Developing a successful, repeatable, reliable process for pouring aluminum into a permanent mold is no simple task. The many variables include metal and mold temperatures, shrink factors, mold coatings, casting cycle times and melt quality. Considering the numerous factors that influence quality and quantity, introducing sand cores to permanent molds only complicates matters.
Beyond the obvious issues of cost and binder material, a sand core can slow the heat transfer (and extend solidification time for the casting), change shrinkage factors for the casting, introduce potential core coatings and require additional venting for gases in semi-permanent molds—so-called because of the disposable core. Additional issues arise in handling cores, such as trimming, insertion and removal from the metal mold. Sand cores also require added tooling, which includes validation and maintenance. The corebox itself is only one concern. Tooling includes trimming methods, c ore assembly fixtures, transport materials, material handling components and verification fixtures.
The core’s effect on dimensional accuracy is a critical consideration for components with tight dimensional requirements. One determination for any given casting part is whether the mold or the sand core will have the more significant influence on dimensional shrinkage. Among the determining factors are the core material, resin percentage, casting size, geometry, wall thickness and whether the core is knocked out after solidification or remains in the casting during cooling. Additionally, some castings may include axis-specific shrink factors.
Gas and Venting
All sand cores generate gas due to binder decomposition when contacting molten metal; the amount depends on the binder system and coating. Various core binders not only generate different amounts of gas, but also show different gas evolution profiles (i.e., when and at what rate gas is released). The gas amount and its evolution rate are affected by binder type, melt temperature, core geometry and processing conditions. Binders that generate more gas need increased venting—in total number and/or size—to allow for proper gassing of the mold cavity. Vents should be placed to ease cleaning of core gas residue. Engineers also must determine if the mold requires active or passive gas venting, that is, if the mold design and filling pattern will push the gas out naturally or a more aggressive vacuum assist is necessary. Gas evolution also can correlate with condensation in the mold cavity.
Core Handling and Placement
Handling sand cores and placing them into a hot metal mold can be sources of trouble. Some practical suggestions include:
Core Positioning: A cold core may fit differently into a cold mold versus a hot one. The core should be positioned in the mold so it’s fully supported during pouring. Core prints should locate the core without constraining it in a way that could lead to fracture. Prints should be easy to clean and discourage residue buildup, which could lead to misalignment. Core prints also must allow for the thickness of the mold core coating and ensure adequate core print clearances. It is important to focus on both halves of the mold and include proper core clearances, at least 0.005–0.015 in. (0.127–0.381 mm) per side. Some metalcasting facilities provide additional close-over clearances of 0.005–0.01 in. (0.127–0.254 mm). Because prints wear excessively, they should be easy to access and repair. Also, since closing the mold will likely shave some sand from the core, prepare for residue in the mold. The core print may be an ideal spot for venting. Because there is no liquid metal surrounding the core print, it provides an easier path for venting core gasses that won’t mark the casting surface.
Core Handling: Handling is a concern with cores that are heavy, fragile, easily contaminated and/or prone to damage resulting in a defective casting. One option includes molding a steel bushing into the core and using a ball detent pin to lift and place the core into the mold (Fig. 1a-1b).
Trimming: Hand trimming is an economical approach for low quantities of cores, but trim plates and stripping dies can be helpful (Fig. 2) when dealing with larger volumes or trying to limit hand trimming variability. Automation, including CNC machines and robotics, is another viable option for higher volume cores.
Core Coating: The first consideration is to determine if a coating is needed, with the surface finish of the casting around the core the essential factor. Coatings usually are added in areas that require a smoother casting surface finish, or to prevent molten metal penetration issues such as erosion, veining or burn-in-type defects. Coating also can help direct the core gas toward vents.
Core Removal: How the core will be removed from the casting and how soon after solidification can affect casting dimensions. Most metalcasting facilities use a combination of vibration, abrasion (e.g., hammers, drills, blasting) and bake-out ovens. Knockout is faster and allows for quicker inspection times. Shot blasting removes final residues after knockout.
Large Shell Cores
Large shell cores—those weighing more than 10 lbs. (4.54 kg), longer than 24 in. (61 cm) or with a length-to-diameter ratio greater than four (Figs. 3a-3b)—present a special set of challenges, including the potential for core breakage, cracking, distortion and gas-related porosity. When a core is damaged, the casting must undergo excessive cleaning room operations, which can increase costs significantly, or be scrapped entirely. Such damage can be caused by core and molten metal issues such as the pressure of the molten metal on the core, insufficient core strength, excessive metal temperature, thermal shock or mold-related issues like excessively tight core prints. Insufficient shell core curing or improper venting will lead to gas porosity. A majority of issues relating to large shell cores can be resolved with proper engineering controls and continual process review.
Here are a few basic steps to deal with large shell cores:
Core and Mold Alignment: Maintain proper mold and machine alignment by minimizing contact points in core prints. Establish a formal preventative maintenance program on all permanent mold machines. Use shell core cooling fixtures to maintain dimensional accuracy.
Core Integrity: Determine the parameters for optimal resin content to maximize core integrity, wall thickness, cure time, temperature, etc. Ensure the core is cured fully.
Gating: Review the gating system to avoid excessive stress on the core.
Core Gas: Provide adequate venting for the removal of core gases.
Core Strength: Add features as needed to improve strength, such as a tie bar between the legs of a U-shaped core to prevent distortion. Modify the core binder system and change the resin composition of the shell core as needed to improve strength. Work with the sand provider on new formulations.
Even after implementing these suggestions, large shell cores can remain difficult to use in permanent mold applications. For example, at Wisconsin Aluminum Foundry Co. (WAFCO), Manitowoc, Wis., the large cylindrical core in Figure 3a failed at a 75% rate due to a number of problems. Engineers filled the 46-lb. (20.87-kg) shell core with air-set sand to produce castings, but this process increased both core weight and knockout costs. Similarly, the core in Figure 4, at nearly 26 x 6 in. (66.7 x 15.2 cm), featured a large flat section that developed cracks regularly. While not leading to outright failure, as in the cylindrical core, it required additional inspection to verify the surface finish in the cored casting cavity and extra cleaning room labor to remove flashing caused by broken and cracked cores.
WAFCO continues to minimize variation in the core and molding process by working with suppliers to modify sand recipes to improve core integrity, discussing necessary core support prints with clients and avoiding new high risk jobs based on experience. Though networking with other metalcasting facilities is helpful in learning techniques to overcome challenges, large shell cores remain a complex endeavor.