Common Defects and Improvement Measures of Insert Injection Molding

Insert injection molding embeds metal or plastic prefabricated inserts into plastic parts in one injection cycle. It is widely applied in electronics, electrical appliances, automotive and new energy products. Insert molding combines the rigidity and conductivity of inserts with the molding flexibility of plastics. However, the difference in thermal expansion coefficient between inserts and plastic materials brings many unique defects, including insert displacement, gaps, plastic cracking, poor bonding and air trapping. It is necessary to analyze defects from pre-insert preparation, mold structure and injection process to formulate targeted improvement measures.

1. Insert Offset and Displacement

Insert offset refers to the position deviation of metal insert after injection molding, which is mainly caused by unstable insert positioning. When the melt is filled at high speed, the impact force will push the insert to shift. Insufficient positioning contact area, loose positioning pins and unreasonable gate position will aggravate this problem. Insert deformation before molding will also lead to final position deviation. Improvement measures: design reliable positioning structure for inserts, adopt multi-point positioning, increase contact area. Set the gate away from the thin insert to reduce melt impact. Fix the insert firmly before mold closing, check insert flatness and deformation one by one.

2. Cracks and Wrinkles Around Inserts

Cracks near the insert are one of the most frequent defects. Metal and plastic have different thermal expansion coefficients. After melt cooling and shrinking, large internal stress is generated at the junction. Sharp corners of metal inserts will produce stress concentration, causing plastic cracks after ejection or during storage. Improvement measures: process rounded corners on the edge of metal inserts, avoid sharp edges. Preheat the insert before putting it into the mold to reduce temperature difference between insert and molten plastic. Optimize cooling rate and reduce internal stress by adjusting mold temperature.

3. Gap and Poor Combination between Insert and Plastic

Gaps between insert and plastic are mostly caused by insufficient holding pressure, poor insert surface treatment or oil contamination on insert surface. Oil, release agent and oxide layer on the insert surface will prevent plastic from adhering closely to the insert, forming separation gaps after cooling. Improvement measures: do degreasing and cleaning treatment for inserts, remove oil and oxide on the surface. Carry out knurling, grooving or sandblasting on insert surface to increase mechanical locking force. Optimize holding pressure and holding time to ensure the plastic can cling to insert during shrinkage.

4. Air Trapping and Burning Marks at Insert Junction

When melt flows around the insert, the air at the back of the insert is hard to discharge, resulting in trapped air and burning marks. Unreasonable exhaust position and insufficient exhaust groove depth are the main reasons. Improvement measures: set exhaust grooves at the melt confluence area around inserts. Optimize injection speed, slow down the filling speed when melt flows through insert position, and help air discharge smoothly. Adjust mold temperature and melt viscosity to reduce gas generation.

5. Insert Deformation and Corrosion

Thin metal inserts are easy to deform under the impact of high-pressure melt. Some metal materials will undergo chemical reaction with plastic at high temperature, resulting in corrosion and poor contact. Improvement measures: increase insert thickness or add auxiliary support structure. Select metal material matching the plastic formula. Control the moisture of plastic raw materials to avoid hydrolytic corrosion of metal inserts at high temperature.

Control methods, carry out strict inspection on insert size, flatness and surface cleanliness before production. Verify insert preheating temperature and positioning structure in trial molding. Optimize injection speed, holding pressure and mold temperature. Regularly check the wear of insert positioning structure in mass production, replace worn positioning pins timely and prevent repeated defects.

Summary 

The common defects of insert injection molding mainly come from positioning reliability, stress difference between insert and plastic, surface bonding state and exhaust condition. Standardized pre-inspection of inserts, reasonable mold positioning structure, preheating process and optimized injection parameters can effectively reduce cracks, offset, gaps and burning marks. Full process control can improve the yield of insert molded parts and ensure long-term stable production.