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Experimental study of preform reheat temperature in two-stage injection stretch blow molding.

INTRODUCTION

PET bottles are usually produced in either one-stage or two-stage ISBM machines. The process starts with injection molding of a tube-like "preform." The preform is stretched axially by a stretch rod and radially by pressurized air until it takes up the shape of the bottle mold. During the blow stage, a preblow is applied to prevent the axial stretch rod contacting the inside of the preform, which may result in defects in the bottle. When the rod reaches the bottom of the container, a high-blow pressure is applied to impart intricate details of the bottle mold and to improve the cooling efficiency. In single-stage ISBM process, injection-molded preform is blown in situ once it has cooled to just above its glass transition temperature; whereas in the two-stage ISBM process injection-molded preforms are stored until subsequent blow molding usually at the bottle filling stage. Hence, the preforms require reheating (1).

During ISBM process. PET molecules undergo biaxial orientation and associated strain hardening. Biaxial deformation of PET strongly depends on forming temperature strain rate, stretch ratio, deformation mode, and molecular weight (2). The subsequent biaxial orientation of PET molecules directly influences mechanical and barrier properties of the bottles (3), (4). Strain hardening, which is temperature and strain rate-dependent, provides a self-leveling effect on the stretching preform, which is important in achieving uniform wall thickness. For a two-stage ISBM process, the preform reheat temperature dictates the material distribution in the bottle as well as the clarity and the ease of processing. In industrial applications, the preform is generally reheated using infrared (IR) lights (5), which take advantage of the semitransparent behavior of amorphous preforms, resulting in an axial surface temperature profile between 80 and 115[degrees]C (6), (7). Both the neck above the support ring and the preform base are kept cooler relative to the preform body temperature to prevent the distortion of the bottle closure and to avoid piercing of the preform base by the stretch rod. However, the preform body temperature may also vary because of tapering in the thickness of the wall. When the temperature of the preform body is not quite right, too much material may reside in the sidewalls and not in the base (8). The temperature of the preform is also known to affect the final morphology of the petaloid bases of the CSD bottles (9). In one of the earlier experimental study conducted by Haessly and Ryan 1101. preform reheat temperature was identified as one of the most important processing variable, and the nonuniformities in the temperature distribution in the preform were found to have the most significant impact on the inflation behavior and the resulting wall thickness. Although there are limited number of experimental work devoted to the study of preform temperature, simulation studies of ISBM process incorporating the preform temperature and temperature profile are numerous. In their studies of ISBM, McEvoy et al. (11) used the ABAQUS software to simulate various axisymmetric PET bottles. The temperature for a PET preform ranged from 90 to 110[degrees]C with preform top temperature being lower than that of the main body to encourage more material movement into the bottle base. Other processing parameters, namely the magnitude of the blow pressure, the timing of the blow relative to the stretch rod motion, and stretch rod speed, were also studied to improve the simulation predictions of the bottle wall thickness. Hartwig and Michaeli (12) proposed a theoretical model that allows the investigation of the combined effect of thermal preform conditioning and the molding phase on the wall-thickness distribution of the bottles. The model that uses a temperature-dependent material behavior takes account of the preform temperature profile both in the axial and radial direction. Pham et al. (13) developed a viscohyperelastic material model to simulate the single-stage ISBM process. An optimal preform temperature profile was input based on experimental preform surface temperature. It was assumed that the preform temperature through thickness is approximately close to the experimental surface temperature at the end of conditioning step. However, their simulation results deviated from the experimental data. Pham et al. (13) used Blowsim software for the stretch blow molding to study the model that they proposed. In numerical analysis, for stress and deformation of PET, they found that viscohyperelastic model had given much better results compared to Ogden model and that the numerical results are slightly higher than the thickness of PET bottles. This discrepancy was explained in terms of the increased specific density of PET in final parts. Yang et al. (14) introduced a fully coupled temperature-displacement modeling of ISBM of PET bottles with a view to optimize process parameters. The model incorporating heat transfer between the preform., stretch rod, and mold successfully predicts the side-wall thickness distribution for most part of the bottles studied. In their study, preform surface temperature was measured by means of an IR camera. The discrepancy between the predicted and the experimental data was attributed to the inaccuracy in recording the actual preform temperature. Visualization of preform deformation during stretching and blowing steps was undertaken by Huang et al. (15) via a transparent mold. The deformation mechanisms of the PET preform, which are dependent on preform size, geometry, and temperature profile, were found to be critical in controlling the bottle wall thickness distribution. In a recent study, Bordival et al. (16) proposed a practical methodology to numerically optimize the temperature distribution of the preform in order to provide a uniform thickness for the bottle in a two-stage ISBM machine. They achieved good agreement in the trend between temperature profile experimentally determined within industrial conditions and the temperature distribution computed using their numerical optimization method. However, they did not optimize the process parameters of the heating system. The authors (17) also studied the effect of ISBM process parameters and preform design on the bottle properties; the process parameters comprising the blow pressure, the timing of the blow pressure activation relative to the stretch rod motion was studied to improve the simulation predictions of bottle wall thickness (18), (19).

The present work aims to elucidate the effects of preform reheat temperature on the 1.5-1 CSD bottle properties in a two-stage ISBM process. Details of the bottle and preform molds, process variables, and tests methods are provided. A discussion of the comprehensive experimental results in terms of bottles' mechanical, thermal, and ESCR values is presented.

EXPERIMENTAL

Material

In this experimental study, a PET copolymer (PET 9921W) from Eastman Chemical Company, USA, was used. The polymer is characterized by an intrinsic viscosity of 0.80 Wig: a weight-average molecular weight ([M.sub.w]) of 52,000 g/mol and a number-average molecular weight (Mn) of 26,000 g/mol. It has a melt density of 1.2 g/[cm.sup.3].

ISBM Process Parameters and Reheating Unit

In this study, Sidel SBG-1 Reheat Injection Stretch Blow Moulding machine, located at a local PET bottle manufacturer's site, was used. The ISBM process parameters are given in Table 1. IR Heating Unit of the machine consists of two heating modules, where each module has 10 separate zones. The heating in each zone is controlled by an IR lamp, which is set to operate between 80 and 98% of its capacity depending on its location, that is, the zone. The reheat temperature displayed on the machine is controlled by a pyrometer, which is incorporated into the reheating unit. The pyrometer (Impac Infratherm IN 5 AC) records the temperature of the preform ~30mm away from where the stretching of the preform starts. The digital read-out displayed on the machine as the "Reheat Temperature" refers to the recording obtained from this particular pyrometer. There are two other pyrometers on the reheating unit, which are used for indication purposes only. They provide the temperature range of each preform profile.

TABLE 1. ISBM process parameters for the 1.5 I PET bottle.

Process parameters

Screw                                     38

Diameter (mm)                            100

Screw speed (rpm)                          3

Nozzle diameter (mm)

Hot runner block ([degrees]C)

Sprue                                    275

Block                                    275

Nozzle                                   295

Barrel temperature ([degrees]C)

Front                                    275

Middle                                   275

Rear                                     270

Nozzle                                   275

Injection pressure

Primary (MPa)                           13.7

Secondary (MPa)                          5.9

Injection speed (mis)                    200

Stretch blow molding

Mold cooling temperature ([degrees]C)     12

Preblow (MPa)                           1.25

Final blow (MPai)                          4

Machine oil temperature ([degrees]C)      35

Stretch rod speed (m/s)                  1.0

Stretch rod outside diameter (mm)         14


Bottle and Preform Design; Preform Conditioning

The 1.5-1 PET bottle mold and 40 g preform used for the two-stage ISBM process are shown in Figs. 1 and 2. respectively. The overall temperature profiles of the preforms, which typically range between 30 and 35[degrees]C along the preform length, were adjusted through varying the power output of the IR lamps of the reheating unit. In this study, while maintaining a certain preform temperature profile, the preform reheat temperatures were set to 105, 110, and 115[degrees]C as recorded by means of a pyrometer, which is an integral part of the reheating unit: all the other process variables and preform dimensions were kept constant.

Material Distribution

To measure the material distribution in the bottle, the bottles were cut into three sections (base, body, and top) as shown in Fig. 1 by using a custom-designed hot-wire cutter. Each bottle section was weighed on a precision scale. At least five bottles were measured from each production run and the results averaged.

Thickness Measurements

The thickness of the foot, valley, and clearance regions of the petaloid base is directly related to the Formation of cracks on the bottle base. Therefore, the thickness measurements that belong to foot, valley, and clearance regions of the petaloid base were taken with a Vernier Calliper.

Bottle Burst Strength

Carbonated beverages pressurize the bottle at up to 689 kPa. The pressurized bottle should not expand excessively, or worse still, blow up at the bottler's filler line. A number of factors can lead to low-bottle burst strength; insufficient wall thickness in some areas due to poor material distribution, wide mold parting lines that may be due to loss of mold compensation (neck, shoulder, or side-wall failure), mold damage etc. Similarly, excessive expansion can occur in the shoulder or body regions as a result of low-section weight or poor material distribution. Burst strength, the pressure at which the bottle bursts, provides an assessment of the overall stability of the bottle. It ensures that the bottles do not blow up at the filling stage, and filled bottles do not expand excessively during storage or bottle warming during pasteurization process. An AGR Plastics Burst Tester with a "Ramp Fill" was used to measure the burst pressure of the bottles, and the test head seals the brink of the bottle. The bottle is filled with water and pressurized rapidly to 689 kPa. The pressure is then increased at a controlled rate until the bottle fails or the maximum pressure limit of the tester is reached. At least five bottles were tested from each production run and the results averaged.

Bottle Top-Load Strength

Top-load strength assesses the overall durability of the bottles necessary for filling and stacking the bottles during manufacturing, storage and distribution. Top-load strength tests were conducted using an INSTRON 4466 instrument equipped with a top-load test platform under laboratory conditions of 20[degrees]C at atmospheric pressure. At least five bottles were tested from each production run and the results averaged. The bottle to be tested is positioned into the test instrument. A compression force at 100 mm/min is applied on the top of the bottle. When the bottle reaches the first point of crush, the load is recorded as the top load.

Environmental Stress Cracking Resistance

The tests were conducted under laboratory conditions at 20[degrees]C. The purpose of the test is to ensure that the PET bottles have sufficient resistance to PET stress cracking to meet minimum performance standards. Accelerated Stress Crack Test Unit (ASCRU) and % 0.20 of NaOH solution prepared from the pellets of concentrated NaOH were used to measure the ESCR of the bottles. In each ASCRU compartment, sufficient caustic solution is added to immerse the base of the bottle. Each bottle is labeled and then filled to the flange with cool tap water. Bottles are attached to the unit by screwing the finish to the metallic strap, and the unit is closed to start the test. The unit displays the time when the bottle fails. The detailed test procedure can be obtained from ASTM D 2561 "Standard Test Method for Environmental Stress-Crack Resistance of Blow-Molded Containers" as well as from Ref. (20).

Thermal Stability

The thermal stability test is designed to measure dimensional changes of filled carbonated PET beverage bottles induced by thermal stresses that occur during the life of the filled bottle.

Satisfactory thermal stability performance is considered as a critical requirement by the bottlers. A carbonated product exerts pressure on the inside of a bottle; and as temperature increases, this internal pressure increases, causing the bottle to expand and creep as function of time. Excessive creep will cause the beverage fill-level to drop, which will negatively affect the appearance of the package and reduce shelf-life of the product.

In the thermal stability test, bottle dimensions in terms of clearance of the bottle base to ground, body diameter; fill-point drop (the distance between the scribed reference line and the bottom of the product meniscus); and bottle perpendicularity were measured before and after the carbonation of the bottles. To carry out this test, the required carbon dioxide ([CO.sub.2]) was generated by the reaction of citric acid and sodium carbonate in water according to the Eq. I. The bottles filled with the prepared solutions were kept at 38[degrees]C for 24 hr and later at 22[degrees]C for 4 hr. After the waiting period, the dimensional changes occurring on the bottle were recorded in terms of percentage changes. Further information on this test can be obtained in the literature (20).

3[Na.sub.2]C[O.sub.2] + 2COOHC[H.sub.2](OH)COOHC[H.sub.2]COOH.[H.sub.2]0 [right arrow] 2COONaC[H.sub.2](OH)COONaC[H.sub.2]COONa + 3C[O.sub.2] + 3[H.sub.2]O (1)

EXPERIMENTAL

Following the production of the 1.5-1 PET bottles of differing preform temperatures under three different preform reheat temperature, the bottles were characterized in terms of material distribution, thickness, top-load strength, burst strength, thermal stability, and ESCR.

Material Distribution

The weights of bottle sections for each bottle are given in Table 2. The results indicate that there is slight shift in material distribution from the base to the body with increasing preform reheat temperature; however, the change in the top section of the bottles is relatively small. Time to formation of cracks decreased with increasing preform reheat set temperature along with a decrease in the weights of the bottle base. Therefore, it can be said that decreasing the weight of the bottle base reduced the resistance against stress cracking. On the other hand, top-load strength increased with increasing preform reheat temperature along with an increase in the weights of the bottle body sections. Hence, increasing the weight of the bottle bodies, thus in the body thickness, improved the resistance of the bottles against buckling.

TABLE 2. Bottle sections' weights.

Preform reheat     Base (g)       Body (g)        Top (g)
unit
set-temperature
([degrees]C)

105              12.1 [+ or -]  14.5 [+ or -]  13.4 [+ or -]
                          0.03           0.02           0.05

110              11.8 [+ or -]  14.8 [+ or -]  13.4 [+ or -]
                          0.03           0.03              0

115              11.6 [+ or -]    14.9 t 0.19  13.5 [+ or -]
                          0.15                         0.042


Burst Strength Test

The burst strength and volumetric expansion of each bottle are given in Table 3. Burst pressure indicates the maximum pressure that the bottle can bear before it bursts, and the volumetric expansion gives the change in volume at the time of failure. It can be said that the burst strength decreases and volumetric expansion of all bottles increases as the preform reheat temperature increases. However, the difference in burst strength between the bottles of preform reheat temperature of 105 and 110[degrees]C is not statistically significant.

TABLE 3. Burst strength and volumetric expansions.

Preform reheat unit              Burst pressure     Expansion
set-temperature                     (kPa)              (ml)
([degrees]C)

105                            1406 [+ or -] 124  411 [+ or -] 92

110                             1358 [+ or -] 14  435 [+ or -] 44

115                             1234 [+ or -] 27  510 [+ or -] 66


Top-Load Strength

The top-load strength tests results are given in Table 4. The top-load strength of the bottles increases with increasing preform reheat set temperature. According to Table 4, body section weight decreases as the preform reheat set temperature decreases. This might be explained by thinning of the body section, where the buckling under top-load is most likely to occur.

TABLE 4. Top-load performance.

Preform reheat unit            Maximum loud (kg)
set-temperature ([degrees]C)

105                            31.8 [+ or -] 0.4

110                            33.0 [+ or -] 0.3

115                            33.2 [+ or -] 1.0


Environmental Stress Cracking Resistance

The environmental stress cracking resistance (ESCR) values of the bottles for differing preform reheat set temperatures are given in Table 5. Time to formation of crack decreases with increasing preform reheat set temperature. Not only that the bottle base is lighter for the higher preform reheat temperatures it also shows abrupt changes in thickness as shown in Fig. 3. This may explain the low-ESCR values of the bottles with high-preform reheat temperatures.

TABLE 5. Accelerated stress crack performance.

Preform reheat unit             Time for crack development (min)
set-temperature ([degrees]C)

105                                             92 [+ or -] 08

110                                             79 [+ or -] 18

115                                             68 [+ or -] 15


Thermal Stability of the Bottles

The thermal stability tests were performed for all bottles of differing preform reheat temperatures; percentage changes in the base clearance, bottle height growth, fill-point drop, and body diameter (in terms of upper body, ower body, and base) are given in Tables 6 and 7, respectively.

TABLE 6. Changes in base clearance, bottle height, and fill-point drop.

Set temperature      Base         Bottle      Fill-point
of preform         clearance      height      drop (mm)
reheat uni            (%)       growth (%)
([degrees]C)

105              81.7 [+ or -]   4.2 [+ or  16.6 [+ or -]
                          0.07     -] 0.07            0.1

110              86.7 [+ or -]   4.0 [+ or  15.4 [+ or -]
                          0.09     -] 0.06            0.1

115              87.5 [+ or -]   3.9 [+ or  16.3 [+ or -]
                          0.08     -] 0.05            0.1

TABLE 7. Thermal stability of the bottles; growth in body diameter.

Set temperature                    Growth in
of preform reheat                diameter (%)
unit ([degrees]C)

                   Upper body     Lower body       Base

105                 4.4 [+ or      4.5 [+ or -]  3.5 [+ or
                      -] 0.09              0.22     -] 0.5

110                 4,2 [+ or  4.0 [+ or -] 0.2  2.7 [+ or
                      -] 0.08                       -] 0.6

115                 4.4 [+ or  3.7 [+ or -] 0.2  2.6 [+ or
                      -] 0.08                       -] 0.5


The growth in the lower body and base sections of the bottle decreases with increasing preform temperature, but in the upper body section, the growth is not affected by the preform reheat temperature. The fill-point drop is the most important thermal stability parameter from a visual point of view; it reaches a minimum value for the preform reheat set temperature of HOT. However, the percentage change in clearance increases as the preform reheat set temperature increases. Once the preform reheat temperature reaches 110[degrees]C, the bottle base becomes overly concave-shaped, and the bottle thus loses its self-standing property.

CONCLUSIONS

Although maintaining a certain preform temperature profile, it is possible to control the overall temperature of the preform by changing the reheat temperature of the preform. In this study, it was observed that the ESCR values of the bottles and the burst strength decreased with the increasing preform reheat temperature, whereas the top-load strength increased. Thermal stability tests confirmed that high-preform reheat temperatures had a detrimental effect on the self-standing feature of the bottles. It can be said that the preform reheat temperature should be kept at as low as possible in order to ensure high ESCR and burst strength values and to prevent the concaveness at the bottom of the bottle.

Correspondence to: B. Demirel; e-mail: bilaldemirel@erciyes.edu.tr

DOI 10.1002/pen.23333

Published online in Wiley Online Library (wileyonlinelibrary.com).

[c] 2012 Society of Plastics Engineers

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(20.) B. Demirel. "Optimization of Petaloid Base Dimensions and Process Operating Conditions to Minimize Environmental Stress Cracking in Injection Stretch Blow Moulded PET Bottles," PhD Thesis, RM1T University, Australia (2008).

B. Demirel, (1) F. Daver (2)

(1) Faculty of Engineering, Department of Materials Science and Engineering, Erciyes University, Melikgazi 38039 Kayseri, Turkey 2

(2) School of Aerospace, Mechanical and Manufacturing Engineering, Bundoora, RMIT University, Victoria 3083, Australia
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Author:Demirel, B.; Daver, F.
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:7TURK
Date:Apr 1, 2013
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