Multi-layer container and method of making same

A rigid container is made by injection molding a parison having plural layers of polymers. The parison is blow molded to the final shape of the container. Control during injection is exercised over each of the plural polymers so as to produce uninterrupted layers extending throughout the walls of the parison and to insure that the interior layers are completely encapsulated within the outer layers.

BACKGROUND OF THE INVENTION 
Food product rigid containers generally must be impermeable to oxygen. Most 
common structural polymers for rigid food containers are permeable to 
oxygen which invades the food product causing degradation of spoilage. 
Those polymers which are sufficiently impermeable to oxygen generally are 
not suitable alone for rigid containers for foods because they do not 
possess adequate structural properties, are moisture sensitive, or are not 
approved for or are of questionable safety when used in contact with 
foods. Ethylene vinyl alcohol copolymer (EVOH) is a transparent extrusible 
material possessing high impermeability to oxygen when dry, many times 
less permeable than acrylonitrile copolymers, but is very moisture 
sensitive. The oxygen barrier properties of EVOH are markedly diminished 
in the presence of significant quantities of water. To be useful for food 
packaging, particularly where extended shelf life is required, EVOH must 
be kept dry as by total encapsulation within polymers which have good 
moisture barrier properties. 
Many foods are processed in the container in a pressure cooker or retort. 
Retort conditions commonly are 250.degree. F. at 30 psia steam pressure. A 
rigid container must survive retort conditions. It must not permanently 
distort during cooking or during cooling, and must not suffer an 
alteration of the desirable properties of its components. Polyolefins, 
particularly blends or copolymers of polypropylene and polyethylene, are 
well suited to manufacture of rigid containers and have adequate physical 
properties to survive retorting. Polyolefins are relatively poor oxygen 
barriers, but are relatively good moisture barriers. The use of 
polyolefins with a central core of an oxygen barrier polymer is a desired 
goal of the food packaging industry. 
Nohara et al. U.S. Pat. No. 3,882,259 discloses a three ply plastic bottle 
having a core of EVOH blended with Surlyn A brand ionomer resin and outer 
plies of polyethylene blended with Surlyn A. The Surlyn A ionomer is added 
to both the EVOH and the polyethylene resin materials to improve adhesion 
between layers. The bottle is to be made by extrusion blow molding thereby 
the three layers are simultaneously extruded to produce a three ply tube. 
While still hot from extrusion, the tube is pinched together at the bottom 
to form a seal and inflated in a blow mold having the shape of the desired 
bottle. 
Extrusion blow molding has four serious drawbacks when used to form 
multi-layer containers having a core ply of a moisture sensitive barrier 
material such as EVOH. 
First, the pinch seal at the bottom leaves the core ply of EVOH exposed on 
the bottle exterior. Since EVOH and certain other barrier materials are 
adversely affected by moisture, exposure of the core ply at the container 
bottom renders the container susceptible to loss of barrier quality by 
intrusion of moisture. The risk that the container exterior will encounter 
damp conditions in storage or transport is high and the resulting loss of 
barrier quality will degrade or spoil the food. First, retort conditions 
are such that moisture from the steam will intrude into the barrier layer 
through the exposed barrier at the bottom. 
Second, extrusion blow molding necessarily produces scrap as a result of 
the pinch sealing procedure. Since the scrap contains materials from each 
of the three layers, re-extrusion of the scrap is difficult and expensive. 
Third, the pinch seal produces a bottom of non-uniform thickness and 
strength. The sealing takes place along a line between the abutting faces 
of the inner layer material. The seal line is bordered by regions of 
relatively thick material. When stretched during blow molding, the bottom 
varies in thickness in the vicinity of the pinch seal. Because of the 
thickness variation due to the pinch seal, the stiffness of the bottom is 
not uniform along all diameters. Consequently, the bottom does not evenly 
respond to expansion and contraction as the product changes in 
temperature. This lack of even response causes unpredictable performance 
of the container when retorted. 
Fourth, the pinch seal may create an interruption in the barrier layer. If 
the inside surface layer is interposed between the barrier layer at the 
seal, a line lacking barrier material will result. The area of the 
interruption may be great enough to allow sufficient oxygen to enter to be 
a problem. 
Because of these disadvantages, extrusion blow molding cannot produce an 
entirely satisfactory three layer rigid container having a core barrier 
layer of a moisture sensitive polymer such as EVOH, particularly where the 
container is intended for retorting. 
SUMMARY OF THE PRESENT INVENTION 
The present invention is concerned with apparatus for making a plastic 
container by injection molding or by an injection blow molding technique 
which produces a container whose walls are multiple plies of different 
polymers. In particular, the container walls comprise inner and outer 
layers of structural polymers such as polyolefins or a blend of 
polyolefins on either side of a core layer of a polymer having oxygen 
barrier properties such as EVOH. 
Injection blow molding is a process whereby a preform or parison is formed 
by injection molding in a cavity. The parison is transferred to a blow 
mold cavity and blown to the shape of the desired container. The parison 
can be retained on the core pin of the injection mold and transferred on 
the core pin to the blow molding cavity. The parison can be temperature 
conditioned before blow molding to achieve an optimum temperature or 
profile of temperatures. The core pin can be temperature controlled and 
the exterior of the parison can be temperature conditioned by contact with 
air or other fluid such that blow molding occurs at optimal conditions. 
Orientation can be achieved as the parison is stretched during blow 
molding. Injection blow molding produces no scrap and requires no pinch 
seal. 
According to the present invention, polymer melts for the inside and 
outside surface layers and the core layer of the container walls are 
substantially simultaneously injected into a parison mold cavity through 
an injection nozzle having separate passages for each polymer melt 
arranged to lead to coaxial annular nozzle orifices surrounding the 
central orifice. Additional layers or layers interposed between the 
surface and core layers can also be injected simultaneously to produce a 
container wall having four or more layers. 
The initiation, rate, and termination of flow for each layer are 
independently and continuously controlled to provide control over the 
thickness of each layer and to insure that the core layer or layers are 
totally encapsulated between the surface layers. The injection molded 
parison is transferred on the core pin to a blow mold cavity having the 
shape of the container and is then blow molded into the finished 
container. Temperature conditioning of the parison just prior to blowing 
can result in biaxial orientation of the various polymers to achieve 
desirable improvements in physical properties such as impermeability, 
clarity, tensile strength, impact strength, and resistance to creep. The 
resulting product has a barrier layer or layers which extend without 
interruption throughout the container, yet are completely encapsulated 
within the material of the inside and outside surface layers. Since the 
barrier layer is protected from moisture by the moisture barrier 
properties of the surface layers, the oxygen barrier quality is preserved.

The machine of the present invention injection molds a multi-layer parison 
from a plurality of polymers, each separately plasticated and fed to 
separate injection rams. The rams each force a shot of polymer to 
appropriate nozzle passages which lead to the entrance of the injection 
mold cavity. Conditions are controlled to advance the several polymer 
melts substantially simultaneously in the die cavity under non-turbulent 
flow conditions to preserve the polymers as discrete layers in the 
parison. The following detailed description explains how the foregoing is 
accomplished. 
FIG. 1 shows a portion of the injection blow molding machine (IBM) of the 
present invention. Two core pins 10A, 10B are mounted on a transversely 
moveable plate 40 on the axially moveable platen 42 of the machine. Core 
pin 10A is positioned in an injection mold 20 while core pin 10B is 
positioned in a blow mold 30B. When plate 40 is traversed to the left, 
core pin 10A will be in blow mold 30A and core pin 10B will be in the 
injection mold 20. A parison is removed from the mold by axial retreat of 
the moveable platen 42 and the plate 40 with core pins 10 is traversed 
either left or right to the available blow mold. FIG. 1 shows blow mold 
30A ready to receive the parison and shows blow mold 30B containing a 
parison 60B. Parison 60B is inflated with air to assume the shape of blow 
molding cavity 30B while parison 60A is being injected in cavity 20. The 
blow molds open as the platen retreats to eject the finished container. 
The plate 40 shuttles back and forth each cycle so that a container is 
blown simultaneously each time a parison is injected. 
FIG. 2 shows the general layout of the injection blow molding machine and 
indicates the control means. Plasticators 82A, 82B, 82C feed three rams 
70A, 70B, 70C for three polymer melts which are fed to a manifold block 75 
which contains separate passages leading to a multi-passage nozzle 50 for 
the injection mold 20. The platen 42 is moved axially of the mold by a 
hydraulic press 44. Control circuitry means for the press and blowing 
cycles are indicated at press control block 110. A microprocessor 100 is 
programmed to control the servo hydraulics 120 which control the 
individuall injection rams and to command the press control block 110. 
FIG. 3 shows one of the plural plasticators 82B for melting and supplying 
molten polymer B to an injection ram 70B. The plasticator 82B is a 
conventional reciprocating screw device which forces molten polymer into 
the cylinder 71B of the ram when manifold valve 84B is closed and manifold 
valve 85B is opened and the ram is retreated to the left by hydraulic 
actuator 72B. When the ram cylinder 71B is charged with molten resin, 
valve 85B is closed. Upon a control signal from the microprocessor 100, 
valve 84B is opened and the servo control 120 for the ram causes the ram 
to advance to the right, according to a displacement-time schedule stored 
in the microprocessor program. A displacement transducer 76 provides an 
analog signal proportional to ram displacement to complete a feed-back 
loop for the servo 120. Polymer B forced according to the program flows 
past valve 84B through the manifold passages to the injection nozzle, 
through the nozzle passages and into the injection mold cavity where 
polymer B becomes the outside layer of a parison 60. 
FIG. 4 shows schematically the servo loop where the control signal from the 
microprocessor 100 (shown as voltage as a function of time) and a position 
signal from the displacement transducer 76 are algebraically combined in 
an amplifier 78 and the resulting signal is used to control the hydraulic 
servo 120 for the hydraulic actuator 72. A typical ram position control 
signal is shown in FIG. 5. Since displacement is measured by transducer 
76, the plot is in voltage as a function of time. 
FIG. 6 is a flow chart of the system used to control the machine. The 
injection blow molding machine is indicated as IBM on the chart. Upon 
initiation of the cycle, the program checks position of valves, rams, etc. 
and if all are proper, recharges the ram cylinders 71 from the 
plasticators 82. The IBM control circuit 110 provides an "inject" signal 
to the microprocessor 100. Injection is carried out according to the ram 
displacement-time schedule of the microprocessor and is terminated at the 
end of the schedule. An "injection complete" signal is sent to the IBM. 
The control 110 then causes the IBM to traverse to place the parison in 
the blow mold and to procede with the blow molding phase. The machine 
continues to cycle through this sequence. Keyboard 115 may be used to 
change the displacement-time schedule or to shut down the machine. 
FIG. 7 is a plot of ram displacement as a function of time for three rams. 
The positions of the rams are measured as the voltage analog output of the 
transducers 76 for each ram. The polymer for the inside surface layer is 
"A"; that for the core layer "C"; and that for the outside surface layer 
is "B". In this figure an upward slope indicates a forward motion of the 
ram to deliver polymer, a horizontal slope indicates a stopped ram, and a 
downward slope indicates a retreat of the ram. The significance of FIG. 7 
is perhaps better understood by reference to FIGS. 8-15, which show the 
flow of the polymers at the exit of the nozzle 50 and the entrance 52 of 
the injection mold cavity 20 at the rounded bottom of the parison. FIGS. 
8-15 are taken at different times in the cycle and those times are keyed 
to FIG. 7. 
FIG. 8 represents the conditions at the start of a cycle at time 0. The 
cavity 20 is empty. The entrance 52 of the cavity 20 initially contains 
only the polymers A and B for the inside and outside surface layers. The 
rams for polymers A and B begin to advance to force those polymers into 
the cavity. At about 100 milliseconds into the cycle the ram for the core 
layer, polymer C, begins to advance. FIG. 9 shows that polymer C has 
joined the flow stream in the entrance and polymer C is about to enter the 
cavity. FIG. 10, taken at about 250 milliseconds, shows the flow of the 
three polymers as the cavity continues to be filled. All three polymer 
layers must extend throughout the entire length of the parison. Since the 
flow in the mold cavity is laminar, the velocity in the middle of the 
stream is higher than the velocities at the cavity walls. Therefore, 
initiation of flow of polymer C is retarded enough (i.g., about 100 
milliseconds) so that polymer C will reach the far end of the cavity just 
as the slower moving surface layers (A and B) reach the end. In this way, 
the far end of the parison, that which becomes the mouth end of the 
container, will have all layers present in their proper positions. 
At about 1000 milliseconds into the injection cycle, the ram for polymer A 
(the inside surface layer) is stopped and the ram for polymer C (the core 
layer) can be accelerated slightly to achieve the desired thickness of 
material in the bottom of the container. Polymer A is necked down in the 
entrance 52 as is shown in FIG. 11 until it effectively is severed as 
shown in FIG. 12. At 110 milliseconds the ram for polymer C is stopped and 
the ram for polymer A is restarted. FIGS. 13 and 14 show polymer A 
advancing to pinch off polymer C in the entrance, thereby pushing the last 
of polymer C into the cavity 20 with polymer A to bury or encapsulate to 
isolate polymer C from exposure at the surface of the parison. FIG. 15 
shows polymer A knit to polymer B at the entrance to complete the 
encapsulation of polymer C and to return to the conditions at the start as 
shown in FIG. 8. At the time of FIG. 15 (1300 milliseconds) all three rams 
are retreated to depressurize to cavity to prevent expansion of the 
parison when the cavity is opened and to depressurize the polymers 
remaining in the nozzle and entrance to prevent exudation from the nozzle 
while the cavity is open. This exudation leads to premature flow of 
polymers into the cavity during the next cycle which can lead to smearing 
of polymer C on the surfaces of the container. 
1500 milliseconds marks the end of the injection phase of the container 
cycle for this example. Subsequent to the end of the injection phase of 
the cycle, manifold valves 84, 85 are actuated and the ram cylinders 71 
are recharged with their polymers by the plasticators 82. The injection 
mold is opened by retreating the hydraulic press 44 to withdraw the core 
pin 10 from the cavity 20. The parison just formed is transferred to one 
of the blow mold cavities 30A, 30B and the container which was blow molded 
simultaneously with the injection cycle is ejected from the blow mold in 
which it was finished. 
FIG. 16 shows a nozzle 50 appropriate for injection of a parison having a 
three layer wall. Polymer B, which forms the outside surface layer, is 
delivered by the ram 70B to an annular distribution channel 54B which 
distributes the polymer circumferentially of the nozzle structure. Polymer 
B advances along a conical passage 56B to an annular orifice 58B at the 
exit of the nozzle which leads to the injection cavity. Similarly, polymer 
C, which forms the core layer, is delivered by ram 70C to annular 
distribution channel 54C and thence along conical passage 56C to annular 
orifice 58C. Polymer A, which forms the inside surface layer, is delivered 
by the ram 70A to a passage 56A which exits at the center of the 
concentric flows issuing from orifices 58B and 58C. A nozzle shut off 
valve 59 can be moved axially to arrest flow of polymer A. 
FIGS. 17 and 18 compare the pinion 60 as injection molded with the finished 
container. The neck portion 62 remains virtually unchanged during blow 
molding. The parison is held by the chilled neck portion while the hot and 
soft parison is blown. Thus, the neck 62 including the flange 64 is 
essentially formed in the injection mold. The remainder of the parison 
walls are thinned as the parison is stretched during blow molding. 
FIG. 18 shows that the core layer C extends throughout the flange 64, but 
does not penetrate the flange edge. This is accomplished in large part by 
selection of the delay time in starting the ram for the core polymer. The 
flange 64 will be employed in a double seam seal when a metal end is 
crimped, by well known techniques, onto the container mouth to close the 
filled container. Since the flange represents a significant area, it is 
important that the core layer extend well into the flange. The programmed 
flows of the various polymers also ensure that the core layer is not 
exposed at the sprue mark at the central exterior of the container. 
FIG. 19 is an enlargement of the container wall within the circle of FIG. 
18. Layer A is the inside surface layer formed from polymer A in the 
foregoing description. Layer B is the outside surface layer, formed from 
polymer B. Layer C is the core or barrier layer formed from polymer C. The 
thinnest layer is the relatively expensive barrier polymer C. The relative 
thickness of the three layers is controlled by controlling the relative 
flow rates of the three polymers by microprocessor control of the 
displacement rates of the rams. A preferred wall structure is a layer of a 
blend of high density polyethylene and polypropylene on each face of a 
core carrier layer of ethylene vinyl alcohol copolymer (EVOH). 
FIG. 20 shows how the oxygen barrier quality of EVOH decreases abruptly at 
high levels of moisture. Where the EVOH layer is thin, only a small 
quantity of water will cause a large increase in oxygen permeability. For 
this reason, the EVOH layer must adequately be protected against the 
intrusion of moisture. 
Polyolefins do not adhere well to EVOH. Adhesion can be improved by adding 
adhesion promotors to the polyolefin, the EVOH or both. Another approach 
is to provide an intermediate layer of an adherent polymeric material 
which adheres to the polyolefin and the EVOH. Such materials include 
manifold polyolefins sold under the name Plexar by the Chemplex Company of 
Rolling Meadows, Ill. These comprise a blend of a polyolefin and a graft 
copolymer of high density polyethylene and an unsaturated fused ring 
carboxylic acid anhydride. The polyolefin component of the blend can be 
polyethylene or preferably is an olefin copolymer such as ethylene vinyl 
acetate. Schroeder application Ser. No. 059,375 filed Dec. 28, 1978 
teaches the use of these materials to bond to EVOH. The materials 
themselves are disclosed in U.S. Pat. Nos. 4,087,587 and 4,087,588. We 
have found these modified polyolefins to be suitable as interlayers to 
improve adhesion between the polyolefin surface layers and the EVOH core 
layer. 
Another suitable material for use as a interlayer to improve adhesion 
between the EVOH polyolefins are maleic anhydride grafted polyolefins sold 
under the name Admer by Mitsui Petrochemical Industries of Tokyo, Japan. 
The use of interlayers on each side of the EVOH oxygen barrier layers 
results in a five layer container. To produce such a container, the three 
passage nozzle of FIG. 16 is placed with a five passage nozzle of similar 
construction. Where the inside and outside surface layers are of the same 
polymer one ram can be used for both those layers. The flow from that ram 
is divided and proportioned with part supplying the central axial 
passageway to form the inside surface layer and the balance supplying the 
outermost nozzle annular orifice. The two additional nozzle orifices are 
located just inside and just outside the nozzle orifice for the EVOH 
barrier layer. The two additional annular nozzle orifices can be supplied 
with the interlayer polymer from a single ram, the flow being divided and 
proportioned. Thus, a three ram machine can produce a five layer parison. 
Greater control can be exercised over the polymer flows by using a machine 
with an independently controllable ram for each layer. A nozzle shut off 
valve can be employed to selectively control the polymer flows. The three 
layers of interlayer polymer and the barrier polymer can be treated as a 
single core layer. A five layer wall is shown in FIG. 21 wherein layers A 
and B are the inside and outside surface layers of polyolefin, layer C is 
the barrier layer of EVOH, and two layers D are the interlayer material. 
EXAMPLE I 
Five layer containers having a capacity of about 51/2 ounces, of 
202.times.307 size, weighing about 11 g were made using a five orifice 
nozzle on a three ram machine. The inside and outside surface layers were 
polypropylenepolyethylene block copolymer (Hercules Profax 7631). The 
adhesive interlayers were ethylene vinyl acetate copolymer blended with a 
graft copolymer of high density polyethylene and a fused ring carboxylic 
acid anhydride (Plexar 1615-2). The oxygen barrier was EVOH (Kuraray EVAL 
EP-F, available from Kuraray Co. Ltd., Osaka, Japan). The layers were well 
adhered. The barrier extended to the flange lip and was completely 
encapsulated. 
EXAMPLE II 
Five layer containers similar to those of Example I were made wherein the 
inside and outside surface layers were polypropylene (EXXON E612); the 
interlayer material was Plexar III, a blend of ethylene vinyl acetate 
copolymer and a graft copolymer; and the barrier was EVAL EP-F. The layers 
were well adhered. The barrier layer extended to the lip of the flange and 
was completely encapsulated. 
EXAMPLE III 
Five layer containers similar to those of Example I were made wherein the 
inside and outside surface layers were a 50--50 blend of polypropylene 
(EXXON E612) and high density polyethylene (Chemplex 5701); the interlayer 
material was Plexar III; and the barrier layer was EVAL EP-F. The layers 
were well adhered. The barrier layer extended to the lip of the flange and 
was completely encapsulated. 
EXAMPLE IV 
Five layer containers similar to those of Example I were made wherein the 
inside and outside surface layers were a copolymer of propylene and 
ethylene (Hercules Profax 7631); the interlayer material was maleic 
anhydride grafted polyolefin (Mitsui Admer QB 530); and the barrier layer 
was EVAL EP-F. The layers were well adhered. The barrier layer extended to 
the lip of the flange and was completely encapsulated. 
In the making of the containers of Examples I-IV the injection schedule 
began feeding the inside and outside surface layer polymer then the 
polymer for the adhesive interlayer was started and substantially 
simultaneously the barrier layer polymer was started. The flows of the 
adhesive interlayer polymer and the barrier layer polymer were terminated 
before the outside surface layer polymer flow was terminated.