Fill First Stage

The fill second stage is the second of the injection stages: the pressure-controlled pack and hold phase that follows the velocity-controlled first stage. The screw hands off at the transfer position cut off when the cavity is roughly 95–99 % full, and the machine switches from filling by speed to pressing on the melt by pressure — applying hold pressure to finish filling and compensate for shrink. ## What happens in second stage - Pack: a brief, higher pressure tops off the last 1–5 % of the cavity and densifies the part so it copies the steel. - Hold: pressure is maintained while the gate is still open, pushing extra melt in to make up for the volume the plastic loses as it cools and contracts (contraction). Hold ends when the gate freezes — more hold after that does nothing. - Cushion preserved: a small cushion must remain so the screw can keep transmitting pressure during hold. ## First vs second stage - First stage (fill): velocity-controlled, fills ~95–99 %, sets the flow-front behavior and most cosmetic results. - Second stage (pack/hold): pressure-controlled, finishes the fill and sets part weight, dimensions and sink/voids. Separating the two cleanly at the transfer position cut off is the heart of decoupled, scientific molding. ## Why it matters Second stage governs the things customers measure: dimensions, weight and internal soundness. Too little pack/hold gives short shots, sink and voids; too much gives flash, overpacking, high stress and ejection problems. Hold time is set by a gate-seal study (weigh the part as hold time increases until weight stops rising). ## Related terms - See also: injection stages, hold pressure, transfer position cut off, cushion, contraction ## What is the fill second stage in injection molding? The pressure-controlled pack-and-hold phase after the velocity-controlled first stage; it tops off the last few percent of the cavity and holds pressure to compensate for shrink, setting part weight and dimensions. ## What is the difference between first and second stage? First stage (fill) is velocity-controlled and fills ~95–99 % of the cavity; second stage (pack/hold) is pressure-controlled, finishes the fill and compensates for cooling shrink — they switch at the transfer/cut-off position. ## How do you set second-stage hold time? With a gate-seal (gate-freeze) study: increase hold time and weigh the part each step; once part weight stops increasing, the gate has frozen and that is the minimum effective hold time.

Fill Time is the duration measured between screw motion start and the transfer point, during which the cavity is filled to 95 – 99 %. It is one of the most sensitive indicators of injection-molding process stability. ## Why it is critical - Repeatability: variations <2 % indicate a stable process - Dosing: constant time ensures constant volume - Diagnostics: changes in fill time reveal check-valve wear, viscosity shifts or restrictions ## Typical values - Small parts (<10 g): 0.3 – 1 s - Medium parts (10 – 100 g): 1 – 3 s - Large parts (>100 g): 2 – 6 s - Technical parts with fine detail: 1 – 4 s ## Programmed vs. actual time - Programmed: ideal per velocity profile and volume - Actual: effective, may be longer if injection pressure saturates (velocity drops) - Typical difference: <5 % in a stable process ## How to monitor - Automatic logging on the machine (most modern presses) - External screw-position sensors (high end) - Statistical histogram in SPC - Alerts: ±5 % or ±10 % depending on part criticality ## Variation diagnostics - Gradual increase (weeks): check-valve wear, progressive leakage - Sudden increase: gate blockage, contamination, batch change - Gradual decrease: mold temperature drifting up, resin absorbing moisture - Random variations: inconsistent moisture in resin, irregular virgin/regrind mix ## Optimization Seek the shortest time without defects (jetting, splay, burn marks). Every tenth saved multiplies across thousands of cycles. Rule of thumb: fill time = (thinnest wall) / (critical flow velocity of the resin).

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.

Injection stages are the phases of pushing the shot into the mold, split into two fundamentally different control modes — a velocity-controlled fill (first stage) and a pressure-controlled pack-and-hold (second stage) — with a transfer point between them. ## First stage — fill (velocity-controlled) The screw advances at a set injection speed to fill about 95–99 % of the cavity quickly. Speed, not pressure, is the controlled variable; injection pressure is only the ceiling that allows that speed. ## Transfer (cut-off) At the transfer position cut off the machine switches from speed control to pressure control — the single most important transition for shot consistency. It is usually set by screw position (sometimes by pressure or time). ## Second stage — pack & hold (pressure-controlled) hold pressure packs in a little more melt to compensate shrinkage as the part freezes, until the gate seals — this is the fill second stage. A stable cushion must remain so the pressure keeps transmitting. ## Why it matters Decoupling fill (speed) from pack (pressure) at the right transfer point is the heart of scientific molding: it makes the fill repeatable and lets pack control final weight and dimensions independently. ## Related terms - See also: transfer position cut off, injection speed, hold pressure, fill second stage, molding cycle ## What are the injection stages in injection molding? First-stage fill (velocity-controlled, about 95–99 % full), transfer/cut-off, and second-stage pack and hold (pressure-controlled) until gate seal. ## What is the difference between first and second stage? First stage is speed-controlled filling; second stage is pressure-controlled packing and holding. The transfer point switches between them. ## Why decouple fill and pack? So the fill repeats the same way every shot (by speed) while pack pressure independently sets final part weight and dimensions — the basis of a stable, scientific process.

Transfer position (Cut-Off / V/P switchover) is the screw position at which the controller switches from velocity control (injection phase) to pressure control (hold phase). It is one of the most critical settings in scientific molding: it ends dynamic fill and begins packing. ## Why it matters During injection, velocity (cm³/s or mm/s) is controlled; during hold, pressure (bar) is controlled. Transferring too late over-packs the cavity (flash, internal stress); too early causes short shot or sink marks. ## How to set it - Fill 95 – 99 % of the cavity on velocity, leaving the rest to hold - Final cushion: should be 5 – 10 % of shot size, stable and repeatable - "Pressure vs. time" method: transfer before injection pressure saturates ## Transfer methods - By screw position (most common and reproducible) - By time since start of injection (least precise) - By hydraulic / plastic pressure (V/P switch by pressure) - By cavity pressure sensor (most accurate, advanced scientific molding) ## Indicators of a well-tuned transfer - Cushion stable shot-to-shot (±0.5 mm) - Repeatable fill time - Reproducible injection pressure peaks - No flash on any cavity in a multi-cavity tool ## Common issues Late transfer with flash, early transfer with short shot, cushion drift from check-valve wear, and multi-cavity imbalance requiring per-cavity tuning with pressure sensors.