By applying best practices in design, mould engineering, and process control, these defects are largely predictable and avoidable. This article identifies common injection moulding defects, examines their underlying reasons, and offers practical solutions to help you achieve consistently high-quality moulded parts.

Quick-Reference Defect Comparison Table

The table below summarises the various injection moulding defect types and highlights their causes and possible solution or prevention methods. More details are provided in the next section.

Mould Design-Related vs. Process-Related Injection Moulding Defects

Injection Moulding Defects Caused By Mould Design

Defects rooted in mould design usually emerge from improper initial tooling or inadequate mould maintenance. These often require extensive, costly, and time-consuming corrections, including significant mould modifications or complete retooling. Addressing mould-related issues through comprehensive Design for Manufacturing (DfM) analyses during the early design stage prevents costly production interruptions.

The primary mould design-related defects include:

• Short Shots
• Flash
• Gate Vestige
• Improper Parting Line Placement
• Bubbles and Voids

Injection Moulding Defects Caused By Process

Process-related defects commonly result from improper control or incorrect settings within the moulding cycle. Variables such as injection pressure, injection speed, mould/resin temperatures, cooling rates, and material conditions significantly influence these defects. Unlike mould design issues, process-related problems often can be mitigated through adjustments in machine settings—without extensive mould modifications.

Typical process-related defects include:

• Flow Lines
• Burn Marks
• Warping
• Vacuum Voids
• Sink Marks
• Weld Lines (Knit Lines)
• Jetting
• Discolouration
• Surface Delamination
• Splay Marks (Silver Streaking)

By clearly differentiating mould design-related defects from process-related defects, engineers can effectively pinpoint root causes, streamline troubleshooting, and consistently achieve optimal injection moulding 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 moulding defect that occurs when molten plastic fails to completely fill the mould 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 mould 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 mould.
• Low mould or melt temperature: Adjust temperatures to maintain optimal resin flow during injection.
• Poor venting: Improve mould venting to allow trapped air to escape, reducing resistance in hard-to-reach areas.

Pro Tip: For parts with thin walls or intricate features, selecting low-viscosity resins can help ensure better flow into complex geometries and reduce the risk of short shots.

#2 Flash

Flash, also known as spew, is a designer defect that manifests as thin layers of excess material at the edges of moulded parts. It commonly appears along parting lines, ejector pin locations, or near gates—where molten plastic escapes into the gaps between mould components. Minor flashes can be trimmed, but extensive flash may render parts unusable, especially in high-precision or cosmetic applications.

Key Causes & Solutions:

• Poor mould design or wear on mould surfaces: redesign the mould or refurbish sealing surfaces
• Low clamping force: increase clamping pressure to prevent mould separation during injection
• Excessive injection pressure: reduce pressure to avoid forcing material into unwanted gaps
• High mould temperature: lower the temperature to improve sealing between mould halves
• Improper mould alignment or setup: check mould fit and ensure proper plate alignment

Pro Tip: Use simulation tools to validate pressure balance and optimise clamping force before production.

#3 Improper Parting Line Placement

A parting line is where two mould 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 mould design or misalignment: Ensure precise alignment with robust mould 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 minimise their visual impact.

Pro Tip: During design, use CAD-based parting line analysis tools to pinpoint optimal placement. This approach reduces finishing time and prevents flash.

#4 Bubbles and Voids

Bubbles and voids appear as trapped air pockets in or near the surface of injection-moulded 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 mould 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 minimise 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 localised 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.

Pro Tip: Use mould flow simulations to identify potential air-trap zones and optimise gate placement, venting, and resin flow paths before production begins.

#5 Gate Vestige

Gate vestige describes the small protrusion of plastic left behind once the moulded part is ejected and the gate separates. If not carefully managed, it can require manual trimming or degrade the finished part’s cosmetic appearance.

Example of a gate vestige in injection moulding

Example of a gate vestige in injection moulding

Key Causes & Solutions:

• Excessively large or inappropriately positioned gates: Adopt specialised 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 mould inserts and ensure plates align perfectly to avoid inconsistent shearing at the gate.

Pro Tip: Placing gates on a concealed surface or near part edges can mask vestiges if total elimination isn’t feasible. This approach preserves aesthetics while minimising post-mould finishing steps.

#6 Flow Lines

Flow lines are one of the most common process defects. They manifest as patterns or streaks on the surface of moulded components but can also be observed as ring-shaped bands close to the mould entrances. These injection moulding defects are often characterised by a different shade of colour from the surrounding material.

Inconsistent cooling rates, varying mould wall thickness, and low injection speed and pressure cause flow lines in injection moulding, implying the solution involves increasing the injection speed, pressure, and resin temperature. Moving mould gates farther from the mould coolant can also help extend the material cooling time. Uniform cooling rate and mould 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 mould cavity uniformly and without premature cooling.
• Low mould or material temperature: Raise the temperature of both the mould 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 mould flow analysis software can help engineers optimise gate design, injection parameters, and part geometry early in the development phase—reducing the likelihood of flow-related defects before production begins.

Pro Tip: In high-appearance parts, slightly adjusting the surface texture or applying matte finishes can help visually mask minor flow lines without compromising structural integrity.

#7 Burn Marks

Burn marks are process-related injection moulding defects that appear as yellowish, brown, rusty, or black discolouration on the surface of moulded 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 localised 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 mould, 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 mould 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 mould temperature: Lower the melt temperature or optimise cycle time to avoid polymer degradation near air traps.
• Contaminants or degraded material in the mould: Clean mould 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.

Utilising mould 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 optimisation, reducing the chance of burn marks from the outset.

Pro Tip: If burn marks consistently appear in the same area of a part, consider relocating the gate or modifying the flow path to avoid trapping air in that region—this simple design adjustment can significantly reduce thermal buildup and eliminate burn-related discolouration.

#8 Sink Marks

Sink marks in injection moulding appear as small depressions, dents, or shallow craters on the surface of the moulded 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 optimisation.

Key Causes & Solutions:

• Excessive wall thickness: Optimise 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 mould cooling to allow inner layers to solidify fully.
• Gate temperature too high: Lower gate temperature or reposition the gate to improve flow and reduce localised overheating.

Pro Tip: Use mould temperature control units (TCUs) to ensure uniform and consistent cooling across the mould, especially near thicker features prone to shrinkage.

#9 Surface Delamination

Surface delamination is a cosmetic and structural injection moulding 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 mould 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: Minimise 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 optimised processing conditions.

Pro Tip: Use inline material dryers and closed material delivery systems to minimise airborne contaminants and ensure resin stays dry before entering the barrel.

#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 discolouration. 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 mould temperature: Ensure mould temperature is high enough to prevent premature solidification at the meeting point.
• Poor venting: Improve mould venting near convergence areas to release trapped air and enable stronger bonding.
• Improper gate location: Reposition the gate to optimise the flow path and reduce the likelihood of weld lines forming in critical areas.

Pro Tip: In load-bearing or high-stress components, reposition gates or redesign the part to relocate weld lines away from mechanically critical zones.

#11 Warping

Pro Tip: Use mould flow analysis to identify hotspots and redesign cooling channels accordingly. Well-placed channels ensure uniform heat dissipation across the mould.

#12 Jetting

Jetting is a visual surface defect in injection moulding where molten plastic enters the mould 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 mould is completely filled, causing poor fusion between the front and trailing edges of the flow.

This defect not only affects the aesthetics of the moulded part but can also create localised 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 mould 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.

Pro Tip: Incorporating overlapping flow fronts or a fan gate design can help distribute the resin more evenly, reducing velocity peaks that cause jetting.

#13 Vacuum Voids

Vacuum voids are internal cavities or pockets of trapped air that form within moulded parts during the cooling phase of injection moulding. 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 mould temperature: Lower the melt and mould 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 mould venting and avoid abrupt transitions in part thickness to promote even cooling and air escape.

Pro Tip: For thick parts, consider redesigning with uniform wall thickness or hollowing out solid areas with internal ribs to reduce material accumulation and improve cooling uniformity.

#14 Discolouration

Discolouration appears on the surface of moulded parts as a distinct shade or colour from the surrounding material. It often occurs due to moulding material contamination, which can happen at the handling and storage stage or during processing. Discolouration 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 colour 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.

Pro Tip: Always dry hygroscopic materials—like nylon, PET, or ABS—before moulding to prevent moisture-induced colour changes.

#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-moulded parts. This defect typically occurs when moisture or contaminants in the resin vaporise under heat and shear stress, leaving visible surface imperfections.

Key Causes & Solutions:

• Excessive resin moisture: Most thermoplastics absorb atmospheric moisture, which vaporises during injection moulding, 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. Optimise 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.

Pro Tip: If splay marks persist, consider resin formulation changes or use vented barrel injection machines, which help release entrapped gases before they reach the mould.

Causes of Contamination in Injection Moulding and How to Prevent Them

Contamination is a critical injection moulding 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 discolouration 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 colour 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 Moulding Defects by Severity and Cost Impact

Prevent Injection Moulding Defects with Xometry

Avoiding injection moulding 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, optimise for manufacturability, and ensure that quality is never left to chance.

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