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Troubleshooting Guide
PET Blow moulding
Blown finish
Cause Solution
Overheating of the finish area which allows blowpressure air to stretch the material
Move shield away from finish into a thicker part of thepreforms taper.
Oven ambient temperature to high Bring shield close to preform.
Incorrect loading height, too much of the finish isexposed to the elements
Increase oven exhaust.Reduce neck element heat.
Sealing surface damage
Cause Solution
Misalignment of the preform, blow mould and blownozzle usually caused by poor transfer arm positioning
Realign transfer arm.
Blow mold position Realign blow mold or nozzle if necessary.
Nozzle position
Chocked neck
Cause Solution
Over stretching of the preform by the stretch rod priorto blow
Reduce air delay time.
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Low blow delayed or missing Reduce stretch rod pressure.Move shield into a thicker part of the taper.Increase heat in the body or base.Increase low blow pressure.Reduce heat in the neck.
Bent neck
Cause Solution
Misalignment of the preform to the mould or nose Realignment of the preform to the nose or mould.
Distortion below the flange Reduce material thickness in the shoulder by reducingheat in the body.
Insufficient mould cooling Increase high blow time.
One mould half with poor cooling Improve mould cooling.
Excessive material remained in the neck and shoulder
Hard neck
Cause Solution
Initial stretching occurs too low into the preformstaper
Reduce heat in the body and/ or base untilpearlescence occurs then increase neck temperature.
Preform location incorrect through oven Increase stretch rod pressure.
Air leak into preform before the start of low blow Increase air delay time.Reduce low blow pressure.
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Move shield into the thinner part of the taper.Check height of preform though oven.Check for air leak through nose.
Thin shoulder
Cause Solution
Excessive stretching of the taper. Stations producing asmaller low blow container will be first with the fault orthe thinnest.
Move shield into a thicker part of the preforms taper.
Poor shield positioning, allowing excessive stretchingof the thin part of the taper
Reduce air delay time.Reduce stretch rod pressure.Increase low blow pressure.Reduce heat in the neck. If thickening occurs belowdesired area Increase heat below the neck element.
Hot bottles
Cause Solution
Preform temperature above crystal growth range for tolong
Reduce heat adjacent to the haze until pearlescence isevident somewhere on the bottle.
Insufficient cooling of the outside surface for theamount of heat absorbed by the P.E.T.
Reduce heat over all until pearlescence is evident.
Excessive equilibration time o Preform wall thicknessexcessive blowers
Increase air circulation through oven, increase blowerspeed, clean blowers.
Excessive fill point drop
Cause Solution
Low levels of orientation results in the material nothaving enough strength to resist the pressure applied
Reduce over all perform temperature allowing thebottle to be blown closer to its natural stretch limit.
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Under this condition slight pearlescence may beevident.
Pearlescence
Cause Solution
Pearlescence results from stretching of moleculesfaster than they can respond past it's natural stretchlimit. Small tears appear on the material's surface.
Material to thin - Increase heat other than where pearlis evident.Where the pearlescent area may be allowed to stretchfurther increase heat at pearl.Increase overall preform temperature.Pearlescence opposite a swung gate (see"Swunggate").Pearlescence on the bottle shoulder may be the resultof a blow air leak through nose from either the lowvalve, high blow valve or stretch rod.Reduce low blow volume, increase low blow time andreduce low blow pressure.Ensure low blow bottle is not to large. Excessive petalformation in low blow will cause pearl in the petalarea.Pearlescence in the body in the form of a ring is theresult of insufficient low blow.
Flat sides
Cause Solution
Poor venting due to excessive parting of the blowmoulds allowing air to be trapped along the mouldseams
Ensure excessive mould parting does not occur.
Low blow bottle too small. In this case the volume ofair trying to escape during high blow is more than theventing can allow
Increase low blow pressure ensuring bottle diameter isclose to blow mould size at the end of low blow tominimize air that remains in the blow mould beforehigh blow starts.
Low blow bottle blown to fast Reduce low blow pressure and extend low blow time ifthe body of the bottle has reached the mould.
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Excessive high blow pressure Reduce high blow pressure.
Deformation at mold parting lines
Cause Solution
Pressure remaining in the bottle when blow mouldsopen
Inadequate exhaust time, check exhaust valveactivation.
Insufficient high blow cooling time Increase high blow time or reduce material thickness.
Material too hot and/or too thick Reduce material thickness.Reduce preform temperature.
Chocked body
Cause Solution
Insufficient or no low blow Increase low blow pressure.
Insufficient heat beside choke Increase heat in the body.Reduce heat in the base and/ or neck.Reduce air delay time.Reduce stretch rod pressure.
Heavy base weight
Cause Solution
Incorrect heating profile resulting in a poor distributionof material, over stretching of the body or shoulder
Reduce heat in the body and/ or shoulder untilpearlescence occurs then increase heat at the base.Repeat until the base weight is correct.
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Light base weight
Cause Solution
Poor heating profile resulting in poor distribution ofmaterial, insufficient stretching of the body orshoulder.
Reduce heat near the base of the preform untilpearlescence occurs then increase heat to the shoulderand/or body. Repeat until base weight is correct.Increase air delay time.Increase stretch rod pressure.Reduce low blow pressure.Reduce oven shielding of the taper.
Feet not fully formed
Cause Solution
Insufficient high blow pressure. Rate of high blow tooslow, material movement stalls before reaching thecorners. Often as a result of blow nozzle leak.
Check high pressure. If possible. If station relatedcheck for high blow leaks from nozzle or stretch rod oIncrease high blow time.
Insufficient high blow time. If adding high blow timedoes not fix the problem look for another cause.
Reduce base weight by reducing preform temperatureuntil pearlescence is evident then increase prefromslower body temperature.
Heavy base weight. High blow pressure inadequate tomove material into the corners.
Check low blow bottle size. Reduce if necessary.
Excessive low blow pressure or time. If too much ofthe material will be left to form the feet correctly.Corners are likely to be thin.
Fix swung gate.
Swung gate. Feet will not form in the corners oppositethe gate movement unless extra heat is applied nearthe end cap.
Hot sides
Cause Solution
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Inconsistent preform rotation through the oven Check the preform for a drag marl. Ensure ovenshields are not touching the preform. Check collect,spindle or mandrel rotation.
Insufficient surface cooling of the preform Increase air flow onto the surface of the preform.
Low head load
Cause Solution
Insufficient orientation and/or material thickness todeliver the required physical strength
Reduce over all preform temperature.Reduce heat beside failure point.Where failures occur between neck and shoulderadjust shield into a thicker part of the taper.
Low burst pressure (body)
Cause Solution
Insufficient orientation (too hot) Reduce preform temperature.
Material to thin Reduce body heat.
Confirmation
Low IV
Low burst pressure (base)
Cause Solution
Base of bottle blown too cold producing excessivestress
Increase heat below the preform.
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Stretch rod clearance too short Reset stretch rod clearance.
Excessive crystallinity at the gate Reduce stretch rod pressure.
Low IV
Excessive stress in preform
Poor clearance
Cause Solution
Preform temperature too high Base weight correct or light, reduce heat besideand/or below the gate.
Heavy base weight Base weight to heavy, reduce preform temperature.
End cap temperature too high Increase high blow time.
Aged preforms
Insufficient mould cooling
Excessive inherent preform stress
Fold in base at end cap line
Cause Solution
Preform temperature to hot Reduce heat in the areas other than the base. If thefold remains after pearlescence has formed. Heat may
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be applied to the preform end cap area.
Preform end cap area to cold Increase low blow pressure.
An excess of material in the vicinity of the fold Reduce air delay time.
Lack or loss of low blow volume Reduce stretch rod pressure.
Excessive force applied by the stretch rod
Aged or stressed preforms
Fold around the stretch rod
Cause Solution
Over heating of the preform end cap, material wrapsaround the end of the stretch rod. In effect anotherthick/ thin transition forms.
Reduce heat or below the end cap area.Increase low blow pressure.Reduce stretch rod pressure.
Cracked base
Cause Solution
Preform end cap too cold Increase heat beside or below the gate.
Excessive stretch rod pressure Reduce stretch rod pressure.
Clearance between the stretch rod and mould base toosmall
Check preform for excessive crystallinity, if so changepreforms.
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Thick crystallinity above the gate Increase stretch rod, mould base gap.
Swung gate
Cause Solution
First impacted mark is evident and is not centered tothe gate, an alignment problem is usually the cause.Check for misalignment. Ensure transfer arm locatespreform in mould correctly.
Misalignment of the preform to the mould, nozzle orstretch rod.
Check blow mould cooling Inadequate mould cooling, one half only If the gate iscentered to the stretch rod impact mark and yet notcentered to the mould base at the end of blow thestretch rod has lost control during blow.
If the first impact mark is centered, the stretch rodhas lost control of the gate during blow. Often gatecontrol is lost due to excessive growth in length duringlow blow. Ideally growth should start high in thepreform reaching full diameter before full length iflength is achieved.
Lower half of preform too hot.
before diameter the rod must travel further thannormal to keep up.
Low blow pressure too high.
Reduce low bottle size Air delay time too short.
Increase air delay time Stretch rod length too short.
Reset stretch rod height Excessive stretch rod cushioning.Worn stretch rod guides.Excessive preform eccentricity.Thick crystallinity near gate.
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Stress cracking
Cause Solution
Off center gates. Webs with the thinnest material willstress crack early.
Fix swung gate.
Excessive post mould change. As clearancedeteriorates web shape changes increasing the risk ofstress cracking.
Reduce preform temperature to pearlescence, reducebase weight if necessary. Increase high blow time.
Excessive growth at the base cust line. Temperature ofthe lower half of the preform's body is to high.
Reduce heat in the preform's lower body.
Chemical reaction by some line lubricants Line lubricants generally cause severe stress crackingin all feet.
Low I.V. material Increase base weight.
Light base weight. Inadequate thickness to resistmovement.
Explosions
Cause Solution
Hot bottles (see "Hot bottles")
Folds in the base
Light base weight
Excessive post mold growth
Low I.V.
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Chemical Reaction (see "Stress cracking")
Contamination
Air bubbles above the gate
Stress Cracking
Cracked bases
Excessive crystallinity
Drop test failures
Cause Solution
Preform end cap blown too cold Increasing heat at the gate or below without loosingbase weight usually improves drop test failure.
Light base weight Increase base weight.
Excessive crystallinity above gate
Large air bubbles above the gate
