Core Structure
The core is composed of two parts: the main body and the core head. The main body is used to form the internal cavity of the casting, while the core head provides support, positioning, and venting functions. To increase the strength and rigidity of the core, core irons (reinforcements) should be placed inside the core during manufacturing. To ensure smooth venting, vent passages should be incorporated into the core. In order to improve the refractoriness of the core surface, reduce surface roughness, and prevent sand fusion defects in the casting, a layer of refractory coating is usually brushed onto the core surface.
1. Core Iron
Placing core irons inside the core not only improves its overall strength and rigidity but also makes it easier to lift and position.
To avoid obstructing the shrinkage of the casting during solidification, there should be a certain distance between the core iron and the working surface of the core; this distance is called the “sand allowance”. For cast iron cores, the sand allowance is generally 15–50 mm. Since the linear shrinkage of steel castings is larger, their sand allowance should be 20%–30% greater than that of iron castings.
2. Core Venting
During pouring, in order for the gases inside the core to be discharged smoothly and quickly from the core head, vent holes must be provided inside the core.
When using a split core box for core making, vent passages can be dug or scraped out in the sand cores of both halves of the core box using core-making tools before they are bonded together. In mass production, vent templates can be used to press out vent passages.
For thick-section cores, vent holes alone are not sufficient; materials such as coke or slag should be placed inside the core to enhance venting. This method not only improves venting but also increases the collapsibility of the core and reduces its weight. When making vent passages inside the core, care must be taken to ensure that the passages are interconnected and not interrupted or blocked. Vent passages must lead to the core head and must never be connected to the working surface of the core.
Core Preparation
Core preparation can be classified into two categories according to the forming method: core box core making and sweeping plate core making.
1. Core Box Core Making
Core box core making requires standardized operation according to the type and structure of the core box. This method offers high dimensional accuracy and production efficiency, and it is suitable for producing cores with complex shapes. It is the most widely used core-making method.
2. Sweeping Plate Core Making
Depending on how the sweeping plate moves, there are two sweeping plate core-making methods:
(1) When the dimensions of cylindrical cores are large, sweeping plate core making can be used to save material and labor costs associated with manufacturing the core box.
(2) For cores with round or polygonal cross-sections and no variation in cross-sectional shape, a movable sweeping plate (guided sweeping plate) can be used to produce half-cores.
Core Finishing, Inspection, Assembly, and Installation
1. Core Finishing
Cores that have been baked need to be cleaned and finished. For cores that are manufactured in sections and assembled after baking, the bonding surfaces (joint surfaces) must be leveled using a scraper or grinding wheel before assembly.
2. Core Inspection
Inspection is the final process before placing the core into the mold. In addition to checking whether the core has been baked properly, and whether it is cracked or damaged, it is also necessary to check if the vent passages are blocked. For cores that are prone to deformation, special attention should be paid to dimensional accuracy. For single-piece or small-batch production, standard measuring tools are used, while in mass production, special gauges and templates are employed.
3. Core Assembly
For complex-shaped cores, the core is often divided into several parts for manufacturing and then assembled into one piece after baking. This process is called core assembly. Small cores are bonded together with adhesives, while large cores are assembled using bolts that pass through the core irons.
4. Core Installation
In mass production of thin-walled castings with complex shapes (such as engine cylinder blocks), multiple cores must be placed in the mold. To improve assembly speed and ensure casting dimensional accuracy, the cores are often pre-assembled and fixed together on a platform or in a special fixture, and then lowered into the mold all at once.
Post time: Jul-31-2025