Hold Pressure

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 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)

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).