Injection Molding Characteristics of Elastomer Materials (TPE/TPU)
Thermoplastic elastomers TPE and TPU are versatile flexible feedstock widely used in rubber-plastic converting industries, progressively replacing traditional vulcanized rubber for diverse finished product manufacturing. Combining high elasticity and abrasion resistance of vulcanized rubber with thermoplastic’s melt-processable and recyclable advantages, TPE and TPU scraps can be fully recycled and reused after crushing without complex vulcanization procedure required for conventional rubber. Distinct from rigid engineering plastics, molten TPE/TPU shows prominent viscoelasticity and elastic memory effect whose fluidity changes drastically along with melt temperature and shear speed. Blindly copying molding parameters of ABS or PP easily leads to short shot, flash, sink mark, post-mold shrinkage deformation and poor surface gloss. Mastering exclusive injection features of TPE and TPU lays solid foundation for parameter setup, mold design and stable mass production of flexible rubber-plastic composite parts.
1. Fundamental Rheological & Thermal Property Features of TPE/TPU Pellets
TPE and TPU differ greatly from rigid plastics in raw material physical traits: most TPE grades are hygroscopic with low surface hardness ranging from Shore A 0~95, while TPU possesses stronger water absorption capacity whose molecular chain easily combines ambient moisture to trigger hydrolysis degradation under high melt temperature. Drying procedure becomes mandatory before molding: TPU needs 2~3 hours drying at 80~90℃ to control moisture below 0.15%, and low-absorption TPE still requires dehumidification treatment for outdoor-grade products.
Both materials own wide molding temperature window yet sensitive to temperature fluctuation; excessive barrel heat causes thermal decomposition and sticky melt, while insufficient heating leads to poor plasticization and incomplete cavity filling. Compared with common plastics, TPE/TPU melt viscosity declines rapidly under rising shear force, which is the core factor triggering flash defects at mold parting line.
2. Key Process Parameter Control During Injection Molding
2.1 Barrel Temperature Regulation
TPE molding barrel temperature is commonly set between 160℃~220℃ according to hardness grade, and TPU requires higher melt temperature from 180℃ to 240℃ varying with ester or ether base formulation. Temperature gradient rises gradually from feed throat to nozzle to avoid premature melt blocking feed channel; overheated nozzle triggers material degradation and stringing during mold opening.
2.2 Injection Speed & Pressure Setup
High melt elasticity requires graded injection speed: slow filling at initial cavity entry to prevent air trapping and high-speed feeding for thin-wall section, while packing pressure is set lower than rigid plastics to restrain post-mold rebound shrinkage caused by elastic memory. Over-high packing pressure compresses excessive melt inside cavity and induces severe residual stress and long-term dimensional variation after demolding.
2.3 Mold Temperature Control
Low mold temperature below 40℃ is preferred for most TPE to accelerate surface curing and reduce sticking on mold cavity; TPU demands moderate mold temperature from 50~70℃ to balance surface finish and internal residual stress. Too cold mold surface causes poor melt fusion and internal invisible cracks, whereas overheated mold leads to prolonged cooling cycle and sticking defect.
3. Exclusive Mold Design Specifications for TPE/TPU Products
Elastomer’s high elasticity and sticky melt impose special requirement on mold construction. Enlarged overflow well and deeper vent slot are designed at cavity terminal to exhaust trapped air and absorb redundant flash generated from elastic melt compression; vent depth is controlled within 0.02~0.03mm to avoid material overflow. Draft angle is increased 1°~2° higher than rigid plastic mold to ease demolding and prevent product tearing from elastic rebound. Runner and gate adopt rounded smooth transition to lower melt shear and reduce localized overheating degradation of TPE/TPU. For ultra-soft low-hardness TPE products, anti-adhesion coating is applied on cavity surface to resolve persistent sticking issue.
4. Post-Molding Shrinkage & Post-Curing Management
Elastomer parts own higher post-mold shrinkage rate than ordinary plastic due to elastic rebound after pressure relief; finished products need 24~48h natural placement under constant temperature environment to complete stable dimensional shrinkage before dimension inspection. Rapid low-temperature cooling after demolding locks internal residual stress and brings about delayed warpage in later application, so gradual ambient cooling is recommended for precision elastomer components. Recycled TPE/TPU pellets should be blended within limited proportion after strict drying to avoid fluctuating elasticity and inconsistent shrinkage of molded parts.
Conclusion
TPE and TPU’s unique viscoelasticity, thermal sensitivity and elastic shrinkage decide their differentiated molding rules covering raw material pre-drying, parameter tuning and customized mold design. Strict drying standard eliminates hydrolysis defects induced by residual moisture; graded injection speed and moderate packing pressure control flash and residual stress; modified vent layout and enlarged draft angle on molds solve sticking and incomplete filling problems. Following inherent injection characteristics of elastomers stabilizes finished product dimension and surface quality, expands scrap recycling ratio and cuts overall production cost for flexible plastic component manufacturers.
