Hold Time

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.

Gate is the final section of the feed channel where molten plastic enters the mold cavity. Its geometry —shape, size, location— drives fill behavior, surface quality, visible marks and balance across cavities. ## Most common gate types - Edge gate: simplest, on the parting line. Medium parts - Submarine / tunnel gate: self-shears at ejection, no manual cut needed. Diameter 0.5 – 2 mm - Pin gate: small point on the part face. Needs three-plate or hot runner - Direct / sprue gate: sprue feeds directly. For large single-cavity parts - Fan / film gate: for wide flat parts, avoids flow lines - Diaphragm gate: for cylindrical parts, uniform radial fill - Hot tip (valve gate): hot runner with mechanical shut-off, ideal cosmetics - Ring gate: annular, for long cylindrical parts - Tab gate: intermediate tab that is later cut off ## Typical parameters - Diameter: 0.5 – 4 mm depending on part and resin - Land length: 0.5 – 1.5 mm (short, to avoid premature freeze) - Thickness (fan / edge): 30 – 80 % of wall thickness - Gate vs. wall ratio: <0.5 to minimize visible mark ## How to select a gate - Cosmetic: pin gate or valve gate (minimal mark) - Cost-effective: edge gate (easy to machine, simple maintenance) - Self-separation: submarine or tunnel gate - Large single-cavity part: direct / sprue gate - Balanced multi-cavity: pin gate with hot runner ## Common issues - Jetting: jet stream when gate is too open and velocity too high - Premature gate freeze: gate too small → gate seal not reached - Gate vestige (mark): on cosmetic parts, requires smaller gate or valve gate - Gate wear: with fiber-reinforced resins, requires hardened inserts

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.