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Injection Molding Pressure Settings Guide — Fill, Pack, Hold, and Back Pressure
The pressure settings on an injection molding machine directly determine part quality — fill completeness, dimensional accuracy, cosmetic appearance, and mechanical strength. But many importers accept "process parameters" listed on a first-article report without knowing whether they were optimized or simply copied from a similar mold.
This guide explains each pressure setting, its effect on part quality, and what buyers should verify in process documentation.
The Four Pressure Phases
1. Injection Pressure (First-Stage Pressure)
This is the pressure applied to push molten plastic from the barrel into the mold cavity during the fill phase. Typical range: 800-2,000 bar at the screw tip. The correct injection pressure fills the cavity completely in the designated time without causing flash or overpacking. Pressure too low = short shots. Pressure too high = flash, mold damage, and excessive machine wear.
Velocity control vs. pressure control: During fill, most modern machines operate in velocity control — the screw moves at a set speed, and the machine generates whatever pressure is needed to achieve that speed. The injection pressure setpoint acts as a limit (cut-off). When cavity pressure reaches the setpoint, the machine switches to holding pressure.
2. Packing / Holding Pressure (Second-Stage Pressure)
After the cavity fills, the machine switches to holding pressure — typically 50-80% of injection pressure — to pack additional material into the cavity as the part cools and shrinks. Holding pressure is critical for:
- Part dimensional accuracy (compensating for shrinkage)
- Surface quality (preventing sink marks)
- Minimizing weld line visibility
- Reducing internal voids
The hold time must be calibrated to the gate-freeze point — once the gate solidifies, additional holding pressure has no effect and only wastes cycle time.
| Material | Typical Hold Pressure (% of Injection) | Typical Hold Time (s/mm wall) |
|---|
| PP | 50-70% | 0.5-1.0 |
| ABS | 60-80% | 0.3-0.8 |
| PC | 50-70% | 0.3-0.6 |
| PA6 | 60-80% | 0.2-0.5 |
| POM | 50-70% | 0.4-0.8 |
3. Back Pressure
Back pressure is the resistance applied to the screw as it retracts during the plasticizing phase. It generates shear heating that improves melt uniformity and disperses colorants and additives. Typical range: 5-20 bar.
- Low back pressure (5-10 bar): Faster plasticizing, less shear heating. Suitable for shear-sensitive materials like PVC, POM.
- Medium back pressure (10-15 bar): Good balance for most commodity plastics.
- High back pressure (15-25 bar): Better mixing for masterbatch, glass fibers, or flame retardants. Risk: material degradation and longer recovery time.
4. Clamping Force
The clamping force holds the mold closed against injection pressure. Required force = projected area of the part (including runner) × average cavity pressure. Rule of thumb: 3-8 tons per square inch of projected area, or approximately 5 tons per cm².
- Insufficient clamp force = flash
- Excessive clamp force = mold wear, higher energy consumption, and plate deflection on large molds
Buyer's Tip: When you receive a first-article inspection report from a Chinese molder, the process parameters listed may not represent the production settings. Some factories run the sampling at conservative settings (higher injection pressure, longer hold time) to ensure the parts look good, then reduce the settings during production to increase throughput — changing the final part dimensions and mechanical properties. Request a "process window" document showing the upper and lower control limits for each pressure parameter, not just the single setpoint used during sampling. If the molder can't provide process windows, they aren't using scientific molding methods, and your production consistency will vary across batches. The VDI 7008 standard for process documentation is the most widely accepted format globally.
Pressure Loss Along the Flow Path
Pressure drops measurably from the machine nozzle through the sprue, runner, gate, and into the cavity. The cumulative pressure loss can be 30-50% of the machine's injection pressure. The largest losses typically occur at:
- The gate: Small gate cross-section creates the highest restriction. Gate land length should be minimal (0.5-1.5mm) to reduce pressure drop.
- The runner: Full-round runners have 25% less pressure drop than trapezoidal runners. Runner diameter should be at least part wall thickness + 1mm.
- The sprue: A sprue bore smaller than 3mm at the small end creates significant pressure loss.
Switchover Point — Pressure to Velocity
The switch from velocity-controlled fill to pressure-controlled hold is the most critical process setting. Switch too early = short shot. Switch too late = flash, overpacking, and high molded-in stress.
Common switchover methods:
- Screw position (95-98% fill): Most common. The machine switches when the screw reaches a set position, verifying cavity is 95-98% full.
- Cavity pressure sensor: Most accurate. A sensor in the cavity triggers the switchover at precise cavity pressure. More expensive but essential for precision molding.
- Hydraulic pressure threshold: The machine switches when hydraulic pressure hits a setpoint. Less accurate — hydraulic pressure varies with oil temperature and viscosity.
Process Capability Index (Cpk) for Pressure Settings
When qualifying a mold, request Cpk data on key part dimensions. A Cpk ≥ 1.33 indicates a capable process (all parts expected within tolerance). If Cpk < 1.0, the molder should investigate pressure-related causes: inconsistent switchover, pressure drift from oil temperature changes, or mold temperature variation.
What This Means for Your Project: You'll never see the pressure settings during production unless you require process documentation. In your purchase order, specify: "Molder shall provide process windows (upper/lower control limits) for injection pressure, hold pressure, back pressure, and clamp tonnage for this mold. All production runs shall maintain settings within these windows." This forces the molder to determine the actual process range during sampling — not just a single setpoint. If you're sourcing parts where mechanical strength or dimensional consistency is critical (automotive, medical, electrical), also specify: "Cpk ≥ 1.33 for all critical dimensions, verified at initial sample and every 12 months thereafter."
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