GB2124543A

GB2124543A - Parison for oriented plastic containers

Info

Publication number: GB2124543A
Application number: GB08318749A
Authority: GB
Inventors: Long Fei Chang
Original assignee: Owens Illinois Inc
Current assignee: OI Glass Inc
Priority date: 1982-07-29
Filing date: 1983-07-11
Publication date: 1984-02-22
Also published as: GB8318749D0; DE3327136A1; IT1169322B; ES282172U; IT8348679A0; ZA835139B; AU1632183A; JPS5929132A; FR2531001A1

Abstract

A parison for blow moulding oriented plastic containers comprises an elongated hollow body 24 of a strain-hardenable thermoplastic material having an open-ended threaded finish 20, an annular supporting ledge 21 positioned below on the finish and extending radially outwardly thereof, a neck 21 ring below the supporting ledge, a shoulder 23 below the ledge, and a main body portion 24 which extends to a bottom 25 closing the open end of the main body. The neck ring has a radial thickness less than that of the finish and less than that of the wall of the main body. The shoulder gradually increases in wall thickness radially outwardly from the neck ring to the main body. The wall of the main body has a uniform thickness and a minimal taper on the internal and external surfaces thereof and the bottom has a thickness less than the thickness of the wall of the main body. <IMAGE>

Description

SPECIFICATION Parison for oriented plastic containers Background and summary of the invention It has long been known that plastic containers can be made stronger by forming the plastic container at an elevated temperature known as the orientation temperature to orient the molecules of the plastic and thereby increase the mechanical properties of the container and make the container less permeable to gas transmission. One commonly used method of making oriented plastic containers is injection mold a preform or parison and thereafter reheat the parison to the orientation temperature and blow the heated parison to the final shape of the container.

The resultant container commonly has a finish, a main body which is generally cylindrical and a bottom wall that is generally hemispherical or elliptical to produce optimum strength for containment of fluids under pressure.

In making the parison by injection molding, it has been common to utilize a core pin and a partible mold and to inject the molten plastic between the core pin and the closed mold halves to produce the elongated parison. The parison has a body having a finish, a supporting ledge below the finish, a neck ring portion below the ledge, a shoulder portion below the neck which blends into a main body portion that terminates in a bottom portion closing the main body portion. In order to remove the parison from the mold and to remove the core pin from the parison, it has been common to provide a taper on the core pin and the inner mold surfaces so that the resultant parison surface of the main body portion is also tapered. It has further been believed that it is desirable to provide substantially the same amount of material longitudinally throughout the main body portion.Accordingly, the design of the core pin and the mold has been such that the resultant parison has a main body portion that increases in thickness toward the bottom.

The shape and configuration of the parison affects the injection molding cycle time, the reheating time, and the material distribution throughout the container, as well as the mechanical and barrier property levels of the resultant container.

In the case of the so-called reheat and blow process, the cold parison must be reheated to within the molecular orientation temperature region which is slightly above the glass transition temperature of the selected polymer. In practice, using an infra-red (IR) heating system, the neck region of parison immediately below the finish and supporting ledge and the bottom portion of the parison are the most difficult areas to heat up to a desirable orientation temperature in relation to the main body of the parison. The gross result of the non-uniformity axial temperature distribution along the parison is that excessive material tends to be blown into the neck and shoulder and into the bottom region of the bottle. This material maidistribution of material in the side wall results in non-uniform physical properties in the side wall.This results in less acceptable mechanical and gas transmission characteristics.

In the past, many methods of manufacture and designs for thermoplastic parison have been used.

For example, U.S. Patent 4,034,036 to Farrell discloses a parison control in which an injection molded parison is positioned in the mold cavity having a predetermined length-to-diameter ratio and means to selectively and alternatively exert positive pressure or negative pressure on portions of the parison as it is expanded. The thickness of the expanding parison is controlled so that the finished hollow article will have the desired wall thickness.

U.S. Patent 3,347,966 to Seefluth discloses a method for forming hollow thermoplastic articles by forming a parison having an area with a wall thickness less than that of the rest of the parison and heating the parison in a mold to a temperature below the crystalline melt point of the parison. The parison is then placed in a mold and differential pressure is applied to cause it to conform to the mold so that the area with the thin wall thickness extends first. The Seefluth teaching is particularly concerned with the provision of a thermoplastic container having an oriented bottom with the same thickness throughout.

U.S. Patent 3,754,851 to Reilly et al discloses an apparatus for forming multiaxially oriented containers. The containers are formed from an extruded molecular orientable thermoplastic parison having a programmed wall thickness. Reilly is particularly concerned with a process wherein a selected wall thickness pattern of material in the finished container is achievable by controlling the distribution of material in the parison from which the container is eventually formed. U.S. Patent 3,873,660 to Reilly et al discloses a method for forming multiaxially oriented containers. The 3,873,660 patent relates particularly to a method wherein the effect of wall thickness on orientation temperature is minimal and not critical.

U.S. Patent 4,233,262 to Curto discloses the provision of a method for forming blown polyethylene terephthalate containers. The method uses an injection molded parison to achieve exterior surface indentations on blown containers without using profiled mold cavities. In the last step of the process, the preform is blow molded under conditions so that the polyethylene terephthalate material is strain-hardened in order to cause the different wall thicknesses to expand and stretch proportionately.

Similarly, U.S. Patent 4,330,579 to Ota et al discloses a plastic parison with the bottom made gradually thicker from the center of the outer circumference thereof such that the bottom portion can be uniformly oriented at a similar orientation rate to that of the drum portion of the bottle product. The bottom portion is made to protrude into such a generally hemispherical shape that the radius of the outer spherical surface thereof is made larger than that of the inner spherical surface.

Among the objectives of the present invention are to provide a parison which requires less material resulting in a container that is lighter in weight, yet superior in properties; reduces the injection molding cycle time; reduces the required reheating cycle time; improves the material distribution throughout the container and particularly the side wall; and provides a higher mechanical and barrier property level of the container.

More specifically, the parison embodying the invention has a thickness profile which insures lower injection molding cycle times, improved distribution and effectiveness of material, and makes the reheating of the parison easier.

In accordance with the present invention, the parison for making oriented plastic containers wherein the parison is first formed and thereafter rehated to an orientation temperature and blown to final shape comprises an elongated hollow body of a strain-hardenable thermoplastic material having an open-ended finish, an annular supporting ledge positioned below the finish and extending radially outwardly, a neck ring below the supporting ledge, a shoulder, a main body, and a bottom closing the open end of the main body. The neck ring has a radial thickness less than that of the finish and less than that of the wall of the main body. The shoulder gradually tapers from the neck ring to the main body.The wall of the main body has a uniform thickness and a minimal taper on the internal and external surfaces thereof and the bottom has a thickness less than the thickness of the wall of the main body.

Description of the drawings Fig. 1 is an elevational view of a parison embodying the invention.

Fig. 2 is an elevational view of a container made from the parison shown in Fig. 1.

Fig. 3 is a top plan view of the parison shown in Fig. 1.

Fig. 4 is a vertical sectional view of the parison shown in Fig. 1.

Fig. 5 is an elevational view of another parison embodying the invention.

Fig. 6 is an elevational view of the container made from the parison shown in Fig. 5.

Fig. 7 is a top plan view of the parison shown in Fig. 5.

Fig. 8 is a vertical sectional view of the parison shown in Fig. 5.

Description Referring to Fig. 2, the invention relates to the making of containers such as bottles B of oriented strain-hardenable plastic material such as polyethylene terephthalate wherein the container comprises a finish 10 for receiving a closure, a supporting ledge 11 beneath the finish, a neck region or ring 12, a shoulder 13, a main body portion 14 that is generally cylindrical and an outwardly convex bottom portion 1 5 that is hemispherical or at least elliptical to better withstand the fluid pressure packaged in the container. The package further comprises a base cup 16 on which the container is mounted.

The container is made by first forming a preform or parison, thereafter reheating the parison to the orientation temperature, and finally blowing the reheated parison outwardly against the confines of a mold.

In accordance with the invention as shown in Fig. 1, the parison P comprises a finish 20, a supporting ledge 21 below the finish, a neck ring 22 below the supporting ledge, a shoulder 23, a main body 24, and a bottom 25 closing the lower end of the main body portion.

As shown in Fig. 4, the neck ring 22 has a radial thickness less than the finish 20 and less than the thickness of the wall of the main body 24 and a constant diameter. The external surface of shoulder 23 gradually tapers from the lower end of the neck ring 22 to the main body 24 and is longer than the neck ring in an axial direction. The wall of the main body 24 is of uniform thickness throughout with very small variations, on the order of +0.005 inches and preferably t0.003 inches and has a minimal taper on the order of 15 minutes.

Finally, the bottom wall 25 has a thickness less than the thickness of the main body 24. The thickness of the bottom wall is thinnest at the center and gradually thickens toward the main body.

Where the plastic is injected by a sprue at the bottom wall, the thickness of the bottom wall is measured adjacent the sprue.

Where the bottle is smaller, for example, a 1/2 liter bottle rather than a 2 liter bottle, the parison P-l shown in Figs. 5, 7, 8 can be used to form the bottle B-l shown in Fig. 6. Thus, in this form, the shoulder of the parison tapers to the smaller diameter of the body 24.

The dimensions of the parison, exclusive of the finish and supporting ledge, are determined in accordance with the following relationships: weight of the parison (grams) a) < 0.330 g/Cm2 [volume of the container (cm)12'3 Range Preferred thickness of the neck region b) 0.2-0.8 0.45 thickness of the main body thickness of the bottom c) 0.5-1.0 0.80 thickness of the main body length of the preform d) 0.3-1.0 0.60 length of the container mean main diameter of bottle e) 2-6 mean main diameter of preform mean surface area of container f) > 5 mean surface area of parison As used in items e) and f) "mean" is defined as the midpoint between the inside and outside of the container or parison walls.

The proper or normalized surface area of the parison is determined by the relationship: mean surface area of parison exclusive of finish Proper surface area of parison= [container volume exclusive of finish ] 2/3 The ranges to produce proper strain-hardening are: Proper surface area of parison Greater than Less than Generally 0.5 0.9 Preferable 0.55 0.8 More preferable 0.6 0.7 Where the proper surface area is less than 0.5, the material in the side wall is overstretched and stress whitened. Where the proper surface area is greater than 0.9, the material is not stretched enough to produce strain-hardening.

Optimum strain-hardening results are achieved where the following relationships are maintained excluding the finish and supporting ledge: mean surface of container Area: ~ 10 mean surface of parison length container Length: ~ 2.4 length parison container Diameter: ~ 4.2 parison The uniformity of side wall thickness of the parison results in lowering the heating cycle time.

The ratio of the thickness of the bottom to the thickness of the side wall being less than .8 lessens the injection mold cooling time, it being understood that the thickness of the bottom wall is measured adjacent the sprue where a sprue is used at the bottom wall or at the center of the bottom wall where the sprue is elsewhere on the parison.

The ratio of the thickness of the neck to the thickness of the wall being less than 0.55 results in easier reheating time and a reduced heating cycle time.

The preferred form of strain-hardenable plastic comprises polyethylene terephthalate (PET). Low IV (inherent viscosity) is preferred because it is less expensive and tends to produce less taste sensitive contaminants such as acetaldehyde.

Typical examples of the normalized weight to volume relationships of containers exclusive of the weight of the finish and supporting ledge as defined in (a) and embodying the invention made of 0.7 IV PET are as follows: Size Weight Wt/(volume)213 2 liter 45 0.290 g/Cm2 1 liter 35 0.286 g/Cm2 1/2 liter 18 0.290 g/Cm2 This may be contrasted to a weight to volume relationship of .340 for typical 2 liter commercial bottles made of 0.7 IV PET and weighing 54 grams exclusive of the weight of the finish and supporting ledge.

Since the parisons embodying the invention utilize less material, they necessitate less heat to be reheated to the orientation temperature range. This results in a reduction in cycle time which has been found to be on the order of 10% for containers made of 0.7 IV PET.

With respect to the permeabilities, it has been found that 2 liter containers made from parisons made of 0.7 IV PET embodying the invention exceed the present standards which require less than 15% carbonation loss in twelve weeks of shelf life. Containers made from parisons embodying the invention have resulted in less than 14% carbonation loss over twelve weeks utilizing standard techniques for testing.

Containers made from 0.7 IV PET parisons embodying the invention were tested for weight loss in a standard twelve week storage. It was found that the weight loss was 0.25%, well within the maximum allowable loss of 1% over a twenty-four week shelf life.

Samples of containers made from the 0.7 IV PET parisons embodying the invention were subjected to drop impact after being conditioned for twenty-four hours at room temperature, 400 F, and 1000F and all samples passed.

The results of testing 2 liter 0.7 IV PET containers for carbonation and weight loss are summarized as follows: CO, Weight Initial 4.52 Volumes 24 hours 4.28 24 hours (after removal of contents to standardize the volume) 4.00 2081.8 grams 1 week 3.99 2081.2 3 weeks 3.92 2079.6 6 weeks 3.78 2078.1 8 weeks 3.67 2077.4 12 weeks 3.46 2076.6 Total Loss 13.5% 5.3 grams Thus, the parison embodying the invention achieves: 1) Proper degree of stretch and hence orientation in the blown container wall; 2) Desirable temperature profile along the axial direction of the parison; 3) Improved material distribution (wall thickness distribution) in the blown bottle; 4) Minimize the required injection molding cycle time; 5) Improved effectiveness of material use.

Claims

1. A parison for making oriented plastic containers wherein the parison is first formed and thereafter reheated to an orientation temperature range and blown to final container shape comprising: an elongated hollow body of orientable strain-hardenable thermoplastic material having, an open-ended finish, an annular supporting ledge positioned below the finish and extending radially outwardly, a neck ring below the supporting ledge, a shoulder below the supporting ledge, a main body, and a bottom closing the open end of the main body, said neck ring having a radial thickness less than that of the wall of the main body, said shoulder gradually tapering from the neck ring to the main body, said wall of said main body having a substantially uniform thickness and a minimal taper on the internal and external surfaces thereof, said bottom having a thickness less than the thickness of the wall of the main body.

2. The parison set forth in claim 1 wherein the thermoplastic material comprises polyethylene terephthalate and the dimensions of the parison, exclusive of the finish and supporting ledge, are determined in accordance with the following relationship: weight of the parison (grams) a) < 0.330 g/Cm2 [volume of the container (cm)12/

3 Range Preferred thickness of the neck region b) 0.2-0.8 0.45 thickness of the main body thickness of the bottom c) 0.5-1.0 0.80 thickness of the main body length of the preform d) 0.3-1.0 0.60 length of the container mean main diameter of bottle e) 2-6 mean main diameter of preform mean surface area of container f) > 5 mean surface area of parison 3.The parison set forth in claim 2 wherein the thermoplastic material comprises polyethylene terephthalate and the proper surface area of the parison to produce strain-hardening ranges between 0.5 and 0.9 where the proper surface area of the parison is determined by the relationship: mean surface area of parison exclusive of finish Proper surface area of parison= (container volume exclusive of finish)2/3

4. The parison set forth in claim 3 wherein the proper surface area ranges between 0.55 and 0.8.

5. The parison set forth in claim 3 wherein the proper surface area ranges between 0.6 and 0.7.

6. The parison set forth in claim 1 wherein the thermoplastic material comprises polyethylene terephthalate and the parison has the following relationships exclusive of the finish and supporting ledge: mean surface of container Area: N 10 means surface of parison length container Length: N 2.4 length parison container Diameter: N 4.2 parison

7. The parison set forth in claim 1 wherein the parison is made of polyethylene terephthalate.

8. The parison set forth in claim 1 wherein the taper on the internal and external surfaces is 1 5 minutes.

9. A parison for making oriented plastic containers, substantially as described with refernence to the drawings.

10. An oriented plastic container, produced from a parison as claimed in any of claims 1 to 9.