Injection Mold Components and Applications in Plastic Manufacturing

Injection molds are pivotal tools in modern manufacturing for producing plastic components, extensively used in automotive, home appliance, electronics, and medical industries. Here we systematically introduce the fundamentals of injection molds from four perspectives: mold structure, molding process, common types, and application scope.

Injection mold structure

• Molding System: Directly determines the appearance and dimensions of the plastic part. These components require high-precision machining to ensure the dimensional accuracy and surface quality of the plastic part.

  • Core and Cavity: Respectively form the internal surface and external contour of the plastic part.
  • Inserts and Threaded Components: Used for locally complex, easily worn, or threaded sections.

• Gating System: Responsible for directing molten plastic into the cavity, including the sprue, runner, gate, and cold slug well. Proper flow channel design promotes uniform melt flow and helps prevent defects.

  • Sprue: The channel through which the melt enters the mold from the injection molding machine nozzle; it is the first step of melt entry into the mold.
  • Runner: The channel that distributes the melt from the sprue to each cavity; its shape and dimensions vary depending on the number and layout of cavities.
  • Gate: A narrow opening connecting the runner and the cavity, controlling the flow rate and volume of the melt. Different plastic parts require different gate types.
  • Cold slug well: A reservoir for trapping the cold melt at the beginning of injection, preventing it from entering the cavity and affecting the quality of the plastic part.

• Guiding system: Ensures precise alignment between the moving and fixed molds during mold closing. It mainly consists of guide pins and bushings, and is critical for mold longevity and product precision.

• Ejection system: Used to smoothly eject the cooled and solidified plastic part from the mold. Common mechanisms include ejector pins, ejector sleeves, ejector plates, and pneumatic ejectors. The system must be configured appropriately according to the structure of the plastic part to prevent deformation or damage.

  • Ejector pin: The most commonly used ejection component, it directly contacts the molded part to eject it and is suitable for most standard-shaped parts.
  • Ejector sleeve: Suitable for molding plastic parts with holes, it ejects the part off the core through the sleeve, ideal for structures with internal holes.
  • Ejector plate: Used for large-area or thin-walled plastic parts, it applies ejection force evenly to prevent deformation of the plastic part during ejection.
  • Pneumatic ejection: Utilizes compressed air pressure to eject the plastic part, suitable for complex geometries or situations where mechanical ejection is challenging.
• Lateral parting and core-pulling mechanisms: When the plastic part features side holes, bosses, or similar structures, inclined guide pins, sliders, or gear-driven core-pulling devices are required to facilitate demolding.
• Cooling and heating systems: Regulate temperature via internal water channels or heating elements within the mold. The cooling system is critical for enhancing efficiency, while heating devices are commonly used for special materials or high-gloss products.

• Ventilation system: Removes air and gases from the cavity to prevent defects such as air marks and burns. This is typically achieved through vent grooves on the parting surface or gaps between components.

  • Parting surface vent groove: Machined at the mold parting surface, with a depth of 0.03–0.2 mm and a width of 1.5–6 mm, serving both to vent gases and to prevent melt overflow.
  • Component clearance: Includes fitting clearances such as between ejector pins and ejector holes, or sliders and their guiding slots.

Injection molding process

1. Mold closing: The injection molding machine's moving platen drives the moving mold to close against the fixed mold, forming a sealed cavity and gating system. The guiding mechanism ensures precise alignment of the moving and fixed molds, while the mold clamping unit withstands the high pressure of injection molding.
2. Injection molding: After the mold is closed, the injection molding machine screw rotates to melt the plastic pellets, generating pressure at the front of the screw. When the pressure reaches a specified value, the injection cylinder pushes the screw to inject the plastic melt through the nozzle into the mold's gating system, filling the cavity.
3. Holding pressure: After injection molding, the plastic in the cavity is at high temperature and high pressure and will shrink as it cools; holding pressure is required to compensate for this shrinkage. The injection molding machine screw moves forward slowly, continuously supplying plastic melt to the cavity to ensure the plastic part is dense and dimensionally stable. The holding pressure and time are adjusted according to the material, geometry, and size of the plastic part.
4. Cooling: After holding pressure ends, the cooling system operates, and circulating water removes heat from the mold and plastic part, enabling the plastic to cool and solidify. Cooling time affects both the molding cycle and the quality of the plastic part. The cooling system and duration must be appropriately designed and controlled according to the specific requirements of the plastic part.
5. Mold opening: Once the plastic part has sufficiently cooled, the injection molding machine's moving platen drives the moving mold backward to open the mold. The mold opening speed and distance must be precisely controlled to prevent damage to the mold and plastic part, while the ejection system prepares for operation.
6. Ejection: After the moving and fixed molds are fully separated, the ejection system, actuated by the injection molding machine's ejector pin, uses ejector pins, ejector sleeves, or ejector plates to push the plastic part out of the cavity. The ejection process must be stable and reliable to prevent deformation or damage to the plastic part. After ejection, the plastic part proceeds to subsequent processing.

Two-Plate Mold (Single Parting Surface Mold)

The simplest structure, widely used. The moving mold and fixed mold are separated by a single parting surface, suitable for the production of most standard plastic parts.

• Advantages:
  • Low cost and simple structure;
  • Convenient manufacturing and maintenance.
• Disadvantages:
  • Excess gate waste, requires manual removal;
  • Limited control over appearance.

Three-Plate Mold (Dual Parting Surface Mold)

By incorporating a middle plate in the fixed mold, automatic separation of the plastic part and runner is achieved. This supports point gate feeding and is commonly used for products with high aesthetic requirements.

• Advantages:
  • Enhanced appearance, suitable for automation;
  • Flexible gate positioning.
• Disadvantages:
  • Complex structure and high cost;
  • High requirements for injection pressure and operational technique.

Hot Runner Mold

Employs a heating system to maintain the runner plastic in a molten state, eliminating the need for runner solidification.

• Advantages:
  • No waste, high material utilization.
  • Short molding cycle, high efficiency.
  • Excellent consistency in quality.
• Disadvantages:
  • Complex structure and significant investment.
  • Maintenance and material replacement are relatively challenging.

Injection molds are primarily used for the mass production of plastic components. For example, in the automotive industry, numerous parts such as instrument panels, door panels, and air filter housings are manufactured using injection molding processes. In other sectors, including household appliance housings, packaging containers, and medical devices, injection-molded products are equally prevalent.

Injection molds are critical factors influencing both the quality and production efficiency of plastic products. Understanding its structural principles and molding process contributes to enhancing the professional standards of mold design, operation, and maintenance, providing robust support for production stability and product competitiveness. For further information, please feel free to contact our technical team for further support.