Method and device for producing a particle foam part, method for calibrating a process for producing particle foam parts

Abstract

The invention relates to a method for producing a particle foam part using electromagnetic waves, comprising the steps of: - filling a mould chamber (14); - heating the foam particles using electromagnetic waves in order to fuse them; - removal from the mould, wherein, during heating of the foam particles, a parameter characteristic of the absorption of the electromagnetic waves is monitored and, if this changes by a predetermined amount, heating of the foam particles is stopped. This method ensures, in a simple manner, that the correct amount of heat is supplied to the foam particles.

Description

[0001] Method and device for producing a particle foam part, a method for calibrating a process for producing particle foam parts

[0002] The present invention relates to a method for producing a particle foam part from foam particles, a method for calibrating a process for producing particle foam parts and a device for producing a particle foam part from foam particles.

[0003] WO 2013 / 050 581 A1 describes a process for producing particle foam parts. A mixture of foam particles and a dielectric transfer fluid is heated using electromagnetic waves to fuse the foam particles into a particle foam part. Radio waves or microwaves are used as the electromagnetic waves. The foam particles are made of polypropylene (PP).

[0004] US Pat. No. 3,060,513 A discloses a process for sintering moist thermoplastic foam particles. The particles are dielectrically heated and simultaneously compressed in the mold. Electromagnetic waves are applied at a frequency of approximately 2 to 1000 MHz.

[0005] A similar process is described in US 3 242 238 A, in which foam particles are moistened with an aqueous solution and exposed to an electromagnetic field with a frequency of approximately 5 to 100 MHz.

[0006] GB 1 403 326 A describes a process for welding expandable polystyrene foam particles, in which the particles are moistened with an aqueous solution and exposed to an electromagnetic field of 5 to 2000 MHz.

[0007] WO 01 / 64414 A1 discloses another process in which polymer particles made of polyolefins, wetted with a liquid medium, are heated with electromagnetic waves, particularly microwaves. The temperature in the mold is regulated by controlling the pressure within it.

[0008] In the processes explained above, moist foam particles are heated with electromagnetic waves, whereby the electromagnetic energy is absorbed by the liquid and transferred to the particles.

[0009] US Pat. No. 5,128,073 A discloses thermoplastic particles coated with a material that absorbs high-frequency energy. These particles can be heated with electromagnetic waves, with the coating absorbing the electromagnetic energy and transferring it to the foam particles. Electromagnetic waves in the range of 40 MHz to 2450 MHz are used to weld the foam particles.

[0010] These methods have been known for decades. However, they have not been able to gain widespread acceptance in practice. There are various reasons for this. These methods work very well for laboratory samples.

[0011] In practice, therefore, foam particles are almost exclusively welded using saturated dry steam, as known, for example, from WO 2014 / 128 214 A1. In recent decades, welding using electromagnetic waves has never been able to prevail in practice over welding with steam, even though welding with electromagnetic waves would have significant advantages in principle. With electromagnetic waves, the energy can be transferred much more precisely, making it unnecessary to heat auxiliary bodies. When using steam, this must first be generated in a steam generator. The steam must then be fed to the tool via pipes. All of these parts must be heated to a sufficiently high temperature so that the steam does not condense within them. This causes considerable heat loss.In addition, the steam generation and steam piping devices take up the most space in the device for producing the particle foam part. If steam weren't required to weld the foam particles, the entire device could be designed much more compactly.

[0012] The applicant of the present patent application only recently successfully transitioned to industrial production. They were the first to recognize that a key reason for decades of unsuccessful attempts to weld foam particles using electromagnetic waves is that the heat cannot be uniformly introduced into the foam particles. As a result, uniform welding is not achieved in the particle foam part. Methods and devices for producing particle foam parts that achieve uniform heat distribution in the mold cavity are disclosed in WO 2017 / 125 410 A1, WO 2017 / 125 412 A1, WO 2018 / 100 154 A2, WO 2021 / 073 924 A1, and WO 2022 / 229 030 A1.

[0013] In particular, WO 2018 / 100 154 A2 discloses a device for producing particle foam parts, comprising a mold defining a molding cavity, wherein at least two capacitor plates are arranged adjacent to the molding cavity and are connected to a radiation source for electromagnetic radiation, wherein the radiation source for electromagnetic radiation is configured to emit electromagnetic radiation. This device is provided with a voltage measuring device with which the voltage across the capacitor can be measured. This voltage measuring device is used to tune an oscillating circuit configured with a generator for electromagnetic waves and thus the electrical power supplied to the capacitor.

[0014] WO 2023 / 152 056 A1 relates to a computer-implemented method for controlling and / or monitoring at least one particle foam molding process. Input parameters are provided, comprising at least one simulation model, material-specific parameters of the foam particles, and parameters of the particle foam molding machine. Using the input parameters, a particle foam molding process is simulated, with at least one process parameter being determined. A particle foam molding process is carried out based on the at least one process parameter. The properties of the molded particle foam part are examined and compared with optimal properties. If the properties deviate from the optimal properties, the at least one process parameter is adjusted. Further particle foam molding processes are carried out until the properties of the molded particle foam part sufficiently match the optimal properties.Then, at least one actual process parameter of the particle foam molding process is determined and compared with at least one process parameter from the simulation in order to adapt the simulation model accordingly. Welding can be carried out using various methods, including welding with electromagnetic radiation, especially radio waves, or with hot steam.

[0015] WO 2016 / 083 464 A1 discloses a method for controlling a step of a process automatically performed by a machine (for producing a particle foam part). A sensor is used to measure a parameter characteristic of a step that exhibits an exponential progression. Based on several consecutive measurements of the characteristic parameter, a time constant of the exponential change is determined, and the step is terminated after it has been carried out for a period of time corresponding to a predetermined multiple of the time constant. The characteristic parameter can be the temperature in the molding chamber. Hot steam is used to weld the foam particles. DE 102019 215 845 B4 describes a method for producing foamed particle parts, in particular for the production of shoe soles. The welding of foamed particles takes place using electromagnetic waves.At least one mold half is made of a material transparent to electromagnetic waves and has a partition wall and one or more supports to form multiple mold cavities. One or more trimming bodies or a trimming fluid are provided to influence the electromagnetic field in the mold cavity.

[0016] DE 102013217 864 A1 describes a method for treating hydraulically bound building materials and a treatment device. A hydraulically bound binder, such as cement, and a water-containing building material mixture are heated using electromagnetic radiation. The energy of the electromagnetic radiation is varied depending on chemical reactions within the building mixture.

[0017] EP 1 508 420 A2 discloses a method for vulcanizing latex foam and a device therefor. The foam is poured into a mold made of electrically non-conductive material and heated using electromagnetic waves. During heating, the temperature of the foam is continuously measured, and heating is stopped when a preset temperature is reached.

[0018] The invention is based on the object of providing a method for producing a particle foam part, a method for calibrating a process for producing particle foam parts and a device for producing a particle foam part, with which a reliable industrial production of particle foam parts is possible by welding the foam particles by means of electromagnetic waves, whereby particle foam parts of high quality are obtained.

[0019] The problem is solved by the subject matter of the independent patent claims. Advantageous embodiments are specified in the respective subclaims.

[0020] According to a first aspect, the invention relates to a method for producing a particle foam part, in particular using, comprising the steps

[0021] - Filling a mold cavity,

[0022] - Heating the foam particles with electromagnetic waves to weld them,

[0023] - Demolding.

[0024] The method is characterized in that during the heating of the foam particles, a parameter characteristic of the absorption of electromagnetic waves is monitored, and if this parameter changes by a predetermined amount, the heating of the foam particles is stopped. The present invention is based on the finding that when a material to be welded is melted, the absorption capacity of the material improves dramatically. It has long been known that polar molecules or polar functional groups absorb electromagnetic waves by being excited to oscillate by the electromagnetic waves. When a material is melted, the bonds between the molecules dissolve, making the individual molecules or their functional groups more mobile. In materials such asIn PEEK, which has polar functional groups firmly embedded in the molecular structure, the polar functional groups cannot move, and the material barely absorbs electromagnetic waves. To absorb electromagnetic waves, the polar functional groups must be mobile so that they can vibrate. If the polar functional groups are very mobile, they cause little or no friction, so that the electromagnetic waves can only be absorbed to a limited extent. Therefore, there is usually a peak range in which the respective materials can absorb electromagnetic waves well. Since the mobility of the functional groups is temperature-dependent, the absorption capacity of electromagnetic waves is also generally temperature-dependent.

[0025] Furthermore, the inventors of the present invention recognized that a problem with welding foam particles with electromagnetic waves is that the heat must not only be evenly distributed throughout the mold cavity, but also precisely metered. When welding foam particles with steam, if the welding is insufficient, steam can be applied over a longer period of time to ensure that all areas of the particle foam part are adequately welded. Because the temperature of the steam is limited, overheating can never occur. This is also one of the reasons why welding foam particles with electromagnetic waves has not been able to prevail over welding with steam in the past.

[0026] When welding with electromagnetic waves, the heat supply should therefore be stopped once the material to be welded has melted. Once the foam particles have melted, they bond to form the particle foam part upon cooling, a process known as welding. Further heating is unnecessary and, in fact, harmful, as the material would then be heated to higher temperatures and could burn.

[0027] The fact that the material's ability to absorb electromagnetic waves changes during melting, which usually occurs suddenly, is utilized in the invention to adjust the heating time. This allows for a very simple and precise control of the heat applied, which ensures a good weld and reliably prevents the material from burning.

[0028] The foam particles to be welded to form particle foam parts using this process are preferably foam particles made of a thermoplastic material. This thermoplastic material can be expanded polyurethane (ePU), polyethylene block amide (PEBA), or expanded polyethylene (ePE). These are polar plastics that absorb electromagnetic waves and exhibit a very strong change in their absorption capacity when melted.

[0029] Foam particles made of non-polar plastic, such as those based on expandable polypropylene (ePP) or expandable polystyrene (ePS), can also be welded to form particle foam parts. Since these materials have no polar groups, they absorb electromagnetic radiation only to a very low extent. It is therefore advisable to add a dielectric heat transfer medium, such as water. Small amounts of water to wet the surface are sufficient. The water can be added in liquid form or as vapor. Water molecules form a dipole, so they absorb electromagnetic waves well and heat up quickly when exposed to electromagnetic waves. Even if the water is added to foam particles in the form of vapor, at least some of the water condenses on the surface of the foam particles and forms a liquid film.If the water is added in liquid form, it is in liquid form before the foam particles are heated with electromagnetic waves and usually forms a film on the surface of the foam particles. When the foam particles are heated with electromagnetic waves, the water is heated and evaporates. The phase transition from liquid water to vapor causes a significant change in the absorption capacity of electromagnetic waves. If the water is completely converted to vapor, this means that hot vapor is present in the molding chamber, which is suitable for welding the foam particles together. Since the individual water molecules are freely mobile in vapor, they hardly absorb any electromagnetic waves compared to liquid water. Further heating is no longer necessary. The heating process can be stopped.

[0030] The foam particles can also be made from biological materials, such as industrial popcorn made from corn, which is provided or coated with a binder so that the binder binds the popcorn together after heating.

[0031] The term "popcorn" within the meaning of the present invention encompasses in particular all materials which, like popcorn (Zea mays, convar. Microsperma), possibly after appropriate greasing, explode upon rapid heating to high temperatures, as the water present in the seed suddenly evaporates, thus transforming the starch contained in the seed into a foam-like consistency. Such behavior is known, among other things, from quinoa, amaranth, rice, and wheat. Materials based on these raw materials are explicitly referred to and encompassed within the meaning of the present invention as "popcorn." The term "popcorn" is not intended to be limited to corn and was chosen particularly for reasons of simplicity, clarity, and readability.

[0032] The binder can be a thermoplastic that melts when exposed to heat and solidifies upon cooling, thus binding the popcorn particles together. The binder can also contain glue, which reacts chemically when heated, thus binding the popcorn particles together.

[0033] Particularly suitable binders are thermoplastics, thermosets, aminoplasts, phenolic resins, isocyanates, proteins, tannins, starch, synthetic binders or near-natural binders, or mixtures of binders, such as urea-formaldehyde resin, melamine-formaldehyde resin, melamine-reinforced urea-formaldehyde resin, tannin-formaldehyde resin, phenol-formaldehyde resin, polymeric diphenylmethane diisocyanate or mixtures thereof.

[0034] The proportion of binder in the molded part (in wt.% based on the weight of the molded part) is preferably < 10%, preferably < 5%.

[0035] Biological material typically contains a certain amount of moisture. This water absorbs the electromagnetic waves and heats the popcorn particles and the binder. As the binder evaporates, the water's ability to absorb electromagnetic waves changes in the same way as explained above, which can be determined using the characteristic parameter. This, too, is an indication that the material is being heated in the molding chamber and is bonding to form a particle foam part.

[0036] There are therefore different physical mechanisms that cause a change in the absorption capacity of electromagnetic waves. Since the electromagnetic waves heat the material present in the mold cavity, a significant change in the absorption capacity of electromagnetic waves always indicates a reproducible stage of the heating process. The characteristic parameter is thus a physical quantity that is specific to the foam particles to be welded and changes due to the irradiation of electromagnetic waves. This change can be used to determine a specific process state or processing state of the foam particles to be welded.

[0037] The heating process can then be terminated according to the change in the characteristic parameter. The heating process can be terminated immediately when the characteristic parameter changes, or it may also be expedient to terminate the heating process a predetermined period of time after the change in the characteristic parameter (delayed termination of the heating process). The delay time is material-specific, and the value for the delay time can be determined empirically. It has been shown that for most polar plastic materials that melt under the influence of electromagnetic waves, a very short delay time or a delay time of "0" is expedient. For other materials, particularly those that form a bond between the foam particles through a chemical reaction, a delay time greater than "0" can also be useful.

[0038] The electromagnetic waves can be generated using a capacitor in which the mold cavity is located. The electromagnetic waves can be applied to the capacitor at a predetermined voltage amplitude, and the power introduced into the mold cavity or its first or second derivative over time can be measured as a characteristic parameter. It is also possible to apply the electromagnetic waves to the capacitor at a predetermined power, and measure the voltage drop across the capacitor or its first or second derivative over time as a characteristic parameter.

[0039] If the characteristic parameter is monitored directly, then to detect a change by a predetermined amount, it is checked whether the characteristic parameter changes by a predetermined threshold within a predetermined time interval. This represents an abrupt change in the characteristic parameter.

[0040] If the first time derivative of the characteristic parameter is monitored, then to detect a change of a predetermined amount, it is checked whether the first time derivative of the characteristic parameter exceeds a predetermined threshold. This, in turn, indicates an abrupt change in the characteristic parameter.

[0041] If the second time derivative of the characteristic parameter is monitored, then to determine whether a change exceeds a predetermined threshold, it is determined whether this change exceeds a predetermined threshold. This means that the change in the characteristic parameter's curve itself is changing significantly. If the characteristic parameter's curve is plotted in a diagram, then exceeding the second time derivative of a certain threshold indicates a strong curvature in the characteristic parameter's curve over time. This also indicates a significant change in the characteristic parameter.

[0042] There are therefore different ways to detect a significant change in the characteristic parameter. To obtain a signal that can be used to terminate the heating of the foam particles using electromagnetic waves, it is important that this signal is characteristic of the respective heating state of the foam particles and reproducible. This is the case with all of the methods described above.

[0043] Preferably, the particle foam part produced in the molding chamber is cooled after heating before demolding. This cooling process is also referred to as stabilization.

[0044] The electromagnetic waves are preferably RF radiation. RF radiation has a frequency range of 30 kHz to 300 MHz. In many countries, a frequency of 27.12 MHz is approved for industrial applications. This is a suitable frequency for heating the foam particles.

[0045] The amplitude of the high-frequency voltage applied to the capacitor is preferably in the range of 10 3 V to 10 5 V and especially in the range of 5 x 10 3 V to 1.5 x 10 4 V.

[0046] According to a further aspect, the present invention relates to a method for calibrating a process for producing particle foam parts using electromagnetic waves. In this method, a mold defining a specific mold space and foam particles of a specific material type are used to produce at least one particle foam part according to one of the methods explained above, in which the heating of the foam particles is controlled or terminated based on the characteristic parameter. Further particle foam parts are then produced by heating the foam particles with the same settings for supplying the electromagnetic waves, each for a production heating interval corresponding to the time period determined during the calibration process. These settings are primarily the applied electrical power or the applied electrical voltage.

[0047] This allows a production heating interval to be determined in a calibration process, which defines the period of time during which the foam particles are heated using electromagnetic waves.

[0048] Since the appropriate time for heating the foam particles depends on the molding tool and the material type of the respective foam particles, an individual production heating interval can be determined quickly and easily in order to produce high-quality particle foam parts easily and reliably.

[0049] During a calibration process, several particle foam parts can be produced, and the heating time for each of them can be measured. The production time interval is then determined based on the multiple time periods measured during the calibration process. The mean or median of the individual time periods can be determined as the production time interval. Furthermore, when determining the production time interval, only time periods that lie within a predetermined multiple of the standard deviation around the mean or median of the measured time periods can be considered.

[0050] To determine a quality factor for the production process, a statistical evaluation of the measured time periods can be performed. Based on this quality factor, the production process can be optimized by varying a specific parameter if the quality factor indicates insufficient stability of the production process.

[0051] At least one of the following parameters can be varied and then a production time interval can be determined again according to the calibration procedure explained above:

[0052] - applied power of the electromagnetic waves,

[0053] - applied voltage to generate the electromagnetic waves,

[0054] - amount of heat transfer medium supplied to the mold cavity,

[0055] - Quantity of foam particles fed into the molding chamber,

[0056] - Pressure exerted on the foam particles by means of a mold before they are heated.

[0057] The at least one parameter to be changed and the degree of change can be determined automatically using an expert system.

[0058] According to a further aspect, the invention relates to a device for producing a particle foam part, comprising a molding tool which delimits a molding space, wherein at least two capacitor plates are arranged adjacent to the molding space and are connected to a radiation source for electromagnetic radiation, wherein the radiation source for electromagnetic radiation is designed to emit electromagnetic radiation, and the device has a control device which is designed such that the device automatically carries out a method explained above.

[0059] The invention is explained in more detail below using the drawing as an example. The drawing shows schematically:

[0060] Figure 1 shows an embodiment of a device for producing a particle foam part,

[0061] Figures 2a and 2b each show a molding tool in a partially opened position (Figure 2a) and a closed position (Figure 2b) in a sectional view, Figure 3 shows a generator for electromagnetic radiation which forms a tunable oscillating circuit, and the molding tool in an electrical circuit diagram, and

[0062] Figure 4 shows a control device for regulating the power supply in a circuit diagram.

[0063] The basic structure of a device 1 for producing a particle foam part is shown in Figure

[0064] 1. This device 1 comprises a material container 2, a mold 3, and a line 4 leading from the material container 2 to the mold 3.

[0065] The material container 2 serves to hold loose foam particles. The material container 2 has a base 5, and is connected to a compressed air source 7 in the base area via a compressed air line 6. The compressed air line 6 is connected to several nozzles (not shown) arranged in the base 5, so that several air streams (= fluidizing air) can be introduced into the material container 2, which swirl and thereby separate the foam particles contained therein.

[0066] In the area of ​​the bottom 5 of the material container 2, an opening is formed to which the conveying line 4 is connected. The opening can be closed by means of a slide (not shown).

[0067] Adjacent to the material container, there is a drive nozzle 8 in the conveyor line 4. The drive nozzle 8 is connected to the compressed air source 7 via a further compressed air line 9. Compressed air supplied to this drive nozzle 8 serves as drive air, as it enters the conveyor line 4 through the drive nozzle 8 and flows towards the mold 3. As a result, the drive nozzle 8 at the point leading to the material container

[0068] 2 side creates a vacuum that sucks foam particles out of the material container.

[0069] The feed line 4 opens into a filling injector 10, which is coupled to the molding tool 3. The filling injector 10 is connected to the compressed air source 7 via a further compressed air line 11. The compressed air supplied to the filling injector 10 is used, on the one hand, to fill the molding tool 3 by forcing the flow of foam particles toward the molding tool 3 using the compressed air. On the other hand, the compressed air supplied to the filling injector 10 can also be used to blow the foam particles back from the feed line 4 into the material container 2 once the filling process at the molding tool 3 is complete.

[0070] The molding tool 3 is formed from two mold halves 12, 13. At least one mold cavity 14 is defined between the two mold halves, into which the filling injector 10 for introducing the foam particles opens. The volume of the mold cavity 14 can be reduced by moving the two mold halves 12, 13 together. When the mold halves 12, 13 are moved apart, a gap is formed between the mold halves 12, 13, which is referred to as the crack gap. For this reason, such a molding tool 3 is also referred to as a crack-gap molding tool. The device 1 has no steam generator and no steam supply to the mold cavity 14, as is usual in conventional devices for producing particle foam parts. Moisture can enter the mold cavity 14 through the residual moisture contained in the material of the foam particles and through the moisture contained in the compressed air.However, the device 1 can also be designed with a steam generator and a steam supply to the molding chamber 14 and / or to the conveying line 4 in order to supply saturated dry steam to the molding chamber 14 for additional heating or preheating of the foam particles and / or to wet foam particles during their transport from the material container 2 to the molding chamber 14. The foam particles located in the material container 2 can also be wetted with water in liquid form, with corresponding nozzles arranged in the material container 2 for atomizing the water.

[0071] A capacitor plate 15, 16 is arranged on each of the mold halves 12, 13. These capacitor plates are each made of a highly electrically conductive material, such as copper or aluminum. The filling injector 10 is arranged on the mold half 13. The filling injector 10 extends through a recess in the capacitor plate 16, which is mounted on the mold half 13.

[0072] The capacitor plates 15, 16 are connected to a generator 18 via electrical lines 17 for transmitting high-frequency voltages. These electrical lines 17 are typically waveguides. In the present embodiment, the waveguides are formed from a hollow tube with, for example, a rectangular cross-section made of an electrically conductive material. However, other types of waveguides, such as coaxial cables, are also generally suitable for connecting the generator 18 to the capacitor plates 15, 16.

[0073] The mold halves 12, 13 each have a base body, which can be made of an electrically non-conductive material that is essentially transparent to electromagnetic RF radiation, such as polytetrafluoroethylene (PTFE), polyethylene, especially UHMWPE, or polyether ketone (PEEK). Only the capacitor plates 15, 16 are electrically conductive. The "essentially transparent material" is a material that can be penetrated by electromagnetic radiation, especially RF radiation. However, this material can be deliberately designed with a certain absorption property for electromagnetic RF radiation in order to convert a portion of the electrical RF radiation into heat and to heat the mold halves 12, 13 themselves. This will be explained in more detail below.

[0074] Alternatively, one or both mold halves can also form the capacitor plates if they comprise or consist of an electrically conductive material, such as aluminum or steel. However, the two mold halves are generally electrically insulated from one another. Preferably, one of the two mold halves is electrically conductive and the other half is non-electrically conductive. The electrically conductive mold half can be three-dimensionally contoured so that it forms a corresponding three-dimensionally contoured surface on the particle foam part. The electrically conductive mold half effects a smooth or predetermined exact texture of the surface and is therefore preferably used to create a visible surface on the particle foam part.

[0075] The mold can optionally be connected to a vacuum pump 19 so that a negative pressure or vacuum can be applied to the mold cavity 14. This negative pressure causes moisture contained in the mold cavity 14 to be removed.

[0076] The capacitor plates 15, 16 are preferably provided with a cooling device. In the present embodiment, the cooling device is formed by fans 20, which direct cooling air to the side of the capacitor plates 15, 16 facing away from the molding chamber 14. To increase the cooling effect, cooling fins 21 can be provided on the capacitor plates 15, 16.

[0077] Alternatively or additionally, temperature control lines can be arranged on the capacitor plates 15, 16, through which a temperature control medium is passed. A liquid, such as water or oil, is preferably used as the temperature control medium.

[0078] The following explains a method for producing particle foam parts using the device described above:

[0079] The procedure includes the following basic steps:

[0080] - Filling the mold cavity 14,

[0081] - Welding of the foam particles,

[0082] - Stabilize (optional),

[0083] - demoulding,

[0084] - Cleaning the tool (optional).

[0085] To fill the molding chamber 14, air is injected via the compressed air line 6 into the area of ​​the bottom 5 of the material container to swirl and separate the foam particles contained therein. At the same time, propellant air is also supplied to the propellant nozzle, so that foam particles are drawn from the material container 2 into the conveying line 4 and transported by the propellant air toward the molding tool 3. The molding chamber 14 is closed, whereby the mold halves 12, 13 can be fully closed or spaced apart by a crack gap.

[0086] The slide valve of the material container 2 can be opened and closed sequentially. The opening and closing times are typically in the range of 500 ms to 1 s. This cyclical opening and closing of the slide valve intermittently feeds the foam particles from the material container 2 to the conveying line 4. This can break up any bridging of the foam particles in the material container 2 and separate the foam particles. This is particularly useful for foam particles with an adhesive surface, such as eTPU foam particles.

[0087] Intermittent suction can also alternatively be achieved by intermittently supplying the driving air from the compressed air line 9 to the driving nozzle 8 arranged directly adjacent to the material container 2.

[0088] The molding tool 12, 13 is provided with at least one valve (not shown) that is opened when foam particles are fed in, allowing the compressed air flowing into the molding chamber 14 to escape. This valve can be adjusted when filling the molding chamber 14 such that a counterpressure develops in the molding chamber 14. This maintains a high pressure in the feed line and in the molding chamber 14, thereby keeping the foam particles at a small volume. This allows more foam particles to be fed into the molding chamber 14 than would be possible without the counterpressure. After the counterpressure is released, the foam particles expand in the molding chamber 14.

[0089] Another parameter for adjusting the fill quantity is the crack gap, i.e., the gap by which the two mold halves 12, 13 are spaced apart during filling. The use of a crack gap during filling primarily increases the density in a thin region of the particle foam part being produced.

[0090] As soon as it is determined that the mold cavity 14 is filled with foam particles, the filling injector 10 is closed. The foam particles in the line are blown back into the material container 2 with the compressed air supplied to the filling injector 10.

[0091] The filling of the mold cavity 14 with foam particles is described in detail in the German patent application DE 10 2014 117 332, which is why reference is made to this patent application in this regard.

[0092] After the mold cavity 14 is filled with foam particles, they are heated by applying electromagnetic RF radiation. This RF radiation is generated by applying a high-frequency voltage of approximately 104 V at a frequency of 27.12 MHz to the capacitor plates 15, 16. The electromagnetic RF radiation preferably has a frequency of at least 30 kHz or at least 0.1 MHz, in particular at least 1 MHz or at least 2 MHz, preferably at least 10 MHz.

[0093] The electromagnetic RF radiation preferably has a maximum frequency of 300 MHz.

[0094] The foam particles can be based on polyurethane (eTPU). Polyurethane has a dielectric loss factor D of 0.2 for electromagnetic radiation with a frequency of 1 MHz. In contrast, the dielectric loss factor of polypropylene (PP) for electromagnetic radiation with a frequency of 1 MHz is only 0.00035. The absorption capacity of polyurethane is therefore significantly higher than that of polypropylene. This makes it possible to introduce the heat required to weld the foam particles into the mold cavity 14 without additional heat-transfer materials, in particular without aqueous solutions, since the foam particles themselves absorb the electromagnetic waves.

[0095] Instead of polyurethane-based foam particles, polyethylene block amide (PEBA) or polyethylene (PE)-based foam particles can also be used.

[0096] Foam particles based on ePP (expandable polypropylene) or ePS (expandable polystyrene) can also be welded to form particle foam parts. Since these materials absorb electromagnetic radiation only to a very low degree, it is advisable to add a dielectric heat transfer medium, such as water. The foam particles can be wetted with the heat transfer medium in the material container 2 or during their transport from the material container 2 to the mold 3. Wetting in the line 4 has the advantage that the foam particles are wetted very evenly and the heat transfer medium is evenly distributed in the mold cavity 14. This leads to a correspondingly uniform heating of the foam particles in the mold cavity 14.

[0097] The mold 3 can also be connected to a steam source (not shown), with which saturated dry steam can be supplied to the mold cavity 14. This is useful when welding materials whose dielectric loss factor is temperature-dependent. Such materials include, for example, ePES (expandable polyether sulphone) or expandable polyamide. At low temperatures, the absorption properties of electromagnetic waves are low. Therefore, these foam particles are first heated using steam and then, from a certain temperature, heated to even higher temperatures solely or additionally using electromagnetic radiation. Alternatively, the foam particles can be wetted with a dielectric heat transfer medium, so that the electrical heat transfer medium is heated using electromagnetic radiation to heat the foam particles to a predetermined temperature.Afterwards, the foam particles can be heated directly due to electromagnetic radiation, since the absorption properties of electromagnetic radiation increase with increasing temperature.

[0098] The following describes a mold 3 (Figure 2a, Figure 2b), which has a first mold half 12 and a second mold half 13 and can be used in the device 1 described above. Movement devices, holding elements, thermometers for measuring the temperature in the mold cavity, and other mechanical parts for opening and closing the mold are omitted from Figures 2a and 2b for simplicity of illustration.

[0099] The mold 3 is formed from two mold halves 12, 13, each having a base body 24, 25 made of an electrically conductive material. These base bodies consist, for example, of aluminum, copper, or a highly electrically conductive alloy.

[0100] The two mold halves 12, 13 delimit a mold space 14 by means of an inner boundary surface 26, 27. The inner boundary surfaces 26, 27 of the two mold halves 12, 13 are provided with an electrically insulating coating 28, 29.

[0101] The electrically insulating coatings can be made of a material that is essentially transparent to electromagnetic radiation, in particular RF radiation, such as PTFE, PE, or PEEK. However, they can also be made of a plastic material that has a similar dielectric loss factor when exposed to electromagnetic radiation as the plastic material to be processed in the mold cavity 14, in order to achieve uniform heating throughout the entire mold cavity 14 and at the edge region of the mold cavity when the electromagnetic radiation is applied. For this purpose, the coating 28, 29 is preferably made of a material with a moderate loss factor, such as PET (polyethylene terephthalate), PEEK (polyether ketone), POM (polyoxymethylene), polyimide, and PMMA (polymethyl methacrylate).These coatings 28, 29 are thus essentially transparent to RF radiation, since they absorb only a small proportion of the electromagnetic radiation. Due to the relatively low loss factor, they can be formed with a certain thickness of, for example, at least 2 mm, in particular at least 2.5 mm or at least 5 mm. The coating is preferably no thicker than 20 mm, in particular no thicker than 15 mm, and preferably no thicker than 10 mm, so that the proportion of the electromagnetic wave energy absorbed by the coating is small.

[0102] The electrically conductive base bodies 24, 25 form the capacitor plates of the molding tool 3. They therefore have an electrical connection for connection to the generator 18 or to ground 30. The generator 18 represents a radiation source for generating electromagnetic radiation. The generator is preferably designed to generate RF radiation. The generator can also be designed to generate microwave radiation; with larger mold cavities 14, significantly more uniform heating is possible with RF radiation than with microwave radiation. In addition, most plastic materials can absorb RF radiation significantly better than microwave radiation. Therefore, the use of RF radiation is preferred.

[0103] Because the mold halves 12, 13 both define the mold cavity 14 and simultaneously form the capacitor plates, the distance between the capacitor plates and the mold cavity 14 is very small and is determined solely by the electrically insulating coatings 28, 29. This minimizes electromagnetic radiation losses, resulting in a very high proportion of the power transferred as heat into the foam particles to be welded. Such a tool thus allows for very efficient welding of the foam particles into a particle foam part.

[0104] In the present embodiment of the molding tool 3, the first mold half 12 has a bottom wall 31 and a circumferential side wall 32. In the present embodiment, both the bottom wall 31 and the side wall 32 are formed from the electrically conductive base body 24 and the internally arranged coating 28. It is also possible for the side wall 32 to be formed solely from a non-electrically conductive material, in particular plastic material, or to be formed only partially by means of the electrically conductive base body 24. The second mold half 13 forms a punch that can move into the cavity formed by the first mold half 12 and thus tightly seals the mold cavity 14. The tight seal between the two mold halves 12, 13 is at least tight enough that foam particles located therein cannot escape. The mold cavity 14 is not necessarily sealed gas-tight.

[0105] The two mold halves 12, 13 can be moved relative to each other by means of a press (not shown) and subjected to a predetermined force.

[0106] A through-opening for supplying foam particles is arranged on the first mold half 12, which is referred to below as the filling opening 33. The filling injector 10 is connected to the filling opening 33. This filling injector 10 differs from conventional filling injectors in that it does not have a closing mechanism for closing the filling opening 33, as explained in more detail below.

[0107] The first mold half 12 has one or more through openings for the escape of air, which are referred to below as vent opening 34.

[0108] The filling opening 33 and the vent openings 34 are arranged in a section or region, in particular an edge region, of the first mold half 12, which is concealed or covered by the second mold half 13 when the mold 3 is closed (Figure 2b). As a result, the filling opening 33 and the vent opening 34 are automatically closed when the mold 3 is closed by inserting the second mold 13 into the cavity formed by the first mold 12. This eliminates the need for the filling injector 10 to have a closing mechanism with which the filling opening 33 is closed.

[0109] This section or area is the area of ​​the first mold half 12 which is not covered by the second mold half 13, is the area around which the mold halves are opened when moving apart into the crack gap position.

[0110] Preferably, the first mold half 12 is connected to ground 30. The filling injector 10 is coupled to the electrically conductive base body 24 of the first mold half 12, so that the filling injector 10 is also electrically connected to ground 30. The generator 18 generates electromagnetic waves or an alternating electrical voltage relative to ground 30, which is applied to the base body 25 of the second mold half 13. This creates an alternating electromagnetic field, in particular RF radiation, in the mold cavity 14.

[0111] With such a design of the molding tool 3, it is important that the electrically conductive base bodies 24, 25 of the two mold halves 12, 13 are electrically insulated from each other. In the present embodiment, this is achieved by means of the coatings 28, 29.

[0112] Preferably, the inner boundary surface 26, 27 of one of the two mold halves 12, 13 is contoured. In the context of the present invention, "contoured" means any shape that deviates from a flat boundary surface. In the present embodiment, the inner boundary surface 27 of the second mold half 13 is contoured. The inner boundary surface 26 of the first mold half 12 is not contoured in the region of the bottom wall 31.

[0113] Such a molding tool 3 further differs from known molding tools for welding foam particles by means of electromagnetic waves in that the second molding tool 13 is stamp-like and with its electrical base body 25 is located at least partially within the cavity delimited by the first mold half 12 and thus the electrically conductive base body 25, which functions as a capacitor plate, is located very close to the molding space 14 or to the foam particles to be welded.

[0114] Figure 3 shows a schematic electrical circuit diagram of the generator 18, the mold capacitor formed by the capacitor plates 15, 16, which encloses the mold halves 12, 13, and a line (hollow waveguide or coaxial line) 46 suitable for transmitting the electromagnetic waves, with which the electromagnetic waves are transmitted from the generator 18 to the mold capacitor 15, 16. The hollow waveguide forming the line 46 is preferably designed as a coaxial air line with an electrically conductive inner tube and an electrically conductive outer tube. The coaxial air line is dimensioned such that high-voltage signals can be reliably transmitted. The characteristic impedance is preferably set to approximately 50 Ω.

[0115] A generator-side inductance 47 and a tool-side inductance 48 are symbolically shown in this line 46. These inductances are caused by the line itself, with the length of the respective line sections determining the magnitude of the respective inductance. A tool-side capacitor 49 is connected in parallel to the tool capacitor 15, 16. This capacitor 49 represents the electrical capacitance between the capacitor plate 15 and the housing 35 of the molding tool 3. The tool capacitor 15, 16, the capacitor 49, and the tool-side inductance 48 form a tool resonant circuit 50.

[0116] A generator-side capacitor 51 is connected in series with the generator 18 and the generator-side inductance. The generator-side capacitor 51 and the generator-side inductance 47 form a generator resonant circuit 52. At least the generator-side capacitor 51 or the generator-side inductance 47 is designed to be variable, for example by providing a capacitor with capacitor plates whose distance is variable or by providing line sections of different lengths. It is also possible for both the generator-side capacitor 51 and the generator-side inductance 47 to be designed to be variable. The generator-side capacitor 51 can be provided with a servomotor, the actuation of which changes the distance between the two capacitor plates, for example byone of the two capacitor plates is moved in a straight line, whereby both capacitor plates are always parallel to each other, or one of the two capacitor plates is pivoted.

[0117] By varying the capacitance of capacitor 51 or inductance 47, the resonant frequency of generator resonant circuit 52 can be changed or tuned. If the resonant frequencies of the generator resonant circuit and the tool resonant circuit match, the maximum electrical power is transferred from generator 18 to tool resonant circuit 50 and thus to tool capacitor 15, 16. By varying the resonant frequency of generator resonant circuit 52, the transfer of electrical power can be specifically controlled, whereby the greater the difference between the resonant frequencies of the two resonant circuits 50, 52, the lower the transferred power. Tuning generator resonant circuit 52 can thus be used specifically to adjust the electrical power introduced into mold cavity 14.

[0118] In the present embodiment, the resonant frequency of the generator resonant circuit 52 is varied. It is equally possible to vary the resonant frequency of the tool resonant circuit 50. This has the same effect on the transmission of electrical power. However, it is more difficult to provide a variable capacitor or inductor on the tool side than on the generator side.

[0119] Figure 4 shows a device for regulating the electrical power supplied to the tool capacitor 15, 16 in a simplified schematic circuit diagram. The generator 18 is connected to the tool capacitor 15, 16. A measuring capacitor 53 is connected in parallel with the tool capacitor 15, 16, the electrical capacitance of which is a fraction of the electrical capacitance of the tool capacitor 15, 16. The measuring capacitor 53 is connected via a coaxial line 54 to a voltage measuring device (voltmeter) 55. A diode 56 is preferably connected in parallel with the measuring capacitor 53. The coaxial line 54 is connected in series with an inductor 58, which serves to filter high-frequency signals.

[0120] The measuring unit formed by the measuring capacitor 53 and the diode 56 is separated from the tool capacitor 15, 16 by an isolating capacitor 59. The isolating capacitor has a high dielectric strength. The capacitance of the isolating capacitor 59 is smaller than the capacitance of the measuring capacitor 53. As a result, a higher voltage drop occurs across the isolating capacitor than across the measuring capacitor 53. The ratio of the capacitance of the isolating capacitor 59 to the capacitance of the measuring capacitor 53 is preferably 1:100 or 1:1,000 or 1:10,000. As a result, the voltage present across the tool capacitor 15, 16 is reduced in the measuring unit 53, 56 such that it lies within a measuring range of the voltage measuring device 55 and can be reliably detected by the latter.

[0121] In this circuit, a voltage drops across the measuring capacitor 53 that corresponds to the voltage present at the tool capacitor 15, 16 and is reduced according to the ratio of the capacitance of the measuring capacitor 53 to the capacitance of the isolating capacitor 59. By providing the diode 56, only the oscillation halves of a specific polarity are present. The diode 56 thus rectifies the voltage present at the measuring capacitor 53. This measuring voltage is measured by the voltage measuring device 55 and converted into a measuring signal. The measuring signal is forwarded to a control device 57, which automatically controls the generator 18 to output a predetermined electrical power in order to generate a specific voltage at the tool capacitor or a specific measuring voltage at the measuring capacitor, which is a fraction of the voltage at the tool capacitor.

[0122] The control device 57 regulates the power of the generator 18 based on the measured voltage. Different targets can be set for the predetermined power to be regulated. On the one hand, the power can be regulated to a certain constant value, or on the other hand, the power can be varied such that the voltage drop across the measuring capacitor 53 and rectified by the voltage measuring device 55 is essentially constant. In other words, depending on the measured voltage, either the output electrical power or the measured voltage can be kept constant.

[0123] As explained at the beginning, the absorption capacity of the material to be welded located in the tool capacitor 15, 16 changes depending on the temperature, with a particularly strong change in the absorption capacity of the electromagnetic waves occurring in certain processing situations. In the equivalent circuit diagram shown in Figure 4, the change in the absorption capacity of the electromagnetic waves can also be viewed as a change in the capacitance of the tool capacitor 15, 16. This changing capacitance at the tool capacitor 15, 16 leads either to a change in the voltage (= voltage amplitude) or to a change in the delivered electrical power, depending on whether the control device 57 is designed to keep the output electrical power or the voltage applied to the tool capacitor 15, 16 constant (= voltage amplitude).

[0124] In principle, it is easier to deliver a predetermined constant electrical power using the generator 18, because then the tuning of the tool resonant circuit 50 to the generator resonant circuit 52 can be kept constant, so that a constant electrical power is delivered by the generator 18. In this embodiment, the measurement voltage measured by the voltage measuring device 55 can represent a parameter characteristic of the absorption capacity of the material to be welded, which parameter is monitored by the control device 57. If the measurement voltage changes by a predetermined threshold value within a predetermined time interval, this is determined by the control device 57 and assessed as a trigger point for terminating the heating process. The heating process can be terminated immediately upon determination of the trigger point or delayed by a predetermined period of time.The heating process is terminated by switching off the supply of electrical power from the generator 18, wherein the switching off of the generator 18 is controlled by the control device 57.

[0125] In a modification of this embodiment, the control device 57 can be configured to first derive the measurement voltage over time. If the signal exceeds a certain threshold after the first time derivation of the measurement voltage, this indicates a predetermined abrupt change in the measurement voltage applied to the tool capacitor 15, 16, which in turn is assessed by the control device 57 as the trigger point for terminating the heating process.

[0126] In a further modification of the embodiment, the control device 57 can also be configured to determine the second time derivative of the measured voltage. If the second time derivative of the measured voltage exceeds a certain threshold, this indicates a predetermined curvature in the measured voltage curve and thus an abrupt change in the voltage signal. This can also be interpreted as a trigger point for terminating the heating process.

[0127] If the control device 57 is configured such that the electrical power delivered by the generator 18 is varied such that a substantially constant voltage (= voltage amplitude) is dropped across the tool capacitor 15, 16, the entire system reacts to an abrupt change in the absorption capacity of the electromagnetic waves of the material to be welded located in the tool capacitor 15, 16 by a corresponding change in the electrical power. This power could, in principle, be determined by measuring the current delivered to the tool capacitor 15, 16, since the voltage applied to the tool capacitor 15, 16 is already known and is measured by the voltage measuring device 55. The power could then be determined from the corresponding current value and the corresponding voltage value.

[0128] However, it is simpler to tap the control signal output by the control device 57, which controls the power output of the generator 18. This control signal corresponds to the power to be delivered, and a predetermined significant change in the control signal can be interpreted as a trigger point for terminating the heating process. Since this control signal is present in the control device 57, it can also be monitored there simultaneously.

[0129] Here, too, different embodiments are possible for determining the trigger point. The control signal can be monitored to determine whether it changes by a predetermined threshold within a predetermined time interval; the first or second derivative can also be monitored to determine whether they exceed a predetermined threshold.

[0130] In the same way as the trigger point is determined (based on voltage or power; monitoring the directly measured signal or the first derivative or the second derivative), the heating process can be stopped immediately upon detection of the trigger point or can be stopped with a predetermined time delay.

[0131] This method for automatically determining the heating time of the foam particles using electromagnetic waves can be carried out individually and repeatedly for each manufacturing process for producing a particle foam part. However, within the scope of the invention, it is also possible to use this method for calibrating the heating time of the foam particles using electromagnetic waves for a specific mold and for a specific type or batch of foam particles. The duration of the heating process until all foam particles are completely welded to form a particle foam part depends primarily on the mold used, which defines the mold space, and on the type or batch of foam particles used.

[0132] The calibration process refers to the production of a particle foam part in which the heating time of the foam particles with electromagnetic waves is individually adjusted according to the method described above. The time spent heating the foam particles with electromagnetic waves is recorded and stored using a clock or timer. The thus determined time for heating the foam particles with electromagnetic waves is hereinafter referred to as the "production time interval."

[0133] A single calibration process or multiple calibration processes can be performed, each of which records the duration of heating the foam particles with electromagnetic waves. If multiple heating intervals are recorded, these can be statistically evaluated, and, for example, the mean or median of the recorded heating intervals can be stored as a production time interval for subsequent production processes. In subsequent production processes, the characteristic parameter is then no longer monitored; instead, the duration of heating the foam particles with electromagnetic waves is controlled solely based on the production time interval. Thus, the duration of heating the foam particles with electromagnetic waves has been calibrated.This simplifies further control and can, if necessary, be transferred to other devices for producing particle foam parts that use the same tool and in which the same type or batch of foam particles is used to produce the particle foam part. These additional devices can be equipped with a simple control device that controls the heating process solely based on the existing production time interval.

[0134] As explained above, several production time intervals, each recorded with a calibration process, can be statistically evaluated. With such a statistical evaluation, it is also possible to determine a quality factor for the production process. If the different production time intervals differ significantly from one another, this is a sign that the production process is unstable and particle foam parts of varying quality are being produced. If, on the other hand, the production time intervals are essentially the same, this means that the production process is stable and particle foam parts of the same quality are being produced. Thus, the standard deviation of the production time intervals of the different calibration processes can be varied as a quality factor to assess the quality of the production process if the quality factor indicates that the stability of the production process is too low.

[0135] One or more of the following parameters can be varied to optimize the production process: - applied power of the electromagnetic waves,

[0136] - applied voltage to generate the electromagnetic waves,

[0137] - amount of heat transfer medium supplied to the mold cavity,

[0138] - Quantity of foam particles fed into the molding chamber,

[0139] - Pressure exerted on the foam particles by means of a mold before they are heated.

[0140] The lower the applied power or voltage to generate the electromagnetic waves, the slower the foam particles are heated. The slower they are heated, the better heat can be equalized between the different areas of the mold cavity, and the more uniform the welding process is. Therefore, reducing the power or voltage can often stabilize the production process.

[0141] However, with some molds, particularly those with an electrically conductive mold half, heat may be dissipated very well at certain points, resulting in certain areas of the mold cavity being cooled more than others. With such molds, however, it may be advantageous to heat the foam particles as quickly as possible and to heat the foam particles with the highest possible electromagnetic wave power or the highest possible applied voltage. In this situation, increasing the power or voltage can stabilize the manufacturing process.The quantity of foam particles in the mold cavity and / or the amount of binder used to bond the foam particles can also have a significant impact on the stability of the manufacturing process. There are cases where increasing or decreasing the quantity of foam particles or the amount of binder leads to a stabilization of the manufacturing process. This can be identified using statistical analysis based on the quality factor and adjusted accordingly.

[0142] This optimization can be performed automatically using an expert system. It can also be based on machine learning, whereby the quality of the manufactured particle foam parts is recorded manually or using suitable measurement methods and fed into the system as feedback, which can then be used to execute the optimization and learning process.

[0143] It has been shown that the duration of application of the electromagnetic RF radiation depends on the volume of the molding cavity 14, the density of the foam particles, and the applied electrical power or voltage. Tests have shown that, depending on the volume and the material from which the foam particles are formed, approximately 30 seconds to approximately 2 minutes are required to reliably and completely weld the foam particles. Typically, an electrical voltage (= voltage amplitude) of 5 kV to 20 kV is applied.

[0144] During welding, the temperature of the foam particles can be measured and the electrical power adjusted accordingly. The electrical power is preferably adjusted so that the foam particles are at a temperature slightly above their softening temperature.

[0145] The surface bordering the mold cavity 14 can also be temperature-controlled. For this purpose, heating wires 34 can be arranged in the mold adjacent to the surface bordering the mold cavity 14. The heating wires 34 are connected to a power source 35, which can be used to feed a heating current into the heating wires.

[0146] Instead of heating wires, fluid channels can also be provided in the mold halves 12, 13, through which a correspondingly temperature-controlled fluid flows. The fluid is preferably water or steam.

[0147] After the electromagnetic RF radiation is applied, the mold cavity 14 is kept closed for a stabilization period, whereby the introduced heat is evenly distributed throughout the particle foam part and a very uniform weld is formed between all foam particles. This process step is referred to as stabilization. During stabilization, the particle foam part also cools slightly. Since the mold halves 12, 13 are made of a material that is essentially transparent to electromagnetic RF radiation, which is usually a plastic material that conducts heat poorly, little heat is generally dissipated to the outside when the mold cavity 14 is closed. This is, of course, different if one mold half is made of an electrically conductive and thus highly thermally conductive material.

[0148] Mold halves 12, 13 made of plastic have the advantage over metal mold halves in that they offer significantly better thermal insulation and, at the same time, have a lower heat capacity. This allows the desired temperature cycles to be carried out much faster and with less energy, with the heat supplied being transferred almost entirely to the foam particles.

[0149] During the stabilization period or part of the stabilization period, the capacitor plates 15, 16 can be actively cooled by the cooling device 32, 33, whereby heat is removed from the base bodies of the mold halves 12, 13 and thus also from the particle foam part.

[0150] After stabilization, the particle foam part is demolded by moving the two mold halves 12, 13 apart. The mold can be equipped with demolding rams for demolding, which push the particle foam part out of one of the two mold halves 12, 13. Stabilization is an optional process step. For certain materials and shapes, it can also be omitted. The larger the volume of the particle foam part to be produced, the more expedient it is to stabilize the particle foam part in the mold after welding.

[0151] To increase throughput, electromagnetic RF radiation can be applied during filling and / or closing of a crack gap.

[0152] The electromagnetic radiation, in particular the RF radiation, can be applied during filling or after filling the mold cavity 14 with foam particles, initially with low electrical power or low electrical voltage, in order to preheat the material to a certain temperature and then gradually or suddenly increase the electrical power or electrical voltage.

[0153] It may also be useful to gradually increase the power or voltage of the electromagnetic RF radiation, so that a ramp is carried out over a period of time, for example, 30 seconds to 3 minutes by gradually increasing the electrical power or voltage of the electromagnetic RF radiation. This achieves very uniform heating of the foam particles.

[0154] Optionally, a negative pressure and / or vacuum can also be applied to the molding chamber 14. This is useful if the foam particles and / or the supplied compressed air have a certain moisture content.

[0155] The process described above is a dry process compared to welding with steam alone. This means that the manufactured particle foam parts are dry, or drier, after the production process and can be transferred to further processing steps more quickly. It may also be expedient to demold the warm particle foam parts and transfer them directly to further processing. This can significantly increase efficiency in production, as the pauses between individual process steps can be shortened, and the heat applied to weld the foam particles can also be used, at least in part, for subsequent process steps.

[0156] It may also be useful to provide a water or steam supply line at the welding station, through which water and / or steam is fed to the mold. This is particularly advantageous when foam particles are to be welded that have a low dielectric loss factor or generally only a low dielectric loss factor. In such a case, a small amount of water or steam is added. The electromagnetic radiation heats the water to steam or the steam is heated further. This heats the foam particles to a higher temperature at which the dielectric loss factor is greater, causing them to absorb the electromagnetic radiation themselves and heat up further. It has been shown that just a few hundred grams of water are sufficient for a mold cavity with a volume of 50 liters.For example, if the foam particle material is ePS (expandable polystyrene), 300 g or less of water is sufficient to heat and weld the foam particles in a mold cavity with a volume of 50 liters. With conventional welding, in which the foam particles are heated solely using superheated steam, steam quantities equivalent to several kilograms of water are required for a mold cavity with a volume of 50 liters.

[0157] Therefore, if foam particles that only slightly absorb electromagnetic radiation are to be welded, a single addition of 300 g of water is sufficient for a mold cavity with a volume of 50 liters. For many materials that only slightly absorb electromagnetic radiation, even smaller amounts of water may be sufficient. For mold cavities with other volumes, the maximum required amount of water can be adjusted to the volume in the same ratio.

[0158] If water is heated in the mold cavity using electromagnetic radiation, it is advisable to use a molding tool with a pressure sensor that can measure the pressure in the mold cavity. This pressure is proportional to the temperature. The irradiation of the electromagnetic radiation is then preferably regulated according to the measured pressure value, i.e., preferably set to a specific pressure value.

[0159] List of reference symbols

[0160] 1 device 27 inner boundary surface

[0161] 2 material containers 30

[0162] 3 mold tool

[0163] 4 Line 30 Ground

[0164] 5 Floor 31 Floor wall

[0165] 6 Compressed air line 32 Side wall

[0166] 7 Compressed air source 35 33 Filling opening

[0167] 8 Driving nozzle 34 Ventilation opening

[0168] 9 Compressed air line 35 Housing

[0169] 10 filling injector

[0170] 11 Compressed air line

[0171] 12 mold half 40 46 line

[0172] 13 mold half 47 generator-side inductance

[0173] 14 mold cavity 48 tool-side inductance

[0174] 15 Capacitor plate 49 tool-side capacitor

[0175] 16 Capacitor plate 50 Tool resonant circuit

[0176] 17 electrical line 45 51 generator-side capacitor

[0177] 18 AC voltage source 52 Generator resonant circuit

[0178] 19 Vacuum pump 53 Measuring capacitor

[0179] 20 Fan 54 Coaxial cable

[0180] 21 cooling fin 55 voltmeter

[0181] 50 56 Diode

[0182] 57 Control device

[0183] 24 Base body 58 Inductance

[0184] 25 Base body 59 Isolating capacitor

[0185] 26 inner boundary surface

Claims

Claims 1 . A method for producing a particle foam part from foam particles using electromagnetic waves, comprising the steps - Filling a mold cavity, - Heating the foam particles with electromagnetic waves to weld them, - demoulding, whereby during the heating of the foam particles a parameter characteristic of the absorption of the electromagnetic waves is monitored and when this parameter changes by a predetermined amount, the heating of the foam particles is stopped.

2. Method according to claim 1, characterized in that the electromagnetic waves are generated by means of a capacitor in which the molding space is located, the electromagnetic waves being applied to the capacitor with a predetermined voltage amplitude and the power introduced into the molding space or its first or second derivative over time being measured as a characteristic parameter, or the electromagnetic waves are applied to the capacitor with a predetermined power and the voltage dropping across the capacitor or its first or second derivative over time being measured as a characteristic parameter.

3. Method according to claim 1 or 2, characterized in that after heating and before demoulding, the particle foam part is cooled in the moulding chamber.

4. Method according to one of claims 1 to 3, characterized in that RF radiation is used as electromagnetic waves.

5. A method for calibrating a process for producing particle foam parts using electromagnetic waves, wherein at least one particle foam part according to one of claims 1 to 4 is produced in a calibration process using a mold that defines a specific mold space and foam particles of a specific material type, and the time duration of heating with electromagnetic waves is measured until the characteristic parameter changes by the specific threshold value and further particle foam parts are produced by heating the foam particles with the same settings for supplying the electromagnetic waves for a production heating interval that corresponds to the time duration determined with the calibration process.

6. Method according to claim 5, characterized in that during a calibration process a plurality of particle foam parts are produced according to one of the methods according to claims 1 to 4 and in each case the heating time is measured and the production time interval is determined on the basis of the plurality of time periods measured during the calibration process.

7. Method according to claim 6, characterized in that the mean or median of the individual time periods is determined as the production time interval.

8. Method according to claim 6 or 7, characterized in that for determining the production time interval only time periods are taken into account which lie within a predetermined multiple of the standard deviation around the mean or around the median of the measured time periods.

9. Method according to one of claims 6 to 8, characterized in that a statistical evaluation of the measured time periods is carried out to determine a quality factor of the production process.

10. Method according to claim 9, characterized in that the production process is optimized based on the quality factor by at least one certain parameters of the production process are varied if the quality factor indicates that the production process is too unstable.

11. Method according to claim 10, characterized in that at least one of the following parameters is varied and then a production time interval is again determined by producing several particle foam parts according to a method according to one of claims 1 to 4: - applied power of the electromagnetic waves, - applied voltage to generate the electromagnetic waves, - amount of heat transfer medium supplied to the mold cavity, - Quantity of foam particles fed into the molding chamber, - Pressure exerted on the foam particles by means of a mold before they are heated.

12. The method according to claim 11, characterized in that the at least one parameter to be changed and the degree of change are determined automatically by means of an expert system.

13. Device for producing a particle foam part from foam particles, comprising a molding tool which delimits a molding space, wherein at least two capacitor plates are arranged adjacent to the molding space and are connected to a radiation source for electromagnetic radiation, wherein the radiation source for electromagnetic radiation is designed to emit electromagnetic radiation, and the device has a control device which is designed such that the device automatically carries out a method according to one of claims 1 to 12.

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