FIELD OF INVENTION

THIS invention relates to a paper lid or cover for a beverage container, and to a related method of manufacture using thermoforming.

BACKGROUND OF INVENTION Thermoforming, as a manufacturing process, is well known. Traditionally, thermoforming involves the heating of a plastic to a pliable forming temperature, molding the pliable plastic sheet into the required shape (using vacuum forming, as an example), and then trimming the molded body to create a usable product. In complex and high-volume applications, very large production machines are utilised to heat and form the plastic sheet and trim the formed parts from the sheet in a continuous high-speed process, and can produce many thousands of finished parts per hour depending on the machine and mold size and the size of the parts being formed. This type of thermoforming is primarily used to manufacture plastic disposable cups, containers, lids, trays etc., for the food, medical, and general retail industries.

In the most common method of high-volume, continuous thermoforming of thin- gauge products, plastic sheet is fed from a roll or from an extruder into a set of indexing chains that incorporate pins, or spikes, that pierce the sheet and transport it through an oven for heating to a forming temperature. The heated sheet then indexes into a form station where a mating mold and pressure-box close on the sheet, with vacuum then being applied to remove trapped air and to pull the material into or onto the mold along with pressurized air to form the plastic to the detailed shape of the mold. After a short form cycle, a burst of reverse air pressure is actuated from the vacuum side of the mold as the form tooling opens, commonly referred to as air-eject, to break the vacuum and assist the formed parts off of, or out of, the mold. A stripper plate may also be utilized on the mold as it opens for ejection of more detailed parts or those with negative-draft, undercut areas. The sheet containing the formed parts then indexes into a trim station on the same machine, where a die cuts the parts from the remaining sheet web, or indexes into a separate trim press where the formed parts are trimmed.

With the current trend of moving to more environmentally friendly, namely biodegradable and compostable, products, there is a need to provide alternatives to the plastic currently being used. In this regard, some lids for beverage containers are made from plant-derived sustainable raw materials, such as a PLA (polylactic acid), which is made from corn starch and is relatively compostable. However, the use of paper or paperboard has many advantages over plastic and even over PLA (and its equivalents), the primary advantage (in the context of the present invention at least) being its biodegradability. Other advantages include the fact that paper products can be used in both microwave and convection ovens thus enabling heating, and not only keeps food warmer for longer but can be handled more easily from the oven or microwave than plastic equivalents.

In addition, although paper lids or covers have been suggested in the past, no one appears to have been successful in putting this into practice. In particular, no one appears to have considered using thermoforming to manufacture paper lids or covers.

Using paper or paperboard in thermoforming is itself also not new. There are currently several paperboard thermoforming presses that can be used to form solid paperboard into three dimensional bodies, such as paper plates and trays.

Regardless of the type of thermoforming machine used to thermoform the paper plates and trays, the process revolves around three interrelated variables, which are inversely proportional to some degree, namely heat, dwell and pressure.

Thermoforming and setting a paper plate or tray into the desired shape with an acceptable amount of rigidity is, it has been said, very similar to steam ironing a pair of trousers or a shirt. The heat from a forming die combines with moisture in the paperboard and the pressure and dwell afforded by the paperboard thermoforming press to deform the fibers of the paperboard and steam iron them into the shape of a plate or tray. In particular, as the paper is drawn into the die, the diameter or original outside dimensions of the blank are reduced. This causes overlaps of paperboard to occur in the plate or tray creating wrinkles or pleats on the sidewall and rim of the paperboard container. The heat, dwell time and pressure supplied by the die and the paperboard thermoforming press, steam iron these pleats into the container and essentially“weld” the layers of paperboard in the pleat together to provide structural rigidity. These parameters are inversely proportional to a certain degree. For example, if one of the parameters is reduced, then in order to maintain the same quality plate or tray it is necessary to increase the other two parameters. However, in order to keep the production speed as high as possible with as little waste as possible, it is necessary to determine the best balance between the three parameters. It is also extremely important that the paperboard contain between 8.5% and 13% moisture by weight in order to be soft and pliable for thermoforming and in order to provide the catalyst for welding together the pleats. Too little moisture and the paperboard will simply fracture or the resulting paperboard container will lack strength and rigidity. The heat transfer from the die to the paperboard can only be controlled by the material of the die, the temperature of the die, or the degree of contact between the die and the paperboard. Although each of these can be controlled, they can only be varied to a certain degree. Because paperboard is extremely abrasive, it is very important that the die be manufactured from a material hard enough to withstand millions of cycles of paperboard drawing over it under pressure. Unfortunately, most materials that give up their heat easily are not very abrasive resistant and therefore quickly wear causing the fit between the die and paperboard to degrade. If the temperature of the die is increased too high, the paperboard or the coating on the paperboard will soften and stick to the die creating a jam in the machine. The fit of the die to the paperboard is dependent upon wear of the dies, consistency of the paperboard thickness, and expansion and contraction of the die due to normal temperature fluctuations in the course of production. However, if this heat transfer can be increased, then dwell time in the closed position can be decreased and thus speed can be increased providing the pressure remains constant. If the heat transfer is decreased, then the dwell must be increased and thus the speed decreases, once again provided the pressure remains constant.

If the thermoforming machine can provide for forming pressure variation through force increases or decreases, then there is one more variable that can be adjusted to compensate for changes in dwell or heat transfer. While most thermoforming machines have some type of pressure adjustment, it is of course limited and can only be increased so much before there is risk of overtaxing the machine or dies. Furthermore, tests have proven that pressure on the paperboard can only be increased to a certain point before plate or tray rigidity is decreased due to crushing of the paper fibers themselves. When increasing the pressure, it is very important that the wrinkles in the paperboard caused by the thermoforming process are very carefully controlled so as not to create large overlaps that can result in compression fractures of the paperboard.

Although the details may vary to some degree, the overall concept of thermoforming paper plates and trays remains the same regardless of the machine type. The moistened paperboard is cut to a predetermined shape and creased by the die cutting machine. The creases or scores are strategically placed in the areas where controlled wrinkles called pleats will be formed. This pre-creased, cut blank is stacked, then the stack is put into the blank feeder in the paperboard thermoforming machine. The blank feeder in the paperboard thermoforming machine then picks one blank at a time per lane, and drops the blank onto the draw ring surrounding the male forming die via gravity, guided by an stainless steel channel (blank chute), and is centered between a heated female cavity die on the top reciprocating platen, and a draw ring and heated male punch die on the bottom stationary bolster plate.

The heated female die descends via the top reciprocating platen in the paperboard thermoforming machine and contacts the paperboard resting on the draw ring in order to provide tension to the material as the male die pushes it into the female cavity. The purpose of the draw ring is to hold the paperboard tightly against the draw pad on the top female cavity die to control the formation of wrinkles in the container as the material slips into the heated female cavity. The draw ring forces the excess paper into the scored areas and ensures that neat, even gathers are made, following the precreased lines. The more tension applied to the paperboard through springs or air cylinders under the draw ring, the neater the controlled wrinkles. However, too much tension and the male punch will be pushed through the paperboard, causing fractures and tears. It is also extremely important that the material have enough moisture in it to make it soft and pliable. This level of moisture can range from 8.5% to 13% by weight depending upon the type of material and the geometric shape of the container being made.

After all of the paperboard has drawn into the heated cavity, the press extends the female cavity die solidly into the male punch die and allows the paperboard thermoforming press to build pressure either by springs, hydraulics or pneumatics, and holds the product under this pressure and heat for a period of dwell time. This allows the moisture in the board to turn into steam and escape through vents in the die, "setting" the product in the shape of the die. Essentially, the controlled wrinkles or pleats of board are steam pressed into the product, imparting structural integrity.

Therefore, it requires a combination of moist paperboard, good creasing, heat, pressure, and dwell to form good product.

The press then opens, finished plate or tray is ejected onto the conveyor where it is automatically stacked ready for inspection and packing.

It is accordingly an object of the present invention to provide a thermoforming method of forming paper lids and covers, and in particular sip lids and straw lids for beverage containers, and to paper lids and covers made using this method.

SUMMARY OF INVENTION

At a high level, this invention revolves around using thermoforming to make paper lids and covers for beverage containers, and in particular sip lids and straw lids; significantly, the paper lids and covers are thermoformed from a single piece of paper or paperboard. According to a first aspect of the invention there is provided a thermoforming method to form a paper lid or cover, the method comprising: drawing a piece of paper into a thermoforming die, the piece of paper having an inherent amount of moisture therein; heating and pressing the piece of paper in the die, the heat combining with moisture in the paperboard, so that in combination with the pressure provided by the die, the fibers of the paper deform to ultimately form a paper lid or cover with an acceptable amount of rigidity, the paper lid or cover comprising: a substantially flat, circular face; a peripheral formation around the flat, circular face, the peripheral formation being raised relative to the flat, circular face; and a lip extending around the lower edge of the peripheral formation, to engage a rim of a beverage container to which the lid or cover is to be fitted.

In an embodiment, the lip comprises a ridge to fit over the beverage container rim, with inner and outer flanges extending from either side of the ridge, the inner and outer flanges including at least one strengthening rib, with the heating and pressing method step including the step of forming these strengthening ribs.

In one version, the raised peripheral formation includes an aperture to define a sip hole, so as to define a paper sip lid, with the heating and pressing method step including the step of forming the sip hole, typically by punching the peripheral formation to define the sip hole. The paper sip lid would typically be used on coffee cups.

In another version, the flat, circular face includes a slit arrangement to accommodate a straw, so as to define a paper straw lid, with the heating and pressing method step including the step of forming the slit arrangement, typically by punching the flat, circular face to define the slit arrangement. The paper straw lid would typically be used on cold drink containers.

In an embodiment, as the piece of paper is drawn into the die, the diameter or original outside dimensions of the blank are reduced.

In embodiment, the method comprises printing indicia, information, branding, messages and the like, in any colour, on the piece of paper, typically before the piece of paper is drawn into the die.

The method may further include one or more of the other steps described above.

According to a second aspect of the invention there is provided a paper sip or straw lid formed in accordance with the thermoforming method defined above.

BRIEF DESCRIPTION OF DRAWINGS

The objects of this invention and the manner of obtaining them, will become more apparent, and the invention itself will be better understood, by reference to the following description of embodiments of the invention taken in conjunction with the accompanying diagrammatic drawings, wherein:

Figure 1 shows a flow diagram summarising a thermoforming method to form a paper sip lid, according to a first aspect of the invention; and

Figure 2 shows various views of a paper sip lid, according to a second aspect of the invention, and typically made in accordance with the method shown in Figure 1 . DETAILED DESCRIPTION OF AN EXAMPLE EMBODIMENT

The following description of the invention is provided as an enabling teaching of the invention. Those skilled in the relevant art will recognise that many changes can be made to the embodiment described, while still attaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be attained by selecting some of the features of the present invention without utilising other features. Accordingly, those skilled in the art will recognise that modifications and adaptations to the present invention are possible and can even be desirable in certain circumstances, and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not a limitation thereof.

Referring to Figures 1 and 2, a thermoforming method 10 (Figure 1 ) to form a paper sip lid 20 (Figure 2) is shown. The method 10 comprises drawing a piece of paper or paperboard into a thermoforming die, as shown by block 12, the piece of paper having an inherent amount of moisture therein.

The method 10 then comprises heating and pressing the piece of paper in the die, as shown by block 14, to form a paper sip lid 20 of the type shown in Figure 2. The heat combines with the moisture in the paperboard, so that in combination with the pressure provided by the die, the fibers of the paper deform to ultimately form the paper sip lid 20, the sip lid having an acceptable amount of rigidity.

As best shown in Figure 2, the paper sip lid 20 comprises a substantially flat, circular central face 22. The paper sip lid 20 includes a peripheral formation 24 around the flat, circular face 22, the peripheral formation 24 being raised relative to the flat, circular face 22.

A lip 26 extends around the lower edge of the peripheral formation 24, and is used to engage a rim of a beverage container to which the sip lid is to be fitted, in use. ln an embodiment, the lip 26 comprises a ridge 28 to fit over the beverage container rim, with inner and outer flanges 30, 32 extending from either side of the ridge 28. The inner and outer flanges 30, 32 include at least one strengthening rib, with the heating and pressing method step 14 including the step of forming these strengthening ribs.

In an embodiment, the raised peripheral formation 24 includes an aperture 34 to define a sip hole, with the heating and pressing method step 14 including the step of forming the sip hole, typically by punching the peripheral formation 24 to define the sip hole.

In an embodiment, as the piece of paper is drawn into the die, the diameter or original outside dimensions of the blank are reduced. The method may further include one or more of the other steps described above.