Injection molding is one of the most widely used manufacturing processes for producing plastic parts — but even with well-designed molds and precise machinery, defects happen. Understanding what causes each defect and how to fix it is the difference between a smooth production run and hours of costly troubleshooting.
This guide covers the 12 most common injection molding defects, their root causes, and actionable solutions. Whether you are a mold designer, process engineer, or shop floor technician, these insights will help you identify problems faster and reduce scrap.
What they look like: Localized depressions on thick sections of molded parts, usually opposite ribs, bosses, or internal features.
!Sink marks on injection molded part
Sink marks occur when the outer surface of the part solidifies while the inner core is still molten. As the inner material cools and shrinks, it pulls the surface inward.
| Factor | Detailed Cause |
|---|---|
| Material | Excessive shrinkage, insufficient holding pressure compensation |
| Process | Insufficient hold pressure, short hold time, high melt temperature |
| Mold design | Thick wall sections, inadequate cooling near ribs, undersized gates freeze too early |
| Gate | Gate freezes before sufficient material can pack the cavity |
| Area | Fix |
|---|---|
| Process | Increase hold pressure by 10–20%; extend hold time until gate freezes; reduce melt temperature slightly |
| Mold design | Reduce wall thickness variation; core out thick sections; add cooling channels near heavy areas |
| Gate | Enlarge gate cross-section or move gate closer to thick sections |
| Material | Switch to lower-shrinkage grade or add filler (e.g., glass fiber) |
Real-world case study: A customer running PP automotive interior trim panels was seeing sink marks on all ribbed surfaces. Increasing hold pressure from 60 MPa to 75 MPa and extending hold time from 3s to 6s reduced the defect rate from 18% to under 2%.
What they look like: Visible lines or V-notches where two melt flow fronts meet and rejoin after flowing around an obstruction (core pin, insert, or split flow geometry).
When the melt front splits and recombines, the two fronts may not fully fuse if the material has cooled too much or if the flow front pressure is insufficient.
| Factor | Detailed Cause |
|---|---|
| Material | Low melt flow index, moisture content, incompatible additives |
| Process | Low melt or mold temperature, slow injection speed |
| Mold design | Obstructions in flow path, poor gate locations, inadequate venting |
| Flow | Long melt travel distance before rejoining |
| Area | Fix |
|---|---|
| Process | Raise melt temperature 10–20°C; increase injection speed at weld line position; raise mold temperature 10–15°C |
| Mold design | Add overflow wells or venting at weld line; relocate gates to minimize flow-around obstructions |
| Material | Dry material thoroughly; select higher MFR grade for thin-wall parts |
| Gate | Add additional gates to reduce weld line severity |
Pro tip: For appearance-critical parts, position weld lines away from visible surfaces by adjusting gate location or using sequential valve gating.
What it looks like: Thin fins of excess plastic that escape from the mold cavity along the parting line, ejector pin clearance, or sliding core gaps.
Flash occurs when the clamping force is insufficient to keep the mold closed against the injection pressure, or when mold surfaces do not mate properly.
| Factor | Detailed Cause |
|---|---|
| Process | Injection pressure exceeds clamping force; excessive shot size; high melt temperature reduces viscosity |
| Mold | Parting line damage, worn or dirty mold faces, insufficient clamp force |
| Machine | Toggle linkage wear; tie bar stretch unevenly; platen parallelity out of spec |
| Material | Low viscosity materials flow more easily into gaps |
| Area | Fix |
|---|---|
| Clamp | Increase clamp tonnage; verify mold is mounted flat on platen |
| Mold | Check and dress the parting line; ensure vents are no deeper than 0.02 mm for crystalline materials |
| Process | Reduce injection pressure or switch to a velocity-controlled filling profile; reduce shot size |
| Machine | Check tie bar tension balance (should be within 5%); verify platen parallelism |
Quick checklist when flash appears:
What it looks like: An incomplete part where the plastic did not fill the entire cavity — often showing missing details at the farthest flow point from the gate.
The melt solidifies before completely filling the cavity. This is almost always a flow-length-to-wall-thickness ratio problem.
| Factor | Detailed Cause |
|---|---|
| Process | Insufficient shot size, low injection speed, low melt temperature, premature mold opening |
| Mold | Poor venting (air trap), undersized runners or gates, cold slug not trapped |
| Material | Low MFR, excessive filler content, degraded material |
| Machine | Screw non-return valve leaking (backflow), worn barrel |
| Area | Fix |
|---|---|
| Process | Increase shot volume by 5–10%; raise melt temperature 10–20°C; increase injection speed; increase back pressure |
| Mold | Improve venting; enlarge runner/gate cross-section; add cold slug wells |
| Material | Verify material MFR; pre-dry hygroscopic materials |
| Machine | Check non-return valve condition; verify barrel temperature profile |
Case example: A connector housing mold running LCP was short-shotting on the far side of the cavity. The root cause was an air trap at the last fill point. Adding 0.02 mm deep vents at the end-of-fill location eliminated the problem immediately.
What it looks like: Part distortion or bowing after ejection. The part does not hold its intended shape.
Warpage results from differential shrinkage — different areas of the part cool and shrink at different rates, causing internal stresses that distort the geometry.
| Factor | Detailed Cause |
|---|---|
| Cooling | Uneven cooling rates across the part; insufficient cooling time before ejection |
| Mold design | Non-uniform wall thickness; uneven cooling channel layout; poor gate location |
| Material | High shrinkage rates; anisotropic shrinkage in glass-filled materials |
| Process | Low mold temperature; high melt temperature; insufficient cooling time |
| Area | Fix |
|---|---|
| Cooling | Balance cooling channels for uniform temperature distribution (±5°C max variation); increase cooling time by 20–30% |
| Mold design | Design uniform wall thickness (±15% variation max); add ribs for stiffness |
| Process | Lower melt and mold temperatures; reduce injection speed and pressure |
| Material | Use semicrystalline materials with controlled cooling (they shrink more predictably); consider amorphous materials for tighter tolerance |
Warpage analysis comparison:
| Material Type | Shrinkage Rate | Warpage Risk | Mitigation Strategy |
|---|---|---|---|
| PP | 1.5–2.5% | High | Slow cooling, uniform wall thickness |
| ABS | 0.4–0.7% | Medium | Balanced mold temperature, ribs |
| PC | 0.5–0.7% | Low | Keep mold temp 80–120°C |
| PA6+GF30 | 0.2–0.8% | Medium (anisotropic) | Flow simulation, multiple gates |
| POM | 1.8–2.5% | High | Post-mold annealing |
What they look like: Brown or black discoloration on the part surface, usually at the end of flow or in dead-end cavities.
Burn marks occur when trapped air in the cavity is compressed and heated so rapidly that it ignites or burns the plastic material — known as the diesel effect.
| Factor | Detailed Cause |
|---|---|
| Venting | Inadequate or blocked vents; no venting at end-of-fill |
| Process | Excessive injection speed; high injection pressure at final fill stage |
| Mold | Poorly designed venting; gas traps in deep ribs or blind pockets |
| Material | Highly volatile additives or excess moisture |
| Area | Fix |
|---|---|
| Venting | Add vent slots 0.015–0.03 mm deep at end-of-fill positions; clean vents regularly |
| Process | Reduce injection speed by 20–30% during final 15% of fill; use multi-stage injection |
| Mold | Add vacuum venting or porous mold steel for difficult-to-vent areas |
| Material | Pre-dry materials properly; purge degraded material from barrel |
Tip: Analyze the burn location to determine where to add vents. A simple finger test — cover suspect vent areas with mold release and look for discoloration — can quickly identify blocked vents.
What they look like: Internal cavities or bubbles within the part wall, not visible from the surface on opaque materials but detectable on translucent parts.
Voids form when the skin solidifies before the core can be fully packed. As the core shrinks, it pulls away from itself, creating an internal pocket.
| Factor | Detailed Cause |
|---|---|
| Process | Insufficient hold pressure or time; gate freezes too early |
| Mold | Thick wall sections with fast surface cooling |
| Material | High shrinkage material |
| Gate | Gate solidifies before sufficient material can compensate for shrinkage |
| Area | Fix |
|---|---|
| Process | Increase hold pressure and hold time; raise melt temperature slightly to extend gate open time |
| Mold | Core out thick sections; add inserts in heavy areas |
| Gate | Enlarge gate to delay freeze-off |
| Material | Switch to lower-shrinkage material |
Rule of thumb: Voids are almost always a packing problem. If you see voids, the first thing to check is whether the gate is freezing too early. Increase hold time until the part weight stabilizes.
What it looks like: A snake-like or worm-like pattern near the gate, often with a distinct fold or wrinkle in the flow front.
Jetting happens when the melt enters the cavity so fast that it does not contact the cavity wall immediately — it "jets" into open space, then folds back on itself.
| Factor | Detailed Cause |
|---|---|
| Gate | Gate design directs flow into open space (e.g., center-gated parts with no obstruction) |
| Process | Injection speed too high at the gate; low mold temperature |
| Mold | No flow obstruction (core/pin) near the gate to redirect flow |
| Area | Fix |
|---|---|
| Process | Reduce injection speed during the first 10–20% of fill (slow-first profile); raise mold temperature |
| Mold | Redirect gate to hit a core or cavity wall immediately; use a fan gate instead of a pinpoint gate |
| Gate | Enlarge gate to reduce shear rate; change gate type to submarine or tab gate |
Slow-first profile for jetting prevention:
| Stage | Position (% fill) | Injection Speed |
|---|---|---|
| 1 | 0–15% | 15–30% of max |
| 2 | 15–50% | 60–80% of max |
| 3 | 50–90% | 40–60% of max |
| 4 | 90–100% | 20–40% of max |
What they look like: Silvery or white streaks radiating from the gate or appearing randomly on the part surface.
Silver streaks (also called splay) are caused by moisture or volatile gases expanding during injection, creating micro-bubbles at the surface.
| Factor | Detailed Cause |
|---|---|
| Material | Moisture not dried sufficiently; contaminated resin; regrind with low degradation point |
| Process | Excessive screw rotation speed causing frictional heat; high back pressure; melt temperature too high |
| Mold | Cold slug not trapped; gate freezes then reopens |
| Machine | Nozzle drool; barrel temperature too high |
| Area | Fix |
|---|---|
| Material | Dry hygroscopic materials thoroughly (e.g., PA at 80°C for 4h; PC at 120°C for 4h); reduce regrind ratio below 20% |
| Process | Reduce screw RPM by 20%; reduce back pressure by 10–20 bar; lower melt temperature 5–10°C |
| Mold | Add cold slug well opposite the gate; check gate freeze timing |
| Machine | Clear nozzle drool; check barrel condition |
Drying guidelines for common materials:
| Material | Drying Temp (°C) | Drying Time (h) | Dew Point (°C) |
|---|---|---|---|
| ABS | 80–85 | 2–4 | −30 |
| PC | 120 | 3–4 | −40 |
| PA6/66 | 75–85 | 4–6 | −30 |
| PMMA | 75–85 | 2–4 | −30 |
| PBT | 120–130 | 3–4 | −40 |
What they look like: Glossy spots, indentations, or stress whitening at ejector pin locations.
Excessive ejection force causes localized stress or deformation at the ejector pin contact point. This usually means the part is too tight in the cavity, or the ejector system is unbalanced.
| Factor | Detailed Cause |
|---|---|
| Mold | Insufficient draft angle; unbalanced ejector pin layout; pins too small relative to ejection force; rough cavity surface |
| Process | Part still too hot at ejection; excessive packing causing expansion |
| Cooling | Insufficient cooling time; uneven cooling causing localized sticking |
| Area | Fix |
|---|---|
| Mold | Increase draft angle to minimum 1.5° (2–3° preferred for textured surfaces); redistribute ejector pins evenly; increase pin diameter in sticky areas |
| Process | Extend cooling time by 5–10s; reduce hold pressure slightly; allow part to cool below material Tg before ejection |
| Surface | Polish cavity in sticking areas; apply mold release (temporary fix) |
Quick fix sequence:
What they look like: Concentric rings, wave patterns, or bands near the gate or on the surface.
Flow marks occur when the melt front advances in an erratic, non-uniform manner. This is commonly caused by hesitations in the flow front, cold mold surfaces, or gate freeze-off and re-melting.
| Factor | Detailed Cause |
|---|---|
| Process | Injection speed too slow especially at start; mold temperature too low |
| Gate | Gate too small causing hesitation; gate freeze-off |
| Mold | Cold surface stops flow momentarily |
| Material | Low MFR doesn't allow smooth filling |
| Area | Fix |
|---|---|
| Process | Increase injection speed at the start of injection; raise mold temperature 10–15°C; use multi-stage speed to maintain uniform flow front |
| Gate | Enlarge gate cross-section; move gate to thinner wall section |
| Mold | Verify mold temperature controller (±2°C accuracy); pre-heat mold |
What it looks like: Thin skin layers peeling or flaking off the part surface. Looks like paint peeling, but it's the plastic itself.
Delamination is a layer separation problem — the outer skin separates from the underlying material due to poor bonding, contamination, or moisture.
| Factor | Detailed Cause |
|---|---|
| Material | Contaminated material; incompatible material compounds; excessive regrind; moisture |
| Process | Melt temperature too low for interlayer bonding; too much mold release |
| Mold | Cold mold surface causes premature skin formation |
| Other | Foreign material (different grade pellets) mixed in |
| Area | Fix |
|---|---|
| Material | Verify material purity; reduce regrind ratio below 20%; dry thoroughly |
| Process | Increase melt temperature to promote interlayer fusion; reduce mold release usage |
| Mold | Increase mold temperature |
| Defect | Primary Cause | First Thing to Check |
|---|---|---|
| Sink marks | Insufficient pack | Hold time and pressure |
| Weld lines | Cold melt front | Melt temperature and injection speed |
| Flash | Mold not fully closed | Clamp force and parting line condition |
| Short shot | Material not reaching cavity | Shot size and melt temperature |
| Warpage | Uneven cooling | Cooling time and mold temperature balance |
| Burn marks | Trapped air | Venting at end-of-fill |
| Voids | Early gate freeze | Hold time and gate size |
| Jetting | Melt hits open space | Gate location and first-stage speed |
| Silver streaks | Moisture/gas | Material drying |
| Ejector marks | High ejection force | Cooling time and draft angle |
| Flow marks | Erratic flow front | Injection speed profile |
| Delamination | Layer separation | Material purity and melt temperature |
When a defect appears, follow this step-by-step process rather than guessing:
Step 1 — Identify the defect type. Take a part sample and clearly name the defect. If multiple defects appear, pick the most severe one first.
Step 2 — Check the material. Is it properly dried? Different lot? Contaminated? This rules out 30% of defects immediately.
Step 3 — Review process parameters. Look at the last 20 cycles on the machine. Most process-related defects show a gradual trend before becoming visible.
Step 4 — Inspect the mold. Check venting, gate condition, cooling channels (flow rate and temperature), and parting line condition.
Step 5 — Adjust one parameter at a time. Change one variable, run 5–10 cycles, and evaluate. Changing multiple things at once means you won't know what worked.
Step 6 — Monitor the machine. Hydraulic pressure fluctuations, injection unit stability, and temperature controller accuracy all affect part quality.
A common source of production delays is debating whether the issue is the mold or the process. Here's a practical framework:
| Indicator | Process Problem | Mold Problem |
|---|---|---|
| Appears suddenly | ✓ (likely) | Unlikely |
| Gradual onset over hours/days | Possible | ✓ |
| Same defect on all cavities | ✓ | Unlikely |
| Only one cavity has defect | Unlikely | ✓ |
| Changes with different material batches | ✓ | Unlikely |
| Fixed by adjusting speed/pressure/temp | ✓ | Unlikely |
| Same defect after parameter changes | Unlikely | ✓ |
Injection molding defects are frustrating, but they are almost always solvable with systematic troubleshooting. The key takeaways:
By understanding the root cause of each defect rather than blindly adjusting parameters, you can reduce scrap rates, increase machine uptime, and produce higher-quality parts.
Yes, most of the time. Increasing hold pressure and hold time fixes 80% of sink mark issues. If process changes don't work, then look at mold design (wall thickness, gate size).
Intermittent weld lines are usually related to temperature variation. Check the mold temperature controller stability (±2°C spec), and verify material drying consistency.
Flash is thin plastic escape from the mold cavity. Burrs are raised edges from trimming or machining. In injection molding, the term is always "flash."
Not always, but cooling temperature imbalance is the #1 cause. If uniform cooling doesn't fix it, check gate-induced orientation (especially for glass-filled materials) and consider annealing the part after ejection.
Standard guidelines: amorphous materials — 0.015–0.02 mm; crystalline materials — 0.02–0.03 mm. Going deeper risks flash on the parting line.
Only as a last resort. Material change affects shrinkage, cycle time, and mechanical properties. Always optimize process and mold design first.
Simulation is excellent for predicting fill patterns, weld line locations, and air traps. It is less reliable for predicting cosmetic defects like silver streaks or ejector pin marks. Use simulation for mold design review, not as a substitute for practical troubleshooting.
Run a minimum of 5–10 stable cycles before evaluating. The first 1–3 cycles often show transient behavior due to thermal stabilization of the mold.
Three interventions yield the biggest payoff: (1) install cavity pressure sensors for real-time process monitoring, (2) implement material drying verification (dew point check), and (3) establish a preventive mold maintenance schedule (venting cleaning every 50,000 cycles, cooling channel flushing every 100,000 cycles).
Call a specialist when: the defect disappears with process adjustment but returns unpredictably (suggests mold or machine wear), when measurement data shows no correlation with any process parameter, or when safety-critical parts (automotive airbag housings, medical devices) show inconsistent quality.
This guide is maintained by the Moldkey Engineering Team. For more injection molding resources, visit Moldkey.com.
