Traceability and Troubleshooting Procedure for Demolding Sticking Faults of Plastic Molds in China
Demolding sticking means plastic products adhere to cavity, core or insert surfaces after injection molding and cannot separate smoothly. Minor faults cause product stretching and deformation, while severe cases result in tearing, ejector pin bending and mold cavity damage. This issue occurs more frequently for elastic parts such as soft TPU grommets. The whole troubleshooting follows the principle from simple to complex, checking process factors before mold structure, to quickly locate root causes and reduce downtime.
Preliminary Phenomenon Judgment and Basic Information Collection
When sticking fault occurs, first record the fault scenario and confirm whether it appears in the first trial molding or breaks out suddenly during mass production. Observe the sticking position and judge whether the product adheres to moving mold or fixed mold. Check marks on product surface, distinguish large-area overall sticking or local sticking on ribs, rounded corners and insert positions. Collect production parameters including mold temperature, melt temperature, injection pressure, packing parameters and cooling duration. Verify raw material batch, drying records and recent mold maintenance, polishing and coating repair records, to exclude variables brought by raw material replacement or mold service.
Troubleshooting on Raw Material and Molding Process
Raw material status should be checked first. Moisture absorbed raw materials, excessive plasticizer addition or mismatched material grades will raise adhesion between melt and mold surface. Verify raw material drying temperature and duration, check impurity mixing. For soft TPE and TPU materials, confirm hardness complies with drawing requirements. Then check injection molding process. Too high packing pressure or long packing time increases the wrapping force of plastic parts on core. Excessive mold temperature leads to insufficient cooling and soft surface which easily sticks to cavity. Reduce packing pressure, shorten packing time, lower mold temperature and extend cooling time step by step, and carry out small batch trial to verify improvement. Check injection speed as well. High-speed shearing causes material decomposition, and decomposed substances depositing on mold surfaces gradually worsen sticking.
Troubleshooting of Product Structure Factors
If process adjustment cannot solve the problem, inspect product structure. Check whether draft angle meets standard. Insufficient draft angle is a common cause of sticking. The inner hole ribs and annular inner wall require larger draft angle than outer surfaces. Inspect sharp corners which cause stress concentration and lock product on mold surface after cooling. Check tiny undercut features which hook mold inserts and lead to tearing and sticking during ejection. For annular parts such as wire grommets, focus on inner wall rounded corners. Insufficient radius increases demolding resistance, and soft materials are easy to adhere to core after stretching.
Inspection of Mold Surface Condition
Surface roughness directly affects adhesion. Improper polishing grain direction, polishing scratches, carbon deposit and material precipitates will increase sticking force. Visually inspect cavity and core surface for yellowing, dirt and tiny pits, confirm polishing grain follows demolding direction. Check coating status of chrome plating or nitriding. Peeling or partial damage of coating layer causes sticking easily on damaged spots. Clean mold cavity with dedicated mold cleaner to remove precipitates, polish fine scratches and re-run trial molding to verify effect.
Troubleshooting of Mold Structure and Ejection System
Check mold parting and product retention tendency. If friction on fixed mold surface is larger than moving mold, products will stick to fixed mold. Inspect ejection system, confirm ejector pins and ejector sleeve without jamming or poor reset, and ejection actions keep synchronization. For soft parts with sleeve ejection, check matching clearance between sleeve and core. Too small clearance creates large friction resistance while excessive clearance generates flash, and flash hooks product and triggers sticking. Check vent system. Poor venting causes local burning, and burnt material adheres to cavity surface. Verify cooling channels for blockage. Local hot spots lead to incomplete shaping and partial adhesion.
Troubleshooting of Mold Fitting and Steel Material
Check fitting status of mold inserts, sliders and lifters. Flash generated at insert seams hooks product and leads to sticking. Different mold steel grades show different adsorption characteristics, and some steel types are more prone to sticking with soft plastics. Evaluate clearance variation caused by steel thermal expansion. After mold heating, insert expansion changes fitting gap and creates fine burrs by scraping, resulting in sticking. For molds in long-term mass production, inspect micro grooves from surface wear which lock plastic parts.
Verification and Root Cause Solidification
Only modify one variable and conduct small batch trial after each adjustment to accurately locate root cause. After fault resolution, solidify process parameters, update mold maintenance checklist and shorten cleaning cycle of cavity. Record fault phenomenon, troubleshooting steps and corrective actions in mold file. Optimize demolding draft angle and surface polishing scheme for subsequent similar mold design to avoid repeated sticking faults.
