TAILIEUCHUNG - Handbook of Plastics, Elastomers and Composites Part 6

Tham khảo tài liệu 'handbook of plastics, elastomers and composites part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Elastomeric Materials and Processes 200 Chapter Three machinery equipment and tool and product data with the resin supplier. For example if it is suspected that an improper screw design will be used melt temperature gradient may be reversed. Instead of increasing temperature from rear to front it may be reduced from rear to A higher mold temperature favors a uniform melt cooling rate minimizing residual stresses and improves the surface finish mold release and product quality. The mold cooling rate affects finished product quality. Polyether type TPU can set up better and release better. High pressures and temperatures fill a high surface-to-volume ratio mold cavity more easily but TPU melts can flash fairly easily at high pressures Table . Pressure can be TABLE Typical Temperature Pressure Settings for Pellethane TPU Pressure lb in2 t MPa Injection pressure First stage 8000-15 000 Second stage 5000-10 000 Back pressure 0-100 Cushion in mm Screw speed r min 50-75 Cycle time s injection relatively slow to avoid flash etc. 3-10 Typical temperature and pressure settings are based on Ref. 3. Settings are based on studies using a reciprocating screw general-purpose screw clamp capcity of 175 tons and rated shot capacity of 10 oz 280 g . Molded speciment thickness ranged from to in to . . units refer to line pressures metric units are based on the pressure on the average cross-sectional area of the screw. carefully controlled to achieve a quality product by using higher pressure during quick-fill followed by lower The initial higher pressure may reduce mold shrinkage by compressing the elastomeric The back pressure ranges from 0 to 100 lb in2 0 to MPa . TPU elastomers usually require very little or no back When additives are introduced by the processor prior to molding back pressure will enhance mixing and when the plastication rate of the machine is .

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