The key to pressure transfer efficiency lies in whether gate can inject melt into mold cavity with minimal pressure loss and maintain unobstructed flow during holding phase, allowing subsequent melt to effectively fill volume loss caused by cooling shrinkage. Gate's geometry (cross-section, length), position, and orientation determine melt's flow resistance, shear rate, flow pattern, its own cooling and solidification rate.

 

Impact: Low pressure transfer efficiency.
Causes:
Small Cross-Section: Gate's small cross-sectional area results in extremely high melt flow velocities, generating significant shear stress (significant shear heating) and leading to significant viscous pressure loss.
Narrow Flow Channel: Narrow channel itself creates high flow resistance.
Rapid Solidification: Small cross-section causes gate itself to cool and solidify very quickly. Although filling is possible during injection phase, gate may freeze prematurely during critical holding phase, severely hindering or even cutting off subsequent melt's access to cavity. Even if gate thickness reaches 1/2 or even 2/3 of part thickness, its small cross-sectional area and large surface area to volume ratio make it highly susceptible to freezing.
Applications: Commonly used for small, thin-walled parts with high aesthetic requirements (minimal gate mark), and automated production (three-plate molds with automatic gate cutoff).

 

Impact: Moderate to good pressure transfer efficiency (superior to point gates).
Reason:
Adjustable Cross-Section: Width and thickness (height) are relatively easy to adjust. Increasing thickness (especially to 1/2-2/3 of part thickness) and width can significantly reduce flow resistance, minimize pressure loss.
Solidification Control: Appropriately increasing thickness can delay gate solidification time, maintaining a sufficient access for feeding during holding phase. However, compared to direct gates or fan gates, cross-section is still smaller.
Strong directional flow: May cause jetting after melt enters the cavity, affecting flow front stability and weld line strength.
Application: Most widely used, suitable for most flat-plate and box-type products, especially when the gate is located on parting surface.

 

Impact: Excellent pressure transmission efficiency.
Cause:
Gradual cross-sectional expansion: Gate gradually expands from a narrow entrance to width of cavity. This design effectively reduces flow velocity and shear rate of melt as it enters cavity, minimizing viscosity loss and jetting.
Smooth flow: Provides a more uniform and gentler flow front, facilitating uniform cavity filling and pressure transfer.
Relatively controllable solidification: Although expanded portion is thin, entrance area is typically thicker (designed according to product thickness requirements), which helps delay complete freezing of main portion of gate, provides better pressure retention and shrinkage compensation than side gates.
Applications: Commonly used for flat panels and large, thin-walled products (such as panels and lids) where spraying must be avoided and uniform flow and high surface quality (especially high gloss) are desired.

 

Impact: Low to moderate pressure transfer efficiency (similar to or slightly better than point gates).
Causes:
Long and tortuous flow path: Melt must flow through a curved, often long tunnel, generating significant frictional resistance and pressure loss.
Restricted cross-section: To ensure automatic break-off, gate's cross-section (especially thickness) cannot be very large, limiting flow capacity.
High shear and temperature rise: When passing through a narrow, curved tunnel, melt experiences high shear. While some temperature rise reduces viscosity, pressure loss remains significant.
Solidification risk: Tortuous shape and relatively small cross-section make it more susceptible to premature freezing during holding phase, affecting feeding efficiency.
Applications: Primarily used in automated production (two-plate molds with automatic degating), where appearance is critical and point or side gates are not feasible (e.g., on the outside of part).

 

Impact: Highest pressure transmission efficiency.
Reason:
Largest Cross-Section: Sprue leads directly to mold cavity and has the largest cross-section (usually close to or equal to thickness of part at gate).
Shortest Path: Melt has the shortest flow path from injection molding machine nozzle to mold cavity.
Minimum Resistance: Large cross-section and shortest path mean minimal flow resistance and pressure loss.
Slowest Solidification: Large volume maximizes cooling and solidification times, maintaining excellent feeding channels throughout the entire holding phase, maximizing holding efficiency.
Disadvantages: Gate vestige is large and difficult to remove, resulting in high residual stress and a tendency to cause sink marks and shrinkage cavities near gate (although feeding is good, localized volume also results in significant shrinkage). Suitable only for single-cavity molds or specific large, thick-walled parts.
Applications: Large, thick-walled products (such as buckets and basins), applications requiring extremely high pressure transmission and pressure holding, and products with less demanding appearance.

 

Core objective of "gate thickness must be ≥ 1/2 (no fill) or 2/3 (high-gloss finish) part thickness" is to ensure effective feeding channels during holding phase.
Principle: As part cools and solidifies, material shrinkage creates a volumetric void. This volumetric void requires continued melt flow into cavity under holding pressure.
Gate solidification is a bottleneck: If gate solidifies before part has fully contracted, feeding channel is cut off, resulting in voids (voids) or surface sinks (sinks) forming inside part (especially in thick-walled areas away from the gate).
Thickness affects solidification time: The thicker gate, the greater its heat capacity, the longer it takes to cool and solidify. Achieving a thickness of 1/2 (general requirement) or 2/3 (high requirement) of part thickness allows gate to solidify significantly longer than that of critical areas of part, ensuring that gate remains molten and unobstructed even when shrinkage compensation is required. This is crucial for ensuring efficient pressure transfer during holding phase.

 

Gate selection is a comprehensive trade-off; there's no absolute optimal solution, only the most suitable. Following are systematic steps and key considerations:

Appearance requirements: What is allowable location, size, and visibility of gate marks? Gate design and location for high-gloss finish requirements (point gates, fan gates, and latent gates are often selected, with strict gate thickness and solidification time requirements).
Functional requirements: Are there any stress-bearing areas in part? What are weld line location and strength requirements? Gate location should be avoided in high-stress areas or where it might impact critical functions.
Dimensional accuracy: For products with high deformation control requirements, gate location and type can affect filling balance and holding pressure, which in turn can affect warpage.
Material Properties: Material fluidity (viscosity), shrinkage, thermal stability, and shear sensitivity. Materials with poor fluidity require larger gates or more efficient gates.

Geometry: Is it a flat plate, box, cylinder, or complex three-dimensional shape?
Wall thickness distribution: Identify the thickest areas (prone to shrinkage), the thinnest areas (difficult to fill), and areas of sudden wall thickness changes (prone to air trapping and stress concentration).
Identify filling difficulties and potential shrinkage areas: This directly influences gate location selection (should be close to thick walls or difficult-to-fill areas) and gate type (sufficient pressure transmission efficiency is required for shrinkage compensation).

Based on product requirements (appearance, function), structural characteristics, and mold parting surface, list all technically feasible gate location options.
Consider melt flow path: Optimize flow balance, shorten process flow, avoid air traps, control location and properties of weld line.

For each possible location, consider gate type that is suitable for that location (e.g., side gates/fan gates can be used at the edge of part, pin gates/direct gates can be used at the center, and latent gates can be used on inner side).
Key Trade-offs:
Pressure transfer/holding requirements vs. appearance requirements: Thick-walled parts with high dimensional accuracy require high pressure transfer efficiency (favoring direct gates, side gates, and fan gates), while parts with high appearance requirements require concealed or micro-gates (favoring pin gates, latent gates, and fan gates).
Flow pattern vs. quality requirements: Is jetting to be avoided? Is extremely uniform flow required (a fan gate is advantageous)?
Mold complexity and cost: Pin gates require a three-plate mold, while latent gates are complex to process. Direct gates are simple but require difficult part handling.
Production Efficiency and Automation: Point gates and latent gates facilitate automated demolding; direct gates and side gates typically require manual or post-processing de-gating.

Key Step: After initially selecting several options, be sure to perform simulation analysis using software such as Moldflow.
Analysis Content:
Is filling pattern balanced? Is there a risk of short shots?
Is pressure distribution reasonable? Is maximum injection pressure within machine's capabilities?
Gate solidification time vs. solidification time in critical areas of part? (This is the most direct evidence to verify whether gate thickness/form can effectively maintain pressure.)
Weld line location and predicted strength? Gas entrapment location? Predicted sink mark location and extent? Predicted warpage tendency?
Adjustments based on analysis results: Optimize gate location, size (especially thickness), and form. This may even require adjusting part wall thickness or adding auxiliary measures (such as venting grooves and cooling channels).

Taking all factors into consideration (product requirements, CAE results, mold cost, and production feasibility), select optimal gate form, location, and dimensions.
Refine gate design: precise dimensions (length, width, and thickness), corner radii, and surface finish (especially for high-gloss molds).
Design appropriate runner system (cold or hot runner) to match design.