What Cost Drivers Are Owned by the OEM/Buyer?
It’s often misunderstood what cost factors are owned by the OEM versus those owned by the foundry. Following are items that should be considered by the buyer to make sure they are not driving extraneous costs into a cast component.
Casting Design-Related: For the buyer, the design of the casting has the largest influence on part cost. Multiple cost drivers are related to casting design. Cores, parting lines and section thicknesses are all common places to review.
In relation to the use of cores in the design, does it have a single core, multiple cores or a core assembly? These factors can have a significant impact on casting cost. Multiple cores are not usually a major problem, but they do require extra time to be placed in the mold, which can slow down the manufacturing process. Core assemblies are not generally an issue, but the assembly needs to be robust. Fragile core assemblies can lead to increased core scrap as well as casting scrap if the cores break or dislodge during pouring.
One particularly troublesome design feature is the use of long cored passageways in castings, such as oil channels or long tubular sections. If the mold does not have enough support for a long core, the foundry will probably be forced to use chaplets to support the core along its length. The cost of the chaplet, the time to set it on the pattern or to attach it to the core, and the potential increase in cleaning costs and scrap rate all drive cost up. Core sand needs to be carefully removed from the passageway. If tight bends or narrow openings are in the cored passage, core sand can be trapped in the cored cavity. More costly cleaning methods and/or additional inspection may be required to ensure the spent core sand is removed completely.
The overall level of complexity of the casting has an impact on cost. For instance, the use of multiple offset parting lines or deep offsets will not only increase tooling costs; they will also increase grinding and cleaning costs and can have a negative impact on rejection levels. Isolated heavy sections in the casting can cause soundness issues due to shrinkage porosity. The foundry may need to employ additional risering or chills to eliminate this issue, and these features will always increase costs.
Material type will obviously impact the cost of the casting. This often comes in to play when the material type is over-specified, such as when a ductile iron material is specified for a part that would perform just as well in gray iron. The engineering team should be consulted to make sure the material selected meets design criteria.
Ultimately, the casting has a per pound or kilogram cost associated with it. Removing unnecessary material from the design will obviously reduce cost, but only up to a point. Removing too much material can create isolated heavy sections and potential soundness issues. Thin sections in the casting can restrict metal flow and increase scrap rates.
Grinding and Finishing Related: Often, casting parting lines are required to be ground flush with the adjacent cast surface. Can this grinding be eliminated by working with the foundry to minimize or avoid the parting flash? Do ingates and risers need to be ground flush or can they be left in place? Does your casting need to be shotblasted after grinding to blend the surface appearance or can it be left in the post-grind state?
Inspection and Quality Requirements: Requiring elevated levels of casting inspection will increase casting cost. Does your entire order of parts require Brinell hardness testing or can an audit sample of the order be employed? Every time you require the foundry to touch your casting you will drive added cost.
One cost driver is the level of soundness required on a casting, especially for steel castings. Requiring high levels of soundness can increase costs significantly due to the number of risers used, riser sleeves consumed, chill usage, and validation and rework time. Always consider what soundness level is actually required—using unnecessarily high levels of soundness can easily increase the cost of a casting by up to five times.
Machining and Secondary Processes: Heat treating, paint and special packaging are all valid requirements to produce a functional part, but make sure this is being done in the proper sequence and with a minimum of transportation to avoid adding any unnecessary costs.
Order Sizing: If you only purchase a small number of castings per year, you can expect increased costs due to set-up times and short runs. You may need to consolidate your casting buys into one annual quantity to get the costs to where you feel comfortable with them. However, consult with your supply chain team to make sure you are not driving costs by increasing the amount of inventory you are carrying.
Every casting buyer should understand the role design features play in the overall cost of the casting and the impacts that design decisions can cause. Having a knowledge of the cost factors that are under your control is the first step in developing the best casting design with the optimal cost. CS