CA1083313A

CA1083313A - Apparatus for controlling plastic material distribution in manufacture of biaxially oriented plastic containers

Info

Publication number: CA1083313A
Application number: CA260,006A
Authority: CA
Inventors: Thomas J. Bond; William J. Kelly; Richard C. Adams
Original assignee: Standard Oil Co
Current assignee: Standard Oil Co
Priority date: 1975-09-10
Filing date: 1976-08-27
Publication date: 1980-08-12
Also published as: AU1687476A; NL7609136A; DE2639078A1; SE7609842L; AU505338B2; JPS5233956A; FR2323516A1; ATA662576A; FR2323516B1; GB1561067A; SE421509B; IT1076943B

Abstract

APPARATUS FOR CONTROLLING PLASTIC MATERIAL
DISTRIBUTION IN MANUFACTURE OF BIAXIALLY
ORIENTED PLASTIC CONTAINERS

Abstract of the Disclosure An apparatus which allows for uniform stretch rate and careful temperature control in the production of biaxially oriented plastic containers having uniform wall thickness and good clarity.

Description

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 -1- ~ ,~

16)833~3 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 1~333~3 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, 1~833~3 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 1~833~3 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).

10833~3 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:

~083313 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.

~0 :

Claims

We claim:

1. In a blow molding apparatus useful for the production of biaxially oriented hollow thermoplastic contain-ers from thermoplastic preforms, said apparatus being composed of a hollow mold and means for positioning said thermoplastic preform within the cavity of said mold and means for preheating said preform to the orientation temperature and means for stretching and expanding said preform against the walls of the mold cavity, the improvements comprising a stretch-blow mandrel which an axis, orifices for fluid inlet and moveable blow pin with tip, said blow pin having means responsive to fluid pressure to cause it to move axially to stretch the parison and spring means for controlling the rate of axial movement of the blow pin and for limiting the axial movement of the blow pin so that the blow pin tip cannot force the walls of said preform against the wall of the cavity of the mold, means for controlling the rate of fluid flow through the orifices, said blow pin tip being of an insulating material which reduces the heat loss from the thermoplastic preform to the blow pin tip, mold heating means for preventing the thermoplastic preform material from dropping below orientation temperatures in the last areas to expand into the mold cavity.

2. The apparatus of claim 1 wherein the blow pin tip is constructed of a low heat-transfer material selected from a group consisting of Teflon?, nylon, polyester and transite.

3. The apparatus of claim 2 wherein the mold heating means are zone temperature controls.