Presses for producing sandwich products

Abstract

A press (10) for manufacturing sandwich panels includes first and second press plates (12, 14) movable relative to one another. The press tools (12, 14) include at least one internal flow path (16). The press (10) has heating and cooling fluid circulation loops. The press includes a heater (34) for generating hot pressurized water in communication with the press tools (12, 14), and an outlet pressure control valve (58) configured to convert the hot pressurized water in communication with the press tools (12, 14) to steam during initial rapid cooling.

Description

[Technical Field]

[0001] The present invention relates to a press for the in situ production of sandwich products, in particular thermoplastic sandwich panels, comprising an expanded thermoplastic core between covers, and to a production method using such a press. [Background technology]

[0002] In-situ fabrication of sandwich panels from a starting structure is known in the art. For example, EP 636463 discloses such a method. In-situ fabrication according to EP 636463 includes at least the steps of providing a starting structure including a core layer of a thermoplastic material incorporating a physical foaming agent, particularly a swelling agent, and at least one surface of which is covered with a skin layer. Typically, the core layer is disposed between two skin layers, preferably fiber-reinforced thermoplastic skin layers. The starting structure is placed in a heated press between a lower press plate and an upper press plate. The press is then closed, and the press plates apply pressure to the starting structure. While maintaining pressure on the starting structure to prevent premature foaming, the starting structure is heated to the foaming temperature of the thermoplastic material (higher than the boiling temperature of the swelling agent). Upon heating, migration of the swelling agent from the core layer to the skin layer of the starting structure causes adhesion between the core layer and the skin layer. In this way, a homogeneous foam layer is formed between the skin layers. Once the foaming temperature is reached, foaming is performed by separating the press plates in a controlled manner to a predetermined distance, allowing the thermoplastic material of the core layer to expand and the physical foaming agent to form foam cells. Thus, foaming and bonding occur in the same press. Pressure is then maintained on the foamed structure, and the press plates are cooled. When sufficiently cooled to prevent further foaming, typically at a temperature below the boiling temperature of the swelling agent, e.g., ambient temperature, the pressure can be completely released, and the resulting sandwich panel can be removed from the press. In the production of thermoplastic sandwich panels with physical foaming agents, rapid cooling is necessary to prevent collapse of the foam, particularly at the interface between the foam and the skin layers, and to prevent further migration of the physical foaming agent, if present, into the thermoplastic material of the fiber-reinforced thermoplastic skin.

[0003] For example, in the case of chemical blowing agents, as described in WO 2015 / 065176, the starting structure is placed in a heated press heated to a temperature well above the melting temperature or range of the thermoplastic material in the core. The chemical blowing agent in the core layer is decomposed, and the resulting intermediate structure is then cooled in an intermediate cooling step, typically to a temperature just above the melting temperature of the respective thermoplastic material in the core layer. Foaming is then carried out by separating the press plates in a controlled manner while maintaining pressure on the structure to be foamed, and the press plates are further cooled when the required predetermined foam thickness is reached.

[0004] For productivity reasons, the various process steps of heating, foaming, and cooling should be rapid, for example, in the range of tens of seconds to at most a few minutes, and should be homogeneous, especially during cooling, to prevent large temperature differences on the press plates, which could affect the quality of the final sandwich panel. Rapid cooling is also required to prevent after-foaming, which would have a significant impact on the mechanical properties of the resulting sandwich panel, and in particular to prevent the collapse of the formed cells. WO 2020 / 117048 discloses a press for producing such sandwich panels. This known press has a fluid circulation loop for heating and cooling the press plates. The fluid circulation loop includes a heater for generating a heated fluid, the heater having an outlet connected to a fluid supply conduit and an inlet connected to a fluid return conduit. The fluid supply conduit is in fluid communication with the inlet of at least one internal flow channel in each press plate, and the fluid return conduit is in fluid communication with the outlet of at least one internal flow channel in each press plate. Additionally, the fluid circulation loop is provided with a controlled expansion valve configured to convert the hot pressurized water into steam for cooling the press plate. The water source has an outlet in fluid communication with the inlet of each press plate's internal flow passage via a water supply conduit. Furthermore, it is disclosed that if the volume of hot pressurized water in the flow passages becomes too small and insufficient for the required cooling by phase transformation, further hot pressurized water can be preferably introduced into the flow passages. The purpose of this type of flash cooling, which uses heat of evaporation to convert the hot pressurized water in the press plate flow passages into steam, is to improve temperature uniformity on the surface of the press plate, particularly during the first stage of cooling, after the flow of pressurized hot water through these passages is stopped.

[0005] It has now been discovered that the large amount of steam discharging from the press plate channels and supply conduits, including connections and headers or manifolds, forces the hot water contained in the channels to be drawn in with the steam, resulting in a high flow rate that also causes it to be blown out. As a result, the channels quickly empty and remain insufficiently wetted, preventing the desired rapid cooling rate from being achieved throughout the cooling period and / or resulting in unacceptable temperature unevenness on and / or between the press plates. A cooling rate that is too low, especially in the initial cooling phase, affects the quality of the final sandwich panel. Adding make-up water to compensate for water loss, as disclosed in WO 2020 / 117048, does not provide a satisfactory solution. Summary of the Invention [Problem to be solved by the invention]

[0006] The object of the present invention is to avoid this drawback, and in particular to maintain a sufficiently rapid and homogeneous cooling during the initial cooling of the foam sandwich product. [Means for solving the problem]

[0007] A sandwich product comprising a foamed thermoplastic core between covers, in particular a press for producing thermoplastic sandwich products, more particularly a press for producing in situ thermoplastic sandwich panels, is defined in the accompanying claim 1.

[0008] Embodiments of the press according to the invention are defined in the dependent claims. A method for producing sandwich products, preferably using this press, is also defined in the claims. The press according to the invention comprises press plates (sometimes called press tools) that are movable relative to each other, typically a lower press plate and an upper press plate that are arranged above each other and capable of vertical movement. Typically, one or both of the press plates has a cavity in which a starting structure for the sandwich product can be placed. Generally, the periphery of the cavity engages with the periphery of the starting structure so that only thickness (vertical) expansion can occur during the controlled foaming stage. Typical products that can be produced using this press according to the invention include two-dimensional products (length and width are large compared to thickness), such as sandwich panels. The panels preferably have a flat shape, although slightly curved or corrugated shapes are also contemplated.

[0009] The press according to the present invention includes a heater capable of generating high-temperature water for use in the heating and cooling stages of the manufacturing process, as described in more detail below. The heater has a supply inlet for supplying fresh water, as well as a high-temperature water outlet and return port. The high-temperature water outlet and return port are connected to a fluid circulation loop. The press plate can be heated during the heating and foaming stages of producing the sandwich product using a heated fluid, typically high-temperature water, circulating through the press plate in the fluid circulation loop. Steam can also be used as an alternative means for heating the press plate, but in that case, the fluid circulation loop should be filled with high-temperature pressurized water before cooling. Typically, an electric heater, boiler, or other heat exchanger is used as the heater for heating water to the required temperature range. The heater can be a boiler that generates high-temperature water at a constant, high delivery pressure, thereby avoiding the risk of steam bubbles forming in the supply lines and manifold to the press plate. The press has a fluid circulation loop for heating and cooling the press plate during each operating stage of the press. Each press plate has at least one internal flow passage through which a fluid can flow from a flow passage inlet to a flow passage outlet, and typically there are multiple flow passages in each press plate. Typically, the inlet ends of the multiple flow passages are connected to one or more supply manifolds or headers for connection to fluid supply conduits, and the outlet ends are connected to one or more collector manifolds or headers for connection to fluid return conduits. During heating, foaming, and cooling, adjacent internal flow passages within the same press plate are preferably supplied in counterflow. The hot water outlet of the heater is connected to the inlet end of at least one flow passage in each press plate via a fluid supply conduit. The return port is connected to the outlet end of at least one flow passage via a fluid return conduit. Water circulation is achieved by one or more suitable pumps in the fluid return conduit. To heat the press plate, the heater prepares hot water, which is circulated through the flow passages in the press plate via the fluid supply conduit and returned from the press plate via the fluid return conduit.The temperature and pressure of the hot water supplied to the press plates generally range from 170 to 250°C and 8 to 30 bar, respectively, taking into account the type of blowing agent. For physical blowing agents, lower temperatures, such as 170 to 190°C, are appropriate. For chemical blowing agents that require decomposition, higher temperatures, such as 210 to 230°C, are typically applied. Advantageously, the distribution of the channels within the body of the press plates, and the flow rate through all channels of both press plates during operation, are such that a uniform temperature distribution is achieved on the inner surfaces of the press plates facing the product.

[0010] The press according to the invention also has a cooling mode in which the cooling effect is based in part on the heat of evaporation required for the conversion of hot pressurized water into steam, which is extracted (homogeneously) from the press plate, which thereby cools the foamed structure.

[0011] The fluid return conduit of the fluid circulation loop is in controlled fluid communication with the outlet control pressure valve. In the cooling mode, the connection between the heater and the fluid circulation loop is closed. Then, during the first cooling stage, the outlet control pressure valve is opened while high-temperature pressurized water circulates over the press plate. The high-temperature water is derived from at least one water source for supplying temperature-controlled water, the outlet of which has a valve connected to the fluid return conduit upstream of the circulation pump. The pump pressurizes the high-temperature water. The temperature and pressure of the high-temperature pressurized water are below the saturation pressure at the prevailing temperature of the press plate, such as a water temperature in the range of 80-100°C. Steam is primarily formed in the press plate flow passages and expands through and downstream of the outlet control pressure valve. During the first cooling stage, cooling by flowing high-temperature pressurized water continues until the press plate reaches a predetermined first temperature. To ensure dissolution of the steam formed in the water in the press plate's internal passages while preventing the passages from emptying and insufficient wetting, the outlet pressure control valve is gradually opened, maintaining a relatively low total water flow rate. The predetermined first temperature is typically in the range of 105 to 125°C, for example, 110 to 120°C.

[0012] Optionally, when the outlet control pressure valve is open to atmospheric pressure, a second cooling stage is performed, in which cooling of the at least one press plate from the predetermined first temperature to the predetermined second temperature is performed by introducing water from at least one water source in the fluid circulation loop, and increasing the total water flow rate by controlling the pump.

[0013] Thus, once the press plate reaches a predetermined first temperature, an optional second cooling stage can be initiated, in which the press plate is further cooled to a predetermined second temperature using water from at least one water source having a lower temperature, typically derived from a temperature-controlled water source. The water temperature is typically in the range of 70-90°C. In this manner, evaporative cooling and convection continue until the press plate reaches a temperature of 100°C. However, during the first cooling stage, the temperature difference between the circulating water at approximately 100°C and the press plate being cooled decreases, slowing the cooling rate and preventing the desired high cooling rate from being achieved. Preferably, if present, the flow rate in the second cooling stage is increased and the temperature is set to prevent evaporative cooling. Therefore, during this optional second cooling stage, the water temperature is reduced and the flow rate is increased by adjusting the circulating pump, e.g., the flow rate of the second cooling stage is approximately 8-10 times that of the first cooling stage to obtain the required cooling rate and temperature homogeneity on the press plate. The predetermined second temperature is typically in the range of 75 to 95° C., for example 80 to 85° C. The greater the difference between the inlet temperature of the water supplied to the press plate and the measured temperature of the press plate, the lower the inlet pressure must be, corresponding to a value below the saturation pressure, increasing the risk of water being entrained in any of the steam formed.

[0014] This optional second cooling step can be omitted if the outlet control pressure valve is in communication with a sub-atmospheric environment, for example using a vacuum pump, where the press can be cooled more deeply.

[0015] The press according to the present invention comprises control means arranged to operate the press under heating, foaming and cooling conditions.

[0016] In a preferred embodiment, a vacuum pump is provided in the flash tank, which allows the predetermined first and second temperatures in the first and second cooling stages to be lowered. This additional pressure drop, caused by applying a (partial) vacuum using a vacuum pump connected via the flash vessel described in the fluid return conduit, results in a lower temperature at which steam formation occurs. This allows the cooling due to the phase transformation of water into steam to continue to a lower temperature, for example below 90°C, compared to the above-mentioned predetermined first temperature range, providing a more uniform temperature of the press plate.

[0017] The third slow cooling stage is carried out by water cooling from either a predetermined temperature (or a predetermined second temperature, if a second cooling stage is carried out) using temperature-controlled water from at least one water source, but at a reduced cooling rate. Typically, to ensure uniform cooling in this stage, the temperature difference between the temperature of the circulating water and the temperature of the press plate is kept constant, for example in the range of 1 to 15°C, preferably in the range of 3 to 12°C. If necessary, an increased flow rate can be maintained in the third cooling stage.

[0018] The press according to the invention ensures that during the fast first and optional second cooling stages, water of suitable pressure and temperature flows continuously through the internal channels in sufficient quantity to dissolve the formed steam in the water flow, thereby still cooling the press plates. Expansion takes place at or downstream of the outlet control pressure valve.

[0019] The at least one water source for supplying temperature-controlled water in the first cooling stage is configured to deliver water at approximately 100°C into the fluid circulation loop. In one embodiment, the outlet control pressure vessel is in fluid communication with a downstream flash tank for recovering water from the formed steam. In a further embodiment, the flash tank has a water outlet in fluid communication with a fluid return conduit upstream of the circulation pump and serves as one of the at least one water source. The outlet pressure control valve is connected to the downstream flash tank, which is maintained at atmospheric pressure or at reduced pressure via a vacuum pump. Expansion then occurs primarily in the flash tank. The recovered water is returned to the fluid return conduit upstream of the circulation pump. Because the recovered water from the flash tank has a temperature of 100°C under atmospheric conditions, when the recovered water is repressurized in the fluid return conduit by the circulation pump, its temperature is still lower than the temperature of the press plate (i.e., the saturation pressure of the circulating water to the press is lower than the saturation pressure at the prevailing (current) temperature of the press plate), thereby allowing for continued flashing and subsequent cooling in the first cooling stage down to a predetermined first temperature, such as in the range of 105-125°C. Under conditions below atmospheric pressure, evaporative cooling in the first cooling stage can continue down to a lower predetermined first temperature, such as below 90°C. An additional water source may be present to replace the water lost in the first cooling stage and to mix with water derived from the flash tank to set the appropriate temperature in the optional second cooling stage.

[0020] Instead of using recovered water from the flash tank, a separate temperature controlled water source can be used.

[0021] Thus, during (part of) the first and second cooling stages, pressurized water having a high temperature of approximately 100°C flows through at least one internal flow passage within each press plate to the outlet control pressure valve. The hot pressurized water absorbs heat from the press plate and transforms into steam, thereby cooling the press plate. Steam bubbles, still under pressure from the constantly supplied hot pressurized water, are dissolved in the flow of hot pressurized water. The dissolved steam expands at or downstream of the outlet pressure control valve. This absorbed heat is expelled in the form of steam by the evaporation of the hot pressurized water. The water recovered from the flash tank has a temperature of 100°C at atmospheric pressure and can be used to cool the press plate with 100°C water in the first and second cooling stages until the press plate reaches the predetermined second temperature. This water absorbs heat from the press plate and transforms into steam, which is dissolved in the constant flow of 100°C water passing through the press plate. The entrainment of hot pressurized water in the steam that forms, below its saturation temperature at the prevailing temperature of the press plates, is strongly reduced by maintaining the pressure of the hot water at an appropriate level, thereby avoiding emptying of the flow channels and ensuring sufficient wetting. The actual temperature of at least one press plate, preferably both press plates, is monitored. Typically, the temperature of the press plate is measured by temperature sensor(s) located a few mm, e.g., 3-4 mm, below the press surface in the center of the press plate. From the measured temperature, e.g., the average of the measured temperatures of both press plates, the corresponding saturation pressure at that (averaged) measured temperature can be calculated, e.g., using a look-up table. Advantageously, the temperature and pressure of the circulating water supplied to the press plates are monitored as well.

[0022] The heat withdrawal from the press plate by converting the hot, pressurized recovered water to steam in accordance with the present invention is more uniform than cooling with cold water, yet still fast enough to achieve the required cooling rate. This uniform cooling in accordance with the present invention also results in a press plate surface having a more uniform temperature across and / or between the press plates, resulting in less localized variation in density and cell size in the final sandwich panel and better, more uniform bonding between the foam and the cover. Therefore, the quality of the sandwich panel produced is improved.

[0023] The press operation is typically controlled using a PLC, PC or similar device. In one embodiment, the controller includes a processor and, when executed by the processor, preheating the press plate by flowing a hot fluid through a fluid heating circulation loop; Closing the press by moving the preheated press plates towards each other and applying pressure to the starting structure of the sandwich panel to be produced; heating the starting structure to a foaming temperature while applying pressure to the starting structure (if the amount of heat in the preheated press plate is insufficient to allow the starting structure to achieve the foaming temperature, continuing to heat the press plate by flowing hot fluid through the fluid circulation loop); foaming the starting structure at the foaming temperature by moving the press plates apart a predetermined distance while maintaining pressure on the structure to be foamed; and interrupting the flow of heated fluid from the heater to the fluid circulation loop, typically at the end of the foaming curve when the required thickness has been achieved; In a first cooling stage, cooling the press plate to a predetermined first temperature by opening an outlet pressure control valve and flowing temperature-controlled water from at least one water source, such as a flash tank, through a fluid circulation loop by a circulation pump, the temperature and pressure of the water being below the saturation pressure at the prevailing temperature of the press plate; Optionally, in a second cooling stage, cooling the press plate from the first predetermined temperature to a second predetermined temperature of the press plate by flowing temperature controlled water from at least one water source at an increased water flow rate; in a third cooling stage, water cooling from the predetermined first temperature or the second temperature to ambient temperature at a reduced cooling rate by flowing temperature-controlled water from at least one water source, such as a separate water source; and a memory storing computer-readable instructions for executing a process including:

[0024] Typically, the instructions also include opening the press to allow the manufactured sandwich panel to be removed.

[0025] In that embodiment, the instructions for the process of cooling the press plates include the steps of measuring a prevailing temperature of at least one press plate with a temperature sensor, determining a saturation pressure of high-temperature water at the measured temperature of the at least one press plate, and setting an inlet pressure of high-temperature pressurized water supplied from the heater to the press plate.

[0026] In a further embodiment, the instructions for the process of cooling the press plate include setting an inlet temperature of the hot pressurized water supplied to the press plate to a temperature equal to or lower than the measured temperature of the press plate.

[0027] Advantageously, the press comprises a feed unit for introducing the starting structure into the press and / or a discharge unit for unloading the manufactured sandwich panel from the press, instead of manually placing the starting structure in the press. The instructions then additionally comprise the steps of introducing the starting structure into the press and, if applicable, removing the manufactured sandwich panel from the press.

[0028] The above instructions are typical for manufacturing sandwich panels using a physical blowing agent. A physical blowing agent is a swelling agent (i.e., a solvent with low solubility in the respective thermoplastic) or solvent that volatilizes upon heating, resulting in foam cells. Examples of physical blowing agents include carbon dioxide and various light hydrocarbons such as pentane, as well as swelling agents such as acetone and solvents such as methylene chloride.

[0029] For thermoplastic sandwich panels manufactured using chemical blowing agents, as described, for example, in WO 2015 / 065176, rapid and uniform cooling from the blowing agent's decomposition temperature (or higher) to the foaming temperature, and from the foaming temperature to a temperature below the Tg or Tm of the thermoplastic material in the foam core of the thermoplastic sandwich panel, is advantageous to reduce internal stresses and / or warpage and / or cycle time. In this way, the optimal temperature or range for decomposition and the optimal temperature or range for foaming can be independently selected and set.

[0030] Chemical blowing agents are compounds that decompose to form low-molecular-weight gases such as nitrogen, carbon dioxide, carbon monoxide, oxygen, and ammonia. Examples of chemical blowing agents include azobisisobutyronitrile, diazoaminobenzene, monosodium citrate, and oxybis(p-benzenesulfonyl)hydrazide. Azo-, hydrazine-, and other nitrogen-based chemical blowing agents are preferred. Azodicarbonamide is a preferred example in this category. Other examples include isocyanates and sodium bicarbonate for PU. The first thermoplastic core layer containing the chemical blowing agent can be easily manufactured, for example, by extrusion or calendaring.

[0031] The present invention can be used with any type of blowing agent, however, the production of sandwich panels as described above using physical blowing agents, particularly swelling agents, will benefit most from the press according to the present invention.

[0032] As a starting structure, a core layer of thermoplastic material containing a blowing agent combined with at least one skin layer is used.

[0033] Suitable thermoplastics for the core layer foamed using a physical blowing agent include both crystalline and amorphous thermoplastics, with amorphous thermoplastics being preferred for solubility considerations.

[0034] Suitable thermoplastics for the core layer foamed using a chemical blowing agent include both crystalline and amorphous thermoplastics. Crystalline thermoplastics are preferred because they have a small difference between their glass transition temperature and melting point, allowing the possibility of solidifying the thermoplastic once foamed within a small temperature interval.

[0035] As materials for the skin layers (also called covers, facings, or face sheets), thermoplastics, especially fiber-reinforced thermoplastics, and metal sheets, such as aluminum, can be used. The lower and upper cover layers are preferably made of the same material, although combinations are also contemplated, e.g., a lower cover layer of metal, such as aluminum, and an upper cover layer of (fiber-reinforced) thermoplastic. The choice of materials for the core layer and skins depends, inter alia, on the desired properties of the end product application.

[0036] The thermoplastics for the core and cover layers may be the same or different, including different grades. Hereinafter, for purposes of illustration, the thermoplastic in the core layer will be referred to as the first thermoplastic, while the thermoplastic in the cover layer, if present, will be referred to as the second thermoplastic. Examples include polyetherimide (PEI), polyethersulfone (PES), polysulfone (PSU), polyphenylsulfone (PPSU), polyketones such as polyetheretherketone (PEEK), PPS (polyphenylene sulfide), liquid crystal polymers, polycarbonate (PC), polyolefins (derived from C1-C4 monomers) such as polypropylene (PP), polyvinyl chloride (PVC), polyethylene (PE), thermoplastic polyurethane (TPU), PA polyamide, PC polycarbonate, etc., and combinations thereof. Combinations of different cover layers, such as PEI / PEEK, PPSU / PEEK, and PEI / PC, are also contemplated. Thermoplastic biopolymers are also contemplated. For example, a blend of PEI and PC is also contemplated for fiber-reinforced facings. The skin may include one or more sublayers, the number of which may vary locally, for example, to allow for localized dedicated additional application. For locally different skins, the heat transfer properties, such as heat capacity and / or thermal conductivity, typically also vary locally. In this case, the press can be equipped with non-adherent local compensation elements that equalize the heat transfer properties.

[0037] Advantageously, at least one of the skins is a fiber-reinforced layer of the second thermoplastic material, preferably both skins are fiber-reinforced layers of the second thermoplastic material.

[0038] As indicated above, it is also possible to use a combination of different thermoplastics for the first and second thermoplastics, examples of which include, inter alia, PEI for at least one core layer covered with a (fiber-reinforced) skin made of PPSU, PS, PEEK or PC, PES or PPSU or a combination thereof for at least one core layer covered with a (fiber-reinforced) cover layer made of PSU (polysulfone) or PC, and PP for at least one core layer covered with a (fiber-reinforced) layer made of a polyamide such as nylon.

[0039] For compatibility reasons, advantageously, the type of first thermoplastic material in the core is the same as the type of second thermoplastic material in the facing.

[0040] Glass fibers are the preferred reinforcing means. Other inorganic fibers, such as metal fibers, carbon fibers, and organic fibers, such as aramid fibers, polymer fibers, nanofibers of the aforementioned fibers, and natural fibers, can also be used, provided they can withstand the temperatures experienced during the process according to the invention. Fibers can be used in the form of mats, woven fabrics, chopped fibers, etc. Directional fibers, especially unidirectional fibers whose fiber direction is adapted to the intended use, can also be used advantageously.

[0041] High strength, high elongation steel cords may be present in the fiber reinforced cover layer. Particulates, such as nanoparticles of metal or inorganic origin, may be used instead of or in addition to fiber reinforcement.

[0042] Reinforcements may also be applied between sub-layers of a thermoplastic core layer incorporating chemical or physical blowing agents, especially swelling agents. Examples of such reinforcements include (glass) fiber reinforcement layers, metal layers, and steel cords.

[0043] Another preferred embodiment of the starting structure is a mat made from both inorganic and thermoplastic fibers, for example glass fibers and propylene fibers, or a mat made from inorganic fibers and thermoplastic powder.

[0044] Additives such as nucleating agents, plasticizers, melt strength enhancers and nanoparticles may also be present in the first thermoplastic layer.

[0045] The press plates of a press according to the present invention can have flat surfaces, and it is also contemplated that one or both of the press plates may have a 2D or 3D curved configuration.

[0046] The present invention also relates to a method for producing a sandwich panel in a press with a press plate, in particular in a press according to the invention, the sandwich structure comprising at least one foamed layer of a first thermoplastic material and a cover layer, the method comprising: a) providing a starting structure comprising at least one layer of a first thermoplastic material and two cover layers, wherein the at least one layer of the first thermoplastic material comprises a physical blowing agent; b) contact heating the starting structure between press tools of a press to a foaming temperature while maintaining pressure on the starting structure by the press tools; c) foaming said at least one layer of said first thermoplastic material containing a physical foaming agent at said foaming temperature by moving a press tool a predetermined distance apart, advantageously according to a predetermined foaming curve, while maintaining pressure on the foamed structure, the volume increasing to a final volume and then being kept constant, thereby obtaining said sandwich panel; d) in a first cooling stage, opening the outlet pressure control valve and flowing temperature-controlled water, having a temperature and pressure below the saturation pressure at the prevailing temperature of the press plate, from at least one water source, such as a flash tank and / or recovered water from a separate temperature-controlled water source, through at least one flow path in the press tool to the outlet pressure control valve by means of a circulation pump, to cool the sandwich panel at a constant final volume while in contact with and under pressure with the press plate to a predetermined first temperature of the press plate, for example below the glass transition temperature of the first thermoplastic material comprising the physical blowing agent, for example in the range of about 110-120°C, thereby establishing the conversion of water into steam; e) optionally, in a second cooling stage, cooling said sandwich panel from the first predetermined temperature to a second predetermined temperature of the press plate by flowing temperature-controlled water from at least one water source, for example recovered water from a downstream flash tank in combination with other temperature-controlled water, by means of a circulation pump at an increased flow rate through at least one flow channel in the press plate; f) in a third cooling stage, cooling the sandwich panel of step d) or optionally step e) from the predetermined second temperature to ambient temperature by flowing temperature-controlled water from at least one water source at a reduced cooling rate, advantageously with an increased flow rate; g) opening the press and removing the sandwich panel cooled in step f) from the press. Includes.

[0047] The physical blowing agent in the first thermoplastic lowers the glass transition temperature of the first thermoplastic. In step d) above, the glass transition temperature is that of the first thermoplastic containing the physical blowing agent. As a guideline, the predetermined first temperature in cooling step d) can be set at least several tens of degrees Celsius higher than the boiling point of the physical blowing agent, for example in the range of 20 to 40 degrees Celsius higher.

[0048] The present invention also relates to a method for producing a sandwich panel in a press with a press plate, in particular in a press according to the invention, which sandwich panel comprises at least one foamed layer of a first thermoplastic material and a cover layer, the method comprising: a) providing a starting structure comprising at least one layer of a first thermoplastic material and two cover layers, wherein the at least one layer of the first thermoplastic material comprises a chemical blowing agent having a decomposition temperature higher than the melting temperature or melting range of the first thermoplastic material; b) contact-heating the starting structure between press plates of a press to a temperature higher than the decomposition temperature of the chemical blowing agent, resulting in decomposition of the chemical blowing agent, thereby obtaining an intermediate structure, wherein the decomposed chemical blowing agent is present in at least one layer of the first thermoplastic material while maintaining pressure on the starting structure by the press plates; c) after decomposition of the chemical blowing agent, cooling the intermediate structure thus obtained to a foaming temperature by opening the outlet pressure control valve and flowing temperature-controlled water, having a temperature and pressure below the saturation pressure at the prevailing temperature of the press plate, from at least one water source, e.g., recovered water from a downstream flash tank, through at least one flow passage of the press tool by means of a circulation pump to the outlet pressure control valve, thereby establishing the conversion of water to steam while maintaining pressure on the intermediate structure by the press plate; d) foaming said at least one layer of said first thermoplastic material containing decomposed chemical foaming agent at said foaming temperature by moving press plates apart a predetermined distance, advantageously according to a predetermined foaming curve, while applying pressure to the structure to be foamed, the volume increasing to a final volume and then remaining constant, thereby obtaining said sandwich panel; e) in a first cooling stage, opening the outlet pressure control valve and cooling the sandwich panel at a constant final volume while in contact with and pressurizing it with the press plate, by flowing temperature-controlled water, having a temperature and pressure below the saturation pressure at the prevailing temperature of the press plate, from at least one water source, for example recovered water from a downstream flash tank, through at least one flow path of the press tool to the outlet pressure control valve by means of a circulation pump, to a predetermined first temperature, for example below the glass transition temperature or below the melting temperature or below the range of the first thermoplastic, for example a temperature in the range of about 110-120°C (at such predetermined first temperature below Tg or Tm, the foam cells stabilize and maintain their shape), thereby establishing the conversion of the hot pressurized water into steam; f) optionally, in a second cooling stage, cooling the sandwich panel from the first predetermined temperature to a second predetermined temperature by flowing temperature-controlled water from at least one water source, for example recovered water from a downstream flash tank, by a circulation pump at an increased flow rate through at least one channel in the press plate; g) in a third cooling stage, cooling the sandwich panel of step e) or optionally step f) from the predetermined second temperature to ambient temperature by flowing temperature-controlled water from at least one water source; h) opening the press and removing the cooled sandwich panel from the press in step g) to steam. Includes.

[0049] The process according to the invention, in which at least one stage of cooling after foaming is carried out by converting hot pressurized water into steam, offers similar advantages to the press outlined above. The various advantageous and preferred embodiments of the press are applicable to this process in a similar manner, including condensation of the steam in a flash tank and its reuse in the first and / or second cooling stages.

[0050] For chemical blowing agents, the foaming temperature is above the glass transition temperature for amorphous thermoplastics and above the melting point or range for (semi-)crystalline thermoplastics.

[0051] The foaming occurs under controlled pressure contact with the starting structure by press plates moving away from each other. During foaming, the volume of the structure increases to the final volume, in particular the final thickness, and then remains constant, thereby obtaining the sandwich panel. Due to the difference between the operating temperature of the foaming step and the final low temperature (typically ambient temperature), a temperature-dependent volume reduction (shrinkage) may occur. In any case, no further expansion occurs. As explained above, the cooling step(s) using phase transformation may be carried out under partial vacuum.

[0052] The invention is further illustrated by the accompanying drawings. [Brief explanation of the drawings]

[0053] [Figure 1] 1 is an overall view of an embodiment of a press according to the present invention; [Figure 2] 1 is a schematic diagram showing an embodiment of a press according to the invention in more detail; FIG. DETAILED DESCRIPTION OF THE INVENTION

[0054] In the drawings, like components and parts are designated by the same reference numerals. In FIG. 1 , one embodiment of a press for producing sandwich panels is generally designated by the reference numeral 10. The press 10 includes two press plates 12 and 14, each movable relative to the other. For example, the lower press plate 14 may have a fixed position, while the upper press plate 12 is vertically movable, as indicated by arrow A, for example, by a hydraulic cylinder (not shown). Each press plate 12, 14 has at least one internal flow passage 16 in its body, extending from an inlet 18 to an outlet 20. Typically, a large number of internal flow passages 16 are distributed within the body of each press plate 12, 14 without exceeding a critical value related to the pressures applied during operation and impairing the strength of the press plate. Arrow B indicates the direction of fluid flow through the internal flow passages 16. As can be seen, in this embodiment, adjacent flow passages are fed in countercurrent flow. 1 shows the completed sandwich panel 22 having two thin covers 24, 26 and a thicker foam core 28. The covers 24 and 26 are advantageously glass fiber reinforced thermoplastic layers, preferably the same thermoplastic as that of the foam core layer 28.

[0055] Figure 2 shows an embodiment of a press 10 according to the present invention in more detail. The inlets 18 of the channels 16 in each press plate 12, 14 (only a few are shown in Figure 2 for clarity) are connected to a supply header 30, and the outlets 20 are connected to a collector header 32. A heater 34 for producing hot fluid for heating the press plates 12, 14 is connected with its hot water outlet 36 via a valve 37 and to the supply header 30 via a fluid supply conduit 38 and a branch conduit 40. The heater 34 has a fresh fluid supply line 42 for introducing fresh fluid.

[0056] At the discharge side of the press plates 12, 14, the collector header 32 is connected to a branched discharge conduit 50 and a return conduit 52 with a valve 54, and to a return port 48 with a valve 49 of the heater 34 via a pump 55. A discharge conduit 56 with an outlet pressure control valve 58 connects the return conduit 52 to a flash vessel 60 with an optional vacuum pump 62. A recycle conduit 63 connects a recovered water outlet 65 of the flash tank 60 to the return conduit 52 upstream (suction side) of the pump 55. The press plates 12, 14 are provided with a temperature sensor 64 linked to a control device 66, such as a PC or PLC with a processor 68 and memory 70, which controls the operation of the press, including conditions such as opening and closing the press, the temperature, pressure and flow rate of the hot pressurized water supplied, the hot fluid for (pre-)heating and initial cooling, and the temperature-controlled water for further cooling, and related equipment such as heater(s) and valves.

[0057] The press is operated as follows.

[0058] The starting structure comprises a core layer between skins. In one embodiment, the core layer is made of a thermoplastic material containing a physical blowing agent. The skins are advantageously glass fiber reinforced thermoplastic layers, preferably of the same thermoplastic material as the core layer.

[0059] The starting structure is positioned between preheated press plates 12, 14 in a mating manner around its periphery to prevent lateral (horizontal) expansion / foaming. Press plates 12 and 14 are preheated to a foaming temperature, e.g., 170-190°C, depending on the thermoplastic being used, by using pump 55 to flow a hot fluid through a fluid circulation loop that includes heater 34 and press plates 12, 14. Press 10 is closed so that both press plates 12 and 14 are in contact with the starting structure.

[0060] The press 10 closes quickly to prevent premature and uncontrolled foaming of the starting structure's core layer, including the physical foaming agent, before pressure is applied by the press plates 12 and 14. Once the starting structure's homogeneous foaming temperature (above the boiling temperature of the physical foaming agent) is achieved, the distance between the press plates 12 and 14 is increased in a controlled manner so that the skins maintain contact with their respective press plates 12 and 14 and thus pressure is applied during foaming. Once the distance has increased to its predetermined value, and the starting structure, particularly its core layer, has foamed to a corresponding predetermined thickness, the circulation flow from the heater through the flow passage 16 is shut off by the operating valves 37 and 54, and cooling begins. Once the first cooling phase begins, the outlet pressure control valve 58 begins to open, and the pump 55 maintains circulation in the fluid circulation loop. Advantageously, the volume of the fluid circulation loop from the pump 55 upstream of the press plates 12, 14 is small, so that only a small amount of hot water with a high temperature for heating is present and needs to flow through the flow passages 16 of the heated press plates 12, 14 before actual cooling begins. The pressurized hot water contained in the circulation loop, having a temperature and pressure below the saturation pressure at the prevailing temperature of the press plates 12, 14, expands and converts to steam in the outlet control pressure valve 58 and flash tank 60. When the flash tank 60 is held at atmospheric pressure (no reduced pressure is applied by the vacuum pump 62), the recovered water has a temperature of approximately 100°C. The recovered water is supplied via the pump 55 to the recycle conduit 63, where it is pressurized and returned to the press plates 12, 14, where it again forms steam, which dissolves in water and travels to the outlet pressure control valve 58. This type of evaporative cooling continues until the predetermined first temperature of the press plates 12, 14 is reached. At this time, the temperature difference between the press plates 12, 14 and the water temperature of approximately 100°C is small, and therefore the cooling rate is slow. To maintain the fast cooling rate, an optional second cooling stage is initiated in which temperature-controlled water from water reservoir 72 is introduced into return conduit 52 at a location upstream of pump 55 via conduit 74 and optional pump 76.A combined stream, consisting of recovered water and temperature-controlled water, having a temperature in the range of, for example, 70-90°C, is pressurized by pump 55 and supplied to press plates 12, 14 to further cool them to a second predetermined temperature. At the end of the second cooling stage, the press is switched to a final cooling (third cooling stage) using temperature-controlled water from source 72 or a further temperature-controlled water source (not shown) to cool the press plates from the second predetermined temperature to ambient temperature at a slower cooling rate by water cooling (convection / conduction) the press plates. In the case of a chemical blowing agent, the press is heated to a temperature above the decomposition temperature of the chemical blowing agent.

[0061] Typically, the press plates 12 and 14 are preheated to a temperature well above the melting temperature or melting range of the thermoplastic material being used and above the decomposition temperature of the chemical blowing agent. Alternatively, the press plates 12 and 14 are preheated to a temperature below the melting point of the thermoplastic material being foamed and therefore below the decomposition temperature of the chemical blowing agent, which is higher than said melting temperature. After closing the press 10, the temperature of the starting structure is further increased by heating the press plates 12 and 14 above their decomposition temperature. After the blowing agent decomposes, the structure is rapidly cooled to an appropriate temperature above the melting point / range of the thermoplastic material by evaporative cooling via the outlet pressure control valve 58, as described above. Cooling continues until a predetermined lower limit temperature above the melting temperature of the thermoplastic material is reached. When the starting structure, still under pressure, reaches a homogeneous temperature just above the melting temperature of the thermoplastic material used in the core layer, the distance between the press plates 12, 14 is increased in a controlled manner so that the skins maintain contact with their respective press plates 12, 14, thus maintaining pressure on the structure being foamed. Once the distance has increased to its predetermined value, and the starting structure, particularly its core layer, has thus foamed to a corresponding predetermined thickness, cooling is resumed by operating the outlet pressure control valve 58 in the three cooling stages described above. The intermediate cooling from the decomposition temperature to the melting temperature of the thermoplastic material may be omitted. Foaming then occurs at a relatively high foaming temperature.

[0062] In the case of an embodiment having a vacuum pump, the second cooling stage is omitted.

Claims

1. A press (10) for producing sandwich products, in particular thermoplastic sandwich panels, comprising a foamed thermoplastic core between covers, more particularly a press (10) for producing in situ thermoplastic sandwich panels (22), said press (10) comprising: a first press plate and an opposing second press plate (12, 14) configured to be movable relative to one another, each press plate (12, 14) having at least one internal flow passage (16) with an inlet (18) and an outlet (20), and at least one press plate (12, 14) including a temperature sensor (64); a heater (34) configured to produce hot water, having a hot water outlet (36) with a valve (37), a return port (48) with a valve (49), and a supply port (42) for fresh water; a fluid circulation loop for heating and cooling the press plates (12, 14), the fluid circulation loop comprising: a fluid supply conduit (38) in fluid communication with the outlet (36) of the heater (34) and in fluid communication with the inlet (18) of the at least one internal flow passage (16) in each press plate (12, 14); a fluid return conduit (52) in fluid communication with the outlet (20) of the at least one internal flow passage (16) of each press plate (12, 14) and in fluid communication with the return port (48) of the heater (34); Equipped with the fluid return conduit (52) being in controlled fluid communication with an outlet control pressure valve (58); a fluid circulation loop, the fluid return conduit (52) comprising a circulation pump (55) for pressurizing water in the fluid circulation loop; at least one water source (60, 72) configured to supply temperature-controlled water, the water source having an outlet in fluid communication with the fluid return conduit (52) upstream of the pump (55), the water source including a valve; The temperature sensor (64) is configured to measure the prevailing temperature of at least one press plate (12, 14), and the circulation pump (55) is configured to set a water flow in the fluid circulation loop, and the at least one press plate (12, 14) is configured to supply temperature-controlled water pressurized by the circulation pump (55) and having a temperature and pressure below the saturation pressure at the prevailing temperature of the press plates (12, 14) from the at least one water source (72) through the fluid return conduit (52) to the press plates (12, 14). a control means (66) configured to open the outlet control pressure valve (58) during a first cooling stage of cooling to a predetermined first temperature at a reduced cooling rate, and configured to flow controlled water from the at least one water source (72) having a temperature less than the prevailing temperature of the at least one pressure plate (12, 14) through the fluid circulation loop during a third cooling stage of cooling from a predetermined second temperature of the at least one press plate (10, 12) equal to or less than the predetermined first temperature to ambient temperature at a reduced cooling rate. A press (10) comprising:

2. 2. The press of claim 1, wherein the pressure downstream of the outlet control pressure valve (58) is atmospheric, and the control means (66) is also configured to introduce water from the at least one water source (72) into the fluid circulation loop and control the pump (55) to increase the total water flow rate during a second cooling stage of cooling from the first predetermined temperature to a second predetermined temperature of the at least one press plate (12, 14).

3. 3. The press of claim 1 or claim 2, further comprising a vacuum pump (62) in fluid communication with the outlet control pressure valve (58), wherein the predetermined second temperature is equal to the predetermined first temperature.

4. 3. The press of claim 1 or claim 2, wherein the outlet control pressure valve (58) is in fluid communication with a downstream flash tank (60).

5. 5. The press of claim 4, wherein the at least one water source comprises a flash tank (60) having a water outlet in fluid communication with the fluid return conduit (52) upstream of the circulation pump (55), the control means (66) circulating recovered water from the flash tank (60) to the fluid return conduit (52) during the first cooling stage and having an outlet in fluid communication with the fluid return conduit (52) upstream of the pump (55) during the third cooling stage, and an additional water source (72) comprising a valve is configured to supply temperature-controlled water.

6. 3. The press of claim 2, wherein the predetermined first temperature is in the range of 105 to 125°C.

7. 3. The press of claim 2, wherein the predetermined second temperature is in the range of 75 to 90°C.

8. A press according to any one of the preceding claims, wherein the control means (66) is configured to control the temperature difference between the temperature of the cooling water and the temperature of the at least one press plate during the third cooling stage to a range of less than 15°C.

9. 9. The press of claim 1, wherein the control device (66) is configured to control the outlet control pressure valve (58) so that the saturation pressure of the water supplied to the press plates (12, 14) at an inlet pressure is equal to or lower than the saturation pressure of the water at the prevailing temperature of the press plates (12, 14).

10. The control means (66) includes a processor (68) and, when executed by the processor (66), preheating the press tools (12, 14) by flowing heated fluid from the heater through the fluid circulation loop; closing the press (10) by moving the preheated press plates (12, 14) towards each other and applying pressure to the starting structure of the sandwich product (22) to be produced; heating the starting structure to a foaming temperature while applying pressure to the starting structure; foaming the starting structure at the foaming temperature by moving the press tools (12, 14) apart a predetermined distance while maintaining pressure on the structure to be foamed; interrupting the flow of heated fluid from the heater (34) to the fluid circulation loop; in a first cooling stage, cooling the press plates (12, 14) to a predetermined first temperature by opening the outlet pressure control valve (58) and flowing temperature-controlled water from the at least one water source (60, 72) through the fluid circulation loop by the circulation pump (55), the temperature and pressure of which is less than the saturation pressure of the press plates at the prevailing temperature; Optionally, in a second cooling stage, cooling the press plates (12, 14) from the first predetermined temperature to a second predetermined temperature of the press plates (12, 14) by flowing temperature-controlled water from the at least one water source (60, 72) at an increased water flow rate; in a third cooling stage, water cooling from the predetermined first or second temperature to ambient temperature at a reduced cooling rate by flowing temperature-controlled water from the at least one water source (60, 72); and a memory (70) storing computer readable instructions for performing a process comprising:

11. A method for producing a sandwich product (22) in a press with press plates, preferably a press (10) according to any one of claims 1 to 10, wherein the sandwich structure (22) comprises at least one foamed layer (28) of a first thermoplastic material and a cover layer (24, 26), said method comprising: a) providing a starting structure comprising at least one layer of a first thermoplastic material and two cover layers, wherein the at least one layer of the first thermoplastic material comprises a physical blowing agent; b) contact heating the starting structure between the press tools (12, 14) of the press (10) to a foaming temperature while maintaining pressure on the starting structure by the press tools (12, 14); c) foaming the at least one layer of the first thermoplastic material containing the physical foaming agent by moving the press tools (12, 14) a predetermined distance apart while maintaining pressure on the structure to be foamed at the foaming temperature, allowing the volume to increase to a final volume and then remain constant, thereby obtaining the sandwich panel (22); d) during a first cooling stage, opening an outlet pressure control valve (58) and flowing temperature-controlled water, having a temperature and pressure less than the saturation pressure of the press plates (12, 14) at the prevailing temperature, from at least one water source (60, 72) by means of the circulation pump (55) through the at least one flow path (16) of the press tool (12, 14) to the outlet pressure control valve (58), thereby cooling the sandwich panel (22) at a constant final volume while in contact with and under pressure with the press plates (12, 14) to a predetermined first temperature of the press plates (12, 14), thereby establishing the conversion of the water into steam; e) optionally, in a second cooling stage, cooling the sandwich panel from the first predetermined temperature to a second predetermined temperature of the press plates (12, 14) by flowing temperature-controlled water from the at least one water source (60, 72) by means of the circulation pump (55) at an increased flow rate through the at least one flow channel (16) of the press plates (12, 14); f) in a third cooling stage, cooling the sandwich panel (22) of step d) or optionally step e) from the first predetermined temperature or the second predetermined temperature to ambient temperature at a reduced cooling rate by flowing temperature-controlled water from the at least one water source (60, 72); g) opening the press (10) and removing the sandwich panel (22) cooled in step f) from the press (10); A method comprising:

12. A method for manufacturing a sandwich panel (22) in a press with a press plate, preferably a press (10) according to any one of claims 1 to 10, said sandwich panel (22) comprising at least one foamed layer (28) of a first thermoplastic material and a cover layer (24, 26), said method comprising: a) providing a starting structure comprising at least one layer of a first thermoplastic material and two cover layers, wherein the at least one layer of the first thermoplastic material comprises a chemical blowing agent having a decomposition temperature higher than the melting temperature or melting range of the first thermoplastic material; b) contact-heating the starting structure between the press plates (12, 14) of the press (10) to a temperature higher than the decomposition temperature of the chemical blowing agent, resulting in decomposition of the chemical blowing agent, thereby obtaining an intermediate structure, wherein the decomposed chemical blowing agent is present in the at least one layer of the first thermoplastic material while maintaining pressure on the starting structure by the press plates; c) after the decomposition of the chemical blowing agent, cooling the intermediate structure thus obtained to a foaming temperature by opening an outlet pressure control valve (58) and flowing temperature-controlled water, having a temperature and pressure below the saturation pressure at the prevailing temperature of the press plates (12, 14), from the at least one water source (60, 72) through the at least one flow passage (16) of the press tool (12, 14) by means of the circulation pump (55) to the outlet pressure control valve (58), thereby establishing the conversion of water to steam while maintaining pressure on the intermediate structure by the press plates (12, 14); d) foaming the at least one layer of the first thermoplastic material containing the decomposed chemical foaming agent by moving the press plates (12, 14) apart a predetermined distance while applying pressure to the structure to be foamed at the foaming temperature, allowing the volume to increase to a final volume and then keep constant, thereby obtaining the sandwich panel (22); e) in a first cooling stage, opening an outlet pressure control valve (58) and flowing temperature-controlled water, having a temperature and pressure less than the saturation pressure of the press plates (12, 14) at the prevailing temperature, from the at least one water source (60, 72) through the at least one flow path (16) of the press tool (12, 14) to the outlet pressure control valve (58) by the circulation pump (55), thereby cooling the sandwich panel (22) to a predetermined first temperature at a constant final volume while in contact with and pressurizing the press plates (12, 14), thereby establishing the conversion of the hot pressurized water into steam; f) optionally, in a second cooling stage, cooling the sandwich panel from the first predetermined temperature to a second predetermined temperature by flowing temperature-controlled water from the at least one water source (60, 72) by means of the circulation pump (55) at an increased flow rate through the at least one flow channel (16) of the press plate (12, 14); g) in a third cooling stage, cooling the sandwich panel (22) of step e) or optionally step f) from the second predetermined temperature to ambient temperature at a reduced cooling rate by flowing temperature-controlled water from the at least one water source (60, 72); h) opening the press (10) and removing the sandwich panel (22) cooled in step g) from the press (10); A method comprising:

13. 13. A method according to claim 11 or 12, wherein the cooling step by conversion of hot pressurized water to steam is carried out at a pressure below atmospheric pressure.

14. 14. A method according to any one of claims 11 to 13, wherein the first cooling step by conversion of hot pressurized water into steam is carried out under a continuous supply of water at a temperature of 80 to 100°C.

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