Casting Processes and Tooling
This section of the toolkit is provided to reacquaint the user with basic information regarding the primary casting processes and the impact of tooling on the cost and lead-time of procuring castings. It is not meant to be a total technical tutorial on casting. The user is referred to the Additional Resources section of the toolkit to seek links to more in-depth information.
The choice of a casting process by the original designer of a component is driven by a number of factors: metal alloy, desired as-cast tolerances, desired as-cast surface finish, required mechanical properties and quality level, and quantity required, to name a few. The various tabulations of mechanical properties achievable for alloys cast by different processes are available from the various standards setting organizations like ASTM and AMS. The graphic below shows some of the tolerances associated with the various casting processes and casting sizes which may drive the casting process decision.
Once the alloy and casting process are chosen by the product designer and manufacturing engineers these elements of the design are normally fixed for the remainder of the item's life. In the military systems spare parts world, making changes to these specified materials and processes is not a trivial matter. For a contractor to request such a chance, the process can be complex and time consuming and may, in the end, not yield a deviation from what is called out on the casting drawing. Thus, it is critical that during the bidding process the drawing casting requirements be followed explicitly and a casting source capable meeting these requirements be chosen.
Overview of Casting Processes and Tooling Requirements
One of the most common casting processes specified is Sand Casting. This process is divided into two different methods, typically chosen by the foundry based upon size, quantity and alloy being cast. The two methods rely upon the same type of tooling (called patterns and core boxes), but utilize different sand systems.
Green sand (called that because it uses a binder system to hold the sand together that contains water and clay as the main elements tends to be very cost-effective in both large and small quantity production. It can be used for all commonly cast metal alloys, except for Titanium and Zirconium alloy. The size of castings produced in green sand molds span the full range of casting weights from ounces to tons. Low volume castings are typically produced by hand, with low pressure molding of the sand. It is a forgiving process in terms of using tooling that might have some minor flaws (i.e., older wood patterns with minor surface damage, etc.). Tolerances are typically acceptable (as shown above), but surface finish is considered the coarsest. When higher precision is required, metal pattern equipment is used in lieu of wood with higher pressure sand molding machines and fixtured core assemblies.
Shown below is a schematic of a simple sand casting process without any cores. It should be noted that the gating system that allows the molten metal to fill the mold can either be part of the pattern (preferably) or cut into the sand manually.
No-bake sand casting (also called air-set or chemically bonded) can be used to make any size cores and molds. This process uses resin-coated sand with a chemical binder (similar to a two-part epoxy) which has a finite working time before the binder hardens the sand mold. This process provides good, uniform strength of both cores and molds. Dimensional control is considered better than green sand. The molds need not be used immediately (as is typically required for green sand molds). On the down-side, mold production is slower due to the need for the curing time, which when high production volumes are required, may result in a higher molding cost.
Sand molding requires that each mold be broken away from the metal castings made in the mold and thus each mold makes only one or one set of castings. The patterns and core boxes used to produce the sand molds have a life ranging from hundreds to thousands of molds, depending upon from what they are constructed (wood, plastic or metal), the abrasiveness of the sand system and the care in handling and removing patterns from the sand. Because of the need to remove (draw) the pattern from the sand mold, taper (draft) must be built in to the pattern or core box. This permissible draft is usually called out on the casting drawing.
Investment casting is a high precision process usable for all metal alloys. From the tolerance chart above it can be seen to produce the tightest tolerances with the best surface finish. The process is adaptable to both low and high production quantities, but tends to be limited in terms of the size of casting economically produced. Castings weighing less than 500 lbs are most commonly produced using this casting process. Larger, more complex castings can be produced as investment castings when specialty alloys (Titanium and Nickel-base superalloys) are involved and geometry is such that the net-shape capabilities of the process offset the high cost of the casting. Investment casting, like sand casting, requires tooling (in this case molds to make the wax pattern). This tooling tends to be more expensive than sand casting tooling and is a function more of the complexity, tolerances and size of the casting of the casting required that then dictates what the wax pattern will look like (or, in fact, if multiple wax patterns will need to be made and joined together to form the entire pattern). Each mold produced from the waxes is used only once and must be broken away from the casting. Draft requirements tend to be minimized with the investment casting process.
A schematic of the investment casting process is shown below:
The pattern is produced from the waxes which are themselves made by injecting molten wax into machined aluminum molds. These waxes are then assembled into what is commonly called a tree (which can make multiple castings of smaller components or may be a large assembly of the multiple waxes required to make a very complex casting). This tree also includes the pouring cup and runner system that will allow the molten metal to reach the actual casting. After assembly, the tree is given a dip in refractory slurry and stuccoed (adhesion of fine sand to the wet refractory slurry). This dipping and stuccoing process is repeated a number of times, with controlled drying of each slurry/stucco layer, over several days, to build up a shell of ceramic of sufficient thickness and strength to support the mold under the weight of the molten metal which will ultimately fill the mold. After the stuccoing is completed, the wax is then melted out of the built up green ceramic shell at low temperatures in a steam autoclave. Firing of the shell is undertaken just before the castings are to be poured. The firing at elevated temperature (greater than 1000F) is performed to remove any residual wax from the shell and to cause a chemical reaction in the shell to make it hard and strong. Prior to the firing, the shell is relatively week and unable to withstand the pouring of metal into it. Finally the shell is ready to have metal poured into it and the casting made. After casting and allowing the metal enough time to solidify, the shell can be removed (by water jets, tumbling or other impact creating processing). The castings are then removed by cutting them off from the rest of the tree and are now ready for other secondary operations (more precise gate removal, machining, heat treatment, non-destructive evaluation, etc.)
Permanent Mold Casting and Die Casting
While the sand and investment casting processes described above require that a new mold be produced for every casting or group of castings produced, when the production quantity and geometry dictates, a permanent, semi-permanent mold or die casting process can be specified. In the permanent and semi-permanent mold casting processes, a metal mold is produced in steel, cast iron or tool steel that replicates the outside configuration of the casting. If required, interior features of a casting (for instance, flow passages in a valve body) are created by using either permanent steel or single-use expendable sand cores, depending on the geometry of the internal feature. The permanent mold can used to produce thousands of castings. The permanent molds can be poured identically to sand molds (called static pouring) or via tilt pouring of the mold where metal is poured into a basin while the mold is horizontal and then flows into the mold cavity as the mold is gradually tilted to a vertical position. Variants of the permanent mold process can use low air pressure or vacuum to draw metal from below the mold up into the mold. The permanent mold process provide good accuracy (better than sand, but less precise than investment) and the metal flow into the mold cavity is controlled to reduce the occurrence of casting defects. The permanent mold process is predominantly used for non-ferrous alloys like aluminum and magnesium, but tooling costs are significantly higher than the patterns required for sand casting. In addition mold size is limited, with the large castings being perhaps at most 60" x 24".
Take the concept of the permanent mold process but inject the molten metal into the mold very quickly (less than a second) under high pressure (several thousand psi) plus allow it to solidify under this high pressure and you have the basics of die casting. Precision machined tool steel mold cavities and cores provide extremely tight tolerances (equivalent or better than investment casting) and exceptional fine surface detail and surface finish. The tooling cost of die casting molds (or dies as they are commonly called) is the highest of all the casting processes. However, this high tooling cost is justified when high quantities of a casting are required (typically 1000 or more depending on their complexity and the cost of tooling) and secondary machining operations can be eliminated. Like permanent mold, die casting has its limitations: aluminum, magnesium and zinc alloys are typically the only ones die cast and the size of castings is limited (the largest die castings typically weigh less than 100 lbs and the size of the dies grow proportionally larger as the size of a die casting increases since the dies have to withstand the high pressures involved.
It has been noted that many die castings that were originally specified when military weapon systems were designed and large numbers were being procured are now being procured in small quantities as sand or permanent mold castings. However, it is incorrect to automatically assume that if a drawing calls out a die casting that it can automatically be replaced by a sand casting.
A lower volume, precision process used for aluminum and magnesium castings is plaster mold casting. The mold making tooling is somewhat similar to that used in sand casting, but many times a master pattern or model is produced from electronic data. From this model, plastic patterns are made. A liquid plaster slurry is then poured around the patterns (instead of sand being compacted around the pattern as done in sand casting), allowed to dry and the pattern is removed, leaving a very fine finish plaster mold, which is then baked to remove moisture. The molds are then finished by adding the metal flow passages, assembled together (with cores if needed), and molten metal is poured and allowed to solidify. As with sand casting, a plaster mold can only be used once, as it must be knocked away and destroyed to remove the casting. Limitations in the plaster mold process include it use for non-ferrous low melting temperature alloys like aluminum, magnesium and zinc and overall casting size. Tooling cost is typically low, but the individual mold cost is higher than a sand mold. Therefore, plaster mold casting is typically used for very low production runs and prototyping.
Rapid Manufacturing Technologies
The use of rapid manufacturing technologies has been incorporated into many foundries' business model. Via the use of electronic data and solid models, foundries have begun to replace hard tooling (patterns) when small production quantities are required. Technologies such as "QuickCast" which uses a hollow stereolithography model instead of a pattern or wax allows an investment, sand or plaster casting to be produced without the time required to build a pattern or machine a wax injection mold. Foundries using this rapid manufacturing technology typically request the electronic data in native as well as ProE, Parasolid or STEP format. If the electronic data is not available, foundries can create the CAD file but this adds to the time and cost of the project. In all cases it is the foundry's task to add the applicable shrinkage factor. It is best to consult with the foundry before choosing to go the electronic data route and confirm how draft and shrinkage are to be handled.
Things to Remember:
- The casting process called out on the drawing and in the TDP is the governing data.
- Changes to the manufacturing method may require approval by the engineering authority which is typically not within the procuring agency.
- Tooling is a significant element in the casting process. Lack of tooling leads to additional cost and time.