Your first-article samples arrive, and something is immediately off. There's a sunken crater on a thick rib. One wall has bowed inward. A faint silver streak fans out from the gate. Every one of these issues has a name, a root cause, and — critically — a fix. But if you can't identify what you're looking at, you can't know where to start.

Injection molding defects range from cosmetic nuisances to structural failures that can ground an entire production run. They originate from three overlapping sources: part design decisions, mold tooling, and process parameters. Understanding which lever to pull first is what separates a quick fix from an expensive debugging spiral.

This visual guide covers 12 of the most common plastic injection molding defects — what each one looks like, what causes it, and what needs to change to eliminate it. Whether you're a product designer reviewing first articles, a process engineer troubleshooting a live run, or a program manager evaluating scrap rates, this guide gives you the language and the logic to move fast.

What Causes Injection Molding Defects?

Most injection molding defects trace back to one of three root-cause categories, and understanding this framework is the fastest way to narrow down a fix. Design-related causes include non-uniform wall thickness, inadequate draft angles, improper rib-to-wall ratios, and gate placement that forces material to travel too far before filling. Tooling-related causes include worn parting surfaces, blocked vents, undersized gates and runners, and cooling channel imbalances baked into the mold itself. Process-related causes include incorrect injection speed, pressure, or temperature settings — parameters that can often be adjusted without touching the tool.

A common mistake during troubleshooting is jumping straight to process adjustments when the real culprit is design or tooling. Process changes can mask a symptom but won't eliminate a defect that originates upstream. The most effective strategy is to address design issues before the mold is cut, tooling issues before production begins, and process issues during qualification runs — in that order.

With that diagnostic framework in mind, here's a defect-by-defect breakdown of what to look for and what to do about it.

1. Flow Lines

What It Looks Like

Flow lines appear as wavy streaks, rings, or ripple patterns on the surface of the molded part — often in a slightly different color or sheen than the surrounding material. They're most visible near gates, corners, and areas where wall thickness changes abruptly.

Root Causes

• Design: Abrupt transitions between thick and thin wall sections; sharp corners instead of fillets
• Tooling: Gates placed in thin sections; runners that are too small to maintain flow velocity
• Process: Injection speed too slow, allowing the melt to cool unevenly before the cavity is filled; barrel or nozzle temperatures too low

How to Fix It

On the design side, maintain uniform wall thickness throughout the part and use gradual tapered transitions rather than step changes. Round corners with fillets sized at least equal to the wall thickness. On the tooling side, place gates in thicker sections of the cavity where material is hotter and flow is more consistent, and increase runner diameter if filling speed is insufficient. On the process side, raise injection speed and barrel temperature to keep the melt fluid long enough to fill the cavity evenly before the skin begins to solidify.

2. Sink Marks

What It Looks Like

Sink marks are small depressions, divots, or dimples on the outer surface of a part — typically appearing on flat or gently curved faces opposite a thick internal feature like a rib or boss. The surface appears to have been pulled inward slightly.

Root Causes

• Design: Ribs or bosses that are too thick relative to the nominal wall; excessive wall thickness variation
• Tooling: Gates too small or too far from thick sections; insufficient cooling near heavy cross-sections
• Process: Holding pressure too low or hold time too short, allowing the material to pull away from the mold wall as it shrinks during cooling

How to Fix It

The most durable fix is design-based: keep rib thickness at 50–60% of the nominal wall, and core out thick sections where possible to reduce mass. On the process side, increase holding pressure and extend hold time to push additional material into the cavity as it cools and contracts. Lowering mold temperature can also accelerate surface solidification. If the gate is undersized or poorly positioned, a tooling modification to increase gate diameter or move it closer to the thick section will reduce the pressure drop and improve pack-out.

3. Warping

What It Looks Like

Warping shows up as bowing, twisting, or unexpected curvature in parts that should be flat or dimensionally consistent. A box-shaped part with walls that curve inward is a classic example. Warpage is often only detected after ejection, when the part cools to ambient temperature.

Root Causes

• Design: Non-uniform wall thickness causing differential shrinkage; semi-crystalline materials with high and directionally variable shrinkage rates
• Tooling: Unbalanced cooling channels that cool one side of the part faster than the other; gate location that creates long, asymmetric flow paths
• Process: Insufficient cooling time; mold temperature too low, causing premature freezing before material is fully packed

How to Fix It

Uniform wall thickness is the single most effective design-level countermeasure — it ensures the part cools and shrinks at a consistent rate in all directions. If the material is semi-crystalline (such as nylon or POM), consider whether an amorphous alternative meets the mechanical requirements, as amorphous resins have more isotropic shrinkage. On the tooling side, review cooling channel layout for balance and consider moving the gate to a thicker zone. Process-wise, allow adequate cooling time before ejection and avoid prematurely opening the mold.

4. Short Shots

What It Looks Like

A short shot is an incomplete part — one or more sections of the cavity simply didn't fill, leaving a visible void, thin spot, or missing feature. Thin ribs, fine details, and end-of-fill locations are the most common sites. The part is structurally unusable and must be scrapped.

Root Causes

• Design: Wall sections too thin for the material's viscosity; flow-length-to-thickness ratio too high for the resin being used
• Tooling: Gates or runners too small, restricting flow; inadequate venting trapping air and blocking fill at end-of-flow locations
• Process: Injection pressure or speed too low; melt temperature too low, causing premature solidification before the cavity is full

How to Fix It

Start by verifying that wall thickness meets the minimum required for the chosen resin — a minimum of 1.0–1.5 mm is a common baseline, though this varies significantly by material. Check whether vents are clean and adequately sized to allow air to escape as the cavity fills. On the process side, increase injection speed and melt temperature in small increments. If the problem persists, a tooling modification to enlarge or reposition the gate is often the most reliable long-term solution. For complex geometries, a flow simulation prior to tooling commitment can identify fill issues before steel is cut.

5. Flash

What It Looks Like

Flash is a thin fin or web of excess plastic that forms outside the intended part geometry — most commonly along the parting line, at ejector pin locations, or around shut-off features. Mild flash can sometimes be trimmed, but heavy flash signals a process or tooling problem that will worsen over time.

Root Causes

• Design: Insufficient draft angles at parting surfaces; wall thickness that generates excessive cavity pressure for the selected material
• Tooling: Worn or mismatched parting line surfaces; vents that are too deep, allowing stiffer plastics to protrude; inadequate clamping support
• Process: Injection pressure or melt temperature too high, exceeding the mold's clamping resistance; shot size too large

How to Fix It

Flash is fundamentally a pressure leak: injection force is exceeding the mold's ability to stay sealed. The first process correction is to reduce injection pressure and melt temperature to decrease the force pushing plastic into gaps. On the tooling side, inspect the parting line for wear, dents, or misalignment and resurface as needed. Increasing clamp tonnage (within machine limits) can also help if the press is undersized for the part's projected area. For new tool designs, adding adequate draft early in the design phase prevents parting line interference that becomes progressively worse as the mold cycles.

6. Weld Lines (Knit Lines)

What It Looks Like

Weld lines — also called knit lines — appear as faint lines, seams, or discoloration where two separate flow fronts met inside the mold cavity. They're most common downstream of holes, pins, or cores that split the melt flow. Beyond the cosmetic mark, weld lines represent a bond between two partially cooled fronts, which can be weaker than the surrounding material.

Root Causes

• Design: Part features (holes, slots, inserts) that force the melt to split and rejoin; wall thickness variation that slows one flow front relative to another
• Tooling: Gate placement that generates two flow fronts on a cosmetic surface; insufficient mold temperature allowing fronts to solidify before fusing
• Process: Melt temperature too low; injection speed too slow, allowing the flow fronts to cool before they meet

How to Fix It

Weld lines cannot be fully eliminated if the geometry requires flow fronts to meet, but their location and strength can be managed. Raising melt temperature and increasing injection speed keeps both fronts hotter at the point of convergence, producing a stronger bond. Gate repositioning can move the weld line to a non-cosmetic surface. If structural strength at the weld is critical, selecting a lower-viscosity resin or increasing mold temperature can both improve fusion. Texturing or painting the part surface is a common cosmetic strategy to make minor weld lines invisible in service.

7. Jetting

What It Looks Like

Jetting produces thin, squiggly, worm-like lines or folds on the part surface, originating near the gate. It occurs when the melt enters the cavity as a fast, narrow jet that doesn't immediately contact the mold wall — the stream cools and solidifies mid-cavity before the rest of the material fills in around it.

Root Causes

• Design: Gate positioned opposite a large open cavity with no obstruction for the melt to land against
• Tooling: Gate size too small relative to the part wall thickness, creating a high-velocity jet; gate geometry not optimized to spread the melt across the mold wall
• Process: Injection speed too high, particularly at the beginning of the fill stage; melt temperature too low, reducing flow length

How to Fix It

The most effective tooling fix is to enlarge the gate or change its geometry so the melt fans out against the mold wall rather than shooting across the cavity. Fan gates and overlap gates are specifically designed to prevent jetting in high-risk configurations. On the process side, slowing the initial injection speed (a slow-fill approach at the start of the shot) reduces jet velocity. Repositioning the gate so the melt impinges directly on a core, rib, or wall eliminates the open-cavity run that jetting requires.

8. Vacuum Voids

What It Looks Like

Vacuum voids are air pockets trapped inside the body of the molded part — often invisible from the outside but detectable through cross-sectioning, X-ray inspection, or CT scanning. In transparent materials, voids are visible as bubbles. In structural parts, voids reduce load-bearing cross-section and create stress concentration points.

Root Causes

• Design: Walls too thick, creating a large interior mass that cools slower than the surface skin
• Tooling: Gate positioned far from the heaviest section; runner too small to maintain adequate packing pressure
• Process: Holding pressure too low or hold time too short; high mold temperature keeping the interior molten too long

How to Fix It

Voids and sink marks share the same root cause — insufficient material compensation for volumetric shrinkage — but manifest differently based on whether the outer skin is rigid enough to resist being pulled inward. The fixes are similar: increase holding pressure and hold time to force additional material into the cooling mass, and reduce mold temperature to accelerate skin solidification. Design-wise, coring out thick sections eliminates the thermal mass that creates voids. Moving the gate closer to heavy sections ensures packing pressure reaches the area where it's most needed.

9. Burn Marks

What It Looks Like

Burn marks appear as yellowish, brown, or black discoloration on the part surface — most commonly at end-of-fill locations, around cores, or at the tips of ribs and thin features. In severe cases, the affected area may be charred or show signs of polymer degradation. Burn marks are often a sign of trapped air, not just excess heat.

Root Causes

• Design: Thin ribs or deep blind features that trap air with no escape path
• Tooling: Inadequate or blocked venting at end-of-fill locations; gates or runners that are too restrictive, generating excessive shear heat
• Process: Injection speed too high, compressing trapped air rapidly (the diesel effect); barrel temperature or residence time too high, thermally degrading the resin before injection

How to Fix It

When burn marks appear at the end of the flow path, the most likely cause is a trapped air pocket — adding or cleaning vents at those locations is the primary fix. Reducing injection speed in the final fill stage allows air more time to escape. If burn marks occur near the gate, excessive shear heat is more likely the cause; enlarging the gate or reducing injection speed and barrel temperature addresses this. Regular mold cleaning to remove degraded material residue from the ejector pins and parting line is also essential maintenance for preventing recurrence.

10. Splay (Silver Streaks)

What It Looks Like

Splay marks — also called silver streaks — appear as shiny, silvery, or white streak patterns on the part surface, typically fanning outward from the gate area. They can look like water wave patterns, arrow-shaped sprays, or elongated lines following the material flow direction. Splay affects both cosmetics and, in more severe cases, mechanical strength.

Root Causes

• Design: Sharp corners near gate areas causing turbulent flow; inadequate venting leading to gas entrapment
• Tooling: Gate too small, generating excessive shear heat; blocked or undersized vents around the gate
• Process: Hygroscopic material not properly dried before molding (moisture converts to steam under injection heat); injection speed or pressure too high, causing turbulent melt flow and air entrapment; barrel temperature too high, degrading the resin and releasing gas

How to Fix It

The most common and most preventable cause of splay is insufficient material drying. Hygroscopic resins such as nylon (PA), polycarbonate (PC), and ABS absorb ambient moisture rapidly — in some cases within an hour of being removed from sealed packaging. Pre-drying to the manufacturer-specified moisture content level before molding eliminates moisture-based splay entirely. If the streaks are process-driven, reduce injection speed and back pressure to minimize shear heating. Enlarging the gate and adding venting near the gate area also reduce both shear and gas entrapment-related splay.

11. Surface Delamination

What It Looks Like

Surface delamination causes the outer layer of a molded part to peel or flake away in thin sheets, similar to the layers of a croissant. The effect is most visible on flat or slightly curved surfaces. Beyond being a cosmetic defect, delamination significantly reduces part strength by separating the load-bearing outer skin from the core.

Root Causes

• Design: Sharp corners that cause abrupt flow direction changes, shearing the melt into layers
• Tooling: Runners or gates too small, generating extreme shear that separates material layers; excessive use of mold release agents that contaminate the melt surface
• Process: Incompatible material contamination — most often a different resin from a previous run mixed into the current shot; mold temperature too low, freezing layers before they can bond

How to Fix It

Material contamination is the most frequent root cause of delamination: even small amounts of an incompatible resin — introduced through inadequate purging between material changes — will prevent proper interlayer bonding. Thoroughly purging the barrel when switching materials and establishing strict material handling protocols eliminates this cause. On the tooling side, smooth corners in the runner and gate system reduce shear. Minimizing the use of external mold release agents — and improving the ejection mechanism so release agents aren't needed — also significantly reduces delamination risk.

12. Cracking and Crazing

What It Looks Like

Cracks are visible fractures in the part, either on the surface or propagating through the wall. Crazing appears as a network of fine surface microcracks that create a whitish, hazy appearance — the part hasn't yet fractured, but it's internally stressed. Both typically occur at ejection, during assembly, or in early service.

Root Causes

• Design: Sharp internal corners creating stress concentration points; walls too thin to distribute ejection force adequately
• Tooling: Insufficient draft angles causing high ejection force; ejector pin placement over weak or thin features; overly aggressive ejection system
• Process: Parts ejected before adequate cooling; mold temperature too low, causing the material to be brittle at ejection; chemical incompatibility between the resin and any mold release agent or secondary lubricant

How to Fix It

Adding generous internal radii to corners is the single most effective design-level fix — sharp corners are stress risers that act as crack initiation sites regardless of how well the process is controlled. Ensure draft angles are sufficient (typically 1–2° minimum on vertical walls) to reduce ejection drag. On the process side, extend cooling time before ejection, particularly for thick-walled or semi-crystalline parts that need longer to develop adequate rigidity. Review ejector pin placement with the mold maker to ensure force is applied to robust features rather than delicate walls.

Quick-Reference: Defect Diagnosis at a Glance

When a defect appears on the shop floor or at first article inspection, the table below provides a fast starting point for diagnosis — pairing the visual symptom with the most likely root-cause category and the first corrective action to investigate.

Preventing Defects Before the Mold Is Cut

The most cost-effective defect is the one that never makes it to first article. A thorough design-for-manufacturing (DFM) review before tooling commitment can identify wall thickness violations, problematic gate locations, insufficient draft, and rib geometries likely to cause sink or warp — all without spending a dollar on steel. For teams working with plastic injection molding, integrating DFM early in the design cycle is the highest-leverage action available.

It's also worth considering whether the right manufacturing process is being used at the right stage of development. During early design validation, 3D printing or vacuum casting can produce functional prototypes that validate geometry and assembly without injection mold tooling — allowing teams to resolve design issues before committing to hard tooling. For programs that need bridge parts or first production quantities, low volume manufacturing with soft tooling offers a lower-risk path to learning how a design behaves in the injection molding process before scaling to high volume production.

For applications where injection molding isn't the right fit — or where secondary processes complement it — CNC machining, compression molding, and liquid silicone rubber (LSR) molding each carry their own defect profiles and design requirements. Understanding those differences upfront is part of selecting the right process for the part.

Final Thoughts

Injection molding defects are predictable. That's actually good news — it means they're preventable, provided you understand where in the design-tooling-process chain the problem originates. The 12 defects covered in this guide account for the vast majority of issues that engineering teams encounter across industries, from consumer electronics housings to automotive under-hood components to medical device enclosures.

The most important takeaway is sequencing: address design issues first, tooling issues second, and process issues last. Chasing a sink mark with process adjustments when it's caused by an oversized rib will produce a never-ending game of parameter tuning. Fixing the rib geometry eliminates the issue entirely. That same logic applies across every defect on this list.

Working with a manufacturing partner who provides engineering input at the design stage — not just at first article — is what separates teams that hit production schedules from those that spend months in tooling revisions. Explore NICE Rapid's full manufacturing services to see how expert DFM guidance and scalable production capabilities can help you take your part from CAD file to finished production with fewer surprises along the way.