Method For Making A Flexible Container Portion

Abstract

A method for forming an integral flexible closure for a container, said method comprising the steps of heating an upper cross-sectional portion of said container, stretching said heated portion to form a film, and permitting the stretched portion to cool.

Description

This invention relates to containers. In one aspect it relates to a method for integrally forming a flexible closure in a rigid wall container.

In the packaging of granular material such as detergent, coffee, candy, and the like, it is desirable that the container have the structural strength offered by the rigid wall container and the accessibility offered by twist-wrap closures commonly associated with the film packaging art. The present invention presents a simplified method for providing a rigid wall container with an integral flexible closure portion.

This invention operates on the principle that thermoplastic material becomes soft and pliable when heated and hardened when cool. It has been found that when a cross-sectional upper portion of the container composed of thermoplastic material is heated to the softening point of that material, and stretched sufficiently to cause the heated portion to elongate thereby substantially reducing the thickness thereof, the stretched portion upon cooling remains flexible. The upper flexible material then can be twist-tied, forming the container closure. In addition to providing a closure integrally formed with the container, the flexible closure portion can be folded into the rigid walls to permit nesting of the containers during transit and storage. Thus, it will be appreciated that the container formed by the method of this invention offers the structural strength of the rigid wall container and the convenience of the film twist-tie closure.

FIG. 1 is a sectional view of the apparatus for carrying out the method of this invention; and

FIG. 2 is a perspective view of the container formed by the method of this invention.

The method of this invention will be described with reference to the apparatus shown in FIG. 1, it being understood that the apparatus shown is but one embodiment and that variations and modifications can be made therein. The apparatus includes a stationary frame 10, a pair of friction clips 11 and 12, electric heater 13, a movable container support 14, and power means, e.g., pneumatic cylinder 15, for moving the support 14 up and down as indicated by arrow 16. The clips 11, 12 and heater 13 are secured to the frame 10 vertically spaced above and in axial alignment with the holder 14. The support 14 is provided with an internal passage 17 having upwardly opening ports 18. Line 19 is in fluid communication with passage 18 and leads to means (not shown) for evacuating the passage 18.

The container 19 may be composed of any thermoplastic materials such as styrene, acrylics, cellulosoics, polyolefins, vinyls, nylons, etc. However, orientable polyolefins are preferred. These include high density polyethylene, polypropylene, copolymers of ethylene and propylene, copolymers of ethylene and hexene-1, etc.

Although the method according to this invention may be performed on thermoplastic containers varying in shapes and sizes, it is preferred that the container 19 be particularly shaped to compensate for necking caused by stretching portions of the container. The container 19 may be formed by conventional blow molding or thermoforming techniques to provide a body portion 20, a lip portion 22 and an intermediate transition portion 21. The portions 21, 22 and 23 can have similar cross-sectional configurations ranging from polygons to circles. It is preferable, however, that the configuration be oval or circular so that the elongation be uniformly distributed. For purposes of illustration, it will be assumed that the container 19 is circular in cross section, the diameter of body portion 20 being less than the diameter of lip portion 22 and intermediate portion 21 being in the form of an inverted frustoconico portion.

When formed by conventional techniques, the wall thickness of a polyethylene container 19 will be in the range from 5 to 100 mils. The intermediate transition portion 21 represents the portion to be stretched according to this invention.

The container 19 is placed on the support 14 and passage 18 is evacuated thereby vacuum locking the container 19 to the support 14. The upper lip portion 22 of the container 19 is inserted into the clips 11 and 12 at diametric points placing the frustoconical transition portion opposite the electric heater 13 which in this embodiment is circular in configuration in conformance with the configuration of portion 21. The portion 21 is heated to the softening temperature of the thermoplastic material, e.g., 260.degree. F. for high-density polyethylene. The cylinder 15 is then actuated lowering the support 14 and pulling the container 19 vacuum locked thereto therewith. The upper lip portion 22 is restrained by the clips 11 and 12 so that the downward movement of the container 19 causes the heated, softened portion 21 to stretch in the direction of force application. The portion 21 is stretched to such a degree that the wall thickness thereof is reduced sufficiently to take on the dimensions and hence the characteristic of a film, e.g., 10 mils or less. For a polyethylene container having a starting wall thickness of 20 mils, a stretch ratio in the range of from about 5 to 1 provides a film thickness of from about 4 mils to about 10 mils. The radially outwardly extending transition portion 21 compensates for the necking downed caused by the stretching so that the diameter of the stretched portion 21 is about the same as the inside diameter of the body portion 20.

After the stretched portion 21 has cooled, the container 19 is removed from the apparatus and the upper lip portion 22 cutaway by conventional post-trimming techniques, leaving a rigid container 19 capped by a flexible closure portion 21. As shown in FIG. 2, the closure portion 21 is sufficiently long to permit a twist-tie as illustrated.

In summary, then, the method according to this invention comprises the steps of heating a cross-sectional portion of a rigid container composed of thermoplastic material to the softening temperature of that material, stretching the heated portion to such a degree to reduce the wall thickness of the heated portion to a film thickness, and permitting the heated portion to cool.

The following example illustrates this method.

EXAMPLE

A cylindrical container was made from high density polyethylene (density 0.950 and melt index 0.4 ). The container was about 8 inches long and had a wall thickness of about 20 mils. An upper portion about 11/2 inches long was circumferentially heated to the softening temperature of the material (about 240.degree. F.) by turning the container in a gas burner, the temperature of the lower portion remaining below the softening temperature. The softened (heated) portion was then stretched by hand in a ratio of about 3:1. The container was allowed to cool producing an article having a rigid body portion about 20 mils thick and a flexible closure portion about 4 mils thick integrally formed with the body portion.

Claims

While the preferred embodiment has been described in particular detail, it should be understood that variations and modifications may be made therein without departing from the scope and spirit of the invention as set forth in the appended claims.

1. A method for forming an integral flexible closure on a rigid thermoplastic container, comprising:

Providing a container formed of rigid thermoplastic material having an axis, a lower body portion, an upper lip portion having a greater diameter than a diameter of said lower body portion, and an intermediate transition portion integrally interconnecting said lower body portion and said upper lip portion with said lower body portion and said upper lip portion being in axial alignment one with the other;

restraining one of said upper lip portion or lower body portion;

heating only the intermediate transition portion to the softening temperature of the thermoplastic material;

moving the other said upper lip portion or lower body portion away from said restrained portion along the axis of the container;

continuing to move the upper lip portion or lower body portion away from said restrained portion a distance sufficient for stretching said heated intermediate portion and reducing the thickness thereof to a thickness less than about 10 mils;

maintaining the container at the stretched position until said intermediate portion is cooled to the set temperature of the thermoplastic material; and

removing the upper lip portion from the container and shaping a flexible end portion into a closure.

2. The invention as recited in claim 1 wherein said thermoplastic material is polymers of 1 -olefins.

3. The invention as recited in claim 2 wherein said thermoplastic material is polyethylene and said step of heating said material increases the temperature of the intermediate transition portion of the container to about 240.degree. F.

4. The invention as recited in claim 3 wherein said container prior to stretching has a thickness of at least 20 mils.

5. The invention as recited in claim 4 wherein said step of stretching said film is performed at a stretch ratio of at least 3:1.

6. The invention as recited in claim 1 wherein said body, lip, and intermediate portions prior to stretching are configurated with elliptical cross sections.

7. The invention as recited in claim 1 wherein said body, lip and intermediate portions prior to stretching are configurated with circular cross sections.