Cushion Position

Cushion (residual cushion) is the small amount of melt left in front of the screw at the end of injection and hold, so the screw never bottoms out on the barrel. It is what lets the machine keep transmitting hold pressure into the cavity through the pack phase. ## Typical values A cushion is usually a few millimetres of screw position — commonly 2–10 mm (often 3–6 mm), or roughly 5–10 % of the shot stroke. It should be small but never zero. ## Why it matters - Pressure transmission: with melt still ahead of the screw, hold pressure reaches the cavity. If the cushion goes to zero the screw bottoms out, pack pressure is lost, and you get sink marks, short shots and a weight drop. - Repeatability & diagnostics: a cushion that repeats shot to shot signals a healthy process. A drifting cushion is the classic symptom of a leaking check valve. ## How it is set The cushion is the gap between where dosing / the transfer position cut off leaves the screw and screw-bottom. Adjust the shot size / dosing volume so a consistent few-millimetre cushion remains; the monitored value is the cushion position. ## Related terms - See also: check valve, hold pressure, transfer position cut off, cushion position, shot size ## What is cushion in injection molding? It is the residual melt left in front of the screw at the end of pack so the screw never bottoms out and can keep transmitting hold pressure — typically a few millimetres. ## What is a good cushion value? Usually 2–10 mm (often 3–6 mm) and, above all, stable shot to shot — small but never zero. ## What does a changing cushion mean? A cushion that drifts from shot to shot with no process change usually means the check valve (non-return valve) is leaking and needs inspection.

Output values are the measured results a molding cycle reports back — the readings the machine and the part give you in response to the input parameters you set. Inputs are what you control; outputs are what actually happened. Watching outputs, not just inputs, is the core discipline of scientific method scientific molding, because the same settings can drift into different parts. ## Typical output values - Process readings (per shot): actual fill time, peak injection pressure, the cushion left, recovery time, actual cooling and overall cycle time. - Part results: weight of the molded part (or cavity weight), dimensions, sink/voids, flash and cosmetics. - Trends: shot-to-shot variation of these values, which reveals process stability over a run. ## Outputs vs inputs - input parameters (set, cause): fill speed, hold pressure, temperatures, timers. - Output values (measured, effect): fill time, peak pressure, cushion, part weight, actual cycle. A drifting output with unchanged inputs is the early-warning signal: a rising fill time or falling cushion points to a worn check valve, wet resin or a temperature shift before bad parts appear. ## Why it matters Output values are how a process is verified, not just set. Robust process control (and a quality system) defines acceptable ranges for key outputs and alarms or rejects when they leave the window — catching problems the input settings alone would hide. Monitoring part weight and fill time is one of the simplest, most powerful output checks on the floor. ## Related terms - See also: input parameters, scientific method scientific molding, cushion, cavity weight, cycle time ## What are output values in injection molding? The measured results of a cycle — actual fill time, peak injection pressure, cushion, part weight, real cooling and cycle time — that report what the process and part actually did in response to the input settings. ## What is the difference between output values and input parameters? Input parameters are the settings you control (speed, pressure, temperature); output values are the measured response (fill time, peak pressure, cushion, weight). Inputs are causes, outputs are effects — and outputs are what verify the process. ## Why monitor output values instead of just settings? Because identical input settings can still produce different parts as the material, mold or machine drift; tracking outputs like fill time, cushion and part weight catches that drift early, before scrap is made.

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.