5 Critical Injection Molding Defects & Solutions (Part 2)
Introduction
So you’ve read Part 1 of this series and learned how to tackle warpage, sink marks, short shots, flow lines, and flash. That’s great—those five defects alone cause about 30-40% of quality headaches in most molding operations.
But here’s the thing: the defect troubleshooting journey doesn’t end there. In fact, some of the trickiest problems we see at Rayleap Plastic aren’t the obvious dimensional issues—they’re the surface and structural defects that sneak up on you. We’re talking about parts that look fine at first glance but fail in the field, or transparent housings that suddenly develop streaks that no amount of polishing can fix.
Today, we’re diving into five more critical defects: weld lines, bubble defects, splay marks, burn marks, and jetting. Our engineering team deals with these challenges daily across industries from automotive to medical devices, and we’ve learned one thing: understanding these defects isn’t just about fixing problems—it’s about preventing them before they cost you thousands in scrap and rework. Let’s get into it.
📑 Quick Navigation
Already know which defect you’re dealing with? Jump straight to it:
- → Weld Lines – Strength-reducing flow convergence issues
- → Bubble Defects – Hidden internal voids
- → Splay Marks – Silvery surface moisture streaks
- → Burn Marks – Thermal degradation from poor venting
- → Jetting Defects – Snake-skin flow patterns
- → Quick Reference Table – All 5 defects at a glance
⏱️ Reading time: 8 minutes | Need the fast answer? Check our Quick Reference Table below for instant troubleshooting.
1. Weld Lines: Where Flow Fronts Meet
What Is It?
A weld line—sometimes called a knit line—shows up as a visible line or seam where two or more streams of molten thermoplastic polymers meet and re-join. You’ll see these most often on parts with multiple gates, holes, or complex shapes that force the material to split and come back together.
On dark ABS or PC parts, weld lines usually look like lighter streaks running across the surface. But here’s what really matters: they’re not just cosmetic. Testing consistently shows that weld lines can knock down tensile strength by anywhere from 10% to 40% compared to the base material. That’s a structural weak point you can’t ignore.
Common Causes
Not enough heat or speed at the meeting point. When plastic arrives too cool or too slowly, the molecular chains don’t get enough time or mobility to tangle together properly. You end up with a weak bond that’s visible and mechanically compromised.
Gate placement isn’t optimized. Your gate location determines how material flows through the cavity. If gates are positioned too far from critical areas, the plastic has to travel longer distances—and that means it cools down before it can fuse properly at convergence points.
Trapped air at flow fronts. Without proper venting where flows meet, trapped air blocks complete fusion. The result? A visible line and a structural liability.
How to Fix It
Rethink your gate strategy. Sometimes the fix is as simple as relocating gates to minimize flow path differences, or positioning weld lines in non-critical areas where they won’t show or compromise strength. For high-strength applications with PA or POM, we often recommend hot runner systems with sequential valve gating—this gives you precise control over flow fronts.
Turn up the speed and heat. Increasing injection speed by 15-25% keeps the material hotter when flows converge. Raising mold temperature by 10-20°C helps too—it gives those molecular chains more time to entangle. We’ve seen PP parts where bumping mold temp to 40-60°C completely eliminates visible weld lines.
Get serious about venting. Machine vent channels 0.02-0.05mm deep right at predicted weld line locations. This lets trapped air escape so material can fuse completely.
Hold the pressure longer. Extending holding pressure by 5-10 seconds compresses the weld line area while it’s still molten, improving molecular bonding before everything solidifies.
Pro Tip: If aesthetics are critical, consider switching to a material with better flow characteristics. PC-ABS blends often produce less visible weld lines than pure ABS because of their superior melt flow properties.
2. Bubble Defects and Voids: Hidden Structural Weaknesses
What Is It?
Bubble defects are exactly what they sound like—air pockets, voids, or gas bubbles trapped inside the molded part. The tricky part? Unlike surface defects, these are often internal. You might not spot them until you cross-section the part, X-ray it, or—worse—it fails in service. In transparent materials, bubbles show up as spherical inclusions that scatter light and ruin clarity.
Here’s a scenario we see all the time: a thick-walled component looks perfect externally, passes visual inspection, ships to the customer… and then fails in the field because it’s riddled with dozens of internal voids that compromised structural integrity and dimensional accuracy.
Common Causes
Not enough pressure during cooling. As plastic cools and shrinks, you need adequate holding pressure to compensate. Without it, you get vacuum voids in thick sections—basically, the material pulls away from itself as it contracts.
Moisture in the material. Thermoplastic polymers like PA and ABS are hygroscopic, meaning they absorb water from the air. When that moisture vaporizes during injection, it creates steam bubbles that get trapped as the material solidifies. This is the #1 cause of bubble defects with hygroscopic materials.
Air that can’t escape. Rapid injection without proper venting literally traps air in the melt. That air has nowhere to go, so it forms bubbles within the plastic as it cools.
How to Fix It
Dry your material properly—no exceptions. For hygroscopic materials, this isn’t optional. PA needs drying at 80-90°C for 4-6 hours before you even think about processing it. ABS needs 80-85°C for 2-4 hours. Target moisture content below 0.1% for PA and 0.05% for PC. And here’s the kicker: use desiccant dryers, not hot air dryers, for critical applications. The investment pays for itself in reduced scrap.
Boost holding pressure and time. Increase holding pressure by 10-15% and extend holding time until the gate fully solidifies. For thick sections over 4mm, you might need to increase holding time by 30-50% or more.
Improve your venting game. Add or enlarge vents at last-fill areas and thick sections. Make sure venting is sufficient to let all trapped air escape during filling—not just some of it.
Slow down in thick sections. This might seem counterintuitive, but slightly slower injection (10-20% reduction) in thick-wall areas allows air to vent naturally instead of getting compressed into bubbles.
Material-Specific Tip: POM (polyoxymethylene) is particularly prone to void formation because of its high crystallinity and shrinkage rate. For POM parts, use progressive cavity designs and make sure holding pressure can reach deep into thick sections.
Common Mistake: A lot of operators think the solution to bubbles is faster injection. Wrong. That actually makes gas entrapment worse. The real solution is better venting combined with optimized holding pressure—not just cranking up the speed.
3. Splay Marks and Silver Streaks: Surface Moisture Indicators
What Is It?
Splay marks appear as silvery or white streaks that radiate out from the gate area. They look like frost patterns or spider webs on the part surface—pretty distinctive once you know what you’re looking for. These silver streaks are surface defects caused by moisture or volatile gases trapped in the material, which create micro-bubbles that break through the surface as molten plastic flows into the mold cavity.
On transparent PC or PMMA parts, splay marks completely destroy optical clarity. There’s no salvaging it. On cosmetic housings like consumer electronics, they create this mottled, unacceptable appearance that no amount of post-processing can fix.
Common Causes
High moisture content. This is far and away the number one cause. Even material that’s been properly stored can pick up moisture during handling or from humid ambient air—especially in high-humidity environments or during summer months.
Material breaking down. Excessive melt temperature or prolonged residence time in the barrel breaks down polymer chains, releasing gases that manifest as splay marks on the surface.
Contamination issues. Foreign materials, especially those with different decomposition temperatures than your base resin, create localized gas release that shows up as splay marks.
How to Fix It
Get rigorous about drying protocols. Use desiccant dryers to hit moisture content below 0.02% for PC and 0.05% for ABS. Don’t guess—verify moisture levels with a moisture analyzer before you start production runs. We’ve seen too many scrap runs that could have been avoided with a $500 moisture meter.
Dial back the temperature. Try lowering barrel temperature by 10-20°C to minimize thermal degradation. For ABS, if you’re running at 260°C and seeing splay marks, try dropping to 220-240°C range.
Minimize residence time. Reduce barrel capacity or increase cycle frequency so material doesn’t sit at elevated temperatures too long. And clean that barrel thoroughly between material changes—leftover material from the previous run is a common culprit.
Check for contamination. Purge the barrel completely when switching materials. For difficult-to-clean materials like POM or PA, invest in quality purging compounds. It’s cheaper than scrapping parts.
Adjust injection speed. Reducing initial injection speed by 15-25% helps prevent turbulent flow, which tends to draw moisture to the surface where it becomes visible.
Pro Tip: At Rayleap, we install hopper dryers and use dry air purging systems to maintain material dryness right up to the moment of injection. For really moisture-sensitive materials, we use closed-loop material handling systems that eliminate moisture exposure entirely. It sounds like overkill until you calculate the scrap cost.
4. Burn Marks and Discoloration: Signs of Overheating
What Is It?
Burn marks show up as brown, black, or rust-colored discoloration on molded parts. You’ll typically see them at last-fill areas, sharp corners, or near vents. Unlike splay marks—which are silvery—burn marks indicate actual thermal degradation or combustion of the plastic material.
And they’re not just ugly. Burned material has degraded molecular structure, which means brittleness and reduced mechanical properties in affected areas. If you’re seeing burn marks, you’re literally burning away part strength.
Common Causes
Inadequate venting. As plastic fills the cavity, it compresses air ahead of the flow front. Without adequate venting, that compressed air generates serious heat through diesel effect (adiabatic compression). We’re talking temperatures that can exceed 500°C—hot enough to burn the plastic and sometimes even damage the mold steel.
Injection speed is too high. When you inject too fast, air gets compressed violently before it can escape. That creates extreme localized heating that burns the plastic at compression points.
Material degradation from heat. Overly high melt temperatures or contaminated material can burn during processing, especially in thin-wall sections or sharp corners where heat concentrates.
How to Fix It
Fix your venting first. This is the most critical solution, period. Add vents at all last-fill areas, especially deep ribs and corners. Vent depth should be 0.02-0.04mm for most materials. For high-temperature materials like PA, you can increase vent width, but keep depth shallow to prevent flash.
Slow down the injection. Lower speed by 20-30%, particularly in thin sections. This allows air to escape gradually instead of being violently compressed and heated. Think of it like slowly squeezing air out of a bag versus slamming it—one creates heat, one doesn’t.
Drop mold temperature if needed. If burn marks persist after you’ve improved venting, try reducing mold temperature by 10-15°C. This slows thermal degradation at critical areas.
Optimize barrel temperature. Reduce barrel temperature to the lower end of the manufacturer’s recommended processing range. For PP, instead of running at 240°C, try 200-220°C.
Redesign problematic geometry. If burn marks consistently occur at sharp corners or thin ribs, consider increasing corner radii or wall thickness. Sometimes the geometry itself is creating air traps that no amount of parameter adjustment can fix.
Material-Specific Tip: PC is particularly susceptible to burn marks because of its high processing temperature (280-320°C). Make absolutely sure your barrel is thoroughly cleaned between runs, and avoid temperature spikes during startup.
Common Mistake: Some operators try to solve burn marks by drastically reducing mold temperature. The problem? That can cause short shots or flow lines—you’re trading one defect for another. The root cause is almost always venting. Fix that first, then adjust other parameters if needed.
5. Jetting Defects: The Snake-Skin Appearance
What Is It?
A jetting defect happens when molten plastic squirts through the gate as a narrow stream instead of flowing smoothly against the mold wall. That stream then folds back on itself in this wavy, serpentine pattern that creates what we call a “snake-skin” or “worm track” appearance on the part surface. It’s most visible near the gate area, though it can extend further into the part depending on severity.
Jetting defects are particularly problematic because the folded material doesn’t bond properly with itself. You end up with weak points and an unacceptable surface finish that can’t be hidden or fixed.
Common Causes
Too much speed through the gate. When injection speed is excessive, especially through small gates, plastic shoots across the cavity like water from a hose rather than filling progressively against the mold walls.
Poor gate design. Small gates—we’re talking less than 0.5mm diameter—or gates positioned to shoot directly into open cavities practically invite jetting. Edge gates perpendicular to thin walls are especially prone to this defect.
Mold is too cold. When mold surfaces are cold, the plastic stream solidifies immediately upon contact. It can’t spread and bond properly, so you get that characteristic folded appearance.
How to Fix It
Reduce that initial injection speed. Drop the first-stage injection speed by 30-50% to allow material to contact the mold wall and flow progressively instead of jetting across the cavity. Multi-stage injection profiles work great here—start slow, then ramp up once the cavity starts filling properly.
Rethink gate location and design. Reposition gates so material flows against a mold wall rather than shooting into open space. Increase gate size by 20-30% to reduce shear and velocity at the gate. Fan gates or tab gates that promote material spreading can eliminate jetting entirely.
Raise mold temperature. Increasing mold temperature by 15-25°C keeps the plastic surface molten longer, giving it time to flow and fuse properly. For ABS, try 60-80°C; for PC, go with 90-110°C.
Smooth out your runner geometry. Use gradual taper in runners leading to gates. Avoid abrupt diameter changes that create turbulent flow and velocity spikes.
Use flow control techniques. If you can’t relocate the gate for whatever reason, position a shallow obstruction pin near the gate to slow and redirect the initial flow. It forces material against the mold wall, preventing jetting.
Material-Specific Tip: High-flow materials like PP are way more susceptible to jetting than high-viscosity materials like PC. When you’re molding PP, pay extra attention to injection speed profiles and gate design from the start.
Pro Tip: At Rayleap, we run moldflow analysis software to simulate filling patterns before we even cut steel. This virtual troubleshooting identifies potential jetting areas early, allowing us to optimize gate design before the first shot. It saves weeks of trial-and-error troubleshooting and tooling modifications.
Quick Reference: 5 Defects at a Glance {#quick-reference}
Here’s a quick diagnostic table you can keep handy on the production floor:
| Defect | Primary Causes | Quick Solutions |
|---|---|---|
| Weld Lines | Low injection speed, poor gate design, inadequate venting | Increase injection speed 15-25%, optimize gate location, add vents at convergence points |
| Bubble/Void | High moisture content, insufficient holding pressure, poor venting | Pre-dry material (PA: 80-90°C, 4-6h; PC: <0.05% moisture), increase holding pressure 10-15% |
| Splay Marks | Material moisture, thermal degradation, contamination | Rigorous drying (<0.02% moisture for PC), reduce melt temp 10-20°C, verify material purity |
| Burn Marks | Poor venting, excessive injection speed, diesel effect | Add vents (0.02-0.04mm depth), reduce injection speed 20-30%, check last-fill areas |
| Jetting | High injection speed, small gates, low mold temperature | Reduce initial speed 30-50%, increase gate size 20-30%, raise mold temp 15-25°C |
Use this table as your quick diagnostic tool when troubleshooting on the production floor. It won’t replace detailed analysis, but it’ll point you in the right direction fast.
Frequently Asked Questions About Injection Molding Defects
Q: What causes weld lines in injection molding?
Weld lines occur when two or more flow fronts of molten plastic converge and re-solidify. Common causes include insufficient injection speed, poor gate design, low mold temperature, and inadequate venting at flow convergence points. These lines can reduce part strength by 10-40%.
Q: How do you prevent bubble defects in injection molding?
To prevent bubble defects and voids: (1) Pre-dry hygroscopic materials like PA at 80-90°C for 4-6 hours, (2) Increase holding pressure by 10-15%, (3) Improve mold venting, especially at thick sections, and (4) Extend holding time until the gate fully solidifies. Moisture content must be below 0.1% for PA and 0.05% for PC.
Q: What is the difference between splay marks and burn marks?
Splay marks are silvery or white streaks caused by moisture or volatile gases in the material, while burn marks are brown or black discoloration from thermal degradation or poor venting. Splay marks indicate material drying issues, whereas burn marks signal excessive air compression (diesel effect) or overly high temperatures.
Q: How can I fix jetting defects in plastic injection molding?
To eliminate jetting defects: (1) Reduce initial injection speed by 30-50%, (2) Increase gate size by 20-30% or switch to fan gates, (3) Raise mold temperature by 15-25°C, and (4) Reposition gates so material flows against a mold wall rather than into open space. Multi-stage injection profiles work best for preventing jetting.
Q: What are the most common injection molding defects?
The 10 most common injection molding defects are: warpage, sink marks, short shots, flow lines, flash, weld lines, bubble/void defects, splay marks, burn marks, and jetting. These account for 60-70% of all quality issues in typical molding operations. Each has distinct root causes related to processing parameters, mold design, or material handling.
Conclusion: Mastering the Complete Defect Portfolio
Alright, let’s bring this full circle. Between Part 1 and this article, we’ve now covered 10 critical injection molding defects that account for the vast majority—we’re talking 60-70%—of quality issues you’ll encounter in production. From the dimensional challenges like warpage and sink marks to the surface and structural problems like weld lines, bubbles, splay marks, burn marks, and jetting, each defect has its own signature causes and proven fixes.
What’s the common thread running through all of these? Most defects boil down to a handful of fundamental issues: processing parameters that need tuning, mold design problems (especially venting and gate placement), or poor material handling practices. Success isn’t about random parameter changes or “let’s try this and see what happens.” It’s about systematic troubleshooting—identify the defect accurately, understand its root cause, then implement targeted solutions.
At Rayleap Plastic, our engineering team has refined defect elimination protocols through literally thousands of production runs. We work with everything from commodity-grade PP to engineering plastics like PA and POM, and we’ve learned what works—not just in theory, but on actual production floors under real-world conditions. We combine advanced moldflow simulation, precision tooling, and strict process controls to deliver parts that meet even the most demanding specifications.
Here’s the reality: whether you’re dealing with persistent quality issues that are killing your margins or you’re launching a new product and want to get it right the first time, expert engineering support makes all the difference. It’s the difference between costly trial-and-error that eats up weeks and first-time success that keeps your project on schedule.
If you’re facing any of these defects—or others we haven’t covered—don’t struggle alone. Contact Rayleap’s injection molding specialists to discuss your specific challenges. Our team is ready to apply these proven solutions to your production environment and help you achieve the consistent, defect-free results your customers expect.
Ready to eliminate injection molding defects in your production? Contact Rayleap Plastic for a free defect analysis and engineering consultation. Our team specializes in complex troubleshooting and process optimization for thermoplastic injection molding.
