Extrusion die and method for extruding and foaming polymer material

Abstract

The invention is directed to an extrusion die for extruding and foaming polymer material, comprising a flow channel including a converging section, converging from a hydraulic inlet diameter (D1) to a smaller hydraulic intermediate diameter (D2), and a diverging section, downstream of the converging section, diverging from the hydraulic intermediate diameter (D2) to a larger hydraulic outlet diameter (D3).

Description

[0001] Title: Extrusion die and method for extruding and foaming polymer material

[0002] Field of the invention

[0003] The invention relates to the field of polymer processing. The invention can be suitably integrated in extrusion foam processes as it reduces post-processing of foamed products. In particular, the invention is directed to an extrusion die and method for extruding and foaming polymer material.

[0004] State of the art

[0005] In the polymer processing industry, extrusion is a common process to form polymer products, such as food products, (bio-based) plastic products, and packaging products, including foamed plastics, from a polymer material that is forced out of an extruder and through a die of a desired cross-section. Such a process is more efficient for producing foamed polymer products compared to batchwise foaming processes, as disclosed, for example, in WO 96 / 30186 A1.

[0006] Achieving high dimensional accuracy in extruded foamed polymer products is a major challenge. Examples of dies used to produce foamed products are disclosed in WO 2008 / 034127 and WO 2013 / 148841. Dies strongly affect the product’s shape due to large and abrupt changes in the density of polymer material at their outlet. The shapes of extruded foamed polymer products are typically determined by isotropic expansion, orthogonal swelling due to viscoelastic properties of the polymer material, and flow distribution across the entire cross-section of the die.

[0007] In extrusion dies known in the art, these factors are strictly interconnected and cannot be tuned independent of each other, making die design complex and the product’s cross-section shape unpredictable. In addition, the divergence of the die flow channel affects mechanical properties of foamed extrusion products by controlling the amount of residual elastic stresses in the polymer material and / or the molecular orientation of the foamed polymer material. Hence, there remains a need in the art for achieving high dimensional accuracy in foamed extrusion products.

[0008] Object of the invention

[0009] It is an object of the invention to provide an extrusion die for manufacturing foamed polymer materials that addresses, at least in part, the aforementioned disadvantages. A further object of the invention is to fine-tune the cellular morphology of extruded polymer material. Yet a further object of the invention is to address one or more additional disadvantages associated with prior art extrusion dies for manufacturing foamed polymer materials.

[0010] Detailed description

[0011] The invention provides an extrusion die for extruding and foaming polymer material, comprising a flow channel including: a converging section, converging from a hydraulic inlet diameter (D1) to a smaller hydraulic intermediate diameter (D2), and a diverging section, downstream of the converging section, diverging from the hydraulic intermediate diameter (D2) to a larger hydraulic outlet diameter (D3).

[0012] Due to the dimensions of the hydraulic inlet diameter (D1), hydraulic intermediate diameter (D2), and hydraulic outlet diameter (D3), the extrusion die is suitable for manufacturing foamed products made of polymer material with a dimensionally accurate cross-section. This is because it allows isotropic expansion, orthogonal swelling due to viscoelastic properties of the polymer material, and flow distribution across the entire cross-section of the die to be decoupled from one another. This enables more precise and accurate control of the product shape after extrusion and foaming.

[0013] Along the displacement direction of polymer material through the flow channel of the extrusion die, a converging section is encountered where the cross-sectional area of the flow channel decreases from the hydraulic inlet diameter (D1) along the length of the converging section. At the end of the converging section, the flow channel has a hydraulic intermediate diameter (D2), which is smaller than D1.

[0014] The converging section is configured such that it can compress polymer material, forcing it through a reduced cross section where normal stresses accumulate. This may contribute to the distribution of the polymer material across substantially the entire cross-section of the flow channel. The pressure at which the polymer material can be compressed may be significantly higher than the pressures occurring in extrusion dies known in the art.

[0015] The cross-section of the flow channel at D2 may contribute to a desirable degree of isotropic foaming.

[0016] Downstream of the converging section is a diverging section where the cross-sectional area increases along its length. At the end of the diverging section, the flow channel has a hydraulic outlet diameter (D3) that is smaller than or equal to D1 and larger than or equal to D2.

[0017] The diverging section is configured such that polymer material is allowed to relax by releasing the accumulated mechanical stresses initiated in the converging section. This relaxation enables the polymer material to expand and initiate foaming. The diverging section may substantially prevent excessive swelling of the foamed polymer material, thereby contributing to improved dimensional accuracy.

[0018] The hydraulic diameters D1 , D2, and D3 are shape-corrected cross-sectional dimensions of the flow channel. The hydraulic diameter may be defined as four times the cross-sectional area (A) of the flow channel, divided by the wetted perimeter (P) of the cross-section, which is the length of the perimeter that is in contact with the polymer material in the flow channel.

[0019] If the flow channel has a circular cross-section, then the hydraulic diameter of the flow channel equals the actual diameter of the flow channel. On the other hand, if the flow channel has a square cross-section, then the hydraulic diameter of the flow channel equals the width of the flow channel.

[0020] The ratio of D1 to D2 may be in the range from 6 to 20, for example, between 8 and 12, preferably about 10. Alternatively or additionally, the ratio of the length to the width of the converging section may be less than 5. Advantageously, these ratios result in improved dimensional accuracy of foamed products obtained with the extrusion die of the invention. An additional advantage of these ratios is improved foaming of the extruded polymer material.

[0021] The length of the converging section may be defined as the straight, linear distance along the displacement direction of polymer material. The width of the converging section may be defined as a value between D1 and D2, for example, the average of D1 and D2.

[0022] The ratio of D3 to D2 may be in the range from 2 to 8, for example, between 2.5 and 6, preferably about 3. Alternatively or additionally, the ratio of the length to the width of the diverging section may be less than 5. Advantageously, these ratios result in improved dimensional accuracy of the foamed product. An additional advantage of these ratios is improved foaming of the extruded polymer material.

[0023] The length of the diverging section may be defined as the straight, linear distance along the displacement direction of polymer material. The width of the diverging section may be defined as a value in the range of D2 to D3, for example, the average of D2 and D3.

[0024] The converging section and the diverging section may be positioned adjacent to one another, with D2 located between both sections. The flow channel may feature a direct transition between the converging section and the diverging section to minimize the overall length of the die and to enable a relatively large degree of foaming, due to the compression of the polymer material only occurs over a relatively short distance. This transition may be either abrupt or gradual, but sufficiently short such that, during use, the polymer material is allowed to expand in the diverging section after being compressed in the converging section.

[0025] The flow channel may comprise an intermediate section between the converging section and the diverging section to further improve the stability of the foamed product and to improve the controllability of the foaming. The intermediate section preferably has a substantially constant cross-sectional area with diameter D2. The intermediate section may be configured such that, during use of the extrusion die, the polymer material remains compressed for a certain period of time before entering the diverging section. The optional substantially constant cross-section of the intermediate section may additionally provide substantial relaxation time for the polymer material, allowing sufficient stabilization.

[0026] The flow channel may comprise an outflow section downstream of the diverging section, preferably with a substantially constant cross-sectional area. The outflow section may stabilize the foamed polymer material as it exits the diverging section. By selecting an appropriate length for the outflow section along the displacement direction, expansion of the foamed polymer material can be restrained. The optional substantially constant cross-section of the outflow section may additionally provide substantial relaxation time for the foamed polymer material, allowing sufficient stabilization. This may result in a dimensional stable product and a stable foaming process.

[0027] The invention further provides a method for manufacturing foamed polymer material, comprising the steps of: providing the extrusion die of the invention; extruding polymer material through the flow channel of the extrusion die, thereby forming the foamed polymer material.

[0028] The extrusion die may be positioned downstream of an extruder device that supplies the polymer material to an inlet of the converging section of the flow channel.

[0029] The method of the invention advantageously allows for the manufacturing of foamed polymer products with a dimensionally accurate cross-section, as the invention enables more precise and accurate control over the final product shape after extrusion and foaming.

[0030] The extruded material may be a viscoelastic polymer material, such as a fossil-based thermoplastic polymer or bio-based thermoplastic polymer, for example, a thermoplastic natural polymer, which may be starch-based. Viscoelastic polymer material may absorb mechanical stresses by deforming elastically, thereby storing elastic energy as the material is displaced through the converging section, and release this energy as the material is displaced through the diverging section.

[0031] In particular, the polymer material may comprise starch, for example, a modified starch, starch from a natural source, and / or starch from an artificial source. Furthermore, the polymer material may comprise starch-based formulations comprising cellulose fibres in amounts more than 5%. All of these polymer materials, particularly the starch-based formulations, may exhibit thermoplastic behavior, for example, as thermoplastic starch. Such polymer materials may advantageously result in a foamed polymer product with improved dimensional and mechanical properties. The polymer material may comprise a starch content of more than 10 wt.% by total weight of the polymer material, such as more than 50 wt.% or more than 80 wt.%. The polymer material may comprise a thermoplastic polymer, such as a bio-based thermoplastic polymer, for example, a thermoplastic natural polymer, which may be starch-based. Further examples of materials that the polymer material may comprise include polylactic acid (PLA), polyethylene furanoate (PEF), partially bio-based material, such as polybutylene succinate (PBS) or polyethylene terephthalate (PET), and / or bio-based polyolefins, such as polyethylene (PE) or polypropylene (PP).

[0032] Alternatively or additionally, the polymer material may comprise a natural polymer material, such as starch, cellulose, or a polyhydroxyalkanoate (PHA).

[0033] The extrusion step may comprise: compressing the polymer material in the converging section by increasing the elastic stress in the polymer material, and displacing the compressed polymer material through the diverging section, thereby decreasing the elastic stress and inducing foaming in said material.

[0034] The polymer material may be subjected to normal stress OECOW in the converging section and to normal stress OEdiv in the diverging section. It will be appreciated that the extrusion die of the invention is typically configured to subject polymer material to these normal stresses. The normal stresses, OEconv and OEdiv, may be defined using the Oldroyd-B model for modelling normal stresses in the extensional flow of a viscoelastic fluid, i.e., a viscoelastic polymer material (Bird, R.B. et al., Dynamics of Polymeric Liquids, John Wiley & Son, 1987 (pages 346-349)), using formulas (la) and (lb): in which: rj is the viscosity of the polymer material, rjPis the viscosity of a polymer in the fluid, rjs is the viscosity of a solvent in the fluid, X is the relaxation time, sconvrepresents the extensional deformation in the converging section of the die, sd(-vrepresents the extensional deformation in the diverging section of die, and t is time. The parameters rj, rjP, rjs, and X represent rheological characteristics of the polymer material. The extensional deformation s depends on the geometry of the die and, at least in practice, will be a function of time.

[0035] Under certain circumstances, As can be more than 1 , which may correspond to a situation where the polymer material behaves purely elastic. Under such circumstances, formulas (la) and (lb) can be simplified to: where st = s, and s is the deformation as a dimensionless value. In this case, the polymer material will fully recover the deformation as subjected to during compression at the die exit, as it behaves as a perfectly elastic material. For viscoelastic polymer materials, a value for OEconv can be obtained when the extensional deformation of the polymer material, £conv, is taken for the converging section of the die. Similarly, a value for OEdiv can be obtained when the extensional deformation of the polymer material, sdiv, is taken for the diverging section of the die. For the sake of simplicity, s may be assumed constant over time. sconvcan then be calculated using formula (II) (Cogswell, F.N., Polym. Eng. Sci. 1972, in which: ui is the average velocity at the hydraulic inlet diameter D1 , U2 is the average velocity at the hydraulic intermediate diameter D2, Vi is the volume of the converging section, which may be defined by a trapezoidal shape with D1 and D2 as bases, and Q is the volumetric flow rate. Likewise, sdivcan then be calculated using formula (III): in which: U3 is the average velocity at the hydraulic outlet diameter D3, V2 is the volume of the diverging section, which may be defined by a trapezoidal shape with D3 and D2 as bases.

[0036] A ratio OE of OEconv to OEdiv is defined by formula (IV): preferably wherein OE is in the range of 0.4 - 0.9.

[0037] The inventors found that when the elastic stresses in the polymer material are relatively high, for example, when OEconv is relatively large in relation to OEdiv, the dimensional accuracy of the foamed polymer material may be compromised due to strong swelling at the die exit. Conversely, the inventors found that when the elastic stresses in the polymer material are relatively low, for example, when OEconv is relatively small in relation to OEdiv, the foaming behavior of the polymer material may be poor because of a poor nucleation rate of bubbles in the polymer material, which can lead to pre-foaming of the polymer material inside the die.

[0038] Surprisingly, a OE ratio of 0.4 - 0.9 results in desirable properties of the foamed polymer material, as it provides a beneficial balance between the normal stress OEconv in the converging section (when the polymer material is compressed) and the normal stress OEdiv in the diverging section (when the polymer material is allowed to expand). The ratio OE may be generically applicable, since it is a unitless value corrected for material properties of the polymer material and is substantially independent of kinetic factors, such as the displacement velocity of the polymer material.

[0039] The preferred OE range of 0.4 - 0.9 may provide a well-defined shape and a good nucleation rate of bubbles in the polymer material, thus promoting desirable foaming of the polymer material. A value of OE less than 0.4, especially 0.3 or less, may result in a not sufficiently accurate shape of the foamed polymer material, whereas a OE of more than 0.9, especially 1.0 or more, may lead to insufficient foaming of the polymer material.

[0040] The polymer material may be subjected in the converging section to an extensional deformation rate EDRcon, defined by formula (V), and / or the polymer material may be subjected in the diverging section to an extensional deformation rate EDRdiv, defined by formula (VI), wherein formulas (V) and (VI) are: in which:

[0041] Q represents the volumetric flow rate of polymer material through the flow channel, Vi represents the volume of the converging section, V2 represents the volume of the diverging section, and wherein the sum of EDRconand EDRdiv is in the range of 0.1 s-1- 50 s’1.

[0042] Compression in the converging section and relaxation in the diverging section can be defined by the extensional deformation rate (EDR) in s’1, representing the rate at which the polymer material is compressed and allowed to relax. EDR is calculated by taking the volumetric flow rate of the respective section and dividing it by that section’s volume, then multiplying the result by the ratio of D1 to D2 to D3. Vi may be defined by a trapezoidal shape with D1 and D2 as bases and L1 as length, while V2 may be defined by a trapezoidal shape with D2 and D3 as bases and L2 as length.

[0043] Advantageously, the EDR in formulas (V) and (VI) may respectively approximate formulas (la), (lb), and (II) for certain polymer materials, especially when the polymer material is a viscoelastic polymer material, in particular one comprising starch. When the sum of EDRconand EDRdiv is in the range of 0.1 s-1to 50 s-1, more precise and accurate control of the product shape after extrusion and foaming, e.g., a well-defined cross-section, can be achieved, as well as controlled expansion during foaming.

[0044] The polymer material may be displaced through the flow channel at a volumetric flow rate (Q) such that the resulting mass flow is in the range of 2-10 g / s, for example, 4-8 g / s, or about 5 g / s, about 6 g / s, or about 7 g / s. Advantageously, such volumetric flow rates may also result in more precise and accurate control of the product shape after extrusion and foaming, e.g., a well-defined cross-section, as well as controlled expansion during foaming.

[0045] The invention further provides a foamed polymer material obtainable by the method of the invention. The foamed polymer material may be a foamed polymer product.

[0046] The invention further provides the use of the extrusion die of the invention in food processing, plastic processing, such as bio-based plastic processing, or packaging processing.

[0047] The invention allows manufacturing of foamed polymer products that are made of polymer material with a dimensionally accurate cross-section. This advantageously enables the polymer products to be used for food processing, plastic processing, or packaging processing. The invention has been described by reference to various embodiments, and methods. The skilled person understands that features of various embodiments and methods can be combined with each other.

[0048] Brief description of the drawings

[0049] Embodiments of the invention will be described in detail with reference to the accompanying drawings in which:

[0050] Figure 1a schematically depicts a prior art extrusion die,

[0051] Figure 1 b schematically depicts an embodiment of the extrusion die according to the invention,

[0052] Figure 2a shows a photo of a foamed polymer product manufactured with the prior art extrusion die in figure 1a,

[0053] Figure 2b shows a photo of a foamed polymer product manufactured with the extrusion die in figure 1b according to the method of invention, and

[0054] Figures 3a - 3c depict cross-sectional views of various embodiments of the extrusion die according to the invention.

[0055] Detailed description

[0056] Figure 1a schematically depicts a prior art extrusion die 100. The die 100 includes a cylindrical portion 101 that is configured to be attached to an extruder device (not shown) and a flow channel consisting of a converging section A’. In the converging section A’, the polymer material is compressed since the cross-sectional area of the flow channel in the converging section A' decreases along its length.

[0057] Downstream of the converging section A', the polymer material emerges from the flow channel, where it is suddenly allowed to relax, since the material is no longer confined within the flow channel. Upon relaxation, the polymer material will foam, forming a foamed polymer material. Due to the sudden ending of the flow channel, the foaming of the polymer material may be too sudden and uncontrolled.

[0058] Figure 1 b schematically depicts an embodiment of the extrusion die 1 according to the invention. The extrusion die 1 comprises a cylindrical portion 101 and a converging section A that is longer than the converging section A’ of extrusion die 100.

[0059] The extrusion die 1 further comprises a diverging section B located downstream of the converging section A. In the diverging section B, the cross-sectional area of the flow channel increases along its length, which at least partially releases the compressive pressure acting on the compressed polymer material, allowing the polymer material to relax and foam in a more controlled manner as compared to extrusion die 100, since the foaming polymer material is still confined within the diverging section B. Furthermore, the extrusion die 1 comprises an outflow section C in the flow channel located downstream of the diverging section B. The outflow section C has a substantially constant cross-sectional area along its length. The outflow section C helps stabilize the foamed polymer product as it exits the diverging section B. In the outflow section C, the expansion of the foamed polymer products is delayed. The substantially constant cross-section of the outflow section C may additionally provide a substantial relaxation time for the foamed polymer material, promoting sufficient stabilization.

[0060] The foamed polymer products depicted in figures 2a and 2b are made of the same polymer material comprising a starch content of about 80 wt.% by total weight of the polymer material. Both products were manufactured at the same die temperature and at the same volumetric flow rate (Q) of about 5 g / s. The properties of both products and the parameters of the extrusion process are presented in table 1.

[0061] Table 1 shows that with the extrusion die of the invention, foamed polymer products can be obtained with lower densities and reduced open cell contents. Furthermore, the pressure in the flow channel of the extrusion die of the invention can be higher, which may result in improved distribution of the polymer material inside the flow channel.

[0062] The polymer product shown in figure 2b was manufactured using the inventive extrusion die 1 , with a ratio OE of the normal stress OEconv in the converging section to the normal stress OEdiv in the diverging section of about 0.7. This value lies within the preferred range of OE (between 0.4 and 0.9), allowing for more precise and accurate control of the product shape after extrusion and foaming, e.g., a well-defined cross-section, as well as controlled expansion during foaming.

[0063] The polymer product shown in figure 2a was manufactured using the prior art die 100, which has a ratio OE of more than 0.9. As shown in figure 2a, this foamed polymer product lost dimensional stability due to excessive swelling upon exiting the die 100.

[0064] Figures 3a - 3c depict cross-sectional views of various embodiments of the extrusion die 1. In each figure, from left to right, the flow channel of the extrusion dies comprises the converging section A with a length L1 , where the flow channel tapers from a hydraulic inlet diameter D1 to a smaller hydraulic intermediate diameter D2. Downstream of the converging section A2 is the diverging section B with a length L2, which tapers outward from the hydraulic intermediate diameter D2 to a hydraulic outlet diameter D3. D3 is smaller than or equal to D1 but larger than or equal to D2.

[0065] The outflow section C, with a length L3, is located downstream of the diverging section B. The outflow section C has a substantially constant cross-sectional area over its entire length, with a substantially constant width or diameter that is substantially equal to D3.

[0066] In the extrusion die in figure 3a, the hydraulic inlet diameter D1 is larger than the hydraulic intermediate diameter D2, while the hydraulic outlet diameter D3 is larger than the hydraulic intermediate diameter D2 but smaller than the hydraulic inlet diameter D1. The sum of EDRcon, being the extensional deformation rate in the converging section A, and EDRdiv, being the extensional deformation rate in the diverging section B, for the extrusion die in figure 3a is in the range of 0.1 s-1- 50 s’1.

[0067] In the extrusion die in figure 3b, the hydraulic inlet diameter D1 is larger than the hydraulic intermediate diameter D2, while the hydraulic outlet diameter D3 is substantially the same as the hydraulic inlet diameter D1. The sum of EDRconand EDRdiv is less than 0.1 s’1. In this regime, D3 may be too large for certain polymer materials, possibly disadvantageously affecting the extrusion process by initiating foaming in the converging section A. This may be due to the pressure level of the polymer material being too low in the converging section A.

[0068] In the extrusion die in figure 3c, the hydraulic inlet diameter D1 is larger than the hydraulic intermediate diameter D2, while the hydraulic outlet diameter D3 is substantially the same as the hydraulic intermediate diameter D2. The sum of EDRconand EDRdiv is lower than 50 s1. In this regime, D3 may be too small for certain polymer materials, possibly disadvantageously affecting the dimensional stability of the foamed polymer product. This may result from excessively high pressure in the diverging section B and the outflow section C relative to the ambient pressure. The sudden pressure drop when the polymer material exits the outflow section C may cause the foaming process to become too abrupt and uncontrolled.

Claims

CLAIMS1. Extrusion die for extruding and foaming polymer material, comprising a flow channel including: a converging section, converging from a hydraulic inlet diameter (D1) to a smaller hydraulic intermediate diameter (D2), and a diverging section, downstream of the converging section, diverging from the hydraulic intermediate diameter (D2) to a larger hydraulic outlet diameter (D3).

2. Extrusion die according to claim 1, wherein D1 : D2 is in the range between 6 and 20, for example between 8 and 12, preferably about 10.

3. Extrusion die according to claim 1 or 2, wherein the ratio of a length of the converging section to a width of the converging section is less than 5.

4. Extrusion die according to any one of claims 1-3, wherein D3 : D2 is in the range between 2 and 8, for example between 2.5 and 6, preferably about 3.

5. Extrusion die according to any one of claims 1-4, wherein the ratio of a length of the diverging section to a width of the diverging section is less than 5.

6. Extrusion die according to any of the preceding claims, wherein the converging section and the diverging section are positioned adjacent to one another with D2 provided between the converging section and the diverging section.

7. Extrusion die according to any one of claims 1-5, wherein an intermediate section is provided between the converging section and the diverging section.

8. Extrusion die according to claim 7, wherein the intermediate section comprises a substantially constant cross-sectional area with D2.

9. Extrusion die according to any of the preceding claims, wherein the flow channel comprises an outflow section downstream of the diverging section.

10. Extrusion die according to claim 9, wherein the outflow section has a substantially constant cross-sectional area.

11. Extrusion die according to any one of claims 1-10, wherein the converging section is configured to subject polymer material to a normal stress OEconv in the converging section and the diverging section is configured to subject polymer material to a normal stress OEdiv in the diverging section, wherein the normal stresses OEconv and OEdiv are defined in formulas (la) and (lb): Oin which:>7 represents the viscosity of the polymer material, r]Prepresents the viscosity of a polymer in the fluid, rjs represents the viscosity of a solvent in the fluid, represents the relaxation time,£convrepresents the extensional deformation in the converging section of the die, represents the extensional deformation in the diverging section of the die, and t represents the time.

12. Extrusion die according to claim 11 , wherein a ratio OE of OEconv to OEdiv is defined as13. Extrusion die according to claim 12, wherein OE is in the range of 0.4 - 0.9.

14. Method for manufacturing foamed polymer material, comprising the steps of providing an extrusion die according to any one of the preceding claims; extruding polymer material through the flow channel of the extrusion die, thereby forming the foamed polymer material.

15. Method according to claim 14, wherein the extrusion step comprises: compressing the polymer material in the converging section by increasing the elastic stress in the polymer material, and displacing the compressed polymer material through the diverging section, thereby decreasing the elastic stress and inducing foaming in said material.

16. Method according to claim 14 or 15, wherein the polymer material is subjected to a normal stress OEconv in the converging section and to a normal stress OEdiv in the diverging section, wherein the normal stresses OEconv and OEdiv are defined in formulas (la) and (lb):in which:>7 represents the viscosity of the polymer material, rjp represents the viscosity of a polymer in the fluid, rjs represents the viscosity of a solvent in the fluid, represents the relaxation time,£convrepresents the extensional deformation in the converging section of the die,Edivrepresents the extensional deformation in the diverging section of the die, and t represents the time.

17. Method according to claim 16, wherein a ratio OE of OEconv to OEdiv is defined as18. Method according to claim 17, wherein OE is in the range of 0.4 - 0.9.

19. Method according to any one of claims 14 - 18, wherein the polymer material is a bio-based and / or natural polymer material.

20. Method according to any one of claims 14 - 19, wherein the polymer material comprises starch, preferably comprising a starch content of more than 10 wt.%, for example more than 50 wt.%.

21. Method according to claim 20, wherein the polymer material comprises a starch content of more than 80 wt.%.

22. Method according to any one of claims 14 - 21 , wherein the polymer material in the converging section is subjected to an extensional deformation rate EDRconof formula (V), and the polymer material in the diverging section is subjected to an extensional deformation rate EDRdiv of formula (VI), wherein formulas (V) and (VI) are:in which:Q represents the volumetric flow rate of polymer material through the flow channel, Vi represents the volume of the converging section,V2 represents the volume of the diverging section, and wherein the sum of EDRconand EDRdiv is in the range of 0.1 s-1- 50 s-1.

23. Foamed polymer material obtainable by the method according to any one of claims 14 - 22.

24. Use of the extrusion die according to any one of claims 1 - 13 in food processing, bio-based plastic processing, plastic processing, or packaging processing.

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