Note: Descriptions are shown in the official language in which they were submitted.
lQ833~L3
This invention relates to an apparatus for carefully
controlling the distribution of plastic material in the manufac-
ture of biaxially oriented plastic containers, and more particu-
larly relates to a blow molding apparatus which allows for uni-
~orm stretch rate and careful temperature control which permits
the production of biaxially oriented plastic containers having
uniform wall thickness and good clarity.
It is an object of this invention to provide an appara-
tus which produces plastic containers having a high degree of
10 orientation.
It is also an object to provide biaxially oriented plas- `
tic containers having improved performance properties by utiliza-
tion of lower orientation temperatures.
Another object is the provision of plastic containers
having improved plastic material distribution in the base thereof.
Another object is the provision of a plastics molding
apparatus which gives higher axial stretch ratios because of
carefully controlled stretch rates. The accomplishment of these
and other objects will become apparent from the following.
Thus, in accordance with the present teachings, an
improvement is provided in a blow molding apparatus which is use-
ful for the production of biaxially oriented hollow thermoplastic -
containers from thermoplastic preforms. The apparatus is com-
posed of a hollow mold and means being provided for positioning
the thermoplastic preform within the cavity of the mold with
means for preheating the preform to th~ orientation temperature
with means for stretching and expanding the preform against the
walls of the mold cavity. The improvement which is provided
comprises a stretch-blow mandrel which has an axis, orifices for
30 fluid inlet and movable blow-pin with tip, the blow-pin has means
responsive to fluid pressure to cause it to move axially to
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stretch the parison and spring means for controlling the rate of
axial movement of the blow-pin and for limiting the axial move-
ment of the blow-pin so that the blow-pin tip cannot force the
walls of the preform against the walls of the cavity of the mold.
Means are provided for controlling the rate of fluid flow through
tha orifices. The blow-pin tip is of an insulating material
which reduces the heat loss from the thermoplastic preform to the
blow-pin tip. Mold heating means are provided for preventing the
thermoplastic preform material from dropping below orientation
temperatures in the last areas to expand into the mold cavity.
In the accompanying drawings, an embodiment of this
invention appears wherein:
Figure 1 is a side sectional elevational view of the
mold apparatus. The mold in this particular embodiment is for a
bottle in inverted position.
Figure 2 is another side elevation in section of the
mold apparatus showing the plastic partially inflated within the
mold.
Figure 3 is a view taken along line 3-3 in Figure 1.
Figure 4 is a view taken along line 4-4 in Figure 1.
The circular mold apparatus 1 is composed of two hollow
mold halves 2 and 2' which are equipped with cartridge heaters 3
which can be located at intervals of from 6 to 60 degrees apart
along the circumference of the mold 1. Within the mold there is
positioned a plastic parison or preform 4 which has been pre-
heated to the orientation temperature. The blow pin 5 has an
enlarged lower end 5' which functions as a piston and is movable
in a vertical direction having a tip 6 which is made of a low
heat-transfer coefficient material such as Teflon~ nylon, poly-
ester, transite, and the like, and as the blow pin moves up it
forces the preform 4 toward the upper limit of the cavity of the
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mold 1. There is also provided a blowing mandrel 7 through which
is passed a fluid, usually a gas which enters under positive pres-
sure through port 8 which forces the blow pin to move upwardly to
a position as shown in Figure 2 and is in communication with
annular orifices 8' and 8'', which in turn inflates the preform
4 as shown in Figure 2. As the preform 4 is inflated by the gas
flowing through the blowing mandrel 7, the blow pin 5 by virtue
of force of the gas on the piston 5' moves upward forcing the
preform 4 in an upward direction. The upward travel of the blow
pin 5 is somewhat restricted by spring 9 which becomes compressed
as the blow pin 5 moves upwardly to allow the blow pin tip 6 to
approach the top of the inside of the cavity of the mold 1 but
not to push the upper center part of the expanded preform 4
against the top portion of the cavity of the mold 1.
The apparatus of the present invention is useful in the
manufacture of biaxially oriented containers from orientable
thermoplastic materials such as polypropylene, polystyrene, poly-
vinyl chlorides, polyesters, and particularly thermoplastic
nitrile-gas barrier resins including those described in U. S.
Patents Nos. 3,426,102, 3,451,538, 3,586,737, 3,763,278 and
3,821,348. The production of clear, biaxially oriented bottles
from nitrile-gas barrier resins which have rather narrow orienta-
tion ranges is readily accomplished in the apparatus of this
invention.
By means of the apparatus of this invention, biaxially
oriented plastic containers can be produced with improved control
of plastic material distribution throughout the walls of the
containers, and a high degree of orientation is obtained in the
base of the container by employing materials which have low heat-
transfer coefficients in construction of the tip 6 of the orienta-
tion blow-stretch pin 5. Materials such as Teflon~ , nylon,
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polyester, transite, and the like, can be used for the tip 6 to
reduce the heat loss from the preform 4 to the tip 6 and blow
pin 5, thus allowing more and better stretch of the preform 4 in
the area of the tip 6 before the thermoplastic material in the
expanded preform 4 drops below the orienta~ion temperature of
the thermoplastic material. Use of this apparatus allows for
use of lower orientation temperatures and better utilization of
the thermoplastic material in the base of the preform and higher
performance of the base portion of the finished oriented container
in terms of creep resistance and drop-impact resistance, and
requires less plastic material than normal in this area of the
container which results in a reduction in weight of thermoplastic
material needed for the container.
In controlling the rate of axial stretch of the preform
4, better control over thermoplastic material distribution in the
vertical walls of the oriented container can be obtained at lower
temperatures with the apparatus of this invention. Control of
the longitudinal stretch of a thermoplastic preform is obtained
by the spring loading of the longitudinal stretch device or blow
pin 5. By varying the spring 9 constant and length of spring 9
or using a spring 9 which has a variable spring constant, dif-
erent rates of stretch can be obtained for a given stretch
distance. This improvement combines with the insulated mandrel
tip 6 to markedly improve the orientation and thermoplastic
material distribution in the base of the container. The spring
9 will slow or stop the blow pin 5 before it reaches the bottom
of the cavity of the finished container mold 1, keeping the
thermoplastic material from dropping below orientation tempera-
ture by contacting the wall of the cool mold at this point before
the preform has been fully inflated. This allows the blowing
fluid pressure to stretch the thermoplastic material in the
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preform near the base of the container over the entire area of
the base giving improved distribution of plastic material and
increased orientation in the base of the container.
Because the heated thermoplastic preform is not instan-
taneously expanded into the form of the interior of the container
mold 1, orientation stress whitening can occur when thermo-
plastics having narrow orientation temperature ranges (5-15F)
are used in the manufacture of biaxially oriented plastic con-
tainers. Orientation stress whitening can be eliminated using
the apparatus of the present invention by an increase in the
surface temperature of the mold cavity in the areas where either
high stretch is experienced or in areas where the thexmosplastic
material fills the mold cavity last. The careful control of the
temperature of selected areas on the surface of the mold cavity is
accomplished using electric low-watt density cartridge heaters or
zoned temperature-controlled fluid in the mold areas where stress
whitening ordinarily occurs. By eliminating stress whitening, a
container of uniform clarity is obtained, and improved thermoplastic
material distribution is obtained in the areas of the mold cavity
surface where stress whitening normally occurs.
Example
In an apparatus like that described in the drawings and
specification above, a biaxially oriented bottle was prepared
from a plastic nitrile rubber-modified copolymer of about 75%
acrylonitrile and 25% methyl acrylate (Barex~ 210 resin marketed
by Vistron Corporation). The mold halves each contained 3
electric cartridge heaters. The heaters were e~ually spaced
radially about the mold and each was of a 50-watt capacity and
1/4 inch by 1 inch long. The heaters were located 1 inch from
the top of the blow-mold cavity as shown in the drawings (bottom
of the bottle).
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An injection-molded plastic preform was installed on the
blowing mandrel. The preform was heated by radiant heat to about
2200F on the mandrel before it was placed in the mold cavity and
blow molded. The preform had the following dimensions:
neck finish 28 mm.
neck finish, I.D. .730 inch
body, O.D. .982 inch
wall thickness .120 inch
overall length 6.5 inches
Body of preform had a l-degree draft angle.
The blowing mandrel had an O.D. in the neck finish which provided
a push fit between the preform and the mandrel. The relaxed
length of the blowing mandrel was designed so that the tip did not
quite contact the inside of the preform. The mold halves (blow
cavity for a 32-ounce round bottle with screw neck bottle having
a body diameter of 3.14 inches and an overall height of 10.75
inches with a 28-mm. neck finish) were closed around the mandrel
and preform and the blow-air supply was connected to the port 8,
and the biaxial orientation process was started. The pressure
~0 of the blow air was programmed so that it was at 200 psig for
1/2 second and at 180 psig for 9 seconds. When the blow air was
thus applied to the port, both axial stretching and radial blow-
in~ as shown in Figure 2 occurred simultaneously. Air through
orifices 8' and 8'' was used to adjust or fine tune the radial
flow. The upward stroke of the blow pin was somewhat retarded
by the stretch-control spring. The blow pin had the following
characteristics:
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blow-pin piston diameter .547 inch
mandrel inside diameter .551 inch
orifice area (8'') .003 inch
orifice area (8') .0005 inch
control spring (9) 10 coil, 1.45 inches
long spring constant
2.838 pounds per
inch
tip (6) Teflon
A highly oriented bottle having excellent clarity was
produced.
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