Cushion

Cushion position is the screw position the machine reports at the end of pack/hold — the resting place of the screw when a small cushion of melt still remains in front of it. It is one of the most-watched outputs values on the controller because, shot to shot, a stable cushion position is one of the clearest signals that the process is healthy. ## What it tells you The cushion is the small melt reserve left so the screw can keep transmitting hold pressure; the cushion position is where the screw stops to leave it. Because it is a measured output, not a setting, it reacts to what the plastic and machine actually did: - Stable position = consistent fill, melt and check valve sealing — a repeatable process. - Drifting/wandering position = a problem to chase: a worn or leaking check valve (screw drifts forward, cushion shrinks), inconsistent shot size or recovery, material or temperature variation. - Cushion gone (bottoming out) = the screw hit zero; pressure transfer is lost, giving short shots and weight swings. ## How it is used - Process monitoring: cushion position is trended and alarmed within a window; leaving the window flags trouble before bad parts ship — a core check in a robust process and the injection stages handoff at transfer position cut off. - Setup target: enough cushion is dialed in (via shot size and transfer position) to maintain pressure, but not so much that residence time and waste grow. ## Why it matters A wandering cushion position is often the first visible sign of a failing check valve or unstable shot — catching it early prevents scrap. Together with fill time and part weight, it is one of the simplest, most powerful health indicators on the machine. ## Related terms - See also: cushion, check valve, hold pressure, shot size, outputs values ## What is cushion position in injection molding? The screw position the machine shows at the end of hold, where a small melt cushion remains; it is a monitored output value whose shot-to-shot consistency indicates a stable process. ## Why does cushion position drift? Usually a worn or leaking check valve lets the screw creep forward and the cushion shrink; inconsistent recovery, shot size, or material and temperature variation also move it. A drifting cushion is an early warning of trouble. ## What happens if the cushion bottoms out? If the screw reaches zero cushion it can no longer transmit hold pressure, causing short shots, sink and part-weight variation; the fix is more cushion (shot size/transfer) or addressing the check valve.

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.

Screw is the helical component inside the barrel of the injection unit. It rotates on its axis to feed, plasticize (melt) and meter the resin; during injection it acts as a piston pushing the melt toward the mold. ## Screw anatomy Three functional zones along its length: 1. Feed zone: deep, takes pellets from the hopper. 50 – 60 % of length 2. Compression zone: depth tapers down, compacts and starts melting. 20 – 30 % 3. Metering zone: minimum constant depth, homogenizes and meters the shot. 20 % ## Geometric parameters - Diameter (D): 18 – 200 mm in commercial machines - L/D ratio: 18:1 to 24:1 standard; up to 30:1 for high mixing - Compression ratio: 2.0:1 to 3.5:1 depending on resin - Material: nitrided steel (standard), bimetallic (PVC, flame retardants), tungsten-carbide coatings (glass fiber) ## Special screw types - Barrier screw: divides the channel in two for better melting - Mixing screw: with extra shear/mixing elements - For PVC: low compression ratio, no hot zone - For fiber-reinforced: low shear so as not to break fibers ## Maintenance - Visual inspection every 6 months - Diameter and clearance check with three-point micrometer - Typical replacement: 1 – 3 million cycles depending on resin abrasiveness - Wear indicators: shot-weight variation, unstable cushion, uneven color ## Common issues Flight wear from abrasive resins, corrosion from PVC without proper coating, pellet bridging in feed from moisture or irregular size, and worn check valve allowing material backflow during injection.

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.