When conducting visual inspections of plastic components, the biggest challenge is not the sheer volume of defects, but the inability to clearly identify the root cause. Is the issue a molding defect, or a post-processing handling defect? Is it black specks, sink marks, flash, gas lines, silver streaks, or weld lines?
Plastic cosmetic defects can be sorted into three core categories for quick diagnosis:
Common examples: short shots, black specks, voids/bubbles, sink marks, flash, weld lines, silver streaks, jetting marks, flow ripples, gas burn, glass fiber bloom, poor surface gloss, discoloration, and more. These defects are generally linked to raw material formulation, mold design, temperature profiles, injection pressure, venting, fill speed, and hold pressure settings.
Common examples: scratches, dents, impact scuffs, oil stains, fingerprint marks, untrimmed gates, excessive gate stubs, incomplete deburring, and residual flash. These issues stem from part removal, trimming, packaging, transportation, and daily operator handling practices.
Common examples: incorrect part numbers, wrong product names, quantity discrepancies, missing or invalid production dates and carton lot codes. While these are not physical defects on the parts themselves, incorrect labeling can lead to customer complaints and major traceability risks if shipped to customers.
First identify whether the issue originates from the injection molding process, post-production handling, or labeling/documentation errors.
Closely examine color, shape, texture, edge geometry, surface contours, and contamination locations:
Linear marks are often weld lines
Silvery flow marks are typically silver streaks
Thin excess edge material is flash
Local bulging areas may indicate trapped gas bubbles/voids
Do not rely solely on verbal reports when abnormalities are found.
Retain defective samples
Take photos with clear context
Record defect location, quantity, batch information, and detailed defect descriptions This data enables accurate root cause analysis, traceability, and corrective action implementation.
Visual inspection is far more than a simple check for cosmetic appearance. The core goal is defect identification, root cause determination, and formal evidence documentation. A skilled inspector does not only sort out bad parts, but also clearly communicate defect details to support on-site process improvement.
Determine if the issue is a molding defect, a post-processing handling defect, or a labeling/documentation error.
(Example: Black specks)
Examine color, shape, surface texture, edge condition, and contamination details.
(Example: Flash / burrs)
Retain physical samples, photos, and formal defect records to enable traceability and formal root-cause evaluation.
(Example: Scratches)
Black specks appear on the molded surface, typically caused by degraded resin inside the barrel or foreign contaminants mixing into the melt.
Tiny indentations form on the surface; this defect often occurs at low injection speeds. 
Linear marks form where separate melt flows converge, commonly around cut-out features, and are more likely to develop at low mold temperatures. 
Surface burning at the end of fill, generally caused by insufficient mold venting. 
Glass fibers become visible on the molded part surface. 
Loss of consistent surface finish, often caused by inadequate hold pressure or low mold temperatures. 
Resin seeps along the mold parting line (PL) and forms a thin film residue.
Likely to occur when resin viscosity is low or packing pressure is excessive.
Visible indentation on the part surface.
Common in thick-wall molded components.
Internal empty cavities formed inside the molded part.
Frequently found in thick-wall products.
Separation between the surface layer and inner substrate.
Usually caused by poor material bonding or unfavorable mold surface conditions.
Raised, bubble-like protrusions on the part surface.
Typically related to trapped gas or moisture contained in the resin.
Unwanted shift in part color.
Generally caused by material residence time or resin thermal degradation.
Wavy wrinkles appear near the gate; typically caused by injection speed being too slow.
When injection speed is excessively high, the leading edge of molten resin surges forward, creating streak patterns on the part surface.
The molded part cannot be completely filled by molten plastic.
Fine stream-like marks originating near the gate. Main root cause: entrapped moisture or gas within the material.
Embossed/textured patterns fail to fully replicate onto the part. Common with low pack pressure or low mold temperature.
Surface displacement on partially solidified plastic leaves visible patterns. Frequently seen at ejector pin locations or sharp corners lacking radii.
Black specks refer to black spots or streaks present on molded parts. Any black discoloration visible on the component surface is defined as BS (Black Specks).
Typically appear as black dot-like or linear foreign inclusions
Mostly found on the part surface
Negatively impacts cosmetic uniformity
Degraded / Burnt resin
Carbonized resin
Material stagnation residue inside barrel
Contamination from foreign debris
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Observe color and distribution pattern
Verify whether the defect is a foreign contaminant
Distinguish between surface residue and embedded inclusions
Retain photos and defective samples
Pockmarks are small depressions formed on thick-section molded areas, caused when molten resin fails to fully adhere to the mold cavity surface.
Tiny indentations on the part surface, generally small in size
Commonly found on thick wall sections
Pockmarks can stem from multiple factors; the primary contributor is insufficient pack pressure.
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Observe the location and density of indentations
Focus inspection on thick wall regions
Distinguish pockmarks from contamination or impact marks
Document size, affected zones and quantity
Weld lines form where separate resin flow fronts converge. They inevitably develop around perforations and appear as distinct linear marks on the surface.
Surrounding holes, resin convergence zones, handles and folded angular geometries.
More likely to occur at low mold temperatures, and become more pronounced when resin flow front merging is poor.
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Confirm position of linear marks
Evaluate impact on aesthetics and mechanical strength
Analyze in conjunction with hole locations, corners and melt flow direction
Take photos for archiving
Gas burn describes scorched discoloration that appears on molded part surfaces during injection.
Burn marks occur at melt flow end positions, most often associated with inadequate mold venting.
Impairs surface appearance, local mechanical performance and perceived quality by customers. Severe cases are accompanied by carbonized traces.
Prioritize checking flow ends, corners and wall thickness transition zones
Confirm discoloration depth and affected area
Retain defective samples
Timely escalate venting concerns for mold optimization
Local black or dark brown scorch marks appear on molded components.
1. Normal Venting As resin fills the mold cavity, trapped air and gases generated by the molten material escape through mold vents.
2. Venting Blockage If vents become clogged, or no vents are present at melt flow ends, gases cannot be discharged smoothly.
3. Compression Heating & Burning Trapped gases are compressed, creating extreme localized high temperature, which scorches the resin.
Clean Vents
Regularly clean mold vent grooves and vent ports to ensure unobstructed ventilation.
Verify Venting at Flow Terminations
Install vents at melt flow end positions, avoid blind end sections without venting.
Optimize Fill Speed & Packing Pressure
Set appropriate filling speed and packing pressure to reduce gas entrapment and compression.
Inspect Mold Design & Maintenance Status
Periodically check mold structure, wear and clogging issues to maintain reliable venting performance.
Figure 1: Glass fiber bloom on 120 square flat panel (Example 1)
Figure 2: Glass fiber bloom on 120 square flat panel (Example 2)
Glass fibers emerge onto the molded part surface; localized whitening, fuzziness, rough texture or distinct fibrous grain patterns can be observed.
Glass fiber bloom shares the same root causes as poor surface gloss. Fundamentally, both defects originate from inadequate surface replication and abnormal surface layer conditions.
Check for visible exposed fiber texture; distinguish this defect from general surface dullness.
Focus inspection around gates, melt flow ends and areas with abnormal local texture.
Bottom Inspection Checklist
Observe surface for exposed fiber appearance
Differentiate between surface dullness and glass fiber bloom
Pay close attention to locally rough or whitened zones
Retain defective samples and defect photos
(1) What is Poor Surface Gloss? (Appearance) Poor surface gloss refers to the molded part failing to properly adhere to the mold surface, resulting in poor surface replication.
Figure 1: Poor surface gloss (indentations) on an appearance evaluation sample
(Sample size approx. 20cm / Material: Duracon M90)
Figure 2: Poor surface gloss (glass fiber bloom) on a 120 square flat panel
(Material: Fortron 1140A1)
Loss of surface luster, dullness, and poor replication on the molded part surface.
Localized loss of gloss, hazing, surface unevenness.
May occur concurrently with indentations or glass fiber bloom.
Compare against normal gloss standards.
Focus on local texture, surface replication quality, and results from magnified observation.
Normally, molten resin flows in a laminar fashion. However, when resin flows from a narrow channel into a wide cavity at excessive velocity, the melt may arc forward and travel without contacting the mold surface. This phenomenon is defined as Jetting. Jetting may appear as band-like or hazy patterns on part surfaces, but the root cause remains identical for both forms.
*Figure 1: Jetting on appearance evaluation specimen (Specimen approx. 20cm / Material: Duracon M90)
Observe band-shaped or hazy texture
Focus on regions near the gate
Distinguish between flow marks and jetting
Retain defective samples and photos
When jetting occurs, the filling front initially enters the wide cavity area in a jet-like manner before gradually spreading out.
Figure 2: Filling behavior during jetting (using short shot method)
The melt advances rapidly in a jet-like form, creating flow mark characteristics.
The kinetic energy of the jet diminishes, and the melt begins to spread outwards to the sides.
The flow front continues to advance until the cavity is nearly or completely filled.
Figure 2: Filling behavior during jetting (using short shot method) Remarks: 120 square flat panel, gate size 2mm × 1mm, Duracon M90
Ripples refer to continuous wrinkled traces visible on the surface of molded parts.
They typically occur under conditions of low injection speed, where the surface layer solidifies faster than the advancing molten resin front.
Figure 1: Ripples on Handle Test Sample
Gate
Figure 2: Ripples on Handle Test Sample (2)
Gate
Regular or continuous wrinkled wavy marks on the part surface.
Low injection speed; premature solidification of surface layer; obstruction to subsequent molten resin advance.
Focus inspection on areas near gates, sharp corners, and long flow path regions.
☑ Observe the orientation of texture patterns
☑ Compare against acceptable reference surface
☑ Pay close attention to areas near gates ☑ Take photos for record retention
A short shot means the molten resin cannot fully fill the mold cavity.
Acceptable parts fully replicate the mold geometry, whereas parts with short shot fail to achieve the designed shape.
Short shot occurs when resin fluidity is inadequate or the metered shot volume is insufficient.
Figure 1: Appearance of Short Shot Gate
Short Shot | Good Part
🔹 Poor resin flow Low melt temperature, poor resin fluidity, excessive additives resulting in high flow resistance.
🔹 Insufficient shot volume Inadequate injection volume, inaccurate metering or low injection pressure, preventing melt from reaching the part extremities.
🔹 Difficult filling at part extremities Uneven wall thickness, improper runner/gate design and poor venting, which obstruct filling at the end sections of the part.
☑ Check incomplete forming at part extremities
☑ Compare with the contour of acceptable parts
☑ Confirm whether melt flow reaches all end regions
☑ Keep defective samples for retention
Splay marks refer to the phenomenon where gas or trapped air appears on the surface of molded parts.
The gas originates from decomposition of resin or additives, or contamination by foreign polymer materials. Air is trapped during the metering phase. Moisture is also a contributing factor.
Figure 1: Splay Marks on 120mm Square Flat Plate Gate
Figure 2: Splay Marks on Colored Plate (Approx. 5cm Plate Size) Gate
Silvery-white flow streaks, filament-like textures or splash-like appearance on the surface.
Gas, entrapped air, moisture, resin degradation, or contamination with foreign polymer materials.
Pay close attention to areas near gates, along melt flow direction, and regions with localized whitening or bright surface.
☑ Confirm whether textures show silvery-white appearance
☑ Focus inspection on gate areas
☑ Distinguish between flow marks and splay marks
☑ Retain photos for documentation
Uneven texture replication means the embossed/etched pattern on the mold cannot be sharply reproduced onto the molded part.
Figure 1: Uneven texture replication on texture evaluation specimen (Specimen length: approx. 10 cm)
Gate
Uneven texture replication occurs when the packing pressure required to press resin against the mold cavity surface is inadequate. Conditions leading to insufficient packing pressure are listed below:
Insufficient Packing Pressure → Incomplete Surface Replication → Uneven Texture
Figure 1: Slip marks on appearance evaluation specimen (Pin hole diameter in photo: approx. 2 mm)
Figure 2: Slip marks near gate on 120mm square flat plate
The partially solidified surface layer is pushed by subsequent pressure and generates lateral displacement.
Sliding occurs under injection / packing pressure; the surface is re-pressed onto the mold surface and forms visible patterns.
Check areas around ejector pins, gates, circular holes, and regions with dragging streaks.
☑ Observe the orientation of dragging streaks
☑ Inspect areas around ejector pins
☑ Pay attention to gate regions
☑ Distinguish slip marks from scratch marks
Flash occurs at Parting Lines (PL), sleeves, slide core interfaces and vent slots.
Figure 1: Flash on the parting line of 120mm square flat plate
☑ Focus inspection on parting lines (PL)
☑ Check for thin film excess material along edges
☑ Pay attention to corners and joint interfaces
☑ Distinguish flash from burrs or damaged areas
A void is an empty cavity formed inside a molded part.
Physical Appearance
Gate
Soft X-Ray Transmission Image (Comparison)
Voids / No Voids
Figure 1: Voids formed in thick-wall sections of handle specimen
Explanation for soft X-ray transmission image:
Internal cavities formed due to material shrinkage or trapped air.
Thick wall sections, corners, part extremities, areas behind gates.
Focus inspection on thick wall areas; confirm by X-ray transmission or sectioning when necessary.
Voids are mainly divided into two categories: one formed by entrapment of large volumes of gas, and the other caused by resin shrinkage in thick-wall sections. The former is defined as Gas Voids, while the latter is Vacuum Voids (Shrinkage Voids).
Normal (No Voids)
Abnormal (With Voids)
Gas → Accumulation → Void formation
Solidification → Outward Shrinkage → Void formation
Gas Voids: Entrapped gas congregates within molten resin to form cavities.
Vacuum Voids: Resin in thick sections shrinks outward during solidification and creates cavities.
Gas Voids: Relatively scattered voids with irregular sizes.
Vacuum Voids: Mostly located at the center of thick-wall areas, with relatively regular shape.
Gas Voids: Identified via X-ray inspection, cross-sectioning or dissection analysis.
Vacuum Voids: Identified via X-ray inspection, cross-sectioning or dissection analysis.
Figure 2: Distinction between Gas Voids and Vacuum Voids
As the name suggests, surface delamination is a phenomenon where the surface layer of molded parts peels off.
Injection molded parts generally consist of a surface skin layer and an inner core layer. Molten resin enters the mold cavity in jet flow form. While the surface layer solidifies, the inner material continues flowing. Poor adhesion at the interface between the two layers causes separation, which is defined as interfacial delamination.
Figure 1: Delamination on 120mm Square Flat Plate (The delamination length in this case is approx. 2 cm)
Gate
Poor adhesion at the interface between skin layer and core layer.
Local lifting, sheet-like peeling, abnormal surface texture.
Focus Areas near gates, jet flow paths, local whitening or layered traces.
☑ Compare with acceptable reference surface
☑ Lightly scrape to verify layer separation
☑ Record defect location and area
☑ Keep photos of defective samples
Blisters refer to raised bulges appearing on the surface of molded parts.
Blisters tend to form under two conditions:
In both scenarios, when the surface softens due to temperature rise, trapped internal gas expands and pushes the surface outward to form blisters.
Figure 1: Blisters on 120mm Square Flat Plate Gate
Figure 2: Blisters on Box-Type Test Specimen Gate
Local bulges or raised bumps on the part surface.
Entrapped internal air, thermal expansion, and surface layer softening.
Check the height and location of bulges; verify if the bulge can be pressed; confirm whether it is associated with internal voids.
☑ Pay attention to thick wall sections
☑ Distinguish blisters from surface contamination
☑ Perform cross-section inspection if necessary
☑ Take photos for documentation
Discoloration means the color of molded parts deviates from the normal standard color.
Plastics are chemical substances. When continuously heated above their melting point, they gradually decompose and degrade. Discoloration is a visual defect accompanying this degradation process.
Figure 1: Specimens with Discoloration
Specimen with Normal Color
Specimen Discolored due to Material Residence
Yellowish, darkened appearance or local browning on parts.
Material residence inside barrel, excessive temperature, polymer degradation, abnormal material blending.
Compare against standard color chip; confirm whether color shift occurs locally or over the whole part.
☑ Compare with standard reference sample
☑ Watch for residence traces
☑ Verify process temperature settings
☑ Keep photos of color difference for record
Gate vestige is the phenomenon where gate material remains on the surface of molded parts.
Subgates or tunnel gates normally break off automatically during mold opening. However, improper gate geometry and dimensions may lead to incomplete severance, leaving residual material or protrusions on the part surface.
Figure 1: Gate Vestige on 50mm Square Plate (Pin Gate Φ0.7) Gate
Incomplete gate breakage, leaving small column-shaped residue on the surface.
Impairs appearance, assembly and tactile feel; may cause sharp edges that cut operators.
Check the residual height, sharpness, position, and whether it interferes with part function.
☑ Measure the height of gate residue
☑ Distinguish gate vestige from foreign contaminants
☑ Pay attention to pin gate locations
☑ Take photos for documentation
Sink marks are indentations formed on the surface due to resin shrinkage.
Crystalline resins experience significant volume reduction and high shrinkage rate during cooling and solidification. Thick wall sections have greater shrinkage, so sink marks tend to appear on thicker portions of molded parts.
Figure 1: Sink Marks on Visual Evaluation Specimen (Specimen approx. 20 cm) Gate
Local surface indentation, dullness or loss of gloss.
Thick wall zones, backside of ribs, around bosses, and corner areas.
Compare surface gloss with acceptable reference; check for obvious indentation; analyze combined with the internal structure on the reverse side.
☑ Observe surface indentations
☑ Pay attention to areas with wall thickness transition
☑ Distinguish sink marks from poor gloss
☑ Take photos for documentation
