Cushion

Cushion Percentage as a Daily Process Health Check

Cushion is the cheapest process monitor in the building. This playbook covers the percentage math, the 5 to 10 percent target window, what drift direction tells you about check rings and viscosity, and the shift-start routine that catches failures early.

Cushion is the guarantee that pack pressure actually reaches the cavity. If the screw bottoms out with no melt left in front of it, hold pressure pushes against steel instead of plastic, and the parts show it: sinks, shorts, voids, and dimensions that wander lot to lot. The money math is simple and ugly: a job making 400,000 parts a year at $0.75 each that ships a 1.5 percent dimensional scrap rate from inconsistent packing is losing $4,500 in parts plus containment costs, and cushion instability is among the most common root causes. The countermeasure costs nothing, because every modern press already displays the number; the plant just has to watch it with discipline.

The math: cushion percentage equals the cushion position divided by total shot size, times 100. A press transferring to hold with 0.30 inch of melt in front of the screw on a 4.0 inch total shot stroke is running a 7.5 percent cushion. The Cushion Percentage calculator standardizes this check across presses with different barrel diameters and stroke lengths, which matters because 0.30 inch means completely different melt volumes on a 40 mm screw versus an 80 mm screw. Percentage is the number that travels: it lets a process engineer set one plant standard, typically 5 to 10 percent, and audit every press against it regardless of frame size.

Targets and tolerances: hold cushion between 5 and 10 percent of shot for most work, leaning toward 10 percent on small precision shots where a few hundredths of stroke are a big fraction of the melt. Below 3 percent you are one viscosity shift away from bottoming; above 15 percent you are carrying extra residence time and inviting degradation on sensitive resins. Repeatability matters more than the setpoint itself: shot-to-shot cushion variation should hold within 0.010 inch on a healthy machine, and the best presses hold 0.005. Set soft alarms at plus or minus 0.020 inch from the established mean so drift gets flagged in minutes, not discovered at final inspection.

Read the drift direction like an instrument, because that is what it is. Cushion trending down over hours while everything else holds steady is the classic signature of a leaking check ring: melt slips backward past the valve during pack, and by the time cushion hits zero you have been shipping under-packed parts for a while. Cushion jumping up usually means viscosity rose, a resin lot change, regrind ratio shift, or a cold heater zone, so the cavity filled less before transfer. A cushion that oscillates shot to shot beyond 0.015 inch points at unstable recovery: back pressure set under 50 psi plastic pressure, worn screw flights, or feed throat temperature wandering.

Failure modes on the floor: running zero cushion on purpose because the parts look fine today, which works until the first viscosity shift, then fails catastrophically and intermittently, the worst kind; parking a 20 percent cushion as a comfort blanket, which adds a full extra shot of residence exposure on heat-sensitive materials; and the diagnostic trap of chasing a shrinking cushion by moving the transfer position every shift, which hides a dying check ring behind fresh setpoints. A check ring replacement is a few hundred dollars and half a shift; the sorting, scrap, and customer escape from three weeks of masked leakage costs ten times that, every time.

Cadence: at every shift start, the operator records cushion mean and spread over five shots on the log sheet, 60 seconds of work. The process monitor charts cushion continuously with alarms at the established limits, and every alarm gets a disposition note. Weekly, process engineering reviews cushion trend charts across all presses, looking for slow slopes that daily eyes miss; a cushion drifting 0.002 inch per day is invisible in a shift and obvious in a month. At every mold change, the recorded cushion baseline travels with the job so the next setup has a target, and check rings get evaluated on a schedule, typically every 150,000 to 300,000 cycles depending on resin abrasiveness, instead of run to failure.

World class cushion management is quiet: cushion standard deviation under 0.005 inch on qualified jobs, alarms wired so drift pages a tech before parts drift out of spec, setup sheets carrying cushion target and tolerance next to the temperatures and pressures, and check ring changes appearing on the maintenance plan rather than the downtime report. Plants that run cushion as a first-class control variable typically cut packing-related scrap by half or better within two quarters, and their process techs diagnose check ring wear, viscosity shifts, and recovery problems from a trend chart in five minutes instead of a teardown in five hours.

Published 2026-07-02.