Common Defects and Solutions for Chinese TPE Soft Rubber Injection Molding
Benefiting from the rapid upgrading of China’s precision plastic processing industry, Chinese TPE soft rubber products are widely applied in overmolding sealing parts, non-slip accessories, buffer components and consumer electronic supporting parts. TPE materials integrate rubber elasticity and plastic moldability, covering a full hardness range from ultra-soft 0A to rigid 90A. Different from hard plastic injection molding, Chinese TPE molding production features sensitive melt fluidity, obvious cooling rebound, strong mold adhesion and unstable shrinkage. In actual mass production, defects such as mold sticking, bubbles, sink marks, flow lines, surface precipitation, insufficient overmolding adhesion, flash and warpage frequently occur. Material formula characteristics, raw material pretreatment, mold structure design, injection process parameters and post-molding environment jointly determine the final yield of Chinese TPE products. This paper systematically analyzes common defective causes in domestic TPE molding and summarizes targeted improvement solutions.
1. Mold Sticking and Demolding Scratches
TPE materials have inherent surface viscosity, especially for low-hardness soft grades, which easily adhere to the cavity during mass production. Improper demolding will cause product tearing, surface scratching and residual glue on the mold surface. In view of the characteristics of Chinese conventional TPE formulations with high plasticizer content, material selection should match product structure and molding conditions to avoid excessive fluidity leading to over-filling and vacuum sticking. The mold cavity adopts matte fine blasting instead of mirror polishing to reduce adsorption force. The demolding draft angle is appropriately increased compared with hard glue molds, and rounded transition structures are added at sharp corners and narrow grooves. Process optimization includes properly reducing barrel temperature, injection pressure and holding pressure to prevent the melt from tightly fitting the mold wall. Extend the cooling time to ensure complete surface shaping, adopt low-speed uniform ejection, and control the use of release agents to avoid residual oil stains affecting subsequent bonding and surface quality.
2. Surface Bubbles and Internal Voids
Although Chinese conventional TPE materials have low water absorption, long-term storage in humid domestic production environments will cause moisture absorption. Moisture and shear air entrapment are the main causes of surface bubbles and internal pores. For humid workshop conditions, low-temperature drying at 60–75°C is adopted to remove water, avoiding high-temperature heating that leads to plasticizer volatilization and material agglomeration. Optimize screw speed and back pressure to reduce shear heat and melt decomposition. Adopt segmented injection speed to prevent frontal air entrapment during rapid filling. Reasonable holding parameters assist melt feeding and squeeze internal air out through vent grooves. The vent depth of TPE molds is strictly controlled to adapt to the soft material characteristics, preventing exhaust blockage or flash overflow, and additional vent inserts are set at weld lines and flow terminals.
3. Sink Marks and Surface Depressions
Local thick glue positions, reinforcing ribs and transition corners of Chinese TPE molded parts are prone to shrinkage depressions due to large molding shrinkage and insufficient feeding solidification. In product design, wall thickness mutation is avoided to ensure uniform glue thickness. The gate is arranged close to the thick glue area to shorten the feeding distance. Appropriately increase the holding pressure and holding time to compensate for shrinkage gaps, and match lower barrel temperature and mold temperature to accelerate surface curing. Extend the cooling cycle to ensure full shaping of thick areas. Avoid undersized runners and gates to prevent early freezing of the feeding channel, which leads to invalid holding and persistent sink marks.
4. Bloom, Precipitation and Sticky Surface Defects
Surface bloom and oil precipitation are typical common defects of domestic formulated TPE materials, caused by the migration of internal plasticizers, lubricants and low-molecular additives. High barrel temperature and long melt residence time in Chinese domestic molding equipment will accelerate additive precipitation. During production, stabilize processing temperature to avoid over-shearing and overheating. Maintain appropriate mold temperature to prevent low-temperature quenching that induces additive surface migration. Optimize post-production storage conditions, avoid high temperature and closed stacking, and reduce delayed precipitation. For products with serious bloom, replace with low-precipitation special TPE grades suitable for domestic molding processes.
5. Insufficient Overmolding Adhesion and Delamination
Chinese TPE secondary overmolding products often have peeling and delamination problems when compounded with PP, ABS and PC hard rubber substrates. The core cause is mismatched material grades and unclean bonding surfaces. It is necessary to select special TPE materials matching the substrate, and thoroughly clean oil stains and release agent residues on the surface of hard rubber parts. The bonding surface of the hard rubber mold is appropriately sandblasted to increase mechanical occlusion friction. Preheat the hard rubber insert before injection, properly increase TPE melt temperature and injection pressure, optimize gate position to make the melt directly impact the bonding surface, and improve interface fusion tightness.
6. Flow Marks, Weld Lines and Silver Streaks
Unstable injection speed and large temperature difference between melt and mold are the main causes of flow lines and weld lines in Chinese TPE production. Properly increase mold temperature and melt temperature to reduce the cooling speed of the melt front, stabilize continuous material flow, and avoid intermittent ripples. Optimize the flow channel layout to reduce melt convergence points, set cold material wells and exhaust grooves at weld line positions to eliminate cold material and trapped air. Silver streaks caused by moisture and decomposition gas can be solved by standardized drying process and reduced shear friction heat.
7. Flash and Burr Defects
Due to the soft texture of TPE, tiny mold gaps in domestic high-cycle production will cause flash. Strictly control mold clamping force, check template deformation and insert matching gaps, and ensure tight fit of parting surfaces. The vent groove depth is strictly reduced compared with hard rubber molds to prevent material overflow. Cooperate with low pressure, low speed and constant temperature molding process to reduce melt fluidity and avoid edge extrusion burrs.
8. Deformation, Warpage and Dimensional Instability
Affected by elastic rebound and unbalanced cooling, Chinese TPE products are prone to warpage and distortion after molding. Uniform cooling water channels are used to ensure consistent mold temperature. Extend cooling time to complete shaping before ejection, and adopt multi-point uniform ejection to prevent bending deformation. Use shaping jigs for post-molding static cooling to release internal stress. Stabilize workshop temperature and humidity to ensure stable dimensional tolerance of batch products.
In conclusion, the stable production of Chinese TPE soft rubber injection molding relies on standardized material pretreatment, optimized mold structure and refined segmented process parameters. Targeted improvement of common defects can effectively improve product appearance, elasticity and bonding performance, meeting the quality requirements of domestic precision soft rubber supporting parts.
