CA1180523A

CA1180523A - Injection molding container lids

Info

Publication number: CA1180523A
Application number: CA000374954A
Authority: CA
Inventors: George Cotroneo
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-04-08
Filing date: 1981-04-08
Publication date: 1985-01-08

Abstract

ROGERS, BERESKIN & PARR

INVENTION: INJECTION MOLDING
CONTAINER LIDS
INVENTOR: GEORGE COTRONEO

ABSTRACT OF THE DISCLOSURE

A warpage-resistant method of injection molding an ultra-thin poleolifin lid with reduced cycle time and with a simple stripping process is described. A mold is provided which tapers the thickness of the lid top and which includes a single mold part defining the interior surfaces of a circular lid sidewall and a thin internal flange to be formed with the sidewall.
A molten poleolifin is injected into the mold cavity, and cooled for an emperically determined time so that the sidewall and flange of the resultant lid are partially set but still strongly elastically deformable. Elastic deformation of the two members is relied upon to permit prying of the lid from the from the said mold part without shearing of the internal flange. The tapering of the lid top eliminates the warpage which otherwise occurs in an ultra-thin lid when stripped prior to substantially complete setting from the mold.

Description

FIELD OF THE INVENTION
The invention relates to a me-thod of molding poleolifin container lids, and in particulax to a method of molding ultra-thin poleolifin lids with reduced cycle time, substantially free of warpage.

BACKGROUND OF THE IMVENTION
_ .
Today, plastic container lids commonly found in supermarkets and the like are molded with an internal, annular bead that s~rves to grip container walls which are located under a corresponding annular lip formed about a container rim. The bead is usually of heavy construction to stand shearing forces that arise when a lid is stripped from an injection mold, and might typlcal.ly be bevelled at an angle of 30-45 at an upper surface to further facilitate strippingu ~ Additionally, to..avoid damage during stripping, the bead is cooled in the mold until substantially fully set, and, because of the thickness of the bead, a very long cooling period is required, resultiny in a relatively long cycle time.

BRIEF SUMMARY OF THE INVENTION
The invention provides a warpage-resistant. method of injection molding a thin poleolifin lid with a circular top portion, an annular sidewall and an annular _ 3 _ sidewall and an annular internal flange. A mold is provided having a substantially radially symmetric lid-defining cavity. The cavity is shaped to taper the thickness of the lid top portion, the lid top portion being thickest adjacent its centre and thlnnest adjacent its periphery. The mold also includes a mold part which defines both the interior surface of the sidewall and the internal flange.

A molten poleolifin is injected into the lid-defining cavity, and then cooled for a predetermined period of time. The cooling period is of such a duration that the flange and lid sidewall of the resultant lid are at least partially set and elastically deformable. This elastic deformability in the partially se$ state is relied upon to permit removal of the lid from the mold~ which removal process involves removing the lid from the said mold part by a displacement of the mold part along a predetermined axis relative to the interior of the lid, as by a simple pushing or prying movement, without shearing of the internal flange. To facilitate removal without shearing, the flange is preferrably a very thin construction with a generally rectangular, radial cross-section.

The cooling time prior to stripping is preferrably determined emperically. The cooling time will depend largely on the characteristics of the mold itself, the poleolifin being molded (particularly inherent elasticity in a warm, partially set state), and the general shape and dimensions of the flange. The inventor has noted, however, that very thin lids with a top thickness less than about .025 inches have a tendancy to warp when stripped from a mold in a partially set state. The tapering of the lid top serves to reduce this warpage thereby permitting the reduction of material requirements associated with a thinner lid together with a significantly reduced cycle time.
: BRIEF DESCRIPTION OF DRAWING5 .
The invention will be better understood with reference to drawings in which:
Fig. 1 is a cross-sectional view illus-trating a lid molded according to the invention, seated on a : container;
Fig~ ~ is a cross-sectional view better detailing the structure of the lid;
Fig. 3 is a cross-sectional view diagra~natically illustrating characteristics of a mold used to make the lid; and, figs. 4 and 5 show the lid during stripping from a mold part defining internal surfaces and members of the lid.

DESCRIPTION OF PREFERRED EMBOD_MENT

Figs. 1 and 2 illu~trate a lid 10 molded according to the invention. In molding such lids, polypropelenes ox polyethelenes with densities varying between low and high may be used. The structure of the lid 10 is radially symmetric about an axis A which lies in the plane of the cross-sections of figs. 1 and 2.

The lid 10 has a dished top 12. The top 12 may be regarded as a dished portion 14 with a centre 16 and peripheral sidewall 18, and an annular peripheral portion 20, an alignment shoulder 22 is formed on the annular portion 20 to permit stacking of the lid 10 with substantially identical lids.
The lid 10 has a circumferential sidewall 24 with an inner surface 26. The sidewall 24 is tapered in ; 15 thickness, being about .022 inches at 28 tnear the top 12) and .027 inches at 30 tdistal from the top 12).
The tapering of the lid sidewall 18 produces a pronounced radial contraction and inward inclination of the side-wall 24 at 30 following stripping to produce a better fit with the alignment shoulder of a similar lid during stacking.

An internal circumferential flange 32 extends radially inwardly from the sidewall inner surface 2h.
The flange 32 has an ~longate, generally rectangular -- 6 ~

radial ~ross-section (that is, a cross-section in any plane containing the axis A) which facilitates stripping.
The flange is tapered in thickness, being about .010 inches at its base 34, and .007 inches at 36, distal from the inner sidewall surface 26.

This tapering further facilitates stripping of the lid from the associated mold~

The lid 10 may be molded in an injection mold having the general structure of the mold 38 diagrammatically illustrated in figs. 3 and 4. The mold 38 is substantially symmetric about an axis of radial symmetry B which lies in the plane of the cross-sections illustrated. Figs.
3 and 4 are not intended to fully detail a functioning mold, and detail respecting cooling of the mold, mechanisms needed to open and close the mold parts and the like have been omitted. Such detail will be readily apparent to one skilled in the art of injection molding.

The mold 3~ includes a mold part 40 which defines outer surfaces of a lid, a mold part 42 which defines inner surfaces of the lid including an internal flange, and a mold part 44 provided primarily for prying of a molded lid from the mold part 42. To facilitate stripping without shearing of the flang~, the mold part 42 is gentley rounded at an annular line 46. An injection gate 48 is located in the mold part 40O

The principal steps and considerations in molding the lid 10 in the mold 38 are essentially as follows.
A molten poleolifin is injected via the gate 48 into the cavity defined by the assembled mold parts 40, 42 and 44. The injected poleolifin is allowed to cool for a period of time, determined in advance, until the flange 32 and lid sidewall ~4 are at least partially set, but still elastically deformable as compared to their relative regidity when fully set. As mentioned above, this cooling time is best determined emperically, given a particular type of lid and associated mold, with the general object b~ing to rely on the enhanced elasticity of the lid parts while still warm to permit stripping. When the predetermined cooling period is expired, the mold part 40 is removed and the mold part 44 advanced along the general direction of the axis B to pry the lid 10 from the mold part 42. A portion of this prying process is illustrated in figs. 4 and 5, where the flange 32 is shown fully flexed about its base 34 to the inner sidewall surface 26 and the lid sidewall 24 is shown deflected radially outwardly in the radial direction indicated by the arrows C.

~5 Once stripping is complete, the lid 10 may be allowed to cool until fully set. The lid sidewall 24 and flange 32 will tend to return to substantially the position in which they were initially molded.
However, because strick elastic deformation of the warm poleolifin will not in practice be obtained, the flange 32 may well be displaced from its initial position. In some instances, this may be accommodated by initially molding the 1ange 32 with a greater inclination towards the lid top 14; however, such measures ~ill tend to increase the tendancy for the flange 32 to shear during stripping. To further reduce the possibility of flange shearing, flange 32 may be molded inclined away from the lid top 12 by an angle between 0-30, for example. However, to ensure more positive engagement of the flange 32 in use with the underside of the lip normally surrounding the lid of a container, the general object will be to mold the flange 32 substantially perpendicular to the lid sidewall 24 tparallel to the lid top 12).

The dimensioning of the flange 32 can significantly - 20 facilitate stripping. As mentioned above, the radially elongate, generally rectangular cross-section with a measure of tapering acilitates separation of the flange 32 from the mold part defining it tfor example, the mold part 42). Also, by keeping the flange 32 relatively thin~ less outward deformation o the lid sidewall 18 is required, as the sidewall 18 must expand by the thickness of the flang~ 32 during stripping (see fign 4). Additionally, the flange 32 may be discontinuous, formed in a number of separate flange portions.

Thick lids are not particularly prone to warpage.
However, the inventor has found that when the general thickness of the lid is reduced below about .025 inch~s, there is a marked tendancy for the lid to warp, if stripped from a mold without allowing substantially full setting. The inventor has found that by molding the lid top to be thickest adjacent its centre and tapering the thickness toward the lid periphery there is a marked reduction in warpage. Such dimensioning of a portion of the lid top is believed to cause greater radially inward contraction of the lid adjacent its centre thereby taking in any "slack" created by radially inward shrinkage of lid portions further away from the centre of the lid. Regardless of the exact mechanism by which it operates, this tapering of the thickness of a lid top has proven in practice to markedly reduce warpage. To assist in understanding the extent of tapering involved, it should be noted that the lid 32 has an overall diameter of 4 inches and the annular lid portion 20 a diameter of .25 inches, that the thickness of the annular top portion 20 and the lid sidewall 24 is .015 inches, and that the dished portion 14 has a thickness of .022 inches at its cen+re 16 tapering continuously to .015 inches at its sidewall 18 (which sidewall 18 is uniformly .015 inches in thickness).

A number of advantages attendant with use of the above-described molding method should be noted. In particular, the method is particularly advantageous with materials that are rigid in the sense that the flange formed with the materials would be sheared if pried substantially set from a moldl and is particularly useful with poleolifins which have a flexural stiffness as measured according to ASTM D747 15 in excess o 55,000 PSI. Also, cycle time is significantly reduced by stripping a lid from the associated mold prior to substantially complete setting. Because lid warpage in such circumstances can be accommodated by tapering the thickness of the lid top, very thin lids can be molded, with the lower material requirements expected to follow from thinner construction, while simultaneously obtaining the reduced cycle times.

Fig. 1 shows the lid 10 seated on a corresponding container 50. The container 50 is conventional and has a rim circumscribed by a radially directed lip 52.
The lid 10 is attached to the container 12 in a conventional manner by locating the lid sidewall 24 above the containcr rim and pressing downwardly until the lid flange 32 engages the underside of the containex lip 52. Removal of the lid 10 involves a simply prying movement. If desired, the lid flange 32 may be extended radially to provide sealing engagement with the containex sidewall following closure.

It will be appreciated that a particular embodiment of the invention has been described, and that modifications may be made therein without departing from the spirit of the invention or the scope of the appended claims.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A warpage-resistant method of injection molding a poleolifin lid having a generally circular top portion with a thickness generally less than about .025 inches, an annular sidewall and an internal annular flange integrally formed with an interior surface of the sidewall, comprising:
providing a mold having a substantially radially symmetric lid-defining cavity, the cavity shaped to define the said top portion, sidewall and flange and to taper the thickness of the top portion so that the top portion is generally thickest adjacent the centre of the top portion and generally thinnest adjacent the periphery of the top portion, the mold including a mold part which defines both the interior surface of the sidewall and the flange;
injecting a molten poleolifin into the lid-defining cavity;
cooling the injected poleolifin for a pre-determined period of time so that the flange and lid sidewall of the resultant lid are at least partially set and elastically deformable;
removing the lid formed by injection of the poleolifin at the expiration of the predetermined period of time, including removing the lid from the mold part by displacing the mold part along a predetermined axis relative to the interior of the lid whereby the flange and sidewall deform elastically to permit removal of the mold part; and, allowing the lid removed from the mold to cool until fully set.

2. A method as claimed in claim 1 in which the mold cavity is so dimensioned that each of the sidewall and flange has a thickness generally less than about .025 inches.

3. A method as claimed in claim 1 or 2 in which the flange has an elongate, generally rectangular radial cross-section.

4. A method as claimed in claim 1 in which the flange comprises a plurality of separate flange portions.