Product testing – Preform / Bottle - Y.Somasundaram

Bottle forms the final product and preforms is the intermediate product, we need to check the quality at these two stages to ensure proper performance of the product at the final destination. Preform testing is more critical as the PET resin undergone transformation from a highly crystalline stage to an amorphous stage at higher temperatures and pressures, and hence the chances of occurring defects are more at this stage and identifying a defect at this stage will avoid subsequent wastage of material and energy at the next stage (Blowing).

Preform quality check need to be conducted at various levels to ascertain the quality of the product;

Level 1: Visual inspection (with normal light)Level 2: Visual inspection (With polarized light)Level 3: Dimensional inspectionLevel 4: Chemical inspection

Level1: Visual inspection (Normal Light)

Level 1 inspection is carried out with the naked eye with or without the aid of following tools;1. White light table, with magnification glass, 5 X2. Preform cutter.

Most of the start up defects come under this category, during a regular run, few of the these defects may reappear due to faulty process conditions, which needs periodical check, as per the process machinery consistency, hence frequency of such check to be decided by internal Quality control.

Level 2: Visual inspection (Polarized light)

Level 2, inspection is carried out with aid of polarized light inspection table, and inference charts.

The polarized light inspection indicates the nature of filling of the preforms and packing, by stress pattern made visible by the polarized light. Another critical defect identified by this method is the surface moisture, called the moisture ring due to the unique appearance.

Level 3: Dimensional check

Dimensional checks are one to confirm whether the preforms meet the nominal sizes and tolerances laid down during design. Dimensional checks are done by measuring devices during initial run of the mold / product qualification and during production normally a Go -No Go gauge is used to speed up the process.

The inspection aids for Level 3 process:1. Vernier caliper2. Screw gauge 3. Profile projector4. Preform cutter5. Go – No Go gauge

Level 4: Chemical test

Chemical test are done to ascertain transition undergone by the polymer during drying, plasticization, and molding has not had any detrimental effect on the polymer, to affect the performance of the final product. The two properties which are monitored at this stage are;

1. Intrinsic Viscosity (IV)2. Acetaldehyde (AA)

The equipments used for this purpose and test methods are similar to the procedure done for PET resin, for details refer “Quality Parameter – section 6”.

Defects could be further classified as 1. Indicative defects2. Major defects3. Critical defects

Indicative defects: These defects are indicative of variation in quality of preforms due to variation processing conditions and raw material. These defects may not create problems in final product quality, but if they are not attended on time, may lead to more serious defects.

Major defects: These defects can create problems in blowing and have appearance problem in final product (Bottle). These defects need immediate correction.

Critical defects: These defects will create problem in blowing as well as functional problems in the final bottle quality. These defects need to be attended by stopping the machine in most cases as any delay in correction will lead to wastage of material.

Classification of preform defects & effects

No. Defect Status Test Level Effect of defect1 Bubbles Major Level 1 Bursting.

Double layer in bottle wall. Poor bottle appearance.

2 Un-melt Major Level 1 Bursting. Poor bottle appearance

3 Long gate Major Level 1 Probability to hit upper row of IR lamps. Bottle gate deformation.

4 Stringing Indicative Level 1 Sticking to hot preform wall. Surface lines on the bottle wall.

5 Moisture marks Critical Level 1 / 2 Bursting during blowing. White patch or pearlescence like bottle wall

appearance. Bottle wall weakness.

6 Gate crystallinity Critical Level 1 Stress cracking. Bursting during blowing. Weak gate / failure in drop test.

7 Hazy preform Major Level 1 Dirty bottle appearance. Unstable blowing operation.

8 Knit line / crack in neck finish

Critical Level 1 Bursting of neck finish during blowing. Poor seal integrity.

9 Black specks Critical Level 1 Bursting during blowing. Poor appearance.

10 Flash at neck finish

Critical Level 1 Higher capping force. Improper capping / seal integrity.

11 Flash below NSR Major Level 1 Poor parting line appearance in bottle. Preform holding and transfer problem in high

speed blow molders.12 Gate dimpling Indicative Level 1 Poor gate appearance.13 Gate peel off Indicative Level 1 Poor gate appearance14 Internal gate

deformationMajor Level 1 Weak gate.

Drop test failure.

15 Burn marks Critical Level 1 Bursting during blowing.

Poor appearance Higher AA Lower IV / weak bottle

16 Heat splay Major Level 1 Weak bottle. Poor surface marks.

17 Preforms buckling

Critical Level 1 Preform cannot be blown

18 Gate pin hole Critical Level 1 Bursting during blowing.19 Color variation Major Level 1 Appearance problem.20 Spider web Indicative Level 1 Poor appearance around gate.21 Void in gate Critical Level 1 Bursting during blowing22 Wall thickness

variation, > 0.2 mm

Critical Level 3 Can cause dancing of preform in oven. Gate offset in bottle. Uneven bottle wall thickness and weakness.

23 IV drop > 0.03 dl/g

Critical, CSD

Level 4 Weak bottle. Higher stress cracking probability. Higher gas loss in CSD bottles.

24 Higher AA, > 4 ppm

Critical, Water

Level 4 Flavor change in water packed.

25 Neck finish – Go – No Go failure

Critical Level 3 Capping problem may arise

26 Neck finish ID – Go – No Go failure

Critical Level 3 Preform may not be held properly in blowing mandrel.

Capping may be a problem in case of inner sleeve caps.

All the defects can be present in various degrees of intensity, which may call for difference in opinion than the ones discussed above, which require expert opinion or the final end user comments.

Preform inspection sheetMachine DateMold OperatorCavities TimeWeight Box no.Sample No. Parameter AQL Defective

0.65% 1.5% 4.0%1 Damaged neck finish 0.65%2 Gate problems, Pin hole, void 0.65%3 Full crystallinity, > 10mm diameter 0.65%4 Spider web 4.0%5 Contamination, dirt, oil 1.5%6 Drag marks, sink marks 4.0%7 Wall thickness variation, > 0.2mm 0.65%8 Weight variation, > 0.5gms 0.65%9 Neck finish, Go No Go failure 0.65%

Total defectsSample size, Pcs. Status

32 AcceptanceRejection

01

12

34

50 AcceptanceRejection

12

23

56

500 Acceptance 7 14 21

Rejection 8 15 22Inspected by Approved by

Typical preform inspection sheet

Shelf life:

Normally the shelf life of the preform can be safely taken as 6 months, beyond which it may require further testing to ascertain the suitability for the particular application. The storage ambient conditions of temperature and humidity play a vital role in the shelf life of the preform. High moisture absorption of preforms can lead to increase in “Natural stretch ratio” of the preform, which may lead to not optimally strain hardened bottles, with lower strength, barrier properties etc,.

Bottle testing;

Bottle forms the final product of the PET chain, and it is used in direct contact with the food material, which is stored, transported and protected by the inherent strength of the bottle. Hence the bottle testing should consider the product packed and the conditions it is subjected during the shelf life for design of a specific test method. The bottle test method and specifications will vary from product to product.

All PET bottle testing methods are in-process in nature, to help aid the blow molding engineer to tune the machine to produce bottle to meet specifications.

1. Bottle sectional weight.2. Bottle wall thickness.3. Bottle volume.4. Bottle top load5. Bottle drop test6. Bottle burst test.7. Bottle stress crack test.8. Bottle head space AA9. Bottle shelf life.

Bottle sectional weight:

Bottle sectional weight measurement is an easy method to control the distribution of wall thickness in the bottle, as it can be done much quicker compared to actually measuring the wall thickness at all locations.

Here the bottle is cut into three pieces horizontally;

Section 1: Base section: Along the base mold parting line.Section 2: Panel section: Along the top limit of the panel.Section 3: Shoulder section: The remaining top portion.

The sectioning is done by hot wire cutter, with wires positioned along the exact position. The weights of each section is calculated during the bottle design stage and adequate tolerance imparted based on the process, so as to enable the blowing engineer to take a quick decision on the process changes.

Bottle wall thickness:

Bottle wall thickness is measured to ensure that there is no variation in the wall thickness along the circumference of the bottle in a particular plane / section, due to gate offset or any other blowing variations. It is also helpful to ensure proper wall thickness at intricate sections, which are critical for the strength of the bottle like the petaloid legs. Bottle wall thickness is measured using a magnetic wall thickness tester, or by

means of CCD scanning. As the scanners are quite expensive in normal circumstances a magnetic probe is used.

Bottle volume:

Bottle volume test is done in order to ensure that the bottle is blow properly and also to control the volume of the content, as filling machines fill quantities by means of fill height and not by measurement, hence and variation in bottle volume will affect the product content.

Normally bottle volume is measured at two conditions; filled to brim, which is called the brimful volume, and filled up the fill height, called the fill volume.

Brimful Volume: In brimful volume the weight of the empty bottle is measured, and followed by weight of bottle with water filled to the brim level. The difference in weight give the weight of water, this value divided by the density at the measuring temperature, gives the volume of the bottle. Brimful volume is measured at time zero (immediately after blowing), and after 24 hrs, allowing for the bottle to shrink and set, the volume at “T + 24 hrs” is always lower than the volume measured at “T”. Volume at “T+24” is considered as the final volume.

Fill Volume: Fill volume is the volume of the bottle, when filled up to the fill height. The fill level is measured from the lip surface / top of the bottle, and specified in mm. The fill height is as low as 25 mm in case of water bottle, as there is no need for a head space, to as high as 50 mm in case of carbonated soft drink bottle, to allow for adequate head space for carbon dioxide. The procedure for measurement of fill volume is also same as that of brimful volume, except here the water is filled up to the fill point.

Bottle top load:

Cutting Location

Label SectionBase Section Shoulder Section

Typical CSD bottle with sectioning lines

Top load test is done to understand the stack ability of PET bottle crate, over another crate during storage. Top load strength or the nominal top load depend on the storage requirements, and differ for different bottle capacities and products. Top load test is conducted on a top load tester, with capacity up to 100Kgs.

Typical top load values:Top load (Kgs)

No. Bottle volume Water CSD1 500 ml 7.5 352 1000ml 10 353 1500ml 12 354 2000ml 16 35

Bottle drop test:

Bottle drop test is done to understand failure of bottle in drop fall during handling of bottles. Drop test are conducted under three conditions;

1. Vertical drop2. Horizontal drop3. Oblique drop (45o)

All the drop tests are conducted for 1.5 meter fall of filled bottle up to the fill point.

Vertical drop test: The bottle is filled up to the fill point and capped. The bottle is held by the neck and allowed to fall from 1.5 M on a flat concrete surface. The drop is done for 3 times for the same bottle continuously and the failure if present is recorded.

Horizontal drop test: The bottle is filled up to the fill point and capped. Then it is held by the body horizontally and allowed to fall horizontally on the concrete surface for three times continuously and the failure if any is recorded.

Oblique drop test: The bottle is filled up to the fill point and capped. Then it is held vertically above a concrete slab inclined at 45o to the horizontal plane at distance of 1.5 M, the drop is repeated for 3 times and failure if any is recorded.

Bottle stress crack test:

Stress cracking is the crazing or cracking that can occur when plastic is under tensile stress. PET material is strongest in a highly oriented state, such as in the sidewall of the container. It is most susceptible to stress cracking when it is in an amorphous state, such as in the area surrounding the center of the base (gate area), and under tensile stress.

The following are the factors which can accelerate the stress cracking of bottle and eventually lead to bursting of bottle abruptly, with high force, which is dangerous. Hence it is important to understand the stress cracking strength of the bottle, through accelerated method.

High alkalinity of environment Poor material distribution Excessive IV degradation Over carbonation Contact with incompatible chemicals High temperature exposure

Objective: To determine the level of resistance to sodium hydroxide induced stress cracking, on a carbonated soft drink bottle.

Principle: One known mode of stress crack attack on PET bottles is by hydroxide ion. A bottle that has more resistance to Sodium Hydroxide attack should be more resistant to stress crack initiators that a bottle may be exposed to during its lifetime.

Apparatus:1. Beaker / containers.2. stop watch or timer 3. Compressed air regulated to 5.31 bars4. distilled water 5. bottle closures6. 0.2% NaOH solution prepared with distilled water and solid NaOH.

Sample quantity: 2 set for each cavity of blow mold.

Procedure: 1. Bottles should be less than 2 weeks old, then aged at 50C +/ -1 C and 50% RH for 24 hours. After

aging, the bottles should be stored at 22C +/ -1C for a minimum of 16 hours. Label the bottles.2. Prepare the solution of 0.2% NaOH solution. (Alkalinity 2.4 -2.6 g/l CaCO3). 3. Fill each bottle with the target net contents of water. (2L bottle would contain 2000ml of water) The

water should be equilibrated to 22C +/- 1 C. 4. Pressurize each bottle with compressed air to internal pressure of 5.31 bars. 5. 5 minutes after pressurizing the bottles mark the liquid level on each bottle and then gently place

each bottle into beaker of 0.2% NaOH solution at 22C +/-1C. The solution must cover the base. Start the timer and check at following frequency.

Time Frequency of check0 ~ 30 Continual check30 ~ 60 Every 2 minutes60 ~ 90 Every 5 minutes

6. Record the time to failure in minutes for each bottle. Failure is defined as a burst or a slow leak. A slow leak is evidenced by a visual fill point drop.

Report:

1. Complete fill out report form including Alkalinity of the NaOH solution, Room Temperature, Bottle Temperature, and Caustic Temperature.

2. Preform numbers and blow cavity numbers. 3. Time to failure in minutes 4. Location of failure, choosing one of five categories:

a. Gate through or tangent to it.b. Amorphous region (around gate and stretch rod area)c. Oriented region (base of foot)d. Strap areae. Stretch rod impression

5. Type of failure, i.e. catastrophic (burst) or slow leak. 6. Manufacturing defects, if present.

Bottle head space AA

Bottle head space AA method check the amount of AA that has migrated into the empty space of the bottle over a period of 24hrs or higher depending on the requirement of the customer, under standard temperature (25 degC). This value is used to extrapolate the amount of AA that will migrate to the content after packing in storage. This helps the designers and quality controllers to design and control the impact of AA on the packed contents.

Equipments used: Gas chromatograph with capillary column to sense AA in 1 ~ 10 PPB, head space sampler.

Nitrogen gas for purging Temperature cabinet to maintain 25 deg C. Septum for sealing nitrogen purged bottles.

Procedure: Freshly blown bottles collected from the machine are purged with nitrogen, so that the atmospheric air does not interfere with the analysis. The bottles are closed with septum and kept in controlled environment for 24 hrs. After 24hrs the bottle internal space air is collected by means of a head space sampler with a needle, and injected into the gas chromatograph, which gives the results in PPM (parts per billion).

Shelf life:

Bottle shelf life: Blown bottles do not normally have a fixed shelf life, and can be used after any duration, excluding the case of “Hear set bottles”. But the users may be aware of the fact that bottles undergo continuous secondary shrinkage after the primary post mold shrinkage. Hence brimful volume of the bottles could have considerable variation depending on the period of storage and the ambient conditions.

Product shelf life: Product shelf life with the bottle is a complex analysis which depends on the product packed and the requirements of the product manufacturer, which may differ from case to case. In basic term, a shelf life study for product and package combination is to study the period till which the product can be safely stored in the package in a shelf without any degradation in its preset qualities for consumption.

In case of a carbonated drink, a loss 15% of gas volume of carbon dioxide is considered as the end of the shelf life. i.e. directly proportional to the outward migration of carbon dioxide from the product.

In case of beer packaging, apart from carbon dioxide outward migration, the inwards migration of oxygen is also critical, which causes stalling and settling of beer.

Whereas in case of a crisp product the migration of moisture into the package which cause the loss of crispness of the product is considered for estimating the shelf life.

Hence the equipment and procedure used for estimating and analyzing the shelf life will also differ from product to product.