Introduction to Hot Runner Systems

I. Basic Definitions and Operating Principles

A hot runner is a temperature-controlled feeding system designed for injection molds. It uses electric heating elements to continuously heat the manifold and hot nozzles, ensuring that the plastic melt remains in a molten state throughout its entire journey from the injection molding machine’s barrel to the product cavity.

Traditional cold-runner molds produce solid gate scrap with every molding cycle, whereas the plastic inside a hot runner system remains in a liquid state throughout. When the mold opens, only the finished product is ejected, with no runner scrap, eliminating the need for gate crushing and material recycling processes.

Complete Process: The injection molding machine melts the plastic → the nozzle contacts the hot runner manifold → the manifold evenly distributes the molten material → each hot nozzle delivers the material directly to the mold cavity → the cavity cools and the part is formed; the molten material in the runner is reused directly for the next injection cycle.

The entire system features independent temperature control for each zone, with a temperature control accuracy typically ranging from ±1 to 3°C, making it suitable for all common thermoplastics such as PP, ABS, PC, POM, PA, LCP, and PEEK.

II. Four Major Core Components

1. Manifold (Hot Runner Plate)

Receives the melt from the injection molding machine and distributes it evenly to each hot nozzle, ensuring balanced feeding to multi-cavity molds and reducing pressure loss in the melt.

Structural configurations include linear, H-shaped, Y-shaped, and X-shaped; heating methods include internal and external heating.

The main body is made of H13 or P20 hot-work die steel, with heating elements and thermocouples embedded inside for independent zone temperature control. It is commonly used in multi-cavity molds and molds for large plastic parts with multiple gate locations.

2. Hot Nozzles (Core Molding Components)

They are divided into two main categories: open-type hot nozzles and needle-valve-type hot nozzles:

Open-Type Hot Nozzles

These have no moving mechanical parts, featuring a simple structure, low cost, and easy disassembly, assembly, and maintenance. The gate remains fully open throughout the process, relying on a thin cooling layer on the gate’s surface to prevent melt dripping.

Advantages: High flow rate and minimal injection pressure loss; Disadvantages: Prone to stringing and slight drooling; leaves a distinct circular gate mark on the product surface.

Applications: Internal, non-cosmetic structural parts; high-volume, general-purpose plastic products; plastic parts with low cosmetic requirements.

Needle-Valve Hot Nozzles

A valve needle is driven by a pneumatic or hydraulic cylinder to mechanically open and close the gate; this is considered a high-end, precision molding configuration.

Advantages: Virtually no gate marks on the finished product surface, eliminating the need for manual polishing; completely eliminates stringing, drooling, and cold runner streaks; supports sequential material injection, eliminating weld lines on long products and reducing warpage and deformation; prevents melt backflow during the screw storage phase.

Applications: Automotive interior and exterior trim, transparent optical components, medical consumables, digital device housings, and thin-walled precision injection-molded parts.

3. Temperature Control Components (Temperature Control Cabinet + Temperature Sensing Wires + Heating Elements)

The core of the entire hot runner system’s temperature control, utilizing PID intelligent constant-temperature regulation:

Heating elements include manifold heating rods and nozzle heating rings, which provide uniform heat directly against the steel body;

Thermocouples (temperature sensors) continuously monitor temperatures in various areas of the runner and transmit the data to the temperature control cabinet;

The temperature control cabinet features multiple independent temperature control channels that automatically adjust output power to prevent plastic decomposition at high temperatures and runner blockages at low temperatures.

4. Thermal Insulation and Sealing Accessories

Thermal insulation pads and rings are used to isolate the high-temperature runner from the ambient-temperature mold plates, reducing heat loss and energy consumption while preventing thermal deformation of the plates.配套 high-temperature sealing rings, junction boxes, locating pins, support shims, and anti-leak rubber O-rings are included to prevent melt leakage and positioning misalignment.

III. Main Classification Methods for the System

Classified by the number of hot nozzles

Single-point system: Features only one hot nozzle, suitable for small single-cavity molds and small parts with single-point gate injection; it has a simple structure and the lowest cost;

Multi-point system: Features two or more hot nozzles, combined with a manifold for flow distribution; used for multi-cavity molds and large products with multiple gates.

Classification by Heating Structure

External heating: Heating elements are located outside the manifold and nozzle, making maintenance convenient; this is the most widely used type on the market;

Internally Heated Type: Heating elements are placed inside the runner, offering high heat transfer efficiency but making disassembly and maintenance difficult; this type is primarily used for high-temperature, high-melt-flow plastics.

Classified by Gate Control

Open-system and needle-valve sequential control systems; the latter can be paired with a sequential controller to control the opening and closing times of each needle valve in stages.