TECHNICAL FIELD

The invention relates to an apparatus and a method for thermoforming a material.

In particular, although by no means exclusively, the invention relates to an apparatus and a method for thermoforming a cellulose-containing material. other materials that have similar characteristics that have made it difficult to thermoform the materials into commercially useful products

BACKGROUND ART

Cellulose is an organic compound that is found in the cell walls of plants. The material was discovered in the 1800’s and was used to produce the first successful thermoplastic polymers.

Cellulose film has been manufactured continuously since the mid- 1930’ s and is still used commercially today.

However, since the mid- 1900’ s production of cellulose-derived polymers has been overtaken by petrochemical-derived polymers due to the variety and flexibility of their chemical properties. Petrochemical-derived polymers are extremely durable, this also means that they last a long time and can take tens or even hundreds of years to decompose. Whilst some petrochemical-derived polymers are recyclable, most of these polymers are not recycled and ultimately end up in landfill. In recent times, there has been a demand for polymers which have a lower impact on the environment.

Cellulose is a biodegradable and sustainable material and is therefore considered more environmentally friendly than petrochemical-derived polymers.

Cellulose is typically produced by first grinding plant matter into a pulp and adding a solvent, such as acetone, to the pulp and leaving the mixture for around 24 hours. The mixture is then spread out onto a stainless-steel conveyor belt and a plasticizer, such as vinegar, is added to the mixture. Heat and air pressure is then applied to remove the acetone from the mixture to leave only the cellulose material behind in a film form. The acetone is collected and reused. Thermoforming is a manufacturing process where a polymer (also referred to herein as a plastic) sheet is heated to a pliable forming (softening) temperature and then moulded to form a specific shape. Polyethylene terephthalate (PET) is a commonly used polymer for thermoforming for the manufacture of packaging.

In the process of thermoforming, a plastic material may undergo a phase transformation, due to the application of strain, known as strain crystallization. In this process, the initially disordered, i.e. amorphous, molecular structure becomes more ordered, i.e. crystalline. This process changes the mechanical properties of the material and increases its hardness. Once a material has become crystalline, it is generally accepted that the material has lost its thermoplastic properties and will no longer thermoform into a desired shape.

By way of example, the crystallization of PET occurs at a rate which is gradual enough to draw shapes having deep cavities, making it suitable for thermoforming a range of different packaging articles.

In contrast, it is generally understood and accepted that cellulose is not capable of being thermoformed into a shape having deep cavities (cavities with a depth to width ratio greater than 0.1). This is because, unlike PET, cellulose crystallizes too quickly for known thermoforming technology. In addition to the above, heated cellulose is difficult to transport to a mould while hot, as it sags under its own weight when heated.

It is desirable to provide a method for thermoforming a material comprising cellulose to produce moulded articles having suitably deep cavities.

SUMMARY OF THE INVENTION

The invention is a method for thermoforming a cellulose-containing material to form a product that has a product wall of the cellulose-containing material, the method including the steps of: i) heating a sheet of the material to be within a softening range of the material; ii) moving the heated sheet and/or a mould having a mould wall defining a profile of the product relative to each other so that the sheet is in an operative position to be formed into a product in the mould; and iii) heating an opposite surface of the sheet to a surface of the sheet facing the mould to maintain the temperature of the sheet within the softening range of the material; and iv) using air pressure and/or vacuum to form the heated sheet against the mould wall to form the product.

The sheet may be any suitable thickness, width and length.

The sheet may be up to 200 microns in thickness.

The sheet may be up to 150 microns in thickness.

The sheet may be up to 120 microns in thickness.

The invention extends to arrangements in which the sheet is a stack of a plurality of sheets, with a second sheet on top of a first sheet and so on.

In this context, the applicant has found that sheets of cellulose-containing material that are greater than 150 microns may not biodegrade quickly enough in some situations and this is a potential issue.

Specifically, the applicant has found that the use of two or more sheets, typically two sheets, typically sheets that are less than 150 microns, in a stack with one sheet on top of the other sheet in the operative position in step ii) makes it possible to form the product in steps iii) and iv) as a single unified sheet.

In addition, the applicant has found that providing a lower sheet in the operative position in step ii) with openings through the sheet thickness, for example being formed as perforations, can facilitate heat transfer to an upper sheet or sheets of the stack and contribute to forming the product.

The applicant has found that, when the lower sheet is formed with such openings, the product may include the lower and the upper sheet being secured together via sections of the upper sheet being forced into the perforations during step iv) and thereby holding the sheets together.

It follows from the above that step i) may include heating a stack of a plurality of sheets, with a second sheet on top of a first sheet and so on to be within the softening range of the material.

In addition, step ii) may include moving the heated stack of sheets and/or a mould defining a profile of the product relative to each other so that the stack of sheets is in the operative position to be formed into the product in the mould. In addition, step iii) may include heating an opposite surface of the stack of sheets to the surface of the stack facing the mould to maintain the temperature of the stack of sheets within the softening range of the material.

In addition, step iv) may include using air pressure and/or vacuum to form the heated stack of sheets into the mould to form the product.

There may be two sheets in the stack.

There may be more than two sheets in the stack.

At least one of the sheets, typically a lower sheet in the operative position in step i) may have openings extending through the sheet to facilitate transfer of vacuum or air pressure to the sheets in the stack.

The product may be any suitable product.

The product may be a packaging product, such as a food tray, having a base and a side wall extending upwardly from the base and defining a compartment.

In some embodiments, the method produces the packaging product with a draw depth to width ratio of greater than 0.1.

In other embodiments the method produces the packaging product with a draw depth to width ratio of between 0.1 and 5.0.

The heating in steps (i) and/or (iii) may be conducted by any suitable heat source.

In some embodiments, heating in steps (i) and/or (iii) are conducted using a radiant heat source. An advantage of a radiant heat source is that it reduces the possibility of the contact between the sheet of material and the heat source.

In some embodiments, the softening range of the cellulose material is between 110-140 °C.

In some embodiments, the mould is chilled to be at a temperature below ambient temperature.

In other embodiments, the mould may be chilled to be at a temperature at or below 10 °C.

In some embodiments, the air pressure is produced via a vacuum device removing air from the mould.

In other, but not the only other, embodiments, the air pressure is produced by blowing air. The material may be any suitable cellulose-containing material.

The material may be 100% by weight cellulose or a blend of cellulose and other materials, such as plastic materials.

The material may be at least 90% by weight cellulose.

The material may be at least 95% by weight cellulose.

The invention is not confined to a cellulose-containing material and extends to other materials that have similar characteristics that have made it difficult to thermoform the materials into commercially useful products.

By way of example, the invention is particularly suitable for materials exhibiting high strain crystallisation rates.

However, the method and the apparatus may be adapted to be used with materials having a range of different crystallisation rates by controlling the parameters of the second heating device, i.e. temperature, power, energy, duration, etc.

The invention is also an apparatus for thermoforming a cellulose-containing material to form a product that has a product wall of the cellulose-containing material, the apparatus comprising: a mould having a mould wall that defines a profile of the product; a heat source; and a source of air pressure and/or a vacuum-forming device, wherein, in use the heat source is adapted to heat a sheet of the cellulose-containing material to be within a softening range of the material whilst air pressure from the source of air pressure and/or the vacuum-forming device forms the heated sheet against the mould wall to form the product.

The apparatus may be adapted for thermoforming a stack of a plurality of sheets of the cellulose-containing material, typically two sheets, with a second sheet on top of a first sheet and so on.

The apparatus may include a feed assembly for forming the stack of the sheets and feeding the stack to the mould.

The invention is also a thermoformed product that includes a product wall made from a cellulose-containing material.

The product wall may include at least two sheets of the cellulose-containing material that are thermoformed together.

At least one of the sheets may have openings, such as perforations, and cellulose-containing material from an adjacent sheet may be in the openings as a consequence of thermoforming the product and thereby contribute to holding the sheets together.

The product may be a packaging product, such as a food tray, having a base and a side wall extending upwardly from the base and defining a compartment.

In some embodiments, the packaging product may have a draw depth to width ratio of greater than 0.1.

In other embodiments the packaging product may have a draw depth to width ratio of between 0.1 and 5.0.

BRIEF DESCRIPTION OF THE DRAWINGS

Notwithstanding any other forms which may fall within the scope of the apparatus and method as set forth in the above Summary, specific embodiments are now described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a step-by-step diagram of a prior art thermoforming apparatus and cycle;

Figure 2 shows a step-by-step diagram of a thermoforming apparatus and cycle according to an embodiment of the invention; and

Figure 3 shows a step-by-step diagram of a thermoforming apparatus and cycle according to another, but not the only other possible, embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 shows a step-by-step diagram of a prior art thermoforming apparatus and cycle used to thermoform a plastic material.

With reference to the Figure, the thermoforming apparatus comprises:

- a heat source 10,

- a moulding device 20 having a mould 22 that has an internal cavity 26 bounded by internal walls of the mould 22, with the cavity defining the shape of a product; and

- a sheet holding/transporting device 14 that is configured to hold the perimeter of a sheet of plastic material 12 and to transport the plastic sheet 12. The thermoforming cycle, according to the prior art, has three steps: ‘a’; ‘b’; and ‘c’ as follows.

In step ‘a’, a sheet of plastic material 12a is heated within a softening range of the material by applying heat using the heat source 10 - see Figure 1(a).

In step ‘b’, the now heated sheet of plastic material 12b is transported by the sheet holding/transporting device 14 to the forming device 20 and positioned on top of the mould 22 above the internal cavity 26, as illustrated in Figure 1(b).

In step ‘c’, air is drawn from the internal cavity 26, as shown by the arrows 24 in Figure 1(c), to generate a negative pressure (i.e. partial vacuum) within the internal cavity 26. The partial vacuum draws the heated sheet of plastic material 12 into the internal cavity so that the sheet of plastic material 12 contacts the internal walls of the mould 22 and thereby conforms to the shape defined by the internal walls. The mould is typically chilled below ambient temperature. When the sheet of plastic material 12 contacts the internal walls, the plastic material cools and solidifies into the product shape.

When steps ‘b’ and ‘c’ are applied to a sheet of cellulose material, a heated sheet of cellulose material 12 is drawn out from the sheet and the material undergoes a phase transformation, due to the application of strain. In this strain crystallization process, the initially disordered, i.e. generally amorphous, molecular structure becomes more ordered, i.e. crystalline. This process changes the mechanical properties of the cellulose material and increases resistance to the forming action as the sheet of cellulose material is being drawn out. The end result is that the prior art thermoforming apparatus and cycle shown in Figure 1 (and other prior art thermoforming apparatus and cycles known to the applicant) is not suitable for thermoforming cellulose material.

The suitability of a material for use in a thermoforming apparatus and cycle, such as that described in relation to Figure 1, is largely dependent on the rate at which crystallization occurs in the material. For example, the crystallization of PET occurs at a rate which is gradual enough to draw shapes having deep cavities, making it a suitable material for thermoforming. In contrast, the crystallization of cellulose occurs at a much higher rate, making it less suitable for thermoforming processes. Figure 2 shows a step-by-step diagram of a thermoforming apparatus and cycle for thermoforming cellulose material according to a first embodiment of the invention. The apparatus for thermo forming comprises: a first heat source 110, a moulding device 120 having (a) a mould 122 that has an internal cavity 128 bounded by internal walls of the mould 122, with the cavity defining the shape of a product, and (b) a second heat source 126; and a sheet holding/transporting device 114 that is configured to hold the perimeter of a sheet of plastic 112 and to transport the sheet.

The first and second heat sources 110, 126 are radiant heat sources.

The thermoforming cycle according to the first embodiment of the invention, has three steps: ‘a’; ‘b’; and ‘c’ as follows.

In step ‘a’, a sheet of cellulose material 112a is heated, within a softening range of the material, by applying heat to a surface of the sheet of cellulose material 112a using the first heat source 110.

In step ‘b’, the now heated sheet of cellulose material 112b is transported by the sheet holding/transporting device 114 to the moulding device 120 and positioned on top of the mould 122 above the cavity 128.

In step ‘c’, air is drawn out of the internal space 128 as shown by the arrows 124 to form a negative pressure (i.e. vacuum) within the internal space 128. The heated sheet of cellulose material is drawn into the mould by the negative pressure and contacts and adopts the shape defined by the internal walls of the mould. The mould is chilled below ambient temperature, typically at or below 10 °C, using cold water. As a consequence, when the cellulose material contacts the internal walls of the mould, the cellulose material cools and solidifies into the product shape.

During steps ‘b’ and ‘c’, the second heat source 126 applies heat to a surface of the heated sheet of cellulose material.

The applicant made a surprising finding that maintaining the application of heat to a heated sheet of cellulose material during step ‘c’ makes it possible to significantly increase the drawing depth of sheets of cellulose material and therefore the commercial usefulness of thermoformed products. Previous attempts to thermoform sheets of cellulose material without this maintained heat, in processes such as that described in relation to Figure 1, were not successful in terms of producing commercially acceptable products.

Whilst not wishing to be bound by the following comments in this paragraph, it is thought that maintaining the application of heat causes crystals formed by strain crystallization are reverted into an amorphous form. This function is particularly useful for materials containing cellulose, which could previously typically only be drawn to a shallow depth because strain crystallization occurs at a relatively high rate.

The invention allows materials containing cellulose to be deep drawn by depth to width ratios of greater than 0.1 and in some cases as much as 5.0.

Figure 3 shows a step-by-step diagram of the thermoforming apparatus and cycle for thermoforming cellulose material according to a second embodiment of the invention.

The thermoforming apparatus shown in Figure 3 comprises: a first heat source 210; a moulding device 220 having (a) a mould 222 with external walls that define a shape of a product, (b) a base 224 forming an internal channel 226 therebetween, and (c) a second heat source 230; and a sheet holding/transporting device 214 that is configured to hold the perimeter of a sheet of plastic material 212 and to transport the sheet.

The thermoforming cycle according to the second embodiment of the invention, functions in much the same way as the first embodiment with the exception that the mould 222 is a positive pressure mould.

With this arrangement, a sheet of cellulose material 212 is pressed onto the external walls of the mould 222 and adopts the shape defined by the external walls.

The mould is chilled below ambient temperature, typically at or below 10 °C, using cold water. As a consequence, when the cellulose material contacts the external walls, the cellulose material cools and solidifies into the product shape.

The applicant has carried out test work on the embodiment of the invention shown in Figure 2 and successfully thermoformed sheets of cellulose material into commercial products.

Another advantage of the invention is that the components which enable the invention to work with cellulose material, in particular the second heating device, can be easily retrofitted into an existing thermoforming apparatus with very few modifications.

Whilst a number of specific apparatus and method embodiments have been described, it should be appreciated that the apparatus and method may be embodied in many other forms.

Whilst the invention is particularly useful for thermoforming materials containing cellulose, it is by no means limited to these materials.

The method and apparatus of the invention is particularly suitable for materials exhibiting high strain crystallisation rates.

However, the method and apparatus may be adapted to be used with materials having a range of different crystallisation rates by controlling the parameters of the second heating device, i.e. temperature, power, energy, duration, etc.

The first and second heat sources described in the above embodiments are radiant heat sources. The applicant has found that radiant heat sources are particularly suitable for thermoforming material containing cellulose because they can be effectuated from a distance and as such, reduce the possibility of contact between the sheet of material and the heat source. Radiant heat sources, include convective, dielectric heating and the like.

However, other heat sources may be used. For example, contact heating may be used, such as in situations where the material has a low tackiness when in the softening range.

The above embodiments describe a method and apparatus for thermoforming a cellulose containing material involving the use of air pressure to create a vacuum. However, it is also envisaged that the thermoforming method could be performed by blowing air against a surface of the sheet to force it against the mould, i.e. positive air pressure. Furthermore, it is also envisaged that the thermoforming method of the invention could be performed without using air pressure at all, for example by using solely the contact pressure from the mould itself.

Unless otherwise stated to the contrary, terms such as polymer and plastic are considered to have the same meaning and are used interchangeably.

In the claims which follow, and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word “comprise” and variations such as “comprises” or “comprising” are used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the apparatus and method as disclosed herein. Further patent applications may be filed in Australia or overseas on the basis of, or claiming priority from, the present application. It is to be understood that the following provisional claims are provided by use of example only and are not intended to limit the scope of what may be claimed in any such future applications. Features may be added to or omitted from the provisional claims at a later date so is to further define or re-define the invention or inventions.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.