Fill Second Stage

Fill (First Stage) is the phase of the cycle in which the screw advances under velocity control, filling the mold cavity approximately to 95 – 99 % of its volume. It ends at the transfer point, where control switches to pressure. ## Key characteristics - Control: velocity (mm/s or cm³/s), not pressure - Purpose: fast and reproducible dynamic filling - Duration: 0.3 – 5 s typically - Volume filled: 95 – 99 % of cavity ## Why separated from hold First stage prioritizes velocity for a uniform flow front; second stage (hold) prioritizes constant pressure to compensate shrinkage. Mixing both in a single-stage process reduces quality and increases variability. ## 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 vents 4. Slow at end for smooth transition ## Typical parameters - Velocity: 30 – 200 mm/s depending on part and resin - Actual pressure (not control): may hit saturation if geometry is restrictive - Time: 0.5 – 3 s on technical parts - Residual volume: 5 – 10 % cushion as margin for hold stage ## Indicators of a good first stage - Uniform flow front (visible in short-shot studies) - Repeatable fill time (±2 % shot-to-shot) - Repeatable pressure peak - Stable final cushion ## Common mistakes - Velocity too high: jetting, splay, burn marks - Velocity too low: cold parts, visible weld lines, short shot - Late transfer: flash, over-pack - Early transfer: sinks, low dimensions

Hold pressure (Packing pressure) is the pressure applied to the material in the cavity after the transfer point, during the hold phase. Its purpose is to compensate volumetric shrinkage as the part cools and solidifies. ## Why it is needed As plastic cools, its volume decreases. Without hold, sinks, internal voids and under-tolerance dimensions appear. Hold pressure pushes additional material to fill this "volumetric deficit" until the gate freezes. ## Typical values - 40 – 80 % of peak injection pressure as starting point - Commodity resins (PE, PP): 300 – 700 bar (plastic) - Engineering resins (ABS, PC, PA): 500 – 1000 bar - Multi-stage: decreasing pressure in 2 – 4 steps as the gate freezes - Time: typically until gate freeze-off (measured by gate-seal study) ## How to set — gate seal study 1. Mold parts with increasing hold times (0.5, 1, 2, 4, 6, 8 s…) 2. Weigh each part 3. Weight rises until it plateaus when the gate freezes 4. The optimal hold time is the first one at which weight no longer grows ## Hold pressure vs. injection pressure These are the two stages of the injection sequence and are easy to confuse: - Injection pressure (first stage) is velocity-controlled and fills the cavity fast — it peaks at the highest value of the shot. - Hold pressure (second stage) is pressure-controlled and only tops up shrinkage after filling — typically 40–80 % of that peak. The instant the machine switches from the first to the second is the transfer (changeover) point. Getting that point right is what separates a stable process from one with flash or short shots. ## Common issues Hold too low: sinks, voids, low dimensions. Hold too high: flash, over-pack, residual stress, ejection difficulty. Hold too long (after gate seal): only wastes cycle time without affecting the part. ## What is hold pressure in injection molding? It is the second-stage pressure that keeps pushing melt into the cavity after filling, compensating for the shrinkage that occurs while the part cools, until the gate freezes. ## How high should hold pressure be? Start at 40–80 % of the peak injection pressure, then fine-tune with a gate-seal study. Commodity resins typically run 300–700 bar plastic pressure; engineering resins 500–1000 bar. ## How do you determine hold time? With a gate-seal (cushion/weight) study: mold parts at increasing hold times and weigh them. Once part weight stops increasing, the gate has frozen and any extra hold time is wasted.

Hold Stage (Packing Phase) is the second fill phase of the mold, after the transfer point, in which the screw applies a controlled pressure (not velocity) to compensate for shrinkage while the material cools. It ends when the gate freezes and material can no longer flow. ## Difference vs. injection phase - Injection (fill): velocity control, dynamic filling to ~95 – 99 % of cavity - Hold (packing): pressure control, packs the last 1 – 5 % and compensates shrinkage ## Typical parameters - Pressure: 40 – 80 % of peak injection pressure - Time: until gate seal (typically 2 – 10 s) - Multi-stage: 2 – 4 steps decreasing as the gate freezes - Final cushion: 5 – 10 % of shot size, stable ## When to raise/lower - Raise if: sinks, voids, low dimensions, weight below target - Lower if: flash, over-pack, internal stress, ejection difficulty ## How to verify good hold — gate seal study Weigh parts at increasing hold times; weight should plateau once the gate freezes. Optimal time = first point at which weight no longer grows. ## Common issues Zero cushion (missing material), large cushion (hold too short or gate sealed early), saturated pressure (upstream restriction), and cavity imbalance in multi-cavity molds.

Hold Time is the duration of the packing/hold phase during which controlled pressure is applied to the material in the cavity to compensate for shrinkage during initial cooling. It ends when the gate freezes and no more material can flow into the cavity. ## How to determine the optimum — gate seal study The most reliable method is weighing parts at increasing hold times: 1. Mold parts with hold of 0.5, 1, 2, 3, 5, 8, 12 s 2. Weigh each (scale with 0.01 g precision) 3. Plot weight vs. hold time 4. Weight rises until it plateaus once the gate freezes 5. Optimal time = first point of plateau + 10 % margin ## Typical values - Small parts (<10 g), wall <2 mm: 1 – 3 s - Medium parts, 2 – 4 mm wall: 3 – 8 s - Large parts, >4 mm wall: 8 – 20 s - Thick parts (>6 mm): up to 60 s - Hot runner: depends on gate type (valve gate shorter) ## Why it matters - Too short (before gate seal): material backs out → sinks, low dimensions - Optimum (at gate seal): maximum weight, repeatable dimensions - Too long (after gate seal): does not affect the part, wastes cycle time ## Relation to other parameters - Wall thickness: thicker wall → longer hold - Gate diameter: bigger gate → longer hold - Mold temperature: colder → gate freezes faster → shorter hold - Gate type: valve gate closes mechanically, time is not freeze-dependent ## Common mistakes - Time "by feel" with no gate seal study, usually over-sized - Not re-validating when changing resin or batch - Multi-cavity: same time for all, but freeze may be asymmetric - Confusing hold time with cooling time (they often overlap)

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.