Last updated: 2026-09-05 · 5 min read
Injection Pressure Profile — What Does the Pressure Curve Reveal About Fill Quality, How Do You Read It to Diagnose Short Shots, Flash and Weld Lines, and What Adjustments Fix Each Defect?
The injection pressure profile is a real-time graph of pressure vs. time (or screw position) during the injection phase. It reveals: fill balance (peak pressure 800–1,500 bar for most thermoplastics), flow front behavior (constant pressure = stable fill, dropping pressure = flow restriction), and pack effectiveness (pressure hold time and decay rate). Reading the pressure profile is the fastest way to diagnose short shots, flash, air traps and weld lines — without opening the mold.
- Pressure profile phases: (1) Injection — screw advances, pressure rises to peak (800–1,500 bar) filling the cavity. (2) Hold/pack — pressure drops to holding (300–800 bar) for 3–15 seconds to compensate shrinkage. (3) Cooling — pressure drops to zero as gate freezes. Shape reveals process health: flat peak = stable fill. Jagged peak = flow instability (air traps). Rapid pressure drop during hold = gate freeze early or check ring leak.
- Diagnostic patterns: Short shots — peak pressure hits machine limit (1,500+ bar), cavity not full. Cause: flow restriction (gate too small, vent blocked, melt temp low). Fix: increase melt temp 10–20°C, enlarge gate, add vents. Flash — pressure does not drop at transfer (over-packed). Cause: transfer too late, holding too high. Fix: reduce transfer position, reduce holding 50–100 bar. Air traps — pressure spike at end of fill (gas burns). Fix: add vents, reduce speed, relocate gate.
- Weld lines — pressure drops where flow fronts meet. Weld strength proportional to meeting pressure: above 500 bar = strong, below 200 bar = weak. Fix: increase melt temp (better fusion), increase injection speed (higher meeting pressure), or relocate gate to move weld to non-critical area. The profile shows the weld problem before ejection — early detection prevents batch defects.
What Does Each Phase of the Pressure Profile Reveal About Process Health?
| Phase | Normal Pattern | Abnormal Pattern | Diagnosis |
|---|---|---|---|
| Injection peak | Flat, 800–1,500 bar | Jagged or oscillating | Air traps, flow instability |
| Injection peak | Reaches target, stable | Hits machine limit (1,500+) | Short shot — flow restriction |
| Transfer point | Smooth drop to hold | No drop (stays at peak) | Over-pack — flash risk |
| Hold phase | Gradual decay, 3–15 sec | Rapid drop to zero | Gate freeze early or check ring leak |
| Cooling | Zero pressure after gate freeze | Pressure persists | Gate not freezing (hot runner issue) |
The pressure profile is recorded by the molding machine controller and can be exported for analysis. A baseline profile (from a known-good production run) serves as the reference — any deviation from the baseline indicates a process shift. Statistical process control (SPC) monitors the peak pressure, hold pressure and gate freeze time across shots to detect drift before defects occur.
Transfer point — the screw position where the machine switches from injection (high speed, high pressure) to hold/pack (lower pressure, slower). Setting transfer too late: cavity over-packs, flash appears. Setting too early: cavity under-packs, sink marks or short shots. The optimal transfer point is at 95–98% cavity fill by volume — determined by a fill study. The packing pressure guide covers how transfer position interacts with holding pressure.
What Three Adjustments Fix the Most Common Pressure Profile Defects?
- Short shots: increase melt temperature or gate size — profile shows peak at machine limit. If peak reaches 1,500+ bar and cavity not full: resin cannot flow further. Increase melt temp 10–20°C (reduces viscosity). If temp does not fix: gate too small (increase width 0.5mm or depth 0.25mm). If gate at max: add second gate or relocate closer to flow end. Always check vents first — blocked vents create back-pressure mimicking flow restriction.
- Flash: reduce transfer position or holding pressure — profile shows no drop at transfer. If pressure stays at peak during transfer: cavity over-packed, resin escapes parting line. Reduce transfer position (switch to hold 2–5mm earlier). Reduce holding pressure 50–100 bar increments until flash disappears. If flash persists: check parting line seal (increase clamp tonnage). Profile confirms fix: transfer shows smooth drop from peak to holding.
- Weld lines: increase melt temperature and injection speed — profile shows pressure drop at weld. If pressure drops below 200 bar at weld: flow fronts meet with insufficient fusion pressure. Increase melt temp 10–20°C (better molecular interdiffusion). Increase injection speed 10–20% (higher meeting pressure). If weld in critical area: relocate gate to move weld to non-critical location. After fix: pressure at weld should be above 500 bar.
Cavity pressure — the actual pressure on the part surface measured by a sensor installed inside the cavity. Cavity pressure is 30–50% lower than hydraulic pressure (measured at the machine cylinder) because hydraulic pressure includes friction losses in the screw, nozzle, runner and gate. Cavity pressure directly measures compaction force on the part. The scientific molding guide covers how cavity pressure is used for decoupled molding control.
Pressure profile: injection peak 800–1,500 bar, hold 300–800 bar, cooling to zero. Short shots: peak at machine limit. Flash: no drop at transfer. Weld lines: pressure drop at meeting point. Read the profile to diagnose without opening the mold.
- Baseline profile recorded — shots 5–15 averaged, saved in machine controller.
- Peak pressure within 80% of machine max — above 80% = short shot risk.
- Transfer point set at 95–98% fill — too late = flash; too early = sink.
- Hold pressure decays gradually — rapid decay = gate freeze or check ring leak.
- SPC monitoring active — peak pressure deviation >±10% triggers investigation.
Frequently Asked Questions
How do I set up a baseline pressure profile for a new mold?
Start with the resin supplier’s recommended processing parameters (melt temp, injection speed, holding pressure). Run 10–20 shots and record the pressure profile for each. The baseline is the average profile from shots 5–15 (first 5 shots are warm-up, not representative). Key parameters to record: peak injection pressure, transfer position, holding pressure, gate freeze time. Save the baseline profile in the machine controller. For production: monitor each shot’s peak pressure against the baseline — if peak pressure deviates ±10% from baseline, the process has shifted and investigation is needed.
What is the difference between hydraulic pressure and cavity pressure?
Hydraulic pressure (measured by the machine) is the pressure in the injection cylinder — includes friction losses in screw, nozzle, runner and gate. Cavity pressure (sensor inside the cavity) is actual pressure on the part surface — 30–50% lower than hydraulic. Cavity pressure is more accurate for part quality — it directly measures compaction force. Hydraulic pressure is useful for machine health (check ring wear, leaks). For critical parts: install cavity sensors and use cavity pressure for control.
Can the pressure profile detect mold wear or damage?
Yes. Mold wear changes the flow path: worn gate (enlarged by abrasive resins) reduces pressure drop — profile shows lower peak and faster fill. Blocked vent (degraded resin) increases back-pressure — profile shows higher peak and slower fill. Leaking check ring causes pressure loss during hold — profile shows rapid decay. Trending the profile over time (SPC) detects changes before defects. Peak pressure drops 5–10% over 10,000 shots: inspect gate for wear. Hold pressure decays faster: check the check ring.
Key Takeaways
- Pressure profile: injection 800–1,500 bar, hold 300–800 bar.
- Short shots: peak at machine limit. Flash: no drop at transfer.
- Weld lines: pressure drop at meeting point. Fix: increase temp/speed.
- Baseline profile + SPC monitoring detects drift before defects.
Send the part drawing and current pressure profile data — the review analyzes the injection, hold and cooling phases, identifies flow restrictions or process instabilities, and specifies the parameter adjustments to eliminate short shots, flash and weld lines.
Sources: Plastics Technology — Injection Pressure Profile Analysis · Plastics Technology Online — Reading Pressure Curves for Process Diagnostics · Society of Plastics Engineers (SPE) — Injection Molding Process Monitoring · AJW Engineering — Injection Pressure Profiling and Process Optimization · ThomasNet — Injection Molding Process Control and Pressure Monitoring
— Arthur, Tooling Engineer at aCatMould