Test device and method for determining the deformation behavior of a plastic sample

Abstract

The invention relates to a testing device and a method for determining the deformation behavior of a plastic sample in the form of a membrane. To facilitate sample changes, prevent workplace contamination by silicone oil, reduce the effort required to evaluate the deformation of the plastic sample, improve test reproducibility, and enable immediate testing of samples from a production batch, it is proposed that the test device comprise a sample holder with a horizontal support surface for the plastic sample and a hollow cylindrical guide device whose cylinder axis is perpendicular to the horizontal support surface. The membrane is deformable into the guide device by a pressure differential on both sides of the membrane. A heating element is thermally connected to the sample holder to heat the clamped plastic sample. A pressurized gas supply provides the pressure differential for deformation.With the help of an evaluation unit, the stretching of the plastic sample in the direction of the cylinder axis of the guide device is recorded depending on the test pressure of the pressurized gas.

Description

[0001] The invention relates to a testing device and a method for determining the deformation behavior of a plastic sample in the form of a membrane.

[0002] Determining the material properties of a plastic is of great importance for selecting the right material for the application. The deformation behavior of the plastic is particularly relevant for thermoforming, extrusion, stretch blow molding, and blown film extrusion. This deformation behavior is described primarily by the stretching and biaxial elongation of a sample of the plastic.

[0003] Hegemann, Bernhard (2004): Deformation behavior of plastics during thermoforming, experimental and virtual determination, dissertation, Institute for Plastics Science and Testing, University of Stuttgart, describes the use of a bubble inflation rheometer (BIR) as one method for determining the deformation behavior of a plastic sample in the form of a membrane. A flat plastic sample in the form of a membrane is clamped in a circular shape, heated, and then freely inflated using a gaseous medium. The stresses in the pole of the plastic sample can be calculated from the pressure difference between the inflation pressure and the ambient pressure, and the bubble height to be measured, using geometric relationships.

[0004] A disadvantage of this setup is the non-constant true strain and thus the strain rate, as controlling the volume increase is difficult. Furthermore, the exact geometric conditions during bubble formation are unknown, and variations in material properties lead to errors. The use of compressed air or nitrogen results in uneven strain rates.

[0005] As a further development of the BIR, the Membrane Inflation Rheometer (MIR) was developed. The MIR allows the description of the true biaxial strain behavior of plastic samples as a function of temperature and strain rate. The plastic sample also has the form of a membrane, but unlike the BIR, it is stretched using temperature-controlled silicone oil. DE 196 05 684 A1 discloses that the plastic sample, in the form of a membrane, is mounted vertically in front of a cylinder. A piston moves forward to displace the oil and deform the sample in a hollow cylindrical, horizontally oriented guide. The multiaxial mechanical material behavior is determined from the relationship between deformation and pressure. The movement of the plastic sample, and thus the deformation, is known. The mechanical behavior is evaluated using the conservation equations for mass and momentum.

[0006] According to Klaus Hartwig (1997): Simulation of the stretch blow molding process and characterization of the process-relevant material behavior, IKV Reports from Plastics Processing Volume 62, Mainz Publishing House, a prerequisite for the reproducibility of the measurement principle with the MIR is that there is complete adhesion between the guide device and the plastic sample. Furthermore, it must be ensured that the silicone oil can escape through the guide device and therefore no additional displacement forces act on the sample. The guide device is therefore designed as a grid, which fulfills the adhesion conditions through its physical interlocking with the grid rods, and through whose sufficiently large grid openings the silicone oil can escape without pressure loss. Due to the special kinematics in the guide device, the membrane rolls along it during the test. The measurement area of ​​the plastic sample is thus determined by the dimensions of the guide device.

[0007] High strain rates under biaxial loading can only be achieved with MIR using silicone oil as the medium. The silicone oil complicates sample changes, which must be performed outside the test fixture. Additionally, it contaminates the work area and must be specifically formulated for each plastic material to minimize measurement distortions caused by density differences. In practice, this is very time-consuming and often not implemented.

[0008] The effort required to evaluate the deformation of the plastic sample is high due to the complex camera technology needed to record the temporal development of the sample shape and the large amounts of data.

[0009] Due to the complexity of its handling and evaluation, the MIR (molten iron reflow) method cannot be used for the direct testing of samples from a production batch of plastic. Therefore, manufacturers or processors of plastics typically use the MVR (melt mass flow rate) measurement to test the processability of production batches. However, the MFR (molten mass flow rate) value determined during this measurement is very limited in its significance; it describes the mass of a thermoplastic material that, at a specified temperature, passes through a nozzle with defined dimensions and properties within 10 minutes under known stress.

[0010] The complete adhesion between the guide device and the plastic sample, required for reproducible MIR testing, can be compromised by the influence of gravity during sample deformation, leading to erroneous MIR test results. This effect is more pronounced at higher sample temperatures.

[0011] Finally, due to the required size of the plastic samples being tested in the MIR, the energy expenditure for temperature control using silicone oil is high.

[0012] Based on the membrane inflation rheometer (MIR) known from DE 196 05 684 A1, the invention aims to propose a test device for determining the deformation behavior of a plastic sample, which does not have or reduces the aforementioned disadvantages and also enables rapid batch testing of plastics. Furthermore, a method for determining the deformation behavior of a plastic sample, in particular for batch testing, using the test device is to be proposed.

[0013] This problem is solved by a testing device having the features of claim 1 and a method having the features of claim 11. Advantageous embodiments are described in the features of the dependent claims.

[0014] The undeformed plastic sample, in the form of a membrane, has a uniform thickness and is bounded on two opposite sides by a flat surface that is extended relative to its thickness. The sample carrier, with its horizontal support surface for the undeformed plastic sample, in conjunction with the guide device, which is a hollow cylinder with a cylinder axis perpendicular to the horizontal support surface, ensures that the complete adhesion between the guide device and the plastic sample, necessary for test reproducibility, is not compromised by the influence of gravity during deformation of the plastic sample. Therefore, the test device according to the invention significantly reduces erroneous test results.

[0015] The heating device, which is thermally connected to the sample carrier, enables precisely controlled temperature control of the plastic sample, which rests directly on the sample carrier. Temperature control of the plastic sample using heated silicone oil, as described in the prior art, is considerably more difficult. In a preferred, compact embodiment of the test device according to the invention, if the guide device has a circular cylindrical cross-section with an inner diameter of at least 10 mm and at most 55 mm, preferably at most 35 mm, and if the free area of ​​the plastic sample that can be deformed into the guide device is dimensioned accordingly, the energy required to temperature control the plastic sample is reduced compared to the prior art.The pressurized gas supply, which provides pressurized gas at a test pressure in the gas chamber on the pressure side of the sample carrier and is connected to the through-holes, eliminates the disadvantages associated with using silicone oil as a propellant when testing plastic samples. These disadvantages include, in particular, more difficult sample changes, contamination of the test device, and the need to individually adjust the propellant for each plastic material. The device according to the invention therefore allows for faster, sequential testing of plastic samples and is thus particularly suitable for batch testing.

[0016] The evaluation unit is designed to detect the stretching of the plastic sample along the cylinder axis of the guide fixture, depending on the test pressure. In the simplest case, the evaluation unit only detects the final state of the plastic sample after stretching, i.e., the change in length along the cylinder axis of the guide fixture. The change in length of the plastic sample deformed within the guide fixture, compared to the undeformed plastic sample, can be measured non-contact, for example, using a digital camera, time-of-flight measurement of electromagnetic or acoustic waves, or inductive or capacitive sensors. However, the change in length can also be measured tactilely, for example, using digital probes. The detected change in length is evaluated taking into account the test pressure specified for the respective test and represents a characteristic value for the respective plastic.However, the evaluation unit can also be set up for a more precise evaluation, especially for examining the pole of the plastic sample.

[0017] In order to distribute the pressurized gas as a propellant for deforming the plastic sample evenly over the free area of ​​the membrane, the sample carrier preferably consists of open-pored material with a pore size in the range of 0.2 mm to 4 mm and a maximum porosity of 65%.

[0018] A sample holder made of an open-pore metal not only allows for the distribution of the pressurized gas, especially compressed air, but also ensures good heat conduction of the heating energy from the heating element to the plastic sample. Besides sintered metal, porous aluminum is a particularly suitable material for the sample holder. Porous aluminum is produced by casting molten aluminum together with crystal salt. The crystal salt is then washed out. The size of the salt grains determines the desired pore size. Due to the manufacturing process, it can be produced in almost any shape and, if necessary, reworked. Porous aluminum exhibits • high strength, • a controllable volume porosity and pore size, • good corrosion resistance and • a homogeneous distribution of pores throughout the entire volume.

[0019] The structure of the pores results in a long contact time and strong turbulence of the pressurized gas flowing through the pores as gas passage openings, and a uniform distribution over the contact surface of the sample carrier.

[0020] The sample holder, made of porous aluminum, can be designed, for example, as a disc with a diameter between 20 mm and 45 mm and a height between 2 mm and 5 mm, particularly 3 mm. The pore size is, for example, in one of the following ranges: 0.30–0.50 mm, 0.40–0.63 mm, 0.40–1.00 mm, or 0.63–1.60 mm. The porous aluminum can be heated to temperatures up to 250°C. The thermal conductivity is between 30 and 50 W / (m*K).

[0021] To allow observation of the plastic sample during deformation, the guide device in one embodiment of the invention has a grid structure and / or is made of a transparent material. The grid structure also facilitates the physical interlocking of the plastic sample with the grid struts and contributes to complete adhesion and thus the reproducibility of the measurement results. The transparent design of the guide device allows the stretching process to be evaluated using a digital camera positioned laterally next to the guide device and aimed at the pole of the deformed plastic sample, without having to remove the guide device after the stretching process is complete.

[0022] To secure the plastic sample to the sample holder, the sample clamping device preferably comprises a plate that surrounds the preferably circular cylindrical guide in a ring-like fashion. In one embodiment of the invention, handling of the test device is simplified by the fact that the guide and the plate of the sample clamping device are permanently connected to each other as an assembly and detachably attached to the frame. The detachable attachment to the frame is achieved using quick-release clamps that press the plate against a flat surface of the frame. Vertical toggle clamps are particularly suitable for this purpose. Due to the large opening angle of the vertical toggle clamps, these allow the entire plate with the integrated guide to be removed, thus facilitating sample changes.

[0023] By clamping the plate, the cylinder axis of the guide device is simultaneously aligned perpendicular to the horizontal support surface above the support surface in such a way that the free area of ​​the membrane can be deformed into the guide device by a pressure difference on both sides of the membrane.

[0024] In one embodiment of the invention, the heating device comprises several electrical heating elements arranged in a metal block. The metal block can simultaneously form the frame of the test device.

[0025] The electric heating elements, especially cartridge heaters, are inserted into bores in the metal block. The cartridge heaters typically consist of a cylindrical stainless steel body, providing corrosion protection. Inside is a heating coil wound around a ceramic core. The number of windings varies depending on the power rating. To prevent a short circuit between the heating element and the metal body, the cartridge heater is filled with magnesium oxide and then compacted.

[0026] To heat a steel block with exemplary dimensions of 100 mm × 100 mm × 30 mm, a 150 W heating cartridge is inserted into each of four horizontal bores offset by 90° to each other. With this total power of 600 W, heating the steel block, which is thermally connected to the sample holder, from 20 °C to 200 °C takes less than 6 minutes.

[0027] Preferably, the total output of the heating device is determined and adjustable such that the temperature required for stretching, depending on the specific plastic sample, is reached within a reasonable time, i.e., a few minutes. The forming temperatures of the various plastics are a maximum of 400 °C, but typically between 100 and 200 °C when stretching the plastic sample with compressed air. Table: Forming temperatures during thermoforming plastic Forming temperature [°C] ABS 130 - 160 PE 140 - 170 PET 110 - 145 PP 150 - 165 PS 120 - 160

[0028] The pressurized gas supply for providing the propellant medium for deformation of the plastic sample includes the following components: - a pressurized gas generator for providing compressed gas with positive overpressure, - a pressurized gas distribution system that connects the pressurized gas generator to the gas space, - an adjustable pressure regulating valve that limits the positive overpressure of the pressurized gas on one inlet side to the test pressure on one outlet side and - an electromagnetically operated 2 / 2-way valve for shutting off the pressurized gas distribution between the pressurized gas generator and the gas space.

[0029] The compressed gas generator, in particular a compressor, produces the compressed compressed gas, preferably compressed air, required for the test operation.

[0030] The compressed pressurized gas is routed from the compressor to the actuators of the test device via the pressurized gas distribution system, for example a pipeline or, in the case of a decentralized compressor, a pipeline network.

[0031] The actuators include the adjustable pressure regulating valve, which limits the positive overpressure of the pressurized gas on one inlet side to the test pressure on one outlet side, and the electromagnetically operated 2 / 2-way valve for shutting off the pressurized gas distribution between the pressurized gas generator and the gas space.

[0032] The electromagnetically actuated 2 / 2-way valve, for example, has a short switching time of 10-20 ms and is designed for a pressure of up to 10 bar. In its first switching position, the electromagnetically actuated 2 / 2-way valve connects the pressurized gas generator to the gas chamber on the pressure side of the sample carrier. In its second switching position, the pressurized gas distribution is shut off.

[0033] The pressure regulating valve is, for example, a pressure reducing valve that ensures a specific, preset test pressure is not exceeded on the outlet side, despite varying pressures of the compressed gas on the inlet side. The pressure regulating valve is preferably installed downstream of the electromagnetically actuated 2 / 2-way valve, which presents additional flow resistance. The pressure regulating valve allows for the setting of a gas pressure in the range of 1 to 9 bar, i.e., the setting of a positive gauge pressure of up to 8 bar.

[0034] A method for determining the deformation behavior of a plastic sample using a compact testing device according to claim 4 is defined by the features of claim 11.

[0035] The step of providing a plastic sample with a constant thickness of at least 50 µm and a free area diameter corresponding to the inner diameter of the guide device of at least 10 mm and at most 55 mm, preferably at most 35 mm, comprises pressing a single granule at a forming temperature dependent on the specific plastic. Producing the miniaturized plastic sample from a single granule eliminates the influence of weld lines during testing. The standard granule is a cylindrical granule of the respective plastic with a diameter of approximately 3 mm and a granule length of 3–4 mm. Spherical or lenticular granules with diameters between 2 and 4 mm are also suitable.

[0036] The granules are pressed into the membrane using a press and spacers to ensure a consistent membrane thickness. However, the miniaturized plastic sample can also be produced using other methods.

[0037] Before the deformation step, the contact surface of the sample holder is heated to the desired processing temperature of the respective plastic, within a range of 100°C to 400°C, using the heating device. However, in certain test scenarios, the contact surface may not be heated, and the deformation of the plastic sample may occur at ambient temperature, for example, during a synthetic rubber test.

[0038] The subsequent deformation of the free area into the guide device is achieved by temporarily pressurizing the gas space on the pressure side of the sample carrier with a pressurized gas at a previously set positive overpressure as the test pressure. The positive overpressure is set to a maximum value of 8 bar, depending on the specific plastic.

[0039] The invention will be explained in more detail below with reference to the figures. They show Fig. 1 a schematic view of a test device according to the invention with an undeformed plastic sample, Fig. 2 a schematic view of the test device according to Fig. 1 with deformed, stretched plastic sample as well as Fig. 3 the test device according to Fig. 1 with deformed, stretched plastic sample and removed guide device and sample clamping.

[0040] Fig.Figure 1 shows a test device 1 according to the invention for determining the deformation behavior of a plastic sample 2, which has the shape of a membrane 2.1, as described in Fig. 1 is recognizable. The undeformed plastic sample 2, designed as a membrane, has a uniform thickness and is bounded on two opposite sides by a planar surface that is extended in relation to its thickness; the membrane 2.1 has a free area 2.2 and a clamping area 2.3.

[0041] The test device 1 comprises a frame 3, which in the illustrated embodiment is designed as a rectangular metal block 3.1 with a flat surface 3.4. The metal block 3.1, for example made of steel and with dimensions of 100 mm × 100 mm × 30 mm, has four horizontal bores 3.2 offset from each other by 90° and extending into the metal block 3.1 from its side walls. The bores 3.2 terminate at a distance from a gas chamber 3.3 in the metal block 3.1.

[0042] A sample holder 4 with a horizontal support surface 4.1 for the plastic sample 2 is attached flush with the surface 3.4 of the metal block 3.1 to the frame 3. A pressure side 4.2 is located opposite the horizontal support surface 4.1 of the sample holder 4. Gas passage openings, not visible in the drawings, extend between the pressure side 4.2 and the horizontal support surface 4.1 of the sample holder 4. In the illustrated embodiment, the gas passage openings are formed by the sample holder 4 being made of an open-pored material, for example, porous aluminum.

[0043] In addition to the sample carrier 4, a guide device 5 in the form of a hollow cylinder with a cylinder axis 5.1, which runs perpendicular to the horizontal support surface 4.1 of the sample carrier 4, is attached to the frame 3. The guide device 5 is arranged above the support surface 4.1 such that the free area 2.2 of the membrane 2.1 can be deformed into the guide device 5 by a pressure difference on both sides of the membrane 2.1, forming a dome, as can be seen by comparing the Fig. 1 and Fig. 2 is recognizable.

[0044] A specimen clamp 6 is detachably attached to the frame 3 to press and clamp the membrane 2.1 with the clamping area 2.3 against the horizontal support surface 4.1 of the specimen carrier 4. In the illustrated embodiment, the specimen clamp 6 is designed as a plate 6.1 that surrounds the guide device 5 in a ring-like manner. The guide device 5 and the plate 6.1 are rigidly connected to each other in this embodiment.

[0045] The sample clamping device 6 also features schematically depicted quick-release clamping devices 6.2, which press the plate 6.1 against the flat surface 3.4 of the frame 3. By releasing the quick-release clamping devices 6.2, the assembly consisting of the guide device 5 and the plate 6.1 can be easily removed from the frame 3 in order to measure the stretch 2.5 of the plastic sample 2 or to place a plastic sample 2 onto the sample carrier 4.

[0046] The test device 1 also includes a heating device 7 which is thermally connected to the sample carrier 4 via the metal block 3.1 and which in the illustrated embodiment is formed by four heating cartridges 7.1 inserted into the bores 3.2 in the metal block 3.1.

[0047] A pressurized gas supply 8 of the test device serves the purpose of supplying the gas space 3.3 within the metal block 3.1 with pressurized gas in a fluid-conducting manner, the test pressure of the pressurized gas supply 8 being adjustable. The pressurized gas is distributed evenly via the gas passage openings, which are fluidly connected to the gas space 3.3, over the support surface 4.1 and thus passes under the free area 2.2 of the membrane, which consequently deforms into the guide device 5.

[0048] Specifically, the compressed gas supply 8 comprises, for the aforementioned purpose, a compressed gas generation unit 8.1 for providing compressed compressed gas with positive overpressure, for example a compressor, a compressed gas distribution unit 8.2, for example in the form of a pipeline and corresponding fittings, which connects the compressed gas generator 8.1 to the gas space 3.3 in the metal block 3.1, an adjustable pressure regulating valve 8.3, which limits the positive overpressure of the compressed gas on an inlet side 8.4 to the test pressure on the outlet side 8.5, and an electromagnetically operated 2 / 2-way valve 8.6 for shutting off the line between the compressed gas generator 8.1 and the gas space 3.3.

[0049] Finally, the test device 1 has an evaluation unit 9 for detecting the stretching 2.5 of the plastic sample 2 in the direction of the cylinder axis 5.1 of the guide device 5 as a function of the test pressure. In the illustrated embodiment, the evaluation unit 9 comprises a digital camera 9.1, a data processing unit (not shown), and a back panel 9.2 for image acquisition using the digital camera 9.1.

[0050] The procedure for determining the deformation behavior of a plastic sample 2 using the described test device 1 comprises the following steps: - Providing the plastic sample 2 by pressing a single granule of the plastic to be tested onto the undeformed membrane 2.1 at a forming temperature specified for the respective plastic. In accordance with the desired miniaturization of the test device 1, the provided plastic sample 2 has a free area 2.2 corresponding to the inner diameter of the guide device 5 of at least 10 mm and at most 55 mm, preferably at most 35 mm. A circular plastic sample 2 with a free area diameter of 10 mm, for example, has an overall diameter of 20 mm to form the required clamping area 2.3, which surrounds the free area 2.2 in a ring shape. - The provided plastic sample 2 is placed on the horizontal support surface 4.1 of the sample carrier 4. The clamping area 2.3 of the membrane 2.1 is pressed against the horizontal support surface 4.1 of the sample carrier using the plate 6.1 of the sample clamping device 6 after closing the quick-release clamping devices 6.2. - The contact surface 4.1 of the sample carrier 4 is optionally heated to a temperature above the ambient temperature in a range typically between 100°C and 200°C, depending on the respective plastic sample 2, using the heating device 7. - The test pressure is set to the constant test pressure intended for the test using the pressure regulating valve 8.3. Then, to deform the free area 2.2 of the plastic sample 2, the 2 / 2-way valve is moved to the open position to pressurize the gas space 3.3 on the pressure side 4.2 of the sample carrier 4 with the pressurized gas at test pressure. The pressurized gas passing through the gas passage openings of the sample carrier 4 deforms the plastic sample 2, guided within the guide device 5. The guide device 5 prevents the pole of the plastic sample's cap from migrating out of the cylinder axis 5.1, as is the case in Fig. 2 is recognizable. Under the given temperature and pressure conditions during deformation, the deformed plastic sample 2, 2.4, stretches in the direction of the cylinder axis 5.1 of the guide device 5. This stretching is detected by the evaluation unit 9, whereby the digital camera 9.1 records the stretched plastic sample 2 in front of the rear wall 9.2, after the guide grid 5 and the plate 6.1 have been removed, as shown in Fig. Figure 3 is shown. Depending on the set test pressure, a material characteristic value can be determined from the measured stretching 2.5 by the evaluation unit 9, which characterizes the tested plastic.

[0051] Due to the low testing effort and the high testing speeds achievable, this characteristic value can be used particularly for batch testing of plastics for different plastic processing methods.

[0052] The following are examples of setting the maximum test pressure, which may be gradually increased to the maximum value in several tests of plastic samples, and the temperature of the sample carrier for different plastic processing methods and the plastics used: 1. Test pressure: up to 6 bar Temperature: 150 to 170 °C Processing: Thermoforming Plastic: Polypropylene (PP) 2. Test pressure: up to 2 bar Temperature: 170-190 °C Processing: blown film extrusion Plastic: low-density polyethylene (LDPE) 3. Test pressure: up to 8 bar Temperature: 250-270 °C Processing: Blow molding Plastic: Polyamide (PA) Reference symbol list 1 test device 2 plastic sample 2.1 Membran 2.2 free area 2.3 Clamping area 2.4 Deformed plastic sample 2.5 Stretching 3 frame 3.1 Metal block 3.2 Drilling 3.3 Gas space 3.4 Surface 4 sample carriers 4.1 Contact surface 4.2 Print page 5 Guide device 5.1 Cylinder axis 6 Sample clamping 6.1 Plate 6.2 Quick-release clamping devices 7 Heating system 7.1 Heating cartridges 8 Compressed gas supply 8.1 Compressed gas generator 8.2 Compressed gas distribution 8.3 Pressure regulating valve 8.4 Homepage 8.5 Home page 8.6 2 / 2-way valve 9 evaluation unit 9.1 Digital Camera 9.2 Back panel QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 196 05 684 A1 [0005, 0012] Cited non-patent literature

[0000] Klaus Hartwig (1997): Simulation of the stretch blow molding process and characterization of the process-relevant material behavior, IKV Reports from Plastics Processing Volume 62, Verlag Mainz shows that a prerequisite for the reproducibility of the measuring principle with the MIR is that there is complete adhesion between the guide device and the plastic sample

[0006]

Claims

[1] Test device (1) for determining the deformation behavior of a plastic sample (2) in the form of a membrane, wherein the membrane (2.1) has a free area (2.2) and a clamping area (2.3) surrounding the free area (2.2), comprising - a rack (3), - a sample carrier (4) arranged on the frame (3) with a horizontal support surface (4.1) for the plastic sample (2) and a pressure side (4.2) opposite the support surface (4.1), - a guide device (5) in the form of a hollow cylinder with a cylinder axis (5.1) attached to the frame (3) which runs perpendicular to the horizontal support surface (4.1), wherein the hollow cylinder is arranged above the support surface (4.1) such that the free area (2.2) of the membrane (2) can be deformed into the guide device (5) by a pressure difference on both sides of the membrane (2.1), - a sample clamping device (6) attached to the frame (3), designed to press the membrane (2.1) in the clamping area (2.3) against the horizontal support surface (4.1), - Gas passage openings extending between the pressure side (4.2) and the horizontal support surface (4.1) of the sample carrier (4), - a gas space (3.3) arranged on the pressure side (4.2) of the sample carrier, which is fluidly connected to the gas passage openings, - a heating device (7) thermally connected to the sample carrier (4), designed to heat the clamped plastic sample (2), - a pressurized gas supply (8) set up to provide pressurized gas in the gas space (3.3) at an adjustable positive overpressure as a test pressure, - an evaluation unit (9) for detecting an elongation (2.5) of the plastic sample (2) in the direction of the cylinder axis (5.1) of the guide device (5) depending on the test pressure. [2] Test device according to claim 1, characterized by , that the sample carrier (4) consists of open-pore material with a pore size in the range of 0.2 mm to 4 mm and a maximum porosity of 65%. [3] Testing device according to claim 1 or 2, characterized by , that the guide device (5) has a lattice structure and / or is made of a transparent material. [4] Test device according to any one of claims 1 to 3, characterized by , that the guide device (5) has a circular cylindrical cross-section, wherein the inner diameter of the circular cylinder is a minimum of 10 mm and a maximum of 55 mm. [5] Test device according to any one of claims 1 to 4, characterized by , that the sample clamping (6) comprises a plate (6.1) which surrounds the guide device (5) in a ring shape. [6] Test device according to claim 5, characterized by, that the guide device (5) and the plate (6.1) of the sample clamping (6) are firmly connected to each other and detachably attached to the frame (3). [7] Test device according to any one of claims 1 to 6, characterized by , that the heating device (7) has several electrical heating elements arranged in a metal block (3.1). [8] Test device according to any one of claims 1 to 7, characterized by , that the power of the heating device (7) is determined and adjustable in such a way as to heat the support surface (4.1) of the sample carrier (4) to a temperature of up to 400°C, preferably up to 250°C. [9] Test device according to any one of claims 1 to 8, characterized by , that the pressurized gas supply (8) comprises the following: - a pressurised gas generator (8.1) for providing compressed pressurised gas at positive overpressure, - a pressurized gas distribution system (8.2) that connects the pressurized gas generator (8.1) to the gas space (3.3), - an adjustable pressure regulating valve (8.3) that limits the positive overpressure of the pressurized gas on an inlet side (8.4) to the test pressure on an outlet side (8.5) and - an electromagnetically operated 2 / 2-way valve (8.6) for shutting off the pressurized gas distribution (8.2). [10] Testing device according to any one of claims 1 to 9, characterized by , that the evaluation unit (9) includes a digital camera (9.1). [11] Method for determining the deformation behavior of a plastic sample (2) in the form of a membrane (2), wherein the membrane has a free area (2.2) and a clamping area (2.3) surrounding the free area, using a test device (1) according to claim 4, comprising the following steps: - Providing a plastic sample (2) with a constant thickness of at least 50 µm and with a free area diameter (2.2) corresponding to the inner diameter of the guide device (5) of at least 10 mm and at most 55 mm, - Applying the plastic sample (2) to the horizontal support surface (4.1) of the sample carrier (4) such that the free area (2.2) covers gas passage openings, - Pressing the clamping area (2.2) of the plastic sample (2) against the horizontal support surface (4.1) using the sample clamping device (6) attached to the frame (3), - Deformation of the free area (2.2) into the guide device (5) by temporarily pressurizing the gas space (3.3) on the pressure side (4.2) of the sample carrier (4) with a pressurized gas at a previously set positive overpressure as test pressure using the pressurized gas supply (8), - Detecting the stretching (2.5) of the deformed free area (2.2) in the direction of the cylinder axis (5.1) of the guide device (5) depending on the test pressure using the evaluation unit (9). [12] Method for determining the deformation behavior according to claim 11, characterized by , that the contact surface (4.1) of the sample carrier (4) is heated to a temperature in a range between 100°C and 400°C before the deformation step using the heating device (7). [13] Method for determining the deformation behavior according to claim 11 or 12, characterized by , that the test pressure is kept constant at least during the deformation step by means of the compressed gas supply (8). [14] Method for determining the deformation behavior according to claim 13, characterized by that the test pressure is kept constant within a range of up to a maximum of 9 bar. [15] Method for determining the deformation behavior according to any one of claims 11 to 14, characterized by , that the step of providing the plastic sample (2) includes pressing a single granule onto the membrane (2.1).

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