Viscosity

Barrel temperature is the set of heater-band temperatures along the injection barrel, zone by zone, that progressively melt the resin as the screw conveys it forward. It is set from the resin's recommended melt temperature and is the operator's main lever on melt quality. ## The barrel zones A barrel is split into three to five controlled zones (plus the nozzle), driven by the barrel heat bands: - Rear / feed zone: takes in pellets and starts softening — kept coolest to avoid bridging at the throat. - Middle / compression zone(s): where most of the melting and mixing happens. - Front / metering zone: homogenizes the melt to target before the nozzle temperature zone. ## Temperature profiles - Increasing (ramped): coolest at the rear, hottest at the front — the common default. - Flat: similar across zones — used for shear-sensitive resins. - Reverse (declining): hotter rear, cooler front — sometimes used for heat-sensitive resins or to fight drooling. ## Why it matters - Too hot: thermal degradation, drool, discoloration and longer cooling, and it raises residence time risk. - Too cold: unmelted pellets, high screw torque, short shots and accelerated screw/barrel wear. Always verify the actual melt with an air-shot probe — the set point is not the same as the real melt temperature. ## Related terms - See also: barrel, barrel heat bands, melt, nozzle temperature, residence time ## What is barrel temperature in injection molding? It is the zone-by-zone heater setpoints along the barrel that melt the resin, set from the resin's target melt temperature. ## How many barrel zones are there? Typically three to five controlled zones plus the nozzle, from the rear feed zone to the front metering zone. ## What happens if barrel temperature is too high? The melt degrades — discoloration, black specks, drooling and weaker parts — while cooling and residence-time problems grow.

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).

Injection Speed (Injection Velocity) is the linear speed at which the screw advances during fill, programmed in mm/s (or volumetric cm³/s). It is one of the parameters that controls fill quality, alongside temperature and hold pressure. ## Why it matters Speed determines: - Fill time: 0.3 – 3 s on technical parts - Shear on the material (faster → more shear → lower effective viscosity) - Cosmetic marks: jetting (excessive speed on small gate), flow marks (too slow or interrupted) - Molecular orientation and residual stress ## Multi-stage profile Modern machines allow 5 – 10 velocity steps along the screw stroke: 1. Slow at gate entry (avoids jetting) 2. Fast in wide cavities 3. Slow near critical vent zones (avoid air trap) 4. Slow at end of fill for smooth transition to hold ## Typical values - Commodity resins, standard wall thickness: 50 – 150 mm/s - Technical detailed parts: 30 – 80 mm/s - Very thin wall parts (<0.8 mm): 200 – 500 mm/s (high-dynamic servo machines) - Shear-sensitive resins (PVC, PMMA): moderate speed ## Optimization Moldflow / Moldex3D analysis to define theoretical profile, iterative tuning with short-shot studies, and melt-temperature monitoring at end of fill (must not rise more than 5 – 10 °C from over-shear). ## Common issues Jetting from high speed on point gates, flow marks from insufficient speed, burn marks from trapped air at end of fill, and delamination if the flow front cools partially.

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.

Relative Viscosity (RV) is the ratio between the viscosity of a polymer solution and that of the pure solvent, measured under standard conditions (concentration, temperature). It is the most practical indicator of molecular weight of a resin and is used to certify polyamide (nylon) lots. ## How it is measured ISO 307 / ASTM D789: - Dissolve 0.5 – 1.0 g of resin in 100 mL of 90 % formic acid or 96 % sulfuric acid - Measure efflux time in an Ubbelohde viscometer at 25 °C - RV = t_solution / t_solvent ## Typical values for PA (nylon) - PA 6 extrusion: RV 230 – 270 (high molecular weight) - PA 6 injection: RV 130 – 200 (low-to-medium molecular weight) - PA 66 injection: RV 40 – 80 (IV / Inherent Viscosity scale) - PA 12: RV 140 – 220 ## Why it matters in molding - High RV → stiff polymer, high mechanical strength, poorer flow (higher pressure, longer cycle) - Low RV → easy filling, ideal for thin or complex parts, but lower toughness - Selection depends on the part: technical engineers pick by RV, not MFI, because it correlates better with final properties. ## Difference vs. MFI (Melt Flow Index) MFI measures melt flow under standard load (g/10 min). RV measures molecular weight via solution viscosity. For PA, RV is more accurate and reproducible than MFI. ## Common pitfalls Mixing PA 6 RV with PA 66 RV (different scales), comparing supplier RV with different methods (formic vs. sulfuric), and forgetting that RV changes with absorbed moisture in PA before measurement.