For previous reading, please refer to A Comprehensive Analysis of Injection Mold Gate Types (Part 1): From Basics to Three Classic Gates.
1. Gate Shape

 

Submerged gates, also known as tunnel gates, are essentially a variation of side gates. Their most significant characteristic is that gate lies submerged at a certain angle outside parting line, guiding melt into cavity through a narrow, tunnel-like channel. Cross-section is typically a small circle or ellipse, with a diameter of 0.8~2.5mm. Angle between tunnel and mold opening direction is 30°~60°. Based on injection location, there are three types:
Outer Submerged: Material is injected obliquely from below outer surface; trace can be concealed by design features. Inner Submerged: Material is injected from inner side or reinforcing rib; outer surface is completely unmarked. Ejector Pin Submerged: Gate is directly located at ejector pin head. During mold opening, ejector pin both ejects product and breaks off gate, achieving high integration.
2. Key Points of Gating Design
Lurking Angle: A core parameter. Too small an angle makes gate prone to breakage; too large an angle weakens formwork and easily leaves residual piles. Recommended angle is 30°~60°. Diameter and Length: Diameter should be 1.0~2.0mm. Tunnel length is determined by wall thickness and angle, generally 2~5mm. Outlet diameter should be slightly smaller than inlet diameter, creating a slight taper to facilitate breakage. Outlet Location: Ideally, outlet should be located at base of reinforcing ribs, at intersection of ribs, or in rounded transition areas where wall thickness is relatively thick. This ensures filling and utilizes thick wall for shrinkage compensation while avoiding appearance surface. Ejector Pin Latent Fit: Ejector pin diameter should be 2~3mm larger than gate diameter; tunnel opening should have a 0.3~0.5mm rounded transition to prevent breakage; ejection stroke must be large enough to ensure gate is completely pulled out. Flow Resistance Compensation: Latent gates have greater flow resistance than ordinary side gates. In multi-cavity molds, gate size should be appropriately increased or a thicker runner should be used. Venting: Air can easily trap near tunnel exit. Venting channels or ejector pin gaps should be used to vent air and prevent scorching or incomplete filling.
3. Advantages and Disadvantages of Gate
Advantages: No gate marks on the outer surface, ideal for appearance parts; Automatic breakage upon mold opening, runner automatically detaches, suitable for fully automated production; Flexible placement, can be hidden using reinforcing ribs, clips, or other non-appearance features; Can be achieved with a two-plate mold, eliminating need for a complex three-plate mold structure, keeping costs under control; Multiple placement points can guide weld lines to non-critical areas.
Disadvantages: Long, narrow tunnels have high resistance, requiring higher injection pressure; High flow rate scouring + ejection shearing can easily lead to tunnel wear expansion or breakage failure, requiring frequent mold repairs; Not suitable for high-viscosity materials such as PC and PMMA, or glass fiber reinforced materials; Improper design may leave scratches, pits, or even cracks on product surface; Sloping tunnels require high machining precision, and angles of each cavity in a multi-cavity mold must be strictly consistent.
4. Applicable Products for Gate Design
Consumer electronics casings with extremely high appearance requirements: mobile phone battery covers, earphone charging cases, smartwatch cases, etc., where no gate marks are allowed on outer surface. Thin-walled products with reinforcing ribs/buckles: Gate is hidden at base of ribs or behind buckles, balancing appearance and feed rate. Small to medium precision parts produced in fully automated processes: disposable medical consumables, precision gears, connectors, etc., requiring zero manual post-processing. Two-color injection molding/insert injection molding: Feeding material at pre-reserved position in the first injection, without damaging appearance of already molded part.

1. Gate Shape
Fan-shaped gate is a variation of side gate. Its core characteristics are: width gradually widens from runner to cavity, forming a fan shape, while thickness gradually decreases along flow direction.

 

Inlet width is relatively narrow, while outlet width increases significantly to 1/4 to 1/2 or even wider than product width. Inlet thickness is approximately 0.6 to 0.8 times product wall thickness, decreasing to 0.3 to 0.5 times at outlet. This gradient allows melt to spread evenly across the entire width, smoothly entering cavity at a lower flow rate.
2. Key Considerations for Gate Design
Width-to-Outlet Ratio: Outlet width W₂ should be 2 to 5 times inlet width W₁, with a width that can reach 30% to 50% of product width. Excessive width makes removal difficult and requires careful consideration. Thickness Gradient: Inlet thickness t₁ should be 0.6 to 0.8 times product wall thickness, and outlet thickness t₂ 0.3 to 0.5 times. Transition should be smooth to avoid abrupt changes that could cause stagnation or jetting. Gate Length: 1 to 2 mm. Excessive length results in significant pressure loss and easy cooling; insufficient length leads to insufficient strength. Runner Connection: End of runner should have a 1 to 2 mm radius rounded transition for a smooth connection to inlet, avoiding dead angles. Anti-center jetting: Center area has the greatest thickness and fastest flow velocity, making it prone to premature flow. A baffle can be placed on opposite side or gate angle adjusted to ensure melt impacts cavity wall first before spreading laterally. Removal of marks: Long strip marks remain at edges after removal. For exterior parts, these marks should be placed on a non-visible surface or covered by subsequent spraying/electroplating. When feeding from exterior surface, a protective ear gate can be used.
3. Advantages and disadvantages of gates
Advantages: Uniform melt distribution, significantly shortening lateral flow path within cavity, reducing pressure and temperature drops; Consistent filling pressure in all areas, uniform molecular orientation, significantly reducing warpage; Fewer weld lines and higher weld strength—fewer junctions in wide-width, single-sided feeds, resulting in higher temperature and pressure at weld; Suitable for wide, flat products, optimal side gate variant for uniform filling of long, thin-walled parts; Only parting surface machining is required, no three-plate mold or angled machining is needed.
Disadvantages: Long and noticeable gate marks, usually requiring placement on non-visible surfaces; Wider gates require sawing or milling removal, increasing post-processing costs. Forcibly breaking them can leave burrs or chipping; Unsuitable for small or round products, causing material waste and flow imbalance; Large inlet thickness leads to slow cooling, extending holding and cooling times; Improper design may cause flow marks or air bubbles in central area.
4. Applicable Products for Gates
Flat-shaped products: Laptop bottom cases, keyboard panels, large decorative strips, etc., requiring uniform feeding to control warping. Shallow box/shallow tray products: Trays, storage boxes, monitor bezels, etc., with smooth filling from a fan-shaped feed along long side. Thin-walled parts requiring high flatness: Printer covers, scanner covers, etc., uniform filling helps control flatness. Products requiring reduced weld lines: Transparent lampshades, display rack panels, etc., significantly reducing weld lines and improving optical quality.

1. Gate Shape
A thin film gate, also known as a sheet gate or flat-slit gate, is an extreme form where gate width is almost equal to the overall width of product—a thin, elongated slit running through edge of product, extremely thin, resembling a film.

 

Cross-section is a flat, narrow slit with a width approximately equal to product width, a thickness of 0.2~0.8mm, and a length of 0.5~1.5mm. Melt spreads across the entire width at an extremely low flow rate. It is usually used in conjunction with a rectangular distribution channel of same width as product to ensure completely consistent pressure across gate width.
2. Key Considerations for Gate Design
Gate Thickness: A critical parameter, ideally 0.2~0.8mm. Too thick, it loses its purpose of uniform feeding and produces large marks; too thin, resistance is too high, leading to incomplete filling. For ultra-thin-walled products, a thickness of 0.15mm can be achieved. Width Matching: Ideally equal to feed edge width. For extremely wide products, gate can be segmented, but each segment must be continuous to avoid additional weld lines. Distribution Runner: Depth is 2-3 times gate thickness, and width is same as gate. Melt first enters distribution runner and is evenly distributed laterally, then simultaneously enters cavity through thin-film gate. Removal Method: Removal leaves a thin, elongated trace, often obscured at parting line or assembly stop. Removal methods include shearing, milling, or punching. Manual breaking easily leaves uneven edges. Ventilation: Melt is evenly pushed from one side, air is smoothly pushed to the other. However, if product is deep, melt entering from top may impact bottom cavity wall, causing air trapping. Sufficient venting channels are needed at the bottom, or feed should be changed to bottom-up. Pressure Holding Control: Due to extremely thin gate and very short freezing time, pressure holding time needs precise control. Premature freezing results in insufficient shrinkage compensation, while delayed freezing leads to melt backflow within cavity. It is recommended to determine optimal value through mold flow analysis.
3. Advantages and Disadvantages of Gate
Advantages: Most uniform filling and minimal warpage: Melt front advances almost parallel, with highly consistent molecular orientation, making it ultimate solution for controlling warpage; Minimal weld lines: A single thin-film gate produces almost no weld lines within mold cavity, highly advantageous for transparent and high-strength parts; Extremely low internal stress: Uniform, low-speed filling minimizes shear stress; Suitable for ultra-thin walls and large, flat products: Provides a stable and uniform melt front; Good venting effect: Smooth melt advancement prevents air entrapment.
Disadvantages: Extremely difficult to remove: Long seams must be sawed or punched; cannot be manually broken or automatically pulled off. Marks cannot be completely eliminated: Visible marks remain after sanding, unsuitable for products with extremely high appearance requirements. High flow resistance, requiring high injection pressure and clamping force from injection molding machine. Unsuitable for deep-cavity or complex-shaped products: Material can only be fed from edge plane, making it difficult to evenly cover deep cavities with multiple ribs. Material waste: High material consumption at gate and distribution channels, and extremely thin waste is difficult to recycle.
4. Applicable Products for Gate
Large, flat, transparent products: Acrylic display racks, PC diffuser plates, PMMA lightbox panels, etc., minimizing weld lines and internal stress, ensuring optical uniformity. Engineering plastic flat sheets with extremely high flatness requirements: Printer paper feed platforms, scanner glass pressure plate supports, etc., a key solution for ultra-low warpage. Ultra-thin wall flat shells: For ultra-thin laptop top/bottom covers with wall thicknesses of 0.4~0.6mm, only thin-film gates can provide sufficiently uniform filling. Precision flat parts requiring extremely low internal stress: For optical instrument filter holders, semiconductor trays, etc., minimal shear ensures long-term dimensional stability.

Submerged, fan-shaped, and thin-film gates were developed to address three major engineering pain points: zero-mark appearance, wide and uniform filling, and extremely low warpage. Starting from fundamental logic of side gates, through shape innovation and extreme dimensional optimization, they expand boundaries of injection molding possibilities. Understanding underlying logic of these gates can help you more accurately predict mold solutions during DFM reviews and provide more professional layout suggestions when communicating with mold manufacturers.