Why Worry About a Parting Line?

Dave Charbauski

As a buyer of castings, you may wonder why you should be concerned about something as simple as a parting line. Did you know a simple change to the parting plane could reduce or increase the cost of your casting? The determination of where the parting line will be located on a casting design seems innocuous, but the parting line can affect 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. 

Let’s start with some definitions: A parting line is the dividing line between the halves of the sand mold, the top half of the mold being referred to as the cope, and the bottom half being the drag. Likewise, sand cores have parting lines that delineate one half of the core box from the other. While selecting a parting line should be relatively simple, several parting line choices often could be used, and determining the proper parting line location on a casting is critical to producing a part that meets the customer’s needs and produces a high quality, sound casting. 

Sometimes the location of the parting line is easy to determine, as would be the case with a tubular shaped casting—the parting line for a tube is always a straight line across the diameter. This is often called a natural parting line. However, the parting line may not always be as straightforward, and several different options for parting line location may be possible in a complex shaped casting. 

As an industry best practice, development of a straight parting line will almost always be the best choice in terms of simplicity and cost avoidance. The design team should attempt to alter the casting design, if necessary, to maintain a straight parting line. 

However, don’t get the wrong impression—offset parting lines are acceptable and sometimes unavoidable. While offset parting lines are very common in the foundry, they do tend to drive cost up because the pattern equipment will be more difficult to produce. Most offset parting lines drive the use of a cast matchplate pattern, which is more expensive than a mounted pattern. Some offset parting lines only can be achieved with an extra core. This situation is not uncommon, but an attempt should be made to design out the need for the parting core, if possible, to reduce overall pattern costs. 

A common attribute of offset parting lines is their tendency to produce fins that will need to be ground off during the finishing process. If the offset is deep, potential problems could occur in drawing the pattern out of the mold because of molding machine depth limitations. Deep offsets create deep pockets along the parting line that can make it difficult to obtain even sand compaction. If variations are present in the mold compaction and hardness between mold halves, the casting may show areas of swell, which will cause dimensional, cosmetic, and perhaps soundness issues.

Reduced scrap rates can be achieved if the design of the part allows the casting to be made in one half of the mold because it 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 require additional grinding to blend the surfaces in the best case and scrapping the casting in the worst case.

When it comes to deciding where to place machining locators on a casting, the casting designer and foundry tooling engineer need to carefully consider the influence of the parting line. Locators positioned on both sides of the parting line will lead to problems at some point in production due to relative movement of the cope and drag halves of the mold. This variation will cause the locator surfaces to be misaligned, which will in turn drive machining problems. The best practice is to place all the machining locators on one half of the casting, preferably the drag half.

When you examine all the issues that can arise from a poor parting line choice, the buyer should indeed worry about the parting line. Always make sure the foundry engineer and the parts designer work together to choose the option that provides the best balance of casting cost, tooling cost and part quality.   CS

Click here to see the story as it appears in the July/August 2020 Casting Source issue.