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No matter how well a mold is designed or how precisely a machine is calibrated, injection molding defects happen. The difference between an average shop and a great one isn’t zero defects — it’s knowing what causes them and how to fix them fast. This guide covers the seven most common injection molding defects, their root causes, and practical solutions your team can implement today.

1. Short Shot — The Mold Doesn’t Fill Completely

A short shot occurs when the molten plastic fails to fill the entire mold cavity before solidifying. You’ll see an incomplete part — often missing edges, thin sections, or the end of flow paths.

Common Causes

• Insufficient injection pressure or speed: The melt front cools before reaching cavity extremities.
• Material viscosity too high: Especially with glass-filled or high-temperature resins.
• Inadequate venting: Trapped air compresses and prevents complete fill.
• Gate size too small or poorly located: Restricts flow into the cavity.
• Barrel temperature too low: Plastic doesn’t reach optimal flow state.

Solutions

• Increase injection pressure and speed incrementally while monitoring for flash.
• Raise barrel temperature by 5–10°C steps within the material’s recommended range.
• Add or enlarge vents — especially at the last point to fill.
• Consider a larger gate or switch to a gate location closer to thin sections.
• For high-viscosity materials, pre-dry thoroughly and consider a hotter mold temperature.

2. Flash — Excess Material at Parting Lines

Flash appears as a thin layer of excess plastic along the parting line, around ejector pins, or at insert boundaries. While small flash can be trimmed, it adds labor cost and signals underlying process problems.

Common Causes

• Clamping force insufficient: The mold halves separate under injection pressure.
• Worn mold components: Damaged parting lines, oversized ejector pin clearances, or aged inserts.
• Excessive injection pressure or speed: Plastic forces its way into gaps.
• Melt temperature too high: Reduces viscosity, allowing material to seep into tiny clearances.

Solutions

• Verify clamping force meets part area × cavity pressure requirements (typically 3–5 tons per square inch).
• Reduce injection pressure, especially during the hold/pack phase.
• Lower melt temperature gradually — high heat may indicate dwell time is too long.
• Inspect and repair worn tooling: parting line surfaces, ejector pin bores, and slide fits.

3. Sink Marks — Surface Depressions Over Thick Areas

Sink marks are localized surface depressions that appear above ribs, bosses, intersections, or any thicker section. They occur when the exterior cools and solidifies while the interior is still shrinking — pulling the surface inward.

Common Causes

• Wall thickness variation > 40%: The classic DFM violation — thick sections shrink more than thin ones.
• Insufficient packing/hold pressure or time: The gate freezes before shrinkage is compensated.
• Mold or melt temperature too high: Prolongs cooling and increases total shrinkage.
• Gate freezing too early: Prevents pack pressure from reaching the cavity.

Solutions

• Redesign to maintain uniform wall thickness — core out thick sections wherever possible.
• Increase packing pressure and extend hold time — verify gate hasn’t frozen prematurely.
• Lower mold temperature at specific locations (use conformal cooling if available).
• Relocate gates to feed thickest section first, allowing pack pressure to reach it before solidification.
• Use a gas-assist process for parts with unavoidable thick sections.

4. Warpage — The Part Twists After Ejection

Warpage is when a part distorts, bends, or twists after ejection. It’s one of the most frustrating defects because the part looks fine in the mold but deforms as it cools to ambient temperature.

Common Causes

• Non-uniform cooling: One side of the cavity cools faster than the other, creating internal stress.
• Non-uniform shrinkage: Different wall thicknesses shrink at different rates.
• Molecular orientation from filling: Fiber-filled materials in particular warp along flow direction.
• Ejection temperature too high: Part hasn’t reached sufficient rigidity.
• Excessive packing pressure: Over-packing creates differential stresses.

Solutions

• Balance cooling circuit design — ensure both cavity halves have comparable cooling rates.
• Reduce packing pressure and increase cooling time.
• For fiber-filled materials, adjust gate location to create more random fiber orientation.
• Use mold flow analysis during design phase to predict and compensate for warpage.
• Add fixturing during post-mold cooling for critical dimensional parts.

5. Weld Lines — Visible Lines Where Flow Fronts Meet

Weld lines (also called knit lines) form where two melt fronts meet — around cores, holes, or when multiple gates are used. They’re not just cosmetic: weld lines are mechanically weaker zones where the two fronts may not have fully bonded.

Common Causes

• Flow obstruction: Pins, cores, or geometry changes split the melt stream.
• Multiple gates: Each gate creates a separate flow front that must rejoin.
• Low melt or mold temperature: Fronts cool before bonding.
• Venting at the meeting point is insufficient: Trapped air prevents proper bonding.

Solutions

• Increase melt and mold temperature to give the fronts more time to fuse.
• Increase injection speed so fronts meet while hotter.
• Add a vent at the weld line location to release trapped air.
• Relocate the gate or switch to a single gate if part geometry allows.
• For structural parts, avoid placing weld lines in high-stress areas — mold flow analysis can predict their location.

6. Burn Marks — Discolored or Charred Surface Spots

Burn marks appear as black, brown, or yellow discoloration — usually at the end of the fill path or near venting areas. They are caused by air trapped in the cavity that becomes superheated under compression and literally burns the plastic.

Common Causes

• Inadequate venting: Air has nowhere to escape and is compressed to extreme temperatures.
• Injection speed too high: Air cannot evacuate quickly enough through existing vents.
• Melt temperature too high: Combined with trapped air, thermal degradation accelerates.
• Excessive screw speed during plasticizing: Air gets entrained in the melt.

Solutions

• Add or deepen vents — standard vent depth is 0.025–0.050 mm depending on material viscosity.
• Reduce injection speed — slower fill gives air time to evacuate.
• Reduce screw RPM during plasticizing and increase back pressure to remove air from the melt.
• Lower barrel temperature if material is degrading thermally.
• Consider vacuum venting for complex or deep-cavity molds.

7. Jetting — Snake-Like Flow Marks on the Surface

Jetting occurs when the melt shoots through the gate as a high-speed jet rather than spreading smoothly across the cavity. The result is visible snake-like or worm-like marks on the part surface near the gate.

Common Causes

• Gate too small or poorly positioned: Creates a high-velocity jet instead of a smooth melt front.
• Injection speed too high at gate entry: The melt doesn’t have time to establish laminar flow.
• Gate lands directly into an open cavity: The melt has no wall to spread against.

Solutions

• Enlarge the gate or change to a fan gate or tab gate that encourages laminar flow.
• Position the gate so the melt impinges against a cavity wall immediately after entering.
• Reduce injection speed during the initial filling phase.
• Increase melt temperature slightly to improve flow behavior.

Building a Defect Prevention System

Treating defects one-by-one is reactive. Building a system that prevents them is what separates consistent manufacturers from inconsistent ones:

1. DFM review before tooling begins: Most defects trace back to part or mold design decisions made before steel is ever cut. A thorough DFM review that flags wall thickness issues, gate locations, venting plans, and cooling layout prevents problems at the source.
2. Mold flow analysis for critical parts: For parts with tight tolerances, cosmetic surfaces, or complex geometries, mold flow simulation predicts fill patterns, weld line locations, air traps, and warpage tendencies — before the mold is built.
3. First-article inspection (FAI) with scientific molding principles: Don’t rely on “this has always worked before.” Use cavity pressure sensors and documented process windows to establish a repeatable, data-driven molding process.
4. Material handling discipline: Moisture, contamination, and inconsistent drying account for a surprising percentage of defects. Maintain strict material handling protocols and verify drying before production runs.
5. Preventive mold maintenance: Worn vents, eroded gates, and damaged parting lines cause defects gradually. A documented maintenance schedule catches them before they catch you.

Summary: Quick Reference Table

Here is a condensed troubleshooting guide you can keep on the shop floor:

Defect	First Thing to Check	Quickest Fix
Short Shot	Injection pressure / venting	Raise pressure 5–10%
Flash	Clamping force / mold condition	Reduce pack pressure
Sink Marks	Wall thickness variation	Increase pack time
Warpage	Cooling balance	Extend cooling time
Weld Lines	Melt/mold temperature	Raise temp 5–10°C
Burn Marks	Venting	Reduce injection speed
Jetting	Gate size / location	Enlarge gate or reposition

Injection molding is a balancing act of pressure, temperature, time, and geometry. When defects appear, resist the temptation to change everything at once. Modify one variable at a time, document the result, and build your process knowledge systematically. The best molding shops aren’t the ones with zero problems — they’re the ones that solve them in minutes, not days.

Need help troubleshooting a specific molding defect or optimizing your part design for production? Contact our engineering team — we bring 20+ years of mold making and injection molding experience to every project.