Why Do ISBM Bottles Have Uneven Wall Thickness?

Ce que cet article doit prouver

Diagnose uneven wall thickness by separating preform variation, thermal imbalance, stretch-rod timing, air timing, orientation, mold cooling and cavity-specific tooling problems. A defensible baseline begins with Cut bottles at repeatable heights and measure several angles to identify exactly where material is thin and where the missing mass accumulated. The first verification method is Record cavity number and orientation on every sample. From there, the article follows only checks that can materially change the answer promised by the title. Where an exact operating value depends on the resin grade, bottle drawing, mold, or delivered machine, the approved project specification controls the final setting.

Map the Thin and Thick Regions Before Adjusting Anything

Symptom map

Symptom map. Cut bottles at repeatable heights and measure several angles to identify exactly where material is thin and where the missing mass accumulated. Record cavity number and orientation on every sample. The diagnostic test should show whether symptom map can physically create the observed defect before a repair or parameter change is accepted. Record the bottle response beside the setting or measurement for symptom map; that record becomes the starting condition when symptom map is reviewed. Calling a bottle uneven without a map encourages random parameter changes.

Check Whether the Problem Follows a Mold Cavity

Cavity pattern

Cavity pattern. Determine whether all cavities show the same thin area or only one cavity does. Compare wall maps, bottle weights and preforms by cavity. The diagnostic test should show whether cavity pattern can physically create the observed defect before a repair or parameter change is accepted. If the symptom or performance target does not move as predicted, return cavity pattern to the baseline and investigate cavity pattern rather than stacking corrections. A single-cavity defect points toward local cooling, alignment, hot-runner or tooling issues rather than a global recipe.

Inspect Preform Mass and Geometry

Preform mass

Preform mass. Weigh preforms and inspect length, concentricity, gate and wall profile. Check injection balance before touching blow settings. The diagnostic test should show whether preform mass can physically create the observed defect before a repair or parameter change is accepted. If a change improves one region but worsens another, compare the material or energy movement between preform mass and preform mass instead of accepting the first visual improvement. A lighter or eccentric preform cannot produce the same bottle distribution as the other cavities.

Axial temperature

Axial temperature. Compare temperature along the preform body using fixed measurement locations. Relate warmer and cooler bands to thick and thin bottle sections. The diagnostic test should show whether axial temperature can physically create the observed defect before a repair or parameter change is accepted. Where the outcome depends on material grade or tooling geometry, confirm the approved project limit and then use axial temperature as the next cross-check. A thermal band that is too hot can over-stretch and become locally thin.

Conditions de travail pour cette tâche spécifique

Symptom map

Cut bottles at repeatable heights and measure several angles to identify exactly where material is thin and where the missing mass accumulated.

Cavity pattern

Determine whether all cavities show the same thin area or only one cavity does.

Preform mass

Weigh preforms and inspect length, concentricity, gate and wall profile.

Axial temperature

Compare temperature along the preform body using fixed measurement locations.

Verify the Axial and Circumferential Temperature Profile

Circumferential temperature

Circumferential temperature. For oval or asymmetric bottles, verify the hot/cold pattern and preform angular orientation. Mark orientation through the transfer path. The diagnostic test should show whether circumferential temperature can physically create the observed defect before a repair or parameter change is accepted. When the project is near a machine or material limit, require a molding trial that isolates circumferential temperature and then challenges circumferential temperature under the same bottle specification. A rotated preform moves preferentially conditioned material to the wrong bottle face.

Check Stretch Rod Position, Speed and Timing

Stretch rod

Stretch rod. Check rod length, tip position, alignment, start time and velocity profile. Compare rod motion or mechanical position across cavities where applicable. The diagnostic test should show whether stretch rod can physically create the observed defect before a repair or parameter change is accepted. The safest interpretation comes from comparing at least several stable cycles and then verifying stretch rod without changing the rest of the recipe. An off-center or late rod can push the bubble sideways and starve the base.

For a compatibility check, the ASB-12 blow mold compatibility shows the kinds of mechanical and thermal interfaces that should be verified before a tooling or process change is released to production.

Check Pre-Blow and Final Blow Timing

Pre-blow timing

Pre-blow timing. Observe whether radial expansion begins before adequate axial draw. Adjust timing in small increments while holding the thermal profile constant. The diagnostic test should show whether pre-blow timing can physically create the observed defect before a repair or parameter change is accepted. A useful production trial keeps the resin lot and cavity identification fixed while pre-blow timing is changed, followed by a separate check of pre-blow timing. Early expansion can freeze material in the upper bottle; excessive delay can cause folds or rod contact.

Blow circuit

Blow circuit. Verify consistent air delivery and valve behavior by cavity. Check pressure at the machine and inspect seals or valves when one cavity behaves differently. The diagnostic test should show whether blow circuit can physically create the observed defect before a repair or parameter change is accepted. The practical value of this check is that it turns blow circuit from a vague setting into evidence that can be compared with blow circuit. A supply setpoint does not prove each cavity receives the same pressure rise.

Check Mold Cooling, Venting and Orientation

Mold cooling

Mold cooling. Compare flow and temperature return from relevant blow-mold circuits. Inspect for blockage or scale where one wall consistently freezes differently. The diagnostic test should show whether mold cooling can physically create the observed defect before a repair or parameter change is accepted. Use this result to narrow the process window, not to create a new universal setpoint; the next constraint to confirm is mold cooling. Uneven cooling can change material contact and post-mold shrinkage.

Make One Corrective Change and Re-Map the Bottle

Correction sequence

Correction sequence. Fix the highest-confidence cause, stabilize the machine, and repeat the same thickness map. Keep a before/after record and do not change multiple variables at once. The diagnostic test should show whether correction sequence can physically create the observed defect before a repair or parameter change is accepted. The expected response should be visible in a bottle measurement, defect map, or machine trend before the team proceeds to correction sequence. Stacked adjustments can create a temporarily good bottle with no reproducible process knowledge.

Questions that arise specifically in Why Do ISBM Bottles Have Uneven Wall Thickness?

Conclusion pratique

For Why Do ISBM Bottles Have Uneven Wall Thickness?, begin by documenting symptom map, then test stretch rod without moving unrelated settings, and release the process only after correction sequence is verified on every active cavity. Stacked adjustments can create a temporarily good bottle with no reproducible process knowledge.