Every defect tells a story about what went wrong in the process, the mold, or the material. Reading those stories accurately separates skilled troubleshooters from parts-swappers who cycle through adjustments hoping something works. A burn mark in the corner speaks of trapped gas. Sink marks over ribs reveal thick sections cooling too slowly. Warpage appearing hours after molding points to frozen-in stress finally releasing. The defect is evidence; the troubleshooter is the detective.
Random parameter adjustments rarely solve injection molding defects because defects have specific causes that require specific remedies. Increasing pack pressure won’t fix burn marks. Raising mold temperature won’t cure moisture-induced splay. Each defect type narrows the investigation to a subset of variables, and understanding those relationships turns troubleshooting from trial-and-error into systematic problem-solving.
A Framework for Defect Analysis
Before diving into specific defects, establish a systematic approach that prevents wasted effort and ensures root causes get addressed rather than symptoms getting masked.
Root cause categories fall into four buckets: machine, mold, material, and process. Machine issues include insufficient clamp tonnage, worn check rings, and barrel heater problems. Mold issues involve venting, cooling, and gate design. Material issues encompass moisture content, contamination, and lot variation. Process issues cover temperature profiles, speed settings, and pressure parameters. Most defects trace to one or two categories, and identifying the category focuses the investigation.
Systematic approach beats random adjustment every time. Document the defect: where on the part does it appear, how frequently, when did it start, what changed before it appeared. Examine samples carefully before touching machine controls. Form a hypothesis based on defect characteristics, then test that hypothesis with a targeted adjustment. One variable at a time, with measurement before and after.
The investigation sequence matters for efficiency. Start with the least expensive interventions first. Process adjustments cost nothing but time. Mold cleaning costs minimal downtime. Material verification requires simple testing. Only after ruling out these categories should investigation move to expensive interventions like mold repair or machine service.
Pattern recognition accelerates diagnosis. Defects that appear in the same location every shot point to mold or design issues. Defects that appear randomly suggest process or material variation. Defects that worsen over a production run indicate progressive mold damage or material degradation. Defects that appear at startup but disappear during production suggest thermal stabilization issues.
Documentation matters for continuous improvement. Without records, the same problems get solved repeatedly by different people, and process knowledge walks out the door when experienced operators leave. A simple defect log capturing date, defect type, cause found, and solution applied builds institutional knowledge over time.
Fill Defects
Fill defects occur when material doesn’t reach all areas of the mold cavity properly or escapes where it shouldn’t.
Short Shots
Short shots, or incomplete fill, produce parts missing material in one or more areas. The missing area might be a thin section, a corner, a rib, or any feature distant from the gate. The root cause is always that material stopped flowing before the cavity filled completely. For detailed diagnosis methodology and solutions by root cause, see the dedicated short shot troubleshooting guide (Article #26).
Pressure-related causes involve insufficient force to push material through the flow path. The machine may lack injection pressure capacity for the application. Pressure drop through long runners and small gates may consume available pressure before material reaches distant areas. Check actual cavity pressure if monitoring is available; peak pressure at transfer indicates whether the machine is pressure-limited.
Flow-related causes involve material that’s too viscous or flow paths that are too restrictive. Low melt temperature increases viscosity and reduces flow length. Undersized gates create excessive pressure drop. Thin sections freeze before material can fill them. Flow hesitation where material pauses and cools mid-fill prevents complete filling.
Venting-related causes produce short shots in specific areas, typically at the end of flow or in last-to-fill locations. Trapped air compresses and heats as the flow front advances, but eventually creates back-pressure that stops flow. The short shot location corresponds to vent locations or lack thereof.
Diagnosing short shots involves short-shot studies: progressively increasing shot size while observing fill pattern. Pressure-limited fills show gradual extension with more pressure. Flow-limited fills show the same pattern regardless of pressure. Vent-limited fills stop abruptly at specific locations.
Flash
Flash is material that escapes the cavity, forming thin webs or fins at parting lines, shut-offs, or around ejector pins. Minor flash might be cosmetically acceptable; heavy flash requires secondary trimming and indicates a serious problem requiring correction. For systematic diagnosis and elimination strategies, see the comprehensive flash troubleshooting guide (Article #25).
Mold-related flash results from gaps that allow material to escape. Parting line damage from previous over-packing or foreign material creates permanent gaps. Worn shut-off surfaces no longer seal tightly. Inadequate parting line contact area can’t withstand cavity pressure. Mold inspection reveals these conditions.
Machine-related flash comes from insufficient clamp force to keep the mold closed against cavity pressure. The calculation is straightforward: clamp force must exceed projected area times cavity pressure. If the machine is under-tonnage for the application, flash results. Worn toggles or hydraulic systems may not deliver rated tonnage.
Process-related flash occurs when conditions create higher cavity pressure than normal. Over-packing, excessive injection speed, or material temperature too high (lowering viscosity) can cause flash even with adequate clamping and good mold condition.
The sequence for diagnosing flash: check mold condition first (parting line, shut-offs), verify clamping force is adequate for projected area, then examine process parameters. Fixing flash by reducing pack pressure may eliminate the flash but create sink marks or dimensional problems. Address root causes rather than compensating.
Jetting
Jetting produces snake-like patterns on the part surface where material squirted through the gate rather than flowing smoothly into the cavity. It occurs when injection speed is too high for the gate geometry, or when the gate directs flow into open space rather than against a wall.
Reducing initial injection speed allows material to form a controlled flow front rather than jetting. Gate redesign to direct flow against an opposing surface prevents jetting by design. Jetting is primarily a gate design and injection speed issue; other process adjustments rarely solve it.
Cooling Defects
Cooling defects result from uneven or insufficient heat removal as the part solidifies in the mold. They’re often the most difficult defects to solve because they involve the interaction of design, tooling, and process.
Warpage
Warpage causes parts to twist, bow, or distort from intended geometry. It may appear immediately upon ejection or develop over hours or days as internal stresses relax. Warped parts fail dimensional specifications and may not function in assembly. This section provides an overview; for comprehensive prevention and correction strategies, see the dedicated warpage troubleshooting guide (Article #23).
Design-induced warpage results from non-uniform wall thickness, asymmetric features, or long unsupported spans. Thick sections cool slower than thin sections, shrinking more and pulling the part out of shape. Asymmetric features create unbalanced shrinkage. Poor design makes warpage nearly inevitable regardless of process optimization.
Process-induced warpage comes from mold temperature differentials, non-uniform cooling, or orientation effects. If the cavity side runs hotter than the core side, differential shrinkage warps the part. Inadequate cooling time ejects parts while still soft enough to deform. High orientation near the gate creates different shrinkage behavior than far from the gate.
Material-related warpage depends on material properties. Semi-crystalline materials like nylon and PP have higher, less uniform shrinkage than amorphous materials. Glass-filled materials shrink differently in flow versus cross-flow directions, creating predictable warpage patterns.
Correcting warpage requires identifying which factor dominates. Design-induced warpage needs design changes. Process-induced warpage responds to cooling optimization and mold temperature balancing. Material-related warpage may require grade changes or post-mold fixturing.
Sink Marks
Sink marks are surface depressions that appear over thick sections, ribs, bosses, and other features where material accumulates. They form because thick sections shrink more as they cool, and if pack pressure doesn’t compensate, the surface follows the contraction, creating visible depressions. For detailed design guidelines and process solutions, see the dedicated sink mark guide (Article #24).
Geometry factors determine sink mark risk. Rib thickness greater than 60 percent of nominal wall (or 40 percent for high-gloss surfaces) creates sink marks. Thick bosses without coring create the same problem. Any feature that creates a thick section behind a visible surface risks sink marks.
Process factors affect whether geometry-driven risk becomes a visible defect. Adequate pack pressure and hold time compensate for shrinkage during cooling. Gate freeze-off must occur late enough to allow packing pressure to reach the thick section. Mold temperature affects cooling rate and shrinkage behavior.
Prevention through design is more effective than process compensation. Coring thick sections, proper rib proportions, and textured surfaces that hide minor sinks address the root cause. Process optimization can reduce sink marks but rarely eliminates them when design creates severe conditions.
Voids
Voids are internal bubbles or cavities within the part, invisible from the surface but detectable through cross-sectioning or imaging. They form in thick sections where material at the center cannot be replenished as outer layers solidify and shrink.
Voids indicate the same conditions that cause sink marks, but with thicker sections where the outer skin is rigid enough to prevent surface depression. Instead, the vacuum from shrinkage creates internal cavities. Pack pressure and hold time are the primary process controls, but design-level solutions (coring, gas assist, foam processing) are often necessary for thick parts.
Surface Defects
Surface defects affect appearance without necessarily compromising function. They’re particularly critical on cosmetic parts where visual quality determines acceptance.
Flow Lines
Flow lines are visible patterns on the surface showing material flow direction. They appear as waves, bands, or color variations corresponding to how material entered and filled the cavity. Cool melt fronts, slow fill speeds, and interactions with mold surface temperature all contribute.
Increasing melt temperature, raising mold temperature, and increasing injection speed typically reduce flow lines by maintaining material fluidity throughout fill. Gate location and design also influence flow line severity.
Weld Lines
Weld lines form where two flow fronts meet, either from multiple gates or from flow splitting around an obstacle and rejoining. They appear as visible lines and create mechanical weak points where the fronts didn’t fully knit together.
Weld line appearance improves with higher melt and mold temperatures that keep the converging fronts hot enough to fuse. Weld line strength improves similarly but may still be 10 to 30 percent weaker than surrounding material. Design can relocate weld lines to less critical areas by adjusting gate location or part geometry.
Burn Marks
Burn marks are brown or black discolorations, typically at the end of flow or in corners, caused by trapped air compressing and heating to the point of igniting material. The compressed air acts like a diesel engine, raising temperature through adiabatic compression until combustion occurs.
Venting is the primary solution. Adequate vents at last-to-fill locations allow air to escape before compression causes burning. Reducing injection speed decreases compression heating but also extends cycle time. Burn marks at consistent locations indicate venting problems; random burn marks may indicate material contamination or degradation.
Splay and Silver Streaks
Splay appears as silver streaks radiating from the gate, caused by volatiles at the flow front. The most common cause is moisture in hygroscopic material, but splay can also result from material degradation, contamination, or volatiles in additives.
Moisture-induced splay requires proper drying before processing. Four hours at specified temperature with adequate dewpoint is the standard for most hygroscopic materials. Degradation-induced splay responds to lower melt temperatures and shorter residence times. Contamination requires material system cleaning.
Gate Blush
Gate blush is a discoloration or texture difference in the area immediately around the gate, caused by high shear stress as material enters the cavity. It’s more visible on lighter colors and glossy surfaces.
Reducing injection speed at gate entry decreases shear stress and blush severity. Larger gates reduce shear stress at a given flow rate but may leave larger vestige marks. Gate blush represents a tradeoff between fill rate and appearance that must be balanced for each application.
Mechanical Defects
Mechanical defects affect part performance rather than appearance, potentially causing field failures that are far more costly than visible defects caught in production.
Stress Cracking
Stress cracks develop over time in parts under load or exposed to chemicals that attack stressed polymer chains. The cracks weren’t present at molding but develop during use. High molded-in stress increases susceptibility.
Molded-in stress comes from fast cooling that freezes orientation, excessive pack pressure, and asymmetric cooling. Reducing molded-in stress through process optimization and design improvements reduces stress cracking risk. Material selection also matters; some grades have better stress crack resistance than others.
Brittleness
Parts that are unexpectedly brittle may have experienced material degradation during processing, over-drying (especially critical for nylon), contamination, or simply received the wrong material grade. Brittleness shows up as unexpectedly low impact strength in testing or field failures.
Checking actual material versus specification, verifying processing temperature history, and testing incoming material properties helps identify brittleness causes. Over-drying nylon at high temperatures for extended periods damages the polymer; following drying specifications precisely prevents this.
Delamination
Delamination causes layers to separate at the part surface, appearing as peeling or flaking. It typically indicates contamination with an incompatible material or severe shear damage during processing.
Material contamination from inadequate purging between materials or cross-contamination in material handling is the most common cause. Verifying material identity and improving contamination prevention addresses the root cause.
Dimensional Defects
Parts outside dimensional tolerance fail regardless of appearance. Dimensional defects challenge troubleshooting because many factors influence final dimensions.
Process variation causes shot-to-shot and day-to-day dimensional changes. Temperature drift, pressure variation, and cycle time inconsistency all affect shrinkage and final size. Statistical process control monitoring key dimensions identifies variation requiring attention.
Material variation between lots causes dimensional shifts even with constant processing. Different lots may have different shrinkage characteristics within specification but enough to shift dimensions. Qualifying lots before production use prevents surprises.
Measurement variation sometimes explains apparent dimensional problems. Verifying measurement method, gage repeatability, and part temperature during measurement ensures dimensional data is meaningful.
When to Adjust Process vs. Mold vs. Material
The most cost-effective solution path depends on defect type and root cause.
Process adjustments are immediate and low-cost but limited in what they can achieve. They’re appropriate for defects caused by parameter drift or suboptimal settings. They cannot compensate for fundamental design or mold problems.
Mold modifications take longer and cost more but address root causes that process adjustments can only compensate for. Improving venting, adding cooling, or enlarging gates provides permanent solutions to recurring problems.
Material changes involve qualification effort but may solve problems no amount of process or mold adjustment can fix. A different grade with better flow, lower shrinkage, or improved stress crack resistance may be the most effective path forward.
The goal isn’t just fixing today’s problem but preventing tomorrow’s. Documenting defects, their causes, and their solutions builds the knowledge base that reduces troubleshooting time on future problems and prevents repeating past mistakes.
Sources
- Rosato, Donald V. “Injection Molding Handbook.” Springer.
- Beaumont, John P. “Runner and Gating Design Handbook.” Hanser, 2004.
- RJG Inc. “Injection Molding Troubleshooting Guide.” https://rjginc.com/
- Plastics Technology. “Troubleshooting Section.” https://www.ptonline.com/
- Kazmer, David O. “Injection Mold Design Engineering.” Hanser, 2007.