Shenzhen Alu Rapid Prototype Precision Co., Ltd.
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Jetting in Injection Molding: How to Find and Fix It
Jetting in injection molding happens when melt shoots through a gate as a narrow stream before spreading against the cavity walls. The stream can fold back on itself and leave a raised or visible wavy line as the rest of the cavity fills around it. On a molded cover, that line may start at a hidden edge gate and run far enough into the part to appear on the customer-facing surface.
The most revealing test is an early short shot, a deliberately incomplete fill. If the sample shows a narrow strand leaving the gate before the surrounding wall fills, test a slower injection stage at gate entry. If the part only fills cleanly at a setting that also creates short fills or other defects, review the gate and the surface the melt meets as it enters. Autodesk Moldflow's jetting troubleshooting guide
Tell jetting apart from other marks near the gate
Start with the shape and origin of the mark, then check when it appears. A photo of a finished part can suggest a cause. A series of early fills can show how the mark formed.
| Mark on the sample | What to look for during the trial |
|---|---|
| Narrow, crooked trail starting at the gate | Look for a thin strand in an early fill, with unfilled cavity space beside it. That pattern supports jetting. |
| Short streak or comet tail near the gate, especially after an interruption | Compare the first shots after restart with shots from a stable run. Check nozzle or hot runner conditions and, on a cold runner, whether the cold slug well captures the first material. |
| Line downstream of a hole, core or other obstacle | Check whether the flow splits at the obstruction and rejoins where the mark appears. That sequence supports a weld line. |
| Patch of altered gloss that hugs the gate | Look for a continuous flow front in early fills and check gate geometry and the entry speed. A change in gloss alone gives little evidence that a strand formed. |
For clear or glossy parts, inspect samples under the same lighting, background and viewing angle. Photograph the gate and the whole trail in the same frame. For a multicavity mold, label every sample with its cavity number. A mark confined to one cavity gives the toolmaker a reason to compare that gate and its runner branch with the others. BASF injection molding troubleshooter
Use a short shot series to locate the start of the defect
Save the baseline. Keep complete parts from a stable run and label the affected cavities. Record resin grade and lot, colorant, the injection speed stages, the transfer position, and the actual fill time and pressure readings available on the press.
Capture gate entry. Ask the processor for several early partial fills that stop just after the gate and then progressively farther into the cavity. Isolate filling from packing using the press team's approved trial procedure. Keep the material and thermal settings comparable across the series.
Compare the flow fronts. Look for a strand with open space on either side. Then inspect the next fill to see whether melt spreads around that strand. A broad front that stays against the cavity walls from the earliest observable fill points toward a different explanation for the surface mark.
Check the cavity pattern. Lay out partial fills by cavity and shot. If one cavity jets before others have even reached the gate, review runner balance as well as the local gate and cavity geometry.
Keep the short shots with the trial report. They help the toolmaker evaluate a proposed gate change against the fill pattern that produced the defect.
Adjust the speed at gate entry first
A slower screw speed as the melt crosses the gate can help it establish contact with the cavity walls. Once a stable front has formed, a faster stage can carry the melt through the rest of the cavity. This can preserve fill in thin or distant sections that would be harder to reach if the entire stroke ran slowly. Autodesk Moldflow guidance on injection profiles
Use the partial fills to relate screw position to gate entry and the point where the melt spreads. Record those positions with the press and mold setup because the same position can correspond to a different point in the fill on another machine or runner system. Compare actual speed and pressure where available. A press that reaches its pressure limit may deliver a slower fill than the programmed speed suggests.
Make one deliberate change at a time. For each candidate profile, collect the early short shot and full parts. Check the original jetting zone, the end of fill and any feature sensitive to the flow pattern. If the jet clears but a clip fails to fill or a weld line moves into a screw boss, include that outcome in the tooling decision.
Change the gate when the acceptable process range is too narrow
Look at the direction of the melt as it exits the gate. An edge gate may point directly into a thick pocket with room for a free stream to form. A gate aimed toward a nearby cavity wall can spread the melt sooner. A tab or fan gate may be suitable when the part has room for a broader entry and the trim operation can leave the required edge finish. Autodesk Moldflow's gate design remedies for jetting
Before changing steel, review where the gate mark will land, how it will be trimmed and where the new flow fronts will meet. Check how the change affects end-of-fill venting and packing. If the tool has a replaceable gate insert, ask whether the alternative can be trialed with a new insert. Keep the original geometry and trial results available for comparison.
Worked example with a clear cover
Consider a clear cover with a small edge gate feeding into a recessed pocket and a thin latch at the far end. A wavy mark crosses the viewing area. The first short shot shows a strand projecting into the pocket. Slowing the gate-entry stage clears the mark, but the latch fills inconsistently across repeated shots.
Before moving the gate, the team tries accelerating earlier once the flow has spread. If the latch still fills inconsistently, the next trial compares an insert that directs the melt toward a nearby wall. The trial uses the same resin and inspection conditions. The team checks the short-shot pattern, the appearance of the viewing area, the trim mark and latch fit. The revised gate earns approval only if the part meets all those requirements over the intended production settings.
What to document for sample approval
Mark the visible zone and agree on the lighting, viewing angle and acceptable surface appearance.
Label the short shots and full samples by cavity, tool revision and process setting.
Verify complete fill, critical dimensions and the fit or strength of any feature the changed flow could affect.
Check parts from a stable run and, where restart marks matter, parts made after a representative interruption.
Record the gate geometry, approved speed stages, and the range tested for appearance and function.
For a new mold or a tool revision, send the part CAD, resin grade, finish requirements and photos showing the gate and the full mark. If you have short shots, include the cavity IDs and the speed stages used for each one. Request an Aluprototype project review
Frequently asked questions
Can increasing hold pressure remove jetting?
Jetting forms during filling. Use an early short shot and a gate-entry speed trial to investigate the cause. Evaluate holding conditions separately for the part's dimensions and appearance.
Can jetting affect part strength?
The strand and surrounding melt may join poorly in the affected area. If the mark crosses a clip, boss or loaded wall, include a functional test in sample approval. The surface appearance alone gives limited evidence of strength. Autodesk Moldflow's explanation of jetting defects