Dimensional Tolerances in Aluminum Castings
Casting purchasers often ask what tolerances they can expect to hold on their final cast components. Metalcasting engineering and casting processes play a critical part in answering this question.
Effective gating and riser design can assure that sufficient feed metal and feed paths are in place to eliminate surface shrinkage. However, patternmakers must carefully consider where these features connect to their castings and avoid machining locators and fixturing points, final component attachment points and other dimensionally
Molding sand and metal mold coatings can be controlled to help improve casting surface finish, but metalcasting process control can only ensure that a final casting is free from external dimensional issues relating to surface finish or imperfections. Process control cannot ensure that a casting will meet the specified blue print dimensions.
Because the capability to hold a tolerance is largely dependent on mold material, the final dimensions of a casting should be an important criterion when selecting a casting process (e.g. sand, permanent mold, etc.). Metal mold processes, such as permanent mold and diecasting, can typically produce a tighter dimensional range than sand casting.
Look to the Standards
All casting specifications should include a reference to final dimensions when negotiating a contract with a metalcasting facility.
For aluminum castings, designers often use the Aluminum Association publication Standards for Aluminum Sand and Permanent Mold Castings to specify tolerances. The document suggests typical tolerances relating to aluminum casting issues such as draft, flatness, straightness, cast-in holes and pockets, ejector-pin marks, parallelism, angularity, perpendicularity and other dimensional considerations.
The Aluminum Association dimensional standards are typically considered to be generous and represent what any metalcasting facility should achieve with accurate tooling.
If a casting does not meet dimensional specifications, first confirm that the tooling accurately meets the print dimensions. Old, worn tooling can create casting problems that affect dimensions, and new tooling also should be validated on receipt from the mold maker to confirm accuracy. Once the tooling accuracy is confirmed, several other issues might affect the final dimensions of the casting.
Warping coming out of the mold—Hot castings are still ductile (pliable), and ejection from a permanent mold can cause casting distortion that may be difficult to correct. Even when a permanent mold is manufactured to exact print specifications, it may include features that make it difficult to eject a casting without warping or distorting it. Sand castings also may distort during mold shakeout due to the distance they are dropped or the weight of the sand on the still-pliable casting. The configuration of a casting may cause it to warp while it is cooling to room temperature after removal from the mold. Castings that are prone to this type of distortion may need to be restrained during the cool down process, as they can be difficult to bring back into the correct dimension without reheating.
Distortion in heat treating—Depending on part geometry and rigidity, some aluminum castings are prone to distortion during the heat treatment solution and quenching process. Castings can experience stress relaxation during the solution process or distortion during the rapid temperature change of the quench cycle. Heat treat distortion can be particularly significant on parts that are flat and broad, with both thick and thin sections. Case studies have shown aluminum castings 20 in. long curl as much as 0.75 in. in quenching. Care must be taken in the heat treat process to assure that casting distortion is held to a minimum and parts are not distorted beyond their ability to be straightened. Orientation in the furnace and furnace heat controls to provide even heating can help minimize the distortion.
Straightening—Castings prone to distortion can be subjected to a straightening process to bring them back into dimensional accuracy. Straightening is done after the heat treat quench cycle and before the artificial aging process, when castings are still somewhat ductile and can be manipulated easily, although heavy or thick-walled castings may be hard to bring back into accurate dimensions. Straightening often is accomplished by use of custom fixtures, and “smart” fixtures can be made that will detect distortion and bend parts back automatically. Metalcasters can robotically place parts into these automatic gauging and straightening fixtures, measure their key points and hydraulically move sections out of specification up or down to bring them into tolerance. These systems can be run on a neural network, so they can learn how much adjustment is needed. Aluminum components experience natural aging over time, so straightening is typically done within a short window before the artificial age cycle (approximately four hours at room temperature for most castings). Castings specified to a more wide open tolerance can save purchasers the additional cost of the straightening process.
The information in this article was provided by the AFS Technical Committee