Basic Structure of Injection Molds

Basic Structure of Injection Molds

An injection mold is a complex system composed of several functional components. Based on their roles, these components can be categorized into the following key systems:

· Runner System

· Molding System

· Temperature Control System

· Venting System

· Ejection System

· Mold Opening/Closing System

· Reset (Return) System

Mold Base (Mold Frame)

The Mold Base serves as the structural backbone of the entire mold. All other mold components must be designed and assembled in coordination with the mold base.

Typically, the mold base accounts for 10% to 30% of the total mold cost. It is manufactured by specialized large-scale suppliers and is highly standardized. Mold makers can simply customize the base according to their specific requirements.

Common Components of a Mold Base:

· Front Plate (Clamping Plate)

· Cavity Plate (A-Plate)

· Core Plate (B-Plate)

· Spacer Blocks (C-Blocks or Pillars)

· Rear Plate (Base Plate)

· Ejector Retainer Plate

· Ejector Plate

· Bushings (Sleeves)

· Guide Pillars

· Return Pins

· Ejector Pins

· Support Pillars (Support Posts)

· Stop Pins (Limit Pins)

Molding System

The Mold Inserts (also known as cavity, core, mold steel, or mold core) are the precision components inserted into the mold base plates to form the actual product shape.

· Cavity (Front Mold Insert) – Typically forms the outer surface of the part.

· Core (Rear Mold Insert) – Typically forms the inner surface or back side.

Why Use Mold Inserts?

Inserts are used primarily for cost efficiency and ease of machining and maintenance. Since the molding surfaces are in direct contact with molten plastic, they require high-performance tool steel with properties such as high hardness, corrosion resistance, and resistance to thermal deformation.

· Hardness of mold inserts: 28–65 HRC

· Hardness of mold base plates: 27–33 HRC

This allows high-grade steel to be used only where necessary, reducing material costs.

Slider (Side Action / Lifter)

A Slider (also called a "side core" or "side action") is a moving component used to form side features such as holes, undercuts, or hooks that cannot be molded with a straight-pull design. It moves laterally during mold opening to allow part ejection.

Angle Lifter (Angled Ejector, Lift Pin)

An Angle Lifter is a mechanism similar to a slider but typically used for internal undercuts or small features. It uses an angled pin that moves at a specified angle to release undercuts during ejection.

Runner System (S.R.G – Sprue, Runner, Gate)

This system delivers molten plastic from the injection machine to the mold cavity.

Key Components:

· Sprue Bushing (Sprue) – The entry point where the injection machine’s nozzle injects molten plastic into the mold.

· Runner – The channel that guides the plastic from the sprue to the individual cavities.

· Gate – The small opening through which plastic enters the mold cavity. The gate design affects part quality, cycle time, and ease of separation.

This standardized structure ensures reliable, repeatable production of plastic parts, while allowing flexibility for complex geometries through features like sliders and lifters.