What’s driving the cost of your casting design?

David M. Charbauski, Process Control Engineer, Caterpillar Inc.
 

(Click here to see the story as it appears in the Sept./Oct. issue of Metal Casting Design & Purchasing.)

Here is a comment that comes up often when talking to new casting buyers: “I have a casting design and I’ve received quotes back from several casting suppliers on the part. The cost of the part seems high to me. How should I go about securing a lower price on this part?”

The reaction some buyers have is to quote the work at additional metalcasters to see if they can find a lower cost. This might not be the best approach. If the quotes you have are from metalcasters you have experience with and all are uniformly high, the reason may be related to the casting design itself. Ask these questions as you determine what could be driving excess cost into the casting and whether you are working with the correct source.

Does the part fit the metalcaster?

There are several key factors in play here: metal type, casting weight and size, and annual volume. The casting sourcing team must make sure the metalcaster being considered produces castings, on a daily basis, that are within these three parameters.

Metal type: Is the cast material an industry standard grade or one unique to your organization? If it is unique or rarely produced, this could be a major cost driver. Consider changing the material to a commonly cast grade to reduce the cost. Another factor to consider is if the material type is correct for the application and if there are alternate material selections available. For example, let’s say a steel fabrication is being redesigned as a casting. The designer specifies steel because that was the material used in the original fabrication. However, the part could possibly function just as well in ductile or gray iron, and it is likely an iron casting will cost less than a steel casting. The engineering team should be enlisted to help determine if a material change is warranted.

Casting weight and size: Does the casting fit within the weight and size range commonly poured by the facility? If the casting is on the high or low end of the range, the part may not be a good fit for that particular metalcaster. Being on one end of the weight/size spectrum may require special handling processes and increase costs. For example, small parts may need to be handled differently through shakeout to prevent the castings from becoming damaged. Large and heavier parts may require extended cooling times in the mold that add delays when the plant is producing the casting in an automated process. Either of these situations can add significant cost to a casting.

Annual volume: The annual volume of parts you plan to buy must fit the chosen metalcaster’s lot size range. For instance, if your part has a small volume, it would be wise to choose a company that specializes in low volume/small lot size casting production. Sourcing a low volume part at a high volume facility will in increase costs, because the metalcaster’s entire manufacturing process is designed around handling larger lot sizes and the low volume work just won’t fit well. Keep in mind that casting suppliers characterize low, medium and high volume differently, so communication is vital on this point.

The casting fits the metalcaster. What’s next?

After you have examined these three categories and determined your casting is a good fit for the chosen supplier, the next step is to collaborate with its engineering team to determine if there are any requirements in your design that could be identified as cost drivers. It is important to identify cost drivers as early in the design stage as possible so they can be minimized or eliminated. In this context, a cost driver is any requirement or combination of requirements with a large influence on total cost of the part, and these can be either design or process related. Following are some of the more common cost drivers that should be considered.

1. The Basics

Don’t overlook these. Most castings have a few basic requirements that need to be addressed correctly during the design phase. In this discussion, two of these would be draft angles and finish stock allowances. If these are chosen correctly, there will not be a problem with the resulting casting, however, if the wrong parameters are applied there can be increased cost driven into the casting.

Draft is added to vertical surfaces of the casting (surfaces that are at a right angle to the parting plane) to facilitate easy removal of the pattern from the mold. Most metalcasters would like to see between 1 to 2 degrees of draft added to these surfaces, but that is dependant on the height of the surface from the parting plane. A good rule of thumb to follow: The shorter the feature, the more the draft angle will need to increase. To illustrate this idea, a feature that is 50 mm high may require 1 degree of draft to properly remove it from the mold, but a feature that is 10 mm high probably will require 4 to 5 degrees. Insufficient draft angles are one of the causes for a casting defect referred to as a sticker. A sticker is defined as excess metal on the surface of a casting caused by a portion of the mold face remaining on the pattern. Having a casting that is prone to forming stickers will increase cost due to additional scrap, grinding and/or rework.

Finish stock is added to those surfaces of a casting that will require subsequent machining. The task here is to design the casting with the amount of finish stock that will allow the casting to be machined successfully without adding too much material for removal. Finish stock allowances will vary with the type of material being poured, the size of the casting, and the molding process used to produce the casting. Sometimes designers will attempt to minimize finish stock allowances on the casting to allow the machine shop to maximize speeds and feeds and thereby speed up the machining process. While this thought process is valid, the designer needs to work closely with the metalcaster’s engineering team so the stock amount does not get too low; the minimum amount of finish stock must be within the limits of normal process variation to avoid additional problems such as surface inclusions in the machined surface and areas of non-cleanup.

2. Parting Lines

Did you realize a simple change to the parting plane could reduce the cost of your casting? The determination of where the parting line will be on a casting design seems innocuous, but the parting line location can effect the cost of the pattern equipment, the number of cores used in the design, the ability to easily remove the pattern from the mold, grinding and finishing costs, and potential scrap rates.

Making an effort in the design stage to keep the parting plane flat is the best practice to adopt. While offset parting lines are common in metalcasting, they tend to drive cost up because the pattern equipment will be more difficult to produce, thus, more expensive. Some offset parting lines can only be achieved through the use of an extra core. This situation should be designed out if at all possible to reduce overall costs.

Offset parting lines are much more likely to produce fins that will need to be ground off during the finishing process. If the offset is deep, there could potentially be problems drawing the pattern out of the mold and perhaps even sand compaction issues because of the difficulty in getting even squeeze pressures during molding.

Reduced scrap can be achieved if the design allows the casting to be made in one half of the mold. This is possible because this will eliminate a defect referred to as shift. Shift is defined as inadvertent movement of one half of the mold in relation to the other half. Shift appears as a step in the casting at the parting line and will cause additional grinding to blend the surfaces in the best case, and scraping the casting in the worst case.

3. Tolerances

Does the design contain dimensional tolerances that are out of the standard tolerance range for the metalcasting operation? Cast tolerances are highly dependant on the type of molding process used to produce the part, but other factors such as casting size and pattern construction also play significant roles. The design engineering team should review readily available industry standards for dimensional tolerances with the metalcasting engineer to determine what can be achieved and then apply the correct tolerances as required to help the facility produce a conforming casting.

In regards to pattern construction, there is a wide spectrum of materials available that can be used to make a pattern, including soft and hard wood, a large variety of cast or machinable plastics, and metals such as aluminum, iron and steel. However, the design team needs take the pattern material into consideration early in the development cycle. For example, a wooden pattern will not be as capable of maintaining tight tolerances like a machined metal pattern can. However, metal pattern equipment will be more costly to produce than wood or plastic patterns. The decision must be made to determine the best balance of tool cost vs. dimensional capability.

4. Surface Finish Requirements  

Are there cosmetic requirements for your casting? Have the surface finish requirements of as-cast surfaces been clearly communicated and understood between the end user and the metalcaster? This often is an area that lacks proper definition and can translate into higher casting costs if incorrect assumptions are made. Surface finishes related to texture are directly related to the molding and finishing processes used, and they should be discussed during collaboration with the supplier at the quoting stage. There are a number of surface finish comparator plates available that can be used to specify the desired surface finish of the casting.

Cosmetic surface finish requirements, such as the amount and type of visual defects allowable on the casting should also be well understood. If the metal casting engineering team is made aware of tighter defect requirements, they can engineer the process to improve the surface finish. Some steps that the metalcaster can take are placing ingates and risers in the correct location in relation to the critical casting surfaces, or the use of alternate metal filtering methods.

Requiring a casting to have high levels of visually defect free surfaces will increase the scrap rate and can easily add 15% or more to the cost of a casting.

5. Section Thickness  

Castings tend to be less prone to a variety of problems when casting wall sections are held to a consistent thickness. If the design has a wide variation in section sizes, this could potentially cause a higher scrap rate, which could translate to increased casting cost.

Thin casting wall sections can cause a problem with incomplete metal flow through the section, resulting in a casting defect referred to as misrun. Thin sections also have the possibility to crack or break during solidification, shakeout and handling.

Transitions between thick and thin sections must be correctly designed to avoid problems with the casting. One  basic engineering concept is that stress will concentrate in areas where there is a drastic change in section size. To limit the stress in a casting, these transition areas should be designed to incorporate a generous fillet. Even better, use a tapered surface to reduce stress and provide better metal feeding characteristics.

Thick sections in castings can often cause problems during solidification, requiring additional feed metal to achieve a proper soundness level. This is especially problematic when the heavy section is isolated, meaning that the section is located in an area of the casting that is difficult to reach with commonly used gating and risering techniques. The metalcaster must then often resort to using additional risers, chills, or padding to produce a conforming casting. These requirements will increase the cost to produce the casting.

A good design practice to address concerns with section size and soundness is to perform solidification modeling on all casting designs unless the design is simple. Modeling will quickly show areas that are the last to solidify, the effect of isolated sections, feed paths and other critical features of the design. The engineering team can then work with the metalcaster on adjusting the design to reduce or eliminate any issues that are uncovered before a physical casting is actually made.

6. Finishing Requirements

Is it clear what your finish requirements are for the casting? Is there the possibility the metalcaster will be performing more finishing work on it than is necessary to fulfill the design intent? Several considerations are:

• Grinding.  Does the parting line need to be ground flush with the surrounding surface, or can it be left as-cast? Similarly, can riser and ingate connections be ground smooth but left as a raised pad instead of being completely removed?
• Cleaning.  Normal processing would be to shot blast the casting after shakeout, then perform grinding operations. If you are requiring an additional shot blast operation after grinding, this will be adding cost to the casting that could possibly be eliminated.
• Packaging.  This area often is critical and needs to be reviewed with the metalcaster to make sure all requirements are understood. If there are specific types of shipping containers, rust protection or packaging methods required, these should be covered during the quoting stage to make sure you receive castings the way you need them.
• Core usage.  While it is not a problem to use cores to create internal passageways throughout your casting, always look for ways to eliminate or minimize them. If your design requires a large amount of cores, is there a way to adjust the design to allow several cores to be incorporated into one? If cores are assembled before being placed in the mold, is the assembly robust? Having a weak or fragile core assembly will cause higher scrap levels and increase costs. If a core is required to allow the mold to be parted, or to provide a base for proper gating or risering, this is an opportunity to work on the design to eliminate these types of cores and reduce costs. Thin or long cores may be prone to movement within the mold, requiring chaplets to hold the cores in place during solidification, which can increase costs. Long cores also can increase costs due to difficulty removing core sand in the cleaning process.

 

7. Inspection

If you have additional inspection requirements that are outside the normal industry parameters, this will drive costs higher. For instance, Brinell hardness testing typically is performed on an audit basis, so if your requirements state that a larger than normal percentage need to be checked, be prepared for costs to increase. Likewise, requiring other tests such as magnetic particle inspection, X-ray or pressure testing on a more frequent basis will increase the overall cost. Work closely with the metalcasting engineering team to determine what is best for your particular casting design and application.

To be successful in your efforts to eliminate casting cost drivers, all the items listed above will require detailed collaboration and communication with the metalcaster. Ideally, this should be done during the design stage and before the quote is finalized so you can be confident that the cost drivers have been addressed and you have received an accurate casting price.