The gate is the most critical yet most overlooked element in injection mold design. It determines how molten plastic enters the cavity, directly affecting fill pattern, part appearance, dimensional stability, and cycle time. A poorly chosen gate can turn an otherwise perfect mold into a source of rejects, while the right gate design delivers consistent quality across thousands of cycles.
What Is a Gate in Injection Molding
A gate is a small, restricted orifice located between the runner system and the mold cavity. It serves as the entry point through which molten polymer flows into the cavity during the injection phase. Despite its small size — typically 0.5 to 6 mm in diameter — the gate plays an outsize role in controlling several critical aspects of the molding process.
First, the gate regulates melt flow rate and direction, determining how the polymer front advances through the cavity. Second, it creates a localized pressure drop that generates shear heating, which can improve melt fluidity for materials that benefit from shear thinning. Third, after the holding pressure phase, the gate solidifies first, sealing the cavity so no backflow occurs during cooling. This freeze-off timing directly affects packing efficiency, sink mark formation, and dimensional repeatability.
The gate also leaves a visible mark on the finished part. The size, shape, and location of this mark — called gate vestige — must be balanced against functional and aesthetic requirements. A gate that is too large leaves an unsightly scar; one that is too small restricts flow, increases injection pressure, and may cause short shots.
Cold Runner Gate Types
Cold runner systems are the most widely used gate architecture in injection molding. In a cold runner, both the runner and the gate solidify during each cycle and are ejected along with the part (or as separate scrap). Each gate subtype offers distinct advantages depending on part geometry, material, and production volume.
1. Edge Gate
Also called a side gate or rectangular gate, the edge gate is the most common and versatile gate type. It is located along the parting line on the side of the part. The gate cross-section is typically rectangular, with a width-to-thickness ratio of 3:1 to 5:1. This geometry provides excellent flow control while minimizing stress concentration at the entry point.
Edge gates are ideal for flat, rectangular parts and are commonly used for housings, enclosures, covers, and panels. Their simplicity makes them easy to machine and modify, and they are compatible with nearly all thermoplastics. The primary disadvantage is the visible gate mark on the part edge, which requires manual or automated degating.
2. Sprue Gate
The sprue gate feeds molten material directly from the nozzle into the cavity through the sprue bushing. It is the simplest and least expensive gating method because it eliminates the runner system entirely. The gate is essentially the sprue itself, tapering from the nozzle orifice down to the cavity opening.
Sprue gates are typically used for single-cavity tools or prototype molds where cost and simplicity take priority over gate mark appearance. They are best suited for large, centrally-fed parts like buckets, basins, and cylindrical components. The major drawback is a large, unsightly gate mark that requires post-processing trim operations to remove.
3. Submarine Gate
The submarine gate — also known as a tunnel gate — is located below the parting line surface. During injection, the polymer flows through a short tunnel that connects the runner (located on the ejector half) to the cavity. At ejection, the gate shears off automatically as the part is pushed off the core, leaving a small, clean witness mark.
Submarine gates are a preferred choice for automatic degating in self-degating molds. They work well for small to medium parts where gate mark profile is a secondary concern but manual trimming should be avoided. The conical shape of a submarine gate typically has a diameter of 0.5 to 2.0 mm at the cavity interface. The gate angle is usually 30 to 45 degrees relative to the parting line.
Ductile materials such as polyethylene, polypropylene, and nylon are well-suited for submarine gating. Brittle materials may break unpredictably at the gate during ejection, causing inconsistent degating or gate fragments that contaminate the mold.
4. Fan Gate
A fan gate spreads the melt across a wider area, distributing the flow evenly into a cavity with a large width-to-depth ratio. The gate cross-section expands from a narrow entry point into a fan-shaped opening at the cavity wall. Typical fan gate depth is 0.25 to 1.5 mm, with the width matching the cavity edge.
Fan gates are the standard choice for flat, wide parts such as panels, covers, trays, and thin-wall housings. They eliminate jetting and reduce flow-induced orientation because the melt front advances as a broad wave rather than a concentrated jet. The trade-off is a large gate area that leaves a prominent gate mark and requires trimming.
5. Tab Gate
The tab gate is a variant of the edge gate that incorporates a small rectangular tab between the gate and the part. The polymer enters the tab, then flows into the part cavity. After ejection, the tab is trimmed off, leaving the part surface entirely free of gate marks.
Tab gates are used when gate mark placement on the part surface cannot be tolerated, such as on cosmetic surfaces or functional mating edges. The tab absorbs shear stress and gate blush, protecting the part from visual defects. The main disadvantage is the additional scrap from the tab material and the secondary trimming operation required.
6. Diaphragm Gate
The diaphragm gate is a ring-shaped gate that feeds material around the entire circumference of a cylindrical or tubular cavity. Material enters through a central sprue, then flows radially outward through a thin diaphragm opening (typically 0.3 to 1.0 mm thick) into the cavity. The diaphragm opening spans 360 degrees around the core.
This gate type is specifically designed for tubular parts such as syringes, pen barrels, pipe fittings, and bottle preforms. The uniform circumferential flow eliminates weld lines along the length of the part and produces excellent concentricity. The diaphragm material is ejected as a disc that must be removed in a secondary operation.
7. Ring Gate
The ring gate is similar to the diaphragm gate but uses a discrete ring-shaped opening rather than a full continuous diaphragm. The ring opening can be partial (180 to 270 degrees) or full, depending on the flow requirements. The melt enters the cavity through the annular gap formed between the core and the cavity.
Ring gates are used for larger tubular parts where the diaphragm gate would create too much scrap. They provide good concentricity and balanced flow while producing less waste than a full diaphragm. The ring mark on the part end face must be removed by secondary trimming or accepted as the part design permits.
Cold Runner Gate Comparison
| Gate Type | Pros | Cons | Am besten für |
|---|---|---|---|
| Edge | Simple machining, flexible placement, low cost | Visible gate mark, manual degating needed | Flat panels, housings, general parts |
| Sprue | No runner system, lowest cost, good for large parts | Large gate mark, high stress at entry | Single cavity, prototypes, buckets |
| Submarine | Automatic degating, small witness mark, fast cycle | Limited to ductile materials, complex runner design | Multi-cavity, PE/PP/nylon, high volume |
| Fan | Even melt distribution, reduces jetting | Large gate mark, trimming required | Wide panels, thin-wall housings, trays |
| Tab | No gate mark on part, absorbs shear | Extra scrap, secondary trim, added cost | Cosmetic surfaces, decorative parts |
| Diaphragm | Perfect concentricity, no weld lines | High scrap (full disc), difficult removal | Cylindrical tubes, syringes, bottle preforms |
| Ring | Good concentricity, less waste than diaphragm | Secondary trim needed, mark on end face | Large tubular parts, pipe fittings |
Hot Runner Gate Types
Hot runner systems maintain the polymer in a molten state inside the manifold and nozzle, eliminating the scrap generated by cold runners. The gate in a hot runner system must balance thermal isolation from the mold steel (to prevent freeze-off) with mechanical shut-off capability (to prevent drool). Three primary hot runner gate architectures dominate the industry.
1. Valve Gate
The valve gate uses a reciprocating pin mechanism to open and close the gate orifice mechanically. During injection, the pin retracts to open the flow path; after packing, the pin advances to seal the gate positively. This mechanical shut-off prevents stringing, drooling, and gate freeze-off issues.
Valve gates produce the cleanest gate vestige among hot runner systems, leaving a small circular mark that is often acceptable for visible surfaces. They offer excellent control over flow timing and can be sequenced independently in multi-gate applications to manage weld line placement and packing balance. The main drawbacks are higher system cost, added maintenance for the moving pin mechanism, and larger nozzle footprint that limits cavity spacing.
2. Thermal Gate (Hot Tip)
Thermal gates, also called hot tip gates, rely on precise temperature control at the nozzle tip to keep the melt at the gate orifice. The tip is heated via an internal heater band or cartridge heater. At the end of the injection cycle, the heater power is reduced briefly to allow a thin skin layer to freeze at the tip, sealing the gate. When injection begins again, the heat restores the melt flow path.
Thermal gates are simpler and less expensive than valve gates, making them the most common hot runner solution. They work well for materials with broad processing windows such as polypropylene, polyethylene, ABS, and nylon. The gate vestige is more pronounced than a valve gate but still smaller than most cold runner marks. Material degradation can occur if the hot tip runs too hot during extended idle periods.
3. Hot Sprue (Sprue Gate in Hot Runner)
A hot sprue gate is essentially a centrally-located hot tip nozzle that feeds directly into a single cavity or into a cold sprue that then feeds the cavity. It combines the simplicity of a sprue gate with the advantages of a hot runner system — no cold runner scrap and reduced cycle time.
Hot sprue gates are commonly found in single-cavity hot runner molds and small family molds. They are economical for parts that would otherwise use a cold sprue gate but benefit from the process stability of a hot runner system. Gate mark size and appearance are similar to a cold sprue gate.
| Hot Runner Type | Kosten | Gate Mark | Maintenance | Material Compatibility |
|---|---|---|---|---|
| Valve Gate | Hoch | Clean, small | Moderate (pin wear) | Most thermoplastics |
| Thermal Gate | Mittel | Visible, circular | Low (heater replacement) | Broad-window materials |
| Hot Sprue | Niedrig-Mittel | Large, similar to cold sprue | Niedrig | General purpose |
Gate Location Principles
Selecting the correct gate location is as important as choosing the gate type. The gate position determines the flow path length, the location of weld lines, the effectiveness of gas venting, and the aesthetic quality of the visible surfaces. These principles guide gate placement decisions.
Flow Length and Fill Balance
The gate should be positioned to minimize the flow-length-to-wall-thickness (L/T) ratio. A high L/T ratio requires higher injection pressure and risks short shots. For a given material, the maximum flow length is determined by the melt flow index, wall thickness, and injection pressure capability. The gate should be placed at the thickest wall section to allow optimal packing, since thicker sections solidify last and benefit from prolonged pressure transmission.
In multi-cavity tools or multi-gate single cavities, flow balance is critical. Runner geometry and gate size must be tuned so all cavities fill simultaneously within approximately 5 percent of fill time. Mold filling simulation software is strongly recommended for complex geometries to validate flow balance before steel is cut.
Weld Lines and Air Traps
When the melt front splits around a core, insert, or flow obstruction, it forms a weld line where the fronts recombine. Weld lines are structurally weaker than the bulk material, with strength typically 30 to 70 percent of the base resin depending on material and processing conditions. The gate should be positioned so that weld lines form in low-stress, non-cosmetic areas. Multiple gates increase the number of weld lines but can also reduce their severity by allowing higher melt temperatures at the knit interface.
Air traps occur when advancing melt fronts surround a pocket of air that cannot escape through the venting system. The gate location determines where the final air traps form. Proper gate placement should drive air toward existing vents or planned vent locations. Core pins and deep ribs are common air trap zones that require careful gate positioning.
Venting Strategy
The gate position dictates the natural flow path of the melt and therefore where the air must be vented. Ideally, the gate is placed opposite the vent locations so air is pushed ahead of the melt front rather than being compressed. Vent depth is material-dependent: amorphous materials like ABS and PC require deeper vents (0.03 to 0.05 mm), while crystalline materials like nylon and POM require shallower vents (0.01 to 0.03 mm).
Aesthetic Surface Considerations
Gate vestige on visible part surfaces is a common cause of customer rejection. On textured, painted, or high-gloss surfaces, the gate mark must be placed on non-critical surfaces or designed for automatic degating to produce a low-profile mark. If the part has a decorative Class A surface, the gate should be placed on the internal surface, in an area covered by a label, or on a surface that is hidden during final assembly.
Gate Size Calculation
Gate dimensions are determined by the part geometry, the material rheology, and the processing parameters. While empirical rules provide starting points, the final gate size is typically validated and fine-tuned during mold trials.
General Formula
The gate cross-sectional area is calculated as a function of the part wall thickness and the gate geometry factor. For a rectangular edge gate, the recommended gate depth (t) is 50 to 80 percent of the nominal wall thickness (T).
t = k × T
Where: t = gate depth (mm), T = part wall thickness (mm), k = gate depth factor (0.50 to 0.80)
The gate width (w) is typically 3 to 5 times the gate depth for edge gates, with a recommended range of 1.5 to 10 mm depending on part size. Gate land length (Lg) — the straight section immediately before the cavity — should be 0.5 to 1.0 mm, with 0.5 mm being the preferred target for most applications. The land should never exceed 1.5 mm, as longer lands create excessive pressure drop and may cause premature freeze-off.
Wall Thickness Ratios
The ratio of gate depth to wall thickness varies by gate type:
| Gate Type | Depth / Wall Thickness Ratio | Typical Depth (mm) |
|---|---|---|
| Edge | 0.50 – 0.80 | 0.5 – 3.0 |
| Fan | 0.25 – 0.50 | 0.25 – 1.5 |
| Submarine | 0.30 – 0.60 | 0.5 – 2.0 |
| Diaphragm | 0.10 – 0.30 | 0.3 – 1.0 |
Material-Specific Gate Guidelines
Gate sizing must account for material-specific flow behavior. The table below provides recommended gate dimensions for common engineering materials, assuming nominal wall thickness of 2.0 to 3.0 mm.
| Material | Min Gate Depth (mm) | Recommended Depth (mm) | Anmerkungen |
|---|---|---|---|
| Nylon (PA6, PA66) | 0.8 | 1.0 – 1.5 | Low viscosity; use smaller gate for unfilled, larger for glass-filled |
| Polypropylen (PP) | 0.5 | 0.8 – 1.2 | Excellent flow; submarine gates work well |
| Polycarbonat (PC) | 1.0 | 1.5 – 2.5 | High viscosity; avoid submarine, prefer edge or fan |
| ABS | 0.8 | 1.0 – 1.8 | Moderate viscosity; balanced flow |
| Acetal/POM | 0.6 | 0.8 – 1.2 | Low viscosity; small gates acceptable |
| Polystyrene (PS) | 0.5 | 0.7 – 1.0 | Brittle; avoid submarine, use edge gate |
For glass-filled nylon (PA6+GF30 or PA66+GF30), the recommended gate depth increases by approximately 25 to 40 percent compared to unfilled resin due to the higher melt viscosity and the abrasive nature of glass fibers. A minimum gate depth of 1.2 mm should be used, and fan gates are preferred over edge gates to distribute fiber orientation more evenly across the cavity.
Common Gate Defects and Solutions
| Fehler | Aussehen | Cause | Lösung |
|---|---|---|---|
| Gate Blush | Cloudy, glossy ring around gate | Excessive shear heating at gate land | Increase gate depth, reduce injection speed, shorten gate land to 0.5 mm |
| Gate Vestige | Protruding or recessed mark at gate | Improper gate freeze-off, excessive packing | Optimize pack/hold profile, reduce pack pressure, increase gate size |
| Jetting | Snake-like flow marks inside cavity | Melt entering cavity faster than the front advances | Use fan or tab gate, increase gate size, reduce injection speed, target gate at cavity wall |
| Gate Freeze-Off | Short shot, sink mark near gate | Gate solidifies before packing is complete | Enlarge gate, reduce gate land length, increase mold temperature, increase melt temperature |
Häufig gestellte Fragen
What is the best gate type for glass-filled nylon?
For glass-filled nylon (PA6+GF30, PA66+GF30, PA6+GF50), a fan gate is generally the best choice. The fan geometry distributes the glass fibers more evenly across the cavity width, reducing fiber orientation anisotropy and minimizing warpage. The gate depth should be 1.2 to 2.0 mm minimum, approximately 25 to 40 percent larger than for unfilled nylon, to accommodate the higher viscosity. Valve gates (hot runner) are also excellent for glass-filled nylon in high-volume production because they eliminate freeze-off issues and provide a clean gate break. Avoid submarine gates for glass-filled materials above 30 percent glass content, as the abrasive fibers accelerate gate wear and cause inconsistent degating.
Can I change the gate type after the mold is built?
Gate type changes are possible in many cases, but the extent of modification depends on the existing mold design and the desired new gate type. Converting between cold runner gate subtypes that share the same basic runner layout — for example, edge gate to fan gate or edge gate to tab gate — is typically feasible with wire EDM or conventional machining to modify the gate insert or cavity block. However, converting from an edge gate to a submarine gate, or from cold runner to hot runner, often requires significant modification to the runner plate, cavity plate, and possibly the ejector system. Such changes can cost 20 to 50 percent of the original mold price. A cost-benefit analysis should be performed, and mold filling simulation should be run on the proposed new gate geometry before making modifications.
How small can a submarine gate be?
The practical minimum diameter for a submarine gate is approximately 0.4 mm, achieved with precision EDM or micro-machining. However, submarine gates below 0.8 mm diameter present several challenges. The small orifice creates high shear rates that can degrade temperature-sensitive materials. Mold alignment becomes critical, as even 0.02 mm of mismatch can shear or obstruct the gate. The gate land length at these dimensions must be held to 0.3 to 0.5 mm to avoid excessive pressure drop. For production applications, a submarine gate diameter of 0.8 to 1.2 mm is the practical sweet spot that balances flow capability, shear control, and reliable automatic degating. Micro-molding of parts under 1 gram may use gates as small as 0.3 to 0.5 mm with specialized tooling.
Do hot runner gates leave bigger marks than cold runner gates?
No, hot runner gates generally leave smaller and cleaner marks than cold runner gates. Valve gates in particular produce the smallest gate vestige of any gating system — a clean, flat circular mark typically 0.5 to 2.0 mm in diameter with no protruding material. Thermal gate (hot tip) marks are slightly more prominent than valve gate marks but still smaller than most cold runner marks. Cold runner edge gates leave a rectangular tab that protrudes 1 to 3 mm from the part surface. Submarine gates leave a small conical witness mark (0.5 to 1.0 mm diameter) that is comparable to or smaller than a thermal gate mark. Sprue gates produce the largest gate mark of any type, typically requiring manual trimming. If gate mark appearance is the primary concern, valve gates are the best option, followed by properly designed submarine gates for cold runner systems.
Schlussfolgerung
Gate design is a foundational element of successful injection mold engineering. The choice of gate type, size, and location determines fill behavior, part quality, cycle time, and production cost. Cold runner gates offer lower tooling investment and material flexibility, with edge and submarine gates covering the majority of applications. Hot runner gates provide superior gate mark quality and eliminate runner scrap at a higher upfront cost.
The decision framework is straightforward: evaluate the part geometry, material rheology, production volume, and aesthetic requirements. Select the gate type that best balances these factors, size it using the guidelines provided, and validate with mold filling simulation before committing to steel. When in doubt, a modular gate insert design allows economical gate type changes during mold trials, providing the flexibility to optimize the gate for production conditions.
For expert assistance with gate design, mold flow analysis, or mold sourcing for your next injection molding project, contact our engineering team for a design review and quotation.
