The gate is the smallest feature in the mold. It’s also where most quality problems originate. This narrow channel controls how material enters the cavity, how long pack pressure can act on the part, and what cosmetic mark remains on the finished product. Selecting the right gate type involves tradeoffs between cost, appearance, material behavior, and automation requirements. Understanding these tradeoffs early prevents problems that no amount of process adjustment can solve later.
What Gates Do
Gates serve multiple functions that extend far beyond simply admitting material into the cavity. Each function influences part quality and production efficiency.
Material entry point. The gate determines where plastic first enters the cavity and how it flows from there. Location affects weld line positions, air trap locations, and filling balance in multi-cavity molds. Poor gate location causes flow imbalances that create quality variations between cavities.
Flow restriction. Gate dimensions control fill rate into the cavity. A smaller gate restricts flow, requiring higher injection pressure but providing better control over fill speed. A larger gate fills faster but may cause jetting or overpacking near the gate area.
Freeze-off mechanism. The gate freezes before thicker sections of the part, ending the ability to pack material into the cavity. Gate size and geometry determine how long pack pressure remains effective. Freeze-off timing affects shrinkage compensation in thick sections and can cause sink marks if pressure transmission ends too early.
Cosmetic consideration. Every gate leaves some mark on the finished part: a vestige where the gate was trimmed, a witness mark from a valve gate pin, or a raised area from thermal gate freeze-off. Gate type and location determine visibility and acceptability of this mark.
Structural impact. Gate location creates molecular orientation patterns as material flows through the restriction. High shear in the gate area can degrade material properties or create visible defects. Some materials, particularly fiber-filled grades, develop strength anisotropy based on gate-induced fiber orientation.
Edge Gates
Edge gates are the most common gate type, machined directly into the parting line at the part edge. Their simplicity makes them the default choice when other factors don’t drive a different selection.
Construction consists of a rectangular or trapezoidal channel cut from the runner into the cavity edge at the parting line. Typical dimensions range from 0.5mm to 3mm height and 1mm to 8mm width, scaled to part size and wall thickness. The gate land (length) is typically short, 0.5mm to 1mm, to minimize pressure drop.
Advantages include straightforward machining, easy modification if gate size needs adjustment, simple tooling maintenance, and visible gate location that simplifies troubleshooting. Edge gates work with virtually all materials and accommodate wide process windows.
Disadvantages center on the visible vestige left at the part edge. Manual trimming is often required, adding labor cost. The vestige location at the parting line means it cannot be hidden on most parts. For high-cosmetic applications, edge gates may not meet appearance standards.
Best applications include technical parts where appearance is secondary, parts with edge features that can incorporate or hide the vestige, parts requiring easy gate modifications during process development, and applications where trimming labor cost is acceptable.
Edge gates suit most prototyping and development programs because they allow easy adjustment. Production molds often start with edge gates, converting to other types only when volume justifies the investment.
Sub-Gates (Tunnel Gates)
Sub-gates, also called tunnel gates or submarine gates, enter the cavity below the parting line through an angled channel. The angled geometry allows automatic degating during mold opening.
Construction involves drilling an angled channel from the runner system to a point below the cavity parting line. Typical angles range from 30 to 45 degrees. The gate cross-section at the part is often circular or elliptical. The angled section acts as a shear point: when the mold opens, the gate stretches and shears at the thinnest section.
Advantages include automatic degating that eliminates manual trimming labor. The gate vestige sits below the parting line surface, improving appearance on the primary cosmetic side. Sub-gates work well in multi-cavity molds where consistent automatic degating across all cavities improves efficiency.
Disadvantages include higher tooling cost than edge gates, limited gate size (typically under 2mm diameter to ensure clean shearing), material restrictions (brittle materials like acetal and general-purpose polystyrene work well, but ductile materials like PP and PE may not shear cleanly), and gate vestige below the surface which may be visible on transparent parts or interfere with mating features.
Best applications include multi-cavity production molds where degating labor savings justify tooling cost, parts with accessible locations for below-parting-line gate entry, materials that shear cleanly (PS, ABS, PC, acetal), and automated production where manual operations add significant cost.
Sub-gates require more design attention than edge gates. Gate location must allow an angled approach that doesn’t interfere with other mold components. Material testing may be needed to confirm clean shearing behavior.
Hot Runner Gates (Valve and Thermal)
Hot runner systems eliminate cold runners by maintaining material in a molten state from the machine nozzle to the gate. This approach offers significant advantages for high-volume production but adds cost and complexity.
Valve gates use a mechanical pin that opens and closes to control material flow. The pin retracts during injection to allow fill, then advances to close the gate and shear the material. This mechanism leaves a small witness mark (typically 1 to 3mm diameter) but no vestige projection.
Valve gate advantages include clean cosmetic appearance, precise fill control in sequential valve gating applications, no degating operation required, and ability to gate directly into visible surfaces. Disadvantages include high system cost ($15,000 to $50,000 or more depending on number of drops), maintenance requirements for valve pins and actuators, and potential for leakage around pin seals.
Thermal gates rely on the material freezing and melting at the gate orifice between cycles. During injection, heat conducts from the manifold to melt the frozen plug and allow flow. After injection, the gate freezes to seal. No mechanical valve is required.
Thermal gate advantages include lower cost than valve gates and fewer moving parts. Disadvantages include a small witness mark that may be slightly raised (cosmetically inferior to valve gates), longer cycle times due to freeze-melt cycle, and sensitivity to process conditions that affect freeze behavior.
Best applications for hot runner systems include high-volume production where runner material savings justify system cost, expensive materials where scrap costs are high, parts requiring direct gating with minimal vestige, multi-cavity molds where balanced filling is critical, and applications where cycle time reduction from eliminating runner cooling provides payback.
Hot runner systems require more sophisticated maintenance than cold runner molds. Manifold heater failures, thermocouple problems, and valve seal wear demand attention that simpler tooling avoids.
Specialized Gate Types
Certain applications require gate designs beyond the common options. These specialized types solve specific problems but come with corresponding tradeoffs.
Cashew gates curve beneath the parting line in an arc (shaped like a cashew nut) to reach hidden locations. The curved channel allows degating while the gate vestige sits where it cannot be seen on the cosmetic surface. The curved path is difficult to machine and prone to breakage if gate size is too large or material doesn’t shear cleanly.
Fan gates spread material across a wide entry point for uniform flow into thin, wide parts. The gate width might span much of the part edge, reducing injection velocity and shear while providing balanced filling. Fan gates eliminate flow hesitation lines but require trimming along the entire gate width.
Diaphragm gates feed around the entire circumference of a round part, providing uniform radial flow for concentricity. Used primarily for cylindrical parts where weld lines from edge gating would be unacceptable. The gate must be trimmed completely around the circumference, typically requiring a secondary operation.
Tab gates extend a sacrificial tab from the part that receives the gate vestige. The entire tab is then removed, leaving no gate mark on the functional part. This approach works for fiber-filled materials where gate shear degrades properties, and for parts requiring completely clean surfaces.
Pin gates are small-diameter direct gates used in multi-cavity molds with three-plate construction. The gate diameter is small enough to shear during mold opening while feeding multiple cavities from a runner system on a separate plate. Pin gates limit gate size and require additional mold complexity but enable high-cavitation molds with automatic degating.
Selection Decision Framework
Gate selection requires balancing competing requirements. A systematic evaluation process prevents overlooking critical factors.
| Criterion | Edge | Sub-Gate | Valve | Thermal | Specialized |
|---|---|---|---|---|---|
| Cosmetic surface | Poor | Good | Excellent | Good | Varies |
| Tooling cost | Low | Medium | High | Medium-High | Medium-High |
| Auto-degating | No | Yes | Yes | Yes | Varies |
| Gate size range | Wide | Limited | Wide | Medium | Varies |
| Material flexibility | High | Medium | High | High | Low |
| Maintenance needs | Low | Medium | High | Medium | Varies |
Decision process:
- Can the gate be hidden? If yes, edge or sub-gate may suffice regardless of cosmetic concerns.
- Is degating labor significant? If manual trimming cost matters, sub-gate or hot runner justifies premium tooling.
- What are the cosmetic requirements? Critical surfaces may require valve gates or careful sub-gate placement.
- What is the material? Some materials shear cleanly for sub-gates; others require edge gates or hot runners.
- What is production volume? High volume justifies hot runner investment; low volume favors simpler tooling.
- What is the budget? Cost constraints may dictate edge gates even when other types would provide technical advantages.
Common Gate Problems and Solutions
Gate-related defects appear frequently in injection molding. Recognizing patterns connects symptoms to correctable causes.
Gate blush is a dull or hazy area around the gate caused by high shear stress as material passes through the restriction. Solutions include enlarging the gate, reducing injection speed, raising melt temperature, or adding a transition radius at the gate entrance.
Jetting occurs when a thin stream of material squirts across the cavity before spreading to fill. The stream solidifies slightly before the bulk fill reaches it, creating a visible serpentine pattern. Solutions include changing gate location to impinge on a wall, enlarging the gate, or slowing injection speed.
Hesitation marks appear as surface defects where material paused during filling. Near-gate hesitation occurs when gate size creates excessive restriction. Solutions include enlarging the gate or repositioning to reduce the flow restriction effect.
Freeze-off problems manifest as sink marks in thick sections or dimensional variation from shot to shot. The gate freezes before adequate packing compensates for shrinkage. Solutions include enlarging the gate, repositioning closer to thick sections, increasing mold temperature near the gate, or redesigning thick sections.
Gate selection is a design decision that affects manufacturing for the life of the mold. The right choice simplifies production; the wrong choice creates problems that no amount of process adjustment can fix. Evaluating requirements systematically, understanding the tradeoffs of each gate type, and anticipating potential problems produces gate designs that work from first sample through end of production.
Sources
- Beaumont, John. “Runner and Gating Design Handbook.” Hanser, 2004.
- Kazmer, David O. “Injection Mold Design Engineering.” Hanser, 2007.
- Rosato, Donald V. “Injection Molding Handbook.” Springer.
- Menges, Michaeli, Mohren. “How to Make Injection Molds.” Hanser, 2001.
- Hot Runner Technology Resources. “Gate Selection Guidelines.”