Hold 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.

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 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)

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.