Relative Viscosity

Injection Pressure is the pressure the screw exerts on the molten material during dynamic filling, up to the transfer point. It is an output of the process, not a setpoint: it rises as much as needed to maintain the programmed injection velocity. ## Pressure types - Plastic (Ppsi): actual pressure on the material, in bar - Hydraulic (Hpsi): oil pressure in the hydraulic cylinder - Relation: Ppsi = Hpsi × intensification ratio (typically 10:1 to 15:1 depending on screw diameter) ## Typical values by resin - Commodity (PE, PP): 400 – 1200 bar plastic - Engineering (ABS, PC, PA): 700 – 1800 bar - Fiber-reinforced: 1000 – 2200 bar - High-viscosity resins (PEEK, PSU): up to 2500 bar - Modern machines: up to 2400 bar maximum ## Why it matters If pressure saturates (hits machine max), velocity drops and the part fills slower → cold part, cold weld lines, short shot. Design to not saturate: enlarge gates, runners or thickness, or reduce flow length. ## Diagnostics - Repeatable peaks shot-to-shot: stable process - Rising peaks: worn check valve, contamination, partially blocked gate - Falling peaks: mold temperature rising, gate wearing ## Optimization Raise melt temperature, enlarge gates if the restriction is there, switch to higher-MFI resin, or move to a higher-pressure machine (rarely needed with well-designed molds).

Melt is the plastic in viscous fluid state obtained by heating the polymer above its glass-transition or melting temperature (Tg for amorphous, Tm for semi-crystalline) inside the injection machine's barrel. Its temperature, pressure and viscosity drive molding quality. ## Typical melt temperatures - PE / PP: 200 – 280 °C - PS: 180 – 260 °C - ABS: 220 – 260 °C - PA 6 / PA 66: 240 – 290 °C - PC: 280 – 320 °C - PET: 270 – 290 °C - PEEK: 360 – 400 °C - Rigid PVC: 165 – 195 °C (low, thermally sensitive) ## Melt vs. barrel temperature Melt temperature is not the same as barrel temperature: - Barrel T: heater-band reading per zone (control) - Melt T: actual polymer temperature leaving the nozzle - Melt T typically 10 – 30 °C higher than barrel T due to shear work ## How to measure actual melt T - Needle pyrometer on a purge shot (most common) - IR sensor at the nozzle - Air shot purged on a hot plate, quick reading - Embedded sensors in the barrel (rare, premium) ## Melt characteristics - Pseudoplastic: viscosity drops with shear rate (shear thinning) - Viscoelastic memory: remembers flow, generates directional shrinkage - Lower density than the solid: 0.7 – 0.9 g/cm³ (vs. 0.9 – 1.4 solid) - Low thermal conductivity: 0.1 – 0.3 W/m·K (limits cooling rate) ## Melt-related issues Thermal degradation if process T is exceeded, over-shearing that reduces molecular weight, air trapping at the flow front, and color heterogeneity from poor mixing in the plasticizing zone.

Viscosity is a fluid's resistance to flow. For a polymer melt it governs how easily the resin fills the mold: high viscosity means stiff flow that needs more pressure, low viscosity flows easily but can flash. ## Polymer melts are shear-thinning Unlike water, a thermoplastic melt is pseudoplastic (shear-thinning): its viscosity drops as the shear rate rises. Faster injection speed shears the melt more and thins it, which is why fast fill can need less pressure than slow fill. Viscosity also drops as temperature rises (barrel temperature). ## What changes viscosity - Temperature: higher melt temperature → lower viscosity. - Shear rate: higher injection speed → lower viscosity. - Molecular weight / grade: higher MW (lower melt-flow index) → higher viscosity. - Moisture and degradation: can lower or raise it unpredictably. ## Why it matters Viscosity sets the fill pressure, the process window and gate/runner sizing. In scientific molding a viscosity curve (relative viscosity vs fill speed) finds the speed where the melt is least sensitive to small changes, for a more robust process. (Note: lab relative viscosity is a different, dimensionless ratio used to grade resins like PA.) ## Related terms - See also: melt, injection speed, barrel temperature, injection pressure, relative viscosity ## What is viscosity in injection molding? It is the melt's resistance to flow; it drops with higher temperature and higher shear (injection speed) and sets how much pressure is needed to fill the mold. ## Why does melt viscosity drop at high injection speed? Polymer melts are shear-thinning: more shear untangles and aligns the molecules, lowering viscosity — so a faster fill can need less pressure. ## What affects melt viscosity? Melt temperature, shear rate (injection speed), the resin's molecular weight / melt-flow index, and moisture or thermal degradation.