Weld Lines in Injection Molding for Thin-Wall Enclosures

Concept render

Weld lines, also called knit lines, form where two flow fronts of molten plastic meet and do not fully fuse. Every through-hole and every pair of gates creates one. They are often cosmetic, but near a screw boss they can crack. Gate placement, uniform walls and a mold-flow review move them to less critical areas.

A small device enclosure has many holes: microphone ports, a speaker grille, an LED window, a button, a charging port and several screw bosses. Each one splits the plastic flow in the mold and leaves a line where the flow meets again. Thin walls add a second problem, warpage, which can lift a lid off its gasket. This guide covers causes, prevention rules and the mold-flow review. It contains no test result and no Duobay price.

What are weld lines in injection molding?

Resin cools as it enters the mold, so the leading edge of the flow is the coolest. When the flow splits around an obstacle and meets again, the two cool fronts may not fully meld. The seam is a weld line, also called a knit line (Protolabs, knit lines, as of November 2026).

Geometry sets the location. There is a knit line downstream of every hole that goes through the part, and one between every two gates. Visibility depends on the resin, resin temperature, mold temperature and fill speed, and ranges from nearly invisible to crack-like (Protolabs, minimizing knit lines, as of November 2026).

Protolabs calls knit lines "as a rule" purely cosmetic, and warns they can become a failure point in a highly stressed area such as the head of a screw (Protolabs, defects guide, as of November 2026). Beaumont adds that a butt weld with little molecular orientation across it is weak, and that air can collect at the weld (Beaumont, weld lines, as of November 2026).

Where do knit lines form on a device enclosure?

The snap-hook risk below is our engineering reading and is labeled as such.

Sources, as of November 2026: holes, gates, additives, filler content and fiber length, bosses, edges and fibers not crossing the line, texture and gate count, sliding shutoff snaps, drilling after molding.

Textured faces need care. The cooler resin at the flow front is not pressed into the mold texture as well as the hotter resin behind it. On a thin part that cools fast, process settings may not fix it (Protolabs, cosmetic appearance, as of November 2026). Keep knit lines off a textured front face.

Are knit lines a cosmetic or a strength problem near screw bosses and snap fits?

They can be either. Flow wraps around a boss and meets behind it. If the boss is near the part edge, the knit line is very short and little surface holds the faces together. A screw then acts as a wedge, and the line can turn into a crack (Protolabs, knit lines, as of November 2026).

Screw bosses

• Move bosses inward from exterior walls. Protolabs advises this for sink (screw bosses, as of November 2026), and by the knit-line guide it also lengthens the line behind the boss.
• Support the boss with gussets. Tie it to a nearby wall with ribs, never with a solid fill (boss design, as of November 2026).
• Take hole size and boss wall from the screw maker's guidelines, so the screw engages without splitting the post (screw bosses, as of November 2026).
• Where a screw head puts high stress on a hole, skip the molded hole and drill it afterward (defects guide, as of November 2026).

Snap fits

Protolabs calls a sliding shutoff probably the best way to mold a snap hook, and the method needs a hole in the part beneath the snap (Protolabs, complex features, as of November 2026). That hole has a knit line downstream, and the hook flexes every time the cover opens. No source we read measured knit line strength in a snap hook, so check it on molded samples. In a glass-filled grade little or no fiber crosses the line, so the seam does not get the benefit of fiber reinforcement (Protolabs, knit lines, as of November 2026). A snap hook is also an undercut. Our undercuts guide compares shutoffs, side actions and other ways to form it.

How do gate location and wall thickness prevent weld lines?

You cannot remove every knit line from a part with holes. You can control how many there are, how strong they are and where they land.

Gate location

Protolabs lists four causes of knit lines: holes, multiple gates, extreme geometry and a poorly placed single gate. Its fixes are to remove holes, change gate locations, use the fewest gates possible and correct geometry that forces poor flow (cosmetic appearance guide, as of November 2026). A hot tip gate near the part center shortens the distance resin travels. A tab gate sits on the parting line, usually in a thicker wall, and suits filled resins (Protolabs, gates, as of November 2026). Beaumont adds that a gate placed so flow keeps moving through the weld after it forms, or a dump outside the cavity, strengthens the weld (Beaumont, as of November 2026).

Wall thickness

Thicker walls slow cooling, which improves knit line strength and appearance (Protolabs, as of November 2026). Too thick and you get sink. These are the published rules we could source:

These are general guidelines. Your molder may set other limits for a given resin or tool, so write the agreed values into the drawing. Ribs and bosses need draft as well as the right thickness. Our draft angle guide covers the values for each feature.

Resin choice

Protolabs names HDPE, LDPE, polypropylene and nylon as resins that flow well through thin areas, and says polycarbonates have difficulty filling thin walls (thin-wall tips, as of November 2026). Xometry adds higher injection pressure, hot runners, a better-flowing resin and changes to gate count and placement (Xometry, defects, as of November 2026).

What causes warpage in injection molding of thin-wall enclosures?

Warpage comes from uneven cooling. Resin against the mold surface cools first and the core cools later, which builds internal stress (Xometry, ribs, as of November 2026). Xometry names rapid cooling as a major cause and notes that shrinkage increases with wall thickness (Xometry, defects, as of November 2026). Design causes from the sources:

• Walls too thin for the resin. The part is likely to curl up like a potato chip (Protolabs defects guide, as of November 2026).
• Non-uniform walls, sharp inside corners, inconsistent draft. Each adds stress that leads to warp (Protolabs wall guide, as of November 2026).
• Glass fill. It helps against sink and makes warp more likely, because fibers line up as the part cools (Protolabs defects guide, as of November 2026).

The fixes the sources give: ribs on large flat faces and gussets on thin walls (Protolabs), uniform walls and consistent draft (Protolabs), a staggered rib pattern (Xometry, ribs), and consistent walls with a symmetric layout, gradual cooling and resins that shrink less (Xometry, defects), all as of November 2026.

For a device, warp matters most where parts mate. A lid that bows away from its housing compresses the gasket unevenly, which can open a leak path. That is our reasoning, and no source we read tested it. If the product has an IP rating, set a flatness tolerance for the sealing faces and measure it on first samples. Our tracker enclosure sealing guide covers the seal side.

What does a mold-flow review check?

Mold flow analysis simulates how resin fills the mold. It helps position gates, predict knit lines and find hard-to-fill spots (Protolabs, mold flow analysis, as of November 2026). The middle column is what that page says the analysis shows. The right column is what we suggest you ask.

Run the review before the tool is cut, while moving a gate or a boss is still a CAD change. See EVT, DVT and PVT for where tooling falls in a build plan.

What should you ask your hardware team before tooling?

• Where will the gate go, and why there?
• Which knit lines cross a boss, a snap fit or a seal surface?
• Was the simulated resin the grade we will buy?
• Where does the wall step, and is each step within the guide ratio?
• Is any boss tied to an outer wall with solid material instead of ribs?
• Can any through-hole be removed or drilled after molding?
• What flatness do the sealing faces need, and how will we measure warp?

If you are planning an enclosure and want these questions answered before tooling, tell us about your device. You can also read how the mechanical work is organized.

Sources. All pages accessed November 5, 2026. Design guides change, so confirm values with your molder.

• Protolabs: Knit Lines and How to Avoid Them; Understanding Knit Lines; Injection Molding Defects; Enhancing Cosmetic Appearance; Using Mold Flow Analysis; Thin Wall Injection Molding; Wall Thickness Guidelines; Plastic Boss Design; Design Better Screw Bosses; Gate Considerations; Designing Complex Features
• Xometry: Plastic Ribs for Injection-Molding Design; Injection Molding Defects and How to Prevent Them
• Beaumont: Weld Lines