The Functional Structure of a Plastic Mold

The Heart of the Plastic Mold: Forming Components

The molding section represents the core of any plastic mold, as it directly shapes the plastic material into the desired product form. This critical part of the plastic mold consists of components that form the mold cavity, where the molten plastic cools and solidifies into the final part geometry.

The primary components of the molding section include型腔 (cavities), 型芯 (cores), 成型 (molding inserts), 成型环 (molding rings), and 镶块 (insert blocks) — all parts that come into direct contact with the molten plastic during the injection process. These components must be manufactured with extreme precision to ensure the final plastic parts meet dimensional requirements and surface finish specifications.

In a typical plastic mold, the cavity is the female part of the mold that forms the outer surface of the plastic part, while the core is the male component that shapes the inner surfaces. For complex part geometries, multiple cores and cavities may be integrated into a single plastic mold to produce multiple parts in a single molding cycle or to create intricate features.

Molding inserts and insert blocks are often used in a plastic mold to create specific features or to facilitate easier maintenance and replacement of high-wear areas. These interchangeable components allow for modifications to part design without rebuilding the entire plastic mold, significantly reducing tooling costs for design iterations.

The materials used for these forming components are carefully selected based on the plastic material being processed, production volume requirements, and part quality specifications. Common materials include various tool steels such as P20, H13, and S7, which offer excellent wear resistance, thermal stability, and machinability — all essential properties for withstanding the harsh conditions inside a plastic mold during production.

Precision Alignment: The Plastic Mold Guiding System

The guiding mechanism is an essential component of any plastic mold, responsible for ensuring precise alignment between the moving and stationary halves of the mold during each production cycle. This system guarantees that the mold closes accurately, maintains proper alignment during injection pressure, and opens consistently — all critical factors for producing high-quality parts and preventing damage to the plastic mold.

Without an effective guiding system, a plastic mold would suffer from misalignment during operation, leading to flash (excess plastic at the part edge), dimensional inaccuracies, and potentially catastrophic damage to the mold components. The guiding mechanism must withstand significant forces during mold closing and injection, requiring robust design and high-quality materials.

The primary components of a typical guiding mechanism in a plastic mold include:

The design of the guiding mechanism in a plastic mold depends on factors such as mold size, part precision requirements, and production volume. Larger plastic molds typically require more guide pins strategically positioned to ensure uniform alignment. High-volume production molds often feature hardened and ground guiding components to withstand millions of cycles without excessive wear.

Complex Geometries: Side Action in Plastic Mold Design

Many plastic parts require features such as undercuts, threads, holes, or projections that are not parallel to the mold's opening and closing direction. These features necessitate specialized mechanisms in the plastic mold that can move perpendicular or at an angle to the main mold opening direction. The侧向分型与抽芯机构 (side core-pulling mechanism) fulfills this critical function, allowing the production of complex geometries that would otherwise be impossible with a simple two-plate plastic mold.

The side core-pulling mechanism in a plastic mold operates by retracting (pulling) side cores or splitting (parting) side cavities away from the part before ejection, and then returning them to their original position for the next molding cycle. This additional movement adds complexity to the plastic mold design but enables the production of highly functional plastic parts with intricate features.

There are several types of side core-pulling mechanisms used in plastic mold design, each with its own advantages and applications:

For more complex plastic mold applications, additional components may be integrated into the side core-pulling mechanism:

• Locks or Clamps (紧块): These components secure the side cores in their extended position during the injection phase of the plastic mold cycle, preventing movement under injection pressure.
• Gear and Rack Mechanisms: Used in plastic molds requiring long core pulls or rotational movement (such as for threading), these systems convert linear motion to rotational motion.
• Springs and Hydraulic/Pneumatic Cylinders: These provide additional force or control for side core movement in specialized plastic mold applications, offering greater flexibility in timing and force.

The design of side core-pulling mechanisms requires careful consideration of factors such as undercut size, required movement, available space in the plastic mold, and production volume. Each additional side action increases the complexity, cost, and potential maintenance requirements of the plastic mold, but enables the production of more functional and complex plastic parts.

Air Management: The Plastic Mold Ventilation System

During the injection phase of the molding cycle, the molten plastic enters the cavity and displaces air and gases that must be properly exhausted from the plastic mold. The ventilation system (排气系统) is responsible for removing these gases to prevent defects in the molded part and ensure complete filling of the cavity. Without adequate ventilation, a plastic mold can produce parts with burns, voids, incomplete filling, or other quality issues that compromise part functionality and appearance.

The gases that need to be vented from a plastic mold include ambient air in the cavity, volatile gases released from the molten plastic, and potentially combustion by-products if the plastic is overheated. These gases must be allowed to escape as the plastic fills the cavity, as trapped gases can reach extremely high temperatures under compression, causing material degradation and surface burns on the part.

The primary components of a plastic mold ventilation system include:

For specialized applications, a plastic mold may incorporate additional ventilation features:

• Vent Pins: These are porous metal pins or specially designed components inserted into deep recesses or thick sections of the plastic mold where conventional vents are ineffective.
• Vacuum Ventilation: In some high-precision plastic molds, a vacuum system is connected to the ventilation channels to actively remove air from the cavity before and during injection.
• Submarine Vents: These are vents located in ejector pins or other moving components, allowing ventilation in areas that are not accessible from the main parting line of the plastic mold.

The design of the ventilation system in a plastic mold requires careful analysis of the filling pattern, with vents strategically placed at the last points to fill in each cavity. Proper venting is essential for achieving complete cavity filling, reducing cycle time, and preventing gas-related defects. However, vents must be carefully designed to avoid creating flash or burrs on the part, which would require secondary processing. In production, the ventilation system of a plastic mold may require periodic cleaning to remove plastic residue that can block the vents over time, ensuring consistent part quality throughout the mold's service life.