Flow Lines and Jetting in Injection Molding: Causes and Fixes

Flow lines in injection molding are responsible for a large share of cosmetic rejects on consumer-facing parts, and in our experience they account for 20 to 35 percent of first-article failures on Class A surface programs. The good news: most cases resolve without cutting steel. Gate relocation, injection speed profiling, and melt temperature adjustments fix the majority of flow line and jetting problems before you spend a dollar on tooling changes.

What Causes Flow Lines in Injection Molding

Flow lines form when the melt front hesitates, splits, or cools unevenly as it travels through the cavity. The result is a visible surface streak, ripple, or wave pattern on the finished part. These marks appear because the outer skin of the flow front freezes at a different rate than the core, locking in the waviness before packing pressure can smooth it out.

The most common root causes fall into four categories: gate placement, injection speed profile, melt and mold temperature, and wall thickness transitions. A cosmetic flow mark near the gate almost always points to a gate that is too small, a melt temperature that is too low, or an injection speed that is too fast for the gate land geometry. A mark that appears mid-cavity usually traces back to a hesitation zone or a wall thickness step.

• Gate too small for the required volumetric flow rate
• Injection speed constant across fill when it should be profiled
• Melt temperature below the resin manufacturer’s recommended processing window
• Wall thickness change of more than 25 percent over a short distance
• Cold mold surface, typically below 100 degrees F for amorphous resins
• Long flow length relative to wall thickness, producing a flow length to wall ratio above 200:1

Wall thickness ratio is worth highlighting. SPI mold classification guidelines and most resin datasheets recommend keeping wall transitions below a 2:1 ratio. When a 0.060-inch wall steps directly to a 0.120-inch wall, the thin section freezes fast, the thick section stays fluid, and you get a visible surface streak at the transition line every time.

Jetting Injection Molding: A Separate but Related Problem

Jetting in injection molding looks like a squiggly worm or snake track frozen into the part surface. It is not the same as a standard flow line, though the two are often confused. Jetting happens when molten plastic shoots through the gate as a free-flowing jet rather than spreading as a controlled, fountain-flow front. The jet hits the far wall, folds back on itself, and freezes into the worm pattern before fresh melt can bond to it.

Jetting is almost always a gate problem. Specifically, it occurs when the gate is positioned so that the initial shot of material enters free space rather than impinging directly on a wall or core. A submarine gate aimed at an open cavity is a classic setup for jetting. The fix is gate relocation or a change to a fan gate or edge gate that spreads the melt immediately on entry.

Injection speed also drives jetting. According to Moldflow simulation data published by Autodesk, reducing fill speed by 30 to 50 percent at the start of injection, then ramping up, eliminates jetting in a large share of cases without any tooling modification. We run a two-stage speed profile as the first corrective step on any jetting complaint before we discuss gate changes with the tool shop.

The Race Track Effect and How It Creates Surface Defects

The race track effect in a mold occurs when plastic fills a thick section much faster than an adjacent thin section because flow resistance is lower in the thick area. The melt races ahead through the easy path, wraps around, and traps air or creates a weld line in the thin section. The trapped gas burns, and the rapid differential fill produces a visible surface defect that looks like a flow line but has a slightly different character, often with a brown or silver tint from gas compression.

The race track effect is common on parts with ribs, bosses, or thick perimeter walls connected to a thin skin. A 0.080-inch nominal wall with a 0.200-inch boss attached to it is a textbook race track setup. The melt hits the boss, accelerates through it, laps around the thin skin, and you end up with a defect on the skin surface directly opposite the boss.

Correcting the race track effect requires one or more of the following actions: add a vent at the last point to fill, reduce the boss wall to 60 percent of nominal wall thickness per standard rib-to-wall design rules, or reposition the gate so both sections fill simultaneously. Simulation identifies the last-fill zone in about two hours; cutting and trialing a vent takes one shift. Do the simulation first.

Diagnosing Flow Lines: A Process-First Approach

Before anyone touches steel, run a systematic process sweep. The table below shows the parameters we adjust first, the direction of change, and the expected result. This sequence resolves approximately 60 percent of flow line complaints in our shops without a tooling change, based on our internal project data from 2019 to 2024.

Parameter	Adjustment Direction	Target Range	Expected Effect on Flow Lines
Melt temperature	Increase 10 to 20 degrees F	Per resin datasheet, upper 25% of window	Reduces skin freeze rate, smooths flow front
Mold temperature	Increase 10 to 30 degrees F	100 to 140 degrees F for ABS; 180 to 220 degrees F for PC	Keeps flow front fluid longer, reduces surface streaks
Injection speed (stage 1)	Slow by 20 to 40 percent at gate	Profile: slow at gate, ramp to fast mid-fill	Eliminates jetting, reduces gate blush
Injection speed (stage 2)	Increase once melt impinges on wall	80 to 95 percent of maximum machine speed	Pushes flow front evenly, reduces hesitation marks
Pack pressure	Increase 5 to 10 percent	60 to 80 percent of fill pressure	Smooths surface texture at flow front stop points
Gate size	Increase land or width	Minimum gate area = shot volume divided by fill time divided by max shear rate	Reduces shear-induced surface streaks at gate

Work through this table in the order listed. Melt and mold temperature are free to change. Speed profiling takes 30 minutes on the machine controller. Gate resizing is the last process lever before you commit to a tooling modification. If you have gone through every row in this table and the cosmetic flow mark persists, the problem is likely geometric and needs a design or tooling fix.

Tooling Fixes When Process Adjustments Are Not Enough

When process optimization fails, the tooling options are gate relocation, gate type change, or wall section redesign. Gate relocation is the most common fix. Moving a gate from a thin section to a thick section, or from an open cavity position to a direct impingement position, resolves both flow lines and jetting in a single modification. Cost to relocate a gate on a P20 tool runs $800 to $2,500 depending on whether you are plugging and re-drilling or adding an insert.

Changing gate type from a pinpoint to a fan gate spreads the melt over a wider front immediately at entry. This change typically costs $1,200 to $3,500 for a new gate insert and eliminates jetting on parts where a narrow gate feeds a wide open cavity. If the part has a visible gate vestige concern, a submarine gate aimed at a rib rather than open space gives you the same flow benefit with a sub-surface break point.

Wall section redesign is the most expensive fix, often requiring a cavity insert swap or a new cavity block. On an H13 hardened tool, a cavity insert to smooth a wall transition typically runs $4,000 to $9,000 and adds 3 to 4 weeks. Weigh that against the cost of 100-percent cosmetic inspection or rework on production parts before deciding. On high-volume programs, a $6,000 insert pays back in two to three weeks of avoided rework labor.

Material and Surface Finish Considerations

Resin choice affects flow line severity. High-viscosity resins like PC and filled nylons show flow lines more readily than low-viscosity resins like HDPE or unfilled PP. Melt flow index matters: a PC with an MFI of 6 g/10 min at 300 degrees C and 1.2 kg will produce far more visible surface streaks than a PC with an MFI of 22 g/10 min under the same conditions. If your cosmetic requirement is tight and your resin is marginal, ask your material supplier for a higher-flow grade before cutting steel.

Mold surface finish interacts with flow lines in a way most engineers underestimate. A high-gloss SPI A1 or A2 finish amplifies every flow mark because specular reflection makes waviness visible. A SPI B2 or C1 texture scatters light and hides minor flow variations. If a part has a functional cosmetic requirement rather than a strict gloss level, specifying a light texture on the tool can eliminate the customer complaint without any process change. This costs $300 to $800 to apply and zero machine time.

Frequently Asked Questions

What is the difference between a flow line and a weld line in injection molding?

A flow line forms along a single flow front as it cools and moves through the cavity. A weld line forms where two separate flow fronts meet and bond, or fail to bond fully. Both are surface defects, but weld lines also reduce structural strength at the meeting point, sometimes by 10 to 50 percent depending on resin and temperature at the meeting zone. Flow lines are primarily cosmetic; weld lines can be both cosmetic and structural.

Can jetting injection molding defects be fixed without changing the tool?

Yes, in most cases. Slowing injection speed by 30 to 50 percent at the start of fill, then ramping up, eliminates jetting without tooling changes in a significant share of cases. Increasing melt temperature by 10 to 20 degrees F and raising mold temperature also helps. If process changes do not resolve jetting after a full sweep, gate relocation is the most reliable tooling fix.

How do I tell if a surface defect is caused by the race track effect versus a simple flow line?

The race track effect produces a defect near the last-fill zone of a thin section adjacent to a thick section, often with a brown or silver discoloration from trapped gas. A simple flow line follows the direction of flow and has no discoloration. Run a short shot study: stop fill at 50 percent and 75 percent of cavity volume to map the fill pattern. If the thick section is significantly ahead of the thin section at 50 percent fill, you have a race track condition.

What draft angle is needed to avoid drag marks that look like flow lines?

Drag marks from ejection are sometimes mistaken for flow lines. Per standard tooling practice, 1 degree of draft per inch of draw depth is the minimum for smooth surfaces on P20 steel. For textured surfaces, add 1.5 degrees per 0.001 inch of texture depth per the SPI texture depth to draft ratio guideline. Insufficient draft produces scratching on ejection, which creates streaks parallel to the draw direction that look like surface streaks from flow.

Does colorant affect flow line visibility?

Yes, significantly. High-contrast colors, especially black and dark blues, show flow lines more readily than medium grays or off-whites because minor surface waviness creates visible light and dark bands. Metallic or pearlescent colorants amplify the problem because the flake orientation changes with the flow pattern. If your color spec is dark and your cosmetic requirement is Class A, plan for more aggressive process development time, typically 20 to 40 percent more development shots, and consider a B2 or finer texture.