Jetting occurs when a fast strand of melt enters the cavity without attaching to a nearby surface, then folds or is surrounded by later flow. The visible rope-like or serpentine mark is evidence of unstable early flow. Correcting it requires control of the first material leaving the gate, not a random combination of pressure and temperature changes.
1. Confirm That the Mark Is Jetting
Distinguish jetting from a weld line, flow mark, splay, burn mark or contamination. Record the mark’s shape, gate relationship, cavity number and repeatability. A short-shot study is especially useful because a full part can hide the early flow path.
2. Map the First Flow After the Gate
Run controlled short shots while holding material preparation and key settings constant. Determine whether the stream shoots into open space, impinges on a wall, hesitates at a thickness transition or accelerates through a restrictive gate. Compare cavities before assuming the part design is the only cause.
3. Review Gate Type, Size and Direction
A gate that directs melt against a core or cavity wall can help the flow fan out and establish a stable front. A gate aimed into a large open volume may allow a free jet. Gate changes must also account for vestige, shear, appearance, packing, freeze time, automatic degating and tool access.
| Test | What it can reveal | Limitation |
|---|---|---|
| Reduced initial velocity | Whether the first stream can attach before speed increases | Too slow may cause hesitation or premature cooling |
| Gate impingement review | Whether geometry can spread the incoming melt | May move the mark or create a new cosmetic issue |
| Melt and mold temperature check | Whether actual temperatures differ from the validated range | Temperature alone may not correct an open-space gate path |
| Short-shot comparison by cavity | Whether runner balance or a local restriction contributes | Requires controlled shot increments and records |
4. Test a Staged Fill-Velocity Profile
A slower initial segment followed by a faster fill can help establish a stable front, but the transition position should be based on screw position or cavity evidence. Changing the whole fill speed may trade jetting for hesitation, weld-line weakness or an incomplete part.
5. Check Material and Actual Temperature
Confirm the exact grade, moisture condition, residence history, colorant and regrind status. Measure actual melt and mold temperature using methods suitable for the process. Work within the material supplier’s processing guidance; increasing temperature without evidence can degrade the resin or extend the cycle.
6. Decide Whether Steel Must Change
If process development cannot produce a stable flow front with acceptable margin, review gate relocation, gate enlargement, a fan or tab feature, local wall transition or an impingement surface. Use steel-safe options where possible. A DFM review should consider the effect on appearance, packing and downstream trimming.
Illustrative Engineering Scenario
This is an illustrative example, not a customer case. A housing shows a curved mark beginning at a side gate. Short shots show a narrow strand crossing an open cavity before touching the opposite wall. A staged initial fill improves but does not create enough process margin. The team compares two gate concepts and chooses one that directs the first melt onto a noncosmetic wall. Trial acceptance then checks the mark, fill balance, gate vestige and dimensions. The scenario demonstrates a decision sequence; it does not prescribe that gate relocation is correct for every part.
Validation After the Mark Improves
- Inspect all cavities and agreed cosmetic zones under defined lighting.
- Confirm part weight and dimensions have not shifted outside requirements.
- Check weld lines, trapped gas, flash, residual stress and gate vestige.
- Demonstrate the process at justified settings, not only one favorable shot.
- Record the change and update setup or maintenance documents.
Jetting Questions
Can more injection pressure eliminate jetting?
Pressure is not the primary decision variable for every jetting problem. The first-flow path, gate geometry and velocity profile should be understood before increasing demand on the process.
Is jetting only cosmetic?
Sometimes, but the folded flow can also affect weld integrity or local structure. Evaluate the mark against the part’s actual load and acceptance criteria.
What information helps remote troubleshooting?
Provide gate and runner layout, short-shot images, cavity identification, material grade, drying record, actual temperatures, fill profile and a defect map.
For persistent jetting, send photographs and process details through CKMOLD’s contact page. If a steel change is likely, review mold repair and modification options.