Understanding the flow pattern of injection molding is fundamental to producing defect-free plastic parts. The way molten plastic flows through the runner system, gate, and cavity determines fill balance, weld line location, fiber orientation, residual stress, and ultimately part quality. This guide explains the key flow patterns in injection molding, what drives them, and how mold designers and process engineers use flow knowledge to prevent defects.
Further Reading
For neutral technical background, see injection molding background.
What Is Flow Pattern in Injection Molding?
Flow pattern refers to the path and behavior of molten plastic as it moves through the mold cavity during the filling stage. Unlike water flowing through a pipe, thermoplastic melt behaves as a non-Newtonian, viscoelastic fluid — its viscosity changes with shear rate, temperature, and pressure, creating complex and sometimes counterintuitive flow behavior.
The three most important flow concepts in injection molding are:
- Fountain flow — the dominant flow mechanism inside the cavity
- Flow front advancement — how the melt front progresses to fill the cavity
- Jetting and hesitation — problematic flow patterns that cause defects
1. Fountain Flow: The Primary Flow Mechanism
The most important flow pattern in injection molding cavity fill is fountain flow (also called plug flow or fountain effect). This is the fundamental mechanism by which molten plastic fills a mold cavity:
- Melt enters the cavity and flows forward in the core of the flow channel
- At the flow front, the melt decelerates, spreads outward, and folds back toward the cold mold wall — like a fountain falling outward
- The material touching the mold wall instantly freezes, forming a thin solid skin layer
- Fresh hot melt continuously flows through the center, keeping the core molten throughout fill
- The result is a layered structure — frozen skin on the outside, oriented shear layer beneath, and relatively relaxed core
Why it matters: Fountain flow determines the molecular orientation of the finished part. Material in the shear layer near the wall is highly oriented along the flow direction — increasing tensile strength in that direction but reducing transverse strength. This anisotropy must be considered in structural design, especially for glass-filled materials.
2. Flow Front Advancement Patterns
How the melt front advances across the cavity depends on gate location, part geometry, and wall thickness variations. The main advancement patterns are:
Radial Flow
When the gate is located at the center of a circular or symmetric part, melt flows outward in a radial pattern — like ripples from a stone dropped in water. Radial flow produces:
- Uniform fill with balanced pressure distribution
- Molecular orientation in the radial direction
- No weld lines (single gate, no converging flow fronts)
- Best dimensional stability and roundness for circular parts
Used for: Gear wheels, caps, discs, round housings — typically using a 3-plate center gate or hot tip gate.
Linear Flow
When the gate is located at one end of a long, flat part, melt flows in a predominantly linear pattern from gate to far end. Linear flow produces:
- Strong orientation along the flow direction
- Predictable weld line positions (at the end of fill or around holes)
- Higher pressure drop along the flow length — risk of short shots at extremities
Used for: Long flat panels, covers, bars — typically using edge or film gates.
Converging Flow
When flow fronts from multiple gates or around obstructions (holes, inserts) meet, they create weld lines (also called knit lines or meld lines):
- Weld line: Two flow fronts meet head-on — material folds together with minimal intermixing. Produces a visible seam and a structural weak point (strength typically 60–90% of base material)
- Meld line: Two flow fronts merge at an acute angle — better intermixing, stronger than a weld line but still visible
Weld lines cannot be eliminated when multiple gates are used or when the part has holes. They can be repositioned (by adjusting gate location) or strengthened (by increasing melt temperature, injection speed, or adding a weld line trap).
Diverging Flow
When melt flows through a restriction and then expands into a wider section, it diverges. This can cause hesitation — the melt front slows or stops in thin sections while faster-filling thick sections continue to advance. Hesitation leads to:
- Premature freezing of the melt front in thin sections
- Short shots or incomplete fill
- Poor surface finish and witness marks at hesitation boundaries
3. Jetting: A Problematic Flow Pattern
Jetting occurs when melt shoots through a small gate into a large open cavity at high velocity without immediately contacting the cavity wall. Instead of the orderly fountain flow, a thin stream of plastic “jets” across the cavity and folds back on itself as the cavity fills around it.
The result is a visible, snake-like surface defect — often called a worm track or jet mark — that cannot be polished away because it is embedded in the part structure.
Causes of Jetting
- Gate too small relative to cavity volume
- Injection speed too high through the gate
- Gate positioned so melt enters open space rather than impinging on a wall or core
Solutions
- Increase gate size to reduce melt velocity through the gate
- Reposition gate to aim melt at a wall or core pin — disrupting the jet and forcing fountain flow
- Reduce injection speed during the initial fill phase (gate seal profile)
- Use a fan gate or film gate instead of a pin gate for wide, flat parts
4. Race-Tracking: Preferential Flow in Thick Sections
Race-tracking occurs when melt preferentially flows through thick sections (which have lower flow resistance) faster than through adjacent thin sections. Like a race car taking the shortest, widest path around a track, the melt “races” along the path of least resistance.
Race-tracking causes:
- Air entrapment in thin sections bypassed by the racing melt front
- Burn marks from compressed, heated air that cannot escape
- Short shots in areas the melt reaches last
- Unbalanced fill leading to differential shrinkage and warping
Solution: Redesign part for more uniform wall thickness (target <25% variation). If thickness variation is unavoidable, add vents in the areas last to fill and optimize gate location using mold flow analysis.
5. Flow Balance in Multi-Cavity Molds
In multi-cavity molds, achieving identical flow patterns in every cavity simultaneously is critical for part consistency. Unbalanced flow causes:
- Different fill times between cavities — some overpacked, some underpacked
- Dimensional variation between cavities
- Defects in some cavities (flash, sink marks) while others are acceptable
Achieving Flow Balance
- Geometrically balanced runner systems (H-pattern runners) ensure equal flow path length and cross-section to every cavity
- Melt flipper technology (static mixers in runners) rotates the melt to correct the thermal imbalance caused by shear heating in curved runners
- Mold flow simulation (Moldex3D, Moldflow) predicts fill imbalance before steel is cut and allows gate and runner optimization
- Cavity pressure sensors measure real-time fill in each cavity during production, enabling closed-loop process correction
How Mold Flow Analysis Predicts Flow Patterns
Modern mold flow analysis software (Autodesk Moldflow, Moldex3D) simulates the complete filling, packing, and cooling cycle before a mold is manufactured. Key outputs include:
- Fill time animation: Shows how the melt front advances through the cavity, revealing race-tracking, hesitation, and air trap locations
- Pressure distribution: Identifies areas of high pressure drop — predicting short shot risk and machine tonnage requirements
- Weld line and air trap locations: Predicts exactly where weld lines will form so gates can be repositioned to move them to non-critical areas
- Fiber orientation: For glass-filled materials, shows the orientation of reinforcing fibers — directly impacting structural anisotropy
- Shear stress and temperature distribution: Identifies areas prone to material degradation, burn marks, or excessive residual stress
Frequently Asked Questions
What is the flow pattern of injection molding?
The primary flow pattern in injection molding is fountain flow — molten plastic flows forward through the cavity center, spreads outward at the flow front, and freezes against the cold mold wall to form a solid skin. The overall cavity fill pattern (radial, linear, or converging) depends on gate location, part geometry, and wall thickness distribution.
What causes weld lines in injection molding?
Weld lines form wherever two flow fronts meet — either from multiple gates converging, or where melt flows around a hole or insert and rejoins on the other side. The strength of a weld line depends on melt temperature, injection speed, and material — typically 60–90% of the base material strength. Gate repositioning can move weld lines to non-critical areas but rarely eliminates them entirely.
What is jetting in injection molding?
Jetting occurs when melt shoots through a small gate at high velocity into a large open cavity without immediately contacting a wall. Instead of orderly fountain flow, a thin plastic stream folds across the cavity, creating a visible snake-like surface defect. It is prevented by enlarging the gate, repositioning the gate to aim at a wall, or reducing initial injection speed.
How does wall thickness affect flow in injection molding?
Thicker walls have lower flow resistance and fill faster — causing race-tracking when adjacent to thin sections. Thinner walls freeze sooner, causing hesitation and potential short shots. Uniform wall thickness (ideally within 25% variation) produces the most balanced, predictable fill pattern and the fewest flow-related defects.
What is race-tracking in injection molding?
Race-tracking is the preferential flow of melt through thick sections of a part, bypassing adjacent thin sections. It causes air entrapment (burn marks) in bypassed areas and uneven fill that leads to differential shrinkage and warping. It is addressed through part redesign for uniform wall thickness, strategic gate placement, and proper venting of last-to-fill areas.
How does gate location affect flow pattern?
Gate location is the single most influential decision in determining cavity flow pattern. A center gate on a round part creates balanced radial flow. An end gate on a long part creates linear flow. Multiple gates allow filling of large or complex cavities but create weld lines where flow fronts converge. Mold flow analysis is used to optimize gate location before tooling is cut.
Summary
The flow pattern of injection molding is governed by fountain flow at the microscale and fill front advancement — radial, linear, converging, or diverging — at the part scale. Understanding these patterns allows engineers to predict and prevent defects including weld lines, jetting, race-tracking, air traps, and hesitation. Mold flow analysis software makes it possible to optimize gate location, runner design, and process parameters before a single piece of steel is machined — saving significant time, cost, and material waste in the tooling development process.