What is a gate? 

 

A gate is "door" through which molten plastic enters mold cavity. If gate is chosen correctly and placed in the right location, plastic part will not have defects such as shrinkage or deformation; if chosen incorrectly, either plastic part will be scrapped, defective products will increase, and cost will rise, or mold will have to be remade. Below are structural details, processing points, advantages, and disadvantages of commonly used mold gates! A practical explanation of selection of different types of plastic parts will also be provided!

 

Main runner of injection molding machine nozzle is directly extended, reaching center of plastic part. Material is poured directly into cavity from center, without any additional branch runners.
Key Points: Taper of main runner should be controlled between 2 and 4 degrees (too large a taper will create eddies in material flow; too small a taper will make it difficult to remove solidified material); thickness of fixed mold plate and fixed mold base should be as thin as possible (to reduce material flow resistance and avoid pressure loss).
Advantages: Minimal material flow resistance, shortest flow path, longest pressure holding and shrinkage compensation time (thick-walled plastic parts are less prone to shrinkage); simplest mold structure, no need to design sub-runners, even small factories can process it, lowest cost.
Disadvantages: Large and hard gate scars (diameter same as large end of main runner), requiring secondary grinding, and easily leaving marks; particularly high internal stress at gate of plastic part, making hard plastics like PC and polysulfone prone to cracking later; only single-cavity molds can be made (one part is produced at a time with one mold, low efficiency).
Applications: Specifically designed for large, thick-walled, deep-cavity plastic parts, such as washing machine inner tubs, television casings, and car bumpers; especially suitable for "stubborn" plastics with poor flowability like PVC and PC.
Tip: Make a rounded corner (R1~R2mm) at large end of main runner to avoid dead spots in material flow, resulting in cold material.

 

A small rectangular slit is made on parting surface of mold, facing side of plastic part. Material is squeezed in from side. This is the most common injection molding method used in injection molding plants.
Key Points: Gate length 0.5~3mm (too long results in high pressure loss, too short makes processing difficult); width 1.5~5mm (adjust according to size of plastic part, choose a smaller value for small parts); thickness 0.5~2mm (generally 1/3~1/2 of plastic part's wall thickness; too thick makes gate cutting trouble, too thin prevents material flow).
Advantages: Extremely simple processing; can be milled with a milling machine; small gate scars, easily broken off with pliers, leaving almost no visible mark; multiple parts can be made from one mold (multi-cavity molds), directly doubling efficiency; material flow rate can be controlled by adjusting gate size (e.g., widening gate for thin-walled parts results in faster material flow).
Disadvantages: Solidified material at gate and molded part is connected, cannot be separated automatically, requiring manual gate trimming (increasing labor costs); material flows into cavity from side, easily merging with another flow, forming weld lines (like two lines glued together, affecting strength); using it on shell-shaped parts can easily cause air trapping (air cannot escape, resulting in air bubbles in molded part).
Applicable Scope: Except for slender, barrel-shaped parts (such as mineral water bottle preforms, where side injection will cause uneven wall thickness), it can be used for most small and medium-sized plastic parts, such as toys, mobile phone cases, and daily necessities casings.
Tips: Make a small bevel (30°~45°) at connection between gate and molded part to prevent damage to molded part when trimming gate.

 

A "wider version" of side gate, one end of gate connects to runner (narrow), and the other end connects to cavity (wide), resembling an open fan, allowing material to spread evenly upon entry.
Key Points: Gate width is generally 1/4 of cavity width, and should not be less than 8mm; gate thickness is 0.25~1.5mm (adjusted according to part wall thickness; the thinner part, the thinner gate); "fan angle" of gate should be controlled between 15° and 30° (too large an angle will cause uneven material flow).
Advantages: Material flows into cavity in a "flat" state, preventing damage to cavity wall; internal stress of plastic part is particularly low, making it less prone to warping and deformation (for example, when making plastic sheets, using a fan-shaped gate prevents sheet from bending); there are almost no flow marks or traces of material flow direction (good appearance).
Disadvantages: If gate is too wide, it needs to be cut little by little during gate trimming, resulting in a large workload; a long shear mark will be left on the side of plastic part, which needs to be sanded off, increasing costs.
Applications: Specifically designed for making long, flat, thin-walled plastic parts, such as rulers, plastic trays, and decorative panels; it can also easily mold plastics with poor flowability, such as PC and PSF.
Tips: Wide end of fan-shaped gate should be aligned with the entire width of cavity to ensure sprue flows evenly across it.

 

Wider than fan-shaped gate, this gate is a long, narrow slit roughly width of part. Sprue flows into cavity in a parallel, "pancake-like" fashion.
Key Points: Gate length 0.3~0.5mm (ultra-short, reducing pressure loss); thickness 0.1~0.3mm (much thinner than part wall); width consistent with part width (maximum 500mm or more).
Advantages: Uniform sprue flow rate; extremely high flatness of part (perfect for large flat panels, such as refrigerator door linings, without unevenness); no weld lines because sprue flows in as a single continuous flow; excellent venting, allowing air to escape along edges of sprue.
Disadvantages: Gate and plastic part are almost connected, making removal extremely difficult and requiring a specialized cutter or grinder; high processing precision is required, and gate thickness must be consistent during milling to prevent material flow deviation.
Applications: Ultra-wide and ultra-thin plastic parts, such as plastic films, large acrylic panels, and billboard base plates.
Tips: Mold should have a heating device (e.g., connected to a hot water pipe) to maintain temperature at gate, prevent material from cooling and solidifying there.

 

A small "ear" (ear groove) is first cut on the side of cavity. Gate is aligned with ear groove, and material flows into ear groove first, hitting wall of ear groove. Direction and speed are adjusted, then it flows smoothly into cavity.
Key Points: Width of ear groove should be 2-3 times width of gate, and depth should be 0.5-1mm thicker than plastic part (to allow for buffering space for material flow); connection between gate and ear groove should be smooth, without sharp corners.
Advantages: Completely solves problem of material flow "jetting" (no white haze or snake-like patterns on transparent plastic parts); material flows into mold cavity slowly and steadily, resulting in minimal internal stress in plastic part (high-transparency acrylic sheets made with this method achieve required light transmittance).
Disadvantages: Solidified material in the ear slots and gates must be removed, resulting in a large workload; ear slot marks will remain on the sides of plastic part, requiring grinding and polishing.
Applications: High-transparency flat plastic parts (such as blanks for optical lenses, acrylic decorative panels), and precision plastic parts with extremely high deformation requirements (such as instrument housings).
Tips: Ear slots should be located on non-visual surfaces of plastic part, such as edge of acrylic sheet, so that grinding does not affect aesthetics.

 

Gate is a small dot (0.5~2mm in diameter) located at the top or in a concealed position on plastic part. Material flows into cavity from this dot. Mold requires an additional "runner plate" (three-plate mold structure). During mold opening, runner plate separates from fixed platen, and gate is automatically broken off.
Key Points: Gate diameter should be adjusted according to size of plastic part (0.5~1mm for small parts, 1~2mm for large parts); guide pillars of runner plate should be 10~15mm longer than guide pillars of fixed platen (to ensure gate breaks off first during mold opening).
Advantages: Gate scar is extremely small (like a pinhead), almost invisible, and requires no grinding; it enables automated production (broken-off sprue can be directly recycled); location is extremely flexible, and it can be placed anywhere on plastic part.
Disadvantages: Mold is a three-plate mold, with a complex structure, costing about 30% more than a two-plate mold; injection pressure is high (material flow must pass through small points, resulting in high resistance), and molding cycle is long (requiring a longer holding pressure time).
Applications: Plastic parts with high appearance requirements, such as mobile phone cases, cosmetic bottles, and cylindrical storage boxes; large-area flat plastic parts, where multiple gate points can be opened (e.g., 4 gate points for a 10-inch flat plate for even material feeding).
Tips: Location of gate points should avoid weak points in plastic part (such as thin-walled areas) to prevent stress concentration and cracking.

 

Gate is "hidden" inside mold core or cavity, not on parting surface; material flows obliquely into cavity from submerged tunnel. When mold opens, plastic part is ejected, and gate is automatically cut off. Solidified material remains in mold and falls out together with material when mold opens again.
Key Points: Angle of tunnel is generally 30°~45° (too small an angle results in high material flow resistance; too large an angle makes it difficult to cut gate); gate outlet should be located in a concealed location on plastic part (such as inside of part, or at the base of reinforcing ribs).
Advantages: No manual gate cutting is required, resulting in high automation efficiency; mold is a two-plate mold, simpler in structure than a three-plate mold, and lower in cost; gate marks are completely concealed (not visible on the surface).
Disadvantages: Not suitable for all plastics—for very tough plastics (such as nylon PA), gate cannot be cut and will string; for very brittle plastics (such as polystyrene PS), gate is prone to breakage inside tunnel, blocking gate; processing is complicated, requiring use of electrodes to create tunnel.
Applicable Scope: Plastic parts on automated production lines, such as internal parts of household appliances and toy accessories; plastic parts requiring a flawless appearance.
Tips: Tunnel outlet should have a small rounded corner to avoid dead zones in material flow, forming cold material.

 

An "arc-shaped" version of submarine gate, tunnel is curved (like a crescent moon); mold is a two-plate mold, with gate located on arc surface of core. When mold opens, plastic part is ejected, and gate automatically breaks off.
Key Points: Radius of arc-shaped tunnel is 5-10mm (adjusted according to size of plastic part); gate outlet diameter is 0.8-1.5mm (slightly larger than a spot gate).
Advantages: Smoother material flow than a submarine gate (arc-shaped tunnel has no sharp corners, preventing material flow blockage); more concealed gate marks (located on arc of core, invisible on the surface of plastic part); two-plate mold structure, lower cost than three-plate mold.
Disadvantages: Arc-shaped tunnel is difficult to process, requiring EDM; only suitable for small to medium-sized plastic parts; too much flow resistance for larger parts.
Applications: Small to medium-sized plastic parts with high appearance requirements, such as gears, knobs in small appliances, and toy wheels.
Tip: Arc of electrode must perfectly match arc of mold core; otherwise, tunnel will be misaligned.

 

Gate is a circular ring that fits over mold core. Sprue flows evenly into cavity from the entire circumference of ring, like "dressing" core.
Key Points: Ring width is 2-5mm (adjust according to size of part); gap between ring and core is 0.5-1mm (to ensure even sprue flow).
Advantages: Even sprue flow from circumference, resulting in no weld lines on part (extremely high strength); excellent venting, allowing air to escape through gaps in the ring; even stress on core, preventing deformation.
Disadvantages: Solidified sprue is a circular ring, wasting material; removing gate is particularly troublesome, requiring scissors or a grinder; only suitable for bottomless cylindrical parts.
Applications: Bottomless cylindrical parts, such as body of a plastic bucket, pipe fittings, and body of a cup.
Tips: Annular gate should be aligned with cavity opening to ensure sprue fills circumference of opening first, then flows along length.

 

An "upgraded" version of annular gate, gate's annular ring is not flat but conical like an umbrella. Sprue flows out from conical ring, first filling opening of part, then flowing evenly into cavity along taper.
Key Points: Angle of taper should be 10°~20° (too large an angle will disperse sprue; too small, it's no different from an annular gate); width of ring should be 3~6mm (slightly wider than an annular gate).
Advantages: More even sprue flow than an annular gate (cone guides flow); lower internal stress in part, less prone to deformation; higher molding quality (suitable for precision tubular parts).
Disadvantages: More difficult to manufacture than an annular gate, requiring a conical electrode; gate removal is more troublesome, requiring cutting tools (such as a lathe) to cut it off.
Applications: Short, thick tubular plastic parts requiring high quality, such as conduits for precision instruments and oil pipe connectors for automobiles.
Tip: Conical surface should be polished (roughness Ra 0.8 or lower) to prevent scratches from sprue flow.

 

By extending conical apex of umbrella-shaped gate to 180°, gate becomes a flat disc that fits over end of core. Sprue flow is evenly injected into cavity from the entire circumference of disc, like "capping" core.
Key Points: Diameter of disc should match inner diameter of plastic part; thickness of disc should be 0.3~0.5mm (thinner than wall thickness of plastic part).
Advantages: Core experiences the most even stress (preventing deformation due to sprue flow impact); sprue flow is completely synchronized, resulting in no weld lines on plastic part; excellent venting, allowing air to escape from edge of disc.
Disadvantages: Solidified material at gate is a disc, which wastes material; a disc mark will be left on inner edge of plastic part, requiring sanding; it can only be used to make cylindrical plastic parts with large inner holes.
Applications: Cylindrical plastic parts with large inner holes, such as body of a trash can, flanges for large pipes, and plastic parts with rectangular inner holes (such as drawer slides).
Tips: Disc gate should be located on inner edge of plastic part so that sanding does not affect appearance.