Injection Molding Defects

QUICK ANSWER
Injection-molding defects rarely have one cause. The same symptom can originate in part geometry, resin condition, tool design, machine settings, cooling, venting or measurement. Diagnose the pattern first, separate the hypotheses, gather evidence and run controlled trials before cutting steel or accepting a permanent process workaround.

1. A Defect Name Is Not a Root Cause

Calling a depression a 'sink mark' or an incomplete edge a 'short shot' describes what the part looks like. It does not explain why the problem occurred. Robust troubleshooting starts by separating symptom from mechanism.

For example, a short shot may result from an overly long thin flow path, frozen gate, inadequate venting, low melt temperature, low injection capability, contamination, a restricted runner or simply an undersized gate. Treating all of these with more pressure may suppress the symptom temporarily while increasing flash, stress or tool wear.

CORE PRINCIPLE
DO NOT TUNE THE MACHINE UNTIL THE FAILURE MODE IS UNDERSTOOD. Process changes should test a hypothesis—not replace one.

2. Defect Families

3. Four Source Buckets: Design, Material, Tool, Process

DIAGNOSTIC DISCIPLINE
EVERY MAJOR DEFECT REVIEW SHOULD CONTAIN ALL FOUR COLUMNS—even if one seems obvious at first.

4. Start With Pattern Recognition

Defect location is evidence. A problem always at the end of fill suggests a different mechanism from a problem that moves randomly shot to shot. A sink opposite a boss points toward local mass; a burn at a dead-end points toward trapped gas; flash that grows around one worn shutoff is different from flash appearing around the entire parting line.

5. Sink Marks

Sink marks are local surface depressions created when a thick region continues shrinking after the outer skin has solidified. Common design drivers include thick walls, heavy rib/boss intersections and abrupt local mass. Process drivers can include insufficient packing, early gate freeze, high melt/mold conditions or inadequate cooling.

Protolabs' current injection-molding guide specifically connects sink to thick walls, heavy bosses and thick ribs and recommends coring thick areas and using structural ribs instead of solid mass.

BEST PREVENTION
REMOVE THE MASS THAT CREATES THE SINK. Process tuning is useful, but a thick local section remains a recurring thermal and shrinkage burden.

6. Sink vs Internal Voids

A sink is visible surface collapse. A void is an internal cavity formed when the center of a thick section shrinks away from itself after the outside has solidified. Both can originate from the same fundamental problem: excessive local mass and insufficient ability to feed contraction.

If a critical thick molded section cannot be redesigned, define an inspection strategy proportionate to the real structural risk rather than relying only on external appearance.

7. Warpage

Warpage is shape distortion caused by non-uniform shrinkage, cooling or orientation. The mechanism can be geometric, thermal or anisotropic—and in fiber-reinforced materials, all three can interact.

• Uneven wall thickness creates different cooling and shrinkage histories.

• Asymmetric cooling can pull the part toward one side.

• Gate location changes flow direction and packing.

• Glass fibers align with flow and create direction-dependent shrinkage/stiffness.

• Ejection can permanently deform a part that is still too hot or flexible.

• Post-mold moisture conditioning can change dimensions in hygroscopic polymers.

WARPAGE PRINCIPLE
A FLAT CAD MODEL DOES NOT CREATE A FLAT MOLDING. Balance geometry, filling, cooling and material orientation.

8. How to Root-Cause Warpage

Avoid 'straightening' every part as the first serial solution. Secondary straightening may be acceptable for some products, but it should be a conscious route—not compensation for an unstable mold/process.

9. Short Shots

A short shot is an incompletely filled cavity. BASF's current Injection Molding Troubleshooter includes short shot as a dedicated defect category, while Protolabs identifies thin walls, long flow paths and poor gate location as common design causes.

TROUBLESHOOTING RULE
MORE PRESSURE IS NOT THE FIRST DESIGN FIX. Improve the flow path, gate, venting or wall strategy if the part demands excessive pressure to fill.

10. Flow Length and Freeze-Off

As melt flows through the cavity, it loses heat to the mold. Thin sections freeze earlier and consume pressure rapidly. A long thin end-of-fill region may be impossible to fill robustly even if one highly tuned trial shot succeeds.

A robust design provides a manufacturable flow path for the exact resin. Gate position, wall transitions and local thickness should be reviewed before tooling steel is finalized.

11. Flash

Flash is excess material escaping at the parting line, shutoff, vent, ejector or other tool interface. It can be a tool-sealing issue, wear issue, clamp-force issue, excessive cavity-pressure issue or a symptom of a difficult-to-fill design.

Protolabs notes that very thin parts can require high injection pressure, increasing flash risk, and recommends simplifying difficult shutoff/parting-line geometry where possible.

QUALITY PRINCIPLE
DO NOT ACCEPT PERMANENT DEFLASHING AS A DEFAULT if the flash mechanism can be corrected at the tool/process/design level.

12. Burn Marks / Dieseling

Burn marks often appear dark brown or black near trapped-gas regions. BASF lists diesel effect/burning and air entrapment separately in its troubleshooting system. Rapid compression of trapped air can generate very high local temperature; polymer degradation from excessive shear or residence time can create similar discoloration.

Venting correction, flow redirection and removal of dead-end geometry are more robust than simply slowing the process until the burn disappears.

13. Weld / Knit Lines

A weld or knit line forms when separate melt fronts meet after flowing around a hole/core or from multiple gates. It can be cosmetic, structural or both.

Line strength depends on temperature at reunion, pressure, resin, fiber orientation, contamination/air and the local load direction. A visually faint knit line can still matter mechanically in a high-stress region.

FUNCTIONAL RULE
DO NOT JUDGE A KNIT LINE BY APPEARANCE ALONE. Place it away from critical stress/cosmetic zones or validate its performance.

14. Splay / Silver Streaks

Splay appears as silver or whitish streaking on the molded surface. Moisture, volatiles, contamination, shear and thermal degradation can all produce similar-looking streaks.

BASF's troubleshooting library distinguishes moisture streaks, burning streaks, colored streaks and reinforcement-related streaks, which is a useful reminder that 'splay' is a visual category rather than one guaranteed root cause.

MATERIAL RULE
DRYING IS A CONTROLLED PROCESS—not 'putting resin in a dryer.' Validate time, temperature, dew point, storage and transfer for the exact resin.

15. Black Specks / Contamination

Black specks can originate from degraded polymer, contamination, dead spots in the barrel/hot runner, dirty regrind, color change residue or tool deposits.

Location and frequency matter. Random specks across cavities point differently from a repeatable mark at one cavity feature.

Before polishing the mold, inspect the entire material path: hopper, dryer, conveying, screw/barrel, nozzle, hot runner and regrind handling.

16. Delamination

Delamination is separation of thin surface layers. Incompatible contamination, moisture, excessive release agent, degraded material or poorly bonded blended streams can contribute.

Do not simply increase temperature. Confirm material identity, contamination risk and whether a multi-material or overmolding interface is chemically/mechanically compatible.

17. Jetting

Jetting occurs when melt shoots into open cavity space as a free stream rather than establishing a stable advancing flow front. It can leave snake-like or wavy surface marks.

Protolabs identifies poor gate entry/geometry as an important design cause. The corrective goal is to make the incoming melt impinge on a wall or spread progressively instead of projecting as a jet.

• Review gate type and direction.

• Move gate so the stream meets a wall.

• Use geometry that encourages fountain flow.

• Control initial injection velocity as a process trial.

• Verify that the process fix remains stable across the production window.

18. Flow Marks / Hesitation

Flow marks can arise from changes in flow velocity, wall transitions, cooling, gate behavior or material front hesitation. Rings or wave-like patterns may indicate repeated acceleration/deceleration or partial skin freezing.

Map the mark to local geometry and flow simulation before treating it as a purely cosmetic surface-finish issue.

19. Gloss Variation

Gloss differences can come from mold surface, pressure contact, temperature, flow orientation, cooling or local geometry. BASF includes gloss variations and dull spots as separate troubleshooting categories.

If cosmetics are critical, define the visible surface and acceptable appearance early; otherwise the molder may meet dimensional requirements while the part still fails customer perception.

20. Ejector Marks

Ejector marks are witness marks or local deformation where ejection force transfers into the part. They become worse when the part is too hot, ejector area is small, draft is insufficient or the part grips the core strongly.

21. Drag Marks / Scuffing

Drag marks occur when the part rubs against the mold during ejection. Zero draft, inadequate draft for texture, rough tool surface, local undercut or misalignment may be involved.

EJECTION PRINCIPLE
A PART THAT ONLY EJECTS CLEANLY WITH RELEASE SPRAY OR OPERATOR ASSISTANCE IS NOT YET A ROBUST SERIAL PROCESS.

22. Sticking / Demolding Problems

BASF's troubleshooter includes demolding problems as a dedicated category. A part can stick on the cavity side, core side, a slider or textured surface.

Root cause may involve draft, surface finish, mold temperature, packing, shrinkage, geometry, undercut, ejection balance or vacuum. Confirm where the part is gripping before changing process settings.

23. Stress Whitening / Cracking During Ejection

White marks around snaps, bosses or ejector locations can indicate excessive strain. Cracks may initiate if the polymer is brittle in the molded condition or if ejection force is concentrated.

Review draft, radii, ejector support, residual stress and material conditioning. A clean cosmetic surface does not guarantee low internal stress.

24. Dimensional Drift

Dimensional drift is not always an obvious 'defect' like a burn or sink, but it can be one of the most expensive serial problems. Dimensions can shift with material lot, moisture, mold temperature, process setup, cooling-water condition, tool wear, machine change or measurement conditioning.

CONTROL PRINCIPLE
A DIMENSION THAT MOVES WITH THE PROCESS WINDOW IS A PROCESS CHARACTERISTIC—not just an inspection number.

25. ISO 20457:2026 and Defects

ISO 20457:2026, Edition 2, was published in August 2026 and is now the current international standard for tolerances and acceptance conditions of plastic molded parts within its scope. ISO explicitly notes that molded-part surface imperfections such as sink marks, undesired flow structures, roughness and joint lines are not addressed by the standard.

That distinction matters: dimensional acceptance and cosmetic/defect acceptance are separate engineering topics. A drawing should not assume that citing ISO 20457 automatically defines acceptable sink, knit lines, burns or surface quality.

SPECIFICATION RULE
DEFINE COSMETIC / SURFACE DEFECT ACCEPTANCE EXPLICITLY. ISO 20457:2026 DOES NOT DO THIS FOR YOU.

26. Measurement Conditioning

Plastic parts can change dimension with temperature, moisture and restraint. ISO 20457:2026 is built around plastics-specific behavior, including shrinkage and warpage, rather than treating parts as rigid metal bodies.

For hygroscopic polymers, define whether parts are measured dry-as-molded, conditioned or after a specified stabilization period. For flexible parts, define fixture/restraint if acceptance depends on it.

27. Tool Wear and Defect Drift

A process that starts clean can develop flash, dimensional drift, vent blockage, deposit, gate wear or ejection damage over tool life. Preventive maintenance should therefore be part of the quality plan.

• Track shots/cycles where meaningful.

• Inspect gates, vents, shutoffs and ejectors.

• Control lubrication/cleaning methods.

• Document repairs and steel changes.

• Requalify critical characteristics after major maintenance.

28. The Corrective-Action Hierarchy

DO NOT CUT STEEL FIRST
STEEL CHANGE SHOULD FOLLOW EVIDENCE. A tool correction can be the right permanent action—but only after the defect mechanism is understood.

29. Symptom Suppression vs Root-Cause Correction

Temporary process adjustments can be valid containment or development tools. The mistake is allowing a narrow workaround to become the serial specification without testing robustness.

30. Controlled Trials: Change One Mechanism at a Time

When many settings are changed together, the team may create a good part without learning why. A structured trial changes one variable or designed group of variables and records the response.

• Define the defect metric before the trial.

• Record cavity and material lot.

• Capture machine/process data, not only nominal setpoints.

• Use enough shots to reach thermal stability.

• Retain labeled samples.

• Return to baseline between major hypotheses where practical.

• Confirm the final fix across normal material and environmental variation.

31. Process Monitoring and Cavity Pressure

For high-risk or high-volume parts, pressure/time data can help distinguish fill, transfer, packing and gate-freeze behavior. Machine traces or cavity-pressure sensors can reveal changes that final inspection detects only after defects occur.

Monitoring does not replace DFM; it improves control of a process that is already fundamentally manufacturable.

32. Moldflow / Simulation

Simulation can predict flow fronts, pressure, weld-line locations, air traps, temperature, fiber orientation and warpage tendencies. Protolabs highlights mold-flow analysis as a way to identify difficult flow, knit-line, flash, sink and short-shot risk before tooling.

Simulation must be calibrated to the actual resin and tooling assumptions. It is evidence for a hypothesis—not certification that production will be defect-free.

33. When a Defect Requires a Design Change

• Thick local mass repeatedly creates sink/voids.

• Flow path is fundamentally too long/thin for the required resin.

• Critical weld line is geometrically unavoidable at a high-stress location.

• Part lacks adequate draft or ejection surfaces.

• Asymmetric geometry creates persistent warpage beyond practical process correction.

• Tolerance is incompatible with plastic behavior and product function can be redefined.

• Side action / shutoff geometry creates recurring flash or wear risk.

34. When a Defect Requires a Tool Change

• Gate size/location prevents robust fill/pack.

• Vents are absent, undersized, blocked by geometry or cannot reach trapped gas.

• Cooling layout creates persistent temperature imbalance.

• Shutoff/parting surfaces cannot contain cavity pressure.

• Ejection layout damages the part.

• Steel dimension requires correction after validated shrinkage data.

• Wear/damage has changed the tool from the qualified condition.

Before tool change, verify that resin preparation, machine condition and process setup are controlled. Cutting steel to compensate for a material or setup problem can create a second problem.

35. When Process Adjustment Is the Right Fix

Some defects genuinely result from process setup: incorrect transfer, inadequate hold, inappropriate injection-speed profile, wrong melt/mold temperature, insufficient cooling, unstable screw recovery or poor startup discipline.

The right fix is a documented stable process window—not a single magic setting. BASF also cautions that troubleshooting recommendations are initial guidance rather than binding guarantees, which reflects the case-specific nature of molding.

36. When Material Handling Is the Root Cause

• Hygroscopic resin not dried to supplier requirement.

• Dried material reabsorbs moisture before molding.

• Wrong resin or color concentrate loaded.

• Excess/uncontrolled regrind changes flow or contamination risk.

• Material lot or formulation change shifts shrinkage/viscosity.

• Thermal degradation occurs from residence time or temperature.

• Foreign material enters conveying or hopper system.

37. Defect Acceptance Must Be Functional

Not every visible mark is a reject, and not every visually perfect part is functional. Defect acceptance should reflect product purpose.

38. Cosmetic Standards and Boundary Samples

For visible consumer/OEM surfaces, words such as 'no visible defects' are too subjective. Use approved boundary samples, viewing distance, lighting, surface class, color/texture and defined defect zones where appropriate.

Separate cosmetic acceptance from dimensional tolerance. A part can be dimensionally conforming under ISO 20457 and still fail the agreed appearance standard.

39. Defect Risk and Cost Escalation

VALUE-AT-RISK
RESIN → MOLDING → INSERT / MACHINING → PRINT / COAT → ASSEMBLY → TEST → PACKAGING

The later a defect is discovered, the more accumulated value is at risk.

Critical molding characteristics should therefore be checked as early as practical, especially before expensive decoration, assembly or functional testing.

40. Supplier Quality Questions

41. Worked Example A — Sink Opposite a Boss

A cosmetic housing shows a depression directly opposite a screw boss. Increasing hold pressure reduces the mark but creates flash at the parting line.

Section analysis confirms a heavy boss-to-wall junction. The boss is cored and supported with gussets. The process returns to a lower, more stable cavity-pressure requirement.

LESSON
THE PROCESS WAS COMPENSATING FOR GEOMETRY. The robust correction removed the thermal mass.

42. Worked Example B — Burn at End of Fill

A dark mark appears at the same dead-end rib on every shot. Lowering injection speed improves appearance but increases short-shot risk.

Tool inspection shows insufficient venting at the trapped-air region. Venting and local flow path are corrected; normal fill speed is restored.

LESSON
THE DEFECT WAS TRAPPED GAS—not simply 'too much speed.'

43. Worked Example C — Warped Glass-Filled Bracket

A glass-filled PA bracket twists after molding even though nominal wall thickness is reasonably uniform.

Simulation and measurement show directional fiber orientation from an asymmetric gate, combined with uneven cooling. Gate/cooling strategy and rib orientation are revised; inspection is performed after defined conditioning.

LESSON
FIBER ORIENTATION + COOLING + CONDITIONING can dominate geometry alone.

44. Worked Example D — Random Splay

Silver streaks appear intermittently after production interruptions. Resin moisture checks are acceptable, but the problem correlates with long barrel residence.

Startup/purge discipline and thermal history are revised. The team confirms the issue was degradation/volatile-related rather than a tool-surface defect.

LESSON
SIMILAR-LOOKING STREAKS CAN HAVE DIFFERENT MECHANISMS. Use evidence, not labels.

45. Worked Example E — Flash After a Short-Shot Fix

A thin-wall cover initially short-shots. Injection pressure and speed are raised until fill completes, but flash appears around the mold.

The team reviews gate size, flow length and wall transition instead of continuing to increase pressure. The flow path is improved and cavity pressure drops.

LESSON
A 'FIX' THAT CREATES A SECOND DEFECT IS DIAGNOSTIC EVIDENCE.

46. Defect-to-Action Matrix

47. RFQ / Tool-Transfer Defect-Prevention Checklist

• Exact resin grade, filler and color/additive system

• Annual/lifetime volume and cavity strategy

• Critical-to-function dimensions and tolerance standard

• Cosmetic zones, texture and boundary samples

• Known historical defects and approved deviations

• Gate/runner/hot-runner concept and gate vestige limits

• Parting line, shutoffs, undercuts and side actions

• Cooling concept and critical thermal zones

• Draft and ejection strategy

• Drying and material-handling requirements

• Regrind policy

• Process-window / validation requirements

• FAI / capability / dimensional conditioning method

• Defect reaction plan and traceability

• Tool ownership, PM, spare inserts and repair history