Chapter 3: Aluminum Casting Processes
3.4 Expendable Mold Gravity-Feed Casting Process and
3.4.1 Sand Casting
Green and Dry Sand. Sand casting involves the forming of a geometrically dimensioned impression in sand. The process in-cludes green sand and dry sand casting. Green sand refers to the use of uncured bonding systems, usually a blended mixture of sand, clay, and water. In the dry sand process, resins, oil, or other chemi-cal binding agents are used to precoat the molding sand. The dry sand mold is then thermally or chemically cured. Another dry sand method reacts carbon dioxide (CO2) gas with sodium silicate in the sand blend to form a silica gel bond.
The important parameters of molding sand are compressive strength and permeability. Tests for each are routinely performed
Fig. 3.3 Typical die casting gating. Source: Ref 3
Chapter 3: Aluminum Casting Processes / 23
for quality assurance. The mold must have sufficient strength to maintain its shape through the casting process and sufficient per-meability to permit the air, and gases formed during pouring, to evacuate the mold cavity as the metal enters. Compressive strength and permeability in green sand are functions of sand particle size and shape, moisture and binder contents, and the degree of com-paction applied in forming the mold.
The advantages of typical sand casting are versatility in a wide variety of alloys, shapes, and sizes. Alloys considered hot short because of cracking tendencies during solidification are more eas-ily cast in green sand since molds offer reduced resistance to di-mensional contraction during solidification. Lower mold strength is also advantageous for parts with widely varying section thick-nesses and intricate designs. These advantages are diminished in chemically bonded molds, which display greater rigidity than green sand molds.
There are only practical limitations in the size of the parts that can be cast. Minimum wall thickness is normally 0.15 in. (4 mm), but thicknesses as little as 0.090 in. (2 mm) can be achieved.
Among disadvantages are relatively low dimensional accuracy and poor surface finish; basic linear tolerances of⫾0.03 in./in.
(⫾30 mm/m) with a minimum tolerance of 0.020 in./in. (20 mm/
m), and surface finishes of 250 to 500 μin. (6 to 13 μm) root mean square (rms). Chemically bonded sands offer improved surface finish and dimensional accuracy, and relatively unlimited shelf life, depending on the type of binder used. Achievable dimensional tolerances can be substantially improved using precision methods in the forming and assembly of dry sand mold components. Surface quality can be improved by using a finer grade of facing sand in the molding process.
Strength is typically lower as a result of slow solidification rates.
Mechanical properties are improved by using sands such as zircon and olivine with higher heat capacity than silica and by the use of copper, iron, or steel chills at strategic locations in the mold.
Sand casting involves minimum tooling and equipment cost when smaller numbers of castings are to be produced.
Sand Molding. In both green and dry sand processes, the mold is formed by compacting the preconditioned sand over the pattern.
In floor or hand molding, the sand is compacted using manual or pneumatic rams; for pattern plates (match plates), machines use jolt/squeeze mechanisms to ensure mold integrity. Automatic mold-ing machines provide a high degree of uniformity and very high mold production rates.
Patterns may consist of wood, composite, or metal plates con-taining the casting impression or may consist of loose pieces as-sembled in the form to be cast. After compaction of the molding sand, the pattern is carefully removed, leaving a cavity in the shape of the casting to be made. A dusting of calcium carbonate or other parting compound on the pattern surface is helpful in facilitating the separation of the pattern from the mold. A small amount of vibration through transducers attached to the match plate also fa-cilitates pattern removal.
The runners and in-gates are usually integral to the pattern, but may also be manually cut into the sand by the molder. A pattern of the sprue is separately placed on the pattern before sand is introduced.
If the casting contains internal passages or undercuts, dry sand cores may be used. In these cases, the mold includes locating points and additional impressions or prints for precisely positioning the cores after pattern removal and before final mold assembly. In rare cases, aluminum chaplets may be used to support the core position.
Molten metal is poured into the mold, and after it has solidified, the mold is physically removed from the casting. Removal is by physical means including vibration.
Casting quality is determined to a large extent by foundry tech-nique. Proper molten metal processing, metal-handling and gating design and practices including the selective use of chills are nec-essary for obtaining sound castings. Complex castings with varying section thicknesses will be sound only if proper techniques are used.
While the principles of sand casting are relatively simple, a large number of process variations are in use that typically involve ex-pendable pattern materials and molding methods.
3.4.2 Evaporative (Lost-Foam) Pattern Casting (EPC) Lost foam is a sand casting process that uses an unbonded sand mold with an expendable polystyrene pattern. This process is some-what similar to investment casting in that an expendable pattern is used to create the mold cavity. Unlike investment casting, the pattern vaporizes during the pouring of molten metal rather than before pouring.
The EPC process employs a foamed polystyrene pattern packed in unbonded sand. The polystyrene model is coated with a thin layer of ceramic or refractory wash that seals the pattern surface. The pattern is sequentially decomposed by the heat of the molten metal, thus replacing the foam pattern and duplicating the features of the pattern in the solidified casting. Use of the process has increased rapidly, especially in large-volume automotive foundries, and many casting facilities are now dedicated to the production of castings by this method (Fig. 3.4).
The major difference between sand castings and castings made by the EPC process is the extent of subsequent machining and cleaning operations required. Evaporative pattern castings are
con-Fig. 3.4 Typical castings produced by the evaporative pattern casting (EPC) process
sistently poured at closer tolerances with less stock for grinding, machining, and finishing. Dimensional variability associated with core setting and the mating of cope and drag are eliminated. The use of untreated, unbonded sand simplifies sand processing, han-dling, and reclamation. Casting cleaning is also greatly reduced and can often be eliminated because of the absence of flash, sand adherence, and resin stains. Further benefits of the EPC process result from the freedom in part design offered by the process.
Assembled patterns can be used to make castings that cannot be produced by any other high-production process.
3.4.3 Shell Mold Casting
In shell mold casting, molten metal is poured into a shell of resin-bonded sand only 0.4 to 0.8 in. (10 to 20 mm) thick. The mold is formed by introducing the chemically coated sand to a heated pattern that thermally cures the bond. By controlling the core mold temperature and cycle, the depth of cure can be controlled to the desired thickness. Cured mold sections are removed and assembled for pouring, usually backed by unbonded or green sand (Fig. 3.5).
Shell mold castings surpass ordinary sand castings in surface finish
and dimensional accuracy and cool at slightly higher rates; how-ever, equipment and production costs are higher, and the size and complexity of castings that can be produced are limited.
3.4.4 Plaster Casting
In this method, either a permeable (aerated) or impermeable plaster is used for the mold. The plaster in slurry form is poured around a pattern. When the plaster has set, the pattern is removed and the plaster mold is baked to remove free water and reduce waters of hydration. The high insulating value of the plaster allows castings with thin walls to be poured. Minimum wall thickness of aluminum plaster casting is typically 0.060 in. (1.5 mm). Plaster molds have high reproducibility, permitting castings to be made with fine details and close tolerances; basic linear tolerances of
⫾0.005 in./in. (⫾5 mm/m) are typical. The surface finish of plaster castings is excellent; aluminum castings attain finishes of 50 to 125 μin. (1.3 to 3.2 μm) rms.
For complex shapes, such as some precision impellers and elec-tronic parts, mold patterns made of rubber are used because their flexibility makes them easier to withdraw from the molds than rigid patterns. Intricate plaster castings may also be produced using polystyrene or other expendable pattern materials such as those used in investment casting.
Mechanical properties and casting quality depend on alloy com-position and foundry technique. Slow cooling due to the highly insulating nature of plaster molds magnifies solidification-related problems such as hydrogen pore formation and shrinkage voids and reduces strength and ductility.
For many plaster cast parts, there are only limited capabilities for improving internal soundness and properties through traditional gating and risering approaches. The configuration of impellers and other rotating parts subject to strict dimensional requirements as well as strengths compatible with high rotational stresses permits the use of extensive chilling of the shaft and base for purposes of improving internal soundness and mechanical property perfor-mance. These techniques are further enhanced when combined with nonturbulent mold filling by low-pressure or other counter-gravity methods. Figure 3.6 shows plaster cast alloy 224.0 impel-lers that were produced through a low-pressure method.
3.4.5 Investment Casting
Investment casting of aluminum most commonly employs ce-ramic molds and expendable patterns of wax, plastic, or other low-temperature melting materials. In the ceramic shell method, assembled patterns are invested in a ceramic slurry by repetitive immersion and air drying until the desired shell thickness has been formed. In the solid mold investment method, the assembled pat-tern is immersed in a container of sufficient size for ceramic slurry to encase and set around the pattern. In either case, the mold is placed in an autoclave to remove the pattern and then fired at high temperature to remove all free water and organic materials and to cure the binding system being used. Molds are typically preheated and poured under partial vacuum. Christmas-tree gating systems are employed to produce small multiple parts in one mold.
Fig. 3.5 Shell molds assembled before pouring. Source: Ref 1
Chapter 3: Aluminum Casting Processes / 25
Aluminum investment castings can have walls as thin as 0.015 to 0.030 in. (0.40 to 0.75 mm), basic linear tolerances of⫾5 mils/in.
(⫾5 mm/m) and surface finishes of 60 to 90 μin. (1.5 to 2.3 μm).
Because of porosity and slow solidification, the mechanical prop-erties of many aluminum investment castings are typically lower than those demonstrated by other casting processes. The interest of the aerospace and other industries in the combination of accurate dimensional control with controlled mechanical properties has re-sulted in the use of improved technologies to produce quality castings by investment methods. Castings in the
premium-strength range can be achieved with molten metal treatments, gating, and solidification conditions that are not typical for conventional investment castings.
Investment casting applications include: instrument parts, im-pellers, compressor vanes, gears, ratchets, pawls, scrolls, speed brakes, wing tips, and aircraft pylons.
3.4.6 Vacuum Mold (V-Mold) Casting
A heated plastic film is drawn over the pattern by vacuum.
Unbonded sand is filled against the plastic-covered pattern within a vented flask and compacted by vibration. A vacuum is drawn through the flask after an unheated plastic film is placed over the back of the mold, creating a mold vacuum package. Pouring takes place with the vacuum retained or reapplied (Fig. 3.7).
Advantages are surface finish, minimum wall thickness, and reduced draft requirements. Disadvantages are tooling costs and size limitations imposed by maximum flask dimensions.