Injection Molding Melt Delivery System

This melt delivery system ensures precise molten plastic flow from the injection machine to the mold cavity. It consists of four key components: the sprue (main entry channel), runner (distribution network), gate (flow control point), and cavity (final shaping zone). Optimizing this melt delivery minimizes waste, improves filling efficiency, and enhances product quality in both cold and hot runner setups

Sprue (Main Runner): The Primary Entry Point for Melt

Core Function

Serves as the sole channel for molten plastic to enter the mold from the injection machine nozzle, guiding high-pressure melt initially.
In cold runner systems, sprue waste accounts for 15%-30% of material loss (hot runners eliminate this issue).

Key Design Parameters

Parameter	Typical Value/Requirement	Design Basis
Taper Angle	2°-5°	Facilitates ejection, reduces stringing
Outlet Diameter	0.5-1mm larger than nozzle bore	Prevents flash
Length	≤150mm	Avoids excessive pressure loss

Advanced Considerations

Hot Runner Sprue: Equipped with built-in heaters to maintain melt temperature; no waste but higher cost.
Z-Puller: Used in cold runner molds to hook sprue waste, ensuring synchronized ejection.

Runner (Distribution Channel): The Melt Distribution Network

System Classification & Features

Type	Cold Runner	Hot Runner
Structure	Unheated, cools with part	Heated, maintains melt state
Waste	Generated	None
Application	Low-volume, multi-material molding	High-volume, high-precision molding

Runner Cross-Section Design Comparison

Circular: Highest flow efficiency (lowest surface-area ratio), but requires bilateral machining (higher cost).
Trapezoidal: Easier machining, common in three-plate molds, but increases pressure loss by ~20%.
U-Shaped: Balances cost and flow performance, often used in small/medium molds.

Balancing Strategies for Multi-Cavity Molds

Natural Balancing: Identical runner lengths/diameters (H-pattern layout).
Artificial Balancing: Adjusts runner diameters to compensate for flow variations (for irregular cavity layouts).

Gate: The Final Control Valve for Melt

Gate Types & Selection Guide

Type	Characteristics	Application
Edge Gate	Simple machining, requires trimming	Large flat parts (e.g., enclosures)
Pin Gate	Auto-separation, no visible marks	Appearance-critical parts (e.g., phone casings)
Submarine Gate	Enters from parting line, hidden	Automotive interiors, multi-cavity small parts

Golden Rules for Gate Dimensions

Thickness = 50%-80% of part wall thickness (too thin causes shear overheating; too thick leads to sink marks).
Width = 1/10 of part projected area (empirical; requires mold flow analysis).

Gate-Related Defects & Solutions

Weld Lines: Formed at melt fronts meeting; adjust gate position or increase melt temperature.
Jetting: Caused by undersized gates; use fan gates or adjust injection speed.

Cavity: The Ultimate Forming Zone

Filling Dynamics

Jetting Flow: Melt shoots forward without contacting walls, causing surface defects.
Laminar Flow: Ideal radial advancement of melt front.

Packing & Shrinkage Compensation

Packing Pressure = 50%-80% of injection pressure, held until gate solidifies (~30% of cycle time).
Shrinkage Rate: 1.5%-3% for crystalline materials (e.g., PP/PE); pre-compensated in mold dimensions.

Venting Design

Vent Depth: Material-dependent (e.g., ABS: 0.02-0.03mm; PVC: 0.04-0.06mm).
Optimal Locations: Melt flow ends, insert junctions.

Comparison of Sprue, Runner, Gate, and Cavity

Component	Function	Location	Generates Waste?	Impact on Product
Sprue	Links nozzle to runner	Mold center (direct to nozzle)	Cold: Yes	Affects injection pressure loss & filling efficiency
Runner	Distributes melt to gates	Branching network (multi-cavity)	Cold: Yes	Determines multi-cavity filling balance
Gate	Controls melt flow into cavity	Cavity entrance	Usually No	Affects appearance (weld lines, sink marks)
Cavity	Final product-forming space	Core mold area (part geometry)	No	Determines dimensional accuracy & surface quality

Melt Delivery Optimization Case Study

Case: Automotive Dashboard Molding (Multi-Cavity Hot Runner System)

Sprue: Hot runner main nozzle (Φ8mm) minimizes pressure loss.
Runner: H-pattern natural balance, circular section (Φ6mm), ensures synchronized 8-cavity filling.
Gate: Submarine gate (1.2mm thick × 3mm wide), hidden behind clip structures.
Cavity: 80°C mold temp, 0.025mm vent depth, eliminates weld lines.
Result: Cycle time reduced by 18%, waste rate dropped from 5% to 0.3%.

The melt delivery system is critical for efficient and high-quality injection molding. By optimizing the sprue, runner, gate, and cavity design, mold manufacturers can reduce waste, improve cycle times, and ensure consistent part performance. Proper melt delivery system design directly impacts production efficiency and final product quality.