Plastics : Surface Blemishes and Defects in Plastics Manufacturing

Surface defects in plastics manufacturing are visible imperfections on the part exterior that compromise appearance and potentially functionality. 

Common types include flow lines (visible patterns showing material flow paths), jetting (worm-like patterns from high-velocity material injection), and orange peel texture (rough, uneven surface resembling citrus skin).

Flow Lines:

Flow lines are visible patterns or lines on the surface of a molded part that reveal the path molten plastic took while filling the mold cavity.

Causes:

• Temperature variations within the melt
• Directional cooling and solidification
• Varying flow velocities around obstacles
• Hesitation in thin-to-thick transitions
• Melt front disruption near gates, ribs, or bosses

Directional cooling and solidification occurs when plastic cools and hardens in a sequential pattern following the flow direction, rather than uniformly throughout the part. This creates visible layers or lines on the surface as each section solidifies at a different time or rate.
Hesitation in thin-to-thick transitions happens when molten plastic flows through a thin section and then slows down or pauses momentarily as it enters a thicker area of the mold. This sudden flow rate change creates visible marks or flow lines on the part surface.

Gates: entry points where molten plastic flows from the runner system into the mold cavity, leaving small marks on the finished part that sometimes require removal.
Ribs: thin, wall-like projections added to plastic parts to increase strength and stiffness without significantly increasing weight or material usage.
Bosses: cylindrical projections or raised areas on plastic parts designed to accept screws or provide mounting points for assembly with other components.

Characteristics:

• Often appear as wavy lines or bands
• More noticeable in colored or transparent materials
• Typically radiate outward from gate locations
• May have different gloss levels or color intensity
• More pronounced in crystalline materials

Prevention:

• Optimizing melt and mold temperatures
• Balanced gate locations and sizes
• Controlled injection speed profiles
• Proper part and runner design
• Material selection with appropriate MFI (measurement of how easily melted plastic flows under standard conditions when subjected to specific temperature and pressure, helping predict processing behavior)

Runner is a channel that transports molten plastic from the injection point to the part cavity in a mold. It acts as a pathway for material flow before entering the part through gates

Jetting:

Jetting occurs when molten plastic shoots into an empty mold cavity at high velocity, solidifying before properly filling the space, creating snake-like or worm-like patterns.

Causes:

• Improper gate design (too small or poorly positioned)
• Excessive injection speed
• Low melt temperature
• High material viscosity
• Sudden changes in flow direction or cross-section

Characteristics:

• Worm-like or snake-like patterns
• Different surface gloss than surrounding areas
• Often visible near gate locations
• Can create weak spots or structural issues
• May trap air along the flow path

Prevention:

• Gate design modifications (size, location, type)
• Reducing initial injection speed
• Increasing melt temperature
• Optimizing mold temperature
• Using baffles or flow leaders to control material entry

Orange Peel Texture:

Orange peel is a rough, uneven surface texture resembling the skin of an orange, typically occurring in thicker sections or highly filled materials.

Causes:

• Uneven cooling and shrinkage
• High filler content with poor distribution
• Material moisture content
• Incompatible material additives
• Mold surface issues (poor polish, damage, or wear)

Characteristics:

• Irregular dimpled appearance
• Reduced surface gloss
• Uneven light reflection
• Typically worse in thicker sections
• More pronounced with certain materials (e.g., highly filled)

Prevention:

• Controlling cooling rate and uniformity
• Optimizing process temperatures
• Proper material drying
• Modifying filler content or type
• Improved mold surface finish
• Balanced wall thickness

Detection and Quality Control:

• Visual inspection under controlled lighting
• Surface profilometry for quantitative measurement
• Gloss measurement for consistency
• Image analysis systems for automated detection
• Reference standards for acceptable appearance

Impact on Production:

Surface defects often require balancing aesthetic requirements against production efficiency which:

• May necessitate slower cycle times for high-quality surfaces
• Can increase post-processing requirements (painting, polishing)
• Might require more expensive materials or equipment
• Could drive mold modifications or redesigns
• May establish acceptance criteria for different applications

For critical applications (medical, automotive, consumer electronics), even minor surface defects can result in rejected parts, while industrial applications may tolerate certain imperfections if functionality is not compromised.