Last updated: 2026-09-01 · 4 min read

How Does Back Pressure Affect Melt Quality — and What Happens When It Is Wrong?

Back pressure is the resistance applied to the screw during the plasticizing (recovery) phase — it controls how much mixing, shearing, and compression the melt receives before injection. Too little back pressure produces unmelted pellets, air entrapment, and shot-to-shot weight variation. Too much back pressure generates excessive shear heat, resin degradation, and color streaking. The correct back pressure is the minimum value that produces a homogeneous melt with no unmelted particles — typically 5–15 bar for most commodity and engineering resins.

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.

Key Facts

  • Back pressure forces the screw to work harder during recovery — the resin is compressed, sheared, and mixed more thoroughly. This improves melt homogeneity and color dispersion but increases melt temperature through shear heating.
  • Every 5 bar increase in back pressure raises the actual melt temperature by approximately 5–10 °C due to shear heating — even when the barrel heater setpoints are unchanged. This can push the melt outside the recommended processing window.
  • The recovery time increases with higher back pressure because the screw retracts more slowly against the resistance. This can become the cycle-time bottleneck if recovery takes longer than the cooling phase.

    • 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. The related guide covers adjacent topics.

    What Back Pressure Actually Does to the Melt

    During the recovery phase, the screw rotates and conveys resin pellets forward through the barrel. Back pressure is the hydraulic (or electric) resistance applied to the screw’s rearward movement. With higher resistance, the screw rotates against a denser, more compressed melt column — the pellets are subjected to more mechanical work per unit of screw rotation.

    Back Pressure Level Effect on Melt Symptom on Part
    0–3 bar (too low) unmelted pellets, trapped air, poor mixing splay marks, weight variation, color streaks
    5–15 bar (typical) homogeneous melt, good color dispersion consistent weight, uniform color, no splay
    20–30 bar (high) excessive shear heat, thermal degradation burn marks, black specks, yellowing
    30+ bar (excessive) severe degradation, screw-tip wear brittle parts, gas burns, shot-to-shot drift

    When to Adjust Back Pressure

  • If the part shows splay marks (silver streaks radiating from the gate) and the resin is properly dried, the root cause is often trapped air in the melt — insufficient back pressure allows air pockets to remain in the melt column. Increasing back pressure to 10–15 bar compresses and dissolves the air into the melt, eliminating splay.
  • If color streaks appear on a colorant-matched part, the pigment or masterbatch is not fully dispersed. Increasing back pressure improves mixing — but if the required back pressure exceeds 20 bar for good dispersion, the screw speed may be too high or the L/D ratio of the barrel may be inadequate for the colorant.
  • If the melt temperature (measured at the nozzle with a pyrometer) is 15–20 °C above the barrel setpoint, the shear heating from back pressure and screw speed is excessive. Reduce back pressure first, then reduce screw RPM, and re-measure the melt temperature before making any other changes.
  • If the recovery time is longer than the cooling time (making recovery the cycle-time bottleneck), reduce back pressure and increase screw RPM to compensate — the target is recovery time equal to or slightly less than cooling time.

    • Back Pressure and the Check Ring

    The check ring (non-return valve) at the screw tip seals during injection to prevent melt from flowing back over the screw flights. During recovery, the check ring opens to allow melt to flow forward.
    Back pressure affects the check ring in two ways: higher back pressure increases the pressure differential across the ring, which increases wear on the ring seat; and if the check ring does not seal completely during injection, back pressure history matters — a melt column that was compressed at high back pressure has more stored energy and is more likely to leak past a worn check ring during the holding phase.

    Check-ring wear is diagnosed by shot-weight variation. If the part weight varies by more than 0.5 % shot-to-shot with consistent process parameters, the check ring is likely leaking. The fix is to inspect and replace the check ring — not to increase back pressure, which would accelerate the wear further.

    Frequently Asked Questions

    Is the back-pressure setting on the machine the actual back pressure?

    The machine display shows the hydraulic pressure setting. The actual melt pressure depends on the screw diameter and the hydraulic-cylinder area ratio. On most machines, the actual back pressure on the melt is 1.5–3× the displayed hydraulic pressure. Check the machine manual for the conversion factor — a setting of 10 bar hydraulic may correspond to 15–30 bar actual melt back pressure.

    Do glass-filled resins need different back pressure?

    Yes. Glass-filled resins (PA66 GF30, PBT GF30) should use lower back pressure (3–8 bar) to prevent fiber breakage. High back pressure breaks the glass fibers, reducing their reinforcing effect and lowering the mechanical properties of the molded part. The fiber-length distribution degrades visibly above 15 bar back pressure in most glass-filled grades.

    Does back pressure affect melt density?

    Yes. Higher back pressure compresses the melt column, increasing the melt density and the mass of resin per shot volume. This improves shot-to-shot weight consistency but also means that a process change in back pressure changes the effective shot weight — if you reduce back pressure by 10 bar, the shot volume must be increased slightly to maintain the same part weight.

    Key Takeaways

    1. Back pressure is a melt-quality parameter, not a process-fine-tuning parameter. It controls mixing, homogeneity, and air removal from the melt. The correct setting is the minimum value that produces a homogeneous melt — typically 5–15 bar for unfilled resins, 3–8 bar for glass-filled grades.
    2. Every back-pressure increase adds shear heat. The actual melt temperature must be verified at the nozzle after any back-pressure change — relying on barrel-heater setpoints alone misses the shear-heating component.
    3. Shot-to-shot weight variation that cannot be explained by process-parameter drift is often a check-ring problem, not a back-pressure problem. Increasing back pressure on a worn check ring accelerates wear and worsens the variation.

    A process engineer who understands back pressure treats it as a set-once parameter — established during process development at T1, documented in the process sheet, and verified at each production start-up. A process that adjusts back pressure every shift to chase a defect is a process that has an underlying problem (worn check ring, wrong screw design, inadequate drying) that back pressure cannot fix.

    Sources
    Plastics Technology — injection unit parameters
    Beaumont Technologies — process optimization
    RJG — scientific molding principles

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