Types of Injection Molds
Injection molds are the core equipment of injection molding production and can be divided into various types according to different classification standards. Each type of mold has its own unique structural characteristics and application range. Understanding the types of injection molds can help you select the appropriate mold based on factors such as the structure, material, and production batch of the plastic part, thereby improving production efficiency and product quality. Injection molds are generally classified based on the number of cavities, gating system type, mold opening and closing method, and structural characteristics. Common types include single-cavity molds, multi-cavity molds, hot runner molds, cold runner molds, two-color molds, and stacked molds.
Single-cavity molds are molds with a single cavity. They are suitable for producing large, complex, or high-precision plastic parts, such as automobile bumpers and large appliance housings. The advantages of single-cavity molds are their simple structure, short manufacturing cycle, and low cost. They also ensure dimensional accuracy and consistency of the plastic parts, as the machining precision of a single cavity is easier to control and the melt flow and cooling conditions within the cavity are consistent. For example, when producing high-precision medical device housings, using a single-cavity mold ensures that the dimensional tolerance of each plastic part is controlled within ±0.02mm, meeting assembly requirements. However, single-cavity molds have the disadvantage of low production efficiency, with each mold only producing one plastic part. They are therefore suitable for small-batch production or trial production. When designing a single-cavity mold, special considerations must be given to the cavity layout, gate location, and size to ensure uniform melt filling and reduce internal stress.
A multi-cavity mold contains two or more cavities. It’s suitable for producing small, simple, and high-volume plastic parts, such as bottle caps, buttons, and electronic connectors. The greatest advantage of a multi-cavity mold is its high production efficiency. It can produce multiple parts within the same molding cycle, significantly reducing unit production costs. For example, molds for producing mineral water bottle caps typically contain 32, 48, or even more cavities, capable of producing tens of thousands of caps per hour, meeting the demands of large-scale production. Multi-cavity molds come in two types: balanced and unbalanced. A balanced layout evenly distributes the cavities, allowing the melt to reach each cavity simultaneously, ensuring consistent quality across all parts. An unbalanced layout, however, offers uneven distribution, with the melt arriving at different times, making it suitable for applications where part consistency is less critical. When designing a multi-cavity mold, a well-designed runner system is crucial to ensure even melt distribution across the cavities, avoiding inconsistent filling.
A hot runner mold is an advanced injection mold whose runners in the gating system utilize heating devices to maintain the melt in a molten state, eliminating the need for runner slurry removal. Therefore, it is also called a runnerless mold. The advantages of hot runner molds are that they conserve raw materials and reduce runner slurry waste, significantly reducing production costs, especially for large plastic parts or those made of precious materials (such as engineering plastics). Furthermore, by eliminating the need for runner slurry removal and handling, they shorten molding cycles and improve production efficiency. The runners of a hot runner mold are divided into main runners, branch runners, and gates, each equipped with heating coils or rods. A temperature control system precisely controls the runner temperature to prevent the melt from solidifying within the runners. Hot runner molds are suitable for high-volume production of plastic parts requiring high dimensional accuracy, such as mobile phone casings and automotive parts. However, their disadvantages include complex structure, high manufacturing costs, and difficult maintenance. Furthermore, they place high demands on the fluidity and stability of the raw materials, making them less commonly used in small-batch production.
Cold runner molds are traditional injection molds. Their gating system’s runners lack heating. The melt cools and solidifies within the runners, forming runner aggregates that need to be removed along with the molded part before separation. Cold runner molds offer a simple structure, low manufacturing costs, and strong adaptability to raw materials, making them suitable for nearly all injection-molded plastic materials. They are particularly well-suited for small-batch or high-variety production. Cold runner molds offer relatively flexible runner design, allowing for various runner shapes (such as circular, trapezoidal, and U-shaped) and sizes to suit the part’s structure. However, their drawbacks include significant raw material waste, with runner aggregates sometimes weighing more than the part itself, increasing material costs. Furthermore, the removal and handling of runner aggregates increases production processes and reduces production efficiency. When designing a cold runner mold, it’s important to optimize runner size and layout, minimizing runner length to minimize runner aggregate formation while ensuring smooth melt flow and uniform filling.
Two-shot and multi-shot molds are molds capable of molding two or more plastic parts of different colors or materials within the same mold. They are suitable for producing parts with complex appearances and diverse colors, such as automotive dashboards and mobile phone keypads. Two-shot molds typically have two cavities. A rotating table or moving core is used to sequentially inject the two materials. The first injection molds a portion of the part, and then the mold rotates or the core moves to perform the second injection mold, combining the two materials. The advantage of two-shot molds is that they enable integrated molding of plastic parts, improving the product’s appearance and structural strength, and reducing subsequent assembly steps. However, their disadvantages are their complex structure, high manufacturing costs, and difficulty in commissioning. They are therefore suitable for the production of high-volume, high-value-added plastic parts. Stack molds are molds with multiple parallel cavities within the same mold. By increasing the number of cavities, production efficiency is improved, making them suitable for the production of small, flat parts, such as compact discs and small gears. The advantage of stack molds is that they can significantly increase production efficiency and reduce energy consumption per unit product without increasing the mold footprint. However, the increased mold thickness requires higher clamping force and opening stroke of the injection molding machine, resulting in relatively high manufacturing costs.
Specialty injection molds, including gas-assisted, water-assisted, and foaming molds, are suitable for specific molding processes and part requirements. After filling the mold cavity with melt, gas-assisted injection molds inject high-pressure gas into the melt. The gas pressure propels the melt to fill the cavity and compensate for shrinkage. This makes them suitable for producing thick-walled, large parts, reducing defects like shrinkage cavities and sink marks, and reducing part weight. Water-assisted injection molds are similar to gas-assisted injection molds, but use high-pressure water for enhanced cooling, making them suitable for parts requiring high cooling rates. Foaming injection molds create microporous structures within the part by adding a foaming agent or injecting gas into the melt. This makes them suitable for producing lightweight, shock-absorbing parts, such as packaging materials and automotive interiors. Specialty injection molds have complex structures and high technical requirements, solving molding problems difficult to solve with traditional molds. However, their manufacturing and maintenance costs are high, and their application range is relatively narrow, primarily used in the production of high-end parts in specific applications.
