How to Reduce Scrap Rate on an ISBM Machine: Root Cause Analysis Guide
Scrap rate is one of the most telling indicators of ISBM process health. A well-optimised ISBM machine running a stable process on a validated product should produce scrap rates of 1–3% in steady-state production — primarily startup and shutdown losses. Scrap rates above 5% signal a process problem that is costing real money: every rejected bottle represents a complete cycle of material, energy, and machine time invested with no return.
The challenge with ISBM scrap reduction is that the defects can originate from any of the machine’s four stations, from the tooling, from the material, or from process parameter drift — and diagnosing which root cause is driving a particular defect requires systematic investigation rather than trial-and-error parameter adjustment. This guide provides a structured root cause analysis methodology for the most common ISBM scrap defects, enabling production teams to identify and fix the actual cause rather than just suppressing the symptom.
The Root Cause Analysis Framework for ISBM Scrap
Before investigating individual defect types, establish a consistent root cause analysis framework. This prevents the common error of jumping to process parameter changes without confirming the defect source:
Define the Defect Precisely
What does it look like? Where on the bottle? Which cavity? What fraction of output? Started suddenly or gradually? Worse at startup or steady state? Document with photos.
Identify When It Started
What changed? New material lot? Parameter change? Maintenance action? Mold change? If something changed when the defect started, it is the primary suspect.
Isolate to One Station
Is the defect in the preform (injection station fault) or only in the blown bottle (blow station fault)? Pull a preform sample from the conditioning station and inspect it before it is blown.
Change One Variable at a Time
Once a suspected cause is identified, change one parameter, run 10+ cycles, then assess the effect before making another change. Multi-variable changes make it impossible to determine which variable caused any observed improvement.
Defect 1: Short Shots (Incomplete Preform Fill)
| Possible Cause | Test / Evidence | Fix |
|---|---|---|
| Gate frozen (nozzle tip too cold) | Short shots on specific cavity; opaque gate spot on partial preform | Raise nozzle tip temperature 3°C |
| Insufficient injection pressure or speed | Short shots on all cavities; preform weight low | Increase injection speed; check injection pressure setting and hydraulic pressure |
| Insufficient shot size (screw over-retracted) | All cavities short; check screw retraction position vs target | Increase metering stroke; check material feed from hopper |
| Melt temperature too low | All cavities short; high injection pressure reading; preform has flow marks | Raise barrel temperature by 5°C; check all barrel heater zones are working |
Defect 2: Uneven Wall Thickness (Bottle)
| Possible Cause | Test / Evidence | Fix |
|---|---|---|
| Non-uniform preform conditioning temperature | Wall variation follows same circumferential pattern consistently; IR scan of preform shows temperature asymmetry | Adjust conditioning zone temperatures; check heater element output uniformity |
| Stretch rod not centred in preform | Wall variation has consistent thick/thin side relationship; preform gate shows off-centre mark from rod | Check and re-align stretch rod to preform axis; inspect rod for bending |
| Preform wall thickness non-uniform (injection) | Wall variation is present in preform cross-section before blowing; mold cavity or core eccentricity | Measure preform wall thickness at multiple points; mold re-centering or core replacement needed |
| Insufficient pre-blow pressure for preform size | Heavy base on bottle; thin shoulder; rod marks at gate | Increase pre-blow pressure in 1-bar steps; adjust pre-blow start timing |
Defect 3: Pearlescence / Haze in PP Bottles
Pearlescence — the milky, pearl-like opaque appearance in areas of PP bottles — is one of the most common and commercially damaging defects in PP ISBM production. It indicates uncontrolled cold crystallisation of the PP during blowing. The root causes are:
The preform enters the blow station below the PP process window (below ~125°C). PP crystallises rapidly below this temperature and the oriented crystallites appear as pearlescence. Fix: raise conditioning temperature by 3°C increments until pearlescence clears. PP window is typically 125–140°C.
PP homopolymer or nucleated grades are inherently prone to pearlescence regardless of temperature — they crystallise too rapidly. Only random copolymer PP (rPP) high-clarity grades are suitable for ISBM. If pearlescence cannot be eliminated despite correct conditioning, suspect a wrong grade has been loaded.
If the preform spends too long in the blow mold before air pressure contacts it, it cools below the PP orientation window before full stretch is achieved. Fix: reduce any delay between stretch rod full stroke and main blow initiation; verify pre-blow valve response time is within specification.
Defect 4: Top-Load Failure in Service
Top-load crush strength below specification is a hidden defect — the bottle looks visually acceptable but fails under pallet stacking loads in distribution. Root causes and investigation steps:
- Under-orientation (BSR too low): Calculate actual BSR from preform and bottle dimensions; if below target, preform design change is required to increase stretch ratio
- Conditioning temperature too high: Over-heated preform blows with less orientation (chains have too much thermal mobility to lock in oriented configuration); reduce conditioning temperature by 2°C and re-test top-load
- Heavy shoulder wall (material not distributed to sidewall): Cut cross-sections through 3 bottles to measure shoulder vs sidewall thickness; if shoulder is heavy, adjust conditioning zone profile to heat shoulder area more aggressively
- Incorrect material grade: Low-IV PET or wrong-grade PP produces weaker walls at the same wall thickness; confirm material grade against specification
Building a Scrap Reduction Action Plan
After using root cause analysis to identify the primary defect driver, a structured action plan ensures the fix is durable rather than temporary:
| Action | Purpose |
|---|---|
| Document confirmed root cause and corrective action taken | Creates institutional knowledge; prevents recurrence from same cause |
| Update the Master Process Record with new confirmed parameters | Ensures future operators start from confirmed-optimal parameters |
| Add defect-specific check to in-process inspection frequency for 2 weeks post-fix | Confirms that the corrective action is durable under production conditions |
| If root cause was a worn component, add replacement to PM schedule | Converts reactive repair to preventive maintenance for future cycles |
| Track scrap rate weekly for 4 weeks post-fix to confirm sustained improvement | Confirms that the OEE Quality rate has measurably improved |
For support with persistent scrap problems or complex multi-defect situations on your ISBMマシン, our process engineers offer remote troubleshooting consultations and on-site process audits. Contact our technical support team to arrange a scrap reduction consultation.
Frequently Asked Questions
What is a realistic target scrap rate for a well-run one-step ISBM machine?
My scrap rate is only high at the start of each shift. What should I check?
How do I distinguish a process fault from a tooling fault when investigating a scrap defect?
High Scrap Rate on Your ISBM Machine? Get Expert Help
Describe your defect, its frequency, when it started, and what has already been tried — our process engineers can guide you to the root cause faster than trial-and-error parameter changes.
