Table of Contents
Jetting is a common defect in injection molding.Jetting is often noticed first as a cosmetic defect, but severe cases can also affect local fusion and part quality. Jetting in injection molded parts is closely related to how the melt exits the gate and enters the cavity.
What Is Jetting in Injection Molding?
Jetting is one of several common injection molding defects, occurring when molten plastic exits the gate as a high-speed stream instead of spreading smoothly against the cavity surface.The melt does not spread evenly along the cavity walls, but instead folds repeatedly, forming a turbulent flow pattern.
Typical signs of jetting include: snake-shaped or worm-shaped marks on the surface of the product, curled or serrated lines near the gate, sometimes accompanied by silver streaks or a dull surface and uneven texture.
Why Jetting Happens
During normal filling, the melt spreads against the cavity wall and advances with a calm, fountain-shaped flow front. The mold wall guides the material, keeping the flow front stable. Jetting occurs when the melt loses contact with the mold surface during initial filling. The melt shoots from the gate like water from a hose, flies across open space, and freezes in a folded state. Later-arriving melt cannot fully fuse with those frozen folds.
In short, jetting occurs when the melt loses contact with the mold surface during initial filling.
Key Factors That Trigger Jetting
Multiple factors work together to cause jetting. The table below organizes them by priority.
| Factor Category | Specific Trigger | Why It Matters |
| Gate design | Gate aimed directly into open cavity/No flow redirection feature | No wall or core to guide the melt/Melt exits without touching steel |
| Injection speed | Excessively high first-stage speed | High momentum throws melt into open space |
| Temperature | Low melt temperature/Low mold temperature near gate | High viscosity resists spreading/Skin freezes too fast, folds remain |
| Part geometry | Abrupt thick-to-thin transition near gate/Long flow path starting cold | Sudden resistance change destabilizes flow/Melt front cools before flattening |
How to Avoid Jetting?
Avoiding jetting injection molding requires a layered strategy. The most effective actions target the root cause, not the symptoms.
Gate Design Optimization
The gate is the final transition into the cavity, so injection molding gate design directly affects melt direction, exit velocity, surface appearance, and jetting risk. In various types of plastic injection molding applications, proper gate optimization is particularly important for controlling issues such as jetting, flow marks, and uneven filling.
1.Changing the Gate Location
The gate location determines how the melt front enters the cavity and how it diffuses within the cavity. Placing the gate where the material can immediately impact the core pin, reinforcing ribs, or cavity walls helps dissipate flow energy and promotes uniform filling. An improperly placed gate can cause the melt to freely eject into open spaces, increasing the risk of jetting and surface defects.
2.Adding Flow Guides
Structures such as baffles or deflectors can be used to intentionally alter the flow path of the material entering the cavity. By forcing the melt to change direction earlier, the flow rate can be reduced, allowing the material to diffuse more evenly across the mold surface, thus improving the plastic mold parts consistency.
3. Increase Gate Size
A larger gate cross-sectional area reduces the shear rate, slowing the melt flow into the cavity. This helps achieve smoother flow and minimizes turbulence, especially when molding higher-viscosity materials or complex geometries.
4. Increase Gate Radius
Sharp edges at the gate accelerate melt flow and create unstable flow patterns. Introducing a smooth radius at the gate transition reduces abrupt changes in flow direction and allows for more controlled material entry.
Mold Design Improvements
While gate design is important, the overall mold structure also plays a crucial role in controlling flow behavior and ensuring stable production. Optimizing the entire mold system helps distribute pressure and temperature more evenly throughout the production process.
1.Optimizing the Runner System
A balanced runner system ensures that molten material reaches each cavity simultaneously with similar pressure conditions. This reduces localized pressure buildup near the gate and helps prevent uneven filling, especially in multi-cavity molds.
2.Controlling Cooling Near the Gate
The temperature around the gate affects the material’s solidification rate. If cooling is too rapid, the gate may freeze prematurely, restricting flow and causing defects. In some cases, localized heating or optimized cooling channel design is used to maintain proper flow conditions during filling.
Material Selection Considerations
Material properties significantly influence melt behavior during injection molding. Choosing the right material is crucial for minimizing flow defects and ensuring consistency across different injection molding processes.
1.Choose Materials with Higher Flow Rates
Materials with better flow rates fill complex or thin-walled plastic injection mold parts more easily, reducing the likelihood of flow stagnation or jetting. This is especially important for complex designs or parts with long runners.
2.Add Lubricants or Flow Modifiers
When the base material has a high viscosity, additives can be used to improve its flow properties. These modifiers reduce internal friction, allowing the melt to flow more smoothly through the gate and cavity.
3.Avoid Using Rapidly Cooling Materials
Materials that solidify too quickly create flow resistance, especially in parts with long runners or complex gate locations. Materials with slower cooling rates allow for longer processing times, resulting in fully stable filling and improved overall part quality.
Design Strategies to Prevent Jetting
Preventing defects during the plastic injection mold design phase is the most cost-effective approach. Professional injection molding manufacturers conduct thorough inspections before mold making to minimize the occurrence of mold defects such as jetting marks.
Gate placement
The gate must not be directly facing an open area and must always be aligned with a steel surface—the core, ribs, sidewalls, or ejector pins.
Flow direction control
A fan-shaped gate or overlapping gate is used to allow the melt to spread out before entering the cavity. At the same time, guide bumps or ribs are placed opposite the gate to change the direction of the melt.
FAQs
1.Is jetting only a cosmetic defect?
No. Jetting affects both appearance and structural integrity. The folded melt creates weak fusion lines and potential air traps inside the part. These hidden defects can reduce mechanical strength, leading to part failure under load or impact.
2.Can I fix jetting just by slowing down the injection speed?
Slowing down the first-stage injection speed helps, but it is not always a complete solution. If the gate design is the root cause — for example, a gate aimed directly into open space — process adjustments alone will not eliminate jetting permanently. The best approach combines gate optimization with proper process settings.
3.How do I tell the difference between jetting and a flow mark?
Jetting appears as curled, worm-like or snake-like folds, usually right at or near the gate. By comparison, flow lines in injection molding usually appear as wavy or rippled surface patterns farther along the flow path, often near ribs, bosses, or wall-thickness transitions.Jetting is a flow control failure at the gate exit; flow marks are usually caused by low temperature or hesitation further into the cavity.
4.Can simulation really predict jetting before cutting steel?
Yes. Mold flow analysis can help visualize melt-front behavior during filling and identify areas where gate direction, flow path, or geometry may increase jetting risk before tooling is finalized. If the animation displays a snake-like or free-stream jet exiting the gate, that is a clear prediction of jetting. Engineers can then adjust gate location, size, or add redirection features in the digital model — saving tooling rework costs and production delays.
Conclusion
Jetting can be reduced through proper flow control, gate design, and molding parameters. For projects requiring DFM, mold development, trials, and repeat production, plastic injection molding services from HingTung can help identify and address jetting risks early. Send your drawings or project requirements to the HingTung engineering team for a tailored molding solution.