pet troubleshooting
DESCRIPTION
PolyethyleneTerephthalate (PET)• Processing Guidelines• Troubleshooting GuideInjection MouldingStretch Blow Moulding• Recommended Tests& EquipmentTRANSCRIPT
Polyethylene
Terephthalate (PET)
• Processing Guidelines • Troubleshooting Guide Injection Moulding Stretch Blow Moulding • Recommended Tests & Equipment
INDEX
PROCESSING GUIDELINES Drying
Drying Conditions
Dryer Startup Dryer Specification Injection Moulding
Resin Inlet Temperature Barrel Temperature
Nozzle Temperature
Manifold Hot Runner Temperature Injection Speed
Stretch Blow Moulding
TROUBLESHOOTING GUIDE Injection Moulding
1) Short Shots 2) Sink Marks 3) Flash 4) Water Marks 5) Black Specs / Contamination 6) Flow Lines 7) Heat Splay 8) PET Inclusion 9) Concentricity 10) Crystalline Gate 11) Long Gate 12) Air Bubbles 13) Stress Pattern 14) Stringing 15) Cloudy Preforms
16) Hollow Gates 17) Discolored Preforms 18) Line Over Finish 19) Burnt Gates 20) Oval Finish 21) Gas Burns 22) High Acetaldehyde
Stretch Blow Moulding 1) Blown Finish 2) Sealing Surface Damage 3) Chocked Neck 4) Bent Neck 5) Hard Neck 6) Thin Shoulder 7) Hot Bottles 8) Excessive fill point drop 9) Pearlescence 10) Flat Sides 11) Deformation at mold parting lin 12) Chocked Body 13) Heavy Base Weight 14) Light Base Weight 15) Feet not fully formed 16) Hot Sides 17) Low Head Load 18) Low Burst Pressure (Body) 19) Low Burst Pressure (Base) 20) Poor Clearance 21) Fold in End Cap Line 22) Fold Around Stretch Rod 23) Cracked Base 24) Swung Gate 25) Stress Cracking
26) Explosion 27) Drop Test Failures
RECOMMENDED PET TESTS & EQUIPMENT FOR DOWNSTREAM CONVERTERS
Intrinsic Viscosity Acetaldehyde Analysis Colour Measurement
Moisture Analysis Headload Testing
Burst Testing
PROCESSING GUIDELINES
THE processing guidelines set out below cover the two major processes involved
in the conversion of PET resin into preforms and containers, namely:
1. INJECTION MOLDING
2. STRETCH BLOW MOLDING.
The resin preparation conditions and barrel temperature profile recommended on
the injection
molder could also be used on extrusion barrel, in converting PET resin into
amorphous sheet.
DRYING : Drying Conditions:
This is the first and most crucial step in converting PET resin into preform/
container/sheet. If the drying equipment is not adequate or is not functioning
efficiently then it is highly unlikely that you will produce an acceptable product.
The dried resin should have a moisture content not more than 50 PPM.
Recommended Drying Temperature: 160 - 180 °C Recommended Drying Time 5
- 6 hours.
Dryer Start-Up :
The time / temperature profile recommended when first starting the dryer or
starting from a major shut down is as below.
The first hour at 120 °C
The next two hours 150 °C
The next two hours 180 °C
Dryer Specification:
Able to heat air upto 200 °C
Able to deliver air flow rate in the order of 0.062 metre cube/ min per kg/ hr of
resin being processed.
Able to deliver hot dehumidified air with a minimum of -40.0 °C dew point.
Dryer Resin Level :
To give approximately 5 to 6
times the kg/hr capacity of the
injection molder or extruder in
case of sheet extrusion.
INJECTION MOLDING PROCESS:
This is the process by which the resin is converted from dried pellets to preforms.
Resin Inlet Temperature:
It is better to have a temperature of 160 -170 °C on the resin entering the feed
zone of the ex-truder.
Barrel Temperature Profile:
For other than water grade resin it is recommended to use a temperature of 265
°C in the feed zone gradually increasing towards the metering zone 280 °C. For
the water grade resin however, it is recommended to use a reverse temperature
profile, with highest temperature in the feed zone, say 280 °C and gradually
lowering towards the metering zone.
Recommended temperature :
265 - 280 °C
Nozzle Temperature :
Recommended temperature:
270 - 280 °C
Manifold Hot Runner Temperature :
Recommended temperature :
270 - 280 °C
Mold Cooling Water Inlet Temperature :
Recommended temperature :
7 to 8 °C
Injection Speed :
As required to fill the cavity with minimum shear and dependent on the gate size.
STRETCH BLOW MOLDING PROCESS:
The most commonly available stretch blow molding machines are the:
Single stage stretch blow molder ( preform & bottle made on the same machine).
They are AOKI, NISSEI, UNILOY, etc.
Two stage stretch blow molder (reheating and blowing of preforms made on
Injection molder).
They are SIDEL, KRUPP, SIPA, etc.
The processing conditions depend on the type of process used and the most
common parameters
to be controlled are:
o Preform Temperature.
o Re-heating oven profile and over all oven percentage.
o Oven sheilds o Bottle blowing speed, bottles/hour.
o Stretch rod speed o Air delay time o Pre-blow pressure.
o High blow time.
o High blow pressure.
o Blow mold cooling.
Optimizing of the above parameters depend on several factors such as :
o Preform design.
o Preform intrinsic viscosity.
o Preform infrared absorbing characteristics.
o Bottle design.
Troubleshooting Guide
Injection Moulding
1 Problem: SHORT SHOTS
POSSIBLE CAUSES
o Injection Pressure low
o Barrel temperature low
o Mould manifold temp. low
o Mould cavity temp. low
POSSIBLE SOLUTIONS
o Increase injection pressure
o Increase barrel temperature
o Increase mould manifold temperature
o Increase mould cavity temperature
o Injection speed low
o Back pressure low
o Increase injection speed
o Increase back pressure
o Clean vent on split, core and cavity
o Check that no foreign object is in gate
area.
o Check that the gate pin is moving all the
way back
2 Problem: SINK MARKS
POSSIBLE CAUSES
o Holding pressure low
o Holding time low
o High mold temperature
o Inadequate coolant supply
o Cooling time low
o Blocked venting
POSSIBLE SOLUTIONS
o Increase holding pressure
o Increase holding time
o Decrease mould temperature
o Check cooling water
o Increase cooling time
o Clean vents on split, core and cavity
3 Problem: FLASH
POSSIBLE CAUSES
o Injection pressure high
o High injection speed
o Unwanted material in cores
o Unwanted material in splits
o Unwanted material in cavity
o High mold temperature
o High barrel temperature
o Clamping pressure low
POSSIBLE SOLUTIONS
o Decrease Injection pressure
o Decrease injection speed
o Clean cores
o Clean splits
o Clean cavity
o Decrease mould temperature
o Decrease barrel temperature
o Increase clamp pressure
4 Problem: WATER MARKS
POSSIBLE CAUSES
o Leaking hoses
o Leaking mould
o Insufficient dehumidification
o Mold enclosure inadequate
POSSIBLE SOLUTIONS
o Check hoses on mould for damage
o Check mould for leaks
o Check that dehumidifier is working
properly
o Check that the machine enclosures
has no major gaps
5 Problem: BLACK SPECS / CONTAMINATION
POSSIBLE CAUSES
o Contaminated resin
o High residence time prior to
restart of the machine
o Degradation of resin
POSSIBLE SOLUTIONS
o Check for resin cleanliness
o Clean the resin hopper
o Purge the barrel thoroughly on restart
o Reduce melt temperature
o Reduce back pressure
6 Problem: FLOW LINES
POSSIBLE CAUSES
o Injection pressure low
o Mould temperature low
o Barrel temperature low
o Injection speed low
o Drier inefficient
o Back pressure low
POSSIBLE SOLUTIONS
o Increase injection pressure
o Increase mould temperature
o Increase barrel temperature
o Increase injection speed
o Check resin drier
o Increase back pressure
7 Problem: HEAT SPLAY
POSSIBLE CAUSES
o Manifold and mold heater
Malfunction.
o Barrel temperature high
o Foreign matter in the gate
POSSIBLE SOLUTIONS
o Check temperature of mould cavity heaters
and Manifold
o Check barrel temperature
o Check that no foreign material is in gate area
8 Problem: PET INCLUSIONS
POSSIBLE CAUSES
o Low barrel temperature
o Low mould temperature
o Back pressure low
o Mixing head damage
o Check valve damage
POSSIBLE SOLUTIONS
o Check barrel temperature
o Check mould temperature
o Increase back pressure
o Check that the mixing head of the
screw or the screw is not broken.
of Inspect the check valve on the
screw (not Husky Machine)
9 Problem: CONCENTRICITY
POSSIBLE CAUSES
o Injection speed high
o Injection core bent
POSSIBLE SOLUTIONS
o Decrease injection speed
o Check that the injection core is not bent
and straighten it.
10 Problem: CRYSTALLINE GATE
POSSIBLE CAUSES
o Cavity heater malfunction
o Cavity temperature low
o Manifold temperature low
o Drier malfunction
o Hold pressure high
o Foreign material in gate
o Cooling insufficient
o Core cooling inadequate
o Fountain in core rod damage
POSSIBLE SOLUTIONS
o Check that the cavity heater is working
o Increase the cavity temperature
o Increase the manifold temperature
o Check that the drier is working
o Decrease injection hold pressure
o Check that no foreign object
obstructs the cavity gate
o Increase cooling time
o Check water flow through the injection
core and cavity
o Check that the fountain in the injection
core is not bent
11 Problem: LONG GATE
POSSIBLE CAUSES
o Cavity heater malfunction
o Cavity heater low
o Gate valve malfunction
o Shut-off nozzle malfunction
o Short decompression time
POSSIBLE SOLUTIONS
o Check cavity heater
o Increase cavity temperature
o Check that the gate valve is working
o Check that the barrel shut-off nozzle is
working
o Increase decompression time
12 Problem: AIR BUBBLES
POSSIBLE CAUSES
o Barrel temperature low
o Inefficient drying
o Low back pressure
o Barrel temperature
o Mixing head damage
POSSIBLE SOLUTIONS
o Check barrel temperature
o Check drier temperature
o Increase back pressure
o Lower temperature at last extruder zone
o Check extruder mixing head
13 Problem: STRESS PATTERN
POSSIBLE CAUSES
o High injection pressure
o High shot size
o Long hold time
POSSIBLE SOLUTIONS
o Decrease injection pressure
o Reduce shot size
o Decrease injection hold time
14 Problem: STRINGING
POSSIBLE CAUSES
o High cavity temperature
o Low cooling time
o Low hold time
o Inadequate coolant to cavity
o Gate valve malfunction
POSSIBLE SOLUTIONS
o Reduce cavity temperature
o Increase cooling time
o Increase injection hold time
o Check water flow to cavities
o Check that gate valves are working
15 Problem: CLOUDY PREFORMS
POSSIBLE CAUSES
o Inefficient Drying
o Blocked drier filters
o Air leaks on the drier
o Air leaks on machine hoppers
o Low air flow to drying hopper
o Low process air temperature
o Barrel temperature low
o Insufficient drying
o Back pressure low
POSSIBLE SOLUTIONS
o Check drier temperature and dew point
o Clean drier air filters
o Check for air leaks on drier
o Check for air leaks on machine hopper
o Check air flow to hopper
o Increase drier process temperature
o Increase barrel temperature
o Shut machine down for 1 hour and dry
material
o Increase back pressure
16Problem: HOLLOW GATES
POSSIBLE CAUSES
o Packing pressure low
o Hold time low
o Insufficient cooling time
o High cavity temperature
o Foreign material in gate
POSSIBLE SOLUTIONS
o Increase packing pressure
o Increase holding time
o Increase cooling time
o Lower cavity temperature
o Check for foreign material in gate area.
17 Problem: DISCOLORED PREFORMS
POSSIBLE CAUSES
o High barrel temperature
o High mold temperature
o High drying temperature
POSSIBLE SOLUTIONS
o Lower barrel temperature
o Lower injection mould temperature
o Lower drier process temperature
18 Problem: LINE OVER FINISH
POSSIBLE CAUSES
o Contaminated splits
o Low packing pressure
o Insufficient packing time
POSSIBLE SOLUTIONS
o Clean splits
o Increase packing pressure
o Increase packing time
19 Problem: BURNT GATES
POSSIBLE CAUSES
o High cavity temperature
POSSIBLE SOLUTIONS
o Reduce cavity temperature
20 Problem: OVAL FINISH
POSSIBLE CAUSES
o Insufficient cooling
o Hold time low
o Insufficient cooling
POSSIBLE SOLUTIONS
o High core and cavity temperatures
o Increase cooling time
o Increase injection hold time
o Check water temperature and water flow
to injection Mould
o Check temperature of injection cores
and cavities
21Problem: GAS BURNS
POSSIBLE CAUSES
o High injection speed
o Contaminated splits
POSSIBLE SOLUTIONS
o Decrease injection speed
o Clean splits
22 Problem: HIGH ACETALDEHYDE
POSSIBLE CAUSES
o High residual AA in resin
o High barrel temperature
o High back pressure
o High screw speed
o High injection speed
o High mold manifold temperature
o High nozzle tip temperature
o Long cycle time
o High extruder / screw cushion
o Low inlet resin temperature
o Unsuitable screw
POSSIBLE SOLUTIONS
o Check for resin AA levels
o Reduce the barrel temperature
o Reduce back pressure
o Reduce screw RPM
o Reduce injection speed
o Reduce mold manifold temperature
o Reduce cycle time
o Reduce cushion
o Increase inlet resin temperature
TROUBLESHOOTING GUIDE
Stretch Blow Moulding
1 Problem: BLOWN FINISH
POSSIBLE CAUSES
o Overheating of the finish area which
allows blow pressure air to stretch the
material.
o Oven ambient temperature to high
o Incorrect loading height, too much of
the finish is exposed to the elements
POSSIBLE SOLUTIONS
o Move shield away from finish into a
thicker part of the preforms taper
o Bring shield close to preform
o Increase oven exhaust
o Reduce neck element heat
2 Problem: SEALING SURFACE DAMAGE
POSSIBLE CAUSES
o Misalignment of the preform, blow
mould and blow nozzle usually caused
by poor transfer arm positioning.
o Blow mold position.
o Nozzle position.
POSSIBLE SOLUTIONS
o Realign transfer arm
o Realign blow mould or nozzle if
necessary
3 Problem: CHOCKED NECK
POSSIBLE CAUSES
o Over stretching of the preform by
the stretch rod prior to blow.
o Low blow delayed or missing
POSSIBLE SOLUTIONS
o Reduce air delay time
o Reduce stretch rod pressure
o Move shield into a thicker part of the taper
o Increase heat in the body or base
o Increase low blow pressure
o Reduce heat in the neck
4 Problem: BENT NECK
POSSIBLE CAUSES
o Misalignment of the preform to the
mould or nose.
o Distortion below the flange
a) Insufficient mould cooling
b) One mould half with poor cooling
o Excessive material remained in the
neck and shoulder.
POSSIBLE SOLUTIONS
o Realignment of the preform to the
nose or mould.
o Reduce material thickness in the
shoulder by reducing heat in the body
o Increase high blow time
o Improve mould cooling
5 Problem: HARD NECK
POSSIBLE CAUSES
o Initial stretching occurs too low
into the preforms taper.
o Preform location incorrect
through oven.
o Air leak into preform before the
start of low blow.
POSSIBLE SOLUTIONS
o Reduce heat in the body and/ or base until
pearlescence occurs then increase neck
temperature.
o Increase stretch rod pressure
o Increase air delay time
o Reduce low blow pressure
o Move shield into the thinner part of the taper
o Check height of preform though oven.
o Check for air leak through nose
6 Problem: THIN SHOULDER
POSSIBLE CAUSES
o Excessive stretching of the
taper. Stations producing a smaller
low blow container will be first with
the fault or the thinnest.
o Poor shield positioning, allowing
excessive stretching of the thin
part of the taper.
POSSIBLE SOLUTIONS
o Move shield into a thicker part of the
preforms taper.
o Reduce air delay time
o Reduce stretch rod pressure
o Increase low blow pressure
o Reduce heat in the neck. If thickening
occurs below desired area Increase heat
below the neck element.
7 Problem: HOT BOTTLES
POSSIBLE CAUSES
o Preform temperature above crystal
growth range for to long
o Insufficient cooling of the outside
surface for the amount of heat
absorbed by the P.E.T.
o Excessive equilibration time o
Preform wall thickness excessive
blowers.
POSSIBLE SOLUTIONS
o Reduce heat adjacent to the haze until
pearlescence is evident somewhere on
the bottle
o Reduce heat over all until pearlescence
is evident.
o Increase air circulation through oven,
increase blower speed, clean blowers.
8 Problem: EXCESSIVE FILL POINT DROP
POSSIBLE CAUSES
o Low levels of orientation results in
the material not having enough
strength to resist the pressure
applied.
POSSIBLE SOLUTIONS
o Reduce over all perform temperature
allowing the bottle to be blown closer to its
natural stretch limit. Under this condition
slight pearlescence may be evident
9 Problem: PEARLESCENCE
POSSIBLE CAUSES
o Pearlescence results from
stretching of molecules
faster than they can respond
past it's natural stretch limit.
Small tears appear on the
material's surface.
POSSIBLE SOLUTIONS
o Material to thin - Increase heat
other than where pearl is evident.
o Where the pearlescent area may be allowed to
stretch further increase heat at pearl.
o Increase overall preform temperature
o Pearlescence opposite a swung gate (Refer
Swung Gate)
o Pearlescence on the bottle shoulder may be the
result of a blow air leak through nose from either
the low valve, high blow valve or stretch rod o
Reduce low blow volume, increase low blow time
and reduce low blow pressure.
o Ensure low blow bottle is not to large.
Excessive petal formation in low blow will cause
pearl in the petal area.
o Pearlescence in the body in the form of a ring is
the result of insufficient low blow.
10 Problem: FLAT SIDES
POSSIBLE CAUSES
o Poor venting due to excessive
parting of the blow moulds allowing
air to be trapped along the mould
seams
POSSIBLE SOLUTIONS
o Ensure excessive mould parting does not
occur
o Increase low blow pressure ensuring
bottle diameter is close to blow mould size
o Low blow bottle too small. In this
case the volume of air trying to
escape during high blow is more
than the venting can allow.
o Low blow bottle blown to fast.
o Excessive high blow pressure
at the end of low blow to minimize air that
remains in the blow mould before high blow
starts.
o Reduce low blow pressure and extend
low blow time if the body of the bottle has
reached the mould
o Reduce high blow pressure
11 Problem: DEFORMATION AT MOULD PARTING LINES
POSSIBLE CAUSES
o Pressure remaining in the
bottle when blow moulds open.
o Insufficient high blow cooling
time
o Material too hot and/or too
thick
POSSIBLE SOLUTIONS
o Inadequate exhaust time, check exhaust
valve activation.
o Increase high blow time or reduce
material thickness
o Reduce material thickness
o Reduce preform temperature
12 Problem: CHOCKED BODY
POSSIBLE CAUSES
o Insufficient or no low blow
o Insufficient heat beside
choke
POSSIBLE SOLUTIONS
o Increase low blow pressure
o Increase heat in the body
o Reduce heat in the base and/ or neck.
o Reduce air delay time
o Reduce stretch rod pressure
13 Problem: HEAVY BASE WEIGHT
POSSIBLE CAUSES
o Incorrect heating profile
resulting in a poor distribution of
material, over stretching of the
body or shoulder
POSSIBLE SOLUTIONS
o Reduce heat in the body and/ or
shoulder until pearlescence occurs
then increase heat at the base. Repeat
until the base weight is correct.
14 Problem: LIGHT BASE WEIGHT
POSSIBLE CAUSES
o Poor heating profile
resulting in poor
distribution of material,
insufficient stretching of the
body or shoulder.
POSSIBLE SOLUTIONS
o Reduce heat near the base of the preform
until pearlescence occurs then increase heat to
the shoulder
and/or body. Repeat until base weight is correct
o Increase air delay time
o Increase stretch rod pressure
o Reduce low blow pressure
o Reduce oven shielding of the taper
15 Problem: FEET NOT FULLY FORMED
POSSIBLE CAUSES
o Insufficient high blow pressure. Rate of
high blow too slow, material movement stalls
before reaching the corners. Often as a
result of blow nozzle leak
o Insufficient high blow time. If adding high
POSSIBLE SOLUTIONS
o Check high pressure. If possible.
If station related check for high
blow leaks from nozzle or stretch
rod o Increase high blow time
o Reduce base weight by reducing
blow time does not fix the problem look for
another cause.
o Heavy baseweight. High blow pressure
inadequate to move material into the corners
o Excessive low blow pressure or time. If too
much of the material will be left to form the
feet correctly. Corners are likely to be thin.
o Swung gate. Feet will not form in the
corners opposite the gate movement unless
extra heat is applied near the end cap.
preform temperature until
pearlescence is evident then
increase prefroms lower body
temperature
o Check low blow bottle size.
Reduce if necessary
o Fix swung gate
16 Problem: HOT SIDES
POSSIBLE CAUSES
o Inconsistent preform rotation
through the oven
o Insufficient surface cooling
of the preform
POSSIBLE SOLUTIONS
o Check the preform for a drag marl. Ensure
oven shields are not touching the preform.
Check collect, spindle or mandrel rotation
o Increase air flow onto the surface of the
preform
17 Problem: LOW HEADLOAD
POSSIBLE CAUSES
o Insufficient orientation and/or
material thickness to deliver the
required physical strength
POSSIBLE SOLUTIONS
o Reduce over all preform temperature
o Reduce heat beside failure point
o Where failures occur between neck and
shoulder adjust shield into a thicker part of the
taper
18 Problem: LOW BURST PRESSURE (BODY)
POSSIBLE CAUSES
o Insufficient orientation (Too hot)
o Material to thin
o Confirmation
o Low IV
POSSIBLE SOLUTIONS
o Reduce preform temperature
o Reduce body heat
19 Problem: LOW BURST PRESSURE (BASE)
POSSIBLE CAUSES
o Base of bottle blown too cold
producing excessive stress
o Stretch rod clearance too short
o Excessive crystallinity at the gate
o Low IV
o Excessive stress in preform
POSSIBLE SOLUTIONS
o Increase heat below the preform
o Reset stretch rod clearance
o Reduce stretch rod pressure
20Problem: POOR CLEARANCE
POSSIBLE CAUSES
o Preform temperature too high
o Heavy baseweight
POSSIBLE SOLUTIONS
o Base weight correct or light, reduce heat
beside and/or below the gate
o Endcap temperature too high
o Aged preforms
o Insufficient mould cooling
o Excessive inherent preform stress
o Base weight to heavy, reduce preform
temperature
o Increase high blow time.
21Problem: FOLD IN BASE AT END CAP LINE
POSSIBLE CAUSES
o Preform temperature to hot
o Preform endcap area to cold
o An excess of material in the vicinity
of the fold
o Lack or loss of low blow volume
o Excessive force applied by the
stretch rod
o Aged or stressed preforms
POSSIBLE SOLUTIONS
o Reduce heat in the areas other than the
base. If the fold remains after
pearlescence has formed. Heat may be
applied to the preform endcap area.
o Increase low blow pressure
o Reduce air delay time
o Reduce stretch rod pressure
22Problem: FOLD AROUND THE STRETCH ROD
POSSIBLE CAUSES
o Over heating of the preform
end cap, material wraps around
the end of the stretch rod. In
effect another thick/ thin
transition forms
POSSIBLE SOLUTIONS
o Reduce heat or below the end cap area
o Increase low blow pressure
o Reduce stretch rod pressure
23Problem: CRACKED BASE
POSSIBLE CAUSES
o Preform endcap too cold
o Excessive stretch rod pressure
o Clearance between the stretch rod
and mould base too small
o Thick crystallinity above the gate
POSSIBLE SOLUTIONS
o Increase heat beside or below the gate
o Reduce stretch rod pressure
o Check preform for excessive crystallinity,
if so change preforms.
o Increase stretch rod, mould base gap
24Problem: SWUNG GATE
POSSIBLE SOLUTIONS
o First impacted mark is evident and is
not centered to the gate, an alignment
problem is usually the cause. Check for
misalignment. Ensure transfer arm
locates preform in mould correctly.
o Check blow mould cooling
o If the first impact mark is centered, the
stretch rod has lost control of the gate
during blow. Often gate control is lost
due to excessive growth in length
during low blow.
Ideally growth should start high in the
preform reaching full diameter before
full length if length is achieved
before diameter the rod must travel
further than normal to keep up.
o Reduce low bottle size
o Increase air delay time
o Reset stretch rod height
POSSIBLE CAUSES
o Misalignment of the preform to the
mould, nozzle or stretch rod.
o Inadequate mould cooling, one half
only If the gate is centered to the stretch
rod impact mark and yet not centered to
the mould base at the end of blow the
stretch rod has lost control during blow.
o Lower half of preform too hot
o Low blow pressure too high
o Air delay time too short
o Stretch rod length too short
o Excessive stretch rod cushioning
o Worn stretch rod guides
o Excessive preform eccentricity
o Thick crystallinity near gate
25Problem: STRESS CRACKING
POSSIBLE CAUSES
o Off center gates. Webs with the thinnest
material will stress crack early.
o Excessive post mould change. As
clearance deteriorates web shape
changes increasing the risk of stress
cracking.
o Excessive growth at the base cust line.
Temperature of the lower half of the
preform's body is to high
o Chemical reaction by some line
lubricants.
o Low I.V. material
o Light base weight. Inadequate thickness
to resist movement.
POSSIBLE SOLUTIONS
o Fix swung gate.
o Reduce preform temperature to
pearlescence, reduce base weight if
necessary. Increase high blow time
o Reduce heat in the preform's lower body
o Line lubricants generally cause severe
stress cracking in all feet.
o Increase base weight
26Problem: EXPLOSIONS
POSSIBLE CAUSES
o Hot bottles (Refer Hot Bottles)
o Folds in the base
o Light base weight
o Excessive post mould growth
o Low I.V.
o Chemical Reaction (Refer stress
Cracking)
o Contamination
o Air bubbles above the gate
POSSIBLE SOLUTIONS
o Refer Causes
o Stress Cracking
o Cracked bases
o Excessive crystallinity
27Problem: DROP TEST FAILURES
POSSIBLE CAUSES
o Preform end cap blown too cold
o Light base weight
o Excessive crystallinity above gate.
o Large air bubbles above the gate
POSSIBLE SOLUTIONS
o Increasing heat at the gate or below
without loosing base weight usually
improves drop test failure.
o Increase base weight.
RECOMMEDED PET TESTS & EQUIPMENT FOR DOWNSTREAM CONVERTERS Many preform and bottle pro- producers ducers will have QC laboratories
already, or may be thinking of setting up one if they do not have one already. The
following gives an indication of essential tests.
Intrinsic Viscosity
The intrinsic viscosity (I.V.) is a primary parameter that is monitored. The I.V. is a
measure of molecular weight. SABIC measures I.V. by solution viscometry. The
PET is dissolved in 3:2 phenol : 1,2 dichlorobenzene, at 25°C and 0.5%
concentration, and its flow time through a capillary is measured and compared
against the that of the solvent.
The higher the molecular weight of the polymer, the higher the flow time and
hence the I.V. This method requires high consumption of chemicals. However,
I.V. can also be measured by melt viscomtery. In this method, the polymer (resin
or preform) is powdered after cooling in liquid nitrogen, dried, then melted and
extruded through a capillary. The melt viscosity can be calculated from the flow
properties. As the melt flow properties are related to the molecular weight, a
correlation with the I.V. can be made. Consistency of drying is Melt Viscometer
very essential in getting good
results. The melt viscometer is r e c o m - mended for our downstream PET
customers for measuring the I.V. of resins and preforms. Melt viscometers that
can measure and give a readout of the I.V. are made by Lloyds Instruments
(U.K.) and Kayeness (U.S.A.).
Acetaldehyde Analysis
The acetaldehyde (AA) is an important property that needs to be monitored
specially for water packaging applications. The AA level in most commercial
resins is < 1 ppm. Most resin manufacturers today meet this target, and hence
the AA in the resin need not be measured by the down stream converter.
However, the preform AA and the AA in the bottle may be important. The preform
AA is measured by grinding the sample into a powder after cooling in liquid
nitrogen, by headspace gas chromatography (GC). There are critical AA levels
for water.
The bottle head space test is different. A freshly blown bottle is capped with a
septum and stored at 23°C for 24 hours. The AA migrates into the bottle
headspace. The AA concentration is measured by sampling the gas in the bottle
and injecting into a GC. It must be noted that Solution Viscometer Gas
Chromatograph
the AA achieved in the bottle depends on resin and injection moulding conditions.
The equipment needed for AA measurement is a GC with head space assembly.
In addition, a grinder for powdering the resin or
preforms is needed. Liquid nitrogen in addition to the glass vials and septa of the
GC are major consumables. Manufacturers of GC with head space assembly are
Hewlett Packard and Perkin Elmer.
Colour Measurement
The colour of the resin and preform may be measured for consistency.
The colour is measured in terms of L* (brightness), b* (yellowness or blueness)
and a* (redness or greeness) For resin, the material needs to be powdered.
Equipment for colour measurement is supplied by Hunter Lab. Colour in preforms
may be measured by crystallising the preform in an oven at 170°C for 30
minutes, powdering it and using the same equipment. Alternatively, preform
colour can be measured without sample reparation
using the ColorQuest XE spectrophotometer. Colour Spectrophotometer Sample
Preparation Acetaldehyde Analysis.
Moisture Analysis
is The moisture content in PET has to be reduced to < 50 ppm by drying before it
is injection moulding. If the moisture content exceeds this, there is a large drop in
the I.V. in the perform. Injection moulders generally have driers, but to check the
efficiency of the drier, it is worth sampling and analysing the resin after it is dried.
There are several bench top equipment available such as from Karl ischer, TA
Instruments and Arizona Instruments. Moisture Analyzer Burst Strength
Testing This equipment is widely used in the carbonated soft drinks industry by
the fillers, to assess the performance of the bottles in filling and withstanding
pressures of carbonated beverages, which are normally filled to 4 volumes of
gas. The burst pressure requirement is generally in the range of 140 to 170 psi
depending on the bottle size. This unit can apply the test pressure and hold it for
a given time and also measure the free volume expansion, the pressure could
also be applied in a ramped profile.
Head Load Testing
This unit - as the name suggests - is used to measure the axial load taken by the
bottle before failure. The speed of load application is critical and so is the axial
positioning of the bottles. The average acceptable values for carbonated bottles
are again in the order of 200 N (20 kg). This test once standardized could notify
the QC personnel of variation in material distribution on the bottle.
This test is crucial, as filled bottles have to take the top load in transportation and
storage of these bottles. Inadequate head could crush the bottles and causes
leakage of the beverage.
Burst Strength Tester
This equipment is widely used in the carbonated soft drinks industry by the fillers,
to assess the performance of the bottles in filling and withstanding pressures of
carbonated beverages, which are normally filled to 4 volumes of gas. The burst
pressure requirement is generally in the range of 140 to 170 psi depending on
the bottle size. This unit can apply the test pressure and hold it for a given time
and also measure the free volume expansion, the pressure could also be applied
in a ramped profile.