METHOD AND DEVICE FOR PROCESS MONITORING DURING THE MANUFACTURING OF A FINISHED PART FROM A HOT-CURING MATERIAL IN A PRIMARY FORMING PROCESS

Description

[0001] The present invention relates to a method and a device for process monitoring during the production of a finished part from a hot-curing material using a primary forming process. The method and the device enable continuous monitoring of the finished part's quality in series production. The present invention is particularly relevant for the production of components in high volumes in the aerospace and automotive industries, as well as in medical technology. Further applications in the aerospace and automotive industries, medical technology, the electronics industry, the consumer goods industry, the cosmetics industry, and the hygiene products industry are also conceivable. State of the art

[0002] Generally, process monitoring is of crucial importance in primary forming processes, particularly within the framework of quality assurance and monitoring of safety-relevant components in the automotive and aerospace industries or in medical technology. Accordingly, numerous approaches exist that address the topic of process monitoring.

[0003] A reliable method for investigating the crosslinking reaction, as an important process parameter and quality characteristic, is to perform differential thermal analysis to measure the amount of heat released or absorbed by a sample during heating, cooling, or an isothermal process. In this method, an encapsulated container with a sample quantity, for example, in the range of 5 mg to 40 mg, and a second encapsulated reference container without contents are placed together in a test chamber and subjected to the same temperature change program. Due to the thermal properties of the sample and exothermic or endothermic processes or phase changes such as melting or evaporation, temperature differences inevitably arise between the sample and the reference, since thermal energy flows into or out of the sample during the process under investigation. In differential thermal analysis, the heat flow is used as the measured quantity.Another option for process monitoring of crosslinking materials in a primary forming process is, in principle, indirect measurements using a rotational rheometer. A further option for process monitoring of crosslinking materials in a primary forming process is, in principle, the use of dielectric analysis (DEA).

[0004] However, both described methods require specialized measuring equipment and corresponding software. These methods cannot be integrated into the manufacturing process. Furthermore, they cannot be used to determine the degree of cross-linking within the manufacturing process and therefore cannot provide traceable quality and process data.

[0005] Various methods are known from the state of the art which use different measured variables and parameters, such as temperature, flow behavior, electrical conductivity or pressure, for process monitoring.

[0006] DE 10 2015 107 024 B3 describes a method for determining a number of process parameter values ​​in an injection molding process within a mold. Geometric data of the mold and / or a molded part to be produced in the mold are determined. Using this geometric data, a virtual, part-specific pressure curve of an injection molding process is calculated. Based on this curve, a part-specific event pattern is determined, comprising a plurality of singular virtual events linked to characteristic event locations in the part geometry. Each virtual event is associated with at least relative time information and at least one positional data point, defining the position of a melt front of the injection molding material within the mold.

[0007] EP 0 897 786 A2 describes a method for controlling an injection molding machine. The injection unit introduces molten metal into a cavity in a controlled manner. The cavity pressure is measured and monitored. The novel aspect is the analysis of one or more control parameters influencing this pressure profile after each cycle. These parameters include the injection speed, the holding pressure level, the holding pressure interval, and the mold temperature. A control variable obtained in this way determines whether the relevant parameter needs to be readjusted. A suitable adjustment is then made.

[0008] EP 0 854 778 B1 describes a measurement of the internal pressure of a tool during a cycle of a clocked machine as a function of time and / or the conveying distance traveled. The tool's internal pressure is then differentiated as a function of the time or distance traveled, and the results are checked for deviations from a monotonic curve. If a deviation is found, the subsequent compression time or switching point is adjusted until the tool's internal pressure curve becomes monotonic.

[0009] The publication by P. Rosenberg et al., "Investigating cavity pressure behavior in high-pressure RTM process variants," 2015, AIP Conference Proceedings, addresses new variants of the high-pressure RTM process, namely high-pressure injection RTM (HP-IRTM) and high-pressure compression RTM (HP-CRTM) for the production of carbon fiber reinforced composites with a high fiber volume content. Both processes utilize high-pressure RTM systems for the precise metering and mixing of highly reactive epoxy resins and amine curing agents at relatively high throughput rates.

[0010] WO 2010 / 005375 A1 concerns a method for determining the curing state of a material using ultrasonic transit-time measurement. During a primary forming process, an ultrasonic signal is transmitted through a mold cavity filled with the material, and its transit time is recorded. A diagram is generated from the temporal development of the transit time, which allows conclusions to be drawn about the crosslinking progress or the curing of the material.

[0011] WO 2010 / 005375 A1 further describes the identification of specific parameters of this diagram to determine characteristic features of the material, such as the maximum crosslinking rate or the time of complete reaction. Additionally, a pressure signal can be recorded, which, together with the ultrasonic signal, is used to determine the temperature in the tool. Further diagrams can be derived from the pressure and temperature profiles.

[0012] DE 37 38 248 A1 discloses a method for controlling the fill level of a mold based on the pressure values ​​measured in the mold. During the heating phase to initiate the crosslinking reaction, the volume expansion of the mold material leads to varying pressure increases. A pronounced pressure increase indicates a completely filled mold, while a smaller or absent pressure increase suggests underfilling.

[0013] DE 37 38 248 A1 describes a method for controlling the internal mold pressure, in which the pressure profile over time and / or a distance traveled by a conveying medium is recorded and subsequently differentiated over time. The derived curves are checked for discontinuities or deviations from a monotonic profile. If such deviations are present, process parameters such as holding pressure time or switchover time are adjusted until a continuous or monotonic pressure profile is achieved.

[0014] From DE 37 82 040 T2, a method and a device for testing or controlling the crosslinking of elastomeric molded products are known. The disclosure enables both laboratory analyses of typical crosslinking parameters and the process-integrated control of the crosslinking duration during production.

[0015] DE 37 82 040 T2 further describes the use of a pressure sensor that is in direct contact with the crosslinking elastomeric material. The evaluation includes, among other things, the detection of a stage in which the gradient of pressure decrease reaches a zero value or a predefined threshold value.

[0016] However, the publications mentioned generally describe an analysis of tool cavity pressure in RTM and injection molding processes with thermoplastic and thermoset plastics, and not in relation to liquid silicone rubber. Object of the invention

[0017] The object of the present invention is therefore to overcome, at least partially, the disadvantages and limitations known from the prior art. In particular, a method and a device for process monitoring during the production of a finished part from a hot-curing material in a primary forming process are to be proposed, which enables precise component and / or process monitoring. Disclosure of the invention

[0018] This problem is solved by a method and a device for process monitoring during the production of a finished part from a hot-curing material, with the features of the independent claims. Advantageous embodiments are found in the dependent claims.

[0019] In the following, the terms "have," "exhibit," "comprise," or "include," or any grammatical variations thereof, are used in a non-exclusive manner. Accordingly, these terms can refer both to situations in which, apart from the features introduced by these terms, no other features are present, and to situations in which one or more additional features are present. For example, the expression "A has B," "A exhibits B," "A comprises B," or "A includes B" can refer both to the situation in which, apart from B, no other element is present in A (i.e., a situation in which A consists solely of B) and to the situation in which, in addition to B, one or more other elements are present in A, such as element C, elements C and D, or even further elements.

[0020] Furthermore, it should be noted that the terms "at least one" and "one or more," as well as grammatical variations of these terms, when used in connection with one or more elements or features and intended to express that the element or feature may be present once or multiple times, are generally used only once, for example, when the feature or element is first introduced. Upon subsequent mention of the feature or element, the corresponding term "at least one" or "one or more" is generally no longer used, without restricting the possibility that the feature or element may be present once or multiple times.

[0021] Furthermore, the terms "preferably," "in particular," "for example," or similar terms are used in the following in conjunction with optional features without limiting alternative embodiments. Features introduced by these terms are optional features, and it is not intended that these features limit the scope of protection of the claims, and in particular the independent claims. As the person skilled in the art will recognize, the invention can also be implemented using other embodiments. Similarly, features introduced by "in one embodiment of the invention" or by "in an exemplary embodiment of the invention" are understood as optional features without limiting alternative embodiments or the scope of protection of the independent claims.Furthermore, these introductory expressions are intended to leave all possibilities of combining the features introduced herein with other features, whether optional or non-optional features, unaffected.

[0022] In a first aspect, the present invention relates to a method for process monitoring during the manufacture of a finished part from a hot-curing material in a primary forming process.

[0023] The term "primary forming process" generally refers to any manufacturing process in which a solid body with a geometrically defined shape is produced from a formless material. A primary forming process can be, in particular, an injection molding process. In this process, polymers are injected under pressure into a mold or cavity, which can also be referred to as an injection molding tool. Within the mold or cavity, the material can transition into a solid state through cooling or a cross-linking reaction and can be removed as a finished part after the tool is opened. The cavity of the tool can essentially determine the shape and surface structure of the finished part. In particular, the tool can have at least two joinable mold plates, specifically at least one nozzle-side mold plate and at least one closing-side mold plate.The nozzle-side mold plate can have an opening into which an injection device (nozzle) is inserted, which enables the introduction of the starting materials of the hot-curing material into the cavity.

[0024] The term "process monitoring" generally refers to any technical procedure for evaluating a manufacturing process. Process monitoring enables a decision to be made as to whether intervention in the manufacturing process is necessary and / or whether the manufacturing process should be modified.

[0025] The term "finished part" here refers to a three-dimensionally extended structure to be examined, which is produced from a suitable material using a primary forming process. The finished part has a shape defined by the form of a cavity in a tool designed for this primary forming process. The finished part could, for example, be a sample or workpiece for a laboratory, or a product or prototype, such as a motor vehicle component, whose production using this method is subject to online monitoring. The finished part can also be referred to as a molded part.

[0026] The process includes the following steps: a) Providing at least one tool set up for the primary forming process, wherein the tool has at least one cavity for receiving at least one starting material for the hot-curing material, and wherein a device for determining an internal tool pressure is further integrated into the tool; b) Heating the tool; c) Introducing the at least one starting material for the hot-curing material under pressure into the cavity, such that the finished part is produced; d) Recording a time course of the internal tool pressure occurring in step c); e) Differentiating the course of the internal tool pressure at least once or at least twice to determine at least one derivative selected from the group consisting of: the first-degree derivative; the second-degree derivative;and f) characterization of the course of a chemical crosslinking reaction using at least one derivative selected from the group consisting of: the first-degree derivative; the second-degree derivative.

[0027] The procedure can include the steps described below. These steps can be performed in the specified order, but other sequences are also possible. Furthermore, one or more steps can be performed simultaneously or overlapping in time. Additionally, one, several, or all of the steps can be performed once or repeatedly. The procedure may also include further steps.

[0028] As explained above, the device for determining the internal mold pressure is integrated into the mold. The term "integrated" here refers to an arrangement of the device that can be fixedly or, preferably, detachably received into the mold, particularly into the mold cavity. This means that the device is preferably designed to withstand the temperature and pressure conditions that can occur in the mold cavity. In particular, the device can be arranged in the mold cavity in such a way that it is in contact with a wall of the cavity. The mold can, in particular, include an injection device such as a nozzle. The device can be arranged in close proximity to the injection device. The device can also be referred to as an internal mold pressure sensor.

[0029] The device for determining the tool cavity pressure can be used, in particular, for direct and / or indirect determination of the tool cavity pressure. The device for determining the tool cavity pressure can be configured for both direct and indirect determination of the tool cavity pressure.

[0030] Direct measurement of the tool cavity pressure offers many advantages. In particular, it provides precise and reliable readings. The pressure sensor can be placed directly inside the tool to capture the actual internal pressure. Furthermore, by placing pressure sensors at various points within the tool, the cavity pressure can be measured at specific locations, particularly useful for identifying local variations or problems.

[0031] The indirect determination of the mold cavity pressure can be achieved, for example, by means of an ejector, such as an ejector pin. Within the scope of the present invention, an "ejector" is understood to be an element that, during a primary forming process, exerts force to push a finished part out of a cavity of a mold. For the indirect determination of the mold cavity pressure, the ejector can include one or more force sensors. Furthermore, the indirect determination of the mold cavity pressure can be achieved, for example, by means of a closure, in particular a needle valve, of the cavity. Alternatively, the indirect determination of the mold cavity pressure can be achieved, for example, by measuring the clamping force. By placing pressure sensors in clamping units for the mold, a force exerted on the mold can be measured. and on theThe mold cavity pressure can be calculated from the mold cavity pressure. Alternatively, the mold cavity pressure can be determined indirectly, for example, using hydraulic pressure measurement. The mold cavity pressure is fundamentally correlated with the hydraulic pressure of the injection molding machine. If a hydraulic pressure sensor is installed in the hydraulic system of the injection molding machine, the mold cavity pressure can be determined indirectly by measuring the hydraulic pressure.

[0032] Indirect measurement of tool cavity pressure offers numerous advantages. For example, it can prevent damage to sensors, such as when using abrasive materials and / or high temperatures. Eliminating or reducing the number of pressure sensors can lower costs. Furthermore, it can reduce maintenance and calibration efforts. Finally, eliminating or reducing the number of pressure sensors can reduce the required space and lower the precision requirements for tool design and manufacturing.

[0033] The device for determining the internal tool pressure can, in particular, be or comprise a piezoelectric pressure sensor. The piezoelectric pressure sensor can include a crystal that generates an electrical charge proportional to an applied pressure. The device for determining the internal tool pressure can, in particular, be selected from the group consisting of: a piezoelectric pressure sensor using a direct and / or an indirect measurement method; a piezoresistive pressure sensor using a direct and / or an indirect measurement method. Various piezoelectric pressure sensors are described in detail in the Kistler Group brochure "Test & Measurement Pressure, Measuring Equipment for Demanding T&M Applications", 960-695d-06.20 © 2018 .... 2020 Kistler Group.

[0034] The term "intra-mold pressure" generally describes the pressure that arises within a mold, particularly a mold with a single cavity, during a primary forming process. As mentioned above, this primary forming process can be injection molding, and the intra-mold pressure can refer to the pressure inside the injection mold. Intra-mold pressure can be an indicator of the quality of a finished part. It is a meaningful process parameter that provides users with comprehensive process transparency and can contribute to zero-defect production. Intra-mold pressure essentially describes processes occurring within the mold cavity. Therefore, it allows conclusions to be drawn about the conditions under which pressure arises during the primary forming process.The pressure profile during a filling, compression, and / or holding pressure phase can, in principle, be associated with specific quality-relevant properties of the finished part, such as dimensional accuracy, surface finish, weight, or degree of deformation. The mold cavity pressure profile thus represents a part-specific fingerprint of quality, allowing for precise statements regarding optimal process parameters throughout the entire primary forming process. This mold cavity pressure profile can be, in particular, a temporal profile or a profile of the mold cavity pressure as a function of temperature.

[0035] As described above, the tool is heated in step b). The tool can, for example, have a temperature of 100 °C to 300 °C, preferably 120 °C to 250 °C, and particularly preferably 140 °C to 230 °C. Other temperatures are also conceivable in principle. The choice of temperature can depend on the type of material. The tool can have at least one heating device, which may, for example, comprise one or more heating circuits. The heating device may, in particular, be selected from the group consisting of: an oil heater; a water heater; a frame heater, in particular an electric frame heater; a heating cartridge, in particular an electric heating cartridge; a heating wire. Other embodiments are also conceivable in principle. The heating device may, for example, be arranged in at least one of the mold plates. Furthermore, the heating device may be arranged near the cavity.

[0036] As described above, in step c), the at least one starting material for the hot-curing material is introduced into the cavity under pressure to produce the finished part. In particular, the at least one starting material can be introduced into the cavity under a pressure of 5 bar to 700 bar, preferably 10 bar to 400 bar. Other pressures are also conceivable in principle. Introducing the material into the cavity can be carried out, in particular, by means of a conveying device, especially an extruder. When processing liquid silicone rubber, unlike thermoplastic processing, it is generally possible to work without holding pressure, since, in particular, no shrinkage needs to be compensated for. Although the starting material is injected into the cavity under pressure, the pressure in the cavity generally increases due to volume expansion.Essentially, only a needle valve nozzle is closed, preventing material from escaping the cavity. Typical curing times are generally 20 to 40 seconds. However, curing times are highly dependent on the component or finished part thickness.

[0037] The term "hot-curing material" generally refers to any material in which the constituent materials cross-link at high temperatures. These high temperatures can be, in particular, between 100 °C and 300 °C, preferably between 120 °C and 250 °C, and especially preferably between 140 °C and 230 °C. The hot-curing material can, in particular, be a reactive material that can be injected into a hot mold at temperatures below its specified curing temperature. Liquid silicone rubber is typically injected at near-room temperature (15 °C to 30 °C). The hot-curing material can, in particular, exhibit thermal expansion within the mold.

[0038] The term "starting material" generally refers to any substance that forms the basis for a specific material. In the production of this material, the starting material may, in particular, participate in a chemical reaction with other starting materials. The starting material may be selected from the group consisting of: a polymer, a catalyst, or a crosslinking agent. Other starting materials are also conceivable.

[0039] The hot-curing material can be selected from the group consisting of: a liquid silicone rubber; a solid silicone rubber; an epoxy resin; a polyurethane; a polyurethane foam; a thermoset, in particular a free-flowing thermoset; a polyester resin; a phenolic resin. Preferably, the hot-curing material can be a liquid silicone rubber, in particular a fast-curing liquid silicone rubber. Other materials are also conceivable in principle. In particular, the hot-curing material, especially the liquid silicone rubber, can have a cycle time of less than 60 s. Other cycle times are also conceivable in principle. The cycle time can depend in particular on the mold temperature and the design of the finished part. Other parameters are also conceivable.Thin-walled components, for example, can have a cycle time of up to 10 s, while thick-walled components can have a cycle time greater than 60 s. Additional parameters influencing the cycle time can include: material composition, one or more process parameters such as the tool temperature. Within the scope of the present invention, "cycle time" is generally understood to mean the time span or duration required to produce a finished part from the hot-curing material.

[0040] The term "liquid silicone rubber" (LSR) generally refers to any material based on two-component silicone elastomers. During injection molding of these two-component silicone elastomers, cross-linking of the two polymers occurs within the mold. This cross-linking can also be described as addition cross-linking. Poly(dimethyl)siloxane (PDMS), for example, is used as a cross-linking polymer, which is cross-linked using the crosslinker H-siloxane and a platinum catalyst. Liquid silicone rubber can be molded and processed at high temperatures and very fast cycle times. The high mold temperatures significantly accelerate the cross-linking reaction, allowing finished parts to be produced within seconds.Liquid silicone rubber can be used to manufacture plastic components for various industries, from sealing lips for headlights to contact lenses.

[0041] Liquid silicone rubber can be processed, in particular, with two components, A and B, which are mixed in a 1:1 ratio and crosslink three-dimensionally in a platinum-catalyzed addition reaction. The reaction can be accelerated, in particular, by processing temperatures of 150 °C to 230 °C. At room temperature, the pot life can take several days. The base polymer can be, in particular, a poly(dimethyl)siloxane (PDMS), which can be adapted to desired requirements by substitution of various functional groups. The crosslinking reaction of the two components is generally exothermic, irreversible, and without degradation products. It can be described by the Chalk-Harrod catalysis. Component A typically comprises a catalyst, in particular a platinum complex, while component B comprises a crosslinker in the form of H-siloxane.Compared to the well-researched thermoplastics, there is generally little process knowledge about the processing of liquid silicone rubber. Although the curing kinetics of liquid silicone rubber have been investigated in detail and studies have been conducted to simulate liquid silicone rubber, the application of these findings to a real injection molding process is lacking. Liquid silicone rubber generally undergoes a crosslinking reaction without byproducts. However, other processes and / or reaction mechanisms, such as polycondensation, exist in which byproducts are formed during the crosslinking reaction. Finished parts or components made of liquid silicone rubber can optionally be tempered in a further process step.

[0042] The liquid silicone rubber can be selected from the following groups: self-lubricating liquid silicone rubber; self-adhesive liquid silicone rubber; optical liquid silicone rubber; medical-grade liquid silicone rubber; and insulating liquid silicone rubber. Furthermore, the liquid silicone rubber can be selected from the following groups: high-tensile liquid silicone rubber; non-curing liquid silicone rubber; heat-resistant liquid silicone rubber; cold-resistant liquid silicone rubber; flame-retardant liquid silicone rubber; and low-viscosity liquid silicone rubber. Other types of liquid silicone rubber are also conceivable.

[0043] Injection molding processes for thermoplastics and liquid silicone rubbers differ fundamentally in key aspects, such as chemical properties, process control, and parameters. In liquid silicone rubber injection molding, various polymers undergo chemical cross-linking, resulting in the solid form. Thermoplastics, on the other hand, solidify. The raw materials for liquid silicone rubber are generally not heated before injection into the mold cavity and are therefore injected cold, whereas thermoplastics are melted at higher temperatures to achieve flowability. Furthermore, the internal mold pressures and cycle times in liquid silicone rubber injection molding are generally longer than those for thermoplastics.A key difference lies in the pressure profiles of the mold cavity, which can vary significantly between liquid silicone rubbers and thermoplastics. This is primarily evident in the fact that, with liquid silicone rubbers, the mold cavity pressure generally increases during the injection molding process due to the thermal expansion of the initially cold raw material in the hot mold. In contrast, with thermoplastics, the mold cavity pressure can decrease again after the injection process is complete during the holding pressure phase. The pressure profile when processing liquid silicone rubbers is fundamentally different due to the reversed temperature profile (cold raw materials are injected into the hot mold). This results in thermal expansion of the component within the mold due to the constant temperature input as soon as the raw materials are in the hot mold.As the process progresses, the internal tool pressure therefore increases steadily until the finished part is ejected.

[0044] As explained above, in step d) the temporal profile of the mold cavity pressure is recorded in step c). The mold cavity pressure profile can thus be monitored during the primary forming process itself, particularly during the production of the finished part, especially to track the progress of the chemical crosslinking reaction.

[0045] The term "crosslinking reaction" refers to a chemical reaction in which a large number of macromolecules are linked together to form a three-dimensional network, whereby the linking can occur particularly at already existing macromolecules. The crosslinking reaction takes place under specific parameters, especially pressure and temperature, with the degree of linkage between the macromolecules generally increasing steadily as the crosslinking reaction progresses. As a result of the crosslinking reaction, the chemical, physical, and / or mechanical properties of the material change; these are also referred to as "material parameters." A value for the degree of crosslinking is generally specified as a measure of the material's change during the manufacturing of the finished part, where the "degree of crosslinking" is defined as the proportion of crosslinked sites relative to the total amount of the plastic.

[0046] A drop in the mold cavity pressure is a characteristic feature indicating that the crosslinking reaction, particularly between the two polymers of the liquid silicone rubber, is complete. The crosslinking reaction typically occurs abruptly and is completed quickly. If a peak in the mold cavity pressure is recorded during measurement, it can be assumed that the crosslinking reaction is complete after a short waiting period. Different materials exhibit different pressure and temperature profiles and therefore have different cycle times.

[0047] In step e), as described above, the pressure profile of the tool is differentiated at least once or at least twice to determine the first and / or second derivative. The term "derivative" generally refers to a limit of the difference quotient of a function. This means that for each x-value of a function, one checks whether the y-value of the preceding and following x-value is greater than, less than, or equal to the y-value of the x-value under investigation. A derivative function describes a function that describes the slope of the function under investigation at every point. For the function f(x), the derivative function is f'(x). The derivative function f'(x) is called the first derivative or derivative of the first degree. For the derivative of the first derivative, the derivative function is f"(x). The derivative function f"(x) is called the second derivative or derivative of the second degree.

[0048] As explained above, in step f) the course of the chemical crosslinking reaction is characterized by means of first-degree and / or second-degree derivatives. In step f), the characterization of the course of the chemical crosslinking reaction can be determined in particular by ascertaining at least one of the following specific quantities: a zero crossing of the second-degree derivative; a course of the second-degree derivative; an extremum of the second-degree derivative.

[0049] Furthermore, in step f), the characterization of the course of the chemical crosslinking reaction can be determined in particular by determining at least one of the following specific quantities: a zero crossing of the first-degree derivative; a curve of the first-degree derivative; an extremum of the first-degree derivative.

[0050] In particular, the temporal profile of the tool's internal pressure can exhibit at least one first phase, at least one second phase, and at least one third phase. The second phase can follow the first phase. The third phase can follow the second phase. The first, second, and third phases can exhibit different average pressure increases. The first phase can be referred to as the heating phase. In the first phase, the starting material for the hot-curing material can heat up and thereby expand. The second phase can be referred to as the crosslinking phase. In the second phase, the starting material for the hot-curing material can crosslink in such a way that the hot-curing material is formed. The crosslinking reaction can be an exothermic crosslinking reaction. In this case, expansion of the hot-curing material can occur.Furthermore, shrinkage due to density changes can occur in the second phase. In the third phase, the hot-curing material can be heated to a set mold temperature, causing it to expand. The profile of the mold cavity pressure in each of the first, second, and third phases can depend on one or more of the following parameters: machine parameters, material, finished part geometry, and mold design. Other parameters may also have an influence.

[0051] In particular, in step f), the course of the chemical crosslinking reaction can be characterized by observing and / or evaluating the first-degree derivative and / or the second-degree derivative during the first phase, especially the heating phase, and / or during the second phase, especially the crosslinking phase. Thus, the crosslinking of the hot-curing material can, in principle, be detected in a phase using the method according to the invention. In the first and second phases, there are generally only small deviations in the course of the tool cavity pressure. These deviations can, in principle, be reliably and automatically detected using the first-degree derivative and / or the second-degree derivative.Therefore, it is generally not necessary to wait until the finished part, especially the entire finished part, has reached a tool temperature and the thermal expansion of the finished part approaches zero. At that point, the internal tool pressure generally no longer increases.

[0052] In particular, step f) can be used to determine when the chemical crosslinking reaction has been completed. Specifically, if step f) determines when the chemical crosslinking reaction has been completed, step c) can be terminated. Furthermore, step f) can be used to optimize the cycle time for manufacturing the finished part from liquid silicone rubber in the primary forming process.

[0053] This type of evaluation can, in principle, allow conclusions to be drawn about material changes during the crosslinking reaction. In this way, the cycle time of primary forming processes can be analyzed and optimized using measurement technology.

[0054] To carry out the method according to the invention, an electronic device, in particular a computer, may preferably be provided which has a program code that is set up to carry out the present method.

[0055] A computer program can be configured to perform the described steps of the method, in particular steps b) to f), and, if necessary, further steps. For details, reference is made to the description of the method according to the invention.

[0056] In a further aspect of the present invention, a device for process monitoring during the production of a finished part from a hot-curing material in a primary forming process is proposed. The device comprises at least one tool configured for one embodiment of the primary forming process. The tool has at least one cavity for receiving at least one starting material for the hot-curing material and the tool further comprises a device for determining an internal tool pressure. The device also comprises at least one control unit. The control unit is configured to perform at least steps b) to f) of the process as already described or as will be described below.

[0057] For further details regarding the device according to the invention, reference is made to the description of the method according to the invention. Advantages of the invention

[0058] The present method and the associated device enable continuous monitoring of finished part quality in series production, aiming for zero-defect production with 100% quality. Visualizing the crosslinking of the raw materials of the hot-curing material during the injection molding process allows for precise component and process monitoring. Additionally, the cycle time of finished parts can be accurately determined, and the progress of the crosslinking reaction can be tracked. In particular, crosslinking can be determined and / or read from a measured mold cavity pressure curve.

[0059] The internal mold pressure, which is sometimes measured as standard for process monitoring, can be specifically evaluated. The internal mold pressure curve can be derived twice, and the largest peak within the expected range can be determined as the point at which cross-linking is complete. This works both in injection molds for series production and in smaller measurement setups. Information about the cross-linking can be obtained for each manufactured component and, if necessary, documented, particularly for quality assurance or traceability purposes.

[0060] The optimal cycle time can be determined based on the set tool temperature, and the crosslinking reaction can be tracked live during production. This allows for cycle time optimization, resulting in cost savings and more efficient resource utilization. In addition to quality assurance, the analysis of the tool cavity pressure can also be used to set and optimize cycle times when implementing new production processes.

[0061] In contrast, the present invention allows the temporal profile of the tool cavity pressure to be used to monitor the chemical crosslinking reaction itself. In particular, it is possible to search for a characteristic segment and / or a characteristic region in the tool cavity pressure profile.

[0062] In particular, discontinuities such as inflection points, declines, increases and / or gradient profiles can be searched for during the heating phase.

[0063] There is no need to wait until the material is fully heated and thus expanded. Brief description of the characters

[0064] Preferred embodiments of the present invention are illustrated in the figures and are explained in more detail in the following description without limitation of generality. These include: Figure 1 shows a schematic representation of a preferred embodiment of a device according to the invention for process monitoring during the production of a finished part from liquid silicone rubber in a primary forming process; Figures 2A and 2B show a schematic progression of a pressure signal against the temperature in a cavity, divided into three phases ( Figure 2A) and schematic representation of the second derivative of the pressure signal with characteristic curve profile, which marks the crosslinking of the liquid silicone rubber ( Figure 2B ); Figures 3A and 3Legs CAD sectional view of a test chamber ( Figure 3A ) and a component used for pressure measurement during the injection molding process ( Figure 3B ); Figure 4 Results of DSC measurements for Silopren 2050, in particular a temperature-dependent representation of the cross-linking for different heating rates; Figures 5A to 5C a measurement of pressure and temperature of Silopren 2050 over the measured time during heating from 25 °C to 150 °C with a heating rate of 2.9 K min⁻¹, holding at 150 °C and cooling to 25 °C ( Figure 5A ); a pressure of Silopren 2050 applied against the temperature during heating and cooling ( Figure 5B ) and a smoothed pressure curve with first and second derivatives for Silopren 2050 and Silastic MS-1002 ( Figure 5C); and Figure 6 smoothed pressure curves of Silopren 2050 in the injection mold at 150 °C and 180 °C mold temperature. Embodiments of the invention

[0065] Figure 1 Figure 1 shows a schematic representation of a preferred embodiment for a device 110 according to the invention for process monitoring during the production of a finished part 112 made of liquid silicone rubber in a primary forming process.

[0066] In Figure 1 A tool 114 of the device 110 is shown. The tool 114 has a cavity 116, wherein the cavity 116 is in Figure 1The mold is designed, for example, as a cavity between two joined mold plates 118, which are usually referred to as the nozzle-side mold plate 120 and the closing-side mold plate 122. The nozzle-side mold plate 120 has an opening 123 into which an injection device 124 is inserted, enabling the introduction of components 130 of the liquid silicone rubber into the cavity 116. The injection device 124 can, in particular, include a conveying unit 126, especially an extruder 128. The mold 114 can, in particular, include a heating device and a device for determining an internal mold pressure. These are described in Figure 1 not shown.

[0067] The components 130 can be mixed, in particular, by means of a mixing device 132 of the device 110, which may in particular comprise a static mixer 134. Furthermore, the device 110 may have a metering device 136 for ink or additives, which is connected to the mixing device 132.

[0068] Figure 2A shows a schematic curve of a pressure signal p versus temperature T in a cavity, divided into three phases 1, 2 and 3.

[0069] There are fundamentally significant differences between the mold pressure curves of thermoplastic components and those of liquid silicone rubber components: Unlike thermoplastic products, shrinkage during injection molding is masked by the thermal expansion of the liquid silicone rubber in the hot mold. Therefore, the mold pressure curves look fundamentally different than when processing thermoplastics. As the cold liquid silicone rubber is heated to mold temperatures of up to 220 °C, the pressure in the mold increases continuously. Depending on the mold geometry, the position of the pressure sensor, and the component thickness, different phases in the pressure curve can be identified. These phases can generally be described by the following three material stages, which are described in Figure 2A schematically represented are: Phase 1: Liquid material is heated and expands as a result. Phase 2: Crosslinking phase: The material shrinks due to the change in density, while additional heat is supplied through the exothermic crosslinking reaction. Heating continues, and the liquid silicone rubber expands. An overlap of the effects can be observed. At the point marked with arrow 138, it can be seen that additional exothermic crosslinking energy leads to further expansion. Phase 3: The solid material is heated to the set mold temperature and expands as a result. At the point marked with arrow 140, it can be seen that a decrease in density during curing leads to stagnant pressure.

[0070] Figure 2B shows a schematic representation of the second derivative p" (t) of the pressure signal with a characteristic curve shape that marks the crosslinking of the liquid silicone rubber.

[0071] According to the theory described above, the networking within phase 2 of the pressure curve is visible during the injection molding process. By differentiating the measured mold cavity pressure profile twice, a characteristic point can be determined by a zero point in the second derivative, accompanied by larger deflections both upwards and downwards, as described above. Figure 2B As can be seen in Box 142. Around this point, the tool cavity pressure curve initially shows a significantly steeper slope, then the slope drops abruptly and shortly afterwards returns to a continuous slope.

[0072] The following Figures 3A to 6These results are based on conducted experiments. A commercial two-component liquid silicone rubber material from Momentive (Silopren™< LSR 2050) with a hardness of 50 Shore A was used. Components A and B were mixed in a 1:1 ratio. To demonstrate the generalizability of the results, an optical liquid silicone rubber formulation, Silastic™< MS-1002, with a hardness of 72 Shore A from Dow, was also investigated. The two components were also mixed in a 1:1 ratio.

[0073] Differential scanning calometry (DSC) is the state of the art for determining the crosslinking reaction of reactive materials such as liquid silicone rubber. DSC measurements are used as the reference method below. The DSC measurements were performed using a DSC 214 Polyma differential scanning calometry analyzer from NETZSCH Gerätebau GmbH. Sample masses of 10 mg were analyzed at four heating rates (1 K min⁻¹, 2.9 K min⁻¹, 5 K min⁻¹, and 10 K min⁻¹) in single-drilled and welded aluminum crucibles. For dynamic measurements, the sample crucibles and an empty, single-drilled aluminum crucible were used as a reference. The temperature profile includes an initial heating from -70 °C to 220 °C, a cooling period and subsequent holding time of 15 minutes at -70 °C, followed by a second heating to 220 °C.The second heating step ensures that the liquid silicone rubber is fully cross-linked after the first heating. During the measurement, the system was purged with nitrogen. Each measurement series was performed at least twice. The sample material was taken directly from the injection molding unit. For sample preparation, a 20-minute interval was observed between material removal and the start of the measurement for all measurements. Only the two heating curves were used for the evaluation of the DSC measurements. The first heating curve shows the cross-linking reaction as an exothermic spike, while the second heating curve remains unchanged at this point because the material is already fully and irreversibly cross-linked.

[0074] Figure 3A shows a CAD sectional view of a test chamber 144 and Figure 3Bshows a component 146 used for pressure measurement during the injection molding process.

[0075] To determine the pressure behavior of liquid silicone rubber under constant volume and temperature changes, a test device 148 was developed. As in Figure 3AAs can be seen, the test device 148 comprises a lower part 150 with a centrally arranged injection molding pressure sensor 152, Kistler type 6157C, and an upper part 154 with a centrally arranged temperature sensor 156, Kistler type 6193A. The test device 148 also includes a sample chamber 158. The sample chamber 158 has a diameter of 19.89 mm, a height of 3.01 mm, and thus a volume of 935.25 mm³. For the measurements, 1.04 g of liquid silicone rubber was placed in the lower part of the sample chamber 158, the upper part was placed on top, and fastened with four screws, each with a torque of 10 Nm. Preliminary investigations have shown that this is the optimal tightening torque. If the tightening torque is too low, the test chamber 144 will generally open during the thermal expansion of the liquid silicone rubber and the pressure will drop suddenly.If the tightening torque is too high, the liquid silicone rubber is pre-stressed and the pressure in the test chamber 144 is already high at the beginning of the measurement.

[0076] The test device 148 was set up in a climate chamber (espec SH-241) heated to 25 °C (not shown in Figure 3A Sensor cables were routed in a special cable duct. A type K temperature sensor was also installed in the climate chamber to record the chamber temperature. The two temperature sensors and the pressure sensor were connected to the Kistler ComoNeo type 5887A process monitoring device. Measurement data from the climate chamber and test fixture 148 were continuously recorded while the climate chamber heated from 25 °C to 150 °C at a heating rate of 2.9 K min⁻¹, maintained this temperature for one hour, and then cooled back down to 25 °C.

[0077] To demonstrate the validity of the investigated effect in the actual injection molding process, liquid silicone rubber components were produced with different setting parameters. For this purpose, the [product / method] was used. Figure 3B The component shown, 146, is made of the same material, Silopren LSR 2050. A Kistler 6152B in-mold pressure sensor 160 is located in the area of ​​a sprue 162. The sprue 162 has a thickness of 2.5 mm at this point. Pressure signals were evaluated using a ComoNeo type 5887A process monitoring device.

[0078] Figure 4This shows results of DSC measurements for Silopren 2050, in particular a temperature-dependent representation of the crosslinking for different heating rates. The DSC signal DSC-S in Wmg⁻¹ is shown as a function of the temperature T in °C. Different heating rates were used. The curve marked with squares shows the behavior at a heating rate of 1 K min⁻¹. The curve marked with triangles shows the behavior at a heating rate of 2.9 K min⁻¹. The curve marked with crosses shows the behavior at a heating rate of 1 K min⁻¹. The curve marked with diamonds shows the behavior at a heating rate of 1 K min⁻¹.

[0079] The DSC measurements performed can generally show the crosslinking response at different heating rates, as shown from Figure 4This is evident. Thus, at a heating rate of 2.9 K min⁻¹ (curve with triangles), an exothermic crosslinking reaction begins at 97 °C, reaches a conversion maximum at 107 °C, and is complete at 112 °C. The same result should be demonstrated using the new measurement method via the pressure curve at a constant heating rate.

[0080] Figure 5A Figure 1 shows a measurement of the pressure and temperature of Silopren 2050 over the measured time during heating from 25 °C to 150 °C at a heating rate of 2.9 K min⁻¹, holding at 150 °C, and cooling to 25 °C. The temperature T in the climate chamber (dashed line), the temperature T in test chamber 144 (bold line), and the pressure p in test chamber 144 are shown, each as a function of the measurement time t.

[0081] First, the furnace is heated to 150 °C at a rate of 2.9 K min⁻¹. This temperature is maintained for one hour to allow the furnace temperature to reach the sample chamber. During this time, the pressure in the sample chamber rises to up to 370 bar. The furnace is then cooled to 25 °C. Due to the shrinkage described above, the pressure drops rapidly to 0 bar.

[0082] Figure 5B The graph shows the pressure p of Silopren 2050 plotted against temperature T during heating and cooling. The key points of the DSC measurement are indicated: initiation of crosslinking (98 °C), maximum conversion (106 °C), and end of crosslinking (112 °C).

[0083] Figure 5B shows the in Figure 5AThe measurement series shown is a recording of the pressure curve versus the chamber temperature. The different profiles of the heating and cooling curves are clearly visible. While effects related to the filling of the test device 148 occur at the beginning of the heating process and are generally not of interest, a continuous pressure increase is achieved from 70 °C onwards. Figure 5B The key points of the heating curve are highlighted by extending the linear sections. As shown in the diagram... Figure 5BAs can be seen, the pressure curve begins to rise at 98 °C and reaches its maximum at 106 °C. From 112 °C, the pressure curve rises again to a continuous curve, increasing linearly up to a pressure of 370 bar at the maximum temperature. The key points correspond to the expected temperatures determined by the DSC measurement. These are the start and end times of crosslinking and the peak temperature at which crosslinking shows its maximum development. During cooling, the measured pressure drops until it reaches zero at 75 °C. This can be explained by crosslinking shrinkage, which causes the liquid silicone rubber sample to no longer be in contact with the injection molding pressure sensor 152. The measurements showed that the crosslinked sample has a thickness of 2.92 mm at room temperature. Taking into account the test chamber height of 3.01 mm, this corresponds to a shrinkage of 3.0%.

[0084] Figure 5CThe graph shows a smoothed pressure curve (top) with first derivative (middle) and second derivative (bottom) for Silopren 2050 (dashed line in each case) and Silastic MS-1002 (solid line in each case). The second derivative shows a characteristic profile at 110.1 °C and at 95.3 °C.

[0085] Since networking is of particular interest from a process engineering perspective, the heating curve will be examined in more detail below. To clarify the networking area, the following is shown in Figure 5CThe pressure-temperature curve was derived twice for Silopren 2050 (dashed line in each case) and, for validation, for another liquid silicone rubber type, Silastic MS-1002 (solid line in each case). What is only noticeable as a discontinuity in the slope of the pressure curve at approximately 110 °C for Silopren is clearly visible in the second derivation. The typical curve shape is: a large negative change in slope, a zero point, and a large positive change in slope. This curve shape essentially reflects the material changes during crosslinking.

[0086] The analysis of the derivative of Silopren 2050 shows a zero point of the second derivative at 110.1 °C. Compared with the characteristic temperatures of the DSC measurement, a correlation can be seen between the end of crosslinking in the DSC measurement at 112 °C and the zero point of the second derivative of the pressure curve at 110.1 °C for Silopren 2050. The other liquid silicone rubber material, an optical liquid silicone rubber formulation (Silastic MS-1002 from Dow), was also investigated. The DSC measurement shows the maximum conversion (peak) at a temperature of 95.3 °C. When measuring the low-viscosity material in the test chamber, a characteristic crosslinking profile was found at 95.4 °C using the double derivative method. In contrast to Silopren 2050, here the zero point coincides with the maximum crosslinking rate (peak) in the DSC measurement.

[0087] The shape of the curve before the crosslinking peak appears very different for the two materials investigated. This is presumed to be due to the different viscosities and the associated filling of the measuring device. While the very low-viscosity Silastic MS-1002 can be poured into the measuring device like water and flows out flat due to gravity, the measuring deviceWhile the highly viscous Silopren 2050 fills the measuring device completely, it is not so easy to pour. During pouring, the material rises up in the center and is then flattened when the lid is put on. Even then, the mass is relatively dimensionally stable and does not immediately fill the measuring device completely. The free areas at the edge are only filled by a combination of temperature increase and thus decreasing viscosity, as well as thermal expansion and thus displacement towards the edge, which is noticeable in the pressure sensor signal. Furthermore, the thermal expansion depends on the material formulation. A high filler content leads to comparatively little shrinkage, since the thermal expansion of the filler is less than that of the polymer matrix. Optical silicones, for example, have a high filler content, which, as in Figure 5CThis can be seen as leading to lower thermal expansion and therefore lower pressure in the cavity.

[0088] Nevertheless, it can be shown that the evaluation method of the double derivative of the pressure signal can be used to represent the crosslinking in the pressure signal, regardless of the material. Furthermore, it can be shown that, depending on the liquid silicone rubber formulation, the zero point in the second derivative coincides with both the end of the crosslinking process and the maximum crosslinking conversion.

[0089] Figure 6 This shows smoothed pressure curves of Silopren 2050 in the injection mold at mold temperatures of 150 °C (bold line) and 180 °C. An injection speed of 50 cm s⁻¹ was used to generate both pressure curves.

[0090] To make the interpretation of the pressure signal usable for the liquid silicone rubber processing industry, the measurement methodology and its characteristic curve were transferred to the internal pressure of injection molds. For this purpose, the internal pressure of the mold was measured during the production of liquid silicone rubber components. Since this is also a closed system, like the test chamber, the crosslinking reaction could also be measured in the pressure profile.

[0091] Many tools used in industrial practice are not equipped with temperature sensors. Therefore, pressure profiles over the cycle time are generally available. An evaluation of the second derivative shows two different characteristic curves for the two tool temperatures: At a tool temperature of 150 °C, the zero point of the second derivative is found after 30.8 s, while at a tool temperature of 180 °C, a zero point is found after 19.8 s. To validate the results, the meshing profiles and transformations for both tool temperatures were simulated using a well-fitting simulation method. Regarding the well-fitting simulation method, reference is made to the publication by D.F. Weißer, D. Walz, J. Schmid, D. Mayer, and M.H. Deckert, Jnl Adv Manuf & Process 2020. According to this publication, complete meshing occurs after 31.2 s at a tool temperature of 150 °C and after 19.0 s at a tool temperature of 180 °C.This means that the curing end measured in the pressure curve deviates by 0.4 s (1.3%) at a mold temperature of 150 °C and the curing time by -0.8 s (-4.0%) at a mold temperature of 180 °C compared to the simulation. The simulation takes into account the injection time of 0.6 s until the mold is completely filled.

[0092] The test data show that the analysis of the pressure profiles during the injection molding process indicates the curing process. Furthermore, the measured temperatures for complete curing agree with the simulation values, except for minor measurement uncertainties. Despite smoothing, the measured pressure profiles show a high degree of agreement with the simulation values. List of reference symbols

[0093] 110 Device 112 Finished part 114 Tool 116 Cavity 118 Mold plate 120 Nozzle-side mold plate 122 Closing-side mold plate 123 Opening 124 Injection device 126 Conveyor unit 128 Extruder 130 Component 132 Mixing device 134 Static mixer 136 Metering device 138 Arrow 140 Arrow 142 Box 144 Test chamber 146 Component 148 Test device 150 Lower part 152 Injection molding pressure sensor 154 Upper part 156 Temperature sensor 158 Sample chamber 160 Mold cavity pressure sensor 162 Gate

Claims

1. Method for process monitoring during production of a finished part (112) from a hot-crosslinking material in a primary forming process, the method comprising the following steps: a) providing at least one tool (114) which is set up for the primary forming process, the tool (114) having at least one cavity (116) for receiving at least one starting substance for the hot-crosslinking material, and further an apparatus for determining a tool internal pressure being integrated in the tool (114); b) heating the tool (114); c) introducing the at least one starting substance for the hot-crosslinking material under pressure into the cavity (116), such that the finished part (112) is produced; d) recording a variation over time of the tool internal pressure that arises in step c); e) differentiating, at least once or at least twice, the variation of the tool internal pressure over time to determine at least one derivative selected from the following group: the first order derivative; the second order derivative; and f) characterizing a profile of a chemical crosslinking reaction by means of the at least one derivative selected from the following group: the first-order derivative; the second-order derivative.

2. Method according to the preceding claim, wherein the hot-crosslinking material is selected from the following group: a liquid silicone rubber; solid silicone rubber; an epoxy resin; a polyurethane; a polyurethane foam; a thermoset, in particular a free-flowing thermoset.

3. Method according to either of the preceding claims, wherein the hot-crosslinking material is liquid silicone rubber.

4. Method according to the preceding claim, wherein the liquid silicone rubber is selected from the following group: a self-lubricating liquid silicone rubber; a self-adhesive liquid silicone rubber; an optical liquid silicone rubber; a medical liquid silicone rubber; an insulating liquid silicone rubber.

5. Method according to one of the preceding claims, wherein the hot-crosslinking material has a cycle time of less than 60 s.

6. Method according to one of the preceding claims, wherein, in step f), the profile of the chemical crosslinking reaction is characterized by an evaluation of the at least one derivative selected from the following group: the first-order derivative, the second-order derivative, during at least one phase of the tool internal pressure profile selected from the following group: a heating phase, a crosslinking phase.

7. Method according to one of the preceding claims, wherein the apparatus for determining the tool internal pressure is used to directly and / or indirectly determine the tool internal pressure.

8. Method according to one of the preceding claims, wherein, in step f), the characterization of the profile of the chemical crosslinking reaction is determined by ascertaining at least one of the following specific variables: • a zero crossing of the second-order derivative; • a profile of the second-order derivative; • an extreme value of the second-order derivative.

9. Method according to one of the preceding claims, wherein step f) is used to determine a conclusion of the chemical crosslinking reaction.

10. Method according to the preceding claim, wherein, if step f) is used to determine the conclusion of the chemical crosslinking reaction, step c) is ended.

11. Method according to one of the preceding claims, wherein, by virtue of step f), a cycle time of the production of the finished part (112) in the primary forming process is optimized.

12. Apparatus (110) for process monitoring during production of a finished part (112) from a hot-crosslinking material in a primary forming process, wherein the apparatus (110) comprises at least one tool (114) which is set up for implementing the primary forming process, the tool (114) having at least one cavity (116) for receiving at least one starting substance for the hot-crosslinking material, and the tool (114) further having an apparatus for determining a tool internal pressure, the apparatus (110) also having at least one controller, the controller being set up to carry out steps b) to f) according to one of the preceding method claims.