Core classification of hot runner systems is based on valve control method at gate, mainly divided into two categories: open and needle valve. Each category has more specific subtypes, each with different working principles and applicable scenarios.

 

In an open hot runner system, gate remains open throughout molding cycle, relying on melt's own flow characteristics and thermal balance at the gate to achieve "closure."
1. Working Principle (Core Details)
Thermal balance is key: Gate has an independent heater to maintain its temperature above plastic's melting point, preventing solidification. Simultaneously, an efficient cooling channel needs to be designed near gate to ensure rapid cooling and solidification of product at gate.
Natural solidification sealing: When injection and holding pressure are completed, and screw retracts, pressure inside barrel is released. At this time, due to contact with low-temperature mold cavity, a very thin solidified layer (approximately 0.5-1mm) forms at gate. This solidified layer acts like a "lid," immediately sealing gate after holding pressure is applied, preventing "drooling" or "stringing."
Remelting Opening: At the start of next cycle, injection pressure increases rapidly, and melt front breaks through this thin solidified layer, reopening gate.
2. Specific Types and Characteristics
Straight-through Gate: The simplest structure, with runner leading directly to gate. Disadvantage is a noticeable gate mark (like a small raised dot), making it prone to stringing. Commonly used for products with large gates and less stringent appearance requirements (such as automotive interior parts).
Pointed-nose Gate: Gate has a pointed, tapered structure. Melt generates shear heat as it flows through pointed nozzle, helping to maintain a high temperature. Gate mark is small (typically 0.5-1.5mm in diameter), appearing as a small raised dot. Suitable for small to medium-sized products and applications with slightly higher appearance requirements.
Edge Gate: Gate is located on the side of product or parting surface. Flexible design, and gate is easy to trim later. Commonly found in multi-cavity molds or side-gated molds.
Open-type hot runners:
Advantages: Simple structure, low cost (30%-50% lower than needle valve type), easy maintenance.
Disadvantages: Obvious gate marks, prone to stringing or drooling; unsuitable for high-quality appearance or precision molding.

Needle valve hot runners use a valve needle driven by an external power source to mechanically open and close gate.
1. Working Principle (Core Details)
Drive System: Valve needle is driven by a cylinder (most common), hydraulic cylinder, or servo motor. Cylinder control is simple and fast; servo motors can precisely control valve needle position (e.g., half-open state), but are more expensive.
Timing Control: Opening and closing times of valve needle are precisely set via injection molding machine signals or an independent timing controller. For example: opening gate 0.2 seconds before injection begins and closing it immediately after holding pressure.
Mechanical Cut-off: When closed, valve needle tip presses into gate cone surface, forming a hard seal, 100% preventing drooling and stringing.
2. Specific Types and Characteristics
Cylinder Driven (Standard Type): The most common and mature type. Valve needle is opened and closed by controlling air circuit through a solenoid valve. Response time is approximately 20-50 milliseconds. Suitable for most injection molding scenarios.
Hydraulic Driven (High-Pressure Type): High driving force, suitable for large products or high-viscosity materials (such as PC, PMMA). However, hydraulic systems have a risk of oil leakage, which may contaminate product.
Servo Motor Driven (Precision Type): Can precisely control valve needle opening (e.g., 10%, 50%, 100%), achieving segmented opening or sequential valve control. Commonly used for precision molding to eliminate weld lines and control molecular orientation.
Valve Needle Sleeve Type: Valve needle does not directly contact mold steel but moves within a precision valve needle sleeve. After wear, only valve needle sleeve needs to be replaced, making maintenance convenient and suitable for high-cavity, long-cycle production.
Advantages and disadvantages of needle valve type:
Advantages: Smooth gate mark (a circular mark), no stringing, delayed control to eliminate weld lines, suitable for materials with wide viscosity.
Disadvantages: High cost (approximately 2-3 times that of open type), complex structure, valve needle may jam due to wear or thermal expansion.

Simple Selection Guide:

 

For products with low appearance requirements: Prioritize open type (low cost, reliable). For products with appearance requirements (no gate mark), materials prone to dripping (e.g., PA, PBT), or multi-cavity molds: Choose needle valve type. For products requiring weld line elimination (e.g., long strip-shaped products): Choose servo needle valve type with sequence valve function.