthetics, and more plant capacity in the customer’s welding and fabrication departments.
4. Requires an excess of machining.
Machining time is not inexpensive or particularly quick. Creating a design that reduces or eliminates the need for CNC machining frees up the equipment for other parts. In the case of an originally machined-from-billet steel part used in a piece of machinery, producing it in the investment casting process freed up valuable machine capacity for other critical applications. The conversion resulted in a 22% cost reduction. It also meant as-cast internal heat transfer capabilities could be added which were not possible when machining from solid. The casting supplier, Signicast Investment Castings (Hartford, Wisconsin) was able to incorporate a cooling internal passage with a turbine style directional cooling flow feature and rib features that enabled the part to be cast in steel rather than copper, which would have cost more with lower wear resistance.
5. The assembly is labor-intensive.
A planter row unit for farm machinery was originally made as a 30-piece weldment that utilized fine thread nylock nuts for attachment. This made assembly time consuming and problematic. In addition, the stamped steel components had to be moved and stored multiple times throughout the production process. Dotson Iron Castings (Mankato, Minnesota) worked with its customer to redesign the row unit as a seven-casting assembly. This redesign included reducing 17 stamped steel parts of the main shank to three castings. The savings in assembly takt time ended up between 5 and 10%. Other benefits included improved seed placement accuracy and a more robust part.
6. Too much variation occurring from assembly to assembly.
“In many cases there are quality issues with assembled parts by the time it is all welded together,” said Mark Hildebrand, director of sales, Monarch Industries. “The heat makes the metal warped and twisted. With casting, the holes and features will always line up.”
7. The market is ready for an improvement.
A dental furniture manufacturer identified a need for a high-end arm for reclining dental chairs and conducted surveys to find out what its customers wanted. The primary desires included an attractive part that was lightweight yet strong, ergonomic and cost effective, could hide tubes and wires, and provide a wide range of motion. To achieve these goals, the company departed from the typical bent tube/weldment combo and worked with L A Aluminum (Hayden, Idaho) on a cast aluminum version. The redesigned articulating arm assembly of aluminum castings has a hollow body for electrical wires and pivot points at each end to adjust the height.
When casting can be considered
1. The assembly consists of multiple parts in the same material or with the same property requirements/application demands.
Casting design provides the freedom to design geometrically, unifying multiple pieces as one.
“The more pieces you have to weld together, the better a casting looks, economically,” Hildebrand said. “Geometry is essentially free in a casting. I can add strength where needed. It’s easy to do in a casting, much harder to do in a weldment.”
2. Other similar functioning parts are made as castings.
When Hildebrand gives customers a background on weldment to casting conversions, he likes to show examples. Often, this sparks recognition and familiarity.
The customer sees something similar to their own parts, and the connection is made more easily for the potential of moving to a metal casting.
“The second conversion is always easier,” Hildebrand said. “Or customers start thinking of castings [upon first product concept] and maybe design it that way from the beginning.”
3. Multiple features can be incorporated for increased opportunity for cost reduction.
Carley Foundry (Blaine, Minnesota) worked with an aerospace customer to create an investment cast stator housing that was originally produced as a two-piece hogout with a pressed-in insert. The end design includes extensive external mounting features that act as attachment and support points for all the components of an electronic system and dual motor controller. The casting includes aerodynamic stator vanes, a motor case with ventilation holes, and a cast-in steel bearing liner. The part also includes an integrated fan impeller containment ring and mounting brackets to support installation into the aircraft.
4. Weight reduction would be an advantage.
Magna Cosma International (Troy, Michigan) worked with Ford Motor Co. to design and build prototype vehicles with a 23.5% reduction in weight compared to a 2002 baseline target. As a result, seven aluminum structural body castings were designed to replace steel assemblies. The casting process allowed for reduced mass and reduced part count. Ultimately, the castings provided a 25% weight savings to the vehicle.
5. The potential benefits can make up for the initial casting design and tooling costs.
“The volume has to be enough annually to justify the tooling,” Hildebrand said. “Higher volumes help. Cost is an issue clearly, but we talk quite a bit about the soft advantages too. Casting can make it look better with curves and angles. We can add logos and part numbers. Or, where does the customer get skilled welders? With a casting, you can deploy those welders elsewhere in the shop.”
6. Can another material be considered?
Often steel is chosen in fabrication because of its weldability. But if it is cast, that property is not necessary. The material properties needed by the application might be met by another alloy choice, such as ductile iron (for better castability) or aluminum (for lighter weight).
If you have a part that corresponds with some of the points listed here, reach out to a casting supplier and invite them to your shop for an onsite review of inventory.
Click here to see this story as it appears in the March/April 2017 issue of MCDP.