PET Troubleshooting
Short Description
Polyethylene Terephthalate (PET) • Processing Guidelines • Troubleshooting Guide Injection Moulding Stretch Blow Mo...
Description
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
POSSIBLE SOLUTIONS
o Injection Pressure low
o Increase injection pressure
o Barrel temperature low
o Increase barrel temperature
o Mould manifold temp. low
o Increase mould manifold temperature
o Mould cavity temp. low
o Increase mould cavity temperature
o Injection speed low
o Increase injection speed
o Back pressure low
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
POSSIBLE SOLUTIONS
o Holding pressure low
o Increase holding pressure
o Holding time low
o Increase holding time
o High mold temperature
o Decrease mould temperature
o Inadequate coolant supply
o Check cooling water
o Cooling time low
o Increase cooling time
o Blocked venting
o Clean vents on split, core and cavity
3 Problem: FLASH POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Injection pressure high
o Decrease Injection pressure
o High injection speed
o Decrease injection speed
o Unwanted material in cores
o Clean cores
o Unwanted material in splits
o Clean splits
o Unwanted material in cavity
o Clean cavity
o High mold temperature
o Decrease mould temperature
o High barrel temperature
o Decrease barrel temperature
o Clamping pressure low
o Increase clamp pressure
4 Problem: WATER MARKS POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Leaking hoses
o Check hoses on mould for damage
o Leaking mould
o Check mould for leaks
o Insufficient dehumidification
o Check that dehumidifier is working
o Mold enclosure inadequate
properly o Check that the machine enclosures has no major gaps
5 Problem: BLACK SPECS / CONTAMINATION POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Contaminated resin
o Check for resin cleanliness
o High residence time prior to
o Clean the resin hopper
restart of the machine
o Purge the barrel thoroughly on restart
o Degradation of resin
o Reduce melt temperature o Reduce back pressure
6 Problem: FLOW LINES POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Injection pressure low
o Increase injection pressure
o Mould temperature low
o Increase mould temperature
o Barrel temperature low
o Increase barrel temperature
o Injection speed low
o Increase injection speed
o Drier inefficient
o Check resin drier
o Back pressure low
o Increase back pressure
7 Problem: HEAT SPLAY POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Manifold and mold heater
o Check temperature of mould cavity heaters
Malfunction.
and Manifold
o Barrel temperature high
o Check barrel temperature
o Foreign matter in the gate
o Check that no foreign material is in gate area
8 Problem: PET INCLUSIONS POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Low barrel temperature
o Check barrel temperature
o Low mould temperature
o Check mould temperature
o Back pressure low
o Increase back pressure
o Mixing head damage
o Check that the mixing head of the
o Check valve damage
screw or the screw is not broken. of Inspect the check valve on the screw (not Husky Machine)
9 Problem: CONCENTRICITY POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Injection speed high
o Decrease injection speed
o Injection core bent
o Check that the injection core is not bent and straighten it.
10 Problem: CRYSTALLINE GATE POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Cavity heater malfunction
o Check that the cavity heater is working
o Cavity temperature low
o Increase the cavity temperature
o Manifold temperature low
o Increase the manifold temperature
o Drier malfunction
o Check that the drier is working
o Hold pressure high
o Decrease injection hold pressure
o Foreign material in gate
o Check that no foreign object
o Cooling insufficient
obstructs the cavity gate
o Core cooling inadequate
o Increase cooling time
o Fountain in core rod damage 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
POSSIBLE SOLUTIONS
o Cavity heater malfunction
o Check cavity heater
o Cavity heater low
o Increase cavity temperature
o Gate valve malfunction
o Check that the gate valve is working
o Shut-off nozzle malfunction
o Check that the barrel shut-off nozzle is
o Short decompression time
working o Increase decompression time
12 Problem: AIR BUBBLES POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Barrel temperature low
o Check barrel temperature
o Inefficient drying
o Check drier temperature
o Low back pressure
o Increase back pressure
o Barrel temperature
o Lower temperature at last extruder zone
o Mixing head damage
o Check extruder mixing head
13 Problem: STRESS PATTERN POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o High injection pressure
o Decrease injection pressure
o High shot size
o Reduce shot size
o Long hold time
o Decrease injection hold time
14 Problem: STRINGING POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o High cavity temperature
o Reduce cavity temperature
o Low cooling time
o Increase cooling time
o Low hold time
o Increase injection hold time
o Inadequate coolant to cavity
o Check water flow to cavities
o Gate valve malfunction
o Check that gate valves are working
15 Problem: CLOUDY PREFORMS POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Inefficient Drying
o Check drier temperature and dew point
o Blocked drier filters
o Clean drier air filters
o Air leaks on the drier
o Check for air leaks on drier
o Air leaks on machine hoppers o Check for air leaks on machine hopper o Low air flow to drying hopper
o Check air flow to hopper
o Low process air temperature
o Increase drier process temperature
o Barrel temperature low
o Increase barrel temperature
o Insufficient drying
o Shut machine down for 1 hour and dry
o Back pressure low
material o Increase back pressure
16Problem: HOLLOW GATES POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Packing pressure low
o Increase packing pressure
o Hold time low
o Increase holding time
o Insufficient cooling time
o Increase cooling time
o High cavity temperature
o Lower cavity temperature
o Foreign material in gate
o Check for foreign material in gate area.
17 Problem: DISCOLORED PREFORMS POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o High barrel temperature
o Lower barrel temperature
o High mold temperature
o Lower injection mould temperature
o High drying temperature
o Lower drier process temperature
18 Problem: LINE OVER FINISH POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Contaminated splits
o Clean splits
o Low packing pressure
o Increase packing pressure
o Insufficient packing time
o Increase packing time
19 Problem: BURNT GATES POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o High cavity temperature
o Reduce cavity temperature
20 Problem: OVAL FINISH POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Insufficient cooling
o High core and cavity temperatures
o Hold time low
o Increase cooling time
o Insufficient cooling
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
POSSIBLE SOLUTIONS
o High injection speed
o Decrease injection speed
o Contaminated splits
o Clean splits
22 Problem: HIGH ACETALDEHYDE POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o High residual AA in resin
o Check for resin AA levels
o High barrel temperature
o Reduce the barrel temperature
o High back pressure
o Reduce back pressure
o High screw speed
o Reduce screw RPM
o High injection speed
o Reduce injection speed
o High mold manifold temperature
o Reduce mold manifold temperature
o High nozzle tip temperature
o Reduce cycle time
o Long cycle time
o Reduce cushion
o High extruder / screw cushion
o Increase inlet resin temperature
o Low inlet resin temperature o Unsuitable screw
TROUBLESHOOTING GUIDE Stretch Blow Moulding
1 Problem: BLOWN FINISH POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Overheating of the finish area which
o Move shield away from finish into a
allows blow pressure air to stretch the
thicker part of the preforms taper
material.
o Bring shield close to preform
o Oven ambient temperature to high
o Increase oven exhaust
o Incorrect loading height, too much of o Reduce neck element heat the finish is exposed to the elements
2 Problem: SEALING SURFACE DAMAGE POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Misalignment of the preform, blow
o Realign transfer arm
mould and blow nozzle usually caused
o Realign blow mould or nozzle if
by poor transfer arm positioning.
necessary
o Blow mold position. o Nozzle position.
3 Problem: CHOCKED NECK POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Over stretching of the preform by
o Reduce air delay time
the stretch rod prior to blow.
o Reduce stretch rod pressure
o Low blow delayed or missing
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
POSSIBLE SOLUTIONS
o Misalignment of the preform to the
o Realignment of the preform to the
mould or nose.
nose or mould.
o Distortion below the flange
o Reduce material thickness in the
a) Insufficient mould cooling
shoulder by reducing heat in the body
b) One mould half with poor cooling
o Increase high blow time
o Excessive material remained in the
o Improve mould cooling
neck and shoulder.
5 Problem: HARD NECK POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Initial stretching occurs too low o Reduce heat in the body and/ or base until into the preforms taper.
pearlescence occurs then increase neck
o Preform location incorrect
temperature.
through oven.
o Increase stretch rod pressure
o Air leak into preform before the o Increase air delay time start of low blow.
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
POSSIBLE SOLUTIONS
o Excessive stretching of the
o Move shield into a thicker part of the
taper. Stations producing a smaller preforms taper. low blow container will be first with o Reduce air delay time the fault or the thinnest.
o Reduce stretch rod pressure
o Poor shield positioning, allowing o Increase low blow pressure excessive stretching of the thin
o Reduce heat in the neck. If thickening
part of the taper.
occurs below desired area Increase heat below the neck element.
7 Problem: HOT BOTTLES POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Preform temperature above crystal
o Reduce heat adjacent to the haze until
growth range for to long
pearlescence is evident somewhere on
o Insufficient cooling of the outside
the bottle
surface for the amount of heat
o Reduce heat over all until pearlescence
absorbed by the P.E.T.
is evident.
o Excessive equilibration time o
o Increase air circulation through oven,
Preform wall thickness excessive
increase blower speed, clean blowers.
blowers.
8 Problem: EXCESSIVE FILL POINT DROP POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Low levels of orientation results in o Reduce over all perform temperature the material not having enough
allowing the bottle to be blown closer to its
strength to resist the pressure
natural stretch limit. Under this condition
applied.
slight pearlescence may be evident
9 Problem: PEARLESCENCE POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Pearlescence results from o Material to thin - Increase heat stretching of molecules
other than where pearl is evident.
faster than they can respond o Where the pearlescent area may be allowed to past it's natural stretch limit. stretch further increase heat at pearl. Small tears appear on the
o Increase overall preform temperature
material's surface.
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
POSSIBLE SOLUTIONS
o Poor venting due to excessive
o Ensure excessive mould parting does not
parting of the blow moulds allowing
occur
air to be trapped along the mould
o Increase low blow pressure ensuring
seams
bottle diameter is close to blow mould size
o Low blow bottle too small. In this
at the end of low blow to minimize air that
case the volume of air trying to
remains in the blow mould before high blow
escape during high blow is more
starts.
than the venting can allow.
o Reduce low blow pressure and extend
o Low blow bottle blown to fast.
low blow time if the body of the bottle has
o Excessive high blow pressure
reached the mould o Reduce high blow pressure
11 Problem: DEFORMATION AT MOULD PARTING LINES POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Pressure remaining in the
o Inadequate exhaust time, check exhaust
bottle when blow moulds open.
valve activation.
o Insufficient high blow cooling
o Increase high blow time or reduce
time
material thickness
o Material too hot and/or too
o Reduce material thickness
thick
o Reduce preform temperature
12 Problem: CHOCKED BODY POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Insufficient or no low blow
o Increase low blow pressure
o Insufficient heat beside
o Increase heat in the body
choke
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
POSSIBLE SOLUTIONS
o Incorrect heating profile
o Reduce heat in the body and/ or
resulting in a poor distribution of shoulder until pearlescence occurs material, over stretching of the
then increase heat at the base. Repeat
body or shoulder
until the base weight is correct.
14 Problem: LIGHT BASE WEIGHT POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Poor heating profile
o Reduce heat near the base of the preform
resulting in poor
until pearlescence occurs then increase heat to
distribution of material,
the shoulder
insufficient stretching of the and/or body. Repeat until base weight is correct body or shoulder.
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
POSSIBLE SOLUTIONS
o Insufficient high blow pressure. Rate of
o Check high pressure. If possible.
high blow too slow, material movement stalls If station related check for high before reaching the corners. Often as a
blow leaks from nozzle or stretch
result of blow nozzle leak
rod o Increase high blow time
o Insufficient high blow time. If adding high
o Reduce base weight by reducing
blow time does not fix the problem look for
preform temperature until
another cause.
pearlescence is evident then
o Heavy baseweight. High blow pressure
increase prefroms lower body
inadequate to move material into the corners temperature o Excessive low blow pressure or time. If too o Check low blow bottle size. much of the material will be left to form the
Reduce if necessary
feet correctly. Corners are likely to be thin.
o Fix swung gate
o Swung gate. Feet will not form in the corners opposite the gate movement unless extra heat is applied near the end cap.
16 Problem: HOT SIDES POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Inconsistent preform rotation o Check the preform for a drag marl. Ensure through the oven
oven shields are not touching the preform.
o Insufficient surface cooling
Check collect, spindle or mandrel rotation
of the preform
o Increase air flow onto the surface of the preform
17 Problem: LOW HEADLOAD POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Insufficient orientation and/or
o Reduce over all preform temperature
material thickness to deliver the
o Reduce heat beside failure point
required physical strength
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
POSSIBLE SOLUTIONS
o Insufficient orientation (Too hot)
o Reduce preform temperature
o Material to thin
o Reduce body heat
o Confirmation o Low IV
19 Problem: LOW BURST PRESSURE (BASE) POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Base of bottle blown too cold
o Increase heat below the preform
producing excessive stress
o Reset stretch rod clearance
o Stretch rod clearance too short
o Reduce stretch rod pressure
o Excessive crystallinity at the gate o Low IV o Excessive stress in preform
20Problem: POOR CLEARANCE POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Preform temperature too high
o Base weight correct or light, reduce heat
o Heavy baseweight
beside and/or below the gate
o Endcap temperature too high
o Base weight to heavy, reduce preform
o Aged preforms
temperature
o Insufficient mould cooling
o Increase high blow time.
o Excessive inherent preform stress
21Problem: FOLD IN BASE AT END CAP LINE POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Preform temperature to hot
o Reduce heat in the areas other than the
o Preform endcap area to cold
base. If the fold remains after
o An excess of material in the vicinity
pearlescence has formed. Heat may be
of the fold
applied to the preform endcap area.
o Lack or loss of low blow volume
o Increase low blow pressure
o Excessive force applied by the
o Reduce air delay time
stretch rod
o Reduce stretch rod pressure
o Aged or stressed preforms
22Problem: FOLD AROUND THE STRETCH ROD POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Over heating of the preform
o Reduce heat or below the end cap area
end cap, material wraps around o Increase low blow pressure the end of the stretch rod. In effect another thick/ thin transition forms
o Reduce stretch rod pressure
23Problem: CRACKED BASE POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Preform endcap too cold
o Increase heat beside or below the gate
o Excessive stretch rod pressure
o Reduce stretch rod pressure
o Clearance between the stretch rod o Check preform for excessive crystallinity, and mould base too small
if so change preforms.
o Thick crystallinity above the gate
o Increase stretch rod, mould base gap
24Problem: SWUNG GATE POSSIBLE SOLUTIONS
POSSIBLE CAUSES
o First impacted mark is evident and is
o Misalignment of the preform to the
not centered to the gate, an alignment
mould, nozzle or stretch rod.
problem is usually the cause. Check for o Inadequate mould cooling, one half misalignment. Ensure transfer arm
only If the gate is centered to the stretch
locates preform in mould correctly.
rod impact mark and yet not centered to
o Check blow mould cooling
the mould base at the end of blow the
o If the first impact mark is centered, the stretch rod has lost control during blow. stretch rod has lost control of the gate
o Lower half of preform too hot
during blow. Often gate control is lost
o Low blow pressure too high
due to excessive growth in length
o Air delay time too short
during low blow.
o Stretch rod length too short
Ideally growth should start high in the
o Excessive stretch rod cushioning
preform reaching full diameter before
o Worn stretch rod guides
full length if length is achieved
o Excessive preform eccentricity
before diameter the rod must travel
o Thick crystallinity near gate
further than normal to keep up. o Reduce low bottle size o Increase air delay time o Reset stretch rod height
25Problem: STRESS CRACKING POSSIBLE CAUSES
POSSIBLE SOLUTIONS
o Off center gates. Webs with the thinnest o Fix swung gate. material will stress crack early.
o Reduce preform temperature to
o Excessive post mould change. As
pearlescence, reduce base weight if
clearance deteriorates web shape
necessary. Increase high blow time
changes increasing the risk of stress
o Reduce heat in the preform's lower body
cracking.
o Line lubricants generally cause severe
o Excessive growth at the base cust line.
stress cracking in all feet.
Temperature of the lower half of the
o Increase base weight
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.
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
POSSIBLE SOLUTIONS
o Preform end cap blown too cold
o Increasing heat at the gate or below
o Light base weight
without loosing base weight usually
o Excessive crystallinity above gate.
improves drop test failure.
o Large air bubbles above the gate
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.
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