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By applying best practices in design, mold engineering, and process control, these defects are largely predictable and avoidable. This article identifies common injection molding defects, examines their underlying reasons, and offers practical solutions to help you achieve consistently high-quality molded parts.
Quick-Reference Defect Comparison Table
The table below summarizes the various injection molding defect types and highlights their causes and possible solution or prevention methods. More details are provided in the next section.
| Injection Molding Defect | Description | Causes | Prevention/Solution |
| Flow Lines | Visible as streaks or lines on the surfaces of molded parts. The lines are often of different color or shade compared to the rest of the material | • Inappropriate injection speed, pressure, and temperature • Too small runner and gates |
• Monitor and adjust injection speed, pressure, and temperature accordingly • Increase runner and gate sizes |
| Burn Marks | Appears as yellow or black marks at the end of the flow | • High injection speed and/or pressure • Insufficient venting in the mold • Dirty vents |
• Monitor and adjust injection temperature accordingly • Check and clean restricted vents and air traps • Clean all molding surfaces and parting lines |
| Sink Marks | Visible as depressions or indentions on the surface of molded parts. Often occurs in thicker areas where outer layers rapidly cool and solidify before inner layers do. | • Inappropriate selection of plastic material • Improper mold design • Unideal cooling temperature • High injection speed or pressure |
• Proper Material selection • Better mold design • Using appropriate cooling temperature • Using appropriate injection speed or pressure |
| Surface Delamination | Peeling of the upper layer of the molded part, which exposes layers beneath. | • Inappropriate handling or storage of plastic material • Wrong choice of plastic materials • Poorly maintained equipment |
• Proper material handling and storage • Proper material selection • Proper maintenance of equipment |
| Weld Lines (Knit Lines) | Incomplete bonding of two mold fronts of molten plastic meet and solidify. Lines are visible on the surface of the molded parts. | • Inadequate mold design • Inappropriate cooling temperature • High injection speed or pressure |
• Proper mold design • Monitor and adjust cooling temperature accordingly • Use the right injection pressure |
| Flash | Shows up as thin layers of plastic that escape from the mold cavity. Are usually visible along the parting line, ejector pins, and gate areas. | • Excess supply of materials • Poorly maintained tools and equipment • Inappropriate mold set up • Low injection pressure |
• Use appropriate shot size • Proper maintenance of tools and equipment • Monitor and adjust injection pressure accordingly • Redesigning the mold |
| Short Shots | An incomplete part resulting from shortage of plastic material. Cavities are left on the molded part. | • Inadequate molding material • Low injection pressure and pressure • Inappropriate mold venting |
• Optimize mold design to match material flow • Adjust temperature and pressure accordingly • Use molding equipment with optimized short size |
| Gate Vestige | Small plastic protrusions or remnants left on parts at gate locations after molding | • Improper gate design • Excessive gate size • Incorrect trimming |
• Redesign gate geometry • Optimize gate dimensions • Implement automated trimming |
| Improper Parting Line Placement | Misaligned or incorrectly positioned parting lines causing flash, aesthetic, or dimensional issues | • Poor mold design • Incorrect tooling alignment |
• Optimize parting line placement in mold design • Improve mold alignment and tooling precision |
| Warping | A visible distortion or bending that occurs in parts as they cool and solidify due to uneven cooling rates | • Fluctuating cooling rates • Ineffective mold design • Flawed selection of plastic material |
• Ensure constant cooling rates • Optimize mold design • Use appropriate molding material |
| Jetting | Manifests as snake-like lines or streaks on the surface of the molded part | • High injection speed and pressure • Low mold surface temperature • Gate size too small |
• Monitor and adjust injection speed and pressure • Modify the gate design • Optimize the temperature of the mold and plastic material • Adjust gate size |
| Vacuum Voids | Air pockets that form within the interior of a molded part. Trapped air fails to vent out of the mold cavity during molding | • Uneven cooling • Low injection speed and/or pressure • Air traps in the melt |
• Monitor and adjust cooling temperature and pressure accordingly • Use an appropriate injection speed • Reduce decompression and screw RPM |
| Discoloration | Any visible discoloration on the surface of the molded parts. May indicate underlying problem within the molded parts | • Degradation of the plastic material caused by excessive heat and contamination of the material • Improper thermal stability of colorant • Different molds from a previous production run |
• Monitor and control the temperature of the mold and the plastic material • Ensure plastic material is properly dried and not contaminated • Proper mixing of the masterbatch • Maintain thermal stability of colorant. |
| Splay Marks (Silver Streaking) | Silvery or streaky marks appearing on part surfaces, typically radiating from gate areas | • Moisture or contamination in resin • High shear stress or injection speed • Improper resin drying |
• Ensure proper resin drying •Adjust injection speed and pressure • Improve material handling |
| Bubbles and Void | Air pockets or cavities visible inside or near the surface of molded parts | • Poor venting • Excessive moisture • Rapid injection speeds |
• Enhance mold venting • Optimize injection parameters •Proper drying and storage of resins |
Mold Design-Related vs. Process-Related Injection Molding Defects
Injection Molding Defects Caused By Mold Design
Defects rooted in mold design usually emerge from improper initial tooling or inadequate mold maintenance. These often require extensive, costly, and time-consuming corrections, including significant mold modifications or complete retooling. Addressing mold-related issues through comprehensive Design for Manufacturing (DfM) analyses during the early design stage prevents costly production interruptions.
The primary mold design-related defects include:
- Short Shots
- Flash
- Gate Vestige
- Improper Parting Line Placement
- Bubbles and Voids
Injection Molding Defects Caused By Process
Process-related defects commonly result from improper control or incorrect settings within the molding cycle. Variables such as injection pressure, injection speed, mold/resin temperatures, cooling rates, and material conditions significantly influence these defects. Unlike mold design issues, process-related problems often can be mitigated through adjustments in machine settings—without extensive mold modifications.
Typical process-related defects include:
- Flow Lines
- Burn Marks
- Warping
- Vacuum Voids
- Sink Marks
- Weld Lines (Knit Lines)
- Jetting
- Discoloration
- Surface Delamination
- Splay Marks (Silver Streaking)
By clearly differentiating mold design-related defects from process-related defects, engineers can effectively pinpoint root causes, streamline troubleshooting, and consistently achieve optimal injection molding quality.
Having established the common defects and their causes in the table above, we now turn directly to detailed, practical explorations of each defect—starting with one of the most critical design-related issues: Short Shots.
#1 Short Shots
A short shot is a severe injection molding defect that occurs when molten plastic fails to completely fill the mold cavity, resulting in incomplete or malformed parts. These parts are structurally unsound and usually discarded or reprocessed, leading to wasted material, time, and resources.
Short shots are often the result of poor flow characteristics—either due to restrictive mold designs, material viscosity issues, or suboptimal injection parameters. Inadequate venting or air traps can also block the material flow, preventing the cavity from filling properly.
Key Causes & Solutions:
- Narrow or blocked gates: Redesign gates to allow smoother material flow into the cavity.
- Low injection speed or pressure: Increase both to ensure the material reaches every section of the mold.
- Low mold or melt temperature: Adjust temperatures to maintain optimal resin flow during injection.
- Poor venting: Improve mold venting to allow trapped air to escape, reducing resistance in hard-to-reach areas.
#2 Flash
Flash, also known as spew, is a designer defect that manifests as thin layers of excess material at the edges of molded parts. It commonly appears along parting lines, ejector pin locations, or near gates—where molten plastic escapes into the gaps between mold components. Minor flashes can be trimmed, but extensive flash may render parts unusable, especially in high-precision or cosmetic applications.
Key Causes & Solutions:
- Poor mold design or wear on mold surfaces: redesign the mold or refurbish sealing surfaces
- Low clamping force: increase clamping pressure to prevent mold separation during injection
- Excessive injection pressure: reduce pressure to avoid forcing material into unwanted gaps
- High mold temperature: lower the temperature to improve sealing between mold halves
- Improper mold alignment or setup: check mold fit and ensure proper plate alignment
#3 Improper Parting Line Placement
A parting line is where two mold halves (core and cavity) meet. Incorrectly placing or aligning this line can lead to flash, visible seams, or additional finishing steps—especially if it crosses critical part features.
Key Causes & Solutions:
- Inadequate mold design or misalignment: Ensure precise alignment with robust mold clamps, alignment pins, and well-planned parting surfaces.
- Excessive complexity at the parting boundary: Simplify part geometry or split the design logically so critical features don’t straddle the parting line.
- Aesthetic considerations: Whenever possible, place parting lines along less visible edges or surfaces to minimize their visual impact.
#4 Bubbles and Voids
Bubbles and voids appear as trapped air pockets in or near the surface of injection-molded parts. Bubbles typically occur close to the part’s exterior, while voids form internally—often in thicker cross-sections. Both can weaken structural integrity, compromise dimensional accuracy, and affect final appearance.
Key Causes & Solutions:
- Inadequate mold venting: Improve or add vent channels to let air escape during filling.
- Uneven or rapid injection speeds: Slow down the injection process and ensure consistent speeds to minimize entrapped air.
- Excessive moisture or contaminants: Thoroughly dry resins and keep equipment clean to prevent moisture-induced bubbles.
- Material flow imbalances: Adjust gate design or cavity layout to promote uniform flow and reduce localized air pockets.
- Thick sections: Aim for uniform part thickness or add ribs/bosses instead of solid blocks; apply balanced cooling channels to maintain even temperatures.
#5 Gate Vestige
Gate vestige describes the small protrusion of plastic left behind once the molded part is ejected and the gate separates. If not carefully managed, it can require manual trimming or degrade the finished part’s cosmetic appearance.
Key Causes & Solutions:
- Excessively large or inappropriately positioned gates: Adopt specialized gate designs like tunnel or sub-gate systems, ensuring the leftover plastic is automatically severed during ejection.
- High injection pressure or extended packing times: Fine-tune injection speed, pressure, and pack profiles to reduce material accumulation and ensure a cleaner separation.
- Mould disrepair, worn edges or misalignments: Regularly inspect mold inserts and ensure plates align perfectly to avoid inconsistent shearing at the gate.
#6 Flow Lines
Flow lines are one of the most common process defects. They manifest as patterns or streaks on the surface of molded components but can also be observed as ring-shaped bands close to the mold entrances. These injection molding defects are often characterized by a different shade of color from the surrounding material.
Inconsistent cooling rates, varying mold wall thickness, and low injection speed and pressure cause flow lines in injection molding, implying the solution involves increasing the injection speed, pressure, and resin temperature. Moving mold gates farther from the mold coolant can also help extend the material cooling time. Uniform cooling rate and mold thickness are equally crucial to eliminate flow lines.
Key Causes & Solutions:
- Low injection speed or pressure: Increase injection speed and pressure to ensure molten plastic fills the mold cavity uniformly and without premature cooling.
- Low mold or material temperature: Raise the temperature of both the mold and resin to maintain adequate flow characteristics and prevent early solidification.
- Improper gate location or small gate size: Relocate gates to promote smoother flow paths or increase gate size to reduce shear and hesitation during fill.
- Inconsistent wall thickness: Redesign part geometry to maintain uniform wall thickness and avoid sudden transitions that disrupt flow velocity and cooling.
- Early cooling near coolant channels: Position gates further from the cooling channels to delay premature cooling in critical regions of the part.
Additionally, simulations using mold flow analysis software can help engineers optimize gate design, injection parameters, and part geometry early in the development phase—reducing the likelihood of flow-related defects before production begins.
#7 Burn Marks
Burn marks are process-related injection molding defects that appear as yellowish, brown, rusty, or black discoloration on the surface of molded parts—often near the end of the flow path or around air traps. While primarily considered aesthetic flaws, in more severe cases, they may indicate localized overheating that leads to polymer degradation and even structural weakness in the affected areas.
These marks are typically the result of trapped air or gases igniting due to excessive heat and pressure. This occurs when there is insufficient venting in the mold, overly high injection speeds, or poorly designed runner systems that don’t allow air to escape efficiently.
Key Causes & Solutions:
- Trapped air due to poor venting: Improve venting channels or add air escape vents to allow gases to exit the mold cavity safely before resin arrival.
- Excessive injection speed or pressure: Reduce injection speed and pressure slightly to prevent rapid compression of air pockets that leads to overheating and ignition.
- Excessive melt or mold temperature: Lower the melt temperature or optimize cycle time to avoid polymer degradation near air traps.
- Contaminants or degraded material in the mold: Clean mold surfaces and avoid using degraded resin to prevent burnt residues that mimic burn marks.
- Improper runner or gate design: Redesign runners and gates to ensure smooth resin flow and reduce the chance of air entrapment in dead zones.
Utilizing mold flow simulation tools during the design phase can help predict and eliminate areas where gas traps are likely to form. These simulations can guide better vent placement and flow path optimization, reducing the chance of burn marks from the outset.
#8 Sink Marks
Sink marks in injection molding appear as small depressions, dents, or shallow craters on the surface of the molded parts. These imperfections typically occur in areas with thick wall sections where the outer layers of the part cool and solidify faster than the internal material, which continues to shrink as it cools. While primarily a cosmetic issue, severe sink marks can also affect the dimensional accuracy and mechanical performance of the part.
Sink marks are generally caused by inadequate packing pressure, excessively thick wall designs, insufficient cooling, or high temperatures at the gate. Fortunately, these defects can be addressed both during the design phase and through process optimization.
Key Causes & Solutions:
- Excessive wall thickness: Optimize part design by maintaining uniform wall thickness and using ribs or hollow structures to reduce bulk volume.
- Inadequate packing pressure: Increase packing pressure and hold time to ensure sufficient material is pushed into thicker areas during cooling.
- Short cooling cycle: Extend cooling time and ensure even mold cooling to allow inner layers to solidify fully.
- Gate temperature too high: Lower gate temperature or reposition the gate to improve flow and reduce localized overheating.
#9 Surface Delamination
Surface delamination is a cosmetic and structural injection molding defect where thin layers of the part peel or flake off, resembling separation or blistering on the surface. This usually results from foreign substances—such as mold release agents, moisture, or incompatible materials—being introduced into the plastic melt. These contaminants interfere with molecular bonding, resulting in weak surface adhesion and layered peeling.
While delamination may not always impact mechanical performance, it significantly affects appearance and can be a sign of deeper processing issues.
Key Causes & Solutions:
- Material contamination: Clean hoppers, dryers, and tooling thoroughly before processing; avoid mixing incompatible plastics.
- Excessive or incompatible release agents: Minimize the use of release agents and ensure they’re suitable for the plastic type in use.
- High moisture content in resin: Dry hygroscopic materials (like ABS, PC, or PA) according to recommended drying times and temperatures.
- Improper melt temperature or shear: Excessive shearing or low melt temps can worsen delamination—maintain optimized processing conditions.
#10 Weld Lines (Knit Lines)
Weld lines—also known as knit lines or meld lines—occur when two or more flow fronts of molten plastic meet but fail to fuse properly during the injection process. This leads to a visible line or seam on the part surface, often appearing as a fine hairline or faint discoloration. More critically, the interface between the flow fronts tends to be mechanically weaker, reducing the part’s overall strength and integrity.
This defect is especially common in parts with complex geometries, multiple gates, or features that interrupt the flow of plastic (e.g., holes, bosses, or ribs).
Key Causes & Solutions:
- Low melt temperature: Increase melt temperature to promote better fusion of flow fronts.
- Low mold temperature: Ensure mold temperature is high enough to prevent premature solidification at the meeting point.
- Poor venting: Improve mold venting near convergence areas to release trapped air and enable stronger bonding.
- Improper gate location: Reposition the gate to optimize the flow path and reduce the likelihood of weld lines forming in critical areas.
#11 Warping
Warping is a deformation defect that causes molded parts to twist, bend, or curve undesirably as they cool. It typically results from non-uniform shrinkage within the part—especially in areas with inconsistent wall thickness, uneven cooling, or asymmetrical part geometry. This dimensional instability can render the part non-functional, especially in precision applications.
Warped components are often rejected due to both poor aesthetics and compromised structural performance. Preventing warping starts with consistent cooling and good mold design, but material choice and process control are equally important.
Key Causes & Solutions:
- Uneven cooling rates: Maintain uniform mold temperature and allow sufficient cooling time for all part sections.
- Inconsistent wall thickness: Redesign the part for uniform wall sections or use ribs to balance structural integrity.
- Improper part ejection timing: Avoid ejecting parts before complete solidification to prevent residual stresses from releasing unevenly.
- Material-specific shrinkage: Account for the shrinkage behavior of each resin during the design and simulation stages.
#12 Jetting
Jetting is a visual surface defect in injection molding where molten plastic enters the mold cavity at high velocity and forms a snake-like, wavy pattern on the part surface. This happens when the polymer jet cools and partially solidifies before the mold is completely filled, causing poor fusion between the front and trailing edges of the flow.
This defect not only affects the aesthetics of the molded part but can also create localized areas of weakness due to incomplete bonding between the layers of material.
Key Causes & Solutions:
- High injection speed: Reduce the initial injection speed or apply a slower first-stage fill to allow more uniform flow.
- Small or improperly placed gates: Enlarge the gate or relocate it to improve the melt front’s distribution and reduce turbulence.
- Low mold or material temperature: Ensure optimal temperature settings to delay premature solidification and allow smooth flow.
- Incorrect RAM speed: Adjust the RAM profile to avoid sudden bursts of flow at the start of injection.
#13 Vacuum Voids
Vacuum voids are internal cavities or pockets of trapped air that form within molded parts during the cooling phase of injection molding. These voids are especially common in parts with thicker cross-sections, where uneven cooling causes the outer layers to solidify faster, drawing the molten core material toward the walls and leaving behind a vacuum.
While small voids may not affect non-structural parts, they can significantly weaken load-bearing components, compromise dimensional accuracy, and reduce overall part integrity—particularly in applications requiring airtight or watertight performance.
Key Causes & Solutions:
- High melt or mold temperature: Lower the melt and mold temperatures to slow material flow and reduce expansion that leads to void formation.
- Low packing pressure: Increase packing and holding pressure to compensate for material shrinkage and eliminate trapped air.
- Short holding time: Extend hold time during the packing phase to maintain pressure as the material cools and contracts.
- Poor venting or part design: Improve mold venting and avoid abrupt transitions in part thickness to promote even cooling and air escape.
#14 Discoloration
Discoloration appears on the surface of molded parts as a distinct shade or color from the surrounding material. It often occurs due to molding material contamination, which can happen at the handling and storage stage or during processing. Discoloration may not directly affect the structural performance of a part, but it significantly reduces its aesthetic quality and marketability.
Key Causes & Solutions:
- Contaminated resin or machine residue: Use thoroughly cleaned machines and purge equipment between material or color changes. Store resins in sealed containers to avoid dust, oil, and moisture contamination.
- Excessive heat or prolonged residence time: Lower the barrel temperature and reduce residence time to prevent thermal degradation. Ensure proper cycle times are followed for the selected material.
#15 Splay Marks (Silver Streaking)
Splay marks, also called silver streaking, appear as silvery, streaky patterns radiating from the gate area across the surface of injection-molded parts. This defect typically occurs when moisture or contaminants in the resin vaporize under heat and shear stress, leaving visible surface imperfections.
Key Causes & Solutions:
- Excessive resin moisture: Most thermoplastics absorb atmospheric moisture, which vaporizes during injection molding, causing streaks. Use dedicated resin dryers and ensure materials are processed immediately after drying.
- High shear rates or injection speeds: High shear during injection leads to polymer breakdown, resulting in surface imperfections. Optimize injection parameters, balancing speed and pressure carefully.
- Material contamination: Dirt, oil residues, or incompatible polymers can cause splaying. Maintain stringent hygiene practices in resin handling and processing equipment.
Causes of Contamination in Injection Molding and How to Prevent Them
Contamination is a critical injection molding defect that occurs when foreign substances—such as dust, dirt, metal shavings, oil residues, or degraded plastic—become embedded in the molten material during processing. These contaminants can cause a range of issues, from aesthetic surface defects like discoloration to structural inconsistencies that compromise the part’s mechanical integrity.
Contaminants typically enter the process through poor material handling, improper tooling and equipment maintenance, or wear-related debris. Preventing contamination is essential for achieving consistently high product quality, especially in applications requiring tight tolerances or high visual standards.
Key Causes & Solutions:
- Poor storage or handling of plastic resins: use sealed containers and dry materials thoroughly before processing
- Dirty hoppers, barrels, or screw components: clean machine parts regularly to prevent build-up and contamination
- Cross-contamination from previous runs: purge machines thoroughly between material or color changes
- Wear and tear on tooling or equipment: implement scheduled maintenance and replace worn components proactively
- Environmental contamination (e.g. airborne particles, oil leaks): maintain a clean, controlled production area
Pro Tip: Train staff on contamination awareness and enforce cleanroom-style protocols where necessary.
Injection Molding Defects by Severity and Cost Impact
| Defect Type | Structural Impact | Cosmetic Impact | Production Risk | Typical Cost Increase |
| Short Shots | High | Medium | High | ↑↑ (part rejected) |
| Flash | Low | Medium | Low | → (trimming required) |
| Improper Parting Line Placement | Medium | Medium | Medium | ↑ (mold rework) |
| Bubbles & Voids | Medium | Medium | Medium | ↑ (mold/process changes) |
| Gate Vestige | Low | Medium | Low | → (finishing/gate changes) |
| Flow Lines | Low | High | Low | → (process adjustments) |
| Burn Marks | Medium | High | Medium | ↑ (process tuning) |
| Sink Marks | Medium | High | Medium | ↑ (tool redesign) |
| Delamination | High | Medium | High | ↑↑ |
| Weld Lines | Medium | Medium | Medium | → |
| Warping | High | High | High | ↑↑ (rework or scrap) |
| Jetting | Medium | High | Medium | ↑ |
| Vacuum Voids | High | Low | Medium | ↑ (tool redesign) |
| Discoloration | Low | High | Low | → |
| Splay Marks | Low | High | Low | → (drying/parameter fixes) |
Prevent Injection Molding Defects with Xometry
Avoiding injection molding defects isn’t just about technical precision—it’s about working with a partner who anticipates problems before they happen. At Xometry, our platform connects you with vetted manufacturers and supports your project from quoting to delivery with real-time visibility and expert guidance.
Every part you order through Xometry benefits from a transparent workflow, quality assurance protocols, and on-demand access to engineering support. Whether you’re prototyping or scaling to production, our team works with you proactively to spot risks early, optimize for manufacturability, and ensure that quality is never left to chance.
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