In injection molding, molten plastic is injected from nozzle and flows through runner system into mold cavity. Gate is a short, narrow channel connecting runner and cavity. Although it occupies a very small volume, it is considered throat of injection molding system.

 

Core functions of a gate can be summarized in five points:
1. Controlling material flow rate for rapid filling. Gate's cross-sectional area is much smaller than runner, causing a sudden increase in melt velocity as it passes through, generating shear heat. This results in a localized increase in melt temperature and a decrease in viscosity, making it easier to fill fine structures.
2. A valve for pressure holding and shrinkage compensation. After cavity is filled, melt at gate cools and solidifies first, naturally cutting off material flow. During pressure holding stage, it effectively compensates for shrinkage before gate freezes, and after freezing, it prevents melt backflow, ensuring dimensional stability.
3. Adjusting direction and orientation of melt flow. Location and shape of gate directly affect melt flow pattern and molecular chain orientation, thus influencing internal stress distribution, warpage, and anisotropy of mechanical properties.
4. Facilitates gate removal and post-processing. A well-designed gate should be easy to remove after demolding, and traces should not affect appearance or function.
5. Balances filling consistency in multi-cavity molds. Gate size in a multi-cavity mold directly determines whether each cavity can be filled simultaneously, avoiding flash or insufficient material in one cavity.

1. Gate Shape
Direct gate, commonly known as a large gate, has the simplest structure: main runner extends directly and connects to product surface, without intermediate runners or necking structures. Its geometry is a large-sized conical or near-cylindrical inlet, with a diameter typically Φ4~12mm at product inlet. Melt flows into cavity with almost no throttling after exiting nozzle. Mold structure is: sprue bushing → main runner → product, representing the shortest and simplest runner design.

 

2. Key Points of Sprue Design
Main runner dimensions: Typically 3-5mm in diameter, 2°-4° taper, surface roughness Ra≤0.8μm. Sprue bushing: Use a standard sprue bushing with a positioning step to prevent backflow; spherical radius should be 1-2mm larger than nozzle to ensure a seal. Cold slug well: A cold slug well must be installed at the end of main runner to collect low-temperature cold material and prevent it from entering cavity. Location selection: Located in areas with thicker walls to facilitate shrinkage compensation, and as far away from outer surface as possible. Removal method: Usually requires secondary processing such as shearing, milling, or grinding; sufficient sprue allowance should be reserved in design.

 

3. Advantages and Disadvantages of Gate Removal
Advantages: Minimal pressure loss, suitable for high-viscosity engineering plastics (PC, PMMA, PSU, etc.); Good pressure holding effect, slow gate cooling allows for continuous shrinkage compensation, reducing shrinkage voids; Simple mold structure, low processing cost; Friendly to fiber-reinforced materials, does not damage fiber length; Gentle melt flow rate, less prone to trapped gas and scorching.
Disadvantages: Obvious gate marks, severely affecting appearance; Difficult to remove, increasing labor costs and easily damaging product; Slow gate cooling, extending molding cycle; Melt near gate directly impacts cavity wall, resulting in high residual stress, easy cracking and warping; Each gate can only correspond to one cavity, unsuitable for multi-cavity molds.
4. Applicable Products for Gate Removal
Large deep cavity shells: Washing machine inner tubs, turnover boxes, etc., where appearance requirements are not high and a large amount of melt needs to be filled quickly. High-viscosity engineering plastic parts: PC optical lens blanks, PSU medical devices, etc. Fiber-reinforced structural parts: Glass fiber reinforced PA automotive parts, with large gates to maximize fiber protection. Trial molding or small-batch production: Mold cost is a priority; large gate processing is the fastest and most economical.

1. Gate Shape
Side gates are located on parting surface, allowing material to enter from side of product. They are one of the most widely used gate types. Geometry is a rectangular, flat, narrow slit, with cross-sectional dimensions expressed as width (b) × thickness (t) × length (L). Typical range: width 2~10mm, thickness 0.5~2.5mm, length 0.5~2mm. Melt enters cavity at a high shear rate after being throttled through narrow slit. They can be located on the outside, inside, or even through a tunnel for internal material entry; aspect ratio is flexibly adjustable.

 

2. Key Points of Gate Design
Dimensions: Thickness t should be 1/3 to 2/3 of product wall thickness; width b should be (3~10)t; length L should be 0.5~2mm. Location: Place gate at the thickest point, following a flow from thick to thin; for exterior parts, place it in a non-visible area or under cover of gate. Multi-cavity Balance: Increase gate size appropriately for cavities far from main runner to compensate for differences in flow resistance. Self-Break Design: Maintaining a thickness below 1.5mm allows for automatic breakage during demolding. Anti-Spraying: Avoid direct contact with open areas; offset gate so that melt impacts cavity wall first before spreading, preventing silver streaks and air bubbles.
3. Advantages and Disadvantages of Gates
Advantages: Minimal traces, almost invisible after polishing; Width and thickness can be independently adjusted, allowing for flexible mold trial optimization; Only rectangular grooves need to be machined on parting surface, easily completed by CNC/EDM; Facilitates multi-cavity molds, allowing for differentiated and balanced filling of gate sizes; Self-breaking occurs when thickness is appropriate, eliminating need for manual trimming.
Disadvantages: Throttling causes pressure loss; high viscosity or ultra-thin walls require increased injection pressure compensation; Multiple gates can easily produce weld lines, affecting appearance and strength; High shear near gate leads to high residual stress, making it prone to environmental stress cracking; Limited feed volume; large products require multiple gates, increasing risk of flow marks; Mold wear or improper thickness can lead to uneven fracture surfaces and burrs.
4. Applicable Products for Side Gates
Side gates are the most straightforward choice for small to medium-sized products: shells for remote controls/routers/power adapters, panel covers and other flat, thin-walled parts; appliance shells with certain aesthetic requirements; connectors/switch panels and other multi-cavity standard parts.

1. Gate Shape
Point gates, also known as pin-point gates, are typical restrictive gates. They have a circular cross-section, with a diameter of 0.5~2.0mm and a length of 0.5~1.5mm. Their three-dimensional shape resembles an extremely fine pinhole connecting the end of runner to product surface. Melt generates intense shear heat as it passes through. They are typically used in three-plate molds—runner plate and cavity plate separate first during mold opening, and solidified molten material in runner is pulled off automatically from gate. A tiny pit with a diameter of 1~3mm is left on product surface, often concealed by a countersunk platform.

 

2. Key Considerations for Gate Design
Diameter Selection: A core parameter. For materials with good flowability, use 0.5~1.0mm; for materials with poor flowability, use 1.0~2.0mm. Empirical formula: d = 0.05 × √(projected area) + 0.6 (mm), which needs to be adjusted according to material. Length: 0.5~1.5mm. Too short and it's prone to cracking; too long and pressure loss is large, it's difficult to break. Rounded corner transition: Inlet radius R value 0.2~0.5mm, outlet with small rounded corners or countersunk edges to prevent stress concentration and cracking at sharp corners. Multi-point distribution: When using multiple feed points, diameter should be consistent, and spacing should be large enough to avoid mutual interference. Three-plate mold opening distance: ≥ Total runner length + 50mm, to ensure smooth sprue removal. Product reinforcement: Appropriately thicken wall or add ribs near gate to offset residual stress caused by high shear.
3. Advantages and Disadvantages of Gate Design
Advantages: Minimal traces, almost invisible after polishing/spraying, making it the first choice for high-appearance requirements; Automatic breakage of three-plate mold + automatic runner detachment, fully automated production; High shear heat generation reduces viscosity, facilitating thin-walled filling; Fast gate freezing, controllable pressure holding, and low residual stress; Multi-point feeding significantly shortens process and reduces warpage; Flexible placement, can be placed on the top, side, and inner surfaces.
Disadvantages: Requires a three-plate mold, with multiple mold plates, complex control, and significantly higher cost than a two-plate mold; High pressure loss, requiring high tonnage injection molding machines; High flow rate scouring + glass fiber materials easily leads to increased gate wear or blockage; Heat-sensitive materials (PVC, POM) have risk of shear overheating and decomposition; Direct spraying into open areas easily causes surface defects; Not suitable for high-viscosity materials such as PC and PMMA.
4. Applicable Products for Gates
High-End Consumer Electronics Casings: Mobile phone frames, tablet back covers, laptop A-sides, etc., requiring extremely high surface integrity. Large, Thin-Walled Flat Products: TV bezels, monitor front frames, etc., multi-point gates shorten the process and reduce warping. Transparent Products: Transparent storage boxes, lampshades, etc., where minor imperfections are most concealed. Automated High-Volume Production: Disposable tableware, packaging containers, etc., fully automated with no post-processing. Precision Multi-Point Feeding Products: Printer/projector casings, etc., requiring precise control of filling balance.

Although small, gates have a significant impact. Direct gates, side gates, and point gates represent three distinct design philosophies: high flow rate with low shear, balanced and flexible, and precise and concealed. In practical engineering, gate selection requires comprehensive consideration of product structure, material properties, appearance requirements, mold costs, and production efficiency.
For further reading, please refer to A Comprehensive Analysis of Injection Mold Gate Types (Part 2): Submerged, Fan-Shaped, and Thin-Film.