Injection Molds: Fundamental Concepts & Applications - Longterm Manufacturing Solutions Limited

An injection mold is a critical tool in the injection molding process, functioning by injecting molten plastic into a precision-engineered mold cavity. As the plastic cools and solidifies within the cavity, it takes shape to form the desired plastic component.

Typically composed of two core components—the moving mold and the fixed mold—the former is movable while the latter remains stationary. When these two parts engage, they form a tightly sealed cavity that dictates the final shape of the plastic part.

The design and manufacturing of injection molds require careful consideration of multiple factors, including plastic flow properties, shrinkage rates, as well as the mold’s structural strength and dimensional accuracy.

01 Basic Classification of Injection Molds

By Number of Parting Surfaces

Single-parting surface injection mold: During mold opening, the moving and fixed molds separate to allow easy removal of the finished part. Its simple structure makes it one of the most widely used mold types.
Double-parting surface injection mold: Equipped with an intermediate plate, this design facilitates the removal of gating system residues. It is particularly suitable for molds utilizing pinpoint gate feeding systems.

By Specialized Functions

Molds with lateral parting & core-pulling mechanisms: Designed to shape plastic parts featuring side holes, undercuts, or concave side features.
Automatic thread-demolding injection molds: Enable automated demolding through the rotation of threaded cores or rings, streamlining production of threaded components.
Runnerless injection molds: Encompassing adiabatic runner and hot runner designs, these molds minimize waste by reducing or eliminating gating system residues.

02 Working Principle of Injection Molds

The operational process of an injection mold unfolds in several key stages:

Clamping: The moving and fixed molds close tightly to form a sealed cavity, ready for plastic injection.
Injection: Molten plastic is injected into the mold cavity through the injection machine’s nozzle.
Pressure holding: Post-injection, sustained pressure is applied to compensate for plastic shrinkage and ensure cavity filling.
Cooling: The plastic within the cavity cools and solidifies, taking on the precise shape of the mold.
Opening: The moving and fixed molds separate, creating access to the formed part.
Demolding: Ejector pins or specialized demolding mechanisms push the solidified part out of the mold cavity.

03 Application Fields of Injection Molds

Injection molds play a pivotal role across diverse industries, including automotive, electronics, home appliances, toys, and medical devices.

Practical examples abound: automotive dashboards and bumpers, casings and buttons for electronic devices, and essential plastic components in home appliances—all are precision-manufactured using injection molds.