Figs. 3a-b: Large shell cores present unique challenges for aluminum permanent molders. Fig. 3a (left) weighed 46 lbs. (20.9 kg) with 32 in. (81 cm) between core prints. Fig. 3b was 14 in. (35.5 cm) tall, 22 in. (55.9 cm) wide and weighed 20 lbs. (9.1 kg).
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 robot- ics, is another viable option for higher volume cores. Core Coating: T e fi rst consid-
eration is to determine if a coating is needed, with the surface fi nish of the casting around the core the essential factor. Coatings usually are added in areas that require a smoother casting surface fi nish, or to prevent molten metal penetration issues such as ero- sion, 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
38 | MODERN CASTING September 2013
soon after solidifi cation can aff ect casting dimensions. Most metalcast- ing 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 fi nal 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, distor- tion and gas-related porosity. When a core is damaged, the casting must undergo excessive cleaning room operations, which can increase costs signifi cantly, 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, insuffi cient core strength, excessive metal temperature, thermal shock or mold-related issues like
Fig. 4: The large fl at section on this 26-in. (66-cm) core developed cracks regularly.
excessively tight core prints. Insuf- fi cient 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 con- tinual 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 for- mal preventative maintenance program on all permanent mold machines. Use shell core cooling fi xtures to maintain dimensional accuracy. Core Integrity: Determine the
parameters for optimal resin con- tent 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 vent-
ing for the removal of core gases. Core Strength: Add features as needed to improve strength, such
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