Jetting in Injection Molding

Introduction

Jetting is a common injection molding defect that can affect both the appearance and performance of molded plastic parts.

It often appears as a snake-like, wavy, or irregular flow mark extending from the gate into the molded part. In many cases, the defect develops when molten plastic enters the cavity as a high-velocity stream instead of immediately forming a stable flow front against the mold surface.

Although jetting may initially look like a cosmetic problem, severe cases can create weak interfaces, inconsistent surface quality, or unacceptable parts.

Preventing jetting requires more than simply changing one machine setting. Gate design, part geometry, material behavior, mold temperature, and injection parameters all influence how the melt enters and fills the cavity.

This guide explains what causes jetting, how to identify it, and how engineers can reduce the risk through design, tooling, and process optimization.

What Is Jetting in Injection Molding?

During normal injection molding, molten plastic enters the cavity and spreads against the mold surface in a controlled flow pattern.

Ideally, the melt develops a stable fountain flow, where material at the center of the flow moves forward while the outer material contacts the cooler cavity walls.

Jetting occurs when the first melt entering the cavity travels forward as a narrow stream rather than immediately spreading across the mold surface.

The stream may bend, coil, or fold inside the cavity before the remaining melt surrounds it. Because the initial strand has already started cooling, it may not fully blend with the surrounding material.

The result can be a visible irregular line or pattern on the molded surface.

What Does Jetting Look Like?

Jetting usually begins close to the gate and follows the direction of initial material flow.

Typical visual characteristics include:

• Snake-like or worm-like flow lines
• Wavy streaks extending from the gate
• Irregular lines embedded in the surface
• Different gloss or texture along the flow path
• Visible boundaries where the initial melt stream meets later material

Jetting should not automatically be confused with weld lines, flow lines, burn marks, or gate blush.

The location and shape of the defect provide important clues.

For example, a winding line beginning directly downstream of the gate is more characteristic of jetting than a weld line created where two separate melt fronts meet.

Why Does Jetting Occur?

Jetting develops when the melt enters the cavity without establishing a controlled flow front.

Several design and processing conditions can contribute to this behavior.

1. Injection Speed Is Too High Near the Gate

Excessive initial injection speed is one of the most common contributors to jetting.

If the melt passes through the gate at very high velocity and enters a relatively open cavity, it may shoot across the cavity instead of attaching to a nearby mold wall.

This is especially important during the initial filling stage.

Reducing the entire injection speed is not always necessary. In some applications, a staged injection profile can use a slower initial velocity near the gate and increase speed after a stable flow front develops.

2. Gate Location Directs Melt into Open Space

Gate location strongly affects the way molten plastic enters the cavity.

A gate that directs the melt directly into a large open cavity can allow the polymer stream to travel freely before contacting a mold surface.

This increases jetting risk.

Where practical, the gate can be positioned so the incoming melt reaches a nearby wall or feature quickly, helping the flow spread more gradually through the cavity.

Gate location should therefore be evaluated during DFM and mold-flow review rather than treated only as a tooling detail.

3. Gate Size Is Too Restrictive

A small gate can increase melt velocity as the material passes into the cavity.

If the gate is too restrictive for the material, part geometry, and required flow rate, the resulting high-speed stream may promote jetting.

Potential improvements may include:

• Increasing gate cross-sectional area
• Using a wider gate
• Reviewing gate thickness
• Selecting a more suitable gate style

Gate changes must also consider gate vestige, shear, cycle time, automatic degating, cosmetic requirements, and part geometry.

4. Melt Temperature Is Too Low

When melt temperature is too low for the selected resin and process, viscosity increases and the material may not flow smoothly through the cavity.

The initial jet can also cool rapidly before being surrounded by later material, making the visible pattern more pronounced.

Increasing melt temperature within the material supplier's recommended processing range may improve flow behavior.

However, temperature should not be increased without considering material degradation, residence time, shear, and other processing requirements.

5. Mold Temperature Is Too Low

A cold mold surface can cause the initial melt stream to cool rapidly.

If the jetted strand freezes or develops a skin before the surrounding cavity is filled, the interface may remain visible in the finished part.

Appropriate mold temperature can support better surface replication and more consistent melt-front development.

The correct setting depends on the resin, part requirements, mold construction, and cycle-time targets.

6. Sudden Changes in Flow Area

Part geometry can also influence jetting.

When the melt passes from a narrow section through a gate or runner into a significantly larger cavity, the sudden expansion can reduce the support that normally helps maintain an organized flow front.

Potential problem areas include:

• Thin-to-thick transitions
• Gates entering deep open cavities
• Large internal volumes
• Abrupt changes in cross section

These conditions should be reviewed early because process adjustments alone may not completely correct geometry-driven jetting.

How Gate Design Influences Jetting

Gate design is one of the most important factors in jetting prevention.

Engineers should evaluate not only where the gate is located, but also how the melt exits the gate and where it travels immediately afterward.

A good gate strategy should help the polymer establish a controlled flow front as early as possible.

Depending on the component, possible approaches may include:

• Redirecting the gate toward a cavity wall
• Increasing gate dimensions
• Using a fan gate to distribute flow across a wider area
• Using an overlap or alternative gate configuration
• Avoiding direct high-velocity flow into open cavity space
• Smoothing abrupt transitions between gate and cavity

There is no single gate style that eliminates jetting in every design.

Gate selection must account for material, wall thickness, flow length, cosmetic surfaces, weld-line locations, filling pressure, and production requirements.

How to Troubleshoot Jetting

Effective troubleshooting should be systematic.

Changing multiple parameters at the same time can make it difficult to determine which factor actually improved the defect.

Step 1: Confirm the Defect

Examine:

• Where the mark starts
• Its relationship to the gate
• The direction of material flow
• Whether it occurs consistently
• Whether the defect changes with process settings

Confirming that the defect is truly jetting prevents unnecessary tooling or process changes.

Step 2: Review Initial Injection Speed

Evaluate the velocity used as the melt first passes through the gate.

If jetting is strongest directly downstream of the gate, test whether reducing the initial injection speed allows a more stable flow front to form.

A multi-stage velocity profile may provide better results than simply slowing the entire filling cycle.

Step 3: Review Melt and Mold Temperatures

Confirm that melt preparation and mold-temperature settings are appropriate for the selected polymer.

The goal is not simply to increase temperature, but to establish conditions that allow predictable flow without creating material degradation or excessive cycle time.

Step 4: Evaluate the Gate

If process adjustments provide only limited improvement, inspect:

• Gate location
• Gate dimensions
• Gate orientation
• Runner-to-gate transition
• Melt direction after entering the cavity

A tooling-related problem may require a gate modification rather than continued adjustment of machine settings.

Step 5: Review Part Geometry

Determine whether the gate feeds directly into a thick or open section.

Large changes in cross section or unsupported flow paths may create conditions where the melt can jet through the cavity.

DFM and mold-flow analysis can help identify these risks before tooling is finalized.

Jetting vs. Other Injection Molding Defects

Correct defect identification is critical because similar surface marks can have different root causes.

Jetting usually appears as a winding stream beginning near the gate.

Weld lines form where separate flow fronts reconnect.

Flow lines may appear as bands or patterns associated with changing melt velocity and cooling behavior.

Gate blush is typically concentrated around the gate area and can result from excessive shear or unfavorable entry conditions.

Burn marks generally involve localized dark or brown discoloration and may be associated with trapped and compressed gas or material degradation.

The defect location, pattern, gate relationship, and process history should therefore be considered together.

How to Prevent Jetting Before Production

The most effective jetting solution is often prevention during product and mold development.

Before tooling is released, engineering teams should review:

• Gate location and orientation
• Gate dimensions
• Runner design
• Wall-thickness transitions
• Flow direction
• Large cavity sections
• Expected injection velocity
• Material flow characteristics
• Mold-temperature requirements

For complex parts, mold-flow analysis can provide additional insight into filling behavior and potential problem areas.

This allows engineers to address filling risks while gate locations and tooling geometry can still be changed efficiently.

Why Jetting Should Be Treated as a System Problem

A common troubleshooting mistake is assuming that every molding defect has one isolated cause.

Jetting illustrates why injection molding must be considered as an integrated system.

Changing injection speed may improve one part but create a short shot somewhere else.

Increasing melt temperature may improve flow but affect material stability.

Enlarging a gate may reduce jetting but increase gate vestige or change cycle performance.

Moving the gate may solve the defect but relocate a weld line to a critical feature.

For this reason, successful optimization considers the interaction between:

Part Design → Material → Gate & Runner → Mold → Process → Part Quality

The objective is not simply to remove a visible line. It is to develop a stable molding process capable of consistently producing acceptable parts.

Need Engineering Support?

Jetting can often be reduced through process optimization, but persistent defects may indicate a deeper issue involving gate design, mold geometry, material selection, or part design.

AccuMolds supports injection molding projects from early DFM review through mold development and production.

Our engineering support includes:

• DFM analysis
• Plastic part design review
• Gate and runner optimization
• Mold-flow considerations
• Precision mold design and manufacturing
• Injection molding process optimization
• Prototype and production support

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