Plastics : Defects - Bubbles and air traps in plastic products -

Voids and air traps are internal defects in plastic parts where pockets of gas become trapped within the material during processing. 

These empty spaces compromise the structural integrity of the part, potentially leading to weakened areas, cosmetic issues, and premature product failure.

Formation mechanisms:

Air entrapment during filling:

Air traps occur when air cannot escape from the mold cavity as molten plastic enters. 

This typically happens in:

• Areas far from vents
• Regions where melt fronts converge (knit, weld lines)
• Sudden thickness changes or corners
• End-of-fill regions (End-of-fill regions are the last areas to be filled with molten plastic during the injection molding process)

Gas generation within the plastic from:

• Volatile compounds from polymer degradation
• Moisture turning to steam
• Gases released from chemical reactions (e.g., in foaming or during polymerization)
• Outgassing of residual monomers or additives

Foaming during polymerization occurs when gas bubbles form and expand within a plastic material as it undergoes the chemical reaction that transforms monomers into polymers. This process can be either intentional (as in producing foam products) or unintentional (creating defects).

Outgassing is the release of trapped or dissolved gases from a plastic material. It occurs when volatile compounds within the polymer (such as residual monomers, moisture, solvents, or additives) convert to gas form due to heat or pressure changes during processing, potentially creating bubbles or voids in the finished part.

Shrinkage-related voids:

during cooling, material contracts, creating:

• Internal vacuum bubbles in thick sections
• Sinks that haven't fully collapsed into voids
• Areas where insufficient material was added during packing

Types and characteristics:

Microscopic voids:

• Often not visible to the naked eye
• Distributed throughout the part
• May appear as haziness or opacity in otherwise transparent materials
• Can act as stress concentrators (points in a plastic part where stress becomes intensified. These areas—such as sharp corners, holes, or internal voids)

Macroscopic voids:

• Larger, visible empty spaces
• Usually located in thick sections or near ribs
• May be visible externally as sink marks
• Significantly reduce mechanical properties

Surface-connected air traps:

• Trapped air that creates surface blemishes
• Often appear as flow marks or "dieseling" burns
• May be visible as silver streaks or discoloration

Dieseling (also called gas burning) occurs when trapped air in a plastic mold is rapidly compressed by incoming plastic material, creating enough heat to burn or scorch the plastic. This produces black or brown burn marks on the plastic part, often accompanied by a characteristic diesel-like odor.

Impact on product performance:

Mechanical property reduction:

• Decreased tensile and impact strength
• Reduced load-bearing capacity
• Stress concentration points leading to crack initiation
• Inconsistent mechanical properties across parts

Tensile strength is the maximum amount of pulling or stretching force a material can withstand before breaking

Cosmetic issues:

• Visible surface defects
• Inconsistent transparency in clear parts
• Irregular gloss or texture

Functional Problems:

• Potential leak paths in sealed containers
• Reduced insulation properties
• Compromised barrier properties for packaging
• Weak points under pressure or load

Prevention and remediation:

Mold design solutions:

• Optimized vent locations and dimensions
• Proper gate placement to ensure complete filling
• Vacuum-assisted venting for complex geometries
• Core-out thick sections to maintain uniform wall thickness

Core-out is a design technique in plastic part manufacturing where material is intentionally removed from thick sections to create hollow areas or uniform wall thickness. 

Process parameter pdjustments:

• Optimized injection speed profiles
• Sufficient holding pressure and time
• Proper melt and mold temperatures
• Controlled decompression before part ejection

Holding pressure (also called packing pressure) is the pressure applied to the molten plastic after the mold cavity is filled. It maintains force on the plastic as it cools and solidifies, forcing additional material into the cavity to compensate for shrinkage and ensuring the part has proper dimensions, weight, and density.

Material preparation:

• Thorough drying of hygroscopic resins
• Proper material handling to prevent contamination
• Selection of materials with appropriate viscosity and degassing characteristics

Hygroscopic resins are plastic materials that naturally absorb moisture from the surrounding air. These polymers must be thoroughly dried before processing because any absorbed water will turn to steam during high-temperature molding, causing defects like voids, bubbles, and surface imperfections in the finished parts.

Post-process solutions:

• Annealing to relieve internal stresses
• Vacuum treatment for surface-connected voids

Annealing is a heat treatment process where plastic parts are heated to a temperature below their melting point and then slowly cooled. This process relieves internal stresses, increases crystallinity in semi-crystalline polymers, and improves dimensional stability and mechanical properties of the finished part.
Surface-connected voids are hollow spaces within a plastic part that extend to the exterior surface. Unlike fully enclosed internal voids, these defects create openings on the part surface, potentially causing leakage paths, contamination entry points, and areas of weakness that can compromise both appearance and functionality.

Quality inspection methods like X-ray or ultrasonic testing to detect internal voids
Understanding the causes and prevention of voids and air traps is essential for producing high-quality plastic parts with consistent mechanical properties and appearance.