Injection Molding Defects and Proven Solutions

A comprehensive guide to identifying, troubleshooting, and resolving the most common defects in plastic injection molding processes, optimized for modern injection mold machines.

Defect #1

Silver Streaks (Silver Marks, Material Flowers)

Silver streaks, also known as silver marks or material flowers, are one of the most common defects encountered in injection molding. These appear as thin, silvery lines or streaks on the surface of the molded part, often following the flow path of the molten plastic. They can significantly compromise both the aesthetic quality and structural integrity of the finished product.

The primary cause of silver streaks is typically the presence of moisture or volatile gases in the plastic material as it enters the mold cavity. When the molten plastic encounters sudden pressure drops or temperature changes within the injection mold machines, these gases expand and are forced out, creating the characteristic silverish streaks on the part surface.

Root Causes:

Moisture content in the plastic resin exceeding recommended levels
Contamination from foreign materials or incompatible resins
Inadequate drying of hygroscopic materials before processing
Excessive melt temperature causing thermal degradation
Improper venting in the mold design
Rapid injection speed creating turbulence in the melt flow
Worn or damaged check rings in injection mold machines allowing backflow

Effective Solutions:

Optimize Material Drying: Ensure proper drying of hygroscopic materials using desiccant dryers. Nylon, for example, typically requires 4-6 hours at 120°C (248°F). Verify moisture content with a moisture analyzer before processing.
Adjust Processing Parameters: Reduce melt temperature to prevent thermal degradation while maintaining proper flow. This is especially critical when operating injection mold machines with older heating elements that may have uneven temperature distribution.
Modify Injection Speed and Pressure: Implement a gradual injection speed profile to minimize turbulence. Reduce initial injection speed while maintaining sufficient pressure to fill the mold completely.
Improve Mold Venting: Add or enlarge vents in areas where silver streaks commonly appear, typically at the end of fill. Vents should be 0.025-0.05mm deep to allow gas escape without material leakage.
Check for Contamination: Thoroughly clean the hopper, barrel, and screw of injection mold machines when changing materials. Use a purging compound to remove residual materials that might cause contamination.
Maintain Equipment: Regularly inspect and replace worn check rings, nozzles, and screw tips to prevent backflow and ensure proper melt delivery.
Use Proper Resin Storage: Store materials in sealed containers in a controlled environment to prevent moisture absorption before processing.

Implementing these solutions systematically can eliminate silver streaks in most cases. It's important to document all parameter changes on injection mold machines to identify the most effective combination for specific materials and part designs. For persistent issues, consider conducting a design of experiments (DOE) to isolate the exact cause.

Close-up photograph showing silver streaks on a black plastic injection molded part, demonstrating the characteristic silvery lines following the material flow path

Silver Streak Identification

Silver streaks typically appear as thin, silvery lines extending from the gate area along the material flow path. They are most visible on dark-colored parts.

Check for these defects under direct lighting at various angles

Critical Considerations

Moisture levels above 0.02% can cause silver streaks in many engineering resins
Dryer air dew point should be maintained below -40°C for hygroscopic materials
Modern injection mold machines offer precise drying monitoring systems

Defect #2

Water Waves

Water waves, also known as flow lines or ripples, are cosmetic defects that appear as wavelike patterns on the surface of injection molded parts. These defects typically form near the gate area and propagate outward, creating a series of concentric or parallel ripples that resemble the pattern formed by dropping a stone into water. They are particularly noticeable on large, flat surfaces and can significantly affect the aesthetic quality of the part.

Water waves occur when the molten plastic cools too quickly as it flows through the mold cavity, causing the leading edge of the melt front to solidify prematurely. As new material is injected behind it, it pushes against this partially solidified layer, creating the characteristic wave pattern. This issue is often exacerbated by improper temperature control in injection mold machines or inadequate mold cooling design.

Root Causes:

Low melt temperature causing premature solidification
Insufficient injection speed and pressure
Cold mold surfaces, especially near the gate
Inadequate gate size or improper gate location
Excessive thickness variations in part design
Poor material flow properties for the part geometry
Inconsistent temperature control in injection mold machines

Effective Solutions:

Increase Melt Temperature: Gradually raise the barrel temperature settings on injection mold machines to improve material flow. Be cautious not to exceed the resin's maximum recommended temperature to prevent degradation.
Optimize Injection Speed: Increase initial injection speed to ensure the melt front maintains sufficient temperature as it fills the mold cavity. Implement a multi-stage injection profile with faster filling in areas prone to water waves.
Adjust Mold Temperature: Increase mold temperature to slow down the cooling rate of the molten plastic. This may require upgrading mold cooling systems or adding heater bands in critical areas.
Modify Gate Design: Increase gate size or relocate gates to improve flow path and reduce distance molten plastic must travel. Consider using fan gates or film gates for large flat surfaces.
Improve Part Design: Reduce thickness variations and add gradual transitions between thick and thin sections. Incorporate fillets and radii to facilitate smoother material flow.
Use Material Additives: Consult with your material supplier about flow-enhancing additives that can improve melt flow without compromising mechanical properties.
Calibrate Equipment: Ensure injection mold machines are properly calibrated for consistent temperature control and pressure delivery. Regular maintenance prevents fluctuations that contribute to water waves.

When addressing water waves, it's important to take a systematic approach, changing one variable at a time while monitoring results. Modern injection mold machines with precise process control capabilities can help maintain the stable conditions needed to eliminate these defects. In some cases, a combination of material, process, and mold design adjustments may be necessary for complete resolution.

Plastic part showing characteristic water wave patterns, with ripples extending from the gate area across the surface

Water Wave Characteristics

Water waves appear as sequential ripples that form as the melt front advances, with each new wave pushing against the previous partially solidified layer.

More prominent on large, flat surfaces and dark-colored parts

Process Optimization Tips

Implement a 2-3 stage injection profile with faster initial filling
Increase hold pressure to pack material firmly against mold walls
Modern injection mold machines offer precise multi-zone temperature control

Defect #3

Jetting (Snake-like Patterns)

Jetting, also known as snake-like patterns, is a defect characterized by a twisted or wavy flow path that occurs when molten plastic is injected into a mold cavity with excessive speed, causing the material to "jet" through the gate before properly spreading to fill the cavity. This creates a distinctive snake-like or curled pattern that follows the initial flow path, often resulting in weak bonding between successive layers of material and potential structural weaknesses.

The primary cause of jetting is the high-velocity flow of molten plastic exiting the gate and traveling through the mold cavity without proper contact with the mold walls. As the material jets forward, it begins to cool and solidify, while new material continues to push from behind, creating the characteristic扭曲 pattern. This issue is particularly common in injection mold machines where injection speed control is not sufficiently precise or when mold design does not account for proper flow dynamics.

Root Causes:

Excessively high injection speed, especially at the start of filling
Inadequate gate size relative to part thickness
Gate positioned too far from thick sections or directly opposite a cavity wall
Low melt temperature combined with high injection pressure
Insufficient mold temperature causing rapid cooling of the jetting material
Excessive viscosity differences in the molten plastic
Improper velocity/pressure transition settings in injection mold machines

Effective Solutions:

Implement Staged Injection Profile: Use injection mold machines capability to program a slow initial injection speed until the melt front has spread sufficiently, then increasing speed for the remainder of the fill. This prevents the initial jetting action.
Optimize Gate Design and Placement: Relocate gates to direct flow toward cavity walls rather than into open spaces. Consider using multiple gates for large parts to reduce flow distance and velocity requirements.
Increase Gate Size: A larger gate reduces material velocity by allowing more material to flow at lower pressure, minimizing the jetting tendency.
Raise Melt and Mold Temperatures: Higher temperatures reduce material viscosity and slow cooling, allowing the material to spread more evenly after exiting the gate.
Add Flow Restrictors: Incorporate baffles or flow leaders in the mold to disrupt the jetting stream and promote more even flow distribution.
Adjust Pressure Settings: Reduce initial injection pressure while extending the fill time. Modern injection mold machines allow precise pressure profiling to prevent jetting.
Use Flow-Enhancing Additives: Consult with material suppliers about additives that can improve melt flow characteristics and reduce jetting tendencies.

Jetting defects not only affect part appearance but can also create weak points where layers of material do not properly bond together. This makes proper resolution critical for structural applications. The programmable injection profiles available on modern injection mold machines provide excellent control over the filling process, making them invaluable tools in eliminating jetting defects when combined with appropriate mold design considerations.

Injection molded part demonstrating jetting defect with characteristic snake-like patterns extending from the gate area

Jetting Pattern Identification

Jetting appears as twisted, snake-like patterns originating from the gate, created by high-velocity material that doesn't properly bond as it cools.

Jetting often indicates potential structural weakness in the affected area

Machine Settings Guide

Set initial injection speed to 20-30% of maximum for first 10-15% of fill
Use pressure control rather than speed control for delicate flow sections
Modern injection mold machines offer 5+ stage injection profiling

Defect #4

Tiger Stripes

Tiger stripes, also known as shear lines or flow marks, are distinctive alternating light and dark bands that appear on the surface of injection molded parts, resembling the pattern of a tiger's coat. These defects typically occur in areas where the molten plastic experiences significant shear stress during filling, often near the gate or in regions of complex geometry. Tiger stripes are particularly problematic in parts with high aesthetic requirements and can be challenging to eliminate completely.

The formation of tiger stripes is closely related to the viscoelastic properties of molten plastic when subjected to varying shear rates. As plastic flows through the gate and into the mold cavity, it experiences different shear rates in different regions of the flow path—higher shear near the center and lower shear near the mold walls. This creates variations in molecular orientation and density, which appear as alternating light and dark bands. Modern injection mold machines with precise control systems can help mitigate these effects through careful parameter adjustment.

Root Causes:

Excessive shear stress in the melt due to high injection speed
Sharp transitions in flow path geometry causing velocity variations
Small gate size creating high shear rates as material passes through
Non-uniform cooling leading to differential crystallization in semi-crystalline materials
Material temperature fluctuations in injection mold machines
Improper screw design causing inconsistent melt quality
Recovering too much material between shots, creating temperature variations

Effective Solutions:

Optimize Injection Speed Profile: Use injection mold machines capability to create a gradual, smooth injection profile that minimizes shear stress. Reduce speed in areas where tiger stripes typically appear.
Increase Gate Size: A larger gate reduces shear rate by allowing material to flow more freely, minimizing the velocity differences that create stripe patterns.
Modify Mold Geometry: Add fillets and radii to eliminate sharp corners that cause flow disruptions. Ensure gradual transitions between different cross-sections.
Adjust Temperature Settings: Increase melt temperature to reduce material viscosity and shear sensitivity. Optimize mold temperature to promote uniform cooling.
Optimize Screw Parameters: Adjust screw rotation speed and back pressure to ensure consistent melt quality. Reduce recovery speed to prevent excessive shear heating.
Use Shear-Stable Materials: Consult with material suppliers about grades formulated to be less sensitive to shear-induced defects.
Implement Mold Surface Treatments: Polishing mold surfaces to a higher finish can reduce flow resistance and minimize shear variations.
Consider Gas-Assisted Injection: For large parts prone to tiger stripes, gas-assisted injection can reduce packing pressure and associated shear effects.

Eliminating tiger stripes often requires a combination of material selection, mold design modifications, and process optimization on injection mold machines. It's important to note that some materials, particularly those with high filler content or semi-crystalline structures, are more prone to this defect than others. In such cases, working closely with material suppliers while fine-tuning machine parameters typically yields the best results. Documenting all parameter changes and their effects is crucial for developing a repeatable process that consistently produces defect-free parts.

Plastic part showing tiger stripe defects with alternating light and dark bands following the material flow path

Tiger Stripe Characteristics

These alternating bands are most visible on curved surfaces and areas downstream from the gate, created by varying shear rates in the molten plastic.

Often more visible under direct light at a 45-degree angle

Material Considerations

Semi-crystalline materials like PP and PA are more prone to tiger stripes
Materials with 30%+ glass filler require special parameter adjustments
Work with suppliers to identify grades optimized for injection mold machines with your specific clamp tonnage

Defect #5

Gas Marks (Shadows)

Gas marks, also referred to as shadows or burn marks, are defects characterized by dark discoloration or streaking on the surface of injection molded parts, typically found in areas where air or gases become trapped during the filling process. These defects range from faint shadows to dark, burnt-looking spots and can significantly compromise both the appearance and structural integrity of molded parts.

The primary cause of gas marks is the entrapment of air or volatile gases in the mold cavity during injection. As molten plastic fills the cavity, it displaces air that must escape through vents. If vents are inadequate or improperly placed, air becomes trapped and compressed, generating heat that can burn or char the plastic in contact with it. Additionally, volatile gases released from the plastic itself during melting can contribute to this defect, especially when processing at higher temperatures in injection mold machines.

Root Causes:

Inadequate mold venting in areas where air becomes trapped
Excessive injection speed trapping air before it can escape
High melt temperature causing material degradation and gas formation
Moisture in the material creating steam during processing
Material contamination introducing volatile components
Improper gate location creating turbulent flow and air entrapment
Inconsistent back pressure in injection mold machines allowing gas buildup in the barrel

Effective Solutions:

Improve Mold Venting: Add or enlarge vents in areas prone to gas marks, typically at the last points to fill. Vents should be 0.025-0.05mm deep and 5-10mm wide, extending beyond the cavity to ensure proper gas escape.
Optimize Injection Speed: Implement a staged injection profile with slower speeds in areas where gas entrapment occurs. Modern injection mold machines allow precise speed control to facilitate proper air evacuation.
Reduce Melt Temperature: Lower barrel temperatures to minimize thermal degradation and the release of volatile gases from the material.
Ensure Proper Material Drying: Thoroughly dry hygroscopic materials to prevent moisture-related steam formation during processing.
Adjust Back Pressure: Increase back pressure slightly on injection mold machines to help release trapped gases from the molten plastic before injection.
Modify Gate Location: Relocate gates to create a more linear flow path that allows air to escape toward vents rather than becoming trapped in recesses or corners.
Use Vacuum Venting: For complex molds or problematic materials, consider adding vacuum venting systems to actively remove trapped gases.
Purge Contaminants: Thoroughly clean the barrel and screw of injection mold machines to remove any degraded material that could contribute to gas formation.

Addressing gas marks often requires a combination of mold modifications and process adjustments. The key is to ensure that all air and gases have a clear escape path from the mold cavity as it fills with molten plastic. Modern injection mold machines equipped with advanced process monitoring can help identify exactly when and where gas entrapment occurs, making troubleshooting more efficient. Regular mold maintenance, including cleaning of existing vents to remove material buildup, is also crucial for preventing recurring gas mark issues.

Plastic component showing gas marks as dark discoloration in recessed areas where air became trapped during molding

Gas Mark Identification

Gas marks appear as dark shadows or discoloration, often in corners, ribs, or other recessed areas where air becomes trapped during the filling process.

Severe gas marks may indicate material degradation and potential part weakness

Venting Guidelines

Vents should be placed at last-to-fill areas identified through flow simulation
Total vent area should be approximately 5-10% of gate area for proper evacuation
Consider specialized venting tapes or inserts for difficult-to-vent areas on complex parts

Defect #6

Black Streaks (Black Stripes)

Black streaks, also known as black stripes or carbon streaks, are severe defects characterized by dark, often burnt-looking lines or spots on injection molded parts. These defects are more severe than other discoloration issues and typically indicate material degradation or contamination. Black streaks can render parts unsuitable for their intended use, particularly in visible applications or those requiring strict purity standards.

The primary cause of black streaks is the presence of overheated or degraded plastic material in the melt flow. This can occur when plastic remains in contact with hot surfaces in injection mold machines for too long, causing it to degrade and turn black. The degraded material then mixes with fresh material and is injected into the mold, appearing as dark streaks. Contamination from foreign materials, particularly carbon-based substances, can also cause similar defects.

Root Causes:

Excessive melt temperature causing thermal degradation
Dead spots in the barrel or nozzle where material stagnates and degrades
Worn or damaged screw, barrel, or nozzle creating material hang-up points
Inadequate purging when changing materials or colors
Contamination from previous materials, especially dark-colored resins
Foreign debris in the material, hopper, or feed throat
Excessive residence time in the barrel of injection mold machines
Improper back pressure causing uneven melting and potential degradation

Effective Solutions:

Reduce Melt Temperature: Lower barrel temperatures in injection mold machines to prevent thermal degradation. Verify actual temperatures with an external thermocouple if necessary.
Optimize Screw Speed and Back Pressure: Adjust parameters to minimize residence time while ensuring complete melting. Higher back pressure can sometimes help eliminate streaks by improving melt homogeneity.
Perform Thorough Purging: Use a proper purging compound and follow recommended procedures when changing materials or colors. Purge until no evidence of degraded material appears.
Inspect and Maintain Equipment: Check for worn components, dead spots, or damage in the screw, barrel, and nozzle that could trap and degrade material. Replace worn parts as needed.
Improve Material Handling: Ensure material is stored properly to prevent contamination. Use dedicated hoppers for light-colored materials and keep handling equipment clean.
Reduce Residence Time: Adjust shot size relative to barrel capacity. Ideally, shot size should be 30-70% of barrel capacity to minimize residence time.
Use Processing Aids: Consult with material suppliers about heat stabilizers or processing aids that can improve thermal stability.
Clean the Mold: Check for and remove any degraded material buildup in the mold cavity, runners, or gates that could be causing contamination.

Black streaks should be addressed promptly as they often indicate more serious issues with injection mold machines or processing parameters that could lead to equipment damage or consistent production of defective parts. In severe cases, it may be necessary to completely disassemble and clean the barrel and screw assembly to remove all degraded material. Implementing preventive maintenance schedules for injection mold machines and establishing strict material handling protocols can help prevent recurring black streak problems.

Injection molded part showing distinct black streaks caused by material degradation during processing

Black Streak Characteristics

These dark, often burnt-looking streaks indicate degraded material, typically appearing as irregular lines following the flow path through the mold cavity.

Black streaks often indicate serious material degradation requiring immediate attention

Preventive Measures

Establish maximum residence time guidelines based on material specifications
Implement regular purging schedules for injection mold machines running heat-sensitive materials
Use dedicated feed throats and hoppers for light-colored and virgin materials

Key Troubleshooting Principles for Injection Molding Defects

Temperature Control

Maintain precise control over melt and mold temperatures, as improper temperature management is a root cause of most common defects. Modern injection mold machines offer advanced temperature profiling capabilities.

Injection Profiling

Utilize multi-stage injection profiles to optimize flow characteristics through different sections of the mold, reducing shear stress and preventing defects like jetting and tiger stripes.

Equipment Maintenance

Regular maintenance of injection mold machines prevents worn components from causing defects. Pay special attention to screw, barrel, and nozzle conditions.

Material Handling

Proper drying, storage, and handling of materials prevents contamination and moisture-related defects. Establish clear protocols for material changeovers on injection mold machines.

Mold Design

Optimize gate design, venting, and flow paths to minimize flow disruptions. Consider simulation analysis during mold design to identify potential defect areas.

Process Documentation

Maintain detailed records of parameters and defect occurrences. This data is invaluable for troubleshooting recurring issues and optimizing performance of injection mold machines.