CA1087827A - Preparation of molecularly oriented containers using reheat process - Google Patents

Abstract

APPLICATION FOR
LETTERS PATENT
FOR

PREPARATION OF MOLECULARLY ORIENTED
CONTAINERS USING REHEAT PROCESS

ABSTRACT OF THE DISCLOSURE

Molecularly oriented containers are formed from molded preforms made of an amorphous thermoplastic resin and having a finish neck tubular portion and a body portion which method includes subjecting the preform body to a thermoforming orienting step while at a temperature at which substantial molecular orientation occurs, said temperature being arrived at by first overheating the outside of said preform body using external heating means to an average outside temperature greater than the substantial molecular orientation temperature range thereby creating a temperature gradient across the thickness of said preform body and then cooling said heated preform body until the average outside temperature is within said substantial molecular orientation temperature range and the entire preform body is at a temperature within said sub-stantial molecular orientation temperature range and whereas said temperature gradient is significantly reduced.

Inventor: Thomas J. Stolki

Description

Ca o c L3 5~ ~ 7 ~7 .
PREPARATION OF MOLl~CUI.ARLY ORII~NTE~D
CONTAINEL~S USING REI-IEAT PROCESS
BACKGROUND OF THE INVENTION
This invention relates to an improved ~ethod for heating molded prffaforms to a temperature at which molecularly oriented containers can be produced by subjecting said heated preforrrls to a therrr~oor~ning orienting 5 stfff~fp.
It is known to molecularly orient thermoplastics in systems wherein such materials are being blow molded into hol3ow articles such as containers. Such molecular orientation is highly desirable, when the thermo-plastic is of such a nature that orientation can be developed therein, since ~0 it can represent an attractive route toward improving the strength properties of the finished container. This feature is particularly important when the ~;~ formed containers are to be used for packaging pressuri~ed liquids such as carbonated beverages and beer as well as other products which require low `~ `
permeability and high impact resistance as characteristics of the container.
A system is disclosed in Reilly et al, U.S. Patent 3, 754, 851 for .. , ~ . ' ~ .~
blcffwing articles from molded preforms which are brought to orientation temperature iD an intermediate conditioning step. In this approach heat is removed from the preform during conditioning and such has become known in the art as a "cool-down" process.~ It is likewise known to add heat to 20 preforms to bring them up to orientation temperature prior to finish forming `f as tlrpically disclosed in Gilbert, U.S. Patent 3, 787, 170 and such technique has become known in the art as a ~'reheat" process.
Although it has gcnerally befffen considercd less difficult to achieve orientation on a hoating cycle (reheat procesfs) than on a cooling cycle (cool-~ 25 down procf?ss), the reheat process i8 by no means a sim~fle procedure ,~'''' ~':
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~ase L3$4 10~'78Z7 to ca~ry o7ut, particularly in a commercial operation. Somc factors which aIfect the process are thc preform thickness, thickl~c~R variation~ within a preform (partic7ularly significant in extrusion blown preforms), heating time and thc thermal conductivity of thc preform material. Additionally compli-5 cating the matter are problems associated with forming containers fromelongated tubular preorms having one end closed and made of thermoplastic material. It is extremely difficult to heat such preforms with any degree of uniformity because of the particularly low thermal conductivity of the materials and also because of the difficulty of applying heat to the inside of 10 6aid preforms. Also, measuring the temperature of the preform both on its inside surface and through its thickness is quite difficult thereby hampering the ability to achieve thF conditions necessary to obtain containers having the ` desired properties.
In order to overcome some of the problems noted above, a variety :, .
~`l 15 of processes and apparatus have been designed specifically or the reheat process. For example, Steingiser, U.S. Patent 3,830,893 discloses a ~7 ~ ~ method for rapidly heating nitrile preforms to orientat1on temperature using n~7icrowave energy; Seefluth, U.S. Patent 3,445, 096 discloses a process and apparatus for heating tubular thermoplastic parisons to orientation temperature 20 by alternately passing the parisons between a heating zone and a constant temperature zone maintained at a temperature just below the melting point of the parisons to distribute heat evenly throughout; and Gilbert, U. S. Patent 3,715,109 who discloses apparatus for rapidly heating pFeforms to orientation 25 tempcrature by applying heat to said preforms both extcrnally and internally.~; :
Despite the attention given to the problem of reheating worlcpieces to the ,desired orientation temperature as noted above, there still is the need ::
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C~IBC L354 8 ~'7 ~or a simplificcl tcchniquc which will allow for the prcduction of rnolecularly oricnted containcr3 using the reheat system and whlch i~ suitable for a continuous commercial operation.
SIJMMARY OF THE INVENTION
Now, in accordance with this invention, there has been developed , an improved and simpliied method for eficiently reheating preforms to a ~' '~ molecular orientation temperature in preparation for the ormation of ;~ . multiaxially oriented containers.
Accordingly, a principal object of this' invention is to provide an improved method for preparing molecularly oriented containers using the .
, ~' rehéat system, ~, Another object is to heat preforms made of an amorphous thermo-'" plastic resin to a temperature range at which substantial molecular ,,1 ' orientation occurs using a two step reheating process.
~',, 15 ~other object of this invention is to prepare oriented containers `' made' of high nitrile polymers by subjecting preforms made of such materials '-' to a thermoforming orienting step while at a molecular orientation temperature . j .
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f reached by first heating s~id preforms using external heating means to an average outside temperature above the substantial molecular orientation 20 temperature range and cooling until the average outside temperature is within ,`
aid range thereby reducing the temperature gradient across said preform thickness and bringing the entir'e preor ,m temperature within said range.
','',' ~Anothcr object o this invention is to provide a simpliied method ' for preparing oriented bottles on a continuous basis from high nitrile polymers 25 by axially stretching and exp~nding preforms o~ such polymers at an orienta-tion temperaturc reached by a two step reheatin~ process, .~. ..
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Othcr objccts of this invcntion will in part be Qbvious and will in part ~ppear herein~fter.
These and other objects are accomplished by providing a method o iorming a ~nolccularly oricnted container from a molded preform macle of 5 an amorphous thermoplastic resin and having a finished neck portion and a body portion, which method includes subjecting the preforrn body to a thermo-;- forming orienting step while iIl a temperature range at ~vhich substantial ~: molecular orientation occurs~ said orientation temperature being arrived at by ~irst overheating said preform body using e~eternal heating means to an 10 average outside temperature greater than the su~stantial molecular orien-`~ tation range thereby creating a t~mperature gradient across the thickness ~ of said preform body and then cooling said heated preform body until the :.
average outside temperature is within said substantial molecular orientation temperature range and the entire preforrn body temperature is within said 15 substantial molecular orientation temperature range and wherein said '`! , - temperature gradient is significantly reduced.
, BRIEF DESCRIPTION OF THE DRAWINGS
~- In describing the overall invention, reference will be made to the accompanying drawings wherein: `
Fig. 1 i9 a perspective view with a portion broken away of a typical molded prefor~n shape for use in.the present invention;
Fig. 2 is a schematic diagram illustrating the passage of preforms through heating apparatus use~ul in carrying out the mcthod of this invention;
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t~ig. 3 i8 a schematic view showing one form of heating systen~
useful in the method of this invcntion;
Fig. 4 i8 a schematic diagram illustrating the oricnting and containcr forming stcps of thc invention;
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.~ ,.i Ca~c L354 .lIL~8'71~7 Fi~. 5 is a graph illu~tratin~ the r~aults in ternns of thc time-tempcrature relationship of a typical run usin~ the reheating procedure of this invcntion.
ETAILED Dl~SCRIPTION OF THIS INVENTION
In carrying out the method of this invention~ an elongated ~ubular preform made oi an amorphous thermoplastic material nd having a general ~ -configuration of the type shown in Fig. 1 is molecularly oriented for example by axially stretching and radially expanding while at a ternperature at which ~;ubstantial molecular orientation occurs using a stretch blow mold station of the type illustrated in Fig. 4.
Generally speaking molecular orientation cf an orientable thermo~
, plastic preform Inay occur over a temperature range varying from just above the glass transition temperature (that temperature or narrow temperatu~e ., .
range below which the polymer is in a glassy state) up to the melt temperatuFe 15 of the polymer. However, as a practical matter the formation of oriented containers is success~ully carried out when the preform is at a much narrower temperature range defined as the substantial molecular orientation temperature range~ This is best illustrated by reference to Table I which ! ., shows results obtained when preparing or attempting to prepare bottles over . ,.
20 a wide temperature range. The birefringence stress values as noted in the results sho~vn in Table I are used as an ind;cation of the relative .. . .
orientation properties of thF prepared b~ttles. Such birefringence stress values were determined from the measurements of birefringence for the : 1 `
~; ~ prepared bottlcs and using the technique and formulas for stress as defi~ed ~ -25 by D. C. Drucker in "Photoelastic Separation of Principal Stresses by Oblique Incidcnce", Journal of Applied Mechanics, Sept. 1943, pp. A-1S6 :
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to A-160. Thc strcss-optical cocficient as used in this tcchniqu~ was de~crmincd usîng a curvc of orientatiorl rclease stress (ORS) vs bire~ringcr~ce ~or the material with ORS arrived at by thc ASTM D 1504-70 mcthod.
As Table I indicatcs, when thc preform ternperature is slightly S above the glass transition temperature i. e. 232F,, bulbs rather than bottles were formed. When the preform temperature ~as increased to the - 240 to 250 F. rallge, bottles were prepared having good orientation properties, expressed as birefringence stress, however, they were formed with low quality yields and s~ther disadvantageous characteristics. This 10 result was undoubtedly due to the difficulty to stretoh and process the preforms in this temperature range. As the temperature is increased the processibility ~`
improves greatly and quality yields also improve, however, the orientation properties begin to fall off. It thus becomes necessary to balance the needs îor processibility and quality with orientation results and this is called the 15 temperature range at which substantial molecular orientation occurs.
; In other words the substantial molecular orientation temperature range as used throughout the specification and claims is defined as the temperature lange at which preLorms must be at to effectively and conveniently form oriented containers having suitable orientation properties such as by axially ` 20 ~tretching and radially expanding said preforms. Thus, by forming oriented containers while in the substantial molecular orientation temperature range, ,; .
proees~ and quality probIems related to the need for excessively high forces to stretch the preform at lower temperatures or the rapid relaxation of ~tresses on stretching at higher temperatures are avoided. More :", `~ 25 particularly, the temperature range at which substantial molccular :' :
orientation occurs for thermoplastic amorphous matcrials will vary from about 20 tc- about 60 F~ and preferably Irom about 30 to about 55 F. above :
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Caso L35'1 ~Q~37~ 7 T~BL,E I
Preparation of bottlcs by stretch/~lowing a preforln made o acrylonitrilc/styrcnc (70/30 wt. ratio)h~inga g1ass tr~nsition tcmperature of about 2 3 0 F .
. S ~reform Temper~ture Results 212 F. Stretch rod stopped on contact with prefoxn~
232 F. 8" bulbs formed - large areas of whitened pc~lyme r 240-250 F. Poor bottles formed having stretch lines and 10 (outside/inside gradient) whitening Low quality yi~ld Bire~ringence stress 494/326 psi ; ~50-270 F. Quality yield approximately 50 percent Birefringence stress 450/280 psi 260-275 F. Few quality defects Birefringence stress 420/290 psi - Convenient processability 290-310 F. Excellent processability Birefringence stress 200/160 psi .. . .

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Th~3 method o this invcntion irlvolves thc tc~hnique wherein amorphous thcrmoplastic preforms which have bcen ~ormed earlier and have cooled down are reheated to thc temperature at which substantial molccular orientation will occur. Reference to the graph of Fig. S will show how a 5 typical operation will be carried out. The graph shows the comp~terized res~lts of a heat transfer study made on a simulated run using selected preform material, size and shape and selected heating and cooling conditions.
The graph shows temperature ~s. time with 0 representir~g the outside surface and I the inside surface of a preform having the shape as shown in 10 Fig. 1. As the preform is heated using external heating~ means, the outside surface will be at a higher temperature (0) than the inside surface (temper-ature I) and a temperature gradient across the thickness of the preform will result ~n~te intermediate points between 0 and I). The temperature gradient results from heat being applied externally as well as the low thermal 15 conductivity of thermoplastic materials. At the end of the heat up period, shown by vertical line H, the temperature gradient across the preform will ..
be at its greatest but such gradient will be reduced and equalize itself some-,:
- what during the ensuing cool down period. Lines Tl and T2 represent the temperature range within which substantial molecular orientation occ~rs.
20 By heating the outside surfacc of the preform to a temperature above the 6ubstantial molecular orientation temperature range, a temperature gradient To~TI results. Upon cooling the prefor~ until the outside temperature is within said substantial molecular orientation temperature range, the .
ternperature gradient is significantly reduced. Also the overall preform 25, or the entire preform body reaches a temperature equilibrium or distribution which is within this range for a significant period of time. During thic - ¢xtended period whcn the entire preorm body is at a temperature within the ; ' ' ,/' . ,~ .. 9 _ Ca~o L354 101~78~7 ~ubstantial rnolccular orientation tcmpcraturc ran~c, molecularly oricnted contalners can desirably bo formed using a stretch blow mold of the type illustrated in Fig. 4. The term overall or entire preorm body as used throughout the specification and claims means all or es~entially all of the 5 body portion of the preform.
As noted in the simulated run of Fig. 5, during cool down the outside preform teniperature drops below the inside p~eform temperature and remains that way for an extended period. Under certain conditions the outside temperature may remain higher than the inside and select 10 conditions may even provide an equalization of temperature for an extended : . .
period. The important feature of this reheat method is that the entire preform body has a reasonably small temperature gradient or distribution and is within a temperature range which coincides with the substantial molecular orientation temperature range over an extended period of time 15Fig. 1 illustrates a typical elongated tubular preform 10 used to prepare containers such as bottles in accordance with the method of this inven~ion. Preform 10 comprises a body portion 12 and a ~inish or neck ~ -portion 14 and has an open end 18 and a closed end 16. Outside surface 20 of said preform i9 directly exposed to the external heating means whereas 20 inside surface 22 is not exposed dlrectly to the ~xternal heating means and becomes heated throug~ thermal conduction. Though the wall thickness and .;
weight of the preform 10 may vary widely, it generally has relatively thick walls along body 12 typically ranging from about 120 to about 260 mils, and typically weighing from about 15 to about 110 grams. The preform 10 may 25 be ormed by conventional means such as injection or blow molding and will ., ~
generally comprise any amorphous thermoplastic material such a~ poly-styrene, polyvinyl chloride and nitrile containin~ polymers. Particularly "
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Ca~c L354 u~eful materials or prcparing containers u~ing th~ n~ethocl of this invention arc nitrile polymcrs containing from about 55% to about 85% by weight of a nitrile monomer unit, based on the total polymer weightJ wherein the weight perccnt of nitrile monomcr is calculated as acrylonitrile. The nitrile 5 monomers include acrylonitrile, methacrylonitrile, ethacrylonitxiIe, propacrylonitrile, glutaronitrile, methyleneglutaronitrile, furnaronitrile, ~- etc., as well as mixtures of these monomers. The preferred monomers which are interpolymerized with the nitrile monomers include aromatic:
monomers such as styrene and alpha methlstyrene; lower alpha olefins ~0 containing 2 to 6 carbon atoms such as ethylene, propylene, butylene, isobutylene, etc.; acrylic acid and methacrylic acid and the corresponding acrylate and methacrylate esters containing 1 to 4 carbon atoms such as ` methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate and the corresponding methacrylates; vinyl esters such as vinyl acetate; alkyl :
~ 15 vinyl ethers wherein the alkyl group contains from 1 to 4 carbon a~oms such "
as ~nethyl vinyl ether, ethyl vinyl ether, etc., and mixtures of the foregoing.
Optionally, the high nitrile packaging materials may contain from 0 to about 25% by weight of a synthetic or natural rubber component ~; such as polybutadiene, isoprene, neoprene, nitrile rubbers, acrylate rubbers, 20 natural rubbers, acrylonitrile-butadiene copolymers, ethylen~-propylene copolymers, chlorinated xubbers, etc. ,~which is used to strengthen or toughen the high nitrile packaging materials, This rubbery component may , be incorporated into the polymeric packaging material by any of the methods which are well kno~n to those fikilled in the art, e. g., direct polymerization .~,j . .
`;~ 25 of monomers, grating the nitrile monomer onto the rubbery backbone, .! . polyblend of a rubber graft polymer with a matrix polyme~, etc.

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Cafio L3$4 8'~'7 Tllc preferrcd nitrile E~olymcrs or those packa~ing applications requiring exccllent oxy~cn and water vapor barrier proporties in the packaging m~tcrials, arc those nitril~ polymers containing frorn about 55 to about 85% by weight, based on the total polymer wei6h~, of an 5' acrylonitrile and/or methacrylonitrile monomer (wherein the weight percent c~ rnethacrylonitrile is calculated as acxylonitrile). When acrylonitrile is used as the sole nitrile monomer the preferred range is ~rom about 60 to about 83% by weight whereas with methacrylonitrile the preferred range is ~Erom about 70 to about 98% by weight of methacrylonitrile which corresponds ~- 10 to about 55 to about 78% by weight of nitrile monomer calsulated as acrylonitrile. The pxeferred comonomers are styrene and alpha methyl , ~
styrene, Also preferred are interpolymers such as acrylonitrile/meth-acrylonitrile/styrene; acrylonitrile/styrene/methyl vinyl ether and acrylonitrile/styrene/ethyl vinyl ether.
", 15 The heating of the cold preforms in accordance with the method,,: :
of this invention may ~be carried out by passing said preforms throug}~ a ' heating oven containing conventional external heating means such as radiant ~' heaters or forced hot air. Apparatus for carrying out the method is illustrated in Fig; 2 where preforms 10 are shown passing through oven ` ~;
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" 2,0 or enclosed unit 24. The preforms 10 are manually or automatically placed ~ neck down in holders 30 and conveyed in one or more passes through the ,,:, ~ s~ ^ , '^,1 oven 24 on an endless chain. In a manner not shown (except very generally ~;

in Fig. 3), the preform neck portion 14 (Fig. 1) is either placed within the ~, holdcr or othcrwise masked as it passes throu~h the oven 90 that only the , 25 body portion 12 is exposed to the heatin~ means. Neck portion 14 has been , ' , accurately inished and ormed to close tolerancc~ and is not intended to be ~urther modified Ol altered at thi~ time. The prcforms 10 are conveyed . .

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~ '713Z'7 ~hrough unit 2~ entcring at inlet si(le 32 and discharging through exit 34.
Thc unit 24 iAS shown consists o~ two sections with 26 being the area where the heating takes place and 28 bein~ the cool down area To get more uniIorm hei~t distribution on the outside surface of the preforms, they may S be rotated in a conventional manner such as by provid;ng means to rotate the preform holders 30.
As the preforms lû pass through the heating section 26, the t>utside surface is heated using external heating means ~mtil the average ; temperature is greater than the ten perature range at which substantial 10 molecular orientation occurs~ Heat may be applied using banks of infrared heaters which surround the preform as illustrated by heaters 36, 38 and 40 - in Fig. 3. As a practical upper limit, the outside preform temperature will .. .
not reach the point where sag, lean or distortion of the preform material will occur or where foaming will result due to the presence of small amounts ; ;` 15 of moisture. More particularly the outside surface of the preform is heated :, . .
; until it reaches a temperature of about 65 to about 100 F. greater than its glass transition temperature (Tg) and preferably irom about 70 to about 90 F. greater than Tg. $ince the preforms are heated externally and also . .
because of the low thermal conductivity of the amorphous thermoplastic 20 materials being used, the outside surface of the preform will be at an . ; , . .
; average temperature significantly greater than that of the inside surface, . ~ . .
i. e. a temperature gradient wiIl exist as shown at H in Fig. 5 as it leaves the heating section 25, The heater temperatures are not particularly ;, clltical and may be varied depending on such factors as the position within 25 the unit, the speed at which the preforms ar~ conveyed and the preform `;. :
material. Temperature measuring means (not shown) may be provided to measure the outside preform temperature ater it lcavcs the heating section - and such information can be used to adjust and control the oven conditions ~o that thc deslred tompcratllre can be reached, ; ~
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1a~'~3'~7 The l~cated prc~orm is thcn allowed to cool eithcr in air or by pas8in~ throuE~h a second cnclosurc which may be joined with the heating ~ection 6uch as section 28 of Fig. 2, The temperature in this cool down section is significantly lower tban the oven heating section and pref~rms 5 are slowly passed thl:rethrough until the outside average temperature drops to within the substantial molecular orientation temperature range. At the ~ame time, as illustrat~d by Fig. 5, the temperature gradient across the .
preform thickness is being signiflcantly reduced and the temperature or temperature distribution of the entire preform body tends to equalize or 10 equilibrate within the substantial molecular orientation temperature range : In this operatîon, the outside average temperature of the preform is generally reduced to from about 20 to about 60 F. greater than Tg and preferably from about 30 to about 55 F. greater than Tg. The temperature . .
in this cool down section can vary widely depending on conditions such as ~
j 15 the speed at which the preforms are conveyed, the thickness of such preforms ;
and the material thereof. General1y this temperature will be varied from oom or ambient temperature up to the molecular orientation ~emperature.
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~ Besides reducin~ the temperature gradient and allowing for more .. . .
uniform temperatures throughout the preform tbickness9 this two step .;,, .
20 reheating procedure has another significànt advantage~ That is the ability :: .
to compensate for unintentional thickness variations resulting in the preform ~;
during processing, partlcularly in blow molded preforn~s. Thus, in the ~1 extended heat up process thinner spots will overheat, but conversely in the cool down process they will cool down faster thus allowing them to be lower ~",',!, 2S in temperature a3 desired but still within the substantial molecular orientation tempcrature range when the overall operation is complete.
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Caso L35~1 ~'7~3Z7 The hcatcd and coolcd (lown preform 10 i9 formed into an oricntcd container by enclosing it within partible sections 42 and 44 o blow mold 46, as shown for example in Fig. 4, while the tcmperature is within the substantial molecular orientation temperature range in order to 5 complete the operation by carrying out a thermoforming orienting step.
- The term thermoforming as used in the specification and claims is intended to include all types of molding including blow molding as well as the axially ~tretching and radially expanding operation illustrated in the apparatus of Fig. 4. As shown i~ Fig. 4, a ~tretching mechanism 48 is moved into place 10 over the end opening in the closed mold and xod member 50 is caused to " ~ move downwardly within and against base 16 of each preform (shown in ., .
phantom in its initial position within the mold) to axially stretch the body ~` portion of the preforrn against the lower wall 52 of the mold 46, Simul-. .
taneously therewith or irnmediately thereafter, an expanding medium 15 issuing from orifices 54 in rod 50 is admitted into the interior of the preform to blow it outwardly against the cavity walls of the mold sections to form the container whch, in the illustrated embodiment is a bottle 56.
The particular advantages of the method of this invention include , .
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the ability to obtain oriented containers from preforms using the reheat 20 method in a simplified operation in high ~ield and quality and with improved uniformity in thicknes6.
~ larious modifications and alteratio~s of the invention ~vill be readily suggested to persons skilled in the art. For example, varying conditions in the heating and cool down procedures can be obtained as .
25 desired to allow for different materials and different thic:knesses e. g. higher or lower heating and cooling temperatures and different rates of speed , ~
through eithcr section. Additionally, temperaturc sensing and control :1 . .
means may bc incorporated in thc operation to aid in reaching and mai~tainin~

- tho dcsired oondition~.
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Cawc L354 ~'78Z7 Thc Iollowin~ cx~mplcs are ~ivcn to illu~itrate more clcarly the p~inciples and pri~ctice o this invention and should not be construed as limitations thercof.
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EXl~MPLE I ~ -A series of 35 gram blow molded preforrns shaped as illustrated in Fig. 1 and made of a polymer comprisiing a 70/30 mixture by weight of polymerized acrylonitrile/sty~ene monomer and havin~ wall thickness of a.bout 150 mils ~ 5 at an ambient temperature of about 75 F. were inserted ~-. ' .~n holders mounted on a conveyor and passed through a heating oven which con-10 tained two banks of infrared heaters on opposite sides, each bank being 35in~hes ` long and consisting of eight horizontal heater strips varying in temperature from about 890 to about 1000 F. The ambient oven temperature as measured by a thermometer on top of the unit was 365 F. and the preforms were conveyed through said oven in a period o 2. 5 minutes. The temperature .,, :. .
~ ij 15 of the preform was rnonitored at its outside surface (5 inches from the base . . .
;~ of the neck) using a Williamson infrared recording instrun;sent as it left the ~ :.! o - ~ ~ ovesl with average temperature being in the 300 to 310 F. range. The preforms were then allowed to cool in air for about 2. 5 minutes until the o ~ ~:
outside surface temperature of the preform was at 270 F. as recorded ;
20 by the Williamson instrument. The preform was then immediately removed from its holder and enclosed within a blow mold in a manner illustrated ~n Fig. 4, axially stretched and expanded outwardly within the mold to form a container in the form o a bottle. Over two-thirds of the bottles formed had good material distribution and in no cases were holes or blow outs .;
`; 25 found.
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ISXAMPLI~ II
For the purposes of ~:omparison, another series of prcforms having the same coniguration as thosc of Example I were passed through an oven having the same characteristics as in Example I except the heatcr strips S were at a temperature of from about 770 F. to about 890 F. and ambient oven temperature was 335 F. The outside surface of ~h~ preforms were .. measured at temperatures averaging about 270 F. shortly after leaving the oven and were placed in a blow mold and stretched and expanded as described isl Example I. Blow through holes were obtained in a significant number of 10 preforms t~ 20%) and less than half of the preforms were blown into bottles . . having good material distribution.
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Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In the method of forming a molecularly oriented con-tainer from a molded preform made of an amorphous thermoplastic resin and having a finished neck tubular portion and a body portion which method includes subjecting the preform body to a thermoforming orienting step while in a temperature range at which substantial molecular orientation occurs, the improvement which comprises heating said preform body to said substantial molecular orientation temperature range by first overheating the outside of said preform body using external heating means to an average outside temperature greater than said substan-tial molecular orientation temperature range thereby creating a temperature gradient across the wall thickness of said pre-form body and then cooling said heated preform body until the average outside temperature is within said substantial molecular orientation temperature range and the entire preform body is at a temperature within said substantial molecular orientation temperature range and wherein said temperature gradient is significantly reduced.

2. The method of claim 1 wherein said thermoplastic resin is a nitrile polymer containing 55 to 85% by weight of nitrile monomer units based on the total polymer weight.

3. The method of claim 2 wherein the nitrile polymer is acrylonitrile.

4. The method of claim 3 wherein said acrylonitrile polymer contains a styrene comonomer.

5. The method of claim 4 wherein said preforms are formed by blow molding.

6. In the method of forming a molecularly oriented con-tainer from a molded preform having a finished neck tubular portion and a body portion and is made of an amorphous thermo-plastic resin which has a glass transition temperature Tg, said method including axially stretching and radially expanding said preform body while in a temperature range at which sub-stantial molecular orientation occurs, the improvement which comprises heating said molded preform body to said substantial molecular orientation temperature range by first overheating said preform body using external heating means to an average outside temperature of from about 65 to about 100°F. greater than Tg thereby creating a temperature gradient across the thickness of said preform and then cooling said heated preform body until the average outside temperature is from about 20 to about 60°F. greater than Tg and the entire preform body is at a temperature within said substantial molecular orienta-tion temperature range and wherein said temperature gradient is significantly reduced.

7. The method of claim 6 wherein said thermoplastic resin is a nitrile polymer containing 55 to 85% by weight of nitrile polymer units based on the total polymer weight.

8. The method of claim 7 wherein the nitrile polymer is acrylonitrile.

9. The method of claim 8 wherein said acrylonitrile polymer contains a styrene comonomer.

10. The method of claim 9 wherein said preforms are formed by blow molding.

11. The method of claim 9 wherein said container is a bottle and wherein said preform body is overheated to an average outside temperature of from about 70 to about 90°F.
greater than Tg and said heated preform body is cooled to an average outside temperature of from about 30 to about 55°F.
greater than Tg.