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PET stretch-blow Molding relies on precise thermal conditioning, axial stretching, and biaxial blowing. When any of these are slightly off, the preform’s gate (often called the “bottom nozzle”) can drift from the geometric center of the bottle base after forming. Below is a practical, engineer-oriented explanation of the mechanisms behind gate shift, how to diagnose the root cause, and what to adjust on the line to restore concentricity and visual quality.
During injection, the preform gate sits on the preform’s axial centerline. After reheating and stretch-blow, the gate should appear centered in the finished bottle’s base push-up. “Shift” is the visible offset of the gate from the base center (or a tilt), usually measured as radial runout in millimeters relative to the bottle’s vertical axis. Excessive shift can trigger base-wobble on conveyors, stack instability, and aesthetic rejects.
Stretch rod not coaxial with the cavity due to bent rods, loose guides, or uneven bushing wear.
Mold half mismatch or worn leader pins creates cavity offset when clamped.
Base insert/push-up plate not centered or with uneven planarity, forcing material to flow more to one side.
Clues: Elliptical gate location, consistent direction of offset across bottles from the same cavity, increased scuffing on one side of the base, or rod witness marks off-center.
IR lamp zoning or reflector contamination causes one side of the preform to be hotter.
Spin axis or gripper rotation not functioning properly during reheating, so the same face is repeatedly exposed.
Neck-support ring (NSR) shading and dust on shields create thermal gradients near the gate end.
Clues: Infrared pyrometer/thermal camera shows a radial temperature delta at the preform base; gate shifts randomize with changes in bottle orientation through the oven.
Pre-blow too early or at too high pressure inflates the base before the rod fully reaches depth, dragging the gate off-center.
Stretch speed too high/low alters material flow symmetry into the base push-up.
Rod depth set shallow or varying due to servo drift or mechanical stop wear.
Clues: Changing pre-blow start by ±10–20 ms or pre-blow pressure by ±0.1–0.2 bar visibly alters gate position; gate tilt correlates with rod depth changes.
Petaloid/champagne base geometry with tight radii can amplify minor misalignments.
Worn venting on one side changes local pressure drop and cooling, biasing material distribution.
Push-up curvature out of spec causes non-uniform wall-thickness and pulls the gate off-center as the base crystallizes.
Clues: Cavity-specific issue; fresh tooling resolves shift; smoke/vent tests show uneven venting paths.
Chiller temperature or flow imbalance between mold halves/inserts.
Dirty channels produce local hot spots; the hotter side shrinks more slowly, “walking” the gate during solidification.
Clues: Thermal imaging of closed mold shows 1–3 °C lateral gradient; gate offset worsens on longer runs until a stop-and-cool reset.
Preform ovality or gate eccentricity from injection leads to baked-in asymmetry.
NSR runout or burrs tilt preform seating, shifting the axis before it ever enters the oven.
Clues: Gate shift tracks specific preform lots; CMM/vision of preforms shows ovality or gate concentricity out of tolerance.
Confirm it’s not a preform issue. Measure preform gate concentricity and ovality; swap in a known-good lot.
Check mechanical centering. Blue-dye the stretch rod tip and base insert, jog the rod, and verify contact is concentric. Inspect leader pins, bushings, tie-bar parallelism.
Thermal uniformity test. Use a handheld IR camera on preforms exiting the last oven zone; verify ≤1.5 °C radial delta near the gate. Clean reflectors, confirm spin.
Process window bump. Change one variable at a time:
Pre-blow start ±15 ms
Pre-blow pressure ±0.15 bar
Stretch speed ±0.1–0.2 m/s
Rod depth ±0.5 mm Note the gate movement direction; this triangulates root cause.
Cooling balance. Check mold water in/out temperature delta and flow rates per cavity; target symmetrical flow and ΔT.
| Symptom | Likely Cause | Corrective Action |
|---|---|---|
| Gate consistently off in same direction, same cavity | Mold half mismatch; base insert miscentered | Recenter/ream dowels; reshim base insert; verify clamp parallelism |
| Gate tilts and varies with shift size run-to-run | Stretch rod depth variance; servo zero drift | Re-zero servo; set mechanical hard stop; verify rod straightness |
| Gate offset grows as run continues | Cooling imbalance; fouled channels | Descale channels; balance flow; verify chiller stability |
| Random gate offset across all cavities | Oven rotation or lamp zoning | Restore preform spin, clean reflectors, recalibrate lamp intensities |
| Gate offset tracks preform lot | Preform ovality/gate eccentricity | Tighten preform QC, change lot/mold, adjust oven profile to compensate |
Preform runout at gate: ≤0.15 mm for small bottles (≤600 mL), ≤0.25 mm for larger formats.
Bottle gate concentricity: typically ≤0.5–1.0 mm depending on base design and customer spec.
Stretch-rod concentricity to cavity: ≤0.05 mm at full stroke.
Radial temperature delta (preform base at oven exit): ≤1.5 °C.
Mold water ΔT across halves: ≤0.5–1.0 °C; flow variation ≤5%.
Delay pre-blow start by 10–20 ms so the rod finishes its stroke before base inflation.
Reduce pre-blow pressure slightly to let the rod dominate bottom formation.
Tune stretch speed to stabilize material draw into the push-up (avoid abrupt accelerations).
Normalize oven zoning: even lamp outputs, clean reflectors, verify rotation; slightly increase heat at the gate end only if thickness thins on the “cold” side.
Re-shim and pin the base insert; polish the push-up profile and restore venting.
Balance cooling by matching flow meters and equalizing hose lengths per half/insert.
Weekly: Clean oven reflectors and lamp shields; verify preform rotation; check rod tip wear.
Monthly: Inspect leader pins/bushings; confirm mold half parallelism; gauge rod straightness.
Quarterly: Descale cooling circuits; flow-balance each circuit; verify servo encoder zero and rod depth.
Per lot: Audit preform ovality and gate concentricity; keep SPC on gate offset at the bottle QA station.
Vision system at the base station to measure gate offset and trend by cavity.
Simple jig with concentric rings for quick visual classification on the line.
CMM or optical comparator for periodic golden-sample checks.
SPC charts correlated with process tags (pre-blow timing, rod depth, cavity ID) to detect drift early.
Modern machines with servo-controlled stretch rods, rigid platens, precise mold centering, and closed-loop oven control make it much easier to hold base concentricity within tight limits. If you are upgrading or benchmarking, consider blowers with fine-step pre-blow timing, per-cavity rod depth offsets, and cavity-level air/cooling balancing.
Looking for equipment and technical support to stabilize base concentricity? BOHANG offers stretch-blow systems and application engineering focused on repeatable centering, accurate rod kinematics, and balanced heating/cooling. Learn more on the BOHANG official site.
Verify rod depth and coaxiality; blue-dye contact looks centered.
Balance oven zones; confirm preform rotation and clean optics.
Set pre-blow start after rod entry; trim pressure to where base forms under the rod.
Equalize cooling flows and temperatures; purge air from channels.
Run cavity-by-cavity samples; log gate offset and iterate one parameter at a time.
With disciplined alignment, thermal balance, and kinematic control, bottom nozzle shift becomes rare—and your PET base aesthetics, stack stability, and downstream handling all improve.
