Multi-cavity molds, also known as multi-hole molds, are molds with multiple molding cavities on a single mold base, capable of simultaneously pressing or injection molding multiple products. The core of their design lies in maximizing production efficiency and reducing unit cost while ensuring product quality through a rational cavity layout, runner, and gating system design.
The number of cavities is typically determined by the production batch size and the model of the injection molding machine or extruder used. A common designation is "one-out-of-four" to indicate a four-cavity mold. Cavity layouts are mainly divided into balanced and unbalanced layouts. Balanced layouts achieve even material feeding into each cavity, ensuring consistent plastic parts, but the runners are longer. Unbalanced layouts have compact cavity arrangements, saving mold space, but may lead to differences in filling between cavities, potentially resulting in uneven part size or quality. In aluminum extrusion multi-cavity mold design, cavity placement must consider steel strength, avoiding excessively large or small cavity spacing. A symmetrical layout is generally preferred, with horizontal placement recommended. If a vertical layout is necessary, staggered placement is recommended.
The runner system is crucial for achieving balanced filling in multi-cavity molds. In multi-cavity injection molding, an "H"-shaped runner structure is often used to achieve balanced flow and filling of the molten resin. However, it is important to note the potential problems of flow shear heat generation and uneven melt temperature distribution associated with the "H"-shaped runner. In processes such as rubber injection vulcanization, cold runner design is widely used due to its material-saving and quality-improving properties. Its design requires verification of the feasibility of multi-cavity filling using fluid models (such as pseudoplastic fluid models). For aluminum extrusion multi-cavity molds, the manifold design is divided into two forms: individual manifold feeding and integral manifold feeding. Manifolds and weld chambers are typically machined using CNC centers to ensure precision.
The gate should be located where it is most easily removed from the part and where it should not affect the appearance as much as possible. Its position should ensure that the distance from all parts of the cavity is as consistent as possible (providing equal flow length) to facilitate balanced filling. The design should also consider facilitating gas escape from the cavity and preventing the molten plastic from directly impacting weak cores, inserts, or slides to prevent part deformation.
The number and location of the parting lines need to be determined based on the specific structure of the plastic part. For plastic parts with complex structures, high requirements for appearance quality, or high assembly precision, a multi-cavity, multi-partition surface design may be adopted. For example, some complex parts may require three or more parting surfaces.
The mold material must possess good properties. Aluminum extrusion multi-hole molds require steel with good toughness, and the heat treatment hardness is typically maintained between 48-50 HRC. The machining requirements for critical parts such as cavities are extremely high, and slow wire cutting is usually used to ensure the perpendicularity, parallelism, and surface finish of the working zone [14-15]. For ultra-precision multi-hole array molds (such as BGA packaging molds), micro-hole machining can be achieved using ultra-fine electrode wire (e.g., 0.02 mm diameter wire) technology, combined with a high-precision positioning system.
Temperature control is crucial for ensuring stable production of multi-hole molds. Taking aluminum extrusion multi-hole molds as an example, the mold temperature is typically controlled between 430-480°C, the ingot cylinder temperature is controlled between 410-440°C, and the outlet temperature must be strictly controlled between 520-580°C. Aluminum rods are typically heated using a stepped cooling method from high to low. Die heating time should generally be controlled within 6 hours to ensure uniform heat penetration. During extrusion, the time from removing the die from the furnace to the start of extrusion should not exceed 3 minutes, and the extrusion speed is usually controlled at 30-40 m/min. During production, quenching (air cooling or water cooling) must be strictly performed according to process requirements, and unexplained shutdowns should be avoided.
