Bubbles, voids and bulges on the surface or inside injection molded products are common defects in manufacturing, which occur frequently on thick-walled parts, precision shells and transparent plastic components. Such flaws not only spoil product appearance, but also reduce structural strength and cause dimensional deviations. In severe cases, they will lead to large-scale scrapping. Based on material properties, mold structures, practical processes and on-site experience, this paper classifies bubble types, analyzes their causes and puts forward applicable improvement measures as well as on-site troubleshooting guidelines.
Main Types of BubblesThree typical bubble types can be distinguished quickly by their appearance and locations. Bubbles induced by material moisture are mostly scattered on the product surface with small sizes, which are particularly obvious on transparent products and feature smooth inner walls. Gas entrapment bubbles generally appear at melt flow ends, rib roots and weld lines, often accompanied by slight burn marks and rough inner walls. Shrinkage cavities mainly exist in thick sections and areas with dramatic wall thickness variations. They usually present as single large cavities formed by insufficient melt feeding during cooling. Distinguishing bubble types accurately is the premise to solve defects efficiently.
Cause Analysis of Bubbles
Material moisture is the primary cause of tiny surface bubbles. Plastics such as ABS, PC and PMMA are highly hygroscopic. Moisture inside raw materials vaporizes into steam under high barrel temperature and remains inside molded products. Excessive regrind addition, humid storage environment and excessive barrel temperature will cause thermal decomposition of plastics and generate extra gas, worsening bubble problems.
Poor mold ventilation is the main factor leading to gas entrapment bubbles. When melt fills the cavity, internal air is squeezed. If exhaust grooves are insufficient in size or distribution, trapped air will be compressed and form bubbles and burn marks. Blocked parting surfaces, inserts and ejector pin clearances, as well as missing auxiliary vents at deep cavities, blind holes and corners will further aggravate defects.
Improper process parameters also trigger bubbles. Excessively fast injection speed traps air inside products, while overly slow speed causes melt stratification and air entrainment. Insufficient holding pressure and short holding time result in early gate freeze, so thick sections cannot be fully filled and shrinkage cavities emerge. Low mold temperature accelerates surface solidification, sealing internal gas and shrinkage spaces inside products permanently.
Product structure, gate layout, equipment and on-site management also exert influences. Excessively thick local walls and abrupt thickness transitions increase shrinkage volume. Improper gate position and size weaken the effect of holding pressure feeding. Wear of screw and barrel, poor hopper sealing, inadequate mold maintenance and uneven regrind particle size will continuously produce bubble defects.
Targeted Improvement MeasuresStandardize material management and drying to reduce moisture fundamentally. Set drying parameters according to material types: ABS is dried at 80-90℃ for 2-4 hours, while highly hygroscopic materials like PC and PMMA are dried at 110-120℃ for 4-6 hours. Store materials in sealed conditions and re-dry damp materials. Control the proportion of regrind below 20% for ordinary products and avoid using regrind for transparent products. Lower barrel temperature properly within the range of normal plasticization to prevent gas generation from thermal decomposition.
Optimize mold venting systems by cleaning and adding vent structures. Arrange segmented exhaust grooves along product contours and add extra vents at weld lines and melt flow ends. Use insert joints and ejector pin gaps for auxiliary ventilation, and apply breathable steel for complex structures. Conduct regular maintenance to clear carbon deposits and impurities in exhaust grooves and runners to keep ventilation unobstructed.
Adjust injection process parameters scientifically. Adopt multi-stage injection speed and reduce speed at air entrapment prone areas to reserve time for air discharge. Increase holding pressure and extend holding time gradually for shrinkage cavities to ensure sufficient feeding before gate freeze. Raise mold temperature moderately to slow down surface solidification. Improve screw back pressure properly to remove entrained air from melt, and reduce screw retraction distance to avoid inhaling extra air.
Optimize product and mold structures, and strengthen on-site management. Design products with uniform wall thickness and perform material removal on thick sections. Adjust gate position and size by placing gates near thick sections, increase gate quantity if necessary, and optimize runners to reduce pressure loss. Inspect screw, barrel and hopper sealing regularly, and standardize procedures of material changing, mixing and shutdown to minimize air intake.
On-site Troubleshooting and SummaryFollow standardized steps to troubleshoot bubble defects. First, check material drying and storage conditions to eliminate moisture impact. Second, adjust process parameters including injection speed, holding pressure and mold temperature according to bubble features. Finally, clean and repair mold exhaust grooves, ejector pins and inserts.
Bubbles in injection molded products are caused by multiple interactive factors, which require comprehensive improvement. Implement basic management including material control, mold maintenance and process standardization in daily production. Judge root causes based on bubble positions and appearances and carry out targeted optimization. Refined management and classified solutions can effectively reduce bubble defects and stabilize product quality as well as production efficiency.
