Distinction and Treatment Solutions for Gas Bubbles and Vacuum Voids in Injection‑Molded Plastic Parts

Internal cavities are common defects for injection‑molded plastic products. Gas bubbles and vacuum voids look similar in appearance but differ completely in root causes, occurrence positions and corrective measures. Misjudgment frequently happens in on‑site production, leading to wrong debugging directions and persistent non‑conformity. Gas bubbles are cavities aggregated by gas, while vacuum voids are vacuum cavities formed by material shrinkage. Accurate identification supports targeted adjustment on raw materials, process and molds, reduces internal cavity reject rate and guarantees structural strength of plastic components.

Basic Identification Methods for Gas Bubbles and Vacuum Voids

Gas bubbles are formed by aggregated water vapor, volatile gas or cavity air trapped inside melt. Their cross‑sections are mostly irregular round shapes with bright inner walls, frequently appearing at thick‑wall areas and weld‑line positions. Smooth burning traces can be observed on cavity inner walls after cutting samples open. Bubbles often occur far from gates or around weld seams. They may even appear on thin‑wall parts when raw‑material moisture or poor venting exists.

Vacuum voids, also known as shrinkage voids, are generated when cooling shrinkage cannot be supplemented by incoming melt. They almost exclusively occur at heavy‑wall features such as bosses, screw posts and locally thick 胶 positions. Inner walls of vacuum voids show matte folded shrinkage textures without bright surfaces. After gates freeze, melt feeding gets cut off, and internal material shrinkage pulls to form vacuum cavities. High‑temperature baking test serves as quick verification: gas‑bubble positions will bulge under heating due to internal trapped gas; vacuum voids will only sink further without bulging effect.

Origins and Countermeasures of Gas Bubbles

Excessive raw‑material moisture acts as a major source of gas bubbles. Moisture absorbed by pellets vaporizes inside hot barrel and forms bubbles entering cavities. Hygroscopic materials such as PA, PC and PET require strict drying management. Even non‑hygroscopic ABS and PP produce water‑vapor bubbles after moisture absorption. Verify actual dryer temperature instead of only relying on display value. Seal unused raw materials, restrict re‑material moisture absorption and prohibit damp agglomerated pellets.

Volatile gas comes from thermal degradation. Excessive barrel temperature, high back‑pressure and fast screw rotation generate shear over‑heating and polymer decomposition. Drain melt during long‑time machine standby. Lower barrel and nozzle temperature, reduce screw back‑pressure and melt‑plasticizing speed to cut shear heat. Clean carbon deposits at nozzle and non‑return valve.

Poor mold venting traps cavity air. Insufficient vent depth, carbon‑blocked vents and ineffective auxiliary venting from ejector pins or inserts trap air at melt‑flow terminals. Clean vent carbon and residues, add vents at weld‑line and flow‑end zones, adjust gating positions and make use of pin and slider clearances for auxiliary venting.

Extra air entrainment in production: mismatched nozzle‑sprue‑bushing clearance sucks in ambient air during plasticizing and injection. Correct nozzle centering, optimize screw metering and cushion value to reduce air entrapment.

Origins and Countermeasures of Vacuum Voids

Premature gate freeze‑off blocks packing‑pressure transmission toward thick‑wall zones. Increase packing pressure and extend packing duration to deliver melt compensation before gate solidification. Adjust packing switch‑over position. Avoid unlimited high packing pressure which brings flash and stress cracking.

Undersized gates accelerate gate cooling and block feeding. Enlarge gate and runner cross‑section within mold allowable scope to delay gate solidification. Optimize product structure by thinning over‑thick 胶 positions and adding hollow cut‑outs. If product modification is unavailable, optimize cooling runners to balance cooling rhythm between thick and thin sections.

Unreasonable mold temperature: over‑high mold temperature extends whole cooling cycle; over‑low mold temperature freezes gates rapidly. Match mold temperature according to material characteristics. Never shorten cooling time blindly. Parts must get fully cooled and solidified before ejection to avoid post‑ejection shrinkage cavities.

On‑Site Inspection Notes for Mold‑Structure Defects

Distinguish defect types before parameter modification. Cut open samples and observe inner‑wall texture, apply baking test when necessary. For gas‑bubble problems, prioritize raw‑material drying, venting improvement and thermal‑degradation control. For vacuum voids, focus on packing parameter, gate design and wall‑thickness optimization.

Some components show composite defects with both gas bubbles and vacuum voids at thick‑wall positions. Both gas source and feeding‑compensation conditions need improvement. Increasing packing pressure alone cannot eliminate gas bubbles; improving venting makes no difference for vacuum voids without trapped gas.

Implement periodic sampling section‑inspection for thick‑wall products. Many internal voids remain invisible from outer appearance but weaken impact resistance and cause late‑stage cracking failure.

Gas bubbles and vacuum voids are easily‑confused defects with totally different mechanisms. Judge defect categories correctly and conduct troubleshooting from raw‑material, injection‑process and mold‑structure layers, so as to resolve internal‑cavity non‑conformity efficiently and stabilize inner quality of plastic parts.