Jetting in injection molding is one of those defects you can often spot before the part even cools. The melt shoots across the cavity like a thin rope, cools on contact, and leaves a distorted surface pattern that won’t blend no matter how much packing pressure you add later. If you’ve worked with thin-wall housings or stiff GF-nylon parts, you’ve probably seen this behavior more than once.Among injection molding defects, jetting stands out because it forms the moment the melt loses contact with the steel and can no longer behave like a continuous front.
Across the jetting injection molding cases we’ve handled at Jeek, the same pattern shows up repeatedly: the melt loses contact with the steel, solidifies mid-flight, and refuses to re-melt back into the flow once it lands. Whenever the melt front stops behaving like a front, jetting becomes almost guaranteed.
How Jetting Injection Molding Actually Happens
Jetting starts at the gate. When the melt exits the gate too fast or without a surface to follow, it doesn’t spread—it coils. Some engineers describe the pattern as “rope marks,” and the description isn’t far off.
Inside the cavity, the melt wants a surface to wet. Without one, the stream cools instantly and forms a thin, hardened strand. The rest of the melt arrives too late to fix it, so the defect stays locked in.
You’ve probably seen this in first-shot conditions: cold mold, cold steel, and a narrow gate firing straight into empty space. That combination practically invites jetting.
Why Jetting Affects Surface and Dimensional Quality
A jetting defect affects more than appearance. Once that first rope cools, it disrupts the way later melt packs behind it. This often leads to slight sink variations, flow hesitation, or inconsistent gloss levels across the surface.
On GF-nylon, the issue becomes more obvious. Fibers align with the early jet stream, and that alignment is nearly impossible to correct afterward. If you’ve molded GF parts long enough, none of this comes as a surprise.
Common Conditions That Trigger Jetting
A gate aimed directly into open space
This is the classic cause. When the melt shoots into nothing, it behaves like a free-flying jet instead of a controlled front. Directing the melt along a wall or surface immediately stabilizes the flow. Even a small deflection surface can completely change the result.
The gate does not guide the melt along steel
A controlled surface gives the melt something to wet. When it follows steel, the front stays cohesive rather than breaking apart. Parts with a flat pad, rib wall, or slight radius near the gate almost always show better flow behavior.
Abrupt thick-to-thin transitions near the gate
These create acceleration zones. The melt speeds up, shoots through the thin section, and loses contact with the boundary. This is where you’ll often see jetting appear even when melt temperature is within spec.
Long flow paths starting colder than they should
When steel near the gate is cooler than downstream areas, the initial rope freezes instantly. As the flow continues, temperature recovers—but the first defect is already locked in. If you’ve ever seen jetting soften halfway through the shift, it’s usually mold temperature catching up.
Process Conditions That Commonly Show Abnormal Jetting Behavior
In sampling, jetting becomes most obvious when at least one of these parameters drifts outside a stable window:
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Mold temperature 5–10°C below target near the gate area
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First-stage speed too steep in the first 5–15 mm of stroke
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Melt temperature dropping 10–15°C from extended residence time
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Gate shear rate exceeding material limits, especially on PA66-GF30
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Holding pressure timing too late, causing the early rope to freeze before the front stabilizes
Engineering Adjustments That Actually Work
Increase melt temperature
Raising melt temperature (within material limits) slows early freeze-off and helps the melt spread instead of rope. This is often the quickest improvement you’ll see on the press.
Increase mold temperature near the gate
Localized mold heating lets the melt follow the surface instead of shocking into a solid strand. Even a small boost on steel temperature near the gate can reduce jet definition.
Apply a softer initial fill profile
A steep injection speed curve makes the melt shoot forward before it has a surface to wet. A gentler first-stage speed gives the front time to attach to the steel before higher velocity kicks in.
Modify or polish the gate
A sharp or rough gate outlet disrupts the melt stream. Polishing the gate land or smoothing the gate edge can noticeably reduce turbulence and prevent the initial jet.
Add or adjust gate radius
A small radius at the gate exit guides the melt, essentially “aiming” it along a surface instead of into open air. Many molders underestimate how much a 0.2–0.5 mm radius can change the flow front.
Lengthen the gate land when appropriate
A slightly longer gate land stabilizes the flow by straightening the melt stream before it enters the cavity. Too short, and the melt exits unpredictably. Too long, and you create shear heating—but the right length is often the difference between jetting and clean flow.
Material Behavior and Jetting Sensitivity
ABS often exaggerates jetting visually due to its surface characteristics. PC may show almost nothing visually—but the dimensional disturbance is still there. GF-nylon tends to produce the most obvious jetting because fibers reinforce the early rope pattern.
Moisture plays a role in nylon as well. Over-dried nylon becomes more sensitive to jetting because the melt stiffens faster and refuses to relax once it hits steel.
Design Choices That Reduce Jetting Before the Tool Is Even Built
A gate should rarely point into empty space. Providing a surface—even a small transition pad—gives the melt the control it needs. Designers who handle flow simulation regularly can usually call out jetting zones on sight: anywhere the melt loses steel contact or accelerates suddenly is a risk.
Adding gradual transitions, removing sudden thickness drops, or slightly redirecting gate aim often reduces jetting more than any processing tweak.
Simulation and Flow Analysis
Flow simulation is especially accurate for jetting because jetting happens early in the fill, where simulation data is most precise. Simulation clearly shows melt-front detachment and acceleration zones, and these usually match molded parts almost exactly.
What Flow Simulation Usually Shows When Jetting Starts
On most Moldflow runs, jetting shows up in the first 2–8% of fill. The melt stream detaches from the steel, accelerates, and forms a narrow, high-velocity core. The fringe plot typically shows:
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A sharp velocity spike at the gate outlet
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A low-shear outer zone where the melt never touches the surface
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A short “dead-air” pocket just ahead of the stream
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Early freeze at the same location where the jet later appears on the molded part
If the simulation is run with temperature traces, the rope region usually drops 10–20°C below the incoming melt within milliseconds. That gap is exactly why the strand refuses to re-melt once packing pressure arrives.
FAQs: Jetting Injection Molding
What causes jetting in injection molding?
A high-velocity melt stream exiting the gate without a surface to follow.
How do you avoid jetting in injection molding?
Guide the melt along steel, soften the initial fill speed, raise melt or mold temperature, and optimize gate geometry.
Is jetting only cosmetic?
Often it is cosmetic, but on stiff materials like PC or GF-nylon, it can influence local packing and fiber orientation.
Does gate size affect jetting?
Yes. A narrow or sharp gate often shoots melt too aggressively, while a slightly larger or smoother gate encourages stable flow.
Last!
If you’re troubleshooting jetting on a new tool or preparing for a production transfer, Jeek can review the part geometry, gate strategy, and process window.
You can submit the CAD or drawing for a technical assessment and a detailed quote.