Mold Classification for Casting Systems

　　Depending on the type of gating system, plastic molds can be classified into three categories:

　　(1) Large‑gate molds: The runner and gate are located on the parting line, so they are ejected from the mold along with the molded part when the mold is opened. This design is simple, easy to manufacture, and cost‑effective; therefore, many users opt for large‑nozzle systems. Plastic mold structures are divided into two main parts: the moving half and the fixed half. The movable portion of the injection molding machine is called the moving mold (often equipped with an ejector system), while the stationary portion at the injection end is typically referred to as the fixed mold. Because the fixed section of a large‑gate mold usually consists of two steel plates, this type of mold is also known as a two‑plate mold. Among all large‑gate molds, the two‑plate mold has the simplest structure.

　　(2) Sub‑gate molds: In these molds, the runner and gate are not located on the parting line but are typically integrated directly into the molded part. Consequently, a set of more than one sprue must be designed, resulting in a more complex layout and greater machining challenges. Selection should generally be based on the specific requirements of the product. These molds employ a precision gating system. The fixed half of the mold usually consists of three steel plates, which is why this type of mold is also referred to as a “three‑plate mold.” Among nozzle‑type molds, the three‑plate mold represents one of the simplest configurations.

　　(3) Hot-runner molds: The structure of this type of mold is essentially the same as that of a sub‑gate mold. The key difference is that the runner system is housed within one or more temperature‑controlled hot-runner plates and hot nozzles, eliminating the need to eject cold material from the runner and gate. Instead, the gate and runner are directly integrated into the molded part, so no separate demolding of the runner is required. This system is also referred to as a gateless system, which helps conserve raw materials. It is particularly suitable for applications where raw materials are expensive, product quality requirements are high, design and machining are challenging, and mold costs are substantial. A hot-runner system, also known as a hot-channel system, primarily consists of hot‑runner bushings, hot-runner plates, and a temperature‑control electrical cabinet. Common configurations include single-point hot runners and multi-point hot runners. In a single-point hot runner, a single hot‑runner bushing injects molten plastic directly into the cavity, making it ideal for single‑cavity, single‑gate molds. For multi‑point feeding in single‑cavity molds or for multi‑cavity molds, a split hot-runner system is typically employed, with multiple hot‑runner bushings leading into the cavity.