What Is an Injection Pressure Profile — and Why Does the Fill Need One?

Last updated: 2026-09-01 · Reading time: about 4 min

A modern press does not fill a cavity with one speed and one pressure — it runs a profile: several velocity stages across the fill, a switch into pressure control near the end, and often a shaped hold after that.
The profile exists because the cavity makes different demands along the path — venting air at the start, fast flow through thin sections, gentle arrival at the end — and because the fill-to-pack switch, not the operator’s instinct, decides whether the part is packed or overpacked. Most defect patterns that look mysterious trace back to a flat profile that ignored the geometry.

Key facts

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  • Fill is velocity-controlled, pack is pressure-controlled: the machine pushes at a set speed until the changeover point, then holds a set pressure — one boundary that splits the whole molding physics in two.
  • Switch too early and the part runs short or sinks; switch too late and the cavity is rammed at velocity — flash, stress and burn marks follow.
  • Speed stages serve the geometry: a slower start keeps the stream from jetting and the gates from eroding, a faster middle bridges thin sections before they freeze, and an eased arrival lets air leave through the vents.

    • Flash — Thin excess material that escapes between the mold parting line or around ejector pins. Cause: insufficient clamp force, worn parting line, excessive injection pressure, or contaminated mold surface. Flash thickness: 0.02–0.1mm. Requires secondary trimming to remove. See also related guide.

    Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. See also related guide.

    Why does filling need more than one speed?

    Because the melt meets a different problem in each third of the path. At the start, the stream leaves the gate into open space — too fast and it jets across the cavity instead of building a flow front, and the gate region sees shear it never needed.
    Through the middle, the front crosses thin ribs and long travels where every second of delay freezes another layer; this is where speed earns its keep, bridging the distance before the skin closes. Near the end, the remaining air has to escape, and a front arriving at full velocity compresses it into heat — the classic burns in corners and at the last-to-fill ribs, and a slow final stage gives the vent paths time to do their job.
    A single velocity compromises all three regions at once, which is why one-speed fills show jetting at the gate and burns at the far corner simultaneously: the ends of the same profile disagree about what “fast” should mean. The profile is how the press obeys the geometry instead of arguing with it — and the same logic continues after fill, where the shaped hold takes over per the hold-time discipline.

    Where should the velocity-to-pressure changeover sit?

    Position What happens Verdict
    Too early — before the cavity is mostly full Pressure mode starts with wide-open space; fill slows and starves. Short shots, sinks, dimensions running small.
    Just before complete fill — 90–99% by volume Velocity finishes the bulk; pressure only tops off and packs. The working zone — found by test, not by rule of thumb alone.
    At or past complete fill The cavity is rammed at velocity with nowhere to go. Flash, stuck parts, high stress, edge burns.
    Position drifting shot to shot Cushion unstable or changeover on stroke, not pressure. Everything downstream floats — fix repeatability first.
    Set once at trial, never revisited Resin lot or mold wear shifts the fill behavior. Re-verify at the documented check intervals.
    Tuned by defect response Steps tested against weight, flash and short-shot limits. The honest method — each move attributable and reversible.

    The changeover is found the same way everything honest is found: by stepping its position and watching the part.
    Fill to 90% and switch — if weight still climbs as the point moves later and the parts stay clean, keep stepping; when flash or burns arrive, step back and the working zone is bracketed. The part-weight curve returns here as a second instrument, one position at a time, so the switch sits on evidence rather than on the sales brochure of the press.
    A changeover set this way also becomes the process’s early-warning sensor: when resin lots shift the viscosity, the old switch point suddenly short-shots or flashes, and the drift announces itself at one parameter instead of ten — the same signature-reading discipline the lot-variation page applies to dimensions.

    Which profile checks belong in the record?

  • Changeover point documented — the switch position with its working zone, not a single number remembered by the setter.
  • Speed stages mapped to the path — each stage justified by a region of the part, so future edits edit a reason.
  • Cushion stable and recorded — the repeatable residue that makes every profile setting meaningful shot to shot.
  • Defect bounds tested — short-shot and flash walls found on purpose, working point placed between them.
  • Peak pressure watched, not maxed — fill pressure headroom kept so the machine never masks a change by brute force.
  • Profile archived with the process sheet — stages, switch, cushion and limits beside the mold number per the acceptance file.

    • Frequently asked questions

    Is a pressure-limited fill wrong?

    Not inherently — some processes run fill on pressure on purpose.
    But when the intended mode is velocity control and the machine hits its pressure limit mid-fill, the profile silently degrades to pressure mode with no switch decision at all: speed collapses where resistance rises, and thin sections starve. Headroom on the pressure gauge is what keeps the chosen mode the actual mode.

    Why did the profile that ran clean last quarter flash now?

    Something upstream moved the fill: a resin lot with easier flow, a worn gate passing more material, a warmer barrel. The switch point that used to arrive at 95% now arrives later into a fuller cavity, and the last stage rams steel that used to be eased in. Re-run the changeover steps — the working zone will have moved, and the weight curve shows where.

    Do I need as many speed stages as the controller offers?

    No. Three to five stages cover most parts — start, middle, arrival — and each extra stage must earn its place by fixing a named defect in a named region. Profiles built to look sophisticated rather than to serve the geometry become impossible to troubleshoot; the discipline is fewer stages, each with a reason, documented beside the settings.

    Key takeaways

    1. The fill is velocity-controlled, the pack is pressure-controlled, and the changeover point between them is the single most consequential line in the process.
    2. Speed stages exist to serve the geometry — gentle start, fast middle, eased arrival — not to fill rows on a controller screen.
    3. A profile is honest when its bounds are tested and archived: switch position, cushion, pressure headroom and the defects that bracket them.

    If defects keep migrating around your parts while the settings look unchanged, send the part and the current profile screen. You will get the stage-by-stage read and the changeover re-verified — by a named tooling engineer, before the next shift blames the resin.

    Sources
    ISO 20457:2018 — Plastics moulded parts: Tolerances and acceptance conditions, International Organization for Standardization
    Plastics Industry Association — industry resources on injection molding practice
    Autodesk Moldflow — injection molding simulation, reference for fill velocity and pressure-profile analysis

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet